In vitro monitoring of poly(ortho ester) degradation by electron paramagnetic resonance imaging

Electron paramagnetic resonance (EPR) imaging was applied to investigate further the in vitro degradation process of poly(ortho esters) containing 30 mol % lactic acid units in the polymer backbone (POE(70)LA(30)) and developed for controlled drug delivery. The objective of this study was the direct and continuous determination of pH values inside the degrading POE70LA30. pH-sensitive nitroxide spin probes 4-amino-2,2,5,5-tetramethyl-3-imidazoline-1-yloxy, 2,2,3,4,5,5-hexamethylimidazohdine-1-yloxy, and 2,2,4,5,5pentamethyl-3-imidazoline-1-yloxy were calibrated in buffer solutions in order to cover a pH range between 1.0 and 8.0. Nitroxide spin probes were incorporated in POE(70)LA(30), and polymer samples were incubated in 0.1 M phosphate buffer (pH 7.4) at 37 degreesC. At selected times, polymer samples were removed for the determination of pH values inside the eroding POE(70)LA(30) by EPR at a frequency of 9.4 GHz. EPR imaging showed that the in vitro degradation of POE(70)LA(30) followed a two-phase process: in the first week of incubation, diffusion of water, and in consequence polymer degradation, were limited to the surface of the hydrophobic POE(70)LA(30) where pH values between 6.0 and 7.4 were measured. After 1 week of incubation, water diffused into the core of the sample, allowing the determination of pH values inside the eroding POE(70)LA(30) until complete erosion. Results indicated the formation of a pH gradient, with the most acidic environment inside the eroding sample where the lowest pH value of 3.8 was measured and higher pH at the surface. It was also possible to observe a polymer erosion front moving down within the polymer matrix in the course of time. The pH value of 3.8 measured inside the degrading POE(70)LA(30) remained constant until polymer samples disintegrated at day 23, where no EPR signal was detectable. In conclusion, EPR imaging allows the noninvasive spatially resolved observation of pH changes within POE(70)LA(30), and results confirmed that the in vitro erosion mechanism of POE(70)LA(30) was neither bulk erosion nor pure surface erosion.