Effects of St. John's wort extract on indinavir pharmacokinetics in rats: differentiation of intestinal and hepatic impacts.

AIMS: To evaluate the possible herb-drug interaction of St. John's wort (SJW) extracts with indinavir in rats and to set up a model for characterizing pre-systemic sites for the interactions between orally administered herbs and pharmaceuticals. MAIN METHODS: The in vivo pharmacokinetic study and in situ single-pass intestinal perfusion model were employed in the research. Plasma indinavir concentration and cytochrome P450 3A activities were measured by high-pressure liquid chromatography and spectrophotometric assays, respectively. KEY FINDINGS: Oral administration of either 150 or 300 mg/day SJW for 15 days significantly reduced indinavir plasma levels with certain pharmacokinetic parameter changes. The cytochrome P450 3A analysis suggested that this interaction was attributable to the induction of indinavir metabolism. Further perfusion study demonstrated that both small intestine and the liver contributed significantly to the reduction of indinavir bioavailability and was flow rate-dependent. Moreover, the small intestine was the major site for the pre-systemic metabolism of indinavir, whether with or without SJW pretreatment. SIGNIFICANCE: Herb-drug pharmacokinetic interactions between SJW and indinavir can be clearly observed in the Wistar rat model. Particularly, the respective first-pass effect contributed by the small intestine and the liver could be differentiated and quantified. The application of the animal model to investigating herb-drug interactions or other relevant research purposes is envisioned.

Application of rat in situ single-pass intestinal perfusion in the evaluation of presystemic extraction of indinavir under different perfusion rates.

BACKGROUND/PURPOSE: First-pass effect has been an important concern for oral pharmaceuticals. An in vivo system was developed for measuring different concentrations of pharmaceuticals in the portal vein and hepatic vein (via the inferior vena cava) for delineating presystemic metabolism under different perfusion rates by using indinavir as an exemplary agent. METHODS: An in situ single-pass intestinal perfusion technique was modified from previous studies to concomitantly obtain portal and hepatic venous bloods. Portal and hepatic venous samples were simultaneously taken from rats at appropriate time points using the perfusion model of 1 mg/mL indinavir at flow rates of 0.05, 0.1, 0.5 and 1.0 mL/min. The indinavir concentrations were assayed by binary-gradient high-pressure liquid chromatography with UV detection. RESULTS: The mean indinavir concentrations in portal vein concentration-time profiles at different perfusion times under various flow rates were all higher than those obtained for hepatic veins. At flow rates of 0.5 and 1.0 mL/min, in particular, the area under the curve (AUC) and maximal concentration (Cmax) of indinavir absorption were significantly different between portal veins and hepatic veins (p < 0.05), indicating considerable hepatic involvement in the presystemic extraction of indinavir. The system also has potential for use when estimating the hepatic extraction ratio (E(H)) and hepatic clearance (Cl(H)). CONCLUSION: This in vivo approach could provide another useful tool for improving our basic understanding of the absorption kinetics and hepatic metabolism of pharmaceuticals under development and facilitating the clinical application of such.

Simultaneous determination of indinavir, ritonavir and saquinavir in plasma by high-performance liquid chromatography.

BACKGROUND AND PURPOSE: Antiretroviral regimens consisting of a combination of protease inhibitors have been shown to be highly potent for the treatment of HIV infection. Therapeutic drug monitoring may ensure optimal drug levels to improve efficacy and minimize side effects. The purpose of this study was to establish a method for simultaneous determination of plasma concentrations of indinavir (IDV), ritonavir (RTV) and saquinavir (SQV) by high-performance liquid chromatography (HPLC) in a single run. METHODS: A C(18) column and an ultraviolet detector set at 215 nm were used in the HPLC. The mobile phase consisted of phosphate buffer (50 mM, pH 5.6) and acetonitrile (55:45, v/v), and the flow rate was 1.5 mL/min. Plasma sample (0.5 mL) containing IDV, RTV, and SQV was alkalinized with ammonia water (10%) and propylparaben added as an internal standard (IS), which was then extracted with methyl tert.-butyl ether. The organic layer was taken and evaporated to dryness. The residue was dissolved in the mobile phase, and the solution was washed with n-hexane followed by HPLC analysis. RESULTS: The calibration curves were linear for each of the 3 drugs in the studied concentration range (0.1 to 5 microg/mL). The extraction recoveries were greater than 90%. One HPLC run takes less than 15 minutes. Analysis of plasma samples of patients treated with combinations of these drugs demonstrated that the calibration curves covered the clinical concentration range. CONCLUSIONS: This study established a simple and accurate method for simultaneous determination of IDV, RTV, and SQV in plasma by isocratic HPLC. The method is practical for therapeutic drug monitoring.

Kinetic and dynamic studies of liposomal bupivacaine and bupivacaine solution after subcutaneous injection in rats.

The pharmacodynamics and pharmacokinetics of bupivacaine in solution and in liposome preparations following subcutaneous administration were studied in rats. Multilamellar vesicles entrapping bupivacaine solution were prepared. The local anaesthetic effect was estimated by the tail-flick test in Wistar rats treated with 1 mg bupivacaine in 0.2-mL preparations. Plasma concentrations of bupivacaine were determined by high-performance liquid chromatography. The results showed that both bupivacaine solution and bupivacaine liposomes revealed local anaesthetic effects in the initial tail-flick test (15 min after injection). With bupivacaine liposomes, the duration of action was 5-fold (447+/-28.9 vs 87+/-6.7 min), the maximum possible effect was 2-fold (100+/-0 vs 47.6+/-13%), and the peak plasma concentration (Cmax) was less than one-fifth (0.12+/-0.04 vs 0.65+/-0.04 microg mL(-1)) that with bupivacaine solution. The sensory block effect of bupivacaine solution completely resolved at 90 min, while the plasma concentration of bupivacaine was still more than half the Cmax. Bupivacaine liposomes resulted in a low and relatively constant plasma level (approx. 0.1 microg mL(-1)) and a pronounced local anaesthetic effect throughout the experimental period (> 7 h). In conclusion, bupivacaine liposomes elevated the intensity and prolonged the duration of the local anaesthetic effect of bupivacaine, and suppressed the systemic absorption rate of encapsulated bupivacaine.

Concentration-dependent disposition of glucuronide metabolite of valproate.

The glucuronide conjugation metabolism of valproate (VPA) has been assessed to be non-linear within the therapeutic concentration range. However, disposition of its metabolite, valproic acid glucuronide (VPAG), in relation to VPA doses is unclear. The purpose of this study was to elucidate the characteristics of dose-related disposition of VPAG. Guinea-pigs were treated with an intravenous bolus dose of sodium valproate at 20, 100, 500 or 600 mg kg(-1). Plasma was sampled on a pre-selected time schedule, and bile and urine were collected. Concentrations of VPA and VPAG in plasma, bile and urine were determined by gas chromatography. The pharmacokinetics of VPA and VPAG both were dose-dependent. However, the plasma concentration-time profiles of VPAG and VPA were not parallel. At a usual dose of VPA (20 mg kg(-1)), plasma VPAG declined with plasma VPA, whereas at a high dose of VPA (>500mg kg(-1)), plasma VPAG was elevated against the decline of plasma VPA, which suggested accumulation of plasma VPAG possibly owing to saturated elimination. The biliary and urinary clearances of VPA (vCLb and vCLu) were independent of dose. However, the clearances of plasma VPA (vCLp), plasma VPAG (gCLp), biliary and urinary VPAG (gCLb and gCLu) all were decreased against the increase in VPA doses. The dose-dependent decrease of gCLu (from 3.19 to 1.12 mL min(-1)) was less pronounced than that of gCLp (from 6.72 to 0.86 mL min(-1)) and the gCLu turned to exceed the gCLp at high doses of VPA (> 500 mg kg(-1)). These results suggest that the excess urinary VPAG might be produced in kidney. In conclusion, at a high dose of VPA, plasma VPAG is accumulated. The concentration-dependent biliary and urinary recovery of VPAG might be governed by a saturable elimination process rather than by saturable hepatic biotransformation rate. Glucuronide conjugation metabolism of VPA in kidney is speculated, which might be minor at low levels of plasma VPA, but more obvious after saturation of hepatic glucuronidation.