OTC drugs have an important role in self-medication. However, the pharmacokinetic properties of some OTC drugs have not been fully investigated and reports concerning their drug interactions are insufficient. Several gastrointestinal drugs are available as OTC drugs. Because of their pharmacological properties, these drugs are often used concomitantly with other drugs. Therefore, it is important to predict the possible drug interactions among these drugs. In the current study, we investigated the inhibitory effects of five gastrointestinal drugs, namely loperamide, oxethazaine, papaverine, pirenzepine, and trimebutine, on CYP activities in human liver microsomes. Furthermore, we calculated the ratio of the intrinsic clearance of each CYP substrate in the presence or absence of the gastrointestinal drugs. The possibility of drug interactions in vivo was predicted by cut-off criteria. CYP3A4 activity was markedly inhibited by trimebutine, papaverine, and oxethazaine. Their inhibitory properties were competitive and the K i values were 6.56, 12.8, and 3.08 µM, respectively. Alternative R values of CYP3A4 exceeded the cut-off level. These results suggested that drug interactions mediated by CYP3A4 may occur during treatment with these gastrointestinal drugs, necessitating the confirmation of the clinical significance of these drug interactions to prevent unexpected adverse effects.
Key words gastrointestinal drug; OTC drug; drug interaction; CYPIn recent years, people have become increasingly interested in their own health due to a rapidly aging society and/or the increase of lifestyle-related diseases. As a result, the concept of self-care, which includes self-medication, has attracted popular attention. Self-medication is defined as the selection and use of medicines by individuals to treat self-recognized illnesses or symptoms. 1) Self-medication includes the use of OTC drugs under the advice of healthcare professionals such as pharmacists.2) Therefore, OTC drugs play an important role in self-medication. OTC drugs are considered relatively safer than ethical drugs. However, many OTC drugs, especially those developed in the distant past, have not had their pharmacokinetic properties sufficiently studied. They may be safe to use alone; however, there is no evidence to suggest their safety when combined with other drugs.Many drug interactions affect specific metabolic processes, especially those mediated by CYP. CYP-mediated drug interactions are roughly classified into enzyme inhibition and enzyme induction. Most are based on enzyme inhibition, with many documented clinically significant cases. Therefore, it is important to examine whether OTC drugs have inhibitory effects on CYP activity.A recent study reported that diphenhydramine interacted with metoprolol via CYP2D6 inhibition.3) Diphenhydramine potently inhibited metoprolol α-hydroxylation in vitro. Moreover, a clinical study found that diphenhydramine decreased metoprolol metabolic clearance 2.5-fold in CYP2D6 extensive metabolizers.4) Diphenhydramine, a fir...