The aim of this study is to investigate the effects of verapamil on the pharmacokinetics of tamoxifen following oral administration of tamoxifen with verapamil to rats. Tamoxifen (10 mg/kg) was administered orally in the presence or absence of verapamil (1, 3 or 6 mg/kg). Compared to the control group (given tamoxifen alone), the presence of verapamil significantly (p<0.05 by 1 mg/kg, p<0.01 by 3 and 6 mg/kg) increased the areas under the plasma concentration-time curve (AUC) and the peak concentrations ($C_{max}$) of tamoxifen. Consequently, the relative bioavailability ($RB\%$) of tamoxifen with verapamil was 1.6-2.1 fold higher than that of the control. But the time to reach peak concentration ($T_{max}$) and the terminal half-life ($t_{1/2}$) of tamoxifen were not altered significantly in the presence of verapamil. The increased AUC and $C_{max}$ of tamoxifen in the presence of verapamil might be associated with the inhibition by verapamil of the P-glycoprotein and the firstpass metabolizing enzyme CYP3A4 in small intestinal mucosa. The drug interaction should be taken into consideration when tamoxifen is used to the patient with verapamil in the clinical setting.
