In vitro and in vivo degradation studies of a novel linear copolymer of lactide and ethylphosphate.

Poly(lactide-co-ethylphosphate)s, a new class of linear phosphorus-containing copolymers made by chain-extending low-molecular-weight polylactide prepolymers with ethyl dichlorophosphate, were investigated for their in vitro and in vivo degradation mechanism and kinetics. Microspheres made from poly(lactide-co-ethylphosphate) were studied under both accelerated and normal in vitro degradation conditions. Gel permeation chromatography (GPC), 1H- and 31P-NMR, weight loss measurements, and differential scanning calorimetry (DSC) techniques were used to characterize the change of molecular weight (M(w)), chemical composition, and glass transition temperature (T(g)) of the degrading polymers. The results indicated that the copolymers degraded in a two-stage fashion, with cleavage of the phosphate-lactide linkages contributing mostly to the initial more rapid degradation phase and cleavage of the lactide-lactide bonds being responsible for the slower latter stage degradation. The decrease in the copolymer M(w) was accompanied by a continuous mass loss. Results from the accelerated degradation studies confirmed that the copolymers degraded into various monomers of the copolymers, which were non-toxic and biocompatible. A two-stage hydrolysis pathway was thus proposed to explain the degradation behavior of the copolymers. In vivo degradation studies performed in mice demonstrated a good in vitro and in vivo correlation for the degradation rates. In vivo clearance of the polymer was faster and without any lag phase. These copolymers are potentially advantageous for drug delivery and other biomedical applications where rapid clearance of the polymer carrier and repeated dosing capability are essential to the success of the treatment.