Renal interaction between itraconazole and cimetidine

Renal drug interactions can result from competitive inhibition between drugs that undergo extensive renal tubular secretion by transporters such as P-glycoprotein (P-gp). The purpose of this study was to evaluate the effect of itraconazole. a known P-gp inhibitor, on the renal tubular secretion of cimetidine in healthy volunteers who received intravenous cimetidine alone and following 3 days of oral itraconazole (400 mg/day) administration. Glomerular filtration rate (GFR) was measured continuously during each study visit using iothalamate clearance. Iothalamate, cimetidine, and itraconazole concentrations in plasma and urine were determined using high-performance liquid chromatography/ultraviolet (HPLC/UV) methods. Renal tubular secretion (CLsec) of cimetidine was calculated as the difference between renal clearance (CLr) and GFR (CLioth) on days 1 and 5. Cimetidine pharmacokinetic estimates were obtained for total clearance (CLT), volume of distribution (Vd), elimination rate constant (K-el), area under the plasma concentration-time curve (AUC(0-240) min), and average plasma concentration (Cp-ave) before and after itraconazole administration. Plasma itraconazole concentrations following oral dosing ranged from 0.41 to 0.92 mug/mL. The cimetidine AUC(0-240 min) increased by 25% (p<0.01) following itraconazole administration. The GFR and Vd remained unchanged, but significant reductions in CLT (655 vs. 486 mL/min, p < 0.001) and CLsec (410 vs. 311 mL/min, p = 0.001) were observed. The increased systemic exposure of cimetidine during coadministration with itraconazole was likely due to inhibition of P-gp-mediated renal tubular secretion. Further evaluation of renal P-gp-modulating drugs such as itraconazole that may alter the renal excretion of coadministered drugs is warranted.