Tissue distribution and plasma pharmacokinetics of UCN-01 at steady-state and following bolus administration in rats: influence of human alpha1-acid glycoprotein binding.

The primary focus of this study was to investigate the role of human alpha1-acid glycoprotein (hAGP) on the pharmacokinetics and tissue distribution of the antitumor drug UCN-01 (7-hydroxystaurosporine) in rats, following bolus administration and at steady-state blood concentration. To evaluate plasma pharmacokinetics, the rats received UCN-01 alone, UCN-01 + hAGP (87:1 ratio), or UCN-01 + hAGP (26:1 ratio) i.v. Additional rats were studied after i.m. administration of UCN-01 and i.v. administration of human AGP (87:1 ratio). For tissue distribution, rats received UCN-01 alone, UCN-01 + hAGP (87:1 ratio). One hour after drug administration, blood samples as well as various tissues and organs were collected. Plasma concentrations of UCN-01 as well as tissue accumulation were measured by HPLC using a fluorescence detector. Following i.v. bolus administration, the UCN-01 concentration-time profile declined bi-exponentially. The distribution half-life was 0.2 hours, while the elimination half-life was 6.65 hours. The volume of distribution of the central compartment (Vc) was 1000 ml/kg and the volume of distribution during the elimination phase (Vdb) was 2551 ml/kg. The total body clearance (TBC) was 4.4 ml/min/kg. Co-administration of hAGP with UCN-01 at 1:87 ratio did not affect the elimination half-life of UCN-01 during our sampling period, however the distribution half-life was delayed by approximately 2.7-fold. Furthermore, the Vc, Vd(beta) and TBC were significantly reduced to 395 ml/kg, 735 ml/kg and 1.34 ml/min/kg, respectively. UCN-01 Vd's and TBC were reduced further by increasing human hAGP:UCN-01 ratio to 26:1. Also, hAGP administration did not significantly affect the pharmacokinetic profile of UCN-01 after i.m. administration, which was similar to that measured after i.v. administration. One hour after i.v. bolus administration, UCN-01 was distributed extensively to all tissues with a tissue/plasma ratio ranging from 10-times in the brain to more than 1000-times in the lungs. The presence of hAGP drastically reduced tissue accumulation of UCN-01, although the tissue to plasma ratio remained > 1.0 except for the brain. At steady-state blood concentration following the infusion of UCN-01 over 180 minutes, the ratio of the drug concentration to the concomitant plasma concentration remained > 1.0, even in the presence of hAGP. The data showed that the binding of UCN-01 to hAGP drastically altered its pharmacokinetics and tissue distribution, even at the plasma steady-state concentration.

Cocaine and alcohol interactions in the rat: effect of cocaine and alcohol pretreatments on cocaine pharmacokinetics and pharmacodynamics.

This experiment was designed to investigate the effect of pretreatment with cocaine and alcohol on cocaine pharmacokinetics and pharmacodynamics. Four groups of rats (n = 8 per group) received one of the following pretreatments for two weeks: none, alcohol (10% v/v in drinking water), cocaine (15 mg/kg/day ip), and alcohol+cocaine (10% v/v in drinking water + 15 mg/kg/day ip). On the day of the experiment, cocaine was administered (30 mg/kg, ip) to each rat, either alone or in combination with alcohol (5 g/kg, po), in a balanced crossover experimental design. Plasma and brain ECF concentrations of cocaine and its three metabolites: benzoylecgonine, norcocaine, and cocaethylene were assayed by HPLC-UV. The percent change in brain dopamine concentration, mean arterial blood pressure, and heart rate were determined simultaneously. A sigmoid-E(max) model was used to describe the brain cocaine concentration-neurochemical effect (dopamine) relationship, and an indirect pharmacodynamic response model was used to describe the plasma cocaine concentration-cardiovascular effect relationships. Alcohol pretreatment led to significant increase in cocaine AUC(p), alpha(t1/2), and beta(t1/2). Cocaine pretreatment significantly increased cocaine bioavailability, absorption rate constant, TBC, and the formation clearance of cocaethylene. Acute alcohol coadministration with cocaine increased cocaine AUC(p) and bioavailability, reduced the fraction of cocaine dose converted to benzoylecgonine, and increased the formation of norcocaine. These results indicate that the pharmacokinetics of cocaine, either administered alone or in combination with alcohol, is significantly altered due to prior cocaine and/or alcohol use. Both cocaine and alcohol pretreatments increased the E(max) for dopamine, with no effect on the EC(50). Acute alcohol coadministration with cocaine significantly increased the E(max) for dopamine and reduced the EC(50). Cocaine pretreatment significantly decreased the I(max) for blood pressure, IC(50), and R(max). For the heart rate response, both alcohol and cocaine pretreatments significantly increased the IC(50), with no effect on I(max). These results indicate that both cocaine and alcohol pretreatments as well as acute alcohol coadministration lead to significant alterations in cocaine pharmacodynamics that are due, at least in part, to the changes in cocaine pharmacokinetics. If similar effects occur in humans, chronic cocaine and alcohol abusers may respond differently to cocaine administration compared to naïve users and may be at higher risks of cocaine central nervous system toxicity.

Comparative brain tissue distribution of camptothecin and topotecan in the rat.

PURPOSE: The primary objective of this investigation was to compare the extent of brain distribution of the lactone and the carboxylate forms of camptothecin (CPT) and topotecan (TPT) in awake freely moving rats. METHODS: The plasma concentration-time profiles of the lactone and the carboxylate forms of CPT and TPT were determined simultaneously after a single i.v. administration of the lactone form of each drug. Also, the brain extracellular fluid (ECF) concentration-time profiles were characterized utilizing the microdialysis technique. This technique allowed serial sampling of the brain ECF in awake rats. RESULTS: CPT-lactone in plasma declined biexponentially with a terminal half-life of 102+/-25.2 min. During the elimination phase, the plasma concentration of CPT-carboxylate was approximately ten times the concentration of CPT-lactone. The brain ECF to plasma distribution ratio measured as the ratio of the AUC in the brain ECF to the AUC in plasma was 0.51+/-0.08 for CPT-lactone, and 0.26+/-0.21 for CPT-carboxylate. The terminal half-life for TPT-lactone was 64.0+/-9.4 min. During the elimination phase, the TPT-carboxylate concentration was higher than that of TPT-lactone but the carboxylate to lactone concentration ratio was much lower than that of CPT. The brain ECF to plasma distribution ratio was 0.38+/-0.12 for TPT-lactone, and 0.21+/-0.06 for TPT-carboxylate. CONCLUSIONS: CPT and TPT are distributed to the brain ECF most probably by passive diffusion across the blood-brain barrier. Although the brain ECF to plasma distribution ratio for CPT-lactone was higher than that for TPT-lactone, the brain ECF concentrations of TPT-lactone were significantly higher than the CPT-lactone brain ECF concentrations. The relatively high brain ECF to plasma distribution ratio of these two drugs makes them potential candidates for first-line treatment of CNS tumors.

Cocaine and alcohol interactions in the rat: effect on cocaine pharmacokinetics and pharmacodynamics.

The effect of alcohol coadministration on cocaine pharmacokinetics and pharmacodynamics was investigated in awake, freely moving rats. Cocaine plasma and brain extracellular fluid (ECF) concentration-time profiles were characterized after intraperitoneal (ip) administration of 30 mg/kg cocaine to rats that were pretreated with either normal saline or alcohol at 5 g/kg in a balanced crossover experimental design. The neurochemical response to cocaine administration, measured as the change in dopamine concentration in the nucleus accumbens (N ACC) and the change in the mean arterial blood pressure were monitored simultaneously. Intragastric alcohol administration significantly increased cocaine systemic bioavailability after ip administration from 0.550 +/- 0.044 to 0. 754 +/- 0.071. Also, the absorption rate constant increased from 0. 199 +/- 0.045 to 0.276 +/- 0.059 min-1 due to alcohol coadministration; however, this increase was not significant. Alcohol inhibition of cocaine metabolism caused an increase in cocaine elimination half-life from 26.3 +/- 3.6 to 40.0 +/- 8.1 min. Also, cocaine tissue distribution was enhanced by alcohol, resulting in a significant increase in cocaine volume of distribution. Analysis of the brain cocaine concentration-neurochemical effect relationship by the sigmoid-Emax pharmacodynamic model showed that Emax increased from 850 +/- 200 to 1550 +/- 640% of baseline due to alcohol coadministration, whereas EC50 decreased from 3400 +/- 580 to 2000 +/- 650 ng/mL, indicating higher cocaine potency in the presence of alcohol. The estimates of the indirect inhibitory pharmacodynamic model used to examine the plasma cocaine concentration-change in blood pressure relationship were not significantly different after the two treatments. These results indicate that alcohol significantly alters cocaine absorption, distribution, and elimination, resulting in higher and prolonged cocaine plasma concentration. Alcohol coadministration also potentiates the neurochemical response to cocaine administration.

Cocaine and alcohol interactions in the rat: contribution of cocaine metabolites to the pharmacological effects.

The pharmacokinetics and pharmacodynamics of cocaine and its three metabolites, benzoylecgonine, norcocaine, and cocaethylene, were investigated in awake, freely moving rats. This work was performed to examine the effect of alcohol coadministration on the metabolic profile of cocaine and to determine the contribution of cocaine metabolites to the pharmacological responses observed after cocaine administration. The plasma and brain extracellular fluid concentration-time profiles were characterized after intravenous (iv) administration of cocaine and the three metabolites in a crossover experimental design. The neurochemical response, measured as the change in dopamine concentration in the nucleus accumbens, and the cardiovascular responses, measured as the change in the mean arterial blood pressure, heart rate, and QRS interval, were monitored simultaneously. Cocaethylene had the highest brain-to-plasma distribution ratio, followed by cocaine, norcocaine, and benzoylecgonine. The estimated total body clearances for cocaine, benzoylecgonine, norcocaine, and cocaethylene were 140 +/- 19, 14.7 +/- 1.2, 130 +/- 19, and 111 +/- 16 mL/min/kg, respectively. Alcohol coadministration increased the formation of norcocaine, decreased the formation of benzoylecgonine, and resulted in the formation of the pharmacologically active metabolite cocaethylene. When cocaine was administered with alcohol, 12.9 +/- 3.1% to 15.3 +/- 2.9% of the cocaine dose was converted to cocaethylene. Benzoylecgonine did not have any central nervous system or cardiovascular activities after iv administration. Compared with cocaine, norcocaine and cocaethylene had more potent and prolonged effects on the neurochemical, heart rate, and QRS interval responses, and were equipotent in increasing the mean arterial blood pressure. These results indicate that changes in the cocaine metabolic profile and the formation of the pharmacologically active metabolite cocaethylene are, at least partially, responsible for the more intense and longer lasting effects reported after using this drug in combination with alcohol.

An animal model for simultaneous pharmacokinetic/pharmacodynamic investigations: application to cocaine.

An animal model suitable for pharmacokinetic/pharmacodynamic investigations is described. This model allows drug administration via different routes, serial blood sampling, serial brain ECF sampling, and monitoring the cardiovascular functions without touching the animal. This rat model was utilized to study the relationship between cocaine pharmacokinetics and the neurochemical and cardiovascular responses to cocaine administration via different routes. The pharmacokinetic results showed that the average cocaine bioavailability after i.p. administration was 71% and after oral administration was only 19.2%. Cocaine was rapidly distributed into the brain, and the brain ECF/plasma distribution ratio measured as the ratio of the brain ECF AUC to the plasma AUC was 2.02 +/- 0.59. The relationship between cocaine brain ECF concentration and the change in dopamine brain ECF concentration was described by the sigmoid Emax pharmacodynamic model. When the relationship between cocaine plasma concentration and the change in the cardiovascular functions was examined, hysteresis loops were observed. These hysteresis loops may suggest the existence of an effect compartment for the cardiovascular effects of cocaine or that cocaine metabolites are contributing to cocaine cardiovascular effects. These results indicate that the described animal model is useful in simultaneous pharmacokinetic/pharmacodynamic investigations specifically for studies that involve centrally acting drugs.

A sensitive method for the determination of uranium in biological samples utilizing kinetic phosphorescence analysis (KPA).

Kinetic phosphorescence analysis is a technique that provides rapid, precise and accurate determination of uranium concentration in aqueous solutions. This technique utilizes a laser source to excite an aqueous solution of uranium, and measures the emission luminescence intensity over time to determine the luminescence decay profile. The lifetime of the luminescence decay profile and the linearity of the log luminescence intensity versus time profile are indications of the specificity of the technique for uranium determination. The luminescence intensity at the onset of decay (the initial luminescence intensity), which is the luminescence intensity at time zero after termination of the laser pulse used for excitation, is proportional to the uranium concentration in the sample. Calibration standards of known uranium concentrations are used to construct the calibration curve between the initial luminescence intensity and uranium concentration. This calibration curve is used to determine the uranium concentration of unknown samples from their initial luminescence intensity. We developed the sample preparation method that allows the determination of uranium concentrations in urine, plasma, kidney, liver, bone spleen and soft tissue samples. Tissue samples are subjected to dry-ashing in a muffle furnace at 600 degrees C and wet-ashing with concentrated nitric acid and hydrogen peroxide twice to destroy the organic component in the sample that may interfere with uranium determination by KPA. Samples are then solubilized in 0.82 M nitric acid prior to analysis by KPA. The assay calibration curves are linear and cover the range of uranium concentrations between 0.05 micrograms l-1 and 1000 micrograms l-1 (0.05-1000 ppb). The developed sample preparation procedures coupled with the KPA technique provide a specific, sensitive, precise and accurate method for the determination of uranium concentration in tissue samples. This method was used to quantify uranium in different tissue samples obtained over a period of 90 days following a single intraperitoneal uranium dose of 0.1 mg kg-1 in rats.

Effect of alcohol coadministration on the plasma and brain concentrations of cocaine in rats.

We investigated the effect of intravenous alcohol coadministration on the pharmacokinetics of cocaine in awake, freely moving rats using the microdialysis technique. Alcohol coadministration resulted in faster rate of cocaine absorption after intraperitoneal administration leading to higher cocaine plasma concentration. The higher plasma cocaine concentration resulted in a proportional increase in the cocaine brain extracellular fluid concentration. However, cocaine brain extracellular fluid/plasma distribution ratio, determined from the ratio of the corresponding cocaine area under the concentration-time curves, was not affected by alcohol coadministration. Cocaethylene was detected only after administration of cocaine + alcohol. The brain extracellular fluid/plasma distribution ratio of cocaethylene was similar to that of cocaine. The higher cocaine concentrations in plasma and brain extracellular fluid, in addition to the formation of the pharmacologically active metabolite cocaethylene are, at least partially, responsible for the increased cocaine effects produced after administration of this drug combination.

Sensitive and specific high-performance liquid chromatographic assay with ultraviolet detection for the determination of cocaine and its metabolites in rat plasma.

A sensitive, specific and precise HPLC-UV assay was developed to quantitate cocaine (COC) and its metabolites benzoylecgonine (BE), norcocaine (NC) and cocaethylene (CE) in rat plasma. After adding 50 microl of the internal standard solution (bupivacaine, 8 microg/ml) and 500 microl of Sørensen's buffer (pH 6) to 100 microl of rat plasma sample, the mixture was extracted with 10 ml of chloroform. The organic layer was transferred to a clean test tube and was evaporated under nitrogen. The residue was reconstituted in 100 microl of mobile phase and 35 microl was injected onto the HPLC column. The mobile phase consisted of methanol-acetonitrile-50 mM monobasic ammonium phosphate (5:7:63, v/v/v) and was maintained at a flow-rate of 0.4 ml/min. Separation of COC and its metabolites was achieved using a Supelcosil ABZ+plus deactivated reversed-phase column (250x2.1 mm I.D., 5 microm). Calibration curves were linear over the range of 25-5000 ng/ml for COC and its three metabolites. The absolute extraction efficiencies for BE, COC, NC, CE and bupivacaine were 56.6%, 78.6%, 61.1%, 76.4% and 67.0%, respectively. COC and its metabolites were stable in mobile phase for 24 h at room temperature and in rat plasma for 2 weeks at -20degrees C. The limits of detection for BE, COC, NC and CE were 20, 24, 15 and 12.9 ng/ml, respectively. These values correspond to 0.70, 0.84, 0.525 and 0.452 ng of the according compound being injected on column. The within-day coefficient of variation for the four compounds ranged from 3.0% to 9.9% while the between-day precision varied from 3.6% to 14%. This method was used to analyze rat plasma samples after administration of COC alone and in combination with alcohol. The pharmacokinetic profiles of COC and its metabolites in these rats are also described.

Cocaine and alcohol interactions in naive and alcohol-pretreated rats.

The interaction between cocaine (COC) and ethyl alcohol (ALC) was investigated in ALC-naive and ALC-pretreated rats. In each group, COC (30 mg/kg ip) was administered 15 min after administration of ALC (3 g/kg, by gavage) or normal saline (NS), in a balanced cross-over experimental design. Cocaethylene (CE) was detected only in the rat plasma when ALC was administered with COC. In the ALC-naive rats, COC area under the plasma concentration-time curve (AUC) and maximum plasma concentration (Cpmax) were significantly higher after administration of COC+ALC compared with after administration of COC+NS. However, COC half-life (t1/2) was not different after the two treatments, indicating that the higher COC Cpmax and AUC after COC+ALC primarily resulted from enhanced COC absorption. In the ALC-pretreated rats, COC AUC, Cpmax, and t1/2 were not different after administration of COC+ALC or COC+NS. However, COC t1/2 in the ALC-pretreated rats after COC+ALC and COC+NS were significantly longer than the corresponding COC t1/2 in the ALC-naive rats. This indicates that repeated ALC exposure significantly slows the rate of COC elimination. In the ALC-pretreated rats, CE AUC was significantly larger, and t1/2 was significantly longer than CE AUC and t1/2 in the ALC-naive rats. This indicates that previous ALC exposure inhibits CE elimination and may increase the fraction of COC dose metabolized to CE. Benzoylecgonine formation was significantly reduced, and its t1/2 was significantly prolonged after administration of COC+ALC in the ALC-naive and the ALC-pretreated rats. These results suggest that the combined abuse of COC and ALC leads to higher plasma COC concentrations that can lead to augmentation of cocaine effects in addition to the COC-like effects of CE. Also, repeated use of ALC slows the rate of COC elimination after administration of COC either alone or in combination with ALC, resulting in higher and prolonged COC plasma concentrations that can potentiate COC effects and toxicities. This higher COC concentrations in addition to the formation of CE are, at least partially, responsible for the serious consequences associated with the combined abuse of COC and ALC.

beta-adrenergic antagonism alters the behavioral and neurochemical responses to cocaine.

The effects of the beta-adrenergic antagonist propranolol on the locomotor stimulating, neurochemical, and reinforcing effects of cocaine were examined in rats. In Experiment 1, propranolol (1, 3 and 10 mg/kg, IP) produced a dose-dependent increase in the motor stimulant effects of cocaine without affecting basal motor activity. Atenolol, a peripherally restricted beta 1 antagonist, and (+) propranolol, the inactive isomer of propranolol, did not alter cocaine-induced locomotion. In Experiment 2, propranolol was shown to augment significantly the increase in extracellular dopamine content in the nucleus accumbens that accompanies a cocaine challenge. Experiment 3 demonstrated that propranolol produced a dose-dependent decrease in cocaine self-administration. Atenolol (10 mg/kg, IP) reduced cocaine self-administration but to a much lesser extent than propranolol. Experiment 4 demonstrated that coadministration of propranolol and cocaine did not alter the levels of cocaine in the brain and plasma achieved by cocaine administration alone. These data suggest that the blockade of beta-adrenergic receptors potentiates cocaine-induced elevation of dopamine transmission in the nucleus accumbens, which is associated with an increase in cocaine-induced motor activity and a decrease in cocaine self-administration.

Competitive inhibition of zidovudine clearance by probenecid during continuous coadministration.

The pharmacokinetics of zidovudine in the rabbit were studied during coadministration of probenecid at two infusion rates. Each animal (n = 6) served as its own control during an initial 8-hr infusion of zidovudine. In the second 8-hr infusion period, probenecid was coadministered with zidovudine. Urine samples were collected by bladder flush hourly for 19 hr. Plasma samples were taken at the midpoint of the urine collection interval and at predetermined intervals for 3 hr postinfusion. Plasma concentrations of zidovudine reached steady state during control periods but showed incomplete attainment of steady state during the infusions of probenecid at the higher rate. Total and renal clearance of zidovudine were reduced by 24.0 +/- 4.0 and 20.7 +/- 15%, respectively, during low-dose probenecid treatment and 48.9 +/- 7.4 and 55.7 +/- 3.4%, respectively, with high-dose probenecid treatment. Plasma probenecid concentrations during low-dose and high-dose infusion were 56.9 +/- 12 and 248 +/- 42 micrograms/ml. Postinfusion data showed that the zidovudine terminal half-life during high-dose probenecid treatment was longer than that with low-dose probenecid treatment (58.2 +/- 4.6 vs 39.0 +/- 9.1 min). The volume of distribution of zidovudine also decreased (1.76 +/- 0.27 vs. 1.10 +/- 0.095 L/kg) as a result of probenecid coadministration. The results are consistent with competitive inhibition of renal and nonrenal clearances. A drug interaction model relating zidovudine clearances to plasma probenecid concentrations was derived. Michaelis-type constants for probenecid inhibition of zidovudine renal and nonrenal clearances were 73 and 55 micrograms/ml, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

A sensitive and specific liquid-chromatographic assay for determination of ganciclovir in plasma and urine and its application to pharmacokinetic studies in the rabbit.

A liquid-chromatographic assay for the analysis of ganciclovir in plasma and urine is described. This assay involves the use of acyclovir, an antiviral drug structurally related to ganciclovir, as the internal standard. A two-step sample preparation method is used. After protein is precipitated with acetonitrile and the addition of diethyl ether, ganciclovir and the internal standard are back extracted into a small volume of aqueous ammonium phosphate, taking advantage of their relatively high water solubility. This isocratic method is specific and sufficiently sensitive to allow quantification of ganciclovir throughout the entire range of concentrations observed during therapeutic use of this antiviral drug. There was no interference from various over-the-counter and prescription drugs often prescribed to patients most likely to receive ganciclovir therapy. This assay was used to analyze plasma and urine samples obtained after intravenous administration of ganciclovir to rabbits. Biexponential decay of ganciclovir plasma concentration-time and urinary excretion rate-time profiles was observed, with a mean distribution half-life of 15.8 min and an elimination half-life of 96 min. The mean renal clearance, 9.0 ml/min per kg, exceeds the glomerular filtration rate in the rabbit, indicating that ganciclovir is actively secreted in the renal tubule. Similar results were obtained by determining the renal clearance at steady state during constant-rate intravenous infusion of ganciclovir.

Modeling the enhanced uptake of zidovudine (AZT) into cerebrospinal fluid. 1. Effect of probenecid.

The kinetics of zidovudine (AZT) distribution into rabbit cerebrospinal fluid (CSF) were studied during continuous infusion of AZT and after iv bolus administration. The CSF/plasma steady-state AZT concentration ratio was 0.192 +/- 0.003. That this ratio is less than unity, and the clearance from the CSF due to bulk flow is much smaller than the total CSF-to-plasma clearance, suggests active CSF-to-plasma transport of AZT. Probenecid coadministration significantly enhances AZT distribution into CSF when plasma and CSF concentrations of AZT are at steady state during continuous infusion of AZT or at a transient steady state after a single iv bolus dose administration. A linear pharmacokinetic model which describes the distribution of AZT into CSF and relates intercompartmental clearances between CSF and plasma was developed and was used to analyze the results. This analysis showed that probenecid enhances the distribution of AZT into the CSF by its effect on clearances between plasma and CSF. The CSF exit-rate constant for AZT estimated during probenecid coadministration was significantly different from controls. Probenecid coadministration resulted in a 36% reduction in the CSF-to-plasma transfer-rate constant. Reduction in the CSF to plasma clearance of AZT is probably due to the effect of probenecid on the active CSF-to-plasma transport of AZT. The model analysis also indicates that probenecid may have increased the plasma to CSF clearance of AZT. There was an increasing trend in the steady-state CSF/plasma AZT concentration ratio with increasing plasma probenecid concentrations. These results are consistent with probenecid competitively inhibiting the CSF-to-plasma transport of AZT.

Probenecid inhibits the metabolic and renal clearances of zidovudine (AZT) in human volunteers.

The effect of probenecid on the disposition of AZT was investigated in a pilot study in two healthy volunteers. The pharmacokinetics of AZT were examined after a single oral dose of 200 mg with and without probenecid coadministration in a balanced crossover study. Administration of 500 mg probenecid every 6 hr prior to and during AZT dosing resulted in an increase in the average AUCAZT from 89 micrograms.min/ml (control) to 191 micrograms.min/ml during probenecid treatment. This was manifested by a corresponding decrease in CLTOT/F, which is attributed to the inhibitory effect of probenecid on the glucuronidation and renal excretion of AZT. Average CLR and CLTOT/F of AZT decreased from 4.76 and 28.7 to 2.98 and 14.1 ml/min/kg during control and probenecid treatment, respectively. AZT glucuronidation was affected to a greater extent than its renal excretion, as reflected by the decreased ratio of GAZT/AZT urinary recoveries. The terminal half-life of AZT was slightly longer during probenecid administration. That only a small change in the half-life occurred indicates that probenecid also reduced the volume of distribution of AZT. The CLR of GAZT decreased from an average of 11.3 ml/min/kg (control) to 2.63 ml/min/kg during probenecid treatment, resulting in a greater than 3.5-fold increase in AUCGAZT. Probenecid did not affect the blood/plasma distribution or the plasma protein binding of AZT. These preliminary findings suggest that it may be possible to maintain effective plasma AZT concentrations in AIDS patients receiving a reduced daily dose, in combination with probenecid.

Effect of probenecid on the renal and nonrenal clearances of zidovudine and its distribution into cerebrospinal fluid in the rabbit.

The effect of probenecid on the renal excretion of zidovudine (3'-azido-3'-deoxythymidine; AZT) and its distribution into CSF was studied in the rabbit. Although probenecid is chemically unrelated to AZT, it has been shown that probenecid inhibits AZT elimination in Acquired Immunodeficiency Syndrome (AIDS) patients. The effect of probenecid on the renal clearance of AZT after an iv bolus dose was studied in crossover experiments in the absence (control) and the presence of a continuous iv infusion of probenecid. Probenecid coadministration increased the AZT plasma AUC by 70%, by proportionally decreasing the total body clearance. The renal clearance decreased by 50%. The effect of probenecid on the renal clearance of AZT at steady state was studied by measuring the renal clearance of AZT at different steady-state plasma probenecid concentrations. The renal clearance of AZT decreased with increasing probenecid concentration, suggesting competitive inhibition of the secretion of AZT in the renal tubule. The relationship between AZT renal clearance and probenecid plasma concentrations, during and after probenecid iv infusion in conscious and in anesthetized uretercannulated rabbits, showed hysteresis, indicating that probenecid plasma concentration is different from the concentration at the site of interaction. This suggests the presence of an effect compartment for the inhibition of AZT renal excretion by probenecid. The effect of probenecid on the CSF distribution of AZT was also studied in the rabbit. Probenecid coadministration caused a sevenfold increase in the AZT AUCCSF in probenecid-treated rabbits when compared with controls.(ABSTRACT TRUNCATED AT 250 WORDS)

A sensitive liquid-chromatographic method for determination of 3'-azido-3'-deoxythymidine (AZT) in plasma and urine.

We describe a liquid-chromatographic assay for AZT in human plasma and urine. This assay involves the use of two internal standards, allowing reference of AZT peaks to the appropriate internal standard, the choice depending on the range of concentrations encountered. This method is isocratic, specific, sensitive enough to allow quantification of AZT in concentrations observed clinically, and requires only 13 min of chromatographic time. We saw no interference from various over-the-counter and prescription drugs often used in treating the infectious complications of AIDS.