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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 (POE₇₀LA₃₀) 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-hexamethylimidazolidine-1-yloxy, and 2,2,4,5,5-pentamethyl-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 POE₇₀LA₃₀, and polymer samples were incubated in 0.1 M phosphate buffer (pH 7.4) at 37 Â°C. At selected times, polymer samples were removed for the determination of pH values inside the eroding POE₇₀LA₃₀ by EPR at a frequency of 9.4 GHz. EPR imaging showed that the in vitro degradation of POE₇₀LA₃₀ followed a two-phase process:Â in the first week of incubation, diffusion of water, and in consequence polymer degradation, were limited to the surface of the hydrophobic POE₇₀LA₃₀ 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 POE₇₀LA₃₀ 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 POE₇₀LA₃₀ 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 POE₇₀LA₃₀, and results confirmed that the in vitro erosion mechanism of POE₇₀LA₃₀ was neither bulk erosion nor pure surface erosion.

In Vitro Monitoring of Poly(ortho ester) Degradation by Electron Paramagnetic Resonance Imaging S. Capancioni, K. Schwach-Abdellaoui, W. Kloeti, W. Herrmann, H. Brosig, H.-H. Borchert, J. Heller and R. Gurny Macromolecules, 36 (16) (2003), p6135-6141 DOI:10.1021/ma034365q \| unige:3715 \| Abstract \| Article HTML \| Article PDF
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 (POE₇₀LA₃₀) 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-hexamethylimidazolidine-1-yloxy, and 2,2,4,5,5-pentamethyl-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 POE₇₀LA₃₀, and polymer samples were incubated in 0.1 M phosphate buffer (pH 7.4) at 37 Â°C. At selected times, polymer samples were removed for the determination of pH values inside the eroding POE₇₀LA₃₀ by EPR at a frequency of 9.4 GHz. EPR imaging showed that the in vitro degradation of POE₇₀LA₃₀ followed a two-phase process:Â in the first week of incubation, diffusion of water, and in consequence polymer degradation, were limited to the surface of the hydrophobic POE₇₀LA₃₀ 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 POE₇₀LA₃₀ 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 POE₇₀LA₃₀ 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 POE₇₀LA₃₀, and results confirmed that the in vitro erosion mechanism of POE₇₀LA₃₀ was neither bulk erosion nor pure surface erosion.

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