NO306068B1 - Process for the preparation of pure polylactides - Google Patents

Description

The present invention relates to a method for producing pure polylactides.

European Patent Publication No. 0270987 A2 describes polylactides, e.g. polylactide-co-glycoides, which have been prepared by condensation of lactic acid and glycolic acid or preferably by polymerization of lactide and glycolide in the presence of a catalyst, e.g. tin di-(2-ethylhexanoate), also known as tin noctoate or tin octanoate.

The polylactides are purified by dissolving them in a solvent, which is not or only partially miscible with water, e.g. methylene chloride, and washing the solution with an aqueous solution of an acid, e.g. HC1 or by a metal ion complexing agent such as e.g. EDTA, after which the catalyst-metal cation or its complex is transferred to the aqueous solution in which it is more soluble.

After separation and detachment of the organic solvent layer and precipitation of the polylactide therefrom, e.g. by mixing the layer with an organic solvent, e.g. petroleum ether or an alcohol, e.g. methanol, which displaces the polylactide from solution, however, the precipitated polylactide still contains a certain amount of the catalyst metal cation - about 2 ppm - and additional catalyst anion, e.g. in acid form. Furthermore, the polylactide contains a certain amount of brown-coloured cleavage by-products which have been formed during the manufacturing process for the polymer, particularly under the influence of the catalyst.

After such as polylactides, e.g. polylactide-co-glycolides, preferably used as matrices for pharmacologically active compounds, e.g. in implants or microparticles that are supplied parenterally, the remaining contaminants can give rise to local irritation reactions in the body tissue, and, e.g. depending on the type of catalyst, to an instability of the matrix and thus possibly to an accelerated release of the pharmacologically active compound. The brown contaminants and the catalyst should thus preferably be removed to the greatest extent possible.

Catalyst-free polylactides based on the condensation of lactic acid and possibly glycolic acid are known, but have low molecular weights of approximately 2000 to 4000. Polylactides with higher molecular weight are preferred and can only be produced in the presence of a catalyst.

According to European patent document no. 0026599 B1, lactic acid and glycolic acid were reacted in the presence of a strongly acidic ion exchange resin as a catalyst, after which the other reaction components could be removed from the copolymer product by filtering the molten reaction mixture or by cooling the reaction mixture, dissolving the copolymer in a organic solvent in which the ion exchange resin is insoluble, filtering the solution and removing the organic solvent after which a copolymer was obtained from which the solid phase catalyst had been substantially removed. By this method, however, only polylactides with a molecular weight of approximately 6,000 to 35,000 were obtained.

Polylactides, e.g. polylactide-co-glycolides, with a wider molecular weight range than up to 35,000, are preferably produced by using lactide and optionally e.g. glycolide as monomers, but upon polymerization in the presence of a metal catalyst, this type of reaction leads, as previously mentioned, to considerable contamination of the reaction product.

It has now been found that polylactides, e.g. polylactide-co-glycolides, especially those prepared from lactide and glycolide as monomers, can be obtained in a better purified state.

The present invention provides a method for obtaining a polylactide which is an ester of a polyol containing at least three hydroxyl groups and which is in a purified state, which meets the requirements of

color strengths corresponding to reference solutions B2- B9 in the brown color test according to

European Pharmacopoeia, 2nd ed. (1980) Part I, Section V, 6.2, and

contains one or more metals in cationic form, where the metal ion or ions

has a concentration of no more than 10 ppm,

has an average molecular weight Mw of at least 35,000.

The method is characterized by the fact that it involves bringing a solution of the impure polylactide into contact with a matrix which has carboxylic acid groups on the surface and isolating the purified polylactide from the eluate.

The polylactide has preferred the reduced color strengths of the reference solutions B4-B9, especially of the reference solution B9. The color in a reference solution B9 indicates that the polylactide is a whitish or colorless product.

The polylactides which are preferably produced contain especially divalent metal ions, such as Zn++ and especially Sn++.

To determine the amount of tin, the polymer is split under high pressure with a mixture of hydrochloric acid and nitric acid. The precipitation and concentration of tin from this mixture takes place on a membrane filter and the measurement of the amount of metal is carried out using energy dispersive X-ray fluorescence (EDXRF) as described by H.D. Seltner, H.R. Lindner and B. Schreiber, Intern. J. Environ. Anal. Chem., 1981, Vol. 10, pp. 7-12 supplemented by atomic absorption spectrometry method carried out in a graphite furnace as a reference, as discussed at the 6th Colloquium Atomspektrometrische Spurenanalytik, 8.-12. April 1991 in Konstanz, Germany. Authors: H. Seltner, G. Hermann and C. Heppler.

According to the invention, the concentration of Sn++ in the purified polylactide obtained in accordance with the invention is preferably at most 1.5 ppm, especially at most 1 ppm. The catalyst anion is preferably ethyl hexanoate, which in the purified polylactide obtained by the invention is preferably present in a concentration of no more than 0.5% by weight of the polylactide.

The purified polylactide preferably contains, apart from its lactide units, further structural units, e.g. such as are described in European Patent Publication No. 0270987, second paragraph on page 4, where the glycolide unit is the preferred unit as, depending on its monomer ratio in the polymer chain, it can shorten the cleavage period of the polymer in the body and thus accelerate the release time of the pharmacologically active connection. The glycolide unit is, as is known, the most frequently used additional unit in polylactides.

The molar monomer ratios of the lactide/glycolide units in the purified polymers produced in accordance with the invention are preferably 100-25/0-75, especially 75-25/25-75, especially 60-40/40-60 and more especially 55-45/ 45-55, e.g. 55-50/45-50.

It is known that the polymerization reaction of monomers such as lactide and glycolide is preferably carried out in the presence of a compound with one or more hydroxyl groups, which acts as an initiator when building up a linear polymer chain. Known initiators are e.g. lactic acid and glycolic acid. Other hydroxyl group-containing compounds can also be used, e.g. alcohols. The initiators are actually used to control the chain length of the polylactides. A smaller amount of initiator hydroxyl compound leads to longer chains than larger amounts. Excellent regulators are polyols, e.g. those described in UK patent publication GB 2,145,422A where mannitol and especially glucose are the most preferred.

By using this type of initiator compounds, hard polylactide-co-glycolide materials with a relatively high molecular weight can be obtained, which are very suitable as

implants or microparticulate materials, and has 2 or 3, preferably more than 3, e.g. 4 relatively short polylactide-co-glycolide chains and can hydrolyze in the body tissue during a relatively short release period for the pharmacologically active compound within a few weeks to e.g. 2 months or more, preferably within 4-6, e.g. 5 weeks. Although the purified polylactides produced in accordance with the invention may have a linear structure, the preferred purified polylactides produced in accordance with the invention are those having the structure described in GB patent 2,145,422 A, in the form of esters of a polyol containing at least 3 hydroxyl groups , preferably those which are an ester of a sugar or a sugar alcohol, in particular an ester of glucose. They are star-shaped, with a center of e.g. the glucose residue and radiating linear polylactide chains.

After their production, the star-shaped polymers are to a greater degree than the linear polymers, contaminated by brown colored by-products, as the sugar or sugar alcohol used for their production is also partially decomposed by the catalyst. The star-shaped polymers have molar monomer ratios of lactide/glycolide units which are preferably those indicated above for the linear polymers.

The star-shaped polymers preferably have a weight average molecular weight Mw from 35,000 to 200,000, especially from 35,000 to 60,000, e.g. about 50,000 and have preferred a polydispersity Mw/Mn of from 1.7 to 3.0, especially from 2.0 to 2.5. The polylactide-co-glycolides with a linear structure, and which are not star-shaped polymers in a purified state in accordance with the invention, have preferred a weight average molecular weight Mw from 35,000 to 100,000 and have preferred a polydispersity Mw/Mn of from 1.7 to 2. The weight average molecular weight Mw is determined by gel permeation chromatography, using polystyrene as a standard, e.g. Dupont Ultrastyragel R 1000 or 500 angstrom, in the column and with e.g. tetrahydrofuran as solvent.

It is known from UK patent 1,467,970 and EP 0181,621 A2 to treat polymers, produced in the presence of a catalyst, with activated charcoal.

According to GB patent document 1,467,970, the polymer is a polyether, obtained from alkylene oxides, such as ethylene oxide, propylene oxide or epichlorohydrin with an active hydrogen-containing compound, e.g. glycerol, sorbitol or sucrose, in the presence of a basic catalyst. The polymer is cleaned with a mixture of activated charcoal and magnesium silicate to remove crystals of polyalkylene glycols, e.g. polyethylene glycols, formed as by-products and which give the polymers a cloudy appearance and unsatisfactory viscosity and chemical properties. In a preferred method, the polyethers are pre-treated using another cleaning method not further described to remove unreacted alkylene oxides and catalyst (page 2, lines 16-20). Thus, the basic catalyst clearly could not be removed by means of the charcoal-magnesium silicate mixture.

According to EP 0181 621 A2, a solution of a polyalkylene ether is mixed in a cyclic ether or in a polyhydric alcohol solvent or the polyalkylene ether itself, e.g. polyoxytetramethylene glycol, produced by polymerization of tetrahydrofuran under the influence of a heteropolyacid catalyst, e.g. 12-tungstophosphoric acid, with an organic hydrocarbon or halogenated hydrocarbon solvent. This solvent, which will contain the largest part of the heteropolyacid, is separated from the second phase and the remainder is brought into contact for further purification with a solid adsorbent, such as charcoal, aluminum oxide or oxides, hydroxides or carbonates of e.g. Mg or Ca or with basic ion exchange resins. According to Table 2 on page 23, the polymers contain 0.2 to 1.8 ppm of acidic metal contaminants after cleaning with activated charcoal.

This cleaning process is thus used to clean a polyether and to remove a very specific type of acidic catalyst and it could not be foreseen that activated charcoal could be used to remove cations, such as Sn. Nor could it be foreseen that such low levels of contamination could be achieved as indicated.

The amounts of activated charcoal used in the cleaning method according to the invention are generally from about 10 to 200%, e.g. 70 to 150% of the polymer weight. Any available charcoal can be used, e.g. as described in the Pharmacopoeia. A representative type of charcoal is "Norit" from Clydesdate Co. Ltd., Glasgow/Scotland. Powdered charcoal is typically used, e.g. finely ground charcoal of which at least 75% passes through a 75 micrometer sieve. Suitable charcoal as used in the example below is described in brochures, available from "Norit", e.g. "Summary of methods for testing "Norit" activated carbons on specifications" by J. Visser.

The new charcoal purification process is of particular interest for star-shaped polymers that have a dark brown color. The coloring effect may be partly due to the polyol, e.g. glucose, which is unstable to heat, especially in the presence of a catalyst and is more pronounced than a reference solution with color strength B i.

It is believed that the presence of small amounts of acidic groups in the activated charcoal is responsible for the surprisingly effective removal of the cations. If a solution of catalyst in an organic solvent is treated with charcoal, the catalyst compound is split and the tin is removed together with the charcoal, which is filtered off, while the anionic part of the catalyst is quantitatively found back in the remaining solution.

For this reason, the invention also provides a method for purifying the polylactide by bringing a solution of the impure polymer into contact with a matrix which has acidic groups on its surface and isolating the purified polylactide from the eluate.

If desired, a weakly acidic cationic ion exchanger with a carboxylic acid functionality can be used if this has a suitably small particle size. For example, an ion exchanger with hydrogen ion form, a density in the wet state of approximately 0.69 g/ml (apparent density) and 1.25 g/ml (real density), a shipping weight of 690 g/liter, an effective particle size of 0.33 to 0.50 mm, a moisture content of 43 to 53 percent, a pH tolerance range of 5 to 14, a maximum working temperature of 120°C, a total ion exchange capacity of 10 meq (dry) and 3.5 meq/ml (wet). An example is "Amberlite" IRC-50 methacrylic acid DVB (available from Fluka, Switzerland) which is ground to a particle size diameter down to less than 0.1 mm or 100 micrometers.

The matrix for the purification, e.g. charcoal, has preferred a suitable concentration of from 0.01 to 0.1 millimoles of acidic groups per gram matrix and is suitable in the form of particles that can be finely divided. Typical particle diameters are e.g. from 1 micrometer to 1 mm, e.g. from 10 to 100 micrometers. They therefore have a large surface area. For example, activated charcoal has a surface area of 1000 square meters for each ml of matrix substance.

The purification method in accordance with the invention preferably relates to a polylactide preparation using lactide and glycolide as monomers and metal cations such as Sn++ as a catalyst, after which this polymerization process gives a better yield and, if desired, a higher molecular weight than the production method using lactic acid and glycolic acid as starting compounds and a strongly acidic ion exchange resin as a catalyst as described in European Patent Document No. 0026599.

By starting from an impure polylactide-co-glycolide containing approximately 1800 ppm Sn++, the concentration of Sn++ can be lowered to approximately 200 ppm. A purification with charcoal can reduce the Sn++ content, as already mentioned, to less than 1.5, e.g. less than 1 ppm. The cleaning method according to the invention is preferably carried out with a solution of an impure polylactide in acetone, although other solvents are possible.

The method can be followed by a further purification process, preferably an ultra-filtration process, which reduces the content of low molecular weight compounds, e.g. of lactide and glycolide. A polylactide solution in acetone can also be used in this process.

After the second purification process, purified polylactides can be obtained with a monomer content of at most 1% by weight of the polymer, preferably at most 0.25% by weight of the polymer, e.g. maximum 0.2% lactide and 0.05% glycolide, a water content of maximum 1%, a content of organic solvent such as e.g. methylene chloride or acetone at most 1%, preferably at most 0.5%, e.g. no more than 0.3% and an ash content of no more than 0.1% by weight calculated on the polylactide. Their acid number is preferably no more than 10. The thus purified polylactides are preferably used parenterally, e.g. as a matrix for pharmacologically active compounds, especially those in implants or in the form of microparticles. These forms can be produced in conventional ways which are described in detail in the literature, e.g. in European Patent Publication No. 58481, UK Patent Publication GB 2,145,422, European Patent Publication No. 52510 and US Patent Publication No. 4,652,441 and 4,711,782, further French Patent Publication No. 2,491,351 and US Patent Publication No. 3,773,919.

The medication forms are suitable e.g. for incorporation of a hydrophilic drug such as a peptide, e.g. a cyclopeptide and especially a hormonally active peptide such as a somatostatin, especially ocreotide, or an acid addition salt or a derivative thereof, or a lipophilic drug, such as an ergot alkaloid, e.g. bromocriptine.

The pharmaceutical preparations are formed by working up the purified polylactide with the pharmacologically active compound to form an implant or microparticles.

Example 1:

a) Preparation of PLG-Glu

This is produced as described in the above-mentioned UK patent publication GB 2,145,422.

D,L-lactide and glycolide (60/40 wt%) containing trace amounts of lactide, glycolide, lactic acid and glycolic acid as impurities, are polymerized at 130°C in the presence of 0.2% (w/w) D (+) glucose and 0.6% (w/w) tin octanoate (product T9 from M & T Chemicals which is the tin(II) salt of 2-ethylhexanoic acid; lemon yellow liquid; viscosity (20°C) 1.2636, refractive index 1.4950; tin content 27-29%; 2-ethyl hexanoate content according to NaOH titration 69%).

The product is the polylactide-co-glycolide-glucose ester (PLG-Glu) with a lactide/glycolide ratio of 60/40 g (basis) or 55/45 mol (basis). Mw = 50,000. Lactide/glycolide content approx. 3% by weight. The color is dark brown and, according to the color index used, is more intensely colored than a reference resolution B\. The tin content is 1800 ppm.

b) Treatment with activated charcoal

130 g of PLG-Glu is dissolved in 1950 ml of acetone to give a clear dark brown solution. During

of 5 minutes, 130 g of activated charcoal is added. The mixture is stirred for 3 hours at room temperature and filtered. The filtered material is washed with 1.5 liters of acetone. The filtrate is slightly yellow with color index B9. It is evaporated under vacuum and the residue is dried at 70°C and finally under a vacuum of 1 mm Hg. Mw = 50,000. Lactide/glycolide content is approx. 3%. Heavy metal content: less than 10 ppm.

Resultant analysis: Fe 3 ppm; Zn 1 ppm; Cu 1 ppm; Ni, Pb and Sn each below 1 ppm.

The filtered charcoal contains practically all the tin and the filtrate practically all the 2-ethylhexanoic acid from the catalyst. (If the experiment is repeated with 50% activated charcoal calculated on the weight of polymer, then the product contains 290 ppm Sn.)

c) Ultrafiltration

The product from step b) (180 g) is dissolved in 1.8 liters of acetone to give a pale yellow solution.

The product is subjected to ultrafiltration using a laboratory pressure filtration apparatus using acetone (approx. 4 x 1800 ml) as a solvent under a pressure of 5 bar with a membrane from DDS type FS 81PP (exclusion threshold 6000) diameter 14 cm, with a permeation rate of approximately 110 to 165 ml/hour. The permeation solution contains lactide/glycolide and lactic acid and glycolic acid and is colored yellow. The residual solution in the pressure chamber (2215 ml solution) is evaporated after a period of 46 hours. The product is taken up again in acetone, filtered and dried at 70 to 80°C under a vacuum. The product (148 g) contains 0.2% by weight of lactide; 0.05 wt% glycolide. Acetone content is 0.3% by weight. No 2-ethylhexanoic acid can be detected by gas chromatography (ie its content is less than 0.1%). Mw = 50,000 according to

GPC.

Accordingly, the polymer is "colorless" according to the color test set forth in the European Pharmacopoeia 2nd Edition, Section V.6.2. The product is not more intensely colored than the reference resolution B9.

Example 2:

4 kg of poly(D,L-lactide-co-glycolide) purified in accordance with

the method in example 1 and with Mw 55,100 was dissolved in 53 kg of methylene chloride.

To the filtered solution was added 1 kg of bromocriptine mesylate. The resulting suspension was intensively mixed using an Ultra-Turrax mill and spray dried. The developed microparticles were sieved, washed with 0.01 molar methanesulfonic acid/sodium chloride solution and cleaned with isotonic saline. The microparticles were dried under vacuum at 40-45°C and sieved. The microparticles were filled into vials under nitrogen and sterilized using gamma irradiation (dose: 25 kGy).

The final product is an aseptically filled two-chamber syringe (TCS) consisting of a compartment containing the microparticles and a compartment containing a carrier for suspension of the microparticles.

Carrier mixture:

TCS is suitable for e.g. i.m. (intramuscular) administration once every 4 weeks.

The results of clinical investigations obtained with TCS in postpartum women, patients with hyperprolactinemia/microprolactinomas and patients with macroprolactinomas show a continuous release of active substance and a good systemic and local tolerability as well as good efficiency of single and multiple administrations of bromocriptine microparticles.

Example 3:

One g of poly(D,L-lactide-co-glycolide)glucose, Mw 46,000, (50/50) molar (prepared according to the process in GB 2,145,422, with polydispersity about 1.7, prepared from 0, 2% by weight of glucose and purified according to example 1) was dissolved in 10 ml of methylene chloride with magnetic stirring followed by the addition of 75 mg of octreotide dissolved in 0.133 ml of methanol. The mixture was intensively mixed e.g. using an Ultra-Turrax mill for one minute at 20,000 rpm. minute and this caused a suspension of very small crystals of ochreotide in the polymer solution. The suspension was spray-dried using a high-speed turbine (Niro Atomizer) and the small droplets were dried in a stream of hot air and this developed microparticles. The microparticles were collected using a "cyclone" and dried overnight at room temperature in a vacuum oven.

The microparticles were washed with 1/15 molar acetate buffer pH 4.0 for 5 minutes and again dried at room temperature in a vacuum oven. After 72 hours, the microparticles were sieved (0.125 mm mesh size) to give the final product. The microparticles were suspended in a carrier and added i.m. in 5 mg/kg dose of octreotide to white rabbits (chincilla cross) and s.c. in a 10 mg/kg dose to male rats.

Blood samples were taken periodically and indicated plasma levels of 0.3 to 10.0 ng/ml (5 mg dose) in rabbits and 0.5 to 7.0 ng/ml in rats for 42 days as measured by radioimmunoassay ( RIA).

Claims (19)

1. Process for obtaining a polylactide which is an ester of a polyol containing at least three hydroxyl groups and which is in a purified state, which meets the requirements with color strengths corresponding to reference information B2- B9 in the brown color test according to European Pharmacopoeia, 2nd ed. (1980) part I, section V, 6.2, and contains one or more metals in cationic form, where the metal ion or ions has a concentration of no more than 10 ppm, has an average molecular weight Mw of at least 35,000, characterized in that it includes bringing a solution of the impure polylactide into contact with a matrix which has carboxylic acid groups on the surface and isolating the purified polylactide from the eluate. 2. Method according to claim 1, characterized in that it includes bringing the solution of the impure polylactide into contact with activated charcoal. 3. Method according to claim 1, characterized in that the isolation includes a further cleaning step, which is ultrafiltration. 4. Method according to any one of claims 1-3, characterized in that the impure polylactide is dissolved in acetone. 5. Method according to any one of claims 1-4, characterized in that it is used to obtain a polylactide which has the color strength according to reference solutions B4-B9. 6. Method according to any one of claims 1-5, characterized in that it is used to obtain a polylactide which has the color strength according to reference solution B9. 7. Method according to any one of claims 1-6, characterized in that it is used to obtain a polylactide, in which the metal ion is Sn++. 8. Method according to any one of claims 1-7, characterized in that it is used to obtain a polylactide which has a Sn++ concentration of no more than 1.5 ppm. 9. Method according to any one of claims 1-8, characterized in that it is used to obtain a polylactide in which the salt anion is ethyl hexanoate. 10. Method according to claim 9, characterized in that it is used to obtain a polylactide which has an ethyl hexanoate concentration of at most 0.5% by weight of the polylactide. 11. Method according to any one of claims 1-10, characterized in that it is used to obtain a polylactide which has 12. Method according to any one of claims 1-11, characterized in that it is used to obtain a polylactide which additionally contains glycolide units. 13. Process according to claim 12, characterized in that it is used to obtain a polylactide which has a lactide/glycolide molar ratio of 100-25/0-75. 14. Method according to claim 13, characterized in that it is used to obtain a polylactide which has a molar ratio of 75-25/25-75. 15. Method according to claim 14, characterized in that it is used to obtain a polylactide which has a molar ratio of 60-40/40-60. 16. Method according to claim 1, characterized in that it is used to obtain a polylactide which is an ester of a sugar or of a sugar alcohol. 17. Method according to claim 16, characterized in that it is used to obtain a polylactide which is a glucose ester. 18. Method according to claim 16 or 17, characterized in that it is used to obtain a polylactide which has an average molecular weight Mw of from 35,000 to 200,000. 19. Method according to claim 16 or 17, characterized in that it is used to obtain a polylactide which has a polydispersity Mw/Mw of from 1.7 to 3.0.