Bioabsorbable polydepsipeptides, their preparation and use

Abstract: Novel, semi-crystalline depsipeptide polymers and copolymers particularly suitable for use in fabricating bioabsorbable medical implant devices such as sutures or clips and obtained through ring-opening polymerization of certain 3-substituted-2,5-morpholinediones prepared by reacting a naturally occurring alpha-amino acid with an alpha-halo acid chloride in inert solvent in the presence of an acid acceptor.

Description: This invention concerns certain 3-substituted-2,5-morpholinediones and their polymerization or copolymerization to bioabsorbable depsipeptide polymers which can be fabricated into useful medical implant devices such as sutures. For the past decade or so, synthetic resorbable sutures have been replacing natural catgut in many applications as biodegradable wound closures since the synthetics, unlike the natural polymers, are consistent and predictable in their strength and sorption properties. At present, the synthetic suture market is dominated by high molecular weight polyglycolic acid (PGA) and a modified PGA copolymer containing about 10 mole percent lactic acid. Despite their superiority to catgut, however, these resins lack certain desirable properties. For example, they tend to be resorbed too readily, they are too rigid as monofilaments, and they cannot be sterilized by irradiation. Certain modifications of these resins have been introduced in an attempt to overcome such limitations. For example, U.S. Patent 4,052,988 discloses a poly(p-dioxanone) (PDS) having a structure similar to PGA but purportedly of a lower resorption rate and more readily handled as monofilaments. Such property improvements are also claimed by the modified PGA polymers of U.S. Patents 4,209,607 and 4,343,931, which incorporate amide bonding into the polymer chain. Attempts at preparing resorbable polydepsipeptides include the copolymerization of a racemic N-carboxyanhydride with a racemic anhydrosulfite as disclosed in U.S. Patent 3,773,737. Such a copolymerization, however, produces a heterogeneous product having random and racemic ester/amide groups and is therefore incapable of yielding crystalline polymers. The copolymerization of p-dioxanone with up to 15 mole percent of a 2,5-morpholinedione disclosed in U.S. Patent 4,441,496 was purported to improve the resorption rate of PDS. Recent publications by Helder et al, Makromol. Chem., Rapid Commun., 6 , 9-14, 1985, and Yonezawa et al, ibid, 6 , 607-611, 1985, have demonstrated the preparation of polydepsipeptides from the ring-opening polymerization of 6-substituted-2,5-morpholinediones. Despite such developments, the need still exists for a true alternating bioabsorbable polymer of suitable strength and resorption rate which can be readily melt processed. The primary objective of the present invention, therefore, is to satisfy this need. Preparation of 3-benzyl-2,5-morpholinediones, employed in this invention, by silver oxide catalyzed condensation of N-(bromoacetyl)-L-phenylalanine is reported by Rumsh et al, FEBS Letters, 9 , 64, 1970. It has now been found that certain optically active 3-substituted-2,5-morpholinediones can be readily prepared by a novel and expeditious procedure, then converted by ring-opening polymerization or copolymerization to semi-crystalline polydepsipeptide compositions of well defined melt temperatures which are particularly suitable for fabrication into bioabsorbable medical devices. The present invention provides an optically active semicrystalline, hydrolysable polydepsipeptide having a molecular weight of from about 5,000 to 200,000 and the formula:- wherein R is as defined below, x and y are relative amounts of (D) and (L) optical isomers, respectively, and x/(x+y) is either less than about 0.45 or greater than about 0.55, as well as a depsipeptide copolymer having a molecular weight of from about 5,000 to 200,000 and the formula:- wherein R, x and y are as defined above R₁ is hydrogen or methyl, z/(x+y+z) is from about 0.01 to 0.5, and n is 1 or 5, with the proviso that when n is 5, R₁ is hydrogen. In the above definitions R is C₁-C₁₂ alkyl, C₁-C₆ alkoxy (C₁-C₆ alkyl), aryl, aryl (C₁-C₁₂ alkyl), C₁-C₁₂ alkylaryl, C₁-C₆ alkylaryl (C₁-C₆ alkyl), C₁-C₆ alkylthio (C₁-C₆ alkyl), C₁-C₆ alkylsulfinyl (C₁-C₆ alkyl), C₁-C₆ alkylsulfonyl (C₁-C₆ alkyl), cyano (C₁-C₁₂ alkyl) or aminocarbonyl (C₁-C₁₂ alkyl), with each aryl group having up to 10 carbon atoms in the nuclear ring. R is alkyl of from 1 to 4 carbons, benzyl or 2-methylthioethyl, especially wherein the polymer or copolymer has a molecular weight of from about 10,000 to 50,000 with R being CH₃ and x/(x+y) being from about 0.2 to 0.4 and from about 0.6 to 0.8. The present invention further provides a process for the preparation of the above polydepsipeptide which comprises polymerizing an optically active 3-substituted-2,5-morpholinedione having the formula I in the presence of a catalyst, and a process for the preparation of the above depsipeptide copolymer, which comprises polymerizing 1.0 mole of such a 3-substituted-2,5-morpholinedione with from about 0.02 to 1.0 mole of a cyclic lactone having the formula VII or VIII:- in the presence of a catalyst. The polymerization or copolymerization is preferably conducted neat at a temperature of from about 100 to 250°C. The present invention still further provides a bioabsorbable surgical device fabricated from the above polydepsipeptide or depsipeptide copolymer, preferably in the form of a suture or clip. The above and other objectives, features and advantages of the present invention will be apparent from the following detailed description in conjunction with the accompanying figure, in which resorption rates typical of the present polydepsipeptides are compared with those of prior polymer PGA. The semi-crystalline polymers of the present invention are prepared by the catalytic ring-opening polymerization or copolymerization of certain optically active 3-substituted-2,5-morpholinediones of the formula:- wherein R is C₁-C₁₂ alkyl, C₁-C₆ alkoxy(C₁-C₆ alkyl), aryl, aryl(C₁-C₁₂ alkyl), C₁-C₁₂ alkylaryl, C₁-C₆ alkylaryl(C₁-C₆ alkyl), C₁-C₆ alkylthio(C₁-C₆ alkyl), C₁-C₆ alkylsulfinyl(C₁-C₆ alkyl), C₁-C₆ alkylsulfonyl(C₁-C₆ alkyl), cyano(C₁-C₁₂ alkyl) or aminocarbonyl(C₁-C₁₂ alkyl), with each aryl group having up to 10 carbons in the nuclear ring, with the comonomer for the copolymerization being a cyclic lactone of the formula VII or VIII:- wherein R₁ is hydrogen or methyl. The 3-substituted-2,5-morpholinedione monomers of the present invention, either optically active or inactive, are prepared by a process which comprises reacting an amino acid having the formula:- with an alpha-halo acid halide having the formula:- wherein X is chlorine or bromine to form an intermediate having the formula:- cyclizing the intermediate IV in an inert solvent in the presence of an acid acceptor. When an optically active amino acid is used to prepare the 3-substituted-2,5-morpholinedione monomer, the optical activity is retained in the monomer, and also in the derived polymer. Thus, (L)-alanine gives (L)-3-methyl-2,5-morpholinedione which, upon polymerization, yields poly(glycolic acid-co-(L)-alanine). This optically active polymer is semi-crystalline with a melting transition point of 232°C and a specific rotation of -67.8° (C=1, DMSO). The polymer's semi-crystallinity results in fabricated devices such as sutures and clips with physical/mechanical and resorption properties superior to those realized with racemic polymer. While any amino acid of formula II may be used for preparing the intermediate IV, the amino acids shown in Table I are preferred. Table I indicates the common name of the amino acid, the structural formula, and the radical or substituent group R, the group R being that portion of the amino acid appearing as the appendage in the subsequent polymerization or copolymerization. Of these amino acids, (L)-leucine and (L)-alanine are especially preferred. The amino acid and alpha-halo acid chloride are preferably reacted in a basic aqueous medium at a temperature of from about -5° to +25°C, the resulting intermediate IV being simultaneously extracted from the reaction medium into an inert, water-immiscible solvent such as ethyl ether. Intermediate IV is then isolated by evaporation of the solvent extract and used directly in the subsequent cyclization, although its purification before use, such as by recrystallization from ethyl acetate, may be employed. The intermediate IV is then cyclized in an inert solvent in the presence of an acid acceptor. While any solvent nonreactive to intermediate IV or the acid acceptor may be used, dimethylformamide is preferred. The cyclization will normally be conducted at a temperature of from about 25° to 200°C, preferably about 75° to 125°C, and will require from about 0.5 to 20 hours, usually about 6 to 12 hours. An optically active form of the prepared 3-substituted-2,5-morpholinedione is then either polymerized to a polymer of the formula:- wherein R is as defined above, x and y are relative amounts of (D) and (L) optical isomers and x/(x+y) is either less than about 0.45 or greater than about 0.55, or copolymerized with from about 0.02 to 1.0 mole, preferably from about 0.1 to 0.4 mole, of a cyclic lactone of the formula VII or VIII:- to yield a copolymer of the formula:- wherein R, x and y are as defined above, R₁ is hydrogen or methyl, z/(x+y+z) is from about 0.01 to 0.5, preferably 0.05 to 0.20, and n is 1 or 5, with the proviso that when n is 5, R₁ is hydrogen. By optically active 3-substituted-2,5-morpholinedione monomer is meant such a monomer with an optical purity of greater than 50 percent. This optical purity is expressed as the percent of an optically pure isomer in a mixture of the isomer with its enantiomorph. Thus, an optically active (L)-3-methyl-2,5-morpholinedione of 80 percent optical purity would consist of 80 percent optically pure (L)-3-methyl-2,5-morpholinedione and 20 percent optically pure (D)-3-methyl-2,5-morpholinedione. Such a composition can be realized, for example, by blending 60 parts (L)-3-methyl-2,5-morpholinedione and 40 parts (D,L)-3-methyl-2,5-morpholinedione, prepared as indicated hereinbefore from (L)-alanine and (D,L)-alanine, respectively. Optically active monomers with an optical purity of from about 55 to 100 percent (L) or (D), especially 60 to 80 percent (L), are preferred. The polymerization, or copolymerization, is conducted, either neat (without solvent) or in an inert solvent, in the presence of an organometallic catalyst. Preferably, the polymerization is conducted neat. In this case, the temperature of polymerization will depend on the melting point of the intermediate IV, since the reaction mixture must be in a molten state. The polymerization is therefore normally carried out above about 120°C, and preferably from about 150° to 250°C, higher temperatures resulting in excessive decomposition of the product polymer. Under such temperature conditions, the polymerization will normally require from about 1 to 10 hours. Solvent polymerization employs similar conditions, suitable inert solvents including such as dimethylformamide, dimethyl sulfoxide and dodecanol. Any lactone polymerization catalyst may be used for the polymerization. Such catalysts include, for example, zirconium acetoacetonate, stannous chloride and especially stannous octoate. The resulting polymers and copolymers preferably are semi-crystalline, showing a defined melt temperature, and are bioabsorble to safe by-products, the resorption rate of the polymer being dependent on the nature of the R radical. Such properties make these polymers highly suitable for use in fabricating bioabsorbable medical devices such as sutures and clips. The following Examples illustrate the present invention. Example 1 (L)-3-Methyl-2,5-morpholinedione Into a 5-liter, 4-neck, round-bottom flask equipped with a mechanical stirrer, pH probe, thermometer, two 1-liter addition funnels and a nitrogen bubbler were charged 157 g (3.93 mole) of sodium hydroxide and 1800 ml of water followed by 350 g (3.93 mole) of L-alanine. After all the solids dissolved, 600 ml of diethyl ether was added and solutions of chloroacetyl chloride (500 g, 4.43 mole) in 600 ml of diethyl ether and dilute sodium hydroxide (225 g in 500 ml water) were added concomitantly while maintaining the pH at about 11 and the temperature at about 0°C by external cooling. The resulting mixture was allowed to warm to room temperature, and the ether layer separated. The aqueous layer was acidified with concentrated hydrochloric acid to pH 1 and extracted with four 1-liter portions of ethyl acetate. The combined organic extract was dried over anhydrous magnesium sulfate and rotoevaporated to yield 550 g (85%) of 2-chloroacetyl-(L)-alanine as a white crystalline solid, melting point (mp) 93-96°C. This material can be purified by recrystallization from ethyl acetate, but this is not necessary for the next step. A total of 404 g (2.44 mole) of the 2-chloroacetyl-(L)-alanine was dissolved in 8 liters of dimethylformamide in a 12-liter round-bottom flask. With stirring, 250 g (2.47 mole) of triethylamine was added and the resulting solution was heated to and maintained at 100°C for 6 hours. Upon cooling, some solid triethylamine hydrochloride salt crystallized out. This side product was filtered off, and the filtrate was concentrated on a rotoevaporator to give a mixture of (L)-3-methyl-2,5-morpholinedione and triethylamine hydrochloride. The desired product was isolated by adding 530 ml of chloroform to the mixture, filtering the resulting slurry, solvent stripping the filtrate, and finally recrystallizing the residue from isopropanol. This yielded 157 g (50%) of (L)-3-methyl-2,5-morpholinedione having the following characteristics: mp = 153.5-154.5°C [α] D = -102° (c=2, acetone) elemental analysis: calc. for C₅H₇NO₃: C,46.49; H,5.47; N,10.85% found : C,46.58, H,5.23; N,10.75% Purification of the crude (L)-3-methyl-2,5-morpholinedione may also be accomplished by sublimation. Example 2 (D,L)-3-Methyl-2,5-morpholinedione This monomer was prepared as described in Example 1, but with (D,L)-alanine rather than (L)-alanine as starting material. The product had a melting point of 137.5-138.5°C. Example 3 (L)-3-Isobutyl-2,5-morpholinedione This monomer was prepared as described in Example 1 but with 515 g (3.93 mole) (L)-leucine as starting material. The product had the following characteristics: mp = 127-128°C [α] D = -6.6° (c=2, acetone) elemental analysis: calc. for C₈H₁₃NO₃: C,56.11; H,7.66; N,8.18% found: C,56.08; H,7.62; N,8.14% Example 4 (L)-3-Phenylmethyl-2,5-morpholinedione This monomer was prepared as described in Example 1, but with 649 g (3.93 mole) (L)-phenylalanine as starting material. The product had the following characteristics: mp = 145-146°C [α] D = -11.3° (c=2, acetone) elemental analysis: calc. for C₁₁H₁₁NO₃: C,64.38; H,5.40; N,6.83% found: C,64.14; H,5.46; N,6.73% Example 5 Poly(glycolic acid-co-(L)-alanine) Into a flame-dried polymerization tube was charged 10.0 g of the (L)-3-methyl-2,5-morpholinedione prepared in Example 1 and 0.01 g of stannous octoate as catalyst. The tube was evacuated, then flushed with dry nitrogen gas several times and finally sealed under vacuum. The polymerization tube was placed in a 180°C oil bath for 2 hours. The resulting orange-colored polymer plug was dissolved in 50 ml of hexafluoroisopropanol (HFIP). The HFIP solution was added to 3 liters of acetone with agitation, and the resulting solids were filtered and air dried at room temperature to yield 5.47 g (55%) of poly(glycolic acid-co-(L)-alanine) as a powder. The semi-crystalline polymer had the following characteristics: T m (mp) = 232°C (DSC measurement) inh = 0.61 (50 mg/dL dichloroacetic acid) [α] D = -67.8° (c=2, DMSO) Racemic 3-methyl-2,5-morpholinedione prepared in Example 2 was polymerized to poly(glycolic acid-co-(D,L)-alanine) in similar fashion using zirconium acetoacetonate as catalyst. The polymer, isolated in 60% yield, was amorphous and showed no T m by DSC analysis. Example 6 Relationship of Monomer Enantiomeric Composition to Polymer Crystallinity and Melt Temperature A mixture consisting of various ratios of (L)-3-methyl-2,5-morpholinedione (A) prepared in Example 1 and (DL)-3-methyl-2,5-morpholinedione (B) prepared in Example 2 were polymerized by the method of Example 5 to give polymers of different melting temperatures (Tm) as shown in Table II. A:B Monomer [α] D (1) Polymer T m , °C (2) %(L) %(D) 100:0 100 0 -102 232 60:40 80 20 -61.2 203 50:50 75 25 -51.0 190 40:60 70 30 -40.8 168 30:70 65 35 -30.6 152 20:80 60 40 -20.0 140 0:100 50 50 0 amorphous Example 7 Poly(glycolic acid-co-(L)-leucine) (L)-3-Isobutyl-2,5-morpholinedione prepared in Example 3 was polymerized by the method of Example 5 to give semi-crystalline poly(glycolic acid-co-(L)-leucine) in 58% yield and with the following characteristics: inh = 0.77 (50 mg/dL dichloroacetic acid) [α] D = -39.8° (c=2, DMSO) Example 8 Poly(glycolic acid-co-(L)-phenylalanine) (L)-3-Phenylmethyl-2,5-morpholinedione prepared in Example 4 was polymerized by the method of Example 5 to give a semi-crystalline poly(glycolic acid-co-(L)-phenylalanine) in 65% yield with an [α] D = -2.05° (c=2, DMSO). Example 9 Copolymerization of (L)-3-Isobutyl-3,5-morpholinedione and Glycolide A mixture of 9 g of (L)-3-isobutyl-3,5-morpholinedione and 1 g of glycolide was polymerized by the method of Example 5. The resulting semi-crystalline copolymer was isolated in 72% yield and had the following characteristics: T m = 114°C (DSC) inh = 0.69 (50mg/dL, dichloroacetic acid) [α] D = -27.1° (c=2, DMSO) Example 10 Melt-spinning of Poly(glycolic acid-co-(L)-leucine) The polymer prepared in Example 7 was dried in a vacuum oven overnight (55°C, 0.05 mm Hg) and then placed in a small scale laboratory melt-spinner at 150°C to produce strands of fiber with an average diameter about 0.07 mm. Straight pull tensile strength of as-spun fiber was 834 bar (12,000 psi), which increased to 1807 bar (26,000 psi) after drawing to 2.5 times its original length at 85°C. Example 11 Melt-spinning of Poly(glycolic acid-co-alanine) A polymer (T m =155°C) prepared as in Example 6 from a 30:70 mixture of (L)- and (DL)-3-methyl-2,5-morpholinedione was dried and spun into a fiber at 160°C. The fiber had a tensile strength of about 834-1390 bar (about 12,000 to 20,000 psi). Example 12 Measurement of Relative Resorption Rates Relative resorption rates of selected polymers were measured by monitoring the decrease in molecular weight with time in 1N aqueous saline solution at 37°C. Samples of the polymers were fabricated by melt pressing a given polymer at the appropriate temperature to produce a clear 20 mil (0.8 mm) film. The film (5 cm squares) was submerged in the saline solution and the molecular weight of the saline incubated sample was measured by size exclusion chromatography as a function of time. The comparative results, shown in the accompanying figure, indicate that the polydepsipeptides hydrolyze with time, and that the resorption rates can be controlled and varied by choice of polymer type. Thus, the poly(glycolic acid-co-(L)-alanine) of Example 5 shows a rate slightly slower than polyglycolic acid (PGA), a common resorbable suture material. In contrast, the poly(glycolic acid-co-(L)-leucine) of Example 7 shows a rate considerably slower than PGA. Example 13 The procedure of Example 1 is repeated except that D-alanine is substituted for L-alanine in the preparation. The resulting (D)-3-methyl-2,5-morpholinedione is then polymerized by the procedure of Example 5 to yield semi-crystalline poly(glycolic acid-co-(D)-alanine).

Claims: A optically active, semicrystalline, hydrolysable polydepsipeptide having a molecular weight of from about 5,000 to 200,000 and the formula:- wherein R is C₁-C₁₂ alkyl, C₁-C₆ alkoxy(C₁-C₆ alkyl), aryl, aryl(C₁-C₁₂ alkyl), C₁-C₁₂ alkylaryl, C₁-C₆ alkylaryl(C₁-C₆ alkyl), C₁-C₆ alkylthio(C₁-C₆ alkyl), C₁-C₆ alkylsulfinyl(C₁-C₆ alkyl)), C₁-C₆ alkylsulfonyl(C₁-C₆ alkyl), cyano(C₁-C₁₂ alkyl) or aminocarbonyl(C₁-C₁₂ alkyl), with each aryl group having up to 10 carbons in the nuclear ring, x and y are relative amounts of (D) and (L) optical isomers, respectively, and x/(x+y) is either less than 0.45 or greater than 0.55. A polydepsipeptide according to claim 1 wherein R is alkyl of from 1 to 4 carbons, benzyl or 2-methylthioethyl. A polydepsipeptide according to claim 2, wherein the molecular weight is from about 10,000 to 50,000, R is methyl and x/(x+y) is from 0.2 to 0.4 or from 0.6 to 0.8. An optically active hydrolysable, semicrystalline depsipeptide copolymer having a molecular weight of from about 5,000 to 200,000 and the formula:- wherein R is C₁-C₁₂ alkyl, C₁-C₆ alkoxy(C₁-C₆ alkyl), aryl, aryl(C₁-C₁₂ alkyl), C₁-C₁₂ alkylaryl, C₁-C₆ alkylaryl(C₁-C₆ alkyl), C₁-C₆ alkylthio(C₁-C₆ alkyl), C₁-C₆ alkylsulfinyl(C₁-C₆ alkyl), C₁-C₆ alkylsulfonyl(C₁-C₆ alkyl), cyano(C₁-C₁₂ alkyl) or aminocarbonyl(C₁-C₁₂ alkyl), with each aryl group having up to 10 carbons in the nuclear ring, R¹ is hydrogen or methyl, x and y are relative amounts of (D) and (L) optical isomers, respectively, x/(x+y) is either less than 0.45 or greater than 0.55, z/(x+y+z) is from 0.01 to 0.5, and n is 1 or 5, with the proviso that when n is 5, R₁ is hydrogen. A depsipeptide copolymer according to claim 4, wherein R is alkyl of from 1 to 4 carbons, benzyl or 2-methylthioethyl. A process for the preparation of a polydepsipeptide according to claim 1, which comprises polymerizing an optically active 3-substituted-2,-5-morpholinedione having the formula:- in the presence of a catalyst. A process for the preparation of a depsipeptide copolymer according to claim 4, which comprises polymerizing 1.0 mole of an optically active 3-substituted-2,5-morpholinedione having the formula:- with from about 0.02 to 1.0 mole of a cyclic lactone having the formula VII or VIII:- in the presence of a catalyst. A bioabsorbable surgical device fabricated from a polydepsipeptide according to claim 2 or claim 5.

Inventors: FUNG FU-NING, GLOWAKY RAYMOND CHARLES, FUNG, FU-NING, GLOWAKY, RAYMOND CHARLES
Applicants: PFIZER, PFIZER INC.
Application year: 1988
Main classification: C07D265
IPC codes: C08G61, C08G63, A61L31, C07D265, A61L27, A61L17, C08G69, C07K11