In vivo degradation of massive poly(alpha-hydroxy acids): validation of in vitro findings

The degradation of various high-molecular-weight aliphatic polyesters derived from glycolic acid and/or lactic acid enantiomers was previously investigated in vitro. It was demonstrated that the bulk degradation mechanism proposed in the literature actually proceeds heterogeneously and proceeds faster in the centre than at the surface of large specimens. In order to compare them, similar compression-moulded specimens were implanted intramuscularly in the backs of rabbits, namely PLA50 (poly(DL-lactic acid)), PLA37.5GA25 (75% DL-lactide and 25% glycolide in the feed) and PLA75GA25 (75% L-lactide and 25% glycolide). These three intrinsically amorphous compounds exhibited faster central degradation. Furthermore, preferential degradation of glycolic acid units and induced crystallization of L-lactic acid enriched fragments were observed in the case of PLA75GA25. These findings are comparable to phenomena observed in vitro and are conclusively supported by the re-examination of some old in vivo results. Accordingly, data reported in this paper validate both the in vitro modelling and new understanding of the degradation of lactic acid/glycolic acid-based aliphatic polyesters reported previously.