DECREASED SYSTEMIC CLEARANCE OF DILTIAZEM WITH INCREASED HEPATIC-METABOLISM IN RATS WITH URANYL NITRATE-INDUCED ACUTE-RENAL-FAILURE

The effect of uranyl nitrate (UN)-induced acute renal failure (ARF) on the pharmacokinetics of diltiazem (DTZ) was examined in rats through in vitro and in vivo studies. In vitro homogenate studies demonstrated that DTZ was metabolized to deacetyl diltiazem (DAD) predominantly in the liver. Metabolism in the small intestine, kidney, or blood pool was negligible compared with that in the liver. UN-induced ARF (UN-ARF) increased the in vitro hepatic clearance (CL(vit)) of DTZ 1.4-fold. In vivo pharmacokinetic studies following intravenous (iv) and portal venous (pv) administration revealed that UN-ARF increased the intrinsic clearance (CL(i)) of DTZ from 243.0 to 414.5 ml/min/kg but decreased its total plasma clearance (CL(t)) from 90.3 to 64.3 ml/min/kg. The increase in CL(i) was consistent with the increase in CL(vit) of the liver. The in vitro plasma free fraction of DTZ (f(p)) was decreased from 0.25 to 0.14 by UN-ARF, but the in vitro blood/plasma partition of DTZ (R(b)) remained constant at unity. From the CL(i) and f(p) changes, the plasma intrinsic clearance for unbound DTZ (C(Li)') was calculated to be increased 2.7-fold, from 1104.5 to 2960.7 ml/min/kg, by UN-ARF. The f(p) decrease was also reflected in the steady-state distribution volume (Vd(ss)) of DTZ, which was decreased significantly from 3595.5 to 2528.3 ml/kg. The absolute bioavailability of pv DTZ (F(pv)) was decreased by UN-ARF from 37.5 to 15.5% but was still much higher than the reported oral bioavailability (6%), indicating poor absorption of DTZ from the GI tract. From the calculation based on a well-stirred pharmacokinetic model, DTZ was found to increase the hepatic blood flow (HBF) of the control rats more than twofold at doses of 3 mg/kg (iv) or 10 mg/kg (pv), possibly due to the vasodilating effect of DTZ. However, the effect of DTZ on HBF was not present in the UN-ARF rats. It is not clear at present whether this could be attributed to vasoconstricting effects of UN-ARF or blockade of the vasodilating effect of DTZ.