Study on in situ and in vivo absorption kinetics of phenytoin by modulating P-glycoprotein with verapamil in rats

P-gp is playing significant role in the development of the drug resistance by altering the absorption of drugs. The objective of present investigation was to study the effect of verapamil on the pharmacokinetics of phenytoin in order to evaluate the role of P-glycoprotein (P-gp) in phenytoin absorption. An in situ single pass intestinal perfusion study was carried out to determine the effect of verapamil on the functional status of intestinal P-gp. Phenytoin (30 μM) and propranolol (100 μM) co-perfused with and without verapamil (200 μM) in rat’s ileum. An in vivo study, verapamil (25 mg/kg, per oral) was administered 2 h before phenytoin (30 mg/kg, per oral) dosing in male Wistar rats. Plasma samples were collected at 0, 1, 2, 3, 4, 5, 6, 8, 12 and 24 h time points from control and treated animals to determine phenytoin concentrations. An in situ single pass intestinal perfusion study indicated phenytoin to be a P-gp substrate and the function of intestinal P-gp was significantly inhibited in presence of verapamil. After per oral phenytoin dosing, the mean area under the plasma concentration–time curve (AUC) was found to be 35.03 ± 1.83 h μg/ml which was increased significantly, i.e. 62.33 ± 2.49 h μg/ml (P < 0.001) when verapamil was co-administered. Similarly the mean maximum plasma concentration of phenytoin increased from 3.2 ± 0.148 μg/ml without verapamil to 6.983 ± 0.272 μg/ml (P < 0.001) with verapamil. These results are quite stimulating for further development of a clinically useful oral formulation of phenytoin based on P-gp inhibition.