The microclimate pH in poly(D,L-lactide-co-hydroxymethyl glycolide) microspheres during biodegradation

The microclimate pH (μpH) in biodegradable polymers, such as poly(D,L-lactic-co-glycolic acid) (PLGA) 50/50, commonly falls to deleterious acidic levels during biodegradation, resulting in instability of encapsulated acid-labile molecules. The μpH distribution in microspheres of a more hydrophilic polyester, poly(D,L-lactide-co-hydroxymethyl glycolide) (PLHMGA), was measured and compared to that in PLGA 50/50 of similar molecular weight and degradation time scales. pH mapping in the polymers was performed after incubation under physiological conditions by using a previously validated ratiometric method employing confocal laser scanning microscopy (CLSM). Confocal μpH maps revealed that PLHMGA microspheres, regardless of copolymer composition, developed a far less acidic μpH during 4 weeks of incubation compared with microspheres from PLGA. A pH-independent fluorescent probe marker of polymer matrix diffusion of μpH-controlling water-soluble acid degradation products, bodipy, was observed by CLSM to diffuse ~3-7 fold more rapidly in PLHMGA compared to PLGA microspheres, consistent with much more rapid release of acids observed from the hydrophilic polymer during bioerosion. Hence, PLHMGA microspheres are less susceptible to acidification during degradation as compared to similar PLGA formulations, and therefore, PLHMGA may be more suitable to deliver acid labile molecules such as proteins.