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

Comparative Pharmacokinetic Study for Linezolid and Two Novel Antibacterial Oxazolidinone Derivatives in Rabbits: Can Differences in the Pharmacokinetic Properties Explain the Discrepancies between Their In Vivo and In Vitro Antibacterial Activities?

This is a comparative pharmacokinetics study of linezolid (Lzd), and two novel oxazolidinone antibacterial agents—PH027 and PH051—in rabbits to determine if the discrepancy between the in vitro and in vivo activities of the novel compounds is due to pharmacokinetic factors. The pharmacokinetics after IV and oral administration, plasma protein binding and tissue distribution for the three compounds were compared. The elimination half-lives were 52.4 ± 6.3, 68.7 ± 12.1 and 175 ± 46.1 min for Lzd, PH027 and PH051, respectively. The oral bioavailability for Lzd, PH027 and PH051 administered as suspension were 38.7%, 22.1% and 4.73%, which increased significantly when administered as microemulsion to 51.7%, 72.9% and 13.9%. The plasma protein binding were 32–34%, 37–38% and 90–91% for Lzd, PH027 and PH051. The tissue distribution for PH027 and PH051 in all investigated tissues were higher than that for Lzd. It can be concluded that the lower bioavailability of PH027 and PH051 compared to Lzd when administered as suspension is the main cause of their lower in vivo activity, despite their comparable in vitro activity. Differences in the other pharmacokinetic characteristics cannot explain the lower in vivo activity. The in vivo activity of the novel compounds should be re-evaluated using formulations with good oral bioavailability.

A validated UPLC-MS/MS method for the analysis of linezolid and a novel oxazolidinone derivative (PH027) in plasma and its application to tissue distribution study in rabbits

OBJECTIVES

Linezolid is the first approved oxazolidinone antibacterial agent, whereas PH027 is a novel compound of the same class that exhibits good in vitro antibacterial activity. The objective of this study was to develop an UPLC-MS/MS assay for the analysis of linezolid and PH027 in plasma and to apply the method for comparative pharmacokinetic and tissue distribution studies of both compounds.

METHOD

Plasma samples and calibrators were extracted with diethyl ether after addition of the internal standard solution. After evaporation of the ether layer, the residue was reconstituted in mobile phase and injected into UPLC-MS/MS. The mobile phase consisted of 2mM ammonium acetate buffer solution and acetonitrile (70:30) at a flow rate of 0.2ml/min. Separation was achieved using UPLC BEH C₁₈ column, and quantitative determination of the analytes was performed using multiple-reaction monitoring (MRM) scanning mode. The method was validated by analyzing quality control tissue homogenate samples, and was applied to analyze tissue homogenate samples obtained following IV injections of linezolid and PH027 in rabbits.

RESULTS

The developed UPLC-MS/MS method was linear in the concentration range of 50-5000ng/ml. Validation of the method proved that the method's precision, selectivity and stability were all within the acceptable limits. Linezolid and PH027 concentrations were accurately determined in the quality control tissue homogenate samples, and analysis of samples obtained following IV administration of the two compounds showed that the tissue to plasma concentration ratio of PH027 was higher than that of linezolid probably due to its higher lipophilicity.

CONCLUSIONS

The developed UPLC-MS/MS method for the analysis of linezolid and PH027 in rabbit's plasma can accurately determine the concentrations of these compounds in different tissues.

Species Differences in the Metabolism and Disposition of Antiviral Nucleoside Analogues: 1. Acyclovir

The acyclic nucleoside analogue, acyclovir, is an antiviral drug with activity against the herpes group of DNA viruses. Clinically, it is used as a selective therapeutic agent for the treatment of herpes simplex and varicella-zoster virus infections. Studies on the disposition of acyclovir, during the course of its preclinical and clinical development, indicated significant species differences in the absorption, metabolism and elimination of the drug. Gastrointestinal absorption was adequate in dogs and in mice; but in rats and primates it was limited to less than 20% of the administered dose. Whereas in some species (mice, rats, and dogs), acyclovir was virtually unmetabolized, significant biotransformation was apparent in guinea pigs, rabbits, and some primates. Acyclovir tissue distribution was extensive and indicated few differences across species. This review summarizes diverse studies on acyclovir absorption, metabolism, and disposition in different species, including humans, and indicates the relevance and importance of such studies in drug development.

Species Differences in the Metabolism and Disposition of Antiviral Nucleoside Analogues: 2. Zidovudine

Preclinical and clinical studies on the disposition of zidovudine, a thymidine analogue with potent activity against human immunodeficiency virus, identified significant species differences in the metabolism and elimination of the drug. Zidovudine was extensively metabolized to the 5′-O-glucuronide in man and other primates. Rabbits and dogs were intermediate in their extent of biotransformation to the glucuronide conjugate, whereas rats and mice excreted the drug largely unchanged. Decreased metabolism was compensated by increased renal elimination, such that plasma elimination phase half-lives for zidovudine were similar (0.6–1.1 h) in all species. Rapid and extensive absorption and considerable penetration into tissues were also observed for all species studied. Only in the brain and testes were drug levels less than in plasma, although effective antiviral concentrations of zidovudine were achieved in brain and CSF. This review summarizes the variety of studies of the absorption, distribution, metabolism, and elimination of zidovudine in several species, including humans.

Advances in brain targeting and drug delivery of anti-HIV therapeutic agents

INTRODUCTION

Human immunodeficiency virus (HIV) is a neurotropic virus that enters the central nervous system (CNS) early in the course of infection. Although antiretroviral drugs are able to eliminate the majority of the HIV virus in the bloodstream, however, no specific treatment currently exist for CNS infections related to HIV. This is mainly attributed to the poor penetrability of antiretroviral therapy across the blood-brain barrier (BBB), and the protective nature of the BBB. Therefore, in order to increase the efficacy of anti-HIV drugs, novel drug delivery methodologies that can exhibit activity in the CNS are most needed and warranted.

AREAS COVERED

In this review article, the authors discussed the challenges with delivering drugs to the brain especially under HIV infection pathophysiology status. Also, they discussed the approaches currently being investigated to enhance brain targeting of anti-HIV drugs. A literature search was performed to cover advances in major approaches used to enhance drug delivery to the brain.

EXPERT OPINION

If drugs could reach the CNS in sufficient quantity by the methodologies discussed, mainly through intranasal administration and the utilization of nanotechnology, this could generate interest in previously abandoned therapeutic agents and enable an entirely novel approach to CNS drug delivery.

CSF penetration by antiretroviral drugs

Severe HIV-associated neurocognitive disorders (HAND), such as HIV-associated dementia, and opportunistic CNS infections are now rare complications of HIV infection due to comprehensive highly active antiretroviral therapy (HAART). By contrast, mild to moderate neurocognitive disorders remain prevalent, despite good viral control in peripheral compartments. HIV infection seems to provoke chronic CNS injury that may evade systemic HAART. Penetration of antiretroviral drugs across the blood-brain barrier might be crucial for the treatment of HAND. This review identifies and evaluates the available clinical evidence on CSF penetration properties of antiretroviral drugs, addressing methodological issues and discussing the clinical relevance of drug concentration assessment. Although a substantial number of studies examined CSF concentrations of antiretroviral drugs, there is a need for adequate, well designed trials to provide more valid drug distribution profiles. Neuropsychological benefits and neurotoxicity of potentially CNS-active drugs require further investigation before penetration characteristics will regularly influence therapeutic strategies and outcome.

Overton's rule helps to estimate the penetration of anti-infectives into patients' cerebrospinal fluid

In 1900, Ernst Overton found that the entry of anilin dyes through the cell membranes of living cells depended on the lipophilicity of the dyes. The brain is surrounded by barriers consisting of lipid layers that possess several inward and outward active transport systems. In the absence of meningeal inflammation, the cerebrospinal fluid (CSF) penetration of anti-infectives in humans estimated by the ratio of the area under the concentration-time curve (AUC) in CSF (AUC(CSF)) to that in serum (AUC(CSF)/AUC(S)) correlated positively with the lipid-water partition coefficient at pH 7.0 (log D) (Spearman's rank correlation coefficient r(S) = 0.40; P = 0.01) and negatively with the molecular mass (MM) (r(S) = -0.33; P = 0.04). The ratio of AUC(CSF) to the AUC of the fraction in serum that was not bound (AUC(CSF)/AUC(S,free)) strongly correlated with log D (r(S) = 0.67; P < 0.0001). In the presence of meningeal inflammation, AUC(CSF)/AUC(S) also correlated positively with log D (r(S) = 0.46; P = 0.002) and negatively with the MM (r(S) = -0.37; P = 0.01). The correlation of AUC(CSF)/AUC(S,free) with log D (r(S) = 0.66; P < 0.0001) was as strong as in the absence of meningeal inflammation. Despite these clear correlations, Overton's rule was able to explain only part of the differences in CSF penetration of the individual compounds. The site of CSF withdrawal (lumbar versus ventricular CSF), age of the patients, underlying diseases, active transport, and alterations in the pharmacokinetics by comedications also appeared to strongly influence the CSF penetration of the drugs studied.

Oxidative stress and toxicity induced by the nucleoside reverse transcriptase inhibitor (NRTI)--2',3'-dideoxycytidine (ddC): relevance to HIV-dementia

Human immunodeficiency virus dementia (HIVD) is the most common form of dementia occurring among young adults. In HIVD, neuronal cell loss occurs in the absence of neuronal infection. With the advent of highly active anti-retroviral therapy (HAART), the incidence of HIVD has drastically reduced, though prevalence of milder forms of HIVD continues to rise. Though these agents have been used successfully in suppressing viral production, they have also been associated with a number of side effects. Here we examine the possible role of NRTIs, in particular 2',3'-dideoxycytidine (ddC), in the neuropathology of HIVD. Synaptosomes and isolated mitochondria treated and incubated for 6 h with CSF-achievable concentrations of ddC, i.e., 6-11 ng/ml, were found to show a significant increase in oxidative stress with 40 nM ddC as measured by protein carbonyls and 3-nitrotyrosine (3NT), effects that were not observed in the more tolerable NRTI, 3TC. Protection against protein oxidation induced by ddC was observed when brain mitochondria were isolated from gerbils 1 h after injection i.p. with the brain accessible antioxidant and glutathione mimetic, tricyclodecan-9-yl-xanthogenate (D609). In addition, there is a significant reduction in the levels of anti-apoptotic protein Bcl-2 and a significant increase in cytochrome c release and also a significant increase in the expression of pro-apoptotic protein caspase-3 after mitochondria were treated with 40 nM ddC. The results reported here show that ddC at 40 nM can induce oxidative stress, cause the release of cytochrome c, and in addition, reduce the levels of anti-apoptotic proteins, increase the levels of pro-apoptotic proteins, thereby increasing the possibility for induction of apoptosis. These findings are consistent with the notion of a possible role of the NRTIs, and in particular, ddC, in the mechanisms involved in HIVD.

Factors affecting delivery of antiviral drugs to the brain

Although the CNS is in part protected from peripheral insults by the blood-brain barrier and the blood-cerebrospinal fluid barrier, a number of human viruses gain access to the brain, replicate within this organ, or sustain latent infection. The efficacy of antiviral drugs towards the cerebral viral load is often limited as both blood-brain interfaces impede their cerebral distribution. For polar compounds, the major factor restricting their entry lies in the tight junctions that occlude the paracellular pathway across these barriers. For compounds with more favourable lipid solubility properties, CNS penetration will be function of a number of physicochemical factors that include the degree of lipophilicity, size and ability to bind to protein or red blood cells, as well as other factors inherent to the vascular and choroidal systems, such as the local cerebral blood flow and the surface area available for exchange. In addition, influx and efflux transport systems, or metabolic processes active in both capillary endothelial cells and choroid plexus epithelial cells, can greatly change the bioavailability of a drug in one or several compartments of the CNS. The relative importance of these various factors with respect to the CNS delivery of the different classes of antiviral drugs is illustrated and discussed.

Effect of Probenecid and Quinidine on the Transport of Alovudine (3'-Fluorothymidine) to the Rat Brain Studied by Microdialysis

Microdialysis was used to sample extracellular unbound concentrations of alovudine in order to study the influence of well-known transport inhibitors (probenecid and quinidine) on the transport of alovudine between the blood and the brain extracellular fluid or whole brain tissue. The AUC (area under the time versus concentration curve) ratio brain extracellular fluid/serum was 0.17+/-0.036 after a subcutaneous injection of alovudine 25 mg/kg in rats treated with probenecid 25 mg/kg subcutaneous (n=5), which was not significantly different from the control group (AUC ratio 0.24+/-0.039). Perfusion through the microdialysis probe with probenecid 100 microM (n=4) also had no effect on the brain extracellular fluid/serum AUC ratio after alovudine 25 mg/kg subcutaneous. The AUC ratio brain extracellular fluid/serum was 0.085+/-0.009 after subcutaneous injection of alovudine 25 mg/kg in rats treated with quinidine 25 mg/kg intraperitoneally (n=8), which was significantly lower than the control group. However, the whole brain tissue concentration was not significantly different between control rats (n=5) and rats treated with quinidine (n=4) 1 hr after subcutaneous injection of alovudine 25 mg/kg (brain to serum ratios being 0.11+/-0.006 and 0.10+/-0.005 respectively). Finally, the microdialysis recovery of alovudine increased with increasing concentrations (10, 50, 250, 1250 microM) of alovudine in the perfusion fluid. The recovery of alovudine was increased in quinidine-treated rats but not in those given probenecid. Thus, probenecid does not significantly influence the concentration gradient of alovudine over the blood-brain barrier in the rat after systemic or after local administration, while quinidine lowered brain extracellular fluid concentration of alovudine, but not total brain tissue concentration. The mechanism behind this phenomenon is not yet known.

Pain and the blood-brain barrier: obstacles to drug delivery

Delivery of drugs across the blood-brain barrier has been shown to be altered during pathological states involving pain. Pain is a complex phenomenon involving immune and centrally mediated responses, as well as activation of the hypothalamic-pituitary-adrenal axis. Mediators released in response to pain have been shown to affect the structure and function of the blood-brain barrier in vitro and in vivo. These alterations in blood-brain barrier permeability and cytoarchitecture have implications in terms of drug delivery to the central nervous system, since pain and inflammation have the capacity to alter drug uptake and efflux across the blood-brain barrier. An understanding of how blood-brain barrier and central nervous system drug delivery mechanisms are altered during pathological conditions involving pain and/or inflammation is important in designing effective therapeutic regimens to treat disease.

Choroid plexus controls brain availability of anti-HIV nucleoside analogs via pharmacologically inhibitable organic anion transporters

OBJECTIVE

In AIDS, early suppression of the viral load in the central nervous system is critical for the efficacy of antiretroviral therapy, in order to prevent the emergence of a reservoir of resistant strains of virus, and brain impairment in late stages of the infection. The blood-cerebrospinal fluid (CSF) interface (i.e. the choroidal epithelium) constitutes the most direct route to reach the ventricular meningeal and perivascular infected macrophages, and may modulate the cerebral biodisposition of antiretroviral drugs through various transport systems. Our aim was to address nucleoside drug transfer specifically across the blood-CSF interface, and identify the possible mechanisms involved in their transport.

METHODS

Drug influx and efflux were measured using an in vitro cellular model that reproduces the barrier and transport properties of the blood-CSF interface in vivo. Transport mechanisms were investigated by competition studies.

RESULTS

The CSF influx rate of zidovudine was the highest, although moderate, followed by that of stavudine. The permeability coefficients of the other drugs tested were low. Zidovudine influx into the CSF is independent of thymidine transport systems, and more importantly is limited by an efflux mechanism. This efflux involves an apical (CSF-facing) carrier belonging to the solute carrier (Slc) 22 family of organic anion transporters, and can be inhibited by a therapeutic concentration of benzbromarone.

CONCLUSIONS

The demonstration and characterization of this efflux mechanism is the basis for the development of specific inhibitory agents in view to increase the delivery of antiretroviral nucleoside analogs to the brain.

The impact of efflux transporters in the brain on the development of drugs for CNS disorders

The development of drugs to treat disorders of the CNS requires consideration of achievable brain concentrations. Factors that influence the brain concentrations of drugs include the rate of transport into the brain across the blood-brain barrier (BBB), metabolic stability of the drug, and active transport out of the brain by efflux mechanisms. To date, three classes of transporter have been implicated in the efflux of drugs from the brain: multidrug resistance transporters, monocarboxylic acid transporters, and organic ion transporters. Each of the three classes comprises multiple transporters, each of which has multiple substrates, and the combined substrate profile of these transporters includes a large number of commonly used drugs. This system of transporters may therefore provide a mechanism through which the penetration of CNS-targeted drugs into the brain is effectively minimised. The action of these efflux transporters at the BBB may be reflected in the clinic as the minimal effectiveness of drugs targeted at CNS disorders, including HIV dementia, epilepsy, CNS-based pain, meningitis and brain cancers. Therefore, modulation of these efflux transporters by design of inhibitors and/or design of compounds that have minimal affinity for these transporters may well enhance the treatment of intractable CNS disorders.

Multiple interactions of cimetidine and probenecid with valaciclovir and its metabolite acyclovir

The effects of probenecid and cimetidine on the pharmacokinetics of valaciclovir and its metabolite acyclovir have been investigated. Twelve healthy male volunteers participated in this open single-dose study with a four-way-crossover randomized and balanced design. At the first of four administrations, volunteers in four groups received 1 g of valaciclovir alone, valaciclovir with 1 g of probenecid, valaciclovir with 800 mg of cimetidine, or valaciclovir with a combination of probenecid and cimetidine. At three subsequent administrations, drug regimens were alternated among groups so that each group received each regimen. Probenecid and cimetidine increased the mean maximum concentrations in serum (C(max)) of valaciclovir by 23 and 53% and the areas under the concentration-time curves (AUC) for valaciclovir by 22 and 73%, respectively; probenecid and cimetidine also increased the mean acyclovir C(max) by 22 and 8% and its AUC by 48 and 27%, respectively. The combination had a greater effect than either drug alone. Their effects may be due to competitive inhibition of membrane transport of valaciclovir and acyclovir in the liver and kidney. Neither cimetidine nor probenecid affected the absorption of valaciclovir. Both probe drugs reduced the rate of valaciclovir metabolism but not its extent. These pharmacokinetic modifications did not affect the tolerability of valaciclovir.

In vivo specific binding characteristics and pharmacokinetics of a 1,4-dihydropyridine calcium channel antagonist in the senescent mouse brain

PURPOSE

To characterize the in vivo specific binding and pharmacokinetics of a 1,4-dihydropyridine (DHP) calcium channel antagonist, PN 200-110, in the senescent brain, using senescence-accelerated prone mice (SAMP8) and senescence-resistant mice (SAMR1).

METHODS

Blood, brain, and heart samples were taken periodically from SAMR1 and SAMP8 following intravenous injection of (+)-[3H]PN 200-110, and the concentration of (+)-[3H]PN 200-110 in the plasma and tissues was determined. In addition, the in vivo specific binding of (+)-[3H]PN 200-110 in the brains of SAMRI and SAMP8 was measured periodically after intravenous injection of the radioligand.

RESULTS

There was very little significant difference between SAMR1 and SAMP8 in terms of the half-life (t(1/2)), total body clearance (CL(tot)), steady-state volume of distribution (Vd(ss)). and AUC for the plasma concentration of (+)-[3H]PN 200-110 after intravenous injection of the radioligand. The brain concentration (AUCbrain) for (+)-[3H]PN 200-110 and the brain/plasma AUC ratio (AUCbrain/AUCplasma) were significantly lower in SAMP8 than in SAMR1, and the heart concentration (AUCheart) and the heart/plasma AUC ratio (AUCheart/AUCplasma) were similar in both strains. Also, the brain/plasma unbound AUC ratio (AUCbrain/AUCplasma-free) for (+)-[3H]PN 200-110 was significantly lower in SAMP8 than in SAMRI. The in vivo specific binding (AUCspecific binding, maximal number of binding sites: Bmax) of (+)-[3H]PN 200-110 was significantly lower in brain particulate fractions of SAMP8 than SAMR1.

CONCLUSIONS

The concentration and in vivo specific binding of (+)-[3H]PN 200-110 was significantly reduced in the senescent brain. The simultaneous analysis of the concentrations of centrally acting drugs and the in vivo specific binding in the brain in relation to their pharmacokinetics may be valuable in evaluating their CNS effects.

Effect of application volume of ethanol-isopropyl myristate mixed solvent system on permeation of zidovudine and probenecid through rat skin

Permeation of zidovudine (AZT) and probenecid from an ethanol-isopropyl myristate (IPM) mixed system through rat skin was studied in a finite system. Several volume sizes of the ethanol-IPM mixed systems containing AZT and probenecid, both as suspensions, were applied on the skin of the hairless rat using a vertical glass cell, and the fractions of the drugs permeated in 8 hr Q%,8 hr were determined. For the systems containing 40% ethanol, the Q%,8 hr value decreased with the reduction of volume of the system applied, and the decreasing profile was similar to that calculated on the assumption that the permeability of the drug does not change with the volume of the sample applied. On the other hand, in the systems containing 10% or 20% ethanol, the Q%,8 hr value showed a maximum when a specific volume of the sample was applied. Therefore, the effect of sample volume on the Q%,8 hr value was different between the 40% ethanol-IPM system and the 10% or 20% ethanol-IPM system. Following pretreatment of the skin with 0.105 ml/cm2 of drug-free 40% ethanol-IPM for 2 hr, several volume sizes of 10% ethanol-IPM systems containing the drugs were applied on the skin to explain why the different profiles were observed in the system containing 10% or 20% ethanol. The results for pretreated skin suggest that the amount of ethanol in the systems with low ethanol concentration and small application volume is too small to exert an effect that enhances permeation of the drugs. In those systems, the integrated effect of ethanol on the skin would be important for the enhancing effect. Total volume, as well as concentration, of an enhancer should be set precisely in designing an efficient transdermal delivery system.

Investigation of distribution, transport and uptake of anti-HIV drugs to the central nervous system

The distribution of currently available anti-HIV drugs into the CNS is reviewed with a focus on transport mechanisms. Among these drugs, nucleoside analogs are most well studied for their CNS distribution. The average reported values of the CSF/plasma steady-state concentration or corresponding AUC ratios are 0.23 (AZT), 0.06 (ddI), 0.04 (ddC), 0.49 (d4T), and 0.08 (3TC). Active efflux transport out of the CNS appears to be a predominant mechanism limiting nucleoside access to the CNS, although poor penetration may contribute to some extent for some polar nucleosides. The nature of the efflux pump for these drugs is speculated to be MRP-like transporter(s) in blood-brain and blood-CSF barriers. For non-nucleoside and protease inhibitors, much research remains to be done on the extent, time course, and mechanisms of their CNS distribution. The CNS penetration of some protease inhibitors is restricted by P-glycoprotein. A better understanding of transport mechanisms of anti-HIV drugs in the CNS is essential to develop approaches to enhance CNS delivery of available drugs and to identify new drugs less subject to active efflux transporter(s) in the CNS.

Brain pharmacokinetics and in vivo receptor binding of 1,4-dihydropyridine calcium channel antagonists

Brain pharmacokinetics of 1,4-dihydropyridine (DHP) calcium channel antagonists and their in vivo receptor binding in mice were characterized. The area under the concentration vs time curve (AUCbrain) for [3H]nifedipine, [3H]nimodipine and [3H]PN 200-110 in mouse brain after intravenous injection was higher than that for [3H]amlodipine. Brain/plasma concentration ratios (AUCbrain/AUCplasm) for [3H]nimodipine and [3H]PN 200-110 were 3 to 5 times higher than those for [ H]nifedipine and [3H]amlodipine. Further, brain/heart concentration ratios (AUCbrain/AUCheart) for [3H]nifedipine, [3H]nimodipine and [3H]PN 200-110 were about 20 times higher than the ratio for [3H]amlodipine. A significant amount of specific binding in particulate fractions of mouse brain was detected in vivo by intravenous injection of [3H]nifedipine, [3H]nimodipine and [3H]PN 200-110 but not [3H]amlodipine. These data suggest that [3H]nifedipine, [3H]nimodipine and [3H]PN 200-110 are more extensively taken up into brain from plasma than [3H]amlodipine and bind to the receptor sites in brain parenchymal cells in a significant amount in vivo. In conclusion, the present simultaneous measurement of pharmacokinetics and in vivo receptor binding in mouse brain suggests an usefulness of calcium channel antagonists such as nimodipine in the pharmacotherapy of brain diseases.

Pharmacokinetics of zidovudine after rectal administration in human immunodeficiency virus-infected patients

We evaluated the pharmacokinetics of rectally administered zidovudine (ZDV) in 10 human immunodeficiency virus-infected adults. After rectal administration of an aqueous ZDV solution (250 mg of ZDV), mean peak ZDV levels were 1.3 +/- 0.7 micromol/liter (mean +/- standard deviation) versus 5.0 +/- 2.2 micromol/liter (P < 0.0001) after oral intake of a 250-mg ZDV capsule. The half-life at beta phase was 87.8 +/- 39.6 min for rectally administered ZDV versus 55.8 +/- 20.1 min (P = 0.035) for orally administered ZDV. The mean area under the concentration-time curve from 0 min to infinity was 232 +/- 181 micromol/liter x min after rectal administration versus 362 +/- 110 micromol/liter x min after oral intake. Although the two routes were not bioequivalent, ZDV was absorbed considerably after rectal administration, with a pharmacokinetic profile resembling that of a sustained-release device.

Application of microdialysis in pharmacokinetic studies

The objective of this review is to survey the recent literature regarding the various applications of microdialysis in pharmacokinetics. Microdialysis is a relatively new technique for sampling tissue extracellular fluid that is gaining popularity in pharmacokinetic and pharmacodynamic studies, both in experimental animals and humans. The first part of this review discusses various aspects of the technique with regard to its use in pharmacokinetic studies, such as: quantitation of the microdialysis probe relative recovery, interfacing the sampling technique with analytical instrumentation, and consideration of repeated procedures using the microdialysis probe. The remainder of the review is devoted to a survey of the recent literature concerning pharmacokinetic studies that apply the microdialysis sampling technique. While the majority of the pharmacokinetic studies that have utilized microdialysis have been done in the central nervous system, a growing number of applications are being found in a variety of peripheral tissue types, e.g. skin, muscle, adipose, eye, lung, liver, and blood, and these are considered as well. Given the rising interest in this technique, and the ongoing attempts to adapt it to pharmacokinetic studies, it is clear that microdialysis sampling will have an important place in studying drug disposition and metabolism.

Renal disposition and drug interaction screening of (-)-2'-deoxy-3'-thiacytidine (3TC) in the isolated perfused rat kidney

PURPOSE

Dideoxynucleoside bases are used for the treatment of acquired immune deficiency syndrome (AIDS), acting by inhibiting reverse transcriptase and preventing human immunodeficiency virus (HIV) replication. Currently, AZT (zidovudine), ddC (zalcitibine), and ddI (didanosine) are available to the medical community to prevent the onset of AIDS in HIV-infected individuals. 3TC (-)-2'-deoxy-3'-thiacytidine, lamivudine), a new dideoxynucleoside base, is currently undergoing Phase II/III trials, and has exhibited anti-HIV replication activity, a favorable adverse event safety profile, and is eliminated via renal mechanisms. Concomitantly administered drugs could potentiate the effects of 3TC due to interaction in the kidney.

METHODS

An isolated perfused rat kidney (IPK) technique was used to screen several clinically relevant drugs for potential interaction with 3TC. The following perfusions were performed: baseline 3TC; and 500 ng/mL 3TC with clinically relevant concentrations of AZT, ddC, ddI, probenecid, trimethoprim, sulfamethoxazole, ranitidine, and cimetidine.

RESULTS

Renal clearance of 3TC was nonlinear between 500 and 5000 ng/mL, decreasing from 3.06 to 1.74 mL/min. Excretion ratio also decreased, from 3.67 (500 ng/mL) to 2.49 (5000 ng/mL), consistent with a decrease in 3TC secretion. AZT, ddI, and ddC elicited no or minimal effects on 3TC elimination at the concentrations studied. However, trimethoprim caused significant reductions in 3TC elimination parameters: clearance and excretion ratio decreased to 1.25 mL/min and 1.43, respectively.

CONCLUSIONS

These results indicate that caution should be exercised when the combination of 3TC and trimethoprim are administered to AIDS patients.

Zidovudine transport in the rabbit brain during intravenous and intracerebroventricular infusion

The distribution of zidovudine (AZT) between plasma, brain extracellular fluid (ECF), and cerebrospinal fluid (CSF) was investigated in a crossover design study (n = 5) in unanesthetized rabbits. Drug was administered by intravenous (iv) and intracerebroventricular (icv) infusions at the same infusion rate (1.5 mg/h.kg). The concentrations of AZT in ECF and CSF were measured by HPLC with microdialysis sampling. Plasma concentrations of AZT were quantitated by HPLC. Following iv infusion, the ECF- and CSF-to-plasma concentration ratios at steady state (SS), were 0.19 +/- 0.05 and 0.29 +/- 0.06, respectively. These values were less than unity, indicating the existence of active transport processes for the transport of AZT from brain to plasma across the blood-brain barrier (BBB) or blood-CSF barrier (BCB). The transport processes were modeled by compartmental model analysis, and the results suggest that the transport efficiency of AZT across the BBB is asymmetric; that is, the efflux clearance was five times greater than the influx clearance. Similarly, the efflux clearance from CSF is three times larger than the influx clearance into CSF. The SS concentrations of AZT in brain ECF in the same animals that received an icv infusion of AZT in the crossover design study were approximately two orders of magnitude greater than those in animals following iv infusion at the same dosing rate. Nevertheless, the SS plasma concentrations of AZT were similar for both routes of administration (1.2 +/- 0.19 and 1.2 +/- 0.13 micrograms/mL for iv and icv routes, respectively), confirming that the brain is not an organ that exhibits first-pass metabolism under the present experimental conditions.

Brain distribution characteristics of xanthine derivatives and relation to their locomotor activity in mice

The relationship between the brain distribution and motor activity in mice of the xanthines, theophylline, enprofylline, 1-methyl-3-propylxanthine (MPX) and oxpentifylline was investigated. Their plasma protein binding and hydrophobicity were also examined. When these xanthines were administered orally, enprofylline and oxpentifylline had no effect on motor activity. While theophylline increased motor activity over 10 mg kg-1, MPX caused a decrease in such activity over 10 mg kg-1. The protein-binding behaviour varied among these xanthines and was closely related to their hydrophobicity, which is represented as a logarithmic partition coefficient (log PC). MPX had the highest hydrophobicity, while oxpentifylline had the lowest. Brain distribution characteristics varied among these xanthines, with the rank order of their brain penetration ratio, calculated as the ratio of brain to unbound plasma concentrations, being theophylline > oxpentifylline > MPX > enprofylline. The inhibition constants (Ki) for adenosine A1 receptors and cyclic 3',5'-adenosine monophosphate (cAMP)-phosphodiesterase (PDE) of these xanthines were 44.6 and 134, > 1000 and 112, 26.4 and 49, and > 1000 and 111 microM for theophylline, enprofylline, MPX, and oxpentifylline, respectively. These findings suggest that the lack of effects of enprofylline and oxpentifylline on motor activity is probably due to their low brain penetration ratio or low adenosine A1 affinity in comparison with theophylline. The decrease in the motor activity by MPX may be, in part, mediated by cAMP or adenosine.

Investigation of the distribution of EAB 515 to cortical ECF and CSF in freely moving rats utilizing microdialysis

A freely moving rat model was developed to study the CNS distribution of EAB 515 (S-alpha-amino-5-phosphonomethyl[1,1'-biphenyl]-3-propanoic acid). Microdialysis (MD) in the frontal cortex (FrC) and in the lateral ventricle (LV) of the rat brain was performed to measure the levels of EAB 515 in the cortical extracellular fluid (ECF) as well as in the cerebrospinal fluid (CSF). The femoral artery and femoral vein were cannulated for serial blood sampling and intravenous (i.v.) drug administration, respectively. EAB 515 was also administered via the intracerebroventricular (icv) route in a cross-over experiment. The in vivo recovery of EAB 515 across the MD probes was determined by simultaneous retrodialysis (RD) performed using a hydroxylated analog of EAB 515, as the RD calibrator (RDC). An extremely sensitive and selective on-line HPLC system with native fluorescence detection was developed for the simultaneous analysis of EAB 515 and RDC in microdialysate samples from rat CSF and cortical ECF. Unbound concentrations of EAB 515 in the rat plasma were determined by direct injection of plasma ultrafiltrate onto the HPLC column. The validity of the use of RDC as the RD calibrator was demonstrated by comparing the results of zero-net flux (ZNF) analysis simultaneously in some experiments. After constant rate i.v. infusion in rats (n = 12) for 900 min, the average (S.D.) ratio of the levels of EAB 515 in the CSF to those in plasma (Ccsf.iv/Cp) was determined to be 17.7 (7.8)% and that in the cortex relative to plasma (Ccortex.iv/Cp) was 8.3 (4.8)%.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatobiliary disposition of 3'-azido-3'-deoxythymidine (AZT) in the rat: effect of phenobarbitone induction

Isolated liver with a recirculating perfusate was used to study 3'-azido-3'-deoxythymidine (AZT) disposition in phenobarbitone-pretreated rats at 68 microM AZT concentration in the reservoir. Clearance of AZT in the livers obtained from control animals was 0.42 +/- 0.01 (mean +/- s.d.) mL min-1/10 g liver. Over the study period of 105 min, 12.7 +/- 2.6% of the dose was excreted in bile and of this 95% was recovered as 3'-azido-3'-deoxy-5'-O-beta-D-glucopyranuronosylthymidine (GAZT). The amount of GAZT found in the perfusate after 105 min of liver perfusion was < 1% of the AZT dose introduced into the reservoir. Phenobarbitone pretreatment of rats resulted in a 5.5-fold increase of AZT clearance. In addition, the area under the perfusate concentration-time curve (AUC0-105 min) for 3'-amino-3'-deoxythymidine (AMT) and for a catabolite of unknown structure was increased 3- and 10-fold, respectively, and the amount of AZT dose excreted in the bile was nearly doubled. Thus phenobarbitone was capable of stimulating both detoxification of AZT to GAZT and bioactivation of AZT to AMT, a catabolite known to be highly toxic to human bone marrow cells. This induction was the result of enhancement of AZT catabolism rather than its transport into the cells, since on incubation of AZT (0-250 microM) with rat isolated hepatocytes, a linear relationship between concentration and amount taken up by the cells was shown. In addition, the rate of AZT uptake was not influenced by KCN, dinitrophenol, or temperature, which is consistent with a simple diffusion of AZT through the hepatocellular membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro and in vivo transport of zidovudine (AZT) across the blood-brain barrier and the effect of transport inhibitors

The transport of the antiviral nucleoside analogue zidovudine (3'-azido-3'-deoxythymidine; AZT) into the central nervous system (CNS) was characterized in vitro and in vivo. The in vitro model consisted of primary cultures of isolated bovine capillary endothelial cells. The transport rate of AZT across the monolayer, expressed as endothelial permeability P, was determined following luminal and abluminal administration. P did not differ between the two administration sites (luminal, 1.65 +/- 0.44 cm/min/10(3); abluminal, 1.63 +/- 0.28 cm/min/10(3)). The transport of AZT across the endothelial cell monolayer was found to be concentration independent in the range between 0.4 and 50 micrograms/mL. AZT transport was not affected by pretreatment of the cells with either metabolic inhibitors (DODG and DODG/NaN3) or probenecid. This suggests that AZT passes the monolayer mainly by passive diffusion. The in vivo transport of AZT across the blood-brain barrier and the blood-CSF barrier was studied in male Wistar rats after coadministration of potential inhibitors of active transport of AZT: probenecid (organic anion transport) and thymidine (nucleoside transport). Intracerebroventricular and intravenous coadministration of probenecid caused a significant (P < 0.001) increase in the CSF/plasma concentration ratio compared to the control phase, indicating that the organic anion carrier is involved in AZT transport from CSF to blood. Since there was no effect of probenecid on the transport of AZT in vitro, it is suggested that this carrier is located at the choroid plexus. Coadministration of thymidine did not affect the CSF/plasma concentration ratio, suggesting that a nucleoside carrier system is not involved in AZT transport into or out of the CNS.

Efflux of zidovudine and 2',3'-dideoxyinosine out of the cerebrospinal fluid when administered alone and in combination to Macaca nemestrina

To determine if there is active efflux of zidovudine (ZDV) and 2',3'-dideoxyinosine (ddI) out of the cerebrospinal fluid (CSF), and if this efflux is saturable, we investigated the steady-state CSF/plasma concentration ratio of the two drugs when administered alone or in combination. Constant-rate infusions of ZDV, ddI or both were administered to seven macaques (Macaca nemestrina) through a chronic venous catheter for a minimum of 28 hr. Antipyrine, a marker of passive diffusion, was coinfused in all experiments. Blood (5 mL) and CSF samples (0.5-1 mL) were collected by venous and lumbar/thoracic punctures, respectively, at 24 and 28 hr after beginning the infusion. When ZDV and ddI were administered alone, the steady-state CSF/plasma concentration ratios were significantly different from unity (ZDV, 0.20 +/- 0.08; ddI, 0.09 +/- 0.04) and were independent of the plasma concentration (P > 0.05). In contrast, the CSF/plasma concentration ratio of antipyrine (0.82 +/- 0.19) was close but significantly smaller than unity (P > 0.05). The CSF/plasma concentration ratios after simultaneous administration of ZDV and ddI were not significantly different (P > 0.05) from those obtained after administration of the drugs alone. These results suggest that ZDV and ddI are actively transported out of the CSF; however, within the concentration range studied, this efflux is neither saturable nor mutually competitive. Concomitant administration of ZDV and ddI did not produce a systemic interaction in the animals, indicating that the pharmacokinetics of either drug is unaffected by the presence of the other.

Pharmacokinetics, oral bioavailability, and metabolic disposition in rats of (-)-cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl] cytosine, a nucleoside analog active against human immunodeficiency virus and hepatitis B virus

The pharmacokinetics and metabolism of the potent anti-human immunodeficiency virus and anti-hepatitis B virus compound, (-)-cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl] cytosine (FTC), were investigated in male CD rats. Plasma clearance of 10 mg of FTC per kg of body weight was biexponential in rats, with a half-life at alpha phase of 4.7 +/- 1.1 min (mean +/- standard deviation) and a half-life at beta phase of 44 +/- 8.8 min (n = 5). The total body clearance of FTC was 1.8 +/- 0.1 liters/h/kg, and the oral bioavailability was 90% +/- 8%. The volume of distribution at steady state (Vss) was 1.5 +/- 0.1 liters/kg. Increasing the dose to 100 mg/kg slowed clearance to 1.5 +/- 0.2 liters/kg/h, lowered the Vss to 1.2 +/- 0.2 liters/kg, and reduced the oral bioavailability to 65% +/- 15%. FTC in the brains of rats was initially less than 2% of the plasma concentration but increased to 6% by 2 h postdose. Probenecid elevated levels of FTC in plasma as well as in brains but did not alter the brain-to-plasma ratio. The urinary and fecal recoveries of unchanged FTC after a 10-mg/kg intravenous dose were 87% +/- 3% and 5% +/- 1.6%, respectively. After a 10-mg/kg oral dose, respective urinary and fecal recoveries were 70% +/- 2.5% and 25% +/- 1.6%. Two sulfoxides of FTC were observed in the urine, accounting for 0.4% +/- 0.03% and 2.7% +/- 0.2% of the intravenous dose and 0.4% +/- 0.06% and 2.5% +/- 0.3% of the oral dose. Also observed were 5-fluorocytosine, representing 0.4% +/- 0.06% of the intravenous dose and 0.4% +/- 0.07% of the oral dose, and FTC glucuronide, representing 0.7% +/- 0.2% of the oral dose and 0.4% +/- 0.2% of the intravenous dose. Neither deaminated FTC nor 5-fluorouracil was observed in the urine (less than 0.2% of dose). The high oral availability and minimal metabolism of FTC encourage its further preclinical development.

Antiviral therapy in human immunodeficiency virus infections. Current status (Part II)

Part I of this article reviewed the targets against which anti-HIV drugs can be directed, problems in assessing active compounds (e.g. resistance development and use of surrogate end-points). and nucleoside analogues effective against HIV reverse transcriptase. Intensive research is currently being undertaken in laboratories and hospitals to design and evaluate new inhibitors of HIV. In this work, combining different drugs is one important approach, both to decrease toxicity and to offset the rate of resistance development, which seems to be a major problem associated with therapy directed against the ever-changing HIV. Therapeutic vaccines and immunomodulators are other modalities being actively evaluated against HIV and AIDS, although this effort has not yet yielded any licensed treatment. It appears likely that new antiviral drugs and immunotherapies will be forthcoming during the next 5 years, that they will be used in a variety of combinations, and that the treatment options available for opportunistic infections will increase. These developments should improve the survival and the quality of life of patients with HIV infection.

Pharmacokinetics of the anti-human immunodeficiency virus nucleoside analog stavudine in cynomolgus monkeys

Stavudine was administered (15 mg/kg of body weight) intravenously and orally to two monkeys in a randomized crossover study. Plasma and urine samples were analyzed for stavudine by high-performance liquid chromatography, and pharmacokinetic parameters were derived by a noncompartmental method. Total body clearance of stavudine was 0.64 liters/h/kg, with a steady-state volume of distribution of 0.68 liters/kg, a terminal half-life of 0.83 h, a urinary recovery of 44%, and an oral bioavailability of 80%. These values were reasonably similar to those reported for patients with AIDS or AIDS-related complex.

Ketoprofen-probenecid interaction in the rat: a probenecid concentration-dependent stereoselective process

Probenecid (PB) is believed to interact with the chiral nonsteroidal anti-inflammatory drug ketoprofen (KT) through competition for glucuronide conjugation and subsequent renal and/or biliary excretion of formed KT conjugates. It is unknown whether the interaction is dependent on PB concentration and whether both KT enantiomers are affected to the same extent. We measured intact and conjugated R-KT, S-KT, and PB in the plasma and urine of female Sprague-Dawley rats after intravenous doses of 10 mg of racemic KT per kg and 0, 25, 50, 100, 150, 175, and 200 mg of PB per kg. Elevated levels of both enantiomers were observed, with S-KT being affected to a much greater extent. Significant positive correlations were found between the concentrations in plasma of KT enantiomers and PB at various sampling times, with the strongest correlations being found at 2 h for R-KT (r = 0.708) and 1.5 h for S-KT (r = 0.913). The areas under the concentration-time curve (AUC) from 0-24 h for R-KT (r = 0.697) and S-KT (r = 0.848) also showed strong correlation with AUC of PB. Our data show that as the dose of PB was increased (0-200 mg/kg), the mean S-KT/R-KT ratios for both the AUC and the fraction of the dose excreted as enantiomer conjugates in urine over 24 h increased progressively from 12.1 +/- 2.3 to 27.6 +/- 5.0 and from 6.8 +/- 0.7 to 36.4 +/- 12.2, respectively. These findings clearly demonstrate that the KT-PB interaction in the rat is a PB concentration-dependent process.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of probenecid on the excretion kinetics of stanozolol, an anabolic steroid, in rats

The pharmacokinetic behaviour and the mechanism of renal excretion of stanozolol (STZ), as affected by co-treatment with probenecid, were studied in male Sprague-Dawley rats. Pharmacokinetic parameters following intravenous (i.v.) administration of STZ (20 mg kg-1 body wt.) were measured in both STZ-treated (control) and STZ plus probenecid-treated (treatment) groups. In order to assess the renal clearance of STZ, bolus doses of STZ and inulin (40 mg kg-1 body wt.) were injected i.v. either in the presence or absence of probenecid (40 mg kg-1 body wt.). The blood and urine concentrations of STZ were determined by capillary gas chromatography-mass spectrometry (GC-MS). In the probenecid treatment group, the area under the plasma disappearance and urinary excretion curves (AUC) of STZ were significantly decreased (P < 0.01) and the volume of distribution (Vd) and total clearance (CLt) were significantly increased statistically (P < 0.05 and P < 0.01, respectively). No remarkable differences in the urine flow rate, urine pH values, glomerular filtration rate (GFR) or renal clearance were observed in the treatment group. However, the clearance ratio in the treatment group was significantly increased from 11.72 to 17.27. From these results, it is suggested that the significant decrease of AUC, i.e. increase of disappearance of STZ in plasma by co-administration with probenecid, is due to the increase of the clearance ratio.

Analysis of zidovudine distribution to specific regions in rabbit brain using microdialysis

The distribution of zidovudine (3'-azido-3'-deoxythymidine; AZT) into two regions of rabbit brain was investigated in crossover using microdialysis. Six rabbits had guide cannulas surgically implanted in the lateral ventricle and thalamus by stereotaxic placement. After recovery, microdialysis probes were positioned and i.v. bolus doses of 5, 10, 20, and 30 mg/kg were administered to each animal over a period of 2 weeks. Blood was drawn via a marginal ear vein catheter for 8 hr. Brain dialysate was collected at 3 microliters/min from ventricle and thalamus dialysis probes every 10 min. Simulated cerebrospinal fluid (CSF), to which 3'-azido-2',3'-dideoxyuridine (AZdU) was added, was used as perfusate. AZdU loss, which was measured during simultaneous retrodialysis, served as a marker for in vivo recovery of AZT. AZT concentrations in plasma, as well as in ventricle and thalamus dialysate, were determined using a sensitive HPLC assay, and AZdU was simultaneously analyzed in the dialysates. Calculation of in vivo recovery of AZT was based on loss of AZdU from the perfusate during retrodialysis and was used to estimate the concentration of drug at both sites in the brain. In vitro loss of AZdU and recovery of AZT showed good agreement, demonstrating a bivariate regression slope of 0.99. The half-lives and AUCs (normalized to dose) achieved in the plasma, ventricle, or thalamus were not significantly different for the four doses.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetic evaluation of drug interactions with anti-HIV drugs, II: Effect of 2',3'-dideoxyinosine (ddI) on zidovudine kinetics in monkeys

The pharmacokinetic basis of a drug interaction between zidovudine (AZT) and 2',3'-dideoxyinosine (ddI) was investigated in normal monkeys. Five animals received 20 mg/kg of AZT intragastrically in the absence and presence of ddI. ddI was administered intravenously to produce steady-state ddI plasma concentrations for 30 min. Plasma and urine samples were analyzed for AZT, its major glucuronide metabolite, GAZT, and ddI by high-performance liquid chromatography (HPLC). Resultant AZT and GAZT concentration data were analyzed by noncompartmental methods. Statistical analysis indicated no differences in AZT's apparent total clearance, apparent volume of distribution at steady-state, and elimination half-life due to ddI, however, the mean apparent total clearance decreased from 2.92 to 1.67 L/h/kg, and the mean apparent volume of distribution at steady-state decreased from 5.79 to 3.43 L/kg in the presence of ddI. Incomplete urine collections in most animals prevented conclusions from being made about ddI's effect on renal elimination parameters. Nonetheless, the urinary GAZT/AZT ratio, a parameter not influenced by incomplete urine collection, was significantly reduced in the presence of ddI. Although additional studies will be useful to characterize the full importance of the interaction, there is evidence to suggest that both renal and metabolic elimination of AZT and renal elimination of GAZT may be inhibited by ddI.

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)

Pharmacokinetics and brain entry of alaptide, a novel nootropic agent, in mice, rats and rabbits

Pharmacokinetics of a novel nootropic agent, alaptide, have been examined in plasma and brain of mice, rats and rabbits following an intravenous dose (1 mg kg-1). First-order equilibration rate constants between plasma and brain (kBO) were calculated by a two-compartment model with a linked compartment (brain). Brain alaptide equilibrates rapidly with the central compartment in mice and rats due to the high kBO/beta ratio. In rabbits the equilibration is much slower (kBO/beta approximately 1). Partition coefficients between brain and plasma calculated from areas under the brain and plasma concentration-time curves, are 0.479, 0.549 and 0.864, in mice, rats and rabbits, respectively.

Pharmacokinetic evaluation of drug interactions with zidovudine. I: Probenecid and zidovudine in monkeys

Pharmacokinetic evaluation of a drug interaction between zidovudine (AZT) and probenecid was conducted in monkeys. Six animals received 20 mg/kg of AZT as single intragastric (ig) and iv doses in the absence and presence of 50 mg/kg of probenecid administered ig. Plasma concentrations of AZT and its 5'-glucuronide metabolite (AZTG) were quantitated for 12 h by HPLC. Amounts of AZT and AZTG in urine were also measured, as were probenecid plasma concentrations. Non-compartmental methods were used to obtain pharmacokinetic parameters for AZT and AZTG. In the presence of probenecid, the total clearance of AZT decreased by 50%, renal clearance decreased, and elimination half-life increased. The volume of distribution at steady-state and systemic bioavailability of AZT were not significantly altered by probenecid. The areas under the plasma concentration-time curves and terminal half-lives of AZTG were increased, and renal clearances of AZTG were decreased. The alterations in AZT and AZTG pharmacokinetic parameters are consistent with inhibition of metabolism and renal tubular secretion by probenecid. Since AZT was administered by both oral and iv routes, clearance, volume of distribution, and bioavailability parameters were independently determined. Based on data reported for humans on the zidovudine-probenecid interaction, monkeys appear to be appropriate animal models for the evaluation of zidovudine drug interactions.

Simultaneous determination of ketoprofen enantiomers and probenecid in plasma and urine by high-performance liquid chromatography

A rapid, sensitive, stereospecific reversed-phase high-performance liquid chromatographic method was developed for simultaneous quantitation of ketoprofen enantiomers, probenecid and their conjugates in biological fluids. Following addition of the internal standard, indoprofen, the constituents were extracted into isooctane-isopropanol (95:5), water-washed, extracted with chloroform, then evaporated and the residue sequentially derivatized with ethyl chloroformate and L-leucinamide hydrochloride. The formed diastereomers were chromatographed on a reversed-phase column with a mobile phase of 0.06 M KH2PO4-acetonitrile-triethylamine (65:35:0.1) at a flow-rate of 1 ml/min and a detection wavelength of 275 nm. The minimum quantifiable concentration was 0.5 micrograms/ml in 100 microliters of rat plasma and urine samples. The intra- and inter-day coefficients of variation for this method are less than 10%. The assay is successfully applied to a pharmacokinetic study. The simultaneous analysis of probenecid with several other non-steroidal anti-inflammatory drugs was also successful.

Pharmacokinetics of 3'-azido-3'-deoxythymidine and its catabolites and interactions with probenecid in rhesus monkeys

The pharmacokinetics and metabolism of 3'-azido-3'-deoxythymidine (AZT) were investigated in rhesus monkeys after subcutaneous administration of 33.3 mg of AZT per kg of body weight alone or in the presence of 100 mg of probenecid per kg. In addition to unchanged drug, two catabolites, 5'-O-glucuronide (GAZT) and 3'-amino-3'-deoxythymidine (AMT), were detected in plasma within 30 min. GAZT exhibited a kinetic profile similar to that of AZT, with an elimination half-life of approximately 1 h, while AMT was more variable, with an apparent half-life of 1.6 +/- 1.5 h. Approximately 90% of the total administered dose was recovered in urine within 24 h as AZT, GAZT, AMT, and the 5'-O-glucuronide of AMT. AZT and AMT demonstrated similar cerebrospinal fluid (CSF) penetration 1 h after AZT treatment, while GAZT poorly crossed the blood-brain barrier. Concomitant administration of probenecid greatly altered the pharmacokinetics of AZT, GAZT, and AMT, resulting in prolongation of their apparent elimination half-lives, increased concentrations in plasma, and marked reduction in renal clearances. In addition, the CSF/plasma concentration ratios for AZT and its catabolites were greatly increased, suggesting that probenecid inhibits efflux of AZT and its catabolites from CSF to plasma. The substantial levels of AMT in plasma suggest that this catabolite affects the pharmacodynamic properties of AZT in relation to its activity against human immunodeficiency virus replication and cytotoxicity to host cells. Enhanced AMT levels in plasma in the presence of probenecid may decrease the therapeutic efficacy of the AZT-probenecid combination.

High-performance liquid chromatographic determination of 2',3'-didehydro-3'-deoxythymidine (d4T) in human and rabbit plasma and urine and its application to pharmacokinetic studies in the rabbit

A rapid and sensitive liquid chromatographic assay for 2',3'-didehydro-3'-deoxythymidine (d4T) in plasma and urine is described. This assay uses thymidine oxetane (TO), a synthetic precursor of d4T, as internal standard. Sample preparation involves a simple extraction of plasma or urine with 5% isopropyl alcohol in methylene chloride. The method is specific and sensitive, allowing a linear response over a 2000-fold range of concentrations in human plasma (5 ng/ml to 10 micrograms/ml) and urine (50 ng/ml to 100 micrograms/ml). This assay, developed for human plasma and urine, is also applicable to rabbit samples with minor modification. Intravenous bolus doses of 10 mg/kg d4T to rabbits showed that the plasma concentration-time profile followed a biexponential decay. Estimates of the distribution and elimination half-lives were 6.7 +/- 0.9 and 51 +/- 6 min, respectively. The total-body and renal clearances were 23.4 +/- 3.6 and 8.82 +/- 3.9 ml/min.kg, respectively. That the renal clearance exceeds the glomerular filtration rate in the rabbit suggests that d4T is actively secreted in the renal tubule. The fraction excreted unchanged in the urine was 36 +/- 8%. Similar results were obtained in the same rabbits at steady state during constant-rate intravenous infusion. Noncompartmental analysis estimates of the MRT and Vdss were 46 +/- 5 min and 1.08 +/- 0.13 L/kg, respectively.