Pharmacokinetics of zidovudine and dideoxyinosine alone and in combination in patients with the acquired immunodeficiency syndrome

Prediction of Clearance in Children from Adults Following Drug-Drug Interaction Studies: Application of Age-Dependent Exponent Model

Background and Objective

Pharmacokinetic drug–drug interaction (DDI) studies are conducted in adult subjects during drug development but there are limited studies that have characterized pharmacokinetic DDI studies in children. The objective of this study was to evaluate if the DDI clearance values from adults can be allometrically extrapolated from adults to children.

Methods

Fifteen drugs were included in this study and the age of the children ranged from premature neonates to adolescents (30 observations across the age groups). The age-dependent exponent (ADE) model was used to predict the clearance of drugs in children from adults following DDI studies.

Results

The prediction error of drug clearances following DDIs in children ranged from 4 to 67%. Of 30 observations, 17 (57%) and 27 (90%) observations had a prediction error ≤ 30% and ≤ 50%, respectively.

Conclusion

This study indicates that it is possible to predict the clearance of drugs with reasonable accuracy in children from adults following DDI studies using an ADE model. The method is simple, robust, and reliable and can replace other complex empirical models.

The crosstalk between antiretrovirals pharmacology and HIV drug resistance

INTRODUCTION

The clinical development of antiretroviral drugs has been followed by a rapid and concomitant development of HIV drug resistance. The development and spread of HIV drug resistance is due on the one hand to the within-host intrinsic HIV evolutionary rate and on the other to the wide use of low genetic barrier antiretrovirals.

AREAS COVERED

We searched PubMed and Embase on 31 January 2020, for studies reporting antiretroviral resistance and pharmacology. In this review, we assessed the molecular target and mechanism of drug resistance development of the different antiretroviral classes focusing on the currently approved antiretroviral drugs. Then, we assessed the main pharmacokinetic/pharmacodynamic of the antiretrovirals. Finally, we retraced the history of antiretroviral treatment and its interconnection with antiretroviral worldwide resistance development both in , and middle-income countries in the perspective of 90-90-90 World Health Organization target.

EXPERT OPINION

Drug resistance development is an invariably evolutionary driven phenomenon, which challenge the 90-90-90 target. In high-income countries, the antiretroviral drug resistance seems to be stable since the last decade. On the contrary, multi-intervention strategies comprehensive of broad availability of high genetic barrier regimens should be implemented in resource-limited setting to curb the rise of drug resistance.

Population pharmacokinetics and pharmacodynamics of zidovudine in HIV-infected infants and children

The purpose of this study was to assess the population pharmacokinetics (PK) and pharmacodynamics (PD) of zidovudine (ZDV) in infants and children. This evaluation includes 394 subjects who participated in Pediatric AIDS Clinical Trials Group (PACTG) Study 152 and received either ZDV alone or in combination with didanosine. The most significant PK covariate was age, with infants < 2 years of age having reduced size-adjusted clearance. ZDV exposure was weakly related to maximal reduction in immune complex-dissociated (ICD) p24 antigen but not to reduction at 6 months. Mild chronic anemia occurred in 7.6% of subjects with ZDV average concentration < 1.3 microM (350 ng/mL) versus in 23.4% subjects with higher ZDV concentrations (p < 0.001). There was a direct linear relationship between hemoglobin and ZDV levels. It was concluded that ZDV oral clearance is reduced in infants compared to older children. This lower clearance leads to higher ZDV exposure in infants and contributes to increased hematologic toxicity.

Improved uptake and retention of lipophilic prodrug to improve treatment of HIV

Dideoxynucleosides currently in use for anti-HIV therapy have been found to be inefficient in passing through the blood-brain barrier to enter and maintain therapeutic drug levels in brain, a very significant reservoir of HIV. The low bioavailability of these drugs combined with the bone marrow toxicity of AZT (3'-azido, 3'-deoxythymidine, Zidovudine), resulting in anemia and leukopenia, pancreatitis with ddI (2',3'-dideoxyinosine, Didanosine) and painful peripheral neuropathy in case of ddC (2',3-dideoxycytosine, Zalcitabine) are the limiting factors in their use. In addition, the emergence of strains of HIV resistant to AZT, the most commonly used drug, further restricts its use. Thus the control of AIDS and its complications, needs special therapeutic approaches to combat the disease. In order to overcome these limitations, AZT and ddI have been synthesized as ester-linked ceramide- and phosphatidylcholine-linked prodrugs possessing therapeutic attributes lacking in the parent compounds. There is greater uptake and longer retention of these prodrugs in NIH/3T3 cells in vitro. Pretreatment with our prodrugs blocked infection of these cells by Moloney murine leukemia virus (M-MuLV) for an extended period, which the parent drugs failed to do. When human CD4+ HeLa cells were continuously exposed to the AZT prodrug, subsequent infection of these cells by HIV was blocked. Similar results were obtained with NIH/3T3 cells exposed to M-MuLV. AE(6)C, a prodrug of AZT linked to ceramide via a cleavable ester bond and a six carbon linker, was less toxic to both mouse and human bone marrow progenitor cells than free AZT. Most significantly, the prodrugs concentration was greater and the retention longer, in well known sanctuaries for HIV, such as the brain, testes and thymus.

Transport, metabolism and elimination mechanisms of anti-HIV agents

Currently available anti-HIV drugs can be classified into three categories: nucleoside analogue reverse transcriptase inhibitors, non-nucleoside reverse transcriptase inhibitors, and protease inhibitors. Knowledge of these anti-HIV drugs in various physiological or pharmacokinetic compartments is essential for design and development of drug delivery systems for the treatment of HIV infection. The input and output of anti-HIV drugs in the biological systems are described by their transport and metabolism/elimination in this review. Transport mechanisms of anti-HIV agents across various biological barriers, i.e., gastrointestinal wall, skin, mucosa, blood cerebrospinal barrier, blood-brain barrier, placenta, and cellular membranes, are discussed. Their fates during and after systemic absorption and their metabolism-related drug interactions are reviewed. Many anti-HIV drugs presently marketed in the US bear some significant drawbacks such as relatively short half-life, low bioavailability, poor penetration into the central nervous system, and undesirable side effects. Efforts have been made to design drug delivery systems for the anti-HIV agents to: (1) reduce the dosing frequency; (2) increase the bioavailability and decrease the degradation/metabolism in the gastrointestinal tract; (3) improve the CNS penetration and inhibit the CNS efflux; and (4) deliver them to target cells selectively with minimal side effects. We hope to stimulate further interests in the area of controlled delivery of anti-HIV agents by providing current status of transport and metabolism/elimination of these agents.

Single-dose pharmacokinetics and safety of abacavir (1592U89), zidovudine, and lamivudine administered alone and in combination in adults with human immunodeficiency virus infection

Abacavir (1592U89), a nucleoside reverse transcriptase inhibitor with in vitro activity against human immunodeficiency virus type-1 (HIV-1), has been evaluated for efficacy and safety in combination regimens with other nucleoside analogs, including zidovudine (ZDV) and lamivudine (3TC). To evaluate the potential pharmacokinetic interactions between these agents, 15 HIV-1-infected adults with a median CD4(+) cell count of 347 cells/mm3 (range, 238 to 570 cells/mm3) were enrolled in a randomized, seven-period crossover study. The pharmacokinetics and safety of single doses of abacavir (600 mg), ZDV (300 mg), and 3TC (150 mg) were evaluated when each drug was given alone or when any two or three drugs were given concurrently. The concentrations of all drugs in plasma and the concentrations of ZDV and its 5'-glucuronide metabolite, GZDV, in urine were measured for up to 24 h postdosing, and pharmacokinetic parameter values were calculated by noncompartmental methods. The maximum drug concentration (Cmax), the area under the concentration-time curve from time zero to infinity (AUC0-infinity), time to Cmax (Tmax), and apparent elimination half-life (t1/2) of abacavir in plasma were unaffected by coadministration with ZDV and/or 3TC. Coadministration of abacavir with ZDV (with or without 3TC) decreased the mean Cmax of ZDV by approximately 20% (from 1.5 to 1.2 microg/ml), delayed the median Tmax for ZDV by 0.5 h, increased the mean AUC0-infinity for GZDV by up to 40% (from 11.8 to 16.5 microg. h/ml), and delayed the median Tmax for GZDV by approximately 0.5 h. Coadministration of abacavir with 3TC (with or without ZDV) decreased the mean AUC0-infinity for 3TC by approximately 15% (from 5.1 to 4.3 microg. h/ml), decreased the mean Cmax by approximately 35% (from 1.4 to 0.9 microg/ml), and delayed the median Tmax by approximately 1 h. While these changes were statistically significant, they are similar to the effect of food intake (for ZDV) or affect an inactive metabolite (for GZDV) or are relatively minor (for 3TC) and are therefore not considered to be clinically significant. No significant differences were found in the urinary recoveries of ZDV or GZDV when ZDV was coadministered with abacavir. There was no pharmacokinetic interaction between ZDV and 3TC. Mild to moderate headache, nausea, lymphadenopathy, hematuria, musculoskeletal chest pain, neck stiffness, and fever were the most common adverse events reported by those who received abacavir. Coadministration of ZDV or 3TC with abacavir did not alter this adverse event profile. The three-drug regimen was primarily associated with gastrointestinal events. In conclusion, no clinically significant pharmacokinetic interactions occurred between abacavir, ZDV, and 3TC in HIV-1-infected adults. Coadministration of abacavir with ZDV or 3TC produced mild changes in the absorption and possibly the urinary excretion characteristics of ZDV-GZDV and 3TC that were not considered to be clinically significant. Coadministration of abacavir with ZDV and/or 3TC was generally well tolerated and did not produce unexpected adverse events.

Pharmacokinetics and potential interactions amongst antiretroviral agents used to treat patients with HIV infection

There are 3 groups of drugs available for the treatment of patients with HIV disease. These are the nucleoside reverse transcriptase inhibitors ('nucleoside analogues') [zidovudine, didanosine, zalcitabine, lamivudine and abacavir]; the non-nucleoside reverse transcriptase inhibitors (nevirapine, delavirdine and efavirenz); and the protease inhibitors (saquinavir, ritonavir, indinavir, nelfinavir and amprenavir). The preferred initial regimen should reduce and maintain plasma HIV RNA below the level of detection. Presently, the regimen of choice consists of 2 nucleoside analogues plus a protease inhibitor with high in vivo efficacy. An alternative combination consists of 2 nucleoside analogues plus a non-nucleoside reverse transcriptase inhibitor. Drug interactions are one of the major problems associated with these multidrug regimens. Changes in plasma concentrations of the nucleoside analogues are unlikely to be of clinical relevance as drug effect is mainly dependent on the rate and extent of intracellular phosphorylation. Combinations of zidovudine plus stavudine, and probably zalcitabine plus lamivudine, should be avoided as competition for phosphorylating enzymes may occur. The antiviral efficacy of some nucleoside analogues, e.g. stavudine, may be compromised by prior treatment with other nucleosides (e.g. zidovudine). However, these data need to be clarified in further studies. It is unlikely that administration of other antiretrovirals will influence the activity of nucleoside analogues. Protease inhibitors are metabolised by hepatic cytochrome P450 (CYP) 3A4. Combination protease inhibitor therapy can result in drug interactions mediated by enzyme inhibition. Ritonavir is the most potent inhibitor, saquinavir the least. The protease inhibitors also interact with the non-nucleoside reverse transcriptase inhibitors. Nevirapine and efavirenz induce drug metabolising enzymes and may reduce plasma concentrations of protease inhibitors. A study in healthy volunteers showed that nelfinavir concentrations are increased by combination with efavirenz. Delavirdine inhibits drug metabolising enzymes and increases the plasma concentration of coadministered protease inhibitors. The nucleoside analogues would not be expected to interact with the protease inhibitors. Apart from the ability of didanosine to reduce the area under the concentration-time curve of delavirdine, there are no reports of clinically significant interactions of other antiretrovirals with the non-nucleoside reverse transcriptase inhibitors. Triple therapy is the current standard of care for patients with HIV disease. However, studies of quadruple therapy are already under way. Drug interactions are likely to remain one of the major considerations when selecting a therapeutic regimen for patients with HIV.

Pharmacokinetic interaction between ritonavir and didanosine when administered concurrently to HIV-infected patients

The effect of coadministration of ritonavir and didanosine (ddI) on the pharmacokinetics of these drugs was investigated in a single-center, three-period, crossover study. Eighteen asymptomatic, HIV-positive men were assigned randomly to 6 different sequences of 3 regimens: ddI (200 mg every 12 hours) alone for 4 days, ritonavir (600 mg every 12 hours) alone for 4 days, and 4 days of ddI with ritonavir under dose-staggering conditions. Although not statistically significant, ritonavir concentrations were slightly higher on average (<10%) with concurrent administration of ddI compared with those of ritonavir alone. In contrast, ddI concentrations were lower with concurrent administration compared with those of ddI alone; maximum concentration and area under the concentration-time curve were reduced by about 15% (p < .05). The ddI elimination rate constant was unaffected by ritonavir, suggesting no change in ddI's systemic metabolism. Adverse events were similar between regimens. The relatively minor changes in ritonavir and ddI pharmacokinetics are probably not clinically relevant; therefore, dosage adjustment of either compound appears unnecessary when administered concurrently. However, the combination regimen of ddI and ritonavir continue to be evaluated clinically.

Relationship between didanosine exposure and surrogate marker response in human immunodeficiency virus-infected outpatients

We used information available from routine clinic visits to characterize the pharmacokinetics of didanosine in 82 human immunodeficiency virus-infected patients. A total of 271 blood samples were collected for the measurement of didanosine concentrations in plasma (mean +/- standard deviation [SD], 3.30 +/- 2.21 samples/patient). Bayesian estimates of didanosine oral clearance (CL[oral]) were obtained for these patients by the POSTHOC option within the NONMEM software package. Population priors from a previous NONMEM analysis of didanosine pharmacokinetics were used. The mean +/- SD CL(oral) was 132 +/- 27.7 liters/h, which agrees reasonably well with estimates obtained from previous pharmacokinetic studies of didanosine. Estimates of individual didanosine exposure were then used to consider potential relationships between drug exposure and surrogate marker response over a 6-month period. No correlations were found between the didanosine area under the concentration-time curve from 0 to 6 months and the absolute CD4 cell count (r = 0.305; 0.1 < P < 0.2), weight response (r = 0.0857; P > 0.4), or percentage of CD4 lymphocytes (r = 0.0559; P > 0.4). Future efforts to characterize didanosine exposure in outpatients by random sampling methods should involve more directed efforts to limit residual variability in the data.

Pharmacokinetic optimisation of the treatment of cancer with high dose zidovudine

The thymidine analogue zidovudine is currently used for the treatment of HIV-infected patients, as the early development of the drug as an anticancer agent yielded modest results. A comprehensive preclinical analysis, however, showed that inhibitors of de novo thymidylate synthesis, including fluorouracil and methotrexate, enhanced the antiproliferative activity of zidovudine in cancer cells. Significant inhibition of tumour growth was obtained in mice bearing human colon cancer xenografts and given intraperitoneal zidovudine 300 to 600 mg/kg weekly in combination with methotrexate 87.5 mg/kg or intraperitoneal fluorouracil 85 mg/kg, and in pharmacokinetic studies high peak drug plasma concentrations (Cmax) of zidovudine were obtained, ranging from 610.3 to 1698.8 mumol/L. In order to exploit the therapeutic activity of zidovudine, phase I and II clinical studies were designed in combination with fluorouracil and the pharmacokinetic-pharmacodynamic profile of zidovudine was investigated. Clinical responses were obtained in patients treated intravenously with bolus fluorouracil 500 mg/m2, leucovorin and short (90 to 120 minutes) infusions of high dose zidovudine (up to 10 g/m2), generating drug Cmax similar to those obtained in preclinical models. However, in chemotherapy-pretreated patients receiving high dose zidovudine by the oral route (1 to 9 g/m2/day) or 48-hourly continuous intravenous infusion (2 to 20 g/m2/day) in combination with fluorouracil and leucovorin, treatment failures were observed despite high systemic exposure, described as the area under the plasma concentration-time curve and the occurrence of DNA strand breaks in peripheral blood mononucleated cells, the biological expression of zidovudine activity. In conclusion, preclinical and clinical evidence suggest that the schedule of administration of zidovudine is a requisite for the expression of its activity, indicating the importance of concentration-monitored trials to optimise chemotherapy dose administration in patients. The likelihood of tumour response appears to be related to the achievement of high peak plasma concentrations of zidovudine, and constant infusions appear less likely to produce clinical results.

Rifabutin absorption in the gut unaltered by concomitant administration of didanosine in AIDS patients

Didanosine (ddI) is currently used in the management of patients infected by the human immunodeficiency virus. Rifabutin (RBT) is being extensively used for prophylaxis against Mycobacterium avium complex (MAC) infections. Due to its acid-labile characteristics, ddI must be administered with a buffer. Recent reports have indicated that absorption of ketoconazole, ciprofloxacin, and dapsone, etc., in the gut is altered by concomitant ddI dosing. We have assessed whether concomitant dosing of ddI as antiretroviral therapy modifies RBT absorption in the gut, its steady-state pharmacokinetics, and/or safety in 15 patients with AIDS. Of the 15 patients enrolled, 12 completed the study and 3 receiving 600 mg of RBT with concomitant ddI administration withdrew prematurely from the study. Steady-state RBT pharmacokinetics were assessed on day 13 (ddI plus RBT) and day 16 (RBT alone). The ddI doses (adjusted for body weight) were 167 to 375 mg twice daily, while RBT was administered as a single 300- or 600-mg daily dose. No statistically significant (P > 0.05) differences were seen in RBT absorption parameter estimates between days 13 and 16: maximum concentration in plasma (Cmax; 511 +/- 341 ng/ml versus 525 +/- 254 ng/ml) and the time at which Cmax was observed (3.0 versus 2.5 h). The mean RBT estimates for area under the concentration-time curve from 0 to 24 h (AUC(0-tau)) (5,650 versus 5,023 ng x h/ml) and for oral clearance (1.28 versus 1.18 liter/h/kg) on both study days were also similar. Assessment based on urinary recovery of RBT (3.1 versus 3.7 mg) and its predominant deacetyl metabolite, LM565 (1.6 versus 1.4 mg), showed no apparent effect of ddI. The fraction of the RBT dose converted to LM565, as suggested by the ratio of AUC of the metabolite to AUC of the parent drug, was also unaltered (0.15 versus 0.12). A ratio analysis (day 13/day 16) of the RBT pharmacokinetic estimates showed that the 95% confidence intervals for all parameters were inclusive of one. Furthermore, the brief interruption of ddI therapy over this short study period at steady state produced no clinically significant changes in body weight, hematology, and renal and pancreatic functions. Therefore, concomitant administration of ddI appears not to affect RBT absorption in the gut and its disposition or safety in patients with AIDS.

Phase I dose-escalation pharmacokinetics of AZT-P-ddI (IVX-E-59) in patients with human immunodeficiency virus

3'-Azido-3'-deoxythymidilyl-(5',5')-2',3'-dideoxy-5'-inosinic acid (AZT-P-ddI, IVX-E-59, Scriptene) is a heterodimer composed of one molecule of 3'-azido-3'-deoxythymidine (zidovudine or AZT) and one molecule of 2',3'-dideoxyinosine (didanosine or ddI) linked through their 5' positions by a phosphate bond. AZT-P-ddI exhibits enhanced antiviral activity and selectivity in vitro compared with AZT and ddI alone. The pharmacokinetics of AZT-P-ddI were studied in 12 patients with human immunodeficiency virus (HIV) who had CD4+ cell counts higher than 200 cells/mm3. Isotopic preparations of (14C)-AZT-P-(3H)-ddI were administered intravenously (50 mg and 100 mg) to eight patients; 1 month later these patients were crossed over to oral administration (100 mg and 200 mg). A second group of patients (n = 4) received only a 450-mg oral dose of AZT-P-ddI. Plasma levels of unchanged AZT-P-ddI after intravenous infusion declined rapidly and were undetectable 0.75 hours after the end of infusion, whereas the parent compound was not detected after oral administration, indicative of a very rapid metabolism. The parent entity was enzymatically cleaved in vivo yielding the two constituent drugs AZT and ddI, which were subsequently subjected to their respective pharmacokinetic and metabolic processes. The beta-glucuronide derivative of AZT (GAZT) represented the major metabolite of AZT, but there were no detectable levels of the toxic metabolite 3'-amino-3'-deoxythymidine (AMT). A major and previously unrecognized in vivo metabolite of ddI, referred as ddI-M, was detected in plasma and urine. Analysis by high-field proton nuclear magnetic resonance and mass spectrometry led to the identification of ddI-M as being R(-)-dihydro-5-(hydroxymethyl)-2(3H)-furanone. The formation of AZT and ddI metabolites was increased after oral administration of AZT-P-ddI compared with the intravenous infusion, with an area under the concentration-time curve (AUC) ratio of metabolite to AZT and metabolite to ddI being 7.7 and 5.7 (oral) and 3.8 and 1.1 (intravenous), respectively. The newly identified ddI-M exhibited sustained plasma levels for extended time periods with an apparent elimination half-life (t1/2) of approximately 10 hours after oral administration of AZT-P-ddI. Recovery of radioactivity associated with 3H and 14C in urine was essentially complete within 48 hours after oral and intravenous administration of AZT-P-ddI. The oral bioavailability of AZT (64.7-67.3%) and ddI (33.6-42.9%) and the other pharmacokinetic parameters were consistent with previous data reported with each nucleoside analog alone or in combination therapy.

Didanosine measurement by radioimmunoassay

Didanosine is commonly prescribed as monotherapy or as part of a combination regimen for patients with human immunodeficiency virus infection. The use of lower doses, either as part of a combination regimen or as a result of dose reduction secondary to clinical intolerance, requires that a sensitive assay method be available for either traditional or population-based pharmacokinetic evaluations. We evaluated a radioimmunoassay technique with a standard curve range of 0 to 100 ng/ml in human plasma, urine, and cerebrospinal fluid and assessed its accuracy and precision for use in pharmacokinetic studies.

Drug interactions with antiviral drugs

Antiviral drug interactions are a particular problem among immuno-compromised patients because these patients are often receiving multiple different drugs, i.e. antiretroviral drugs and drugs effective against herpesvirus. The combination of zidovudine and other antiretroviral drugs with different adverse event profiles, such as didanosine, zalcitabine and lamivudine, appears to be well tolerated and no relevant pharmacokinetic interactions have been detected. The adverse effects of didanosine and zalcitabine (i.e. peripheral neuropathy and pancreatitis) should be taken into account when administering these drugs with other drugs with the same tolerability profile. Coadministration of zidovudine and ganciclovir should be avoided because of the high rate of haematological intolerance. In contrast, zidovudine and foscarnet have synergistic effect and no pharmacokinetic interaction has been detected. No major change in zidovudine pharmacokinetics was seen when the drug was combined with aciclovir, famciclovir or interferons. However, concomitant use of zidovudine and ribavirin is not advised. Although no pharmacokinetic interaction was documented when didanosine was first administered with intravenous ganciclovir, recent studies have shown that concentration of didanosine are increased by 50% or more when coadministered with intravenous or oral ganciclovir. The mechanism of this interaction has not been elucidated. Lack of pharmacokinetic interaction was demonstrated between foscarnet and didanosine or ganciclovir. Clinical trials have shown that zidovudine can be administered safely with paracetamol (acetaminophen), nonsteroidal anti-inflammatory drugs, oxazepam or codeine. Inhibition of zidovudine glucuronidation has been demonstrated with fluconazole, atovaquone, valproic acid (valproate sodium), methadone, probenecid and inosine pranobex; however, the clinical consequences of this have not been fully investigated. No interaction has been demonstrated with didanosine per se but care should be taken of interaction with the high pH buffer included in the tablet formulation. Drugs that need an acidic pH for absorption (ketoconazole, itraconazole but not fluconazole, dapsone, pyrimethamine) or those that can be chelated by the ions of the buffer (quinolones and tetracyclines) should be administered 2 hours before or 6 hours after didanosine. Very few interaction studies have been undertaken with other antiviral drugs. Coadministration of zalcitabine with the antacid 'Maalox' results in a reduction of its absorption. Dapsone does not influence the disposition of zalcitabine. Cotrimoxazole (trimethoprim-sulfamethoxazole) causes an increase in lamivudine concentrations by 43%. Saquinavir, delavirdine and atevirdine appeared to be metabolised by cytochrome P450 and interactions with enzyme inducers or inhibitors could be anticipated. Some studies showed that interferons can reduce drug metabolism but only a few studies have evaluated the pathways involved. Further studies are required to better understand the clinical consequences of drug interactions with antiviral drugs. Drug-drug interactions should be considered in addition to individual drug clinical benefits and safety profiles.

Metabolism of Zidovudine

1. The anti-HIV drug zidovudine (3'-azido-2',3'-dideoxythymidine; ZDV) has three important pathways of metabolism. ZDV is a prodrug and must be phosphorylated in lymphocytes in order to exert its antiviral action. However, in quantitative terms this is a minor pathway probably accounting for less than 1% of the overall metabolic profile. The predominant pathway of metabolism is glucuronidation to GZDV and the metabolite is renally excreted. A further metabolite, derived by reduction of the azido moiety is 3'-amino-3'-deoxythymidine (AMT). 2. Zidovudine glucuronidation has been characterised in human liver microsomes. A number of drugs (e.g., naproxen, indomethacin and probenecid) have been shown to inhibit the in vitro conjugation of ZDV. Some of these drugs have also been co-administered with ZDV in HIV-positive patients. Significant pharmacokinetic interactions have been demonstrated with probenecid, naproxen and fluconazole. 3. 3'-amino-3'-deoxythymidine formation is probably mediated by both cytochrome P450 isozymes and NADPH-cytochrome P450 reductase. Peak plasma concentrations of AMT are approximately 10-15% of ZDV in patients. This is a potentially important metabolite because of its alleged cytotoxicity. 4. Measurement of intracellular ZDV phosphates in patients provides the key to our understanding of both the efficacy and toxicity of ZDV. Important recent work has demonstrated that as patients deteriorate (i.e., CD4 counts decrease below 100 x 10(6)/L), there is a corresponding increase in intracellular ZDV-monophosphate. This could have toxicological implications.

Pharmacokinetic optimisation of antiretroviral therapy in patients with HIV infection

More than 7 years after the introduction of zidovudine for treatment of HIV infection, little use has been made of the pharmacokinetic properties of this or any of the subsequently approved antiretroviral agents to optimise therapy. This is partly because of the limits of technologies developed to measure clinically relevant forms and concentrations of these drugs, and partly because the clinical community has been slow to recognise the potential benefits of pharmacokinetic optimisation of nucleoside analogue therapy in any disease. Nonetheless, for some of these agents, progress in understanding the relationship between pharmacokinetics and pharmacodynamics has been made. With zidovudine, for example, even though plasma concentrations have little clinical utility, evidence suggests that concentrations of active phosphorylated forms of zidovudine inside target cells are related to disease progression and toxicity. Furthermore, a decreased ability to phosphorylate zidovudine might be a prerequisite for the emergence of zidovudine-resistant HIV strains. Measurements of phosphorylated zidovudine inside cells similarly suggest that 100 mg of oral zidovudine every 8 hours approximates the optimal initial dosage regimen in asymptomatic patients. Increased plasma didanosine concentrations have been associated with several measures of clinical improvement in patients, and may be associated with an increased risk of toxicity as well. For zalcitabine and stavudine, however, the picture is much less clear. Their pharmacokinetic and pharmacodynamic relationships have not been studied in patients. Furthermore, there is insufficient data on the effects of age, gender, race and concurrent underlying conditions on the pharmacokinetics of all of these agents. Mounting evidence suggests that monitoring of these compounds could lead to individually optimised intervention strategies. Given the marginal benefits of therapy with these agents, their proven toxic effects and the lack of proven alternatives, it is critical that the clinical community strive to make the most effective use of these agents in the treatment of their patients.

Pharmacokinetics of zidovudine and dideoxyinosine alone and in combination in children with HIV infection

The pharmacokinetics of zidovudine (ZDV) and dideoxyinosine (ddI) were investigated following administration alone and in combination to children with symptomatic HIV disease. The children were studied on three separate occasions and received ZDV 200 mg m-2, ddI 100 mg m2 or a combination of ZDV 200 mg m-2 plus ddI 100 mg m-2. The administration of ddI did not significantly alter ZDV pharmacokinetics. The area under the curve (AUC) was 14.2 +/- 4.9 and 15.8 +/- 7.2 mumol l-1 h and elimination half-life (t1/2, z) was 1.4 +/- 0.4 and 1.2 +/- 0.2 h in the absence and presence of ddI respectively. The peak concentration (Cmax), time to peak (tmax) and apparent oral clearance (CL/F) were also unchanged. The administration of ZDV had no significant effect on ddI Cmax, tmax, t1/2,z, or CL/F, however the AUC was reduced by 19% (5.9 +/- 2.9 to 4.8 +/- 2.7 mumol l-1 h; P < 0.05). This study suggests that ZDV and ddI may be co-administered to children with symptomatic HIV disease without concern of a clinically relevant pharmacokinetic drug interaction.

Effects of dideoxyinosine and dideoxycytidine on the intracellular phosphorylation of zidovudine in human mononuclear cells

1. Zidovudine (3'-azido-2',3'-dideoxythymidine; AZT; ZDV) is a dideoxynucleoside analogue active against human immunodeficiency virus (HIV). We are currently investigating the intracellular metabolism of ZDV to its putative active triphosphate form (ZDV triphosphate) in peripheral blood mononuclear cells and a lymphoblastoid cell line (h1A2v2). 2. Optimal conditions for intracellular phosphate formation in peripheral blood mononuclear cells occurred following a 72 h preincubation with the mitogen phytohaemagglutinin at a concentration of 10 micrograms ml-1. ZDV was metabolized predominantly to the monophosphate with smaller amounts of the di- and triphosphate anabolites. There was considerable inter- and intraindividual variability in phosphate formation in peripheral blood mononuclear cells. A similar pattern of phosphorylation was seen with the h1A2v2 lymphoblastoid cell line with ZDV monophosphate being the major metabolite. 3. With increasing interest in combination nucleoside analogue therapy in HIV-positive patients it is important to know if an interaction occurs at the level of phosphorylation. Neither dideoxyinosine (ddI) or dideoxycytidine (ddC) significantly reduced the intracellular phosphorylation of ZDV in either peripheral blood mononuclear cells or h1A2v2 cells. In contrast thymidine always gave marked inhibition (e.g. at 2.0 microM, 89% inhibition of total phosphate formation in peripheral blood mononuclear cells and 79% in h1A2v2 cells). It is, therefore, unlikely that in vivo either ddI or ddC will perturb ZDV phosphorylation.