Spectroscopic study of drug–drug interactions: influence of two over-the-counter drugs on the albumin binding affinities of carbamazepine and its major metabolite

Abstract Background Multidrug regimens can increase the risk of drug–drug interactions at the level of albumin binding especially for drugs with narrow therapeutic windows such as carbamazepine (CBZ). This risk is particularly heightened for CBZ which is mainly metabolized to the active carbamazepine-10,11-epoxide (CBZE) that has been identified as contributory to both the therapeutic efficacy and severity of toxicity in CBZ-treated individuals. The objective of this study was to investigate the binding affinities of albumin with CBZ and CBZE, and to explore the influence of two competing over-the-counter medicines on the binding characteristics. CBZE was synthesized by epoxidation of CBZ and characterized using IR, NMR and mass spectrometry. The influence of paracetamol and ascorbic acid on the albumin complexes of CBZ and CBZE was investigated using absorption and IR spectrophotometry. Results Protein–ligand complexation produced progressive hyperchromic changes in 278 nm band of bovine serum albumin (BSA) with formation constants of 10.28–10.44 and 12.66–13.02 M −1 for CBZ and CBZE, respectively. Thermodynamic considerations confirmed both binding processes as endothermic, spontaneous and driven by hydrophobic interactions. The presence of ascorbic acid increased the binding constants of both CBZ-BSA and CBZE-BSA complexes by non-competitive interference mechanism. Similarly, paracetamol increased the affinity of CBZ for albumin but then competitively interfered with the CBZE-BSA complex. The ratio of albumin binding affinities of CBZ–CBZE varied from 0.81 in the absence of competing drug to 1.29 and 1.0 with paracetamol and ascorbic acid, respectively. IR study confirmed that both CBZ and CBZE induced a reduction from the 67.34% α -helical content of free BSA to 42.56 and 56.43%, respectively. Competitive binding in the presence of either paracetamol or ascorbic acid induced further reduction in the α -helical content of BSA in the complexes. The most extensive perturbation in the secondary structure of BSA (22.78% α -helical content) which was observed with CBZE-BSA complex in the presence of paracetamol is probably due to the increased interaction of the protein for the analgesic. Conclusion The study has revealed potential interference of paracetamol or ascorbic acid with the albumin binding of carbamazepine and its major metabolite.