curable biodegradable poly(ester

In: Chemistry of Advance Compounds and Materials Editors: N. Lekishvili, G. E. Zaikov

ISBN: 978- 1-60456-67 1-0 O 2008 Nova Science Publishers, Inc.

'I'HERMALLY-AND PHOTO-CHEMICALLY CURABLE

BIODEGRADABLE POLY(ESTER AMIDE)S WITH DOUBLEBONDMOIETIES IN THE LATERAL CHAINS Nino Zavradash vili, Giuli Jokhadze, Tinatin Kviria and Ramaz ~atsarava* Center for Medical Polymers & Biomaterials, Georgian Technical University, 69 Kostava St., 0175 Tbilisi, Georgia

The present work deals with the synthesis of thermally- and photo-chemically cdrable biodegradable poly(ester amide). The copoly(ester amide), on the basis of 1,6hexandiol, sebacic acid, and two amino acids - L-leucine and L-Lysine benzyl ester, was selected for a chain of polymeranalogous transformations. At the first stage of these transformations, catalytic (Pd) hydrogenolysis of the polymeric benzyl ester was carried out, and polyacid, which is the poly(ester amide) with free carboxyl groups, was obtained. Then, the polyacid was transformed into the corresponding P-hydroxyethyl amide (polyol) by the interaction with mono-ethanol amine' in the presence of a condensing agent (carbonyldiimidazole). After acylating the polyol with methacrylic and cinnamic acids' chlorides, the intended thermo- and photo-reactive poly(ester amide) with unsaturated double bonds in the lateral chains were synthesized. The polymers obtained are of interest for both preparing surgical materials with enhanced mechanical characteristics and as macromolecular cross-agents of other unsaturated biodegradable polymers.

' To whom the correspondence should be addressed: [email protected]

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Nino Zavradashvili, Giuli Jokhadze, Tinatin Kviria et al.

AA-BB type bioanalogous heterochain polymers, on the basis of "physiological" monomers like naturally occurring a-amino acids, are promising as biodegradable materials for biomedical applications as both resorbable surgical devices and sustained/controlled drug eluting systems. We have shown [I, 21 that these polymers undergo biodegradation catalyzed by enzymes- hydrolases like trypsin, a-chymotrypsin, lipase and papain. It is especi " important that the ultimate products of PEAs' biodegradation are physiological and nontc compounds - a-amino acids (including essential ones), fatty dicarbxylic acids and diols. Among bifunctional derivatives of a-amino acids that are suitable monomers for constructing biodegradable polymeric backbones, the most important are bis-(L-a-amino acid) a- -alkylene-diesters. These compounds are easily synthesized by the direct condensation of a-amino acids (2 mole) with diols (1 mole) in boiling benzene or toluene in the presence of p-toluenesulfonic acid (2 mole). This latter serves as a catalyst for the condensation (esterification) reaction and protects amino groups via salt formation. Most of the obtained di-p-toluenesulfonic acids salts of a-diamino-diesters are easily purified by recrystallization from water and are stable upon storage. Such monomers contain enzymatically hydrolysable ester bonds that impart the biodegradation property to the polymeric backbones composed of them. Solution poly- condensation of these monomers with "soft" bis-electrophiles - various activated diesters in the presence of p-toluenesulfonic acid acceptors (mostly tertiary amines) and different classes of heterochain polymers: poly(ester amide)s [3, 51, poly(ester urethane)^ and poly(ester urea)s [6] - were synthesized. Among these polymers, the poly(ester amide)s (PEAs) are considered to be the best candidates for numerous biomedical applications. This because of the cheapest of starting biselectrophilic monomers (activated diesters of dicarbonic acids) and the versatility of physical, chemical, mechanical and biochemical properties of PEAs. The PEAs showed a high biocompatibility [7 - 91 exceeding, in this respect, well known biocompatible polymers including lactide-glycolide copolymers [9]. The amino acid derived PEAs were used as matrices for preparing biocomposites containing bacteriophages and other medications that were successfully used to treat refractory diseases like trophic ulcers [lo], radiation burns [I I], as well as osteomyelitis and severely infected wounds [ I 21. However, regardless of these accomplishments, it should be emphasized that most of the PEAs synthesized and studied up to now do not contain any pendant functional groups, with the exception of terminal groups with presumably one amino and one carboxylic (or pnitrophenyl ester) end-groups. These functional end-groups are very low in concentrations that are particularly in higher MW PEAs; the formation of cyclic macromolecules reported by Kricheldorf et al. [I31 results in further reducing their concentration. Hence, terminal functional groups are hardly useful for subsequent transformations. This substantially restricts the scope of applications of these promising biodegradable biomaterials. Therefore, an important task is the incorporation of a variable quantity of lateral functional groups in macro-chains of PEAs. Therefore, the synthesis and transformations of amino acid based biodegradable functional PEAS are the subjects of our systematic study. The present work deals with the synthesis of thermally and photo-chemically sensitive PEAs by means of introducing lateral unsaturated bonds in their macromolecules. Thermal or

Thermally- and Photo-chemically Curable Biodegradable Poly(ester amide)~.. .

175

photo-chemical treatment (cross-linking) will allow us to regulate ~neclianicaland pliysicalchemical characteristics of tlie polymers as well as the rates of their biodegradation. Accordingly, the cross-linked polymers are promising as both surgical construction materials and controlled drug releasing devices.

RESULTS AND DISCUSSION For developing the major principles of tlie synthesis, we selected the copoly(ester atnide) composed of the amino acid L-lysine and containing lateral COOH groups suitable for subsequent transformations. For a starting polymer, we selected the copoly(ester amide) (coPEA) composed of amino acids L-leucine and L-lysine (as benzyl ester), 1.6-hexanediol and sebacic acid of the following structure:

Our PEA contained 75 mole % of a leucine based monomer and 25 mole % of a lysine based monomer, studied also by Chu and one of us [14]. The obtained polymer revealed a high biocompatibility [8, 91, elasticity and good adhesion to stainless steel. Due to these unique properties, it was successfully used as a cardio-stent coating for suppressing restenosis [15]. Currently, the coPEA is being produced in a pilot-scale by the bio-tech company MediVas, San Diego, CA. As one can see from the coPEA structure, L-lysine is incorporated into the polymeric backbones as a diamine in its benzyl ester form. As a result, the polymer contains lateral benzyl ester groups that could be used for subsequent polymer-analogous transformations. At the first stage of the chain of these transformations, we removed the benzyl groups by selective hydrogenolysis - by passing gaseous Hz through a 10 % alcohol solution of the coPEA I and by using Pd-black as a catalyst:

Because the starting coPEA I contains aromatic rings of benzyl groups (that absorb in the UV region of spectrum) and these groups are removed after hydrogenolysis, monitoring the process is possible by UV spectrometry (Figure 1 ,A).

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Figure 1. UV spectra in DMF at c=10" mole/L of: A) coPEAs I (i.e. before hydrogenolysis, curve 1) and 11 (i.e. after hydrogenolysis, curve 2); B) Polyol (Ill).

The degree of the hydrogenolysis of benzyl form I, estimated on the base of these data, is about 80 %. That is, the obtained polyacid - coPEA II contains ca. 80 mole % of free lateral COOH groups. At the next stage, the obtained polyacid II was transformed into the corresponding hydroxyethyl amide III by interactions with mono-ethanol amine in dimethylformamide (DMF) solution using carbonyl diimidazole (ImrCO) as a condensing agent, as shown in scheme below:

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As a result of this reaction, the polymer contained hydroxyl groups in lateral chains polymeric alcohol III, i.e. polyol that in contrast to polyacid II showed excellent solubility in water. Since the polyol III, like the starting acid II, contains no aromatic rings in macrochains, it also does not absorb in the UV region of the spectrum (Figure 1,B; narrow absorption at 269 nm comes from the residual benzyl groups in II), and UV spectrometry is unsuitable for a quantitative study of the transformation of II into III. For this, we used hydroxyl number determination (phthalic anhydride method). The found hydroxyl number

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was 0.66, while the calculated (taking into consideration the 80 % transformation of I to II) was 0.67. That corresponds to 98.5% transformation of II into III. At the final stage of the present study, we carried out the acylation of polyol III using chlorides of unsaturated mono acids - methacryloyl and cinnamoyl cldorides, as is sl~ownin schen~ebelow:

The acylation of polyol III was carried out at room temperature in an amide type solvent dimethylacetamide (DMA) (the ratio OHICOCI was 111) without using tertiary amine as a catalyst. Preliminary model experiments (acylation of aliphatic diols - 1,4-buthan diol, 1,6hexane diol and 1,8-octane diol) showed that, under the said conditions, alcohol hydroxyls are acylated virtually in quantitative yields. The role of a reaction catalyst presumably is played by the solvent (DMA), which is known as an efficient catalyst of acyl-transfer reactions [16]. Polyol acylation and incorporation of unsaturated groups into polymeric chains was determined qualitatively using UV-spectra (Figure 2). In contrast to the starting polyol III (Figure 1,B), the spectra of the acylated products IVa and IVb showed an absorption maxima of unsaturated double bonds in the region of 260-290 nm. As was expected, the cinnamic acid derivative IVb is characterized by a more complex absorption band (Figure 2, B) that is connected with the presence of benzene rings along with the double bonds in the cinnamic acid residue. Quantitatively, the degree of polyol acylation was estimated by the determination of bromine number (using Knopp's method). An average bromine number found for the methacrylic acid derivative IVa was 4.53, while the calculated number was 6.19 (taking into consideration the degrees of transformations at the previous two stages). That corresponds to 73.2% acylation of OH groups. For the cinnamic acid derivative IVb the found bromine number was 5.29 and the calculated was 6.01 (taking into consideration the degrees of transformations at the previous two stages). That corresponds to 88.0% acylation of OH groups. The unsaturated coPEAs (IVa) and (IVb) had undergone both thermal (t = 120°C) and photochemical (photo-initiator - 2, 2-dimethoxy-2-phenylacetophenone "IRGACURE 65 1 ") curing (cross-linking), which was proved by the loss of solubility in DMF (swelled only). The intensity of cross-linkage (that was regulated by varying the heating time) influences the enzyme (a-chymotrypsin) biodegradation rate - the higher the cross-linkage the lower the biodegradation rate (assessed by the quantity of hydrolyzed ester bonds in mollmin, Figure 3). This was expected since increasing the intensity of cross-linkage (i.e. the density of the polymeric network) decreases the mobility of macromolecules, which hinders the interaction of the polymeric substrate with the enzyme's active sites.

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Figure 2. UV spectra in DMF at c=10" mole/L o f the polyol I11 acylated with A) methacryloyl cloride (coPEA IVa) and B) cinnamoyl chloride (coPEA IVb).

Figure 3. The influence of intensity of cross -linkage on the a-chymotrypsin catalyzed biodegradation rate of the unsat~~rated coPEA IVa both intact (non-crosslinked) sample ( 1 ) and the samples heated at 1200C for 1h (2), 6 h (3), and 12 h (4).

The obtained coPEAs with unsaturated double bond moieties in lateral chains belong to the class of thermally and photochemically curable biodegradable polymers and may have the potential to be biodegradable surgical materials with enhanced mechanical characteristics as well as polymeric cross-agents of other unsaturated biodegradable polymers. A systematic study of these and related unsaturated biodegradable coPEAs is in progress now.

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