Procedure for the chromatographic purification of insulins

The present invention relates to an improved procedure for the chromatographic purification of insulins.

In addition to enzymatic and/or genetic engineering procedures, the procedures for preparing insulins essentially comprise chromatographic procedures in order to fulfill the extremely high purity demands.

The term “insulins” is understood here as meaning insulins originating from natural sources or recombinant insulins (i.e., expressed by genetically modified microorganisms) of animal or human origin (e.g., porcine insulin, bovine insulin or human insulin), proinsulins (e.g., insulin precursors, preinsulins), or insulin derivatives.

Insulin derivatives are designated below as derivatives of naturally occurring insulins, namely human insulin or animal insulins, which differ by substitution of at least one naturally occurring amino acid and/or addition of at least one amino acid and/or organic residue from the corresponding, otherwise identical naturally occurring insulin.

Human insulin is a polypeptide which is constructed of 51 amino acids. The so-called A (acidic) chain consists of 21 amino acids, and the B (basic) chain consists of 30 amino acids. In both amino acid chains, 6 cysteine residues occur, each two cysteine residues being bonded to one another via a disulfide bridge (the two chains are linked to one another by two cysteine bridges). In biologically active human insulin, the A and B chains are bonded to one another via two cysteine bridges, and a further bridge occurs in the A chain. The following cysteine residues are linked to one another in the (biologically active) human insulin:

A6-A11

A7-B7

A20-B19.

The letters A and B represent the particular insulin amino acid chain and the number represents the position of the amino acid which is counted from the amino to the carboxyl end of the respective amino acid chain.

The preparation of recombinant insulin is customarily carried out in the steps of fermentation and cell disruption, followed by protein chemistry and process technology processes, customarily chromatographic processes, for the purification of the product.

Genetic engineering procedures allow human proinsulin or proinsulin (proinsulin of insulin derivatives) which has an amino acid sequence and/or amino acid chain length differing from human insulin, to be prepared in microorganisms. The proinsulins prepared from genetically modified Escherichia coli cells do not have correctly bonded cysteine bridges. A procedure for obtaining human insulin having correctly bonded cysteine bridges using E. coli is disclosed, for example, in EP 0 055 945. Improved procedures for the preparation of human insulin and insulin derivatives having correctly bonded cysteine bridges are described in EP 0 600 372 A1 (U.S. Pat. No. 5,473,049) and in EP 0 668 292 A2 (U.S. Pat. No. 5,663,291).

Proinsulin, a precursor of insulin, prepared from genetically modified microorganisms is first isolated from the cells, correctly folded, and then converted enzymatically to human insulin. In addition to undesired by-products, the cleavage mixture obtained in the enzymatic peptidation processes contains both the valuable substance and the undesired insulin-like impurities, which do not markedly differ either in molecular weight or in other physical properties from the valuable product, thereby making separation and purification very difficult, particularly on a large industrial scale.

The process technology processes for purification are a series of various chromatography procedures (e.g., adsorption chromatography, ion-exchange chromatography, reversed phase or reverse-phase high-pressure chromatography or combinations thereof) in some cases in a number of stages using different support materials, in some cases with subsequent crystallization, the actual purification being achieved by chromatography. The removal of the insulin-like impurities in this case takes place on ion exchangers or on reversed phase silica supports.

The end-polishing (removal of very minor impurities, as the last purification stage) is customarily carried out in the high-pressure range using chromatography on reversed phase silica gel (RP-HPLC=reversed phase high-pressure liquid chromatography).

“Reversed phase silica gel” (or reverse-phase, i.e., lipophilically modified, that is hydrophobic) is understood as meaning a silica material to which a hydrophobic matrix has been applied. Examples of a hydrophobic matrix are alkanes having a chain length of 3 to 20 carbon atoms, in particular 4 to 18 carbon atoms. The particle sizes are in the range from 10 to 50 μm, the pore widths 50 to 300 A.

Examples of chromatography procedures that, according to the prior art, utilize RP-silica gels (lipophilically modified silica gels) are EP 0 547 544 A2 (U.S. Pat. No. 5,621,073) or EP 0 474 213 A1 (U.S. Pat. No. 5,245,008). According to the prior art, the high demands on the purity of the insulins to be prepared can only be fulfilled by the use of reversed phase silica gels. The use of reversed phase silica gel, however, has crucial disadvantages:

Reversed phase silica gels are only stable in the range from pH 2 to pH 10. In the chromatography of fermentation products, high molecular weight by-products are always contained which are persistently adsorbed and cannot be desorbed using the customary elution. These by-products concentrate on the RP silica gel with time (referred to as aging of the adsorbent).

Regeneration or cleaning in place (CIP) is usually carried out only by rinsing with dilute sodium hydroxide solution. Thus, in each CIP process, a part of the RP silica gel is destroyed requiring continuous replacement which is very cost-intensive. The danger of denaturation furthermore exists for insulins on silica gels.

Many attempts have been made to replace RP gels based on silica without complete success. Attempts using RP material based on alumina or titanium dioxide (both materials are not completely pH-stable, but at least more stable than silica gel) have shown that the separation is inadequate and that the required purity cannot be achieved.

A further necessary property of chromatography materials is their pressure stability.“Pressure-stable polymeric chromatography materials” are understood as meaning particles of organic polymers, which can occur in all possible forms, e.g., rod form, fragments, or preferably, spherical form, and preferably have diameters between from about 10 μm to about 35 μm, and whose deformation under the action of pressure (up to 70 bar) is only slight. The material located in the chromatography column must be so well packed that no cavities are present (the quality of the packing determines the separation result). For the packing of columns, in principle, two different techniques are known, which can also be used in combination. The first technique is the method of compressing the packing by means of a ram that is usually hydraulically operated (DAC=direct axial compression). The second technique is a method of packing the column hydrodynamically by means of a high-pressure pump, i.e. pressing a suspension of liquid and particles into the column. In both cases, it is necessary for pressures to reach about 70 bar on the cross section of the column in order to avoid cavity formation and to pack the particles as tightly as possible.

Many organic polymer particles are not pressure-stable and deform under high pressure resulting in flat disk spheres that overlap and suppress the flow through the packing. In contrast, reversed phase silica gels are considerably more pressure-stable by nature, and barely deform under the pressures mentioned.

The object of the present invention is to provide a process for the chromatographic purification of insulins on suitable chromatography materials that are pressure-stable and can be employed in the entire pH range from about 1 to about 14. Due to the high separation efficiency of this purification process only one stage is needed.

The object is achieved by a procedure for the chromatographic purification of insulins, using a pressure-stable organic polymeric chromatography material as a stationary phase, and a mobile phase containing at least one water-miscible organic solvent and at least one buffer substance, and a pH from about 7 to about 11 during the purification stage.

Surprisingly, it was found that with chromatography in the pH range from about 7 to about 11, i.e. in the basic range, a very good separation is achieved on pressure-stable organic polymeric chromatography materials. The pH is preferably from about 9 to about 10.

A particular advantage of the procedure according to the present invention is that, in this basic pH range, the formation of the impurity deamido insulin, which is customarily formed in the acidic medium, and which, according to the specifications of insulin preparations, must be removed to very small residual amounts, is suppressed.

The mobile phases which are employed for the elution contain water-miscible organic solvents, for example, alcohols having 1 to 4 carbon atoms, ketones, methyl acetate or acetonitrile. Preferred alcohols are those such as 1 or 2-propanol (n or iso-propanol), methanol, or ethanol. The concentration of the water-miscible organic solvents is between from about 1 to about 90% by volume, preferably between from about 10 to about 50% by volume.

The mobile phases, furthermore, contain a buffer substance that keeps the pH of the eluent constant. Suitable buffer substances are, for example, phosphates, alkali metal or alkaline earth metal salts, such as sodium citrate or potassium acetate, ammonium citrate, acetate, sulfate or chloride.

The pH is adjusted by the addition of hydrochloric acid or sodium hydroxide solution.

The elution can be carried out isocratically, i.e., with constant concentration of the buffer substances and with a constant proportion of the organic solvent, or preferably with a linear gradient, i.e., with an increase in the proportion of solvent.

The average particle size of the pressure-stable organic polymeric chromatography material should advantageously be from about 5 μm to about 300 μm, preferably from about 10 μm to about 50 μm. The smaller the particle size, the sharper and better the separation. However, the pressure stability of smaller particles is lower.

Insulin is a relatively small polypeptide (molecular weight of about 6000) and can diffuse without problems into pores having a diameter of about 10 nm (no steric hindrance). Materials having small pore diameters are more suitable, since the specific surface area, and thus the adsorption capacity, are larger. The average pore size of the pressure-stable organic polymeric chromatography material is advantageously from about 5 nm to about 500 nm, preferably from about 10 nm to about 50 nm.

For the procedure according to the present invention, pressure-stable organic polymeric chromatography materials which preferably consist of polystyrene/divinylbenzene or of polymethacrylate are particularly suitable. Examples of commercially available pressure-stable organic polymeric chromatography materials which can be advantageously employed in the procedure according to the present invention are compiled in Table 1.

The procedure according to the invention is suitable for analytical, for semipreparative, and in particular, for preparative chromatography. The term “preparative chromatography” is understood as meaning the preparation of pure products on a technical scale.

Prior to the discovery of the present invention, in order to achieve the purity necessary for insulin preparations, it was necessary to include at least one additional reversed phase chromatography or cation exchange chromatography step and, if appropriate, a crystallization step before the reversed phase chromatography (for example, see EP 0 547 544 A2 (U.S. Pat. No. 5,621,073) or EP 0 474 213 A1 (U.S. Pat. No. 5,245,008)). In the procedure according to the invention, the same result is achieved with a single stage chromatography on the polymer support. Using the procedure according to the invention, the total yield in the insulin preparation is therefore significantly improved, as losses in yield are eliminated by combining several process stages into one stage.

The procedure according to the present invention is suitable for the chromatographic purification of all insulins according to the definition introduced at the outset, namely insulins originating from natural sources or recombinant insulins (i.e., expressed by genetically modified microorganisms) of animal or human origin (e.g., porcine insulin, bovine insulin or human insulin), proinsulins (e.g., insulin precursors, preinsulins), or insulin derivatives, insulin derivatives being understood as meaning derivatives of naturally occurring insulins, namely human insulin or animal insulins, which differ from the corresponding, otherwise identical naturally occurring insulin by substitution of at least one naturally occurring amino acid residue and/or addition of at least one amino acid residue and/or organic residue.

Examples of such insulins are human insulin, bovine insulin, porcine insulin, insulins according to EP 0 368 187 (U.S. Pat. No. 5,656,722), for example Gly(A21), Arg(B31), Arg(B32) human insulin, insulins according to EP 0 821 006 (ZA 97/6645) or the insulins described in EP 0 547 544 A1 (U.S. Pat. No. 5,621,073), EP 0 474 213 A1 (U.S. Pat. No. 5,245,008), EP 0 600 372 A1 (U.S. Pat. No. 5,473,049) or in EP 0 668 292 (U.S. Pat. No. 5,663,291). (The letters A and B represent the respective insulin amino acid chain, the number for the position of the naturally occurring amino acid residue, which is replaced by the amino acid residue given before the brackets.)