Altered pharmacokinetics of zalcitabine by concurrent use of NSAIDs in rats.

AIM: To investigate the pharmacokinetic interactions between zalcitabine and nonsteroidal anti-inflammatory drugs (NSAIDs) in rats. METHODS: Zalcitabine was administered to rats via an iv injection (20 mg/kg) in the presence or absence of ketoprofen or naproxen (20 mg/kg), and the pharmacokinetic parameters were determined by using non-compartmental analysis. RESULTS: Compared with the control (zalcitabine alone), pretreatment with ketoprofen or naproxen 30 min prior to intravenous administration of zalcitabine significantly altered the pharmacokinetic profiles of zalcitabine in rats. Renal clearance of zalcitabine was reduced by approximately 3-4-fold in the presence of ketoprofen or naproxen. Consequently, systemic exposure (AUC) to zalcitabine in the rats pretreated with ketoprofen or naproxen was significantly greater than that for the control group given zalcitabine alone. The terminal plasma half-life of zalcitabine was also prolonged by 4-5-fold in the presence of ketoprofen or naproxen. CONCLUSION: The NSAIDs ketoprofen and naproxen effectively altered the pharmacokinetics of zalcitabine. Therefore, concomitant use of ketoprofen or naproxen in patients being treated with zalcitabine may necessitate close monitoring for potential drug interactions.

Pharmacokinetics of beta-L-2',3'-dideoxy-5-fluorocytidine in rhesus monkeys.

beta-L-2',3'-Dideoxy-5-fluorocytidine (beta-L-FddC), a novel cytidine analog with an unnatural beta-L sugar configuration, has been demonstrated by our group and others to exhibit highly selective in vitro activity against human immunodeficiency virus types 1 and 2 and hepatitis B virus. This encouraging in vitro antiviral activity prompted us to assess its pharmacokinetics in rhesus monkeys. Three monkeys were administered an intravenous dose of [3H] beta-L-FddC at 5 mg/kg of body weight. Following a 3-month washout period, an equivalent oral dose was administered. Plasma and urine samples were collected at various times for up to 24 h after dosing, and drug levels were quantitated by high-pressure liquid chromatography. Pharmacokinetic parameters were obtained on the basis of a two-compartment open model with a first-order elimination from the central compartment. After intravenous administration, the mean peak concentration in plasma (Cmax) was 29.8 +/- 10.5 microM. Total clearance, steady-state volume of distribution, terminal-phase plasma half-life (t1/2 beta), and mean residence time were 0.7 +/- 0.1 liters/h/kg, 1.3 +/- 0.1 liters/kg, 1.8 +/- 0.2 h, and 1.9 +/- 0.2 h, respectively. Approximately 47% +/- 16% of the intravenously administered radioactivity was recovered in the urine as the unchanged drug with no apparent metabolites. beta-L-FddC exhibited a Cmax of 3.2 microM after oral administration, with a time to peak drug concentration of approximately 1.5 h and a t1/2 of 2.2 h. One monkey in the oral administration arm of the study had a significant delay in the absorption of the aqueous administered dose. The absolute bioavailability of orally administered beta-L-FddC ranged from 56 to 66%.

The effect of probenecid on the pharmacokinetics of zalcitabine in HIV-positive patients.

PURPOSE: The purpose of this study was to determine the potential effect of probenecid on the pharmacokinetics of zalcitabine in HIV-positive patients. METHODS: Twelve patients received single oral 1.5 mg doses of zalcitabine alone and during probenecid treatment (500 mg at 8 and 2 hours before and 4 hours after zalcitabine dosing) in an open-label, randomized two-way crossover study with a one-week washout period between treatments. Serial blood and urine samples were collected over a 24 hour period and assayed for zalcitabine by a modified GC/MS method. RESULTS: Coadministration of probenecid with zalcitabine resulted in a decrease in mean (%CV) renal clearance of zalcitabine from 310 (28%) ml/min when zalcitabine was given alone to 180 (22%) ml/min with probenecid and a prolonged half-life from 1.7 hours to 2.5 hours. Mean AUCs increased from 59 ng.h/ml when zalcitabine was given alone to 91 ng.h/ml when given with probenecid. Considering the short half-life of zalcitabine (1-3 hours) relative to its dosing schedule, the pharmacokinetic changes observed in this study are not expected to result in significant accumulation during chronic dosing. CONCLUSIONS: The results of this study show that co-administration of probenecid with zalcitabine results in a moderate decrease in renal clearance of zalcitabine due to inhibition of renal tubular secretion and a 50% increase in drug exposure. Although well tolerated in this single-dose study, patients taking this combination should be monitored closely for signs of toxicity and dosage reduction should be considered if warranted.

Hplc-nmr identification of the human urinary metabolites of (-)-cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl] cytosine, a nucleoside analogue active against human immunodeficiency virus (HIV).

1. Human urine samples from a clinical trial of the anti-HIV compound (-)-cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]-cyto sin e (BW524W91) have been analysed using 19F-nmr and 1H-hplc-nmr spectroscopy. 2. The identities and relative levels of the xenobiotic species in the urine have been determined by 470-MHz 19F-nmr spectroscopy and by directly coupled 600-MHz 1H-hplc-nmr in the stop-flow mode with confirmation of the metabolite identities being made by comparison with nmr spectra of synthetic standard compounds. 3. The principal urinary xenobiotic was the unchanged drug, but the glucuronide ether conjugate at the 5' position of BW524W91, one of the two diastereomeric sulphoxides and the deaminated metabolite were also characterized. 4. The detection limit of directly coupled hplc-600-MHz 1H-nmr spectroscopy was evaluated by measuring two-dimensional nmr spectra of the glucuronide conjugate of BW524W91 and shown to be approximately 1 microgram material for 1H-1H-TOCSY and 20 micrograms metabolite for 1H-13C-HMQC spectra for overnight (16 h) acquisition.

Calmodulin inhibitors and calcium channel blockers influence dideoxycytidine renal excretion.

Renal handling of 2',3'-dideoxycytidine (ddC), a new anti-HIV dideoxynucleoside which undergoes renal and non-renal clearance, was determined in CF-1 male mice. Since calmodulin inhibitors (CIs) and calcium channel blockers (CCBs) have been shown to influence the flux of pyrimidine nucleosides across mammalian membranes and since the plasma concentration (and hence the efficacy) of therapeutic nucleosides is usually affected by the rate of renal elimination, we decided to determine the impact of the CIs loperamide (LOP) and trifluoperazine (TFP) as well as the CCB verapamil (VER) on the renal excretion of ddC. The ratio of ddC clearance to inulin clearance suggests that ddC undergoes secretion into renal tubules. Pre-exposure of mice to the calmodulin inhibitors loperamide (LOP) and trifluoperazine (TFP) resulted in a decrease in ddC renal secretion while pre-treatment with the calcium channel blocker verapamil increased ddC secretion.

Automated high-performance liquid chromatographic analysis of (-)-2'-deoxy-3'-thiacytidine in biological fluids using the automated sequential trace enrichment of dialysate systems.

A fully automated HPLC procedure was developed for the analysis of a small volume of perfusion solutions from an isolated perfused rat kidney study. The method involved separation of (-)-2'-deoxy-3'-thiacytidine (3TC) from the matrix by dialysis with 10 mM potassium phosphate buffer pH 3.0. 3TC was subsequently separated from the dialysate as it flowed through a SCX cation-exchange cartridge. The trapped 3TC was then eluted with a mobile phase of 50 mM ammonium acetate buffer (pH 5.5)-methanol (90.5:9.5, v/v) at a flow-rate of 1.0 ml/min for 2 min. The eluent was directed to the HPLC system and chromatographed with a BDS C18 analytical column at a temperature of 45 degrees C. Detection of 3TC was carried out by UV absorption at 274 nm. The procedure was validated from 25 to 10,000 ng/ml. Coefficients of variance (C.V.) of 3TC quality control samples were less than 9%. C.V.s of the standard curve samples were also less than 10% except for the 25 ng/ml samples (11.5%). The mean interpolated concentrations were within 8% of the nominal concentrations for all samples. No interference from concurrent drugs was observed. Preliminary results suggested that this procedure may also be used for human serum and urine samples.

Determination of 2'-deoxy-3'-thiacytidine (3TC) in human urine by liquid chromatography: direct injection with column switching.

3TC (GR109714X) is a cytidine dideoxynucleoside analogue which has been shown to have in vitro activity against a variety of strains of HIV-1 and is currently being investigated in clinical trials as a treatment for HIV infection. An HPLC method for the determination of 3TC in human urine has been developed and validated. The method allows direct injection of urine (10 microliters) using HPLC column switching followed by UV detection. On-line extraction is performed using a Spherisorb-SCX (5 microns, 20 x 4.0 mm) eluted with deionized water at 1 ml min-1. 3TC is retained while the bulk of urine constituents are eluted to waste. The SCX column is then backflushed to a BDS-Hypersil-C18 (5 microns, 250 x 4.6 mm) and eluted with 100 mM acetate pH 4.5-methanol (95:5, v/v) for final separation. 3TC is detected by UV absorbance at lambda = 285. The quantitation range of the assay was 0.5-500 micrograms ml-1. The method has demonstrated sufficient ruggedness to be used in support of 3TC clinical trials. Application to other cytidine analogues including DDC has been demonstrated.

Pharmacokinetics of 2',3'-dideoxycytidine after high-dose administration to rats.

The pharmacokinetics of 2',3'-dideoxycytidine (DDC) was characterized after iv administration of a high dose (500 mg/kg) of DDC to rats. The high dose was administered to optimally characterize plasma DDC concentration and urinary excretion rate versus time profiles. Drug concentrations in plasma and urine were determined by HPLC. Plasma DDC concentrations and DDC urinary excretion rates as a function of time were fitted simultaneously to a two-compartment model. Drug concentrations in plasma and urinary excretion rates declined in parallel with a terminal half-life of 1.29 +/- 0.07 h (mean +/- SD). Total, renal, and nonrenal clearances were 1.48 +/- 0.15, 0.73 +/- 0.38, and 0.75 +/- 0.36 L/h/kg, respectively. Renal clearance exceeds glomerular filtration rate in the rat, indicating that DDC undergoes active renal tubular secretion. The unbound secretory intrinsic clearance for DDC renal excretion was moderate, with a value of 0.4 L/h. The steady-state volume of distribution of DDC was 1.25 +/- 0.13 L/kg. Pharmacokinetic parameters after iv administration of 500 mg/kg of DDC were virtually identical to those reported previously after administration of 10-200 mg/kg of the nucleoside to rats. Thus, the disposition of DDC in the rat is independent of dose over a range of 10 to 500 mg/kg. High doses of DDC can be administered to rats to allow for complete characterization of the disposition pattern of the drug without complexities due to any nonlinearity.

Pharmacokinetics of 2',3'-dideoxycytidine in rats: application to interspecies scale-up.

The effects of dose on the pharmacokinetics of 2',3'-dideoxycytidine (DDC), a potent inhibitor of HIV replication, have been studied in rats. DDC was administered intravenously at doses of 10, 50, 100 and 200 mg kg-1. Plasma and urine drug concentrations were determined by HPLC. Non-compartmental pharmacokinetic parameters were calculated by area/moment analysis. DDC plasma concentrations declined rapidly with a terminal half-life of 0.98 +/- 0.18 h (mean +/- s.d.). No statistically significant differences were observed in pharmacokinetic parameters between the four doses. Total, renal and non-renal clearance values were independent of dose and averaged 1.67 +/- 0.24, 0.78 +/- 0.11, and 0.89 +/- 0.27 L h-1 kg-1, respectively. Approximately 50% of the dose was excreted unchanged in urine. Steady state volume of distribution was also independent of dose and averaged 1.2 +/- 0.21 L kg-1. Protein binding of DDC to rat serum proteins was independent of drug concentration with the fraction of drug bound averaging 0.45 +/- 0.12. Thus, the disposition pattern of DDC in the rat is independent of the administered dose even at high doses. Significant interspecies correlations were found for total, renal and non-renal clearance and steady state volume of distribution. Interspecies scaling resulted in superimposable plasma DDC concentration-time profiles from four laboratory animal species and man. Thus, plasma DDC concentrations in humans can be predicted from pharmacokinetic parameters obtained in laboratory animals.