Biodegradation of and tissue reaction to 50:50 poly(DL-lactide-co-glycolide) microcapsules.

The biodegradation of the copolymer 50:50 poly(DL-lactide-co-glycolide)-lypressin microcapsules was studied by light and electron microscopic methods and 14C release. Intramuscular injection sites of microcapsules in rats were studied by dissecting and conventional light microscopy as well as scanning (SEM) and transmission electron microscopy. A minimal localized acute myositis was seen initially at the injection sites. By Day 4, a few small foreign body giant cells were present participating in the minimal foreign body response. Later the inflammatory cells decreased and the individual microcapsules were walled off by immature fibrous connective tissue and large syncytial foreign body giant cells. By Day 35, definitive changes in some microcapsules, consisting of a granular and slightly eroded appearance of the internal matrix, were seen by SEM. By Day 42, the outer rims of the microcapsules were extensively eroded. At Day 56, the inflammatory and connective tissue reactions were almost completely resolved and biodegradation continued so that only remnant pieces of the microcapsules were present at Day 63. The morphologic picture correlated well with loss of 14C radioactivity, which could no longer be detected at the injection sites on Day 56. Phagocytosis did not seem to be an important factor in the biodegradation.