Process for the manufacture of 117Sn diethylenetriaminepentaacetic acids

This invention was made with Government support under contract number DE-AC02-98CH10886, awarded by the U.S. Department of Energy. The Government has certain rights in the invention.

Bone metastases are the most common cause of cancer pain, and primary bone cancers may also cause severe chronic pain. Approximately 75-80% of patients with prostate, breast, and lung cancer develop osseous metastases which cause bone pain during the late stages of their illness. Clinical management of cancer-related bone pain through palliation is necessary to improve the quality of life of terminal cancer patients.

A number of options are currently available for clinical management of bone pain. Nonsteroidal anti-inflammatory agents, opioids, hormones, and cytotoxic chemotherapy are used in the initial stages of bone metastasis, and external beam radiation can be applied locally when bone pain occurs at a single site. As the patient's skeletal tumor burden increases, pain can increase and become multifocal, tending to move from one site to another. Hemibody external beam radiation can afford rapid pain relief for disseminated skeletal tumors. However, such extensive exposure to radiation may affect noncancerous rapidly dividing tissues in the gastrointestinal tract or bone marrow, and morbidity may result from hemibody radiotherapy for bone pain palliation.

Radiation emitted by intravenously administered radionuclides may also be used to treat bone pain palliation. A number of bone-seeking radioisotopes have been studied for their ability to palliate bone pain. For example, 32 P, which emits a 1.7 MeV β particle and has a half-life of 14.3 days, exhibits a 3- to 5-fold increase in uptake in bone around osseous metastases as compared to normal bone. The uptake of 32 P into bone lesions can be increased by pretreatment with androgen, and patients thus treated frequently experienced pain relief within five to fourteen days of 32 P orthophosphate administration, with response duration of two to four months. However, bone marrow receives a disproportionately high dose of 32 P from the surrounding inorganic bone matrix and from the cellular component of the bone marrow space, resulting in myelosuppression as a side effect. Pancytopenia resulting from myelosuppression by 32 P, though reversible, may necessitate transfusions. 32 P orthophosphate is not currently used for palliation of metastatic bone pain.

153 Sm emits an 0.81 MeV β particle, with a half-life of 46.3 hours. The stable 153 Sm ethylenediaminetetramethylenephosphonate (EDTMP) complex has received FDA approval. Patients report clinical benefit within two weeks of 153 Sm-EDTMP treatment, frequently within 48 hours of treatment, and pain relief may last from four to forty weeks. Reversible myelosuppression also results from bone pain palliation treatment with 153 Sm-EDTMP. In addition, at the applied therapeutic doses (35 to 210 mCi) large amounts of radioactivity can be excreted in the patient's urine, creating contamination risks and potentially causing radiation cystitis.

186 Re emits a 1.07 MeV β particle and a 137 keV, 9% abundance γ photon, having a half-life of 89.3 hours. 186 Re forms a stable complex with hydroxyethylidine diphosphonate (HEDP) which rapidly accumulates in osteoblastic metastases. Symptom relief occurs for 40-65% of patients within two weeks, and frequently within 24-48 hours. 186 Re-HEDP causes reversible myelosuppression, and therapeutic doses of 186 Re-HEDP (30 to 70 mCi) also create contamination and radiation cystitis risks. Phase III clinical trials of 186 Re-HEDP have been completed.

89 Sr emits a 1.46 MeV β particle and has a half-life of 50.5 days. The biological half-life of 89 Sr exceeds 50 days in osteoblastic metastases, as compared to 14 days in normal bone. Bone pain relief occurs in 60 to 80% of patients, with onset two to four weeks after injection, though some patients may not experience relief for as much as ten weeks after treatment. The average duration of relief from 89 Sr treatment is from three to six months. Treatment with 89 Sr delays development of new bone pain in pre-existing, but clinically silent metastases. Four weeks after therapy, 89 Sr treatment typically causes a 30% decrease in platelet count, which recovers slowly over 12 weeks. Toxicity from 89 Sr treatment is cumulative, resulting from the total absorbed dose of radiation delivered to the bone marrow and from replacement of marrow by tumor as disease advances. In 1993, the FDA approved an adult dosage of 4 mCi of 89 SrCl 2 for bone pain palliation.

Although pain relief is believed to occur independently from radiation-induced tumor cell killing, administration of bone pain-palliating doses of 153 Sm and 186 Re results in transient changes in levels of certain biochemical markers related to cancer progression. When administered with low doses of cisplatin, 89 Sr also demonstrates reductions in tumor markers. This observation has led to the suggestion that administration of higher doses of these nuclides might result in a tumoricidal effect. However, no anti-tumor effect or improvement in survival has been demonstrated to result from administration of 153 Sm, 186 Re, or 89 Sr, and the ability to increase dosages of these nuclides is limited by their myelosuppressive effects.

117m Sn emits low energy conversion electrons (0.13 and 0.16 MeV) and a 159 keV photon, having a half-life of 14.0 days. 117m Sn (Sn 4+ ) diethylenetriaminepentaacetic acid (DTPA) exhibits higher bone uptake and retention than 32 P orthophosphate, 153 Sm-EDTMP, 186 Re-HEDP, and 89 SrCl 2 . Because of this, therapeutic doses of 117m Sn (Sn 4+ ) DTPA are lower than those of 153 Sm-EDTMP and 186 Re-HEDP, resulting in less risk of contamination and radiation cystitis. The lower energies of the conversion electrons emitted by 117m Sn result in less radiation exposure to the bone marrow and fewer hematologic side effects than are observed with 153 Sm-EDTMP, 186 Re-HEDP, and 89 SrCl 2 . The photon emitted by 117m Sn allows imaging and quantification of the isotope in normal and metastatic bone. 117m Sn(Sn 4+ ) is particularly suitable for the dose escalation necessary to effect cell killing in osseous tumors, since the myelosuppression which limits the benefits of the anti-tumor effects of 153 Sm, 186 Re, and 89 Sr is not a limiting factor for 117m Sn(Sn 4+ ).

Atkins, et al., Radiology (1993) 186, 279-283, discloses biodistribution of low doses of 117m Sn (Sn 4 +) DTPA administered to humans. Atkins, et al. (1995) J. Nucl. Med. 36, 725-729 reports a Phase II pilot study which demonstrates palliation of bone pain resulting from 117m Sn (Sn 4+ ) DTPA treatment. A Phase II study of 117m Sn (Sn 4+ ) DTPA as a bone pain palliation agent is reported in Krishnamurthy, et al. (1997) J Nucl. Med. 38, 230-237. A dose escalation study of 47 patients treated with 117m Sn (Sn4+) DTPA for bone pain palliation is reported in Srivastava, et al. (1998) Clin. Cancer Res. 4, 61-68. All of the 117m Sn (Sn 4+ ) DTPA formulations used in these studies contained a 20-fold molar excess of DTPA over 117m Sn (Sn 4+ ) and an amount of CaCl 2 corresponding to 80% of the molar amount of DTPA. The CaCl 2 was administered with the 117m sn (Sn 4+ ) DTPA to counteract any potential effect of uncomplexed DTPA on bone or blood calcium levels.

U.S. Pat. No. 4,533,541 discloses preparation of 117m Sn (Sn 4+ ) chelates capable of localizing to bone after intravenous injection, which were used for diagnostic purposes. The chelating agents disclosed in U.S. Pat. No. 4,553,541 include DTPA, which was formulated in significant molar excess (8-40-fold) over the concentration of 117m Sn (Sn 4+ ) (i.e., the concentration of total tin) in the radiopharmaceutical composition.

WO 95/29706 discloses 117m Sn (Sn 4+ ) DTPA compositions for bone pain palliation and bone cancer therapy, which employ a molar excess of DTPA over 117m Sn (Sn 4+ ). WO 95/29706 demonstrates dose-dependent relief of bone pain in humans after administration of 117m Sn (Sn 4+ ) DTPA, with particular efficacy at doses of about 9 to about 25 mCi per 70 kg body weight. None of the 117m Sn (Sn 4+ ) DTPA compositions used in WO 95/29706 exhibited bone marrow toxicity, thus providing a significant advantage over the known agents. In a preferred embodiment, CaCl 2 was included with the 117m Sn (Sn 4+ ) DTPA compositions of WO 95/29706, to inhibit or retard possible hypocalcemic effects of the unchelated tin chelating agent DTPA.

U.S. Pat. No. 6,004,532 discloses a process for the manufacture of 117m Sn(Sn 4+ )DTPA which includes the reaction of DTPA at a molar concentration from about eight to about twenty times the molar concentration of 117m Sn(Sn 4+ ) to form a 117m Sn(Sn 4+ )DTPA complex and then removing the excess DTPA by chromatographic means. Because of the toxicity of the DTPA, this process requires the additional step of the removal of the excess DTPA.

The present invention provides a method of making a pharmaceutical composition of 117m Sn(Sn 4+ )DTPA. This first method provided utilizes an aqueous solvent and is therefore termed “aqueous reaction” herein. The method includes the steps:

a) dissolving metallic 117m Sn in a concentrated acid suspended in an aqueous medium to form a 117m SnCl 2 solution;

b) adding the DTPA to 117m SnCl 2 solution in a molar concentration ratio of between about 1.0 to about 3.0 DTPA to 117m SnCl 2 ;

c) allowing the 117m SnCl 2 to react with the DTPA to form a 117m Sn(Sn 2+ )DTPA complex;

d) oxidizing the 117m Sn(Sn 2+ )DTPA to form a composition comprising 117m Sn(Sn 4+ )DTPA; and

e) removing the concentrated acid and water from the solution to form to form a resulting solid composition comprising 117m Sn(Sn 4+ )DTPA complex with a molar ratio of DTPA to 117m Sn(Sn 4+ ) of about 1.0 to about 3.0. In a preferred embodiment, the resulting composition comprises 117m Sn(Sn4+)DTPA with a molar ratio of DPTA to 117m Sn(Sn 4+ ) of about 1 to about 1.2.

In a separate embodiment, another method of making a pharmaceutical composition of 117m Sn(Sn 4+ ) DTPA is provided. This method utilizes organic solvents and therefore is termed the “organic reaction” herein. This method the steps:

a) dissolving metallic 117 Sn in a concentrated acid suspended in an aqueous medium to form a 117m SnCl 2 solution under an inert atmosphere;

b) adding DTPA to the 117m SnCl 2 solution in a molar concentration ratio of between about 1.0 to about 3.0 DTPA to 117m SnCl 2 in an inert atmosphere;

c) removing the concentrated acid and water from the solution to form a solid residue comprising unchelated 117m SnCl 2 and DTPA;

d) dissolving the solid residue in an organic solvent to form an organic mixture,

e) allowing the organic mixture to react sufficient to allow the formation of a 117m Sn(Sn 2+ )DTPA complex;

f) oxidizing the 117m Sn(Sn 2+ )DTPA to form a resulting composition comprising 117m Sn(Sn 4+ )DTPA with a molar ratio of DTPA to 117m Sn(Sn 4+ ) of about 1.0 to about 3.0.

In a preferred embodiment, the resulting composition comprises 117m Sn(Sn 4+ )DTPA with a molar ratio of DPTA to 117m Sn(Sn 4+ ) of about 1 to about 1.2.

The pharmaceutical composition manufactured by each of the aqueous reaction and organic reaction is also provided. These pharmaceuticals can be utilized in a method of treating primary or metastatic tumor in skeletal bone of a mammal. The method includes administering to the mammal a therapeutically effective amount of the pharmaceutical manufactured by either the aqueous or organic reactions.

A method of treating bone pain is also provided herein. This method also includes administering to a mammal a bone palliating amount of a pharmaceutical manufactured by either the aqueous or organic reaction method.

The method of the invention provides the benefit of a one-step reaction which, due to the ease of formulation, will result in minimizing personnel exposure, reduce waste, provide greater contamination control, and greatly simplify the eventual commercial production of 117m Sn(Sn 4+ )DTPA. Unlike previous methods, the purification step to remove excess DTPA is not necessary in the method of the invention because DTPA and 117m Sn(Sn 4+ ) are reacted at an approximately 1:1 ratio. The 117m Sn(Sn 4+ ) DTPA produced by the method of the invention possesses the same chemical (shelf life and stability) and biological (biodistribution in mice) properties, compared to the current 20:1 117m Sn(Sn 4+ )DTPA formulation.

Because the method of the invention can provide a composition having a lower ratio of DTPA to 117m Sn(Sn 4+ ) (i.e. less than 3:1), the composition resulting from the method of the invention contains less of the toxic unchelated DTPA. This reduced toxicity allows more of the composition to be administered which, in turn, also permits the use of low specific activity 117m Sn. Therefore, the radiopharmaceutical produced by the method of the invention can be utilized in the higher quantities to not only provide pain palliation in bone cancer patients, but also be used for bone cancer treatment.

FIG. 1 depicts the structure of the Sn(Sn 4+ ) DTPA of the invention analyzed by x-ray crystallography.

FIG. 2 depicts the analysis of the composition of the invention by 13 C NMR.

The method of the invention provides a radiopharmaceutical composition. More specifically, the method of the invention provides a 117m Sn(Sn 4+ )DTPA composition having a low DTPA to 117m Sn(Sn 4+ ) ratio, i.e. less that 3:1, preferably about 1:1.

The 117m Sn can be obtained by any suitable source. High specific activity 117m Sn is preferred; however, the ability to use low specific activity 117m Sn in the pharmaceutical compositions of the present invention provides greater flexibility in manufacturing for several reasons. First, there are a larger number of low flux reactors and cyclotrons than high flux reactors. Second, nuclear reactions other than the presently preferred 117 Sn(n,n′ γ ) 117m Sn reaction may be employed to produce 117m Sn.

In one embodiment, the 117m Sn has a specific activity from about 0.1 mCi/mg to about 80 Ci/mg. In another embodiment, the 117m Sn has a specific activity from about 2 mCi/mg to about 80 Ci/mg. In yet another embodiment, the 117m Sn has a specific activity from about 2 mCi/mg to about 20 mCi/mg.

117m Sn may be produced in any way for use in the present invention, for example using any of the methods discussed in Mausner, et al. (1992) Appl. Radiat Isot. 43, 1117-1122. As disclosed in WO 95/29706, enriched 117 Sn may be obtained as an oxide or as the metal, for example, from Oak Ridge National Laboratory, Oak Ridge, Tenn. The former may be converted to the metal by reduction at 600° C. in a hydrogen flow for 2.5 hours. A target may be prepared by sealing 4 to 100 mg of the 117 Sn metal in a quartz ampule. To produce 117m Sn, the target is irradiated in a high flux reactor for periods up to 4 weeks. When 117m Sn is produced in a low flux reactor, the 117 Sn target may be irradiated for periods of more than four weeks. 117m Sn may also be produced in a cyclotron, for example, by the nat Sb(p,an/a3n) 117m Sn reaction or by the 114 Cd(a,n) 117m Sn reaction.

The process of the invention can proceed in either aqueous or organic medium. The reaction in aqueous medium is preferred.

In the aqueous reaction, metallic 117 Sn or 117m Sn produced as described above is dissolved in a minimal amount, about 1.0 to 5.0 ml, preferably 1.0 ml, of a concentrated acid, such as hydrochloric acid, in an aqueous medium to form a solution of 117m SnCl 2 . Concentrated acid can be, for example, between 6-12 N. Amounts of tin between 1.0 to 100 mg can be dissolved in up to 5.0 ml, preferably 1.0 ml, of the concentrated acid, with heating.

DTPA, preferably solid DTPA, is then added to the solution of 117m SnCl 2 . The DTPA is added to the 117m SnCl 2 solution in a molar ratio of about 1 to about 3, preferably between about 1.0 to about 1.2, DTPA to 117m SnCl 2 . The 117m SnCl 2 is then permitted to react with the DTPA to form 117m Sn(Sn 2+ )DTPA.

The solution is then permitted to oxidize to allow the oxidation of 117m Sn(Sn 2+ )DTPA to 117m Sn(Sn 4+ )DTPA. This oxidation step can be performed by simple exposure of the solution to open air, e.g. three to 12 hours, or the addition of hydrogen peroxide, as is known in the art. The concentrated acid and water are then removed by conventional means, such as under vacuum, to produce a solid residue.

In a separate embodiment, the reaction can be carried out in an organic solvent. For this organic reaction, 117m Sn is dissolved in concentrated acid to form a 117m SnCl 2 solution as described above. The 117m Sn is dissolved in the concentrated acid under an inert atmosphere, such as Ar or N 2 .

DTPA is then added to the 117m SnCl 2 solution as discussed above. However, because this step also occurs in an inert atmosphere, a substantial quantity of the 117m SnCl 2 and DTPA may remain uncomplexed. At this stage, contrary to the aqueous method, the 117m SnCl 2 /DTPA mixture is not oxidized. The concentrated acid and water are removed by conventional means to produce 117m Sn(Sn 2+ ) and DTPA residue, without previous oxidation.

The solid 117m SnCl 2 /DTPA residue is then dissolved in an organic solvent. Examples of such organic solvents include methylene chloride, ethanol, and others. Methylene chloride is preferred. The organic mixture is then permitted to react for a sufficient amount of time, e.g. 12 hours, to enable 117m Sn(Sn 2+ ) DTPA complexation. The organic mixture is then evacuated to dryness by conventional means to form a resulting solid.

The resulting solid is then taken up in water, and the mixture oxidized from 117m Sn(Sn 2+ ) DTPA to 117m Sn(Sn 4+ ) DTPA. The oxidation of tin from stannous to stannic generally proceeds quite satisfactorily upon letting the mixture sit under air for about 3 hours to 12 hours. Also, an oxidizing agent, such as hydrogen peroxide, may be added to oxidize the 117m Sn (Sn 2+ ) to 117m Sn (Sn 4+ ). Oxidation is allowed to proceed for a suitable time, for example, for ten to fifteen minutes when hydrogen peroxide is employed.

The resulting solid, obtained either using aqueous or the non-aqueous reactions as set forth above, dissolved in water, may optionally be heated, for example in a boiling water bath, to facilitate complexation. The temperature should be sufficient to facilitate complexation, without destroying the desired product. Examples of such temperature ranges are as achieved in a boiling water bath preferably between about 90° C to about 100° C. If a heating step is performed, the 117m Sn (Sn 2+ ) DTPA solution is cooled to approximately room temperature. The pH of the solution containing 117m Sn(Sn 4+ ) DTPA is adjusted to between about 3 to about 5, preferably between about 4 to about 4.5. The solution is then reheated and cooled.

The resulting composition has a molar ratio of DTPA to 117m Sn(Sn 4+ ) of between about 3.0 to about 1.0. That is, in the pharmaceutical composition of the invention, for each mole of 117m Sn (Sn 4+ ) (or total tin) there will be from about one to about three moles of DTPA, either chelated to 117m Sn (Sn 4+ ) or in unchelated form. Preferably, the resulting composition of the invention will contain from about one to about 1.2 moles of DTPA for each mole of 117m Sn (Sn 4+ ).

The structure of the resulting 117m Sn(Sn 4+ ) DTPA was analyzed by x-ray crystallography, shown in FIG. 1, and 13 C NMR, shown in FIG. 2 .

The process of the invention may optionally include the addition of an isotonic vehicle such as Sodium Chloride Injection, Ringer's Injection, Dextrose Injection, Dextrose and Sodium Chloride Injection, Lactated Ringer's Injection, or other vehicle as known in the art. The process can also include the addition of stabilizers, preservatives, buffers, or other additives known to those of skill in the art.

A pharmaceutical composition is also provided that is manufactured by the process described above. The simplified pharmaceutical composition thus produced exhibits substantially the same chemical and biological properties over the composition having a 20:1 ratio of DTPA: Sn(Sn 4+ ) prepared by previously known methods.

A method of treating a primary or metastatic tumor in skeletal bone in a mammal is also provided. The method comprises the step of administering a therapeutically effective amount of the pharmaceutical composition as described above.

As defined herein, a therapeutically effective amount means the total amount of each active component of the pharmaceutical composition that is sufficient to show a meaningful patient benefit, i.e., a reduction in the size of the primary or metastatic tumor or in the incidence of additional metastatic tumors. It is contemplated that the administered dosage of the pharmaceutical composition for treatment of osseous tumors will be from about 10 mCi 117m Sn (Sn 4+ ) to about 1000 mCi 117m Sn (Sn 4+ ) per 70 kg body weight (i.e., a total dosage from about 7 mCi to about 1600 mCi). Preferably, the administered dosage is from about 10 mCi 117m Sn (Sn 4+ ) to about 300 mCi 117m Sn (Sn 4+ ) per 70 kg body weight (a total dosage from about 7 mCi to about 500 mCi). More preferably, the administered dosage is from about 10 mCi 117m Sn (Sn 4+ ) to about 100 mCi 117m Sn (Sn 4+ ) per 70 kg body weight (a total dosage from about 7 mCi to about 150 mCi). Most preferably, the administered dosage is from about 12 mCi 117m Sn (Sn 4+ ) to about 25 mCi 117m Sn (Sn 4+ ) per 70 kg body weight (a total dosage from about 9 mCi to about 50 mCi). Ultimately, specific treatment regimens appropriate for individual patients will be determined by the attending physician, taking into account the nature and severity of the condition being treated, and the nature of the prior treatments which the patient has undergone.

A method of treating bone pain associated with cancer in a mammal is also provided. The method includes the step of administering a bone pain palliating amount of a pharmaceutical composition as described above to the mammal.

As defined herein, a “bone pain palliating amount” means the amount of 117m Sn (Sn 4+ ) DTPA sufficient to show a meaningful reduction in bone pain as determined by objective means, for example, using the Keele pain scale (Keele, K. D. (1948) Lancet 2, 6-8) or the Karnofsky performance scale (Karnofsky, D. A. (1963) Clin. Pharmacol. Ther., 709-712). It is contemplated that the administered dosage of the pharmaceutical composition for relief of bone pain will be from about 6 mCi 117m Sn (Sn 4+ ) to about 50 mCi 117m Sn (Sn 4+ ) per 70 kg body weight (i.e., a total dosage from about 4 mCi to about 130 mCi). Preferably, the administered dosage is from about 10 mCi 117m Sn (Sn 4+ ) to about 50 mCi 117m Sn (Sn 4+ ) per 70 kg body weight (a total dosage from about 7 mCi to about 130 mCi). More preferably, the administered dosage is from about 10 mCi 117m Sn (Sn 4+ ) to about 30 mCi 117m Sn (Sn 4+ ) per 70 kg body weight (a total dosage from about 7 mCi to about 80 mCi). Most preferably, the administered dosage is from about 12 mCi 117m Sn (Sn 4+ ) to about 25 mCi 117m Sn (Sn 4+ ) per 70 kg body weight (a total dosage from about 9 mCi to about 65 mCi). Ultimately, specific treatment regimens appropriate for individual patients will be determined by the attending physician, taking into account the nature and severity of the condition being treated, and the nature of the prior treatments which the patient has undergone.

The host or patient for the pharmaceutical composition generally is mammalian. Mammals include, for example, humans, baboons and other primates, as well as pet animals such as dogs and cats, laboratory animals such as rats and mice, and farm animals such as horses, sheep, and cows. Humans are preferred.

Administration of the pharmaceutical can be by any pharmaceutically acceptable mode of administration. Parenteral administration is preferred. Parenteral administration includes, for example, intravenous injection.