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The purpose of the study was to investigate and identify the interactions within solid dispersions of cationic drugs and anionic polymers processed by hot-melt extrusion (HME) technique. Propranolol HCl (PRP) and diphenhydramine HCl (DPD) were used as model cationic active substances while pH sensitive anionic methacrylic acid based methyl methacrylate copolymers Eudragit L100® (L100) and ethyl acrylate copolymer Eudragit L100-55 (Acryl EZE) (L100-55) were used as polymeric carriers. The extrudates were further characterised using various physicochemical characterisation techniques to determine the morphology, the drug state within the polymer matrices and the type of drug–polymer interactions. Molecular modelling predicted the existence of two possible H-bonding types while the X-ray photon spectroscopy (XPS) advanced surface analysis of the extrudates revealed intermolecular ionic interactions between the API amino functional groups and the polymer carboxylic groups through the formation of hydrogen bonding. The magnitude of the intermolecular interactions varied according to the drug–polymer miscibility.

Human vascular adhesion protein-1 (VAP-1) is an endothelial copper-dependent amine oxidase involved in the recruitment and extravasation of leukocytes at sites of inflammation. VAP-1 is an important therapeutic target for several pathological conditions. We expressed soluble VAP-1 in HEK293 EBNA1 cells at levels suitable for detailed mechanistic studies with model substrates. Using the model substrate benzylamine, we analyzed the steady-state kinetic parameters of VAP-1 as a function of solution pH. We found two macroscopic pKa values that defined a bell-shaped plot of turnover number kcat,app as a function of pH, representing ionizable groups in the enzyme-substrate complex. The dependence of (kcat/Km)app on pH revealed a single pKa value (∼9) that we assigned to ionization of the amine group in free benzylamine substrate. A kinetic isotope effect (KIE) of 6 to 7.6 on (kcat/Km)app over the pH range of 6 to 10 was observed with d2-benzylamine. Over the same pH range, the KIE on kcat was found to be close to unity. The unusual KIE values on (kcat/Km)app were rationalized using a mechanistic scheme that includes the possibility of multiple isotopically sensitive steps. We also report the analysis of quantitative structure-activity relationships (QSAR) using para-substituted protiated and deuterated phenylethylamines. With phenylethylamines we observed a large KIE on kcat,app (8.01 ± 0.28 with phenylethylamine), indicating that C–H bond breakage is limiting for 2,4,5-trihydroxyphenylalanine quinone reduction. Poor correlations were observed between steady-state rate constants and QSAR parameters. We show the importance of combining KIE, QSAR, and structural studies to gain insight into the complexity of the VAP-1 steady-state mechanism.