Solid state interactions between the proton pump inhibitor omeprazole and various enteric coating polymers

The influence of the acidic film formers Eudragit® L 100, HPMCAS-HF, HP-55, and shellac on the stability of the acid-labile proton pump inhibitor omeprazole in solid drug-polymer blends at accelerated storage conditions (40°C/75% RH) was determined by fourier transform infrared spectroscopy (FTIR), modulated temperature differential scanning calorimetry (MTDSC), and high performance liquid chromatography (HPLC). As expected, acidic polymers caused a degradation of omeprazole which was manifested by discolorations and increasing amounts of degradation products. However, MTDSC curves and FTIR spectra did not show additional peaks resulting from the omeprazole degradation products. These methods appeared to be not sensitive enough to separate analytically the drug and polymer signals from those of the decomposition products. With HPLC a sufficient quantification of the degradation products was possible. HP-55 caused the highest degree of omeprazole degradation, followed by shellac, HPMCAS-HF, and Eudragit® L 100. No correlation with the microenvironmental pH values generated by the acidic polymers at the applied storage conditions was found. The melting process and the dissolution of acidic impurities were figured out as possible reasons for the more pronounced decomposition of the drug in presence of HP-55 and shellac.