NONDESTRUCTIVE AND LOCALIZED ASSESSMENT OF ACIDIC MICROENVIRONMENTS INSIDE BIODEGRADABLE POLYANHYDRIDES BY SPECTRAL SPATIAL ELECTRON-PARAMAGNETIC-RESONANCE IMAGING

Spectral spatial electron paramagnetic resonance imaging (e.p.r.i.) wa s applied to characterize the microenvironment inside 2mm thick poly(1 ,3-bis-p-carboxyphenoxypropane-co-sebacic anhydride) (1/4) polymer dis cs. Incorporated nitroxides are highly immobilized in the dry polymer. Exposure to 0.1 M phosphate buffer (pH 7.4) resulted in the formation of a front of degraded polymer from outside to inside. The localized measurement of pH values inside the polymer matrix was performed, usin g a pH sensitive nitroxide (pK(a) 6.1). A pH gradient was found betwee n the degrading polymer and the buffer, but also inside the polymer. T he pH inside the delivery matrix rises with time for 4.7 to 7.4. The r esults demonstrate that difficulties in the characterization of the pH , one of the most important parameters for drug delivery and polymer d egradation, can be overcome by spectral spatial e.p.r.i. (C) 1997 Else vier Science Ltd.