Crystal of pregnane X receptor fused to steroid receptor coactivator-1

This application is a divisional of U.S. application Ser. No. 11/522,698, filed Sep. 18, 2006, now U.S. Pat. No. 7,785,879, which claims the benefit of U.S. provisional patent application No. 60/718,516; filed Sep. 19, 2005, which is herein incorporated by reference in its entirety.

The present invention relates, inter alia, to pregane X receptor (PXR) polypeptides, polynucleotides, crystals, methods of use thereof and assays for identifying PXR modulators.

Two major problems associated with the administration of drugs have been poor pharmacokinetics (PK) and undesired drug-drug interactions. It is known that the cytochrome P450 (CYP) 3A family of enzymes are important in human drug metabolism because they metabolize the majority of commercially available drugs (Wrighton et al., Drug metabolism reviews 32: 339-361 (2000)). In 1998, the pregnane X receptor (PXR), which orchestrates the induction of some of the major genes involved in drug detoxification, was identified. (Lehmann, et al., J. Clin. Invest. 102(5):1016-1023 (1998); Genbank Accession No. AF061056). PXR, also known as steroid and xenobiotic receptor (SXR or PAR), was shown to be activated by many drugs, steroids such as pregnenolone 16α-carbonitrile; RU-486; dexamethasone; and rifampicin (Kliewer et al., Cell 92(1):73-82 (1998); Kliewer et al., J. Lipid Res. 43:359-364 (2002)) and St. John's wort (hyperforin) (Moore et al., Proc. Natl. Acad. Sci. USA; 97(13):7500-7502 (2000)) and recently was shown to be a sensor for bile acids (Staudinger et al., Drug Metab. Dispos. 29(11):1467-1472 (2001); Staudinger et al., Proc. Natl. Acad. Sci. USA. 98(6):3369-74 (2001)). PXR is also activated by other drugs such as clotrimazole (Lehmann et al., J. Clin. Invest. 102:1016-1023 (1998)); 11β-hydroxylase inhibitor metyrapone (Goodwin et al., Mol. Pharmacol. 56:1329-1339 (1999); Wright et al., Biochem. Soc. Trans. 27:387-391 (1998)); troglitazone (Jones et al, Mol. Endocrinol. 14:27-39 (2000)); ritonavir (Dussault et al., J. Biol. Chem. 276:33309-33312 (2001)) and taxol (Synold et al., Nat. Med. 7:584-590 (2001)) as well as by environmental pollutants such as bisphenol A, diethylhexylphthalate, and nonylphenol (Masuyama et al., Mol. Endocrinol. 14:421-428 (2000); Takeshita et al., Eur. J. Endocrinol. 145:513-517 (2001)). PXR plays a key role in the regulation of both drug metabolism and efflux by modulating a plethora of genes encoding cytochrome P450 enzymes (CYPs, especially CYP3A4) and a multidrug resistant gene ABCB1 (Synold et al., Nat. Med. 7(5):584-590 (2001)). Together, these proteins are responsible for the elimination of >50% of all drugs. Identification of candidate pharmaceutical treatments which interact with PXR and are likely to be metabolized by the PXR system is enormously useful information when performing an early evaluation of the safety and pharmacokinetic profile of the treatment. Therefore, high-throughput screening assays detecting PXR-mediated induction have become pivotal at early discovery stages in order to decrease the time line for clinical drug development. Moreover, crystals of PXR are useful for these purposes in that they provide a greater understanding of ligand interactions with the protein and allow computer-assisted, structure-based evaluation of candidate treatments.

Several crystals comprising PXR are known in the art. For example, the structure of apo-PXR-LBD crystal comprising space group P4 3 2 1 2 was solved with a 2.52 Å resolution (Watkins et al., Science 292:2329-2333 (2001)); the structure of PXR-LBD/SR12813 crystal comprising space group P4 3 2 1 2 was solved with a 2.76 Å resolution (Watkins et al., Science 292:2329-2333 (2001)); the structure of PXR-LBD/Hyperforin comprising space group P4 3 2 1 2 was solved with a 2.15 Å resolution (Watkins et al., Biochemistry 42:1430-1438 (2003)).

In addition, the structure of PXR-LBD/SR12813/SRC1 crystal comprising space group P2 1 2 1 2 1 was solved with a resolution of 2.00 Å (Watkins et al., J. Mol. Biol. 331:815-828 (2003)). The crystal in Watkins et al., however, comprised PXR-LBD complexed with SRC1 and not covalently bound in a hybrid.

The present invention provides a polypeptide and crystalline composition comprising a highly advantageous hybrid construct wherein PXR or PXR-LBD (PXR ligand binding domain) is tethered to SRC1 peptide. SRC1 is a coactivator which binds to and stabilizes PXR (see e.g., Watkins, et al.). The solubility of PXR or PXR-LBD is also increased by binding to SRC1. Tethering SRC1 to PXR or PXR-LBD allows PXR to be saturated, with SRC1, to a greater level that when the two polypeptides are merely complexed. This highly-saturated level of binding to SRC1 results in a highly soluble and highly stabilized PXR or PXR-LBD. Moreover, saturation of PXR-LBD with SRC1 peptide is convenient, aids in the generation of high quality crystalline complexes and aids in the generation of high quality binding data (e.g., temperature dependent circular dichroism data) between the hybrid and a candidate binding compound. In an embodiment, the binding of PXR-LBD and SRC1 peptide is optimized in a hybrid of the invention by placing a 10 amino acid peptide linker between the PXR-LBD and the SRC1 peptide.

The present invention provides an isolated fusion polypeptide comprising pregnane X receptor (PXR) polypeptide or a fragment thereof, optionally fused to a linker polypeptide, fused to SRC-1 polypeptide or a fragment thereof. In an embodiment of the invention, the linker comprises from about 8 to about 13 amino acids. In an embodiment of the invention, the PXR fragment is the ligand binding domain (LBD) of pregnane X receptor. In an embodiment of the invention, the pregnane X receptor polypeptide or fragment thereof comprises the amino acid sequence set forth in SEQ ID NO: 1 or 2. In an embodiment of the invention, the SRC-1 polypeptide or fragment thereof comprises the amino acid sequence set forth in SEQ ID NO: 3 or SLTERHKILHRLLQEGSPS (SEQ ID NO: 35). In an embodiment of the invention, the fusion polypeptide is complexed to a member selected from the group consisting of SR12813, hyperforin, clotrimazole, ritonavir, sulfopyrole, pregnane-16α-carbonitrile, RU-486, rifampicin, dexamethasone, paclitaxel, metyrapone, bisphenol A, diethylhexylphthalate, nonylphenol, phthalate, cisplatin, chenodeoxycholic acid, deoxycholic acid, lithocholic acid, 17β-estradiol (estradiol), pregnenolone (5-pregneno-3β-ol-20-one), progesterone, and Medroxyprogesterone-acetate (MPA).

The present invention provides a composition comprising PXR polypeptide or a fragment thereof and SRC-1 polypeptide or a fragment thereof. In an embodiment of the invention, the pregnane X receptor polypeptide or fragment thereof comprises the amino acid sequence set forth in SEQ ID NO: 1 or 2. In an embodiment of the invention, the SRC-1 polypeptide or fragment thereof comprises the amino acid sequence set forth in SEQ ID NO: 3 or SLTERHKILHRLLQEGSPS (SEQ ID NO: 35).

The present invention provides, an isolated polynucleotide encoding the fusion polypeptide of claim 1 . In an embodiment of the invention, the polynucleotide comprises a nucleotide sequence encoding a polypeptide selected from the group consisting of SEQ ID NOs: 22, 24, 26, 28, 30, and 32. The present invention also provides a recombinant vector comprising said polynucleotide. The present invention further provides an isolated host cell comprising said vector.

The present invention provides an isolated polynucleotide comprising a bicistronic cassette comprising a polynucleotide encoding pregnane X receptor or a fragment thereof, which is adjacent to a ribosome binding site, which is adjacent to a polynucleotide encoding SRC-1 or a fragment thereof. In an embodiment of the invention, the polynucleotide encoding the pregnane X receptor is located 5′ of the ribosome binding site. In an embodiment of the invention, the polynucleotide encoding SRC-1 or said fragment thereof is located 5′ of the ribosome binding site. In an embodiment of the invention, the polynucleotide is operably linked to a promoter. The present invention also provides a recombinant vector comprising the polynucleotide comprising the cassette. An isolated host cell comprising the vector also forms part of the present invention.

The present invention also provides a crystalline composition comprising a hybrid polypeptide comprising PXR polypeptide or a ligand binding domain (LBD) thereof tethered, optionally by a peptide linker, to SRC-1 polypeptide or a fragment thereof, optionally complexed with SR12813. In an embodiment of the invention, the crystal comprises a dimer which comprises two monomers comprising said hybrid polypeptides. In an embodiment of the invention, the crystal comprises a tetragonal space group whose asymmetric unit comprises one monomer or orthorhombic space group whose asymmetric unit comprising two monomers. In an embodiment of the invention, the crystal comprises a hybrid polypeptide comprising PXR polypeptide or a ligand binding domain (LBD) thereof tethered by a peptide linker to SRC-1 polypeptide or a fragment thereof, optionally complexed with SR12813 or a structural homologue thereof, wherein the polypeptide three dimensional structure is characterized by structural coordinates comprising a root mean square deviation of conserved or common residue backbone atoms (e.g., backbone atoms of residues common to both structures being compared) or alpha carbon atoms of less than about 1.5 Å when superimposed on backbone atoms or alpha carbon atoms described by structural coordinates of Table 3a, 3b, 4a, 5a or 5b; and wherein the three dimensional coordinates of the SR12813 or the structural homologue thereof are characterized by structural coordinates comprising a root mean square deviation of backbone atoms or alpha carbon atoms over those residues commonly present in the compared structures of less than about 1.5 Å (e.g., about 1 Å, about 0.75 Å, about 0.5 Å, about 0.25 Å or about 0.1 Å) when superimposed on atoms described by structural coordinates of Table 5c. In an embodiment of the invention, the PXR polypeptide or ligand binding domain thereof comprises the amino acid sequence set forth in SEQ ID NO: 1 or 2. In an embodiment of the invention, the SRC-1 polypeptide or said fragment thereof comprises the amino acid sequence set forth in SEQ ID NO: 3 or SLTERHKILHRLLQEGSPS (SEQ ID NO: 35). In an embodiment of the invention, solvent molecules are associated with the crystalline polypeptide. In an embodiment of the invention, the three dimensional structure of the solvent molecules are characterized by the coordinates set forth in Table 3c or Table 5d. In an embodiment of the invention, the crystal comprises a crystal lattice in a space group selected from the group consisting of P2 1 2 1 2 1 and P4 3 2 1 2. In an embodiment of the invention, the crystal comprises a unit cell with dimensions selected from the group consisting of a=84.2 Å, b=90.0 Å, c=106.6 Å, α=β=γ=90°; a=b=94.2 Å, c=88.2 Å, α=β=γ=90°; and a=86.0 Å, b=89.3 Å, c=106.0 Å, α=β=γ=90°. The present invention provides a crystalline composition comprising a dimer which comprises one or more monomers comprising a polypeptide comprising the amino acid sequence of SEQ ID NO: 24, which is optionally complexed with SR12813 wherein: (a) a first monomer is characterized by the structural coordinates set forth in Table 3a and a second monomer is characterized by the structural coordinates set forth in Table 3b; (b) a monomer is characterized by the structural coordinates set forth in Table 4a; or (c) a first monomer is characterized by the structural coordinates set forth in Table 5a, a second monomer is characterized by the structural coordinates set forth in Table 5b; and SR12813 complexed with said polypeptides is characterized by the structural coordinates set forth in Table 5c.

The present invention provides a method for stabilizing a pregnane X receptor polypeptide or fragment thereof comprising providing said polypeptide or fragment in association with SRC-1 or a fragment thereof. The present invention also provides a method for stabilizing a pregnane X receptor polypeptide or fragment thereof comprising tethering said polypeptide or fragment to SRC-1 or a fragment thereof. In an embodiment of the invention, said pregnane X receptor or fragment thereof is tethered to SRC-1 or the fragment thereof by a linker.

The present invention also provides a method for identifying a modulator of PXR, a modulator of CYP3A4 activation or a modulator of in vivo drug half-life comprising: (a) contacting a composition comprising PXR or a fragment or fusion thereof and SRC-1 or a fragment of fusion thereof with a substance to be tested for the presence of the modulator; and (b) determining the ellipticity of the composition contacted with the substance; whereby the substance is selected if the ellipticity of the composition alone is different from the ellipticity of the composition that is in contact with the substance.

The present invention also provides a method for identifying a modulator of PXR, a modulator of CYP3A4 activation or a modulator of in vivo drug half-life comprising: (1) contacting a composition comprising PXR or a fragment or fusion thereof and SRC-1 or a fragment of fusion thereof with a substance to be tested for the presence of a modulator; and (2) determining if the substance binds to said PXR or fragment or fusion thereof; whereby the substance is selected if binding is observed.

The present invention also provides a method for identifying a modulator of PXR, a modulator of CYP3A4 activation or a modulator of in vivo drug half-life comprising: (A) contacting an isolated host cell comprising PXR or a fragment or fusion and SRC-1 or a fragment or fusion thereof and a reporter gene operably linked to a CYP3A4 promoter or to an ABCB1 promoter with a substance to be tested for the presence of the modulator; and (B) determining if the reporter gene is expressed; whereby the substance is selected if the reporter gene, in the cell contacted with said substance, is expressed at a higher or lower level than a reporter gene in a host cell that is not contacted with said substance.

The present invention also provides a method for identifying a modulator of PXR, a modulator of CYP3A4 activation or a modulator of in vivo drug half-life comprising: (i) contacting a composition comprising PXR or a fragment or fusion thereof and SRC-1 or a fragment of fusion thereof with a ligand that is known to bind PXR or said fragment or fusion thereof and with a substance to be tested for the presence of a modulator or binder; and (ii) determining if the substance being tested reduces binding of the ligand to PXR or the fragment or fusion thereof; whereby the substance is selected if binding of the ligand to PXR or the fragmentor fusion thereof, in the presence of the substance, is less than binding of the ligand to PXR or the fragment or fusion thereof in the absence of the substance.

The present invention also provides a method for determining whether the half-life of a drug will be affected by the cytochome P450 pathway when administered to a subject in vivo, comprising (i) contacting an in vitro composition comprising PXR or a fragment or fusion thereof and SRC-1 or a fragment of fusion thereof with the drug; and (ii) determining whether the drug binds to PXR or the fragment or fusion thereof or modulates PXR or the fragment or fusion thereof; whereby the half-life of the drug is determined to be affected by the P450 pathway, when administered in vivo, if said binding or modulation is observed.

The present invention also provides a method for determining whether a first drug will affect the half-life of a second drug when co-administered to a subject in vivo comprising: (i) contacting an in vitro composition comprising PXR or a fragment or fusion thereof and SRC-1 or a fragment of fusion thereof with said first drug; and (ii) determining whether said first drug binds to or modulates PXR; whereby the half-life of the second drug is determined to be affected by in vivo co-administration with the first drug if said binding or modulation is observed.

The present invention also provides a method for determining whether a drug will be metabolized, when administered to a subject in vivo, into a prodrug comprising (i) contacting an in vitro composition comprising PXR or a fragment or fusion thereof and SRC-1 or a fragment of fusion thereof with the drug; and (ii) determining whether the drug binds to or modulates PXR; whereby it is determined that the drug will be metabolized into a prodrug, when administered in vivo, if said binding or modulation is observed.

The present invention also provides a method for identifying a PXR agonist or antagonist comprising the steps of: a) crystallizing PXR or a fragment thereof fused to SRC-1 or a fragment thereof to form at least one crystal; b) irradiating the crystal produced by step (a) to obtain a diffraction pattern of said crystal; c) determining the atomic coordinates of the three-dimensional structure of the PXR or the fragment or fusion thereof from the diffraction pattern; d) using the atomic coordinates and one or more molecular modeling techniques to identify an agent that interacts with the PXR; and e) determining if the agonist or antagonist increases or decreases the in vivo activation of cytochrome P450 enzymes or the multidrug resistant gene ABCB1, binds to or modulates PXR or modulates half life of drugs in vivo; wherein the agonist or antagonist is selected if it increases or decreases the in vivo activation of cytochrome P450 enzymes or the multidrug resistant gene ABCB1, binds to or modulates PXR or modulates half life of drugs in vivo.

The nuclear xenobiotic receptor PXR is activated by a wide variety of clinically used drugs and serves as a master regulator of drug metabolism and excretion gene expression in mammals. Upon activation, PXR binds to DNA and regulates a network of genes in the liver and intestine that are involved in the metabolism and efflux of potentially harmful xenobiotics. In addition, it is also responsible for an important class of harmful drug-drug interactions. The development of high throughput ligand binding assays as well as the determination of crystal structures of PXR ligand binding domain (LBD) will help to prevent the PXR-mediated effects. Two bicistronic constructs (containing PXR-LBD and SRC-1, a fragment of human steroid receptor coactivator-1), and several tethered PXR-SRC constructs were engineered and expressed in E. coli cells. The purified proteins were suitable for ligand binding assays and/or X-ray crystallization.

In accordance with the present invention, there may be employed conventional molecular biology, microbiology, and recombinant DNA techniques within the skill of the art. Such techniques are explained fully in the literature. See, e.g., Sambrook, Fritsch & Maniatis, Molecular Cloning: A Laboratory Manual , Second Edition (1989) Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (herein “Sambrook et al., 1989”); DNA Cloning: A Practical Approach , Volumes I and II (D. N. Glover ed. 1985); Oligonucleotide Synthesis (M. J. Gait ed. 1984); Nucleic Acid Hybridization [B. D. Hames & S. J. Higgins eds. (1985)]; Transcription And Translation [B. D. Hames & S. J. Higgins, eds. (1984)]; Animal Cell Culture [R. I. Freshney, ed. (1986)]; Immobilized Cells And Enzymes [IRL Press, (1986)]; B. Perbal, A Practical Guide To Molecular Cloning (1984); F. M. Ausubel et al. (eds.), Current Protocols in Molecular Biology , John Wiley & Sons, Inc. (1996) (herein “Ausubel et al., 1996”).

The term “PXR-linker-SRC-1” includes any polypeptide or encoding polynucleotide or crystal thereof comprising PXR or a fragment thereof (e.g., PXR-LBD) fused to a polypeptide linker of any size (e.g., 8, 10 or 13 amino acids), fused to SRC-1 polypeptide or a fragment thereof (e.g., SLTERHKILHRLLQEGSPS (SEQ ID NO: 35)). Coexpressing PXR or a fragment thereof (e.g., PXR-LBD or PXR full ) with SRC-1 or a fragment thereof (e.g., SEQ ID NO: 35) or expressing PXR or a fragment thereof (e.g., PXR-LBD) tethered to SRC-1 or a fragment thereof (e.g., tethered by a linker) stabilizes PXR. PXR or the fragment thereof (e.g., PXR-LBD) is stabilized in the sense that its solubility and activity is increased over that of PXR that is untethered or expressed without SRC-1.

The term “subject” includes any organism, such as a mammal (e.g., human, dog, cat, rat, rabbit, monkey, gorilla, chimpanzee or mouse).

The meaning of the term “PXR” or “pregnane X receptor” is well known in the art. In an embodiment of the invention, the amino acid sequence of a human PXR is (PXR-LBD (ligand binding domain) is underscored):

(SEQ ID NO: 1)

In an embodiment, the amino acid sequence of wild-type human PXR-LBD (residue 130-434) is set forth below:

(SEQ ID NO: 2) SERTGTQPLGVQGLTEEQRMMIRELMDAQMKTFDTTFSHFKNFRLPGVLSSGCELPESLQAPSREEAAKWSQVRK DLCSLKVSLQLRGEDGSVWNYKPPADSGGKEIFSLLPHMADMSTYMFKGIISFAKVISYFRDLPIEDQISLLKGA AFELCQLRFNTVFNAETGTWECGRLSYCLEDTAGGFQQLLLEPMLKFHYMLKKLQLHEEEYVLMQAISLFSPDRP GVLQHRVVDQLQEQFAITLKSYIECNRPQPAHRFLFLKIMAMLTELRSINAQHTQRLLRIQDIHPFATPLMQELF GITGS

In an embodiment of the invention, a “fragment” of PXR is PXR-LBD (e.g., amino acids about 169 to about 473 of human PXR (Acc. No. IPI00004364.1 or Q9UNW4; or amino acids about 130 to about 434 of the polypeptide under Acc. No. NP — 003880.3).

The meaning of the term SRC-1 is well known in the art. In an embodiment of the invention, the amino acid sequence of a human SRC-1 is:

The SRC-1 sequence is also disclosed under Genbank Accession Number: NP — 003734. In an embodiment of the invention a “fragment” of SRC-1 comprises the amino acid sequence SLTERHKILHRLLQEGSPS (SEQ ID NO: 35; amino acids 682-700 of human SRC-1) or SSHSSLTERHKILHRLLQEGSPS (SEQ ID NO: 38; amino acids 678-700 of human SRC-1).

Embodiments of the present invention are set forth herein comprising PXR or SRC-1 (e.g., crystals, assays, polypeptides, polynucleotides). Also included within the scope of the present invention are embodiments comprising PXR and SRC-1 variants. Accordingly, the present invention contemplates any superficial or slight modification to the amino acid or nucleotide sequences which correspond to the polypeptides of the invention. In particular, the present invention contemplates function- and sequence-conservative variants of the nucleic acids which encode the polypeptides of the invention. “Sequence-conservative variants” of a polynucleotide sequence are those in which a change of one or more nucleotides in a given codon results in no alteration in the amino acid encoded at that position. Function-conservative variants of the polypeptides of the invention are also contemplated by the present invention. “Function-conservative variants” are those in which one or more amino acid residues in a protein or enzyme have been changed without altering the overall conformation and function of the polypeptide, including, but, by no means, limited to, replacement of an amino acid with one having similar properties. Amino acids with similar properties are well known in the art. For example, polar/hydrophilic amino acids which may be interchangeable include asparagine, glutamine, serine, cysteine, threonine, lysine, arginine, histidine, aspartic acid and glutamic acid; nonpolar/hydrophobic amino acids which may be interchangeable include glycine, alanine, valine, leucine, isoleucine, proline, tyrosine, phenylalanine, tryptophan and methionine; acidic amino acids which may be interchangeable include aspartic acid and glutamic acid and basic amino acids which may be interchangeable include histidine, lysine and arginine.

The present invention includes polynucleotides encoding PXR or SRC-1 and fusions and fragments thereof as well as nucleic acids which hybridize to the polynucleotides. Preferably, the nucleic acids hybridize under low stringency conditions, more preferably under moderate stringency conditions and most preferably under high stringency conditions. A nucleic acid molecule is “hybridizable” to another nucleic acid molecule, such as a cDNA, genomic DNA, or RNA, when a single stranded form of the nucleic acid molecule can anneal to the other nucleic acid molecule under the appropriate conditions of temperature and solution ionic strength (see Sambrook, et al., supra). The conditions of temperature and ionic strength determine the “stringency” of the hybridization. Typical low stringency hybridization conditions are 55° C., 5×SSC, 0.1% SDS, 0.25% milk, and no formamide at 42° C.; or 30% formamide, 5×SSC, 0.5% SDS at 42° C. Typical, moderate stringency hybridization conditions are similar to the low stringency conditions except the hybridization is carried out in 40% formamide, with 5× or 6×SSC at 42° C. High stringency hybridization conditions are similar to low stringency conditions except the hybridization conditions are carried out in 50% formamide, 5× or 6×SSC and, optionally, at a higher temperature (e.g., higher than 42° C.: 57° C., 59° C., 60° C., 62° C., 63° C., 65° C. or 68° C.). In general, SSC is 0.15M NaCl and 0.015M Na-citrate. Hybridization requires that the two nucleic acids contain complementary sequences, although, depending on the stringency of the hybridization, mismatches between bases are possible. The appropriate stringency for hybridizing nucleic acids depends on the length of the nucleic acids and the degree of complementation, variables well known in the art. The greater the degree of similarity or homology between two nucleotide sequences, the higher the stringency under which the nucleic acids may hybridize. For hybrids of greater than 100 nucleotides in length, equations for calculating the melting temperature have been derived (see Sambrook, et al., supra, 9.50-9.51). For hybridization with shorter nucleic acids, i.e., oligonucleotides, the position of mismatches becomes more important, and the length of the oligonucleotide determines its specificity (see Sambrook, et al., supra).

Also included in the present invention are polynucleotides comprising nucleotide sequences and polypeptides comprising amino acid sequences which are at least about 70% identical, preferably at least about 80% identical, more preferably at least about 90% identical and most preferably at least about 95% identical (e.g., 95%, 96%, 97%, 98%, 99%, 100%) to the reference PXR or SRC-1 polynucleotide or polypeptide or fragment or fusion thereof when the comparison is performed by a BLAST algorithm wherein the parameters of the algorithm are selected to give the largest match between the respective sequences over the entire length of the respective reference sequences. Polypeptides comprising amino acid sequences which are at least about 70% similar, preferably at least about 80% similar, more preferably at least about 90% similar and most preferably at least about 95% similar (e.g., 95%, 96%, 97%, 98%, 99%, 100%) to the reference PXR or SRC-1 polypeptide or fragment or fusion thereof, when the comparison is performed with a BLAST algorithm wherein the parameters of the algorithm are selected to give the largest match between the respective sequences over the entire length of the respective reference sequences, are also included in the present invention. Any composition comprising a combination of SRC-1 (or SRC-1 fragment) or a sequence variant thereof and PXR (or PXR fragment) or a sequence variant thereof, fused or unfused, are part of the present invention.

Sequence identity refers to exact matches between the nucleotides or amino acids of two sequences which are being compared. Sequence similarity refers to both exact matches between the amino acids of two polypeptides which are being compared in addition to matches between nonidentical, biochemically related amino acids. Biochemically related amino acids which share similar properties and may be interchangeable are discussed above.

The following references regarding the BLAST algorithm are herein incorporated by reference: BLAST ALGORITHMS: Altschul, S. F., et al., (1990) J. Mol. Biol. 215:403-410; Gish, W., et al., (1993) Nature Genet. 3:266-272; Madden, T. L., et al., (1996) Meth. Enzymol. 266:131-141; Altschul, S. F., et al., (1997) Nucleic Acids Res. 25:3389-3402; Zhang, J., et al., (1997) Genome Res. 7:649-656; Wootton, J. C., et al., (1993) Comput. Chem. 17:149-163; Hancock, J. M., et al., (1994) Comput. Appl. Biosci. 10:67-70; ALIGNMENT SCORING SYSTEMS: Dayhoff, M. O., et al., “A model of evolutionary change in proteins.” in Atlas of Protein Sequence and Structure , (1978) vol. 5, suppl. 3. M. O. Dayhoff (ed.), pp. 345-352, Natl. Biomed. Res. Found., Washington, D.C.; Schwartz, R. M., et al., “Matrices for detecting distant relationships.” in Atlas of Protein Sequence and Structure , (1978) vol. 5, suppl. 3.” M. O. Dayhoff (ed.), pp. 353-358, Natl. Biomed. Res. Found., Washington, D.C.; Altschul, S. F., (1991) J. Mol. Biol. 219:555-565; States, D. J., et al., (1991) Methods 3:66-70; Henikoff, S., et al., (1992) Proc. Natl. Acad. Sci. USA 89:10915-10919; Altschul, S. F., et al., (1993) J. Mol. Evol. 36:290-300; ALIGNMENT STATISTICS: Karlin, S., et al., (1990) Proc. Natl. Acad. Sci. USA 87:2264-2268; Karlin, S., et al., (1993) Proc. Natl. Acad. Sci. USA 90:5873-5877; Dembo, A., et al., (1994) Ann. Prob. 22:2022-2039; and Altschul, S. F. “Evaluating the statistical significance of multiple distinct local alignments.” in Theoretical and Computational Methods in Genome Research (S. Suhai, ed.), (1997) pp. 1-14, Plenum, New York.

Included within the scope of the invention is any embodiment set forth herein comprising a variant of a PXR or SRC-1 polypeptide (e.g., comprising 50, 60, 70, 80, 90 or 99% sequence identity or similarity) wherein said polypeptide comprises any detectable level of PXR or SRC-1 activity. For example, wherein the polypeptide comprises a detectable level of SR12813 binding activity or ABCB1 activating activity. Any polynucleotide encoding such a polypeptide is also within the scope of the present invention.

Assays

The PXR peptides of the invention can be used in assays for the identification of modulators of PXR (e.g., agonists or antagonists); modulators of CYP3A4 activation, and modulators of drug in vivo half-life. The assays set forth herein provide a convenient and cheap in vitro method by which it can be determined if the in vivo half life of a substance (e.g., a clinical drug candidate) will be modulated by cytochrome P450 enzymes such as CYP3A4 and by the multidrug resistant gene ABCB1.

In an embodiment of the assay methods set forth herein, PXR or the fragment thereof (e.g., SEQ ID NO: 1 or 2) is provided with SRC-1 or a fragment thereof (e.g., SEQ ID NO: 35). In an embodiment, PXR or the fragment thereof is fused to SRC-1 or the fragment thereof (e.g., by a polypeptide linker); in another embodiment, they are unfused.

The binding of a substance to be tested for the presence of a PXR agonist or antagonist can be monitored by temperature-dependent circular dichroism. Circular dichroism is observed when optically active matter (e.g., protein) absorbs left and right hand circular polarized light slightly differently. In an embodiment, it is measured with a CD spectropolarimeter. Generally, the instrument needs to be able to measure accurately in the far UV at wavelengths down to 190-170 nm. In addition, the difference in left and right handed absorbance A(l)-A(r) is very small corresponding to an ellipticity of a few 1/1000th of a degree (mdeg). The CD is a function of wavelength. The analysis of temperature-dependent circular dichroism spectra therefore yields valuable information about secondary structure of biological macromolecules as well as the melting temperature (Tm) of the substance being analyzed. Generally, the temperature-dependent circular dichroism spectrum of a given substance takes a sigmoidal shape and the melting temperature of the substance is the point of inflection on the sigmoid curve. Changes in PXR secondary structure can be monitored over a range of temperatures using temperature-dependent circular dichroism. As temperature increases, the temperature-dependent circular dichroism spectra of the protein and/or the Tm will change. Binding of modulators to PXR can be monitored by comparing the temperature-dependent circular dichroism spectrum and/or Tm of native PXR to that of PXR that has been contacted with a substance to be tested for the presence of a modulator. Specifically, a change in the PXR temperature-dependent circular dichroism spectra and/or Tm, in the presence of a substance being tested for the presence of a modulator, as compared to the temperature-dependent circular dichroism spectra of native PXR, will indicate PXR/modulator binding.

In an embodiment of the invention, a method for identifying a modulator or binder of PXR (e.g., agonist or antagonist); a modulator of CYP3A4 activation or a modulator of drug in vivo half-life comprises:

(a) contacting a composition comprising PXR or a fragment or fusion thereof and SRC-1 or a fragment of fusion thereof (e.g., PXR-LBD; PXR full linker-SRC or PXR-LBD-linker-SRC) with a substance to be tested for the presence of the modulator or binder (e.g., agonist or antagonist); (b) determining the ellipticity of said composition;

whereby the substance is selected if the ellipticity and/or the Tm of said composition alone is different from the ellipticity and/or Tm of the composition that is in contact with said substance.

In an embodiment of the invention, ellipticity is measured at several different temperatures (e.g., between 30° C. and 80° C.). The substance is determined to contain a modulator of PXR if the profile that is generated in the presence of the substance is different from the profile that is generated in the absence of the substance.

In an embodiment of the invention, the temperature-dependent circular dichroism assay is performed along with a negative-control method comprising:

(a) contacting a composition comprising PXR or a fragment or fusion thereof and SRC-1 or a fragment of fusion thereof (e.g., PXR-LBD; PXR full -linker-SRC or PXR-LBD-linker-SRC) with a negative-control substance known not to contain a modulator or binder of PXR; and (b) determining the ellipticity of the composition and said negative-control substance;

whereby the method is determined to be functioning properly if the ellipticity and/or Tm of the composition alone is essentially the same as that of the composition that is in contact with the negative-control substance.

In an embodiment of the invention, the temperature-dependent circular dichroism assay is performed along with a positive-control method comprising:

(a) contacting a composition comprising PXR or a fragment or fusion thereof and SRC-1 or a fragment of fusion thereof (e.g., PXR-LBD; PXR-linker-SRC or PXR-LBD-linker-SRC) with a positive-control substance known to contain a modulator or binder of PXR (e.g., SR12813, hyperforin, clotrimazole, sulfopyrole or pregnane 16α-carbonitrile); and (b) determining the ellipticity of the composition and said positive-control substance;

whereby the method is determined to be functioning properly if the ellipticity and/or Tm of the composition alone is different from that of the composition that is in contact with the positive-control substance.

The temperature-dependent circular dichroism methods set forth above can be modified to replace temperature variation with the addition of some other stimulus that affects PXR secondary structure. For example, instead of measuring ellipticity at various temperatures, ellipticity can be measured at different concentrations of protein denaturant. For example, the protein denaturants urea or guanidine HCl can be used.

Modulators or binders of PXR (e.g., agonist or antagonist); modulators of CYP3A4 activation and modulators of drug in vivo half-life can also be identified by direct binding assay. For example, in an embodiment of the invention, a method for identifying a modulator or binder (e.g., agonist or antagonist) of PXR or a fragment or fusion thereof (e.g., PXR-LBD; PXR full linker-SRC or PXR-LBD-linker-SRC); a modulator of CYP3A4 activation or a modulator of drug in vivo half life comprises:

(1) contacting a composition comprising PXR or a fragment or fusion thereof and SRC-1 or a fragment of fusion thereof with a substance to be tested for the presence of a modulator or binder (e.g., a detectably labeled substance); and (2) determining if the substance binds to said PXR or fragment or fusion thereof;

whereby the substance is selected if binding is observed.

In an embodiment of the invention, the direct binding assay is performed along with a negative-control assay comprising:

(1) contacting a composition comprising PXR or a fragment or fusion thereof and SRC-1 or a fragment of fusion thereof with a negative-control substance (e.g., a detectably labeled negative-control substance) that is known not to comprise a modulator or binder; and (2) determining if the negative-control substance binds to said PXR or fragment or fusion thereof;

whereby the method is determined to be functioning properly if no direct binding of the negative-control substance is detected.

In an embodiment of the invention, the direct binding assay is performed along with a positive-control assay comprising:

(1) contacting a composition comprising PXR or a fragment or fusion thereof and SRC-1 or a fragment of fusion thereof with a positive-control substance (e.g., a detectably labeled positive-control substance) that is known to comprise a modulator (e.g., SR12813, hyperforin, clotrimazole, sulfopyrole or pregnane 16α-carbonitrile); and (2) determining if the positive-control substance binds to said PXR or fragment or fusion thereof;

whereby the method is determined to be functioning properly if direct binding of the positive-control substance is detected.

A substance being tested for the presence of a binder or modulator in any of the foregoing assays can be detectably labeled with any of many labels known in the art including, for example, 99 Tc, 90 Y, 111 In, 32 P, 14 C, 125 I, 3 H, 131 I, 11 C, 15 O, 13 N, 18 F, 35 S, 51 Cr, 57 To, 226 Ra, 60 Co, 59 Fe, 57 Se, 152 Eu, 67 CU, 217 Ci, 211 At, 212 Pb, 47 Sc, 109 Pd, 234 Th, and 40 K, 157 Gd, 55 Mn, 52 Tr and 56 Fe. Such labels may be detected, e.g., in a scintillation counter when contacted with a scintillant.

In an embodiment of the invention, binding is determined by chromatographically binding the PXR/modulator or binder complex to a solid support, optionally washing the bound complex, optionally drying the washed complex (e.g., under a vacuum) and then detecting the presence of the modulator or binder on the support. In an embodiment, the solid support is a positively charged membrane or filter such as a nylon (e.g., Immobilon-Ny+ transfer membrane; Millipore; Billerica, Massachusetts).

Bound complex may be detected by any of several methods known in the art. For example, if the complex comprises a radiolabel (e.g., [ 3 H]-SR12813), the bound complex can be detected chemilluminescently (e.g., using Opti-Fluor Scintillation cocktail; Perkin Elmer Life and Analytical Sciences, Inc.; Boston, Mass.). Chemilluminescence can be detected, for example, in a scintillation counter (Packard Top-counter). Specifically, the bound complex can be contacted with scintillant and then analyzed for the occurrence of scintillation.

In an embodiment of the invention, modulators or binders of PXR (e.g., agonist or antagonist); modulators of CYP3A4 activation and modulators of drug in vivo half-life can also be identified by competition or competitive inhibition assay. In a competition assay, a modulator or binder is identified based upon its ability to compete for binding with a substance known to bind PXR (this substance will be referred to as “ligand” for the purposes of discussing this method). In an embodiment, such a method comprises:

(i) contacting a composition comprising PXR or a fragment or fusion thereof and SRC-1 or a fragment of fusion thereof with a ligand that is known to bind said PXR or said fragment or fusion thereof and with a substance to be tested for the presence of a modulator or binder; and (ii) determining if the substance being tested reduces binding of the ligand to the PXR or the fragment or fusion thereof;

whereby the substance is selected if binding of the ligand to the PXR or the fragment or fusion thereof is reduced as compared to ligand binding to PXR or the fragment or fusion thereof in the absence of the substance.

During such a competition assay, a complex between PXR or a fragment or fusion thereof and said ligand is formed. Such a complex is within the scope of and forms a part of the present invention. For example, the scope of the present invention includes complexes comprising PXR, PXR-LBD, PXR full linker-SRC or PXR-LBD-linker-SRC bound to SR12813, hyperforin, clotrimazole, sulfopyrole, pregnane 16α-carbonitrile or any other substance, known in the art, to bind to PXR.

In an embodiment of the invention, the competition assay is performed along with a positive-control assay. In an embodiment, such a positive-control assay comprises:

(i) contacting a composition comprising PXR or a fragment or fusion thereof and SRC-1 or a fragment of fusion thereof with a ligand that is known to bind PXR or said fragment or fusion thereof with a positive-control substance known to bind to PXR or a fragment or fusion thereof; and (ii) determining if said positive-control substance reduces the binding of the ligand;

whereby the assay is determined to function properly if the binding of said ligand is reduced.

In an embodiment of the invention, the competition assay is performed along with a negative-control assay. In an embodiment, such a negative-control assay comprises:

(i) contacting a composition comprising PXR or a fragment or fusion thereof and SRC-1 or a fragment of fusion thereof with a ligand that is known to bind the PXR or said fragment or fusion thereof and with a negative-control substance known not to bind to the PXR or a fragment or fusion thereof; and (ii) determining if said negative-control substance reduces that binding of said ligand;

whereby the assay is determined to function properly if the binding of said ligand is not reduced.

The term “ligand” includes any substance that is known to bind to PXR including, but not limited to, SR12813, hyperforin, clotrimazole, ritonavir, sulfopyrole, pregnane-16α-carbonitrile, RU-486, rifampicin, dexamethasone, paclitaxel, metyrapone, bisphenol A, diethylhexylphthalate, nonylphenol, phthalate, cisplatin, chenodeoxycholic acid, deoxycholic acid, lithocholic acid, 17β-estradiol (estradiol), pregnenolone (5-pregneno-3β-ol-20-one), progesterone and Medroxyprogesterone-acetate (MPA) as well as radiolabeled versions of any of the foregoing.

Any of several method known in the art can be used to determine the binding of the ligand. For example, in an embodiment of the invention, a method similar to that employed in the direct binding assays set forth herein is used. In this embodiment of the invention, the ligand is detectably labeled (e.g., with a radiolabel such as 3 H) and its binding is detected chromatographically. For example, PXR/ligand complexes can be bound to a solid matrix (e.g., a charged membrane), optionally washed and analyzed for the presence of the radiolabel (e.g., by scintillation).

In an embodiment of the invention, the K i value is calculated by the equation set forth in Cheng & Prusoff (Biochem. Pharmacol. 22:3099, 1973): K i =IC 50 /1+([L]/K d ) in which IC 50 value is that concentration (nM) of test substance by which 50% of specific labeled ligand is displaced from PXR, [L] is the concentration of the specific labeled ligand in the assay and the K d is the affinity of radioligand for the receptor.

Modulators of PXR can also be identified using a cell-based assay. An activity of PXR is transcriptional activation of the CYP3A4 gene (Kliewer et al., Cell 92: 73-82 (1998); Lehmann et al., J Clin Invest 102: 1016-1023 (1998); Bertilsson et al., Proc Natl Acad Sci USA 95: 12208-12213 (1998); Pascussi et al., Biochem Biophys Res Commun 260: 377-381 (1999) or the multidrug resistance gene ABCB1 (Synold et al., Nature Med. 7: 584-590 (2001); Geick et al., J. Biol. Chem. 276: 14581-14587 (2001)). Agonism or antagonism of PXR can be observed by monitoring the transcriptional activation of the gene. For example, in an embodiment of the invention, a method for identifying a modulator (e.g., agonist or antagonist) of PXR or a fragment or fusion thereof (e.g., PXR-LBD; PXR-linker-SRC or PXR-LBD-linker-SRC); a modulator of CYP3A4 activation or a modulator of drug half-life comprises:

(A) contacting an isolated host cell comprising PXR or a fragment or fusion thereof (e.g., PXR-LBD; PXR-linker-SRC or PXR-LBD-linker-SRC) and SRC-1 or a fragment of fusion thereof and a reporter gene operably linked to a CYP3A4 promoter or to an ABCB1 promoter with a substance to be tested for the presence of a modulator; and (B) determining if the reporter gene is expressed;

whereby the substance is selected if the reporter gene, in the cell contacted with said substance, is expressed at a higher or lower level than a reporter gene in a host cell that is not contacted with said substance. If the reporter gene is expressed at a lower level, the substance is identified as containing an antagonist and if the reporter gene is expressed at a higher level, the substance is identified as containing an agonist.

In an embodiment of the invention, the cell based method is performed along with a negative-control method comprising:

(A) contacting an isolated host cell comprising PXR or a fragment or fusion thereof (e.g., PXR-LBD; PXR-linker-SRC or PXR-LBD-linker-SRC) and SRC-1 or a fragment of fusion thereof and a reporter gene operably linked to a CYP3A4 promoter or to an ABCB1 promoter with a negative-control substance known not to comprise a modulator; and (B) determining if the reporter gene is expressed;

whereby the method is determined to be functioning properly if a basal level of reporter gene expression is observed in the presence of the negative-control method.

In an embodiment of the invention, the cell based method is performed along with a positive-control method comprising:

(A) contacting an isolated host cell comprising PXR or a fragment or fusion thereof (e.g., PXR-LBD; PXR-linker-SRC or PXR-LBD-linker-SRC) and SRC-1 or a fragment of fusion thereof and a reporter gene operably linked to a CYP3A4 promoter or to an ABCB1 promoter with a positive-control substance known to comprise a modulator (e.g., SR12813, hyperforin, clotrimazole, sulfopyrole or pregnane 16α-carbonitrile); and (B) determining if the reporter gene is expressed;

whereby the method is determined to be functioning properly if the level of reporter gene expression observed in the presence of the positive-control substance is greater than a basal level of reporter gene expression.

A host cell that can be used in the cell-based assay can be any cell type. In an embodiment, the host cell is a mammalian cell such as a HepG2 cell.

In an embodiment of the invention, the reporter gene is, for example, green fluorescent protein (GFP); Renilla luciferase (see e.g., Genbank Accession Nos.: AF416990; AR149562; AF362548 or M63501), firefly ( Photinus pyralis ) luciferase (see e.g., Genbank Accession Nos.: U03687; M15077 and X84846), E. coli lacZ Aequorea Victoria aequorin, human alkaline phosphatase or E. coli β-lactamase. In an embodiment of the invention, chloramphenicol acetyltransferase (CAT) is used as a reporter gene. CAT radioassays are described, for example, by Sleigh (Anal. Biochem. 156(1):251-256 (1986)) and a non-radioactive CAT assay is described by Young et al. (Anal. Biochem. 197(2):401-407 (1991)). In an embodiment of the invention, GFP is green fluorescent protein (GFP) isolated from Aequorea victoria (Chalfie et al. Science 263, 802-805 (1994)) is the reporter gene. In an embodiment of the invention, firefly luciferase reporter gene that has been altered as described in Leskinen et al. (Yeast. 20(13):1109-1113 (2003)) wherein the carboxy-terminal peroxisomal targeting signal, Ser-Lys-Leu (slk), of the firefly luciferase gene was removed is the reporter gene.

Crystals

Crystallization may be accomplished by using a number of methods (See e.g., Giegé, et al., (1994) Acta Crystallogr . D50: 339-350; McPherson, (1990) Eur. J. Biochem. 189: 1-23). Such techniques include microbatch, hanging drop, seeding and dialysis. Hanging-drop vapor diffusion (McPherson, (1976) J. Biol. Chem. 251: 6300-6303) or microbatch method (Chayen (1997) Structure 5: 1269-1274) is used in an embodiment of the invention. In each of these methods, it is important to promote continued crystal growth after nucleation by maintaining a supersaturated solution. In the microbatch method, polypeptide is mixed with precipitants to achieve supersaturation, the vessel is sealed and set aside until crystals appear. In the dialysis method, polypeptide is retained in a sealed dialysis membrane which is placed into a solution containing precipitant. Equilibration across the membrane increases the precipitant concentration thereby causing the polypeptide to reach supersaturation levels. It is desirable to use a PXR protein preparation having a concentration of at least about 1 mg/mL and preferably about 10 mg/mL to about 20 mg/mL. It may also be desirable to include a protein stabilizing agent.

The crystals of the present invention have a wide range of uses. For example, high quality crystals are suitable for X-ray or neutron diffraction analysis to determine the three dimensional structure of PXR or a fragment or fusion thereof (e.g., PXR-LBD-linker-SRC-1) and in particular to assist in the identification of the protein's active and effector sites. Knowledge of these sites and solvent accessible residues allow structure-based design and construction of agonists and antagonists for PXR as well as development of drugs do not interact with PXR and have low incidence of side-effects.

In addition, crystallization itself can be used as a purification method. In some instances, a polypeptide or protein crystallizes from a heterogeneous mixture into crystals. Isolation of such crystals by filtration and/or centrifugation, followed by redissolving the polypeptide affords a purified solution suitable for use in growing high-quality crystals which are preferred for diffraction analysis. Such a method forms part of the present method.

Once a crystal of the present invention is grown, X-ray diffraction data can be collected. One method for determining structure with X-ray diffraction data includes use of synchrotron radiation, under standard cryogenic condition; however, alternative methods may also be used. For example, crystals can be characterized by using X-rays produced by a conventional source, such as a sealed tube or a rotating anode. Methods of characterization include, but are not limited to, precession photography, oscillation photography and diffractometer data collection.

The crystallizable compositions provided by this invention are amenable to X-ray crystallography for providing the three-dimensional structure of a PXR or a fragment or fusion thereof (e.g., PXR-LBD-linker-SRC-1). The present invention includes crystals which effectively diffract X-rays for the determination of the atomic coordinates of PXR or a fragment or fusion thereof (e.g., PXR-LBD-linker-SRC-1) to a resolution of greater than about 5.0 Ångströms (e.g., about 4.5 Å, about 4.0 Å, about 3 Å, about 2.5 Å, about 2 Å, about 1 Å), preferably greater than about 4.0 Ångströms (e.g., about 3 Å, about 2.5 Å, about 2 Å, about 1 Å), more preferably greater than about 2.8 Ångströms (e.g., about 2.5 Å, about 2 Å, about 1 Å).

The present invention includes PXR crystals (e.g., PXR-LBD-linker-SRC-1) whose three-dimensional structure is described by the structure coordinates set forth herein, e.g., in Tables 3 and 4 and 5 (e.g., 3a, 3b, 3c, 4a, 4b, 5a, 5b, 5c, 5d). The scope of the present invention also includes crystals that possess structural coordinates which are similar to those set forth herein, e.g., in Tables 3 and 4 and 5 (e.g., 3a, 3b, 3c, 4a, 4b, 5a, 5b, 5c, 5d); in an embodiment, the crystals or the soluble polypeptides which are used to form the crystals exhibit PXR activity (e.g., binding to SR12813, hyperforin or pregnane 16α-carbonitrile). In an embodiment, the crystals include a polypeptide which includes the amino acid sequence of any of SEQ ID NOs: 1, 2, 22, 24, 26, 28, 30 or 32. Structural similarity between crystals is discussed in detail below.

In an embodiment of the invention, a crystal of the invention comprises a tetragonal space group containing one monomer per asymmetric unit or an orthorhombic space group containing two monomers per asymmetric unit.

The term “structure coordinates” refers to Cartesian coordinates derived from mathematical equations related to the patterns obtained on diffraction of a beam of X-rays by the atoms (scattering centers) of a molecule. The diffraction data are used to calculate electron density maps and to establish the positions of the individual atoms of the molecule.

Those of skill in the art will understand that a set of structure coordinates for an enzyme or an enzyme-complex or a portion thereof, is a relative set of points that define a shape in three dimensions. Thus, it is possible that an entirely different set of coordinates could define a similar or identical shape. Moreover, slight variations in the individual coordinates may have little effect on overall shape.

The present invention includes crystals (e.g., PXR-LBD-linker-SRC-1) exhibiting structural coordinates which are similar to those set forth herein, e.g., in Tables 3 and 4 and 5 (e.g., 3a, 3b, 3c, 4a, 4b, 5a, 5b, 5c, 5d) but for crystallographic permutations of the structure coordinates, fractionalization of the structure coordinates, additions, subtractions, rotations or translations to sets of the structure coordinates or any combinations of the above.

Modifications in the crystal structure due to mutations, additions, substitutions, and/or deletions of amino acids, or other changes in any of the components that make up the crystal may account for variations in structure coordinates. If such variations are within an acceptable standard error as compared to the coordinates of Tables 3 and 4 and 5 (e.g., 3a, 3b, 3c, 4a, 4b, 5a, 5b, 5c, 5d), the resulting three-dimensional shape is considered to be the same and, accordingly, the modified crystal is considered to be within the scope of the present invention.

Various computational analyses may be necessary to determine whether a crystal is sufficiently similar to the crystals whose structural coordinates are set forth in Tables 3 and 4 and 5 (e.g., 3a, 3b, 3c, 4a, 4b, 5a, 5b, 5c, 5d) as to be considered the same. Such analyses may be carried out in current software applications, such as the Molecular Similarity application of QUANTA (Molecular Simulations Inc., San Diego, Calif.) version 4.1, and as described in the accompanying User's Guide.

The Molecular Similarity application permits comparisons between different structures, different conformations of the same structure, and different parts of the same structure. In general, the procedure used in Molecular Similarity to compare structures is divided into four steps: 1) input the structures to be compared; 2) define the atom equivalences in these structures; 3) perform a fitting operation; and 4) analyze the results.

Each structure is identified by a name. One structure is identified as the target (i.e., the fixed structure); all remaining structures are working structures (i.e., moving structures). Since atom equivalency within QUANTA is defined by user input, for the purpose of this invention we will define equivalent atoms as protein backbone atoms (N, Cα, C and O) for all conserved or common residues between the two structures being compared.

When a rigid fitting method is used, the working structure is translated and rotated to obtain an optimum fit with the target structure. The fitting operation uses a least squares fitting algorithm that computes the optimum translation and rotation to be applied to the moving structure, such that the root mean square difference of the fit over the specified pairs of equivalent atom is an absolute minimum. This number, given in Ångströms, is reported by QUANTA.

The term “root mean square deviation” (RMSD) is a commonly known term in the art which, in general, means the square root of the arithmetic mean of the squares of the deviations from the mean distance of corresponding atoms. It is a way to express the deviation or variation from a trend or object.

For the purpose of this invention, any crystalline molecule characterized by a set of structure coordinates that has a RMSD of conserved or common residue backbone atoms (N, Cα, C, O) or of only alpha carbon atoms of less than about 1.5 Å when superimposed—using backbone atoms or alpha carbon atoms (Cα)—on the relevant structure coordinates of Table 3 or 4 or 5 (e.g., any monomer of PXR-LBD-linker-SRC-1 as set forth in Table 3a, 3b, 4a, 5a or 5b) are considered identical and are within the scope of the present invention. In an embodiment, the root mean square deviation is less than about 1.0 Å (e.g., 0.9 Å, 0.8 Å, 0.7 Å, 0.6 Å), less than about 0.5 (e.g., 0.4 Å, 0.3 Å), less than about 0.25 Å (e.g., 0.2 Å, 0.15 Å) or less than about 0.1 Å.

The term “least squares” refers to a method based on the principle that the best estimate of a value is that in which the sum of the squares of the deviations of observed values is a minimum.

Computers

In accordance with the present invention, the structure coordinates of PXR-linker-SRC1 fusion polypeptide and portions thereof may be stored in a machine-readable storage medium. Such data may be used for a variety of purposes, such as drug discovery and x-ray crystallographic analysis of a protein crystal (e.g., for producing a three-dimensional representation of PXR). Accordingly, one aspect of this invention provides a machine-readable data storage medium comprising a data storage material encoded with the structure coordinates set forth in Table 3, 4, or 5 (e.g., 3a, 3b, 3c, 4a, 4b, 5a, 5b, 5c, 5d). The machine-readable data storage medium may also include any set of structure coordinates of a molecule that has a root mean square deviation of conserved or common residue backbone atoms (N, Cα, C, O) or only alpha carbon atoms (Cα) of less than about 1.5 Å, preferably, less than about 1.0 Å, more preferably less than about 0.5 Å and even more preferably less than about 0.1 Å when superimposed—using backbone atoms or alpha carbon atoms—on the relevant structure coordinates of Table 3, 4, or 5 (e.g., 3a, 3b, 3c, 4a, 4b, 5a, 5b, 5c, 5d).

A computer system, useful in reading the machine readable data storage medium, includes a computer comprising a central processing unit (“CPU”) and a memory storage device and is also within the scope of the present invention. In general, the computer system may be any computer with an operating system such as MS-DOS, PC-DOS, Windows, OS/2, Unix, Unix variant or MacOS. Particularly preferred computer systems are the Silicon Graphics Octane workstation or Compaq AlphaServer DS20. Other hardware systems and software packages will be known to those skilled in the art.

Input hardware coupled to the computer system by input line, may be implemented in a variety of ways. Machine-readable data of this invention may be input via the use of a modem or modems connected by a telephone line or a dedicated data line. Alternatively or additionally, the input hardware may comprise CD-ROM drives or disk drives. A keyboard may also be used as an input device.

Output hardware, coupled to the computer system by output lines, may similarly be implemented by conventional devices. By way of example, output hardware may include a display terminal (e.g., a cathode ray tube (CRT)) for displaying a graphical representation of the three dimensional structure of PXR or a portion thereof using a program such as INSIGHT (Molecular Simulations Inc., San Diego, Calif.) or QUANTA as described herein. Output hardware might also include a printer, so that hard copy output may be produced, or a disk drive, to store system output for later use. In preferred embodiments, the computer possesses a display which is displaying a three dimensional representation of PXR or a fragment or homologue thereof.

In operation, the central processing unit (CPU) coordinates the use of the various input and output devices, coordinates data accesses from mass storage and accesses to and from working memory, and determines the sequence of data processing steps. A number of programs may be used to process the machine-readable data of this invention. Such programs are discussed in reference to the computational methods of drug discovery as described herein. Specific references to components of the computer system are included as appropriate throughout the following description of the data storage medium.

A magnetic data storage medium can be encoded with a machine-readable data by a computer system as described above. Storage medium may be, for example, a conventional floppy diskette or hard disk, having a suitable substrate, which may be conventional, and a suitable coating, which may be conventional, on one or both sides, containing magnetic domains whose polarity or orientation can be altered magnetically. The magnetic domains of the coating of medium may be polarized or oriented so as to encode, in a mariner which may be conventional, machine readable data, such as that described herein, for execution by a system as described herein. Storage medium may also have an opening for receiving the spindle of a disk drive or other data storage device. Alternatively, an optically-readable data storage medium can be encoded with such machine-readable data, or a set of instructions. Medium can be a conventional compact disk read only memory (CD-ROM) or a rewritable medium such as a magneto-optical disk which is optically readable and magneto-optically writable.

In general, in the case of CD-ROM, as is well known, disk coating is reflective and is impressed with a plurality of pits to encode the machine-readable data. The arrangement of the pits is read by reflecting laser light off the surface of the coating. A protective coating, which preferably is substantially transparent, is provided on top of the coating.

In general, in the case of a magneto-optical disk, as is well known, disk coating has no pits, but has a plurality of magnetic domains whose polarity or orientation can be changed magnetically when heated above a certain temperature, as by a laser. The orientation of the domains can be read by measuring the polarization of laser light reflected from the coating. The arrangement of the domains encodes the data as described above.

Molecular Modeling

The present invention permits the use of structure-based drug design techniques to design, select, and synthesize chemical entities, including inhibitory compounds that are capable of binding to a PXR polypeptide or fusion thereof (e.g., PXR-linker-SRC-1). Also, de novo and iterative drug design methods can be used to develop drugs from the structure of the PXR crystals of this invention.

Specifically, the crystals of the invention can be used to identify modulators of PXR; modulators of CYP3A4 activation, and modulators of drug in vivo half life by use of molecular modeling techniques that are known in the art. Identification of modulators of PXR by molecular modeling provides a convenient and cheap method by which it can be determined if the in vivo half life of a substance (e.g., a clinical drug candidate) will be modulated by cytochrome P450 enzymes such as CYP3A4 and by the multidrug resistance gene ABCB1.

For example, an embodiment of the invention comprises a method for identifying a PXR agonist or antagonist comprising the steps of a) crystallizing PXR or a fragment thereof fused to SRC-1 or a fragment thereof (e.g., PXR-LBD-linker-SRC1) to form at least one crystal; b) irradiating the crystal produced by step (a) to obtain a diffraction pattern of said crystal; c) determining the atomic coordinates of the three-dimensional structure of the PXR or the fragment or fusion thereof from the diffraction pattern; d) using the atomic coordinates and one or more molecular modeling techniques to identify an agent that interacts with the PXR; and, optionally, e) determining if the agonist or antagonist increases or decreases the in vivo activation of cytochrome P450 enzymes or the multidrug resistance gene ABCB1, or modulates or binds to PXR or modulates drug half life; wherein the agonist or antagonist is selected if it increases or decreases the in vivo activation of cytochrome P450 enzymes or the multidrug resistance gene ABCB1, binds to or modulates PXR or modulates drug half life. In an embodiment of the invention, an agonist or antagonist identified in the present molecular modeling method (steps a-d) is further analyzed by any method set forth in the “Assays” or “Pharmacology” section herein, to determine if it is, for example, a modulator or binder of PXR, a modulator of CYP3A4 activation or a modulator of drug in vivo half life (step e).

The methods of the present invention can also be used to identify drug products that do not interact with PXR. For example, an embodiment of the invention comprises a method comprising the steps of a) crystallizing PXR or a fragment thereof fused to SRC1 or a fragment thereof and complexed with a candidate drug (e.g., a small organic molecule) to form at least one crystal; b) irradiating the crystal produced by step (a) to obtain a diffraction pattern of said crystal; c) determining the atomic coordinates of the three-dimensional structure of the PXR or the fragment or fusion thereof from the diffraction pattern; d) identifying interatomic interactions between the PXR and the candidate drug; and e) modifying the chemical structure of the candidate drug so as to eliminate or reduce that ability of the PXR and the candidate drug to bind.

The candidate drug can be a known substance that is known to interact with PXR or a new chemical entity. For example, in an embodiment of the invention, the drug candidate is SR12813 (or any other known PXR binding compound, e.g., as set forth herein) and the method is used to generate structural homologues of the compound which do not interact with PXR. The method optionally includes the step of performing in vitro or in vivo testing (e.g., a temperature dependent circular dichroism assay) of the modified drug candidate to assess its ability to interact with PXR.

One particularly useful drug design technique enabled by this invention is structure-based drug design. Structure-based drug design is a method for optimizing associations between a protein and a compound by determining and evaluating the three-dimensional structures of successive sets of protein/compound complexes.

Those skilled in the art will appreciate that association of natural ligands or substrates with the binding pockets of their corresponding receptors or enzymes is the basis of many biological mechanisms of action. The term “binding pocket” or “binding domain”, as used herein, includes any region of a molecule or molecular complex, that, as a result of its shape, favorably associates with another chemical entity or compound. Similarly, drugs may exert their biological effects through association with the binding pockets of receptors and enzymes. Such association may occur with all or any part of the binding pockets. An understanding of such associations will help lead to the design of drugs having more favorable associations with the target enzyme, and thus, improved biological effects. Therefore, this information is valuable in designing potential enzyme inhibitors, such as inhibitors of PXR.

In iterative structure-based drug design, crystals of a series of protein/compound complexes are obtained and then the three-dimensional structure of each complex is solved. Such an approach provides insight into the association between the proteins and compounds of each complex. This is accomplished by selecting compounds with inhibitory activity, obtaining crystals of a new polypeptide, solving the three-dimensional structure of the polypeptide, and comparing the associations between the new protein and previously solved protein. By observing how changes in the compound affected the protein/compound associations, these associations may be optimized.

In some cases, iterative structure-based drug design is carried out by forming successive protein-compound complexes and then crystallizing each new complex. Alternatively, a pre-formed protein crystal is soaked in the presence of an inhibitor, thereby forming a protein/compound complex and obviating the need to crystallize each individual protein/compound complex. Advantageously, PXR crystals provided by this invention may be soaked in the presence of a compound or compounds, such as PXR inhibitors, substrates or other ligands to provide novel PXR/compound crystal complexes. As used herein, the term “soaked” may refer to a process in which the crystal is transferred to a solution containing the compound of interest.

The structure coordinates set forth in Table 3, 4 or 5 (e.g., 3a, 3b, 3c, 4a, 4b, 5a, 5b, 5c, 5d) can also be used to aid in obtaining structural information about another crystallized molecule or molecular complex. This may be achieved by any of a number of well-known techniques, including molecular replacement.

The structure coordinates set forth in Table 3, 4 or 5 (e.g., 3a, 3b, 3c, 4a, 4b, 5a, 5b, 5c, 5d) can also be used for determining at least a portion of the three-dimensional structure of molecules or molecular complexes which contain at least some structurally similar features to PXR. In particular, structural information about another crystallized molecule or molecular complex may be obtained by well-known techniques, including molecular replacement.

Therefore, another aspect of this invention provides a method of utilizing molecular replacement to obtain structural information about a crystallized molecule or molecular complex, whose structure is unknown, comprising the steps of generating an x-ray diffraction pattern from said crystallized molecule or molecular complex and applying crystallographic phases derived from at least a portion of the structure coordinates set forth in Table 3, 4 or 5 (e.g., 3a, 3b, 3c, 4a, 4b, 5a, 5b, 5c, 5d) to the x-ray diffraction pattern to generate a three-dimensional electron density map of the molecule or molecular complex whose structure is unknown.

Once the structure coordinates of a protein crystal have been determined, they are useful in solving the structures of other crystals. In addition, the structure of PXR homologues may be determined from the structural coordinates of the present invention. For example, polypeptides may be crystallized and their structure elucidated by, for example, difference Fourier techniques and molecular replacement.

By using molecular replacement, all or part of the structure coordinates of the PXR polypeptide provided by this invention can be used to determine the previously unknown structure of a crystallized molecule or molecular complex more quickly and efficiently than attempting to determine such information ab initio.

Molecular replacement provides an accurate estimation of the phases for an unknown structure. Phases are a factor in equations used to solve crystal structures that cannot be measured experimentally. Obtaining accurate values for the phases, by methods other than molecular replacement, is a time-consuming process. However, when the crystal structure of a protein containing a homologous portion has been solved, the phases from the known structure may provide a satisfactory estimate of the phases for the unknown structure.

Thus, this method involves generating a preliminary model of a molecule or molecular complex whose structure coordinates are unknown, by orienting and positioning the relevant portion of the PXR crystal according to Table 3, 4 or 5 (e.g., 3a, 3b, 3c, 4a, 4b, 5a, 5b, 5c, 5d) within the unit cell of the crystal of the unknown molecule or molecular complex so as best to account for the observed x-ray diffraction pattern amplitudes to generate an election density map of the structure whose coordinates are unknown. This, in turn, can be subjected to any well-known model building and structure refinement techniques to provide a final, accurate structure of the unknown crystallized molecule or molecular complex (Lattman, “Use of the Rotation and Translation Functions”, in Meth. Enzymol., 115: 55-77 (1985); Rossman, ed., “The Molecular Replacement Method”, Int. Sci. Rev. Ser ., No. 13, Gordon & Breach, New York (1972)).

Phase information from the structure coordinates of the present invention may be used to elucidate the structure of other crystals. For example, the structure of PXR in complex with other atoms or molecules may be elucidated. Such complexes include, for example, those containing atoms soaked into or cocrystallized within the crystal lattice. Other structures which can be elucidated using the phase information of the present invention include, for example, other PXR constructs or homologues or mutants thereof having sufficient three-dimensional structure similarity to a PXR complex as to be solved using molecular replacement. Also, these protein molecules, in a complex with a small molecule binder(s), agonist(s), antagonist(s), or analog(s) of any of these, may also be solved using the phase information of the present invention. Complexes containing a combination of the above molecules may also be solved using the phase information of the present invention.

The structure of any portion of any crystallized molecule or molecular complex that is sufficiently homologous to any portion of the PXR protein can be solved by this method. The difference Fourier method simply calculates an electron density map using phases calculated from the structure coordinates and observed diffraction amplitudes from a crystal of an unknown structure. This method is often used to solve structures of protein/ligand complexes where the ligand is small and does not affect the crystal form significantly.

In a preferred embodiment, the method of molecular replacement is utilized to obtain structural information about a molecule wherein the molecule comprises a PXR polypeptide complex. The structure coordinates of PXR provided by this invention are particularly useful in solving the structure of other crystal forms of PXR polypeptide complexes. This approach enables the determination of the optimal sites for interaction between chemical entities, including interaction of candidate inhibitors with PXR.

PXR crystals may be studied using well-known x-ray diffraction techniques and may be refined versus x-ray data to 3 Å resolution or better to an R free value of about 0.40 or less using computer software such as X-PLOR (Yale University, 1992, distributed by Molecular Simulations, Inc.; see e.g., Blundell & Johnson, supra; Meth, Enzymol. , vol. 114 & 115, H. W. Wyckoff et al., eds., Academic Press (1985)). This information may be used to optimize known PXR inhibitors and to design new PXR inhibitors.

The ligand binding pocket or cavity of PXR has been identified and characterized (Watkins et al., Science 292: 23229-2333 (2001); Xiao et al., Drug Metab. and Disposition 30(9): 951-956 (2002); Watkins et al., Curr. Op. Drug Disc. & Dev. 5(1): 150-158 (2002)). In an embodiment of the invention, the binding pocket of PXR comprises the following amino acid residues: L206; S208; L209; V211; L240; M243; A244; M246; S247; F251; F281; C284; Q285; F288; W299; Y306; L308; E321; M323; L324; H327; H407; R410; L411; I414; F420; M425 and F429. The location of such residues in the PXR-LBD-SRC1 polypeptides of the invention (e.g., PXR-LBD-L10-SRC1 (e.g., SEQ ID NO: 24)) can be observed in the structural coordinates set forth in Tables 3a, 3b, 4a, 5b or 5a. Accordingly, the present invention comprise methods for identifying whether a chemical entity will associate with the PXR ligand binding pocket defined by the structural coordinates of PXR-LBD-L10-SRC1 (e.g., SEQ ID NO: 24) amino acids L206; S208; L209; V211; L240; M243; A244; M246; S247; F251; F281; C284; Q285; F288; W299; Y306; L308; E321; M323; L324; H327; H407; R410; L411; I414; F420; M425 and F429 according to Table 3a, 3b, 4a, 5b or 5a; or b) a homologue of said molecule or molecular complex, wherein said homologue comprises a binding pocket that has a root mean square deviation from the backbone atoms of said amino acids of not more than 1.5 Å (e.g., 1.25 Å, 1.0 Å, 0.75 Å, 0.50 Å, 0.25 Å or 0.1 Å) comprising the steps of: (i) employing computational means to perform a fitting operation between the chemical entity and the binding pocket of the molecule or molecular complex; and (ii) analyzing the results of said fitting operation to quantify the association between the chemical entity and the binding pocket. If the chemical entity is determined to fit or be capable of binding the binding pocket, it can be further tested for the ability to bind PXR or a PXR-SRC1 hybrid polypeptide in vitro or in vivo, for example, using any of the assay methods set forth herein.

Another embodiment of the invention comprises a method for identifying a compound capable of associating with a molecule comprising a PXR-LBD-L10-SRC1 (e.g., SEQ ID NO: 24) binding pocket comprising the steps of: a) using the atomic coordinates of PXR-LBD-L10-SRC1 amino acids L206; S208; L209; V211; L240; M243; A244; M246; S247; F251; F281; C284; Q285; F288; W299; Y306; L308; E321; M323; L324; H327; H407; R410; L411; I414; F420; M425 and F429 according to Table 3a, 3b, 4a, 5b or 5a+a root mean square deviation from the backbone atoms of said amino acids of not more than 1.5 Å (e.g., 1.25 Å, 1.0 Å, 0.75 Å, 0.50 Å, 0.25 Å or 0.1 Å), to generate a three-dimensional structure of a molecule comprising a PXR-LBD-L10-SRC1-like binding pocket; b) employing said three-dimensional structure to design or select said compound; c) synthesizing said compound; and d) contacting said compound with said molecule to determine the ability of said compound to interact with said molecule.

Pharmacology

Evaluation of the interaction of PXR with clinical drug candidates provides a fast and convenient method by which to determine if the candidate is likely to exhibit undesirable pharmacological properties. For example, identification of a drug candidate/PXR interaction indicates that the drug may exhibit an undesirably short half-life in vivo, that the drug may cause other co-administered drugs to exhibit an undesirably short half-life in vivo or that the drug candidate would be likely to be metabolized, in vivo, by liver enzymes (e.g., cytochrome P450 enzymes) into potentially toxic prodrugs. Clinical drug candidates that are not likely to exhibit desirable pharmacologic characteristics in vivo can be quickly and conveniently identified, in vitro, before in vivo administration to humans or animals, using the methods set forth herein.

In an embodiment of the pharmacological assay methods set forth herein, PXR or the fragment thereof (e.g., SEQ ID NO: 1 or 2) is provided with SRC-1 or a fragment thereof (e.g., SEQ ID NO: 3 or 35). In an embodiment, PXR or the fragment thereof is fused to SRC-1 or the fragment thereof (e.g., by a polypeptide linker); in another embodiment, they are unfused.

The present invention provides a method for determining whether the half-life of a drug will be affected by the cytochome P450 pathway when administered to a subject in vivo, comprising:

(i) contacting PXR or a fragment or fusion thereof (e.g., PXR-linker-SRC-1) with the drug; and (ii) determining whether the drug binds to PXR or the fragment or fusion thereof or modulates PXR or the fragment or fusion thereof;

whereby the half-life of the drug is determined to be affected by the P450 pathway if said binding or modulation is observed.

The present invention provides a method for determining whether a first drug will affect the half-life of a second drug when co-administered to a subject in vivo comprising:

(i) contacting PXR or a fragment or fusion thereof (e.g., PXR-linker-SRC-1) with said first drug; and (ii) determining whether said first drug binds to or modulates PXR;

whereby the half-life of the second drug is determined to be affected by co-administration with the first drug if said binding or modulation is observed.

The present invention also provides a method for determining whether a drug will be metabolized, when administered to a subject in vivo, into a prodrug comprising

(i) contacting PXR or a fragment or fusion thereof (e.g., PXR-linker-SRC-1) with the drug; and (ii) determining whether the drug binds to or modulates PXR;

whereby it is determined that the drug will be metabolized into a prodrug if said binding or modulation is observed.

Binding or modulation of a drug with PXR or a fragment or fusion thereof can be determined by any of the methods set forth above under “Assays” including circular dichroism, a direct binding assay or a cell based assay.

Any drug can be analyzed using one of the methods set forth herein to determine if its half-life or metabolism is affected by PXR. Such drugs include, but are not limited to, mometasone furoate, loratadine, desloratadine, fexofenadine HCl, cetirizine HCl, temozolomide, ezetimibe, ciprofloxacin, ofloxacin, levofloxacin, norfloxacin, enoxacin, lomefloxacin, grepafloxacin, trovafloxacin, sparfloxacin, temafloxacin, moxifloxacin, gatifloxacin, gemifloxacin, garenoxacin, vardenafil HCl, sildenafil citrate, interferon-2a or 2b, PEG-interferon-2a or 2b, ribavirin, simvastatin, atorvastatin, lovastatin, fluvastatin and pravastatin.

The following examples are provided to more clearly describe the present invention and should not be construed to limit the scope of the invention. Any composition described herein forms part of the present invention as does any method described herein.

Construction of E. coli Bicistron Expression Constructs for Human PXR

D14-PXR-RBS-SRC. Human PXR-LBD DNA fragment P1 (coding for an N-terminal 6× His and amino acid residues 130-434 of SEQ ID NO: 1) was amplified using the primers H6-hPXR(130)-F (5′CACCATGAAAAAAGGTCACCACCATCA CCATCACGGTAGTGAACGGACAGGGACTCAGC3′) (SEQ ID NO: 4) and PBC-PXR-R (5′ CAGCTACCTGTGATGCCGAACAAC3′) (SEQ ID NO: 5); human SRC-1 DNA fragment S1 (containing the amino acid residues 623-710) was amplified using the primers PBC-SRC-F (5′GTTGTTCGGCATCACAGGTAGCTGAATTC AAGAAGGAGATATACCATGAGTAAATACTCTCAAACCAGTCACA 3′) (SEQ ID NO: 6) and SRC710-R (5′CTAATCAGGCTCGACAGACAAAG 3′) (SEQ ID NO: 7). P1 and S1 were mixed at 1:1 ratio, and subsequent PCR was performed on this mixture using primers H6-hPXR(130)-F (5′ CACCATGAAAAAAGGTCACC ACCATCACCATCACGGTAGTGAACG GACAGGGACTCAGC 3′) (SEQ ID NO: 8) and SRC710-R (5′CTAATCAGGCTCGACAGACAAAG 3′) (SEQ ID NO: 9). The final PCR product containing P1, a ribosomal binding sequence (RBS) and S1 was ligated into a Gateway vector pENTR™/SD/D-TOPO® to obtain a plasmid TOPO-PXR-RBS-SRC following the manufacturer's protocol (Invitrogen, Carlsbad, Calif.). The expression plasmid D14-PXR-RBS-SRC was subsequently obtained from an LR in vitro recombination reaction by mixing plasmids TOPO-PXR-RBS-SRC and pDEST14 following the manufacturer's protocol (Invitrogen, Carlsbad, Calif.) and confirmed by DNA sequencing.

D14-SRC-RBS-PXR. Human SRC-1 DNA fragment S2 (coding for an N-terminal 6× His and amino acid residues 623-710 of SEQ ID NO: 3) was amplified using the primers PSRC-F (5′CACCAGTAAATACTCTCAAACCAGTCAC3′) (SEQ ID NO: 10) and PBC-SRC-R (5′ TCAGGCTCGACAGACAAAGTGGTG 3′) (SEQ ID NO: 11); human PXR DNA fragment P2 (containing the amino acid residues 130-434) was amplified using the primers PBC PXR-F (5′CACCACTTTGTCTGTCGAGCCTGATTGAATTCAAGAAG GAGATATACCATGAAAAAAGGTCACCACCATCACCATC 3′) (SEQ ID NO: 12) and hPXR(434)-R (5′ TTAGCTACCTGTGATGCCGAACAAC3′) (SEQ ID NO: 13). S2 and P2 were mixed at 1:1 ratio, and subsequent PCR was performed on this mixture using primers PSRC-F (5′CACCAGTAAATACTCTCAAACCAGTCAC3′) (SEQ ID NO: 10) and hPXR(434)-R (5′TTAGCTACCTGTGATGCCGAACAAC3′) (SEQ ID NO: 13). The final PCR product was cloned into the vector pDEST14 to obtain the expression plasmid D14-SRC-RBS-PXR using the Gateway cloning strategy as described above (Invitrogen, Carlsbad, Calif.) and confirmed by DNA sequencing, respectively, by a PCR reaction using primers H6-hPXR(130)-F (5′CACCATGAAAAAAGGTCACCACCATCACCATCACGGTAGTGAA CGGACAGGGACTCAGC 3′) (SEQ ID NO: 4) and PL10-SRC-R (5′ TTATGAGGGGCTACCCTCCTGTAAGAGCCGGTGTAGAATTTTATGCCGTTCTGTCA ATGAGCTATGAGAAGAGCCACCAGAGCCACCGCTACCTGTGATGCCGAACAACTC 3′) (SEQ ID NO: 14). The resulting PCR product was cloned into the vector pDEST14 to obtain the expression plasmid D14-PXR-L10-SRCp using the Gateway cloning strategy as described above (Invitrogen, Carlsbad, Calif.) and confirmed by DNA sequencing.

The expression cassettes created in the two foregoing plasmids are represented, diagrammatically below:

--PXR(130-434 of SEQ ID NO: 1)→TGAATTCAAG AAGGAG ATA

TACC (SEQ ID NO: 15; Shine-Delgarno in bold

font)--SRC (623-767 of SEQ ID NO: 3)→

--SRC (623-767 of SEQ ID NO: 3)→TGAATTCAAG AAGGAG A

TATACC (SEQ ID NO: 15; Shine-Delgarno in bold

font)-- PXR (130-434 OF SEQ ID NO: 1)→

The nucleotide sequence of the PXR-LBD-RBS-SRC bicistronic construct was:

The nucleotide sequence of the SRC-RBS-PXR-LBD bicistronic construct was:

Construction of Tethered Expression Plasmids

A DNA fragment encoding PXR-LBD with an N-terminal 6× His tag, followed by a linker (GGSGG) (SEQ ID NO: 37) and a peptide fragment of SRC-1 (residues 678-710 of SEQ ID NO: 3) was amplified using the primers hPXR(130)-F (5′CACCATGAAAAAAGGTCACCACCATCACCATCACGGTAGTGAACGGACAGGGAC TCAGC 3′) (SEQ ID NO: 4) and PL10-SRC-R (5′TTATGAGGGGCTACCCTCCTG TAAGAGCCGGTGTAGAATTTTATGCCGTTCTGTCAATGAGCTATGAGAAGAGCCAC CAGAGCCACCGCTACCTGTGATGCCGAACAACTC3′) (SEQ ID NO: 14). The resulting PCR product was cloned into the vector pDEST14 to obtain the expression plasmid D14-PXR-L5-SRCp using the Gateway cloning strategy as described above (Invitrogen, Carlsbad, Calif.) and confirmed by DNA sequencing. D14-PXR-L10-SRCp was used as a template to create D14-PXR-L8-SRCp and D14-PXR-L13-SRCp (which encode 2 aa residues shorter or 3 longer in the translated linker region, respectively) with primer pairs L8-f (5′cacaggtagcggtggtggctcttctc3′) (SEQ ID NO: 18)/L8R (5′gagaagagccaccaccgctacctgtg3′) (SEQ ID NO: 19) and L13-F (5′ GTAGCGGTGGCTCTGGTGGCTCCGGTGGTTCTTCTCATAGCTCATTGACAGAAC 3′) (SEQ ID NO: 20)/L13-R (5′ GTTCTGTCAATGAGCTATGAGAAGAACCACCGGAGCCACCAGAGCCACCGCTAC 3′) (SEQ ID NO: 21), respectively, using the two-stage QuikChange site-directed mutagenesis protocol.

See protocol in Wang et at, Methods Mol. Biol. 182: 37-43 (2002)).

The amino acid sequence of the tethered human PXR-LBD-L8-SRC (the linker sequence is underscored) was:

(SEQ ID NO: 22)

PFATPLMQELFGITG SGGGSSHS SLTERHKILHRLLQEGSPS.

In an embodiment, a polynucleotide encoding PXR-LBD-L8-SRC is

The amino acid sequence of the tethered human PXR-LBD-L10-SRC (the linker sequence is underscored) was:

(SEQ ID NO: 24)

In an embodiment, a polynucleotide encoding PXR-LBD-L10-SRC is:

The amino acid sequence of the tethered human PXR-LBD-L13-SRC (the linker sequence is underscored) was:

(SEQ ID NO: 26)

In an embodiment, a polynucleotide encoding PXR-LBD-L13-SRC is:

The amino acid sequence of the tethered human PXR full -L8-SRC (the linker sequence is underscored) was:

(SEQ ID NO: 28)

In an embodiment, a polynucleotide encoding PXR full -L8-SRC is:

The amino acid sequence of the tethered human PXR full -L10-SRC (the linker sequence is underscored) was:

(SEQ ID NO: 30)

In an embodiment, a polynucleotide encoding PXR full -L10-SRC is:

The amino acid sequence of the tethered human PXR full -L13-SRC (the linker sequence is underscored) was:

(SEQ ID NO: 32)

In an embodiment, a polynucleotide encoding PXR full -L13-SRC is:

In an embodiment of the present invention, a “fusion” of PXR is PXR full -L8-SRC, PXR full -L10-SRC, PXR full -L13-SRC, PXR-LBD-L8-SRC, PXR-LBD-L10-SRC or PXR-LBD-L13-SRC. Embodiments of the present invention also include a PXR “fusion” comprising PXR full -L8-SRC, PXR full -L10-SRC, PXR full -L13-SRC, PXR-LBD-L8-SRC, PXR-LBD-L10-SRC or PXR-LBD-L13-SRC but excluding the N-terminal MKKGHHHHHH (SEQ ID NO: 34) sequence.