Improved fluorometric high-performance liquid chromatographic assay for (-)-carbovir in rat blood and urine

Comparative brain exposure to (-)-carbovir after (-)-carbovir or (-)-6-aminocarbovir intravenous infusion in rats

PURPOSE

Evaluate the ability of (-)-6-aminocarbovir ((-)-6AC) to improve the CNS exposure to (-)-carbovir ((-)-CBV).

METHODS

Activation of (-)-6AC in vitro was assessed by incubations of rat brain tissue homogenates. The in vivo brain exposure to (-)-CBV was then examined in rats after iv infusions of either (-)-CBV (n = 4) or (-)-6AC (n = 5). The drugs were infused to steady-state via the jugular vein. At the end of the infusion, a bolus of [3H]inulin was injected via the femoral vein in order to obtain an estimate of the brain vascular space.

RESULTS

(-)-6AC was converted to (-)-CBV by incubations of rat brain tissue homogenates. After iv infusion of (-)-CBV, the brain/blood concentration ratio of (-)-CBV was 0.032 +/- 0.009. The brain/blood concentration ratio of (-)-CBV after iv infusion of (-)-6AC was 0.080 +/- 0.020.

CONCLUSIONS

(-)-6AC improved the brain delivery of (-)-CBV, although the absolute exposure of the brain tissue to (-)-CBV was still quite low.

Pharmacokinetics and hepatic first-pass effect of (-)-carbovir in the anesthetized rat

Studies were designed to allow an in vivo estimation of the hepatic extraction ratio and to test the hypothesis that the pharmacokinetic parameters of (-)-CBV are significantly different during anesthesia. (-)-CBV was administered as an i.v. bolus followed by i.v. infusion into either the portal (n = 3) or the jugular (n = 3) veins of anesthetized male Sprague-Dawley rats. These studies indicate that (-)-CBV had a very low hepatic extraction ratio, in agreement with previous (-)-CBV in situ liver perfusion studies. Additionally, anesthesia was shown to alter the pharmacokinetics of (-)-CBV by reducing the total body clearance of this drug.

Intestinal absorption of (-)-carbovir in the rat

(-)-Carbovir (CBV) is a carbocyclic nucleoside analogue with in vitro activity against the human immunodeficiency virus. The sites and mechanism of absorption of (-)-CBV from the rat small intestine were studied in the anesthetized male Sprague-Dawley rat. (-)-CBV was perfused through either duodenal, jejunal, or ileal segments at three concentration levels ranging from 1 to 500 micrograms/mL. The fraction remaining to be absorbed at steady-state and the absorptive clearance were calculated for each experiment. The effect of solvent drag on the absorptive clearance was also investigated. Two-way ANOVA for the absorptive clearance per unit length was not significant for either (-)-CBV concentration or site of perfusion. The fraction remaining to be absorbed at steady-state was found to be 0.804 +/- 0.091 (n = 30). A strong correlation was found between the absorptive clearance and the net water absorptive flux. The mechanism of (-)-CBV absorption across the rat small intestine apparently consists of both passive diffusion and convection.

The bioavailability and nonlinear clearance of (-)-carbovir in the rat

The pharmacokinetics and bioavailability of (+/-)-carbovir, a carbocyclic nucleoside active against human immunodeficiency virus, have been described previously. To determine the bioavailability of (-)-carbovir, the biologically active enantiomer, four male Sprague-Dawley rats received 18 mg/kg of (-)-carbovir through the jugular vein and 54 mg/kg orally. Following the pilot studies, five rats were randomly assigned to receive (-)-carbovir in a three-way crossover design as either a single 18-mg/kg iv bolus, a single 54-mg/kg oral dose, or a single iv infusion of 18 mg/kg to achieve a target steady-state concentration (Css) of 1 micrograms/ml, the peak concentration after an oral dose. Blood and urine samples were analyzed by an improved ion-paired reversed-phase HPLC method with fluorescence detection. Blood concentrations of (-)-carbovir declined in a biphasic manner after the iv bolus dose. The terminal half-life was 116 and 106 min after the iv bolus and oral dose, respectively. The blood/plasma distribution ratio was approximately 1.0 in the range of 1 to 10 micrograms/ml of (-)-carbovir in blood. The free fraction in serum was concentration dependent. Significant differences in the renal, nonrenal, and total-body clearances after the iv bolus and iv infusion suggested nonlinear elimination of (-)-carbovir. The oral bioavailabilities derived from blood data were significantly different when the iv bolus was used as a reference rather than the iv infusion. However, the bioavailabilities were not significantly different when the total urinary excretion of unchanged (-)-carbovir after iv bolus or infusion was used as a reference.(ABSTRACT TRUNCATED AT 250 WORDS)