Zidovudine with probenecid: a warning

Inhibitory Effects of Probenecid on Pharmacokinetics of Tenofovir Disoproxil Fumarate and Emtricitabine for On-Demand HIV Preexposure Prophylaxis

In a randomized, crossover pharmacokinetic study in healthy volunteers (N = 14), a single dose of 2 g probenecid (PRO)-boosted 600 mg tenofovir disoproxil fumarate (TDF)/400 mg emtricitabine (FTC) (test (T) +PRO) was compared with the current on-demand HIV preexposure prophylaxis from the IPERGAY study (a 600 mg TDF/400 mg FTC on day 1 and 300 mg TDF/200 mg FTC on days 2 and 3) (control, C IPERGAY). PRO increased mean single-dose area under the plasma concentration-time curve extrapolated to infinity (AUC_0-∞,SD ) of tenofovir (TFV) and FTC by 61% and 68%, respectively. The TFV-diphosphate (TFV-DP) concentrations in peripheral blood mononuclear cells were higher (~30%) at 24 hours in T +PRO but then fell significantly lower (~40%) at 72 hours compared with C IPERGAY. The interaction between FTC and PRO was unexpected and novel. Further study is needed to determine if this PRO-boosted TDF/FTC regimen would be clinically effective.

Combination of probenecid-sulphadoxine-pyrimethamine for intermittent preventive treatment in pregnancy

The antifolate sulphadoxine-pyrimethamine (SP) has been used in the intermittent prevention of malaria in pregnancy (IPTp). SP is an ideal choice for IPTp, however, as resistance of Plasmodium falciparum to SP increases, data are accumulating that SP may no longer provide benefit in areas of high-level resistance. Probenecid was initially used as an adjunctive therapy to increase the blood concentration of penicillin; it has since been used to augment concentrations of other drugs, including antifolates. The addition of probenecid has been shown to increase the treatment efficacy of SP against malaria, suggesting that the combination of probenecid plus SP may prolong the useful lifespan of SP as an effective agent for IPTp. Here, the literature on the pharmacokinetics, adverse reactions, interactions and available data on the use of these drugs in pregnancy is reviewed, and the possible utility of an SP-probenecid combination is discussed. This article concludes by calling for further research into this potentially useful combination.

Antiretroviral drug interactions

It has recently been estimated that persons with the acquired immunodeficiency syndrome (AIDS) receive on average 5.6 prescription medications throughout their disease course, and this number may be as high as 9 [1,2]. With the development and testing of new antiretroviral agents and drugs for opportunistic infections associated with human immunodeficiency virus (HIV) disease, the issue of polypharmacy and multiple drug interactions will become increasingly complex. Since antiretroviral therapy and treatment or prophylaxis of opportunistic infections is lifelong, the nature of these interactions requires delineation to provide an optimal pharmacologic strategy for the use of these agents in combination. This article will address antiretroviral drug interactions from a pharmacokinetic and pharmacodynamic perspective. A background on the clinical pharmacology of antiretroviral agents is provided as is a discussion of selected interactions.

Drug interactions at the blood-brain barrier: fact or fantasy?

There is considerable interest in the therapeutic and adverse outcomes of drug interactions at the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB). These include altered efficacy of drugs used in the treatment of CNS disorders, such as AIDS dementia and malignant tumors, and enhanced neurotoxicity of drugs that normally penetrate poorly into the brain. BBB- and BCSFB-mediated interactions are possible because these interfaces are not only passive anatomical barriers, but are also dynamic in that they express a variety of influx and efflux transporters and drug metabolizing enzymes. Based on studies in rodents, it has been widely postulated that efflux transporters play an important role at the human BBB in terms of drug delivery. Furthermore, it is assumed that chemical inhibition of transporters or their genetic ablation in rodents is predictive of the magnitude of interaction to be expected at the human BBB. However, studies in humans challenge this well-established paradigm and claim that such drug interactions will be lesser in magnitude but yet may be clinically significant. This review focuses on current known mechanisms of drug interactions at the blood-brain and blood-CSF barriers and the potential impact of such interactions in humans. We also explore whether such drug interactions can be predicted from preclinical studies. Defining the mechanisms and the impact of drug-drug interactions at the BBB is important for improving efficacy of drugs used in the treatment of CNS disorders while minimizing their toxicity as well as minimizing neurotoxicity of non-CNS drugs.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

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.

Drug interactions between antiretroviral drugs and comedicated agents

HIV-infected individuals usually receive a wide variety of drugs in addition to their antiretroviral drug regimen. Since both non-nucleoside reverse transcriptase inhibitors and protease inhibitors are extensively metabolised by the cytochrome P450 system, there is a considerable potential for pharmacokinetic drug interactions when they are administered concomitantly with other drugs metabolised via the same pathway. In addition, protease inhibitors are substrates as well as inhibitors of the drug transporter P-glycoprotein, which also can result in pharmacokinetic drug interactions. The nucleoside reverse transcriptase inhibitors are predominantly excreted by the renal system and may also give rise to interactions. This review will discuss the pharmacokinetics of the different classes of antiretroviral drugs and the mechanisms by which drug interactions can occur. Furthermore, a literature overview of drug interactions is given, including the following items when available: coadministered agent and dosage, type of study that is performed to study the drug interaction, the subjects involved and, if specified, the type of subjects (healthy volunteers, HIV-infected individuals, sex), antiretroviral drug(s) and dosage, interaction mechanism, the effect and if possible the magnitude of interaction, comments, advice on what to do when the interaction occurs or how to avoid it, and references. This discussion of the different mechanisms of drug interactions, and the accompanying overview of data, will assist in providing optimal care to HIV-infected patients.

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.

Probenecid hypersensitivity in AIDS: a case report

BACKGROUND

Cidofovir plus probenecid is a new therapeutic alternative for refractory cytomegalovirus and acyclovir-resistant herpes infections in AIDS patients. Probenecid is used in conjunction with the antiherpetic medication (cidofovir) in order to reduce the incidence of nephrotoxicity by cidofovir.

OBJECTIVE

To present therapeutic alternatives for successful administration of probenecid to AIDS patients who develop a hypersensitivity reaction to the medication.

METHODS AND RESULTS

We describe a patient with AIDS who was being treated with the cidofovir/probenecid combination for a peri-anal acyclovir-resistant herpetic infection. The patient subsequently developed a cutaneous hypersensitivity reaction to probenecid alone. A pretreatment regimen consisting of prednisone, H1 and H2 blockers was administered before the dosing of probenecid in order for the patient to continue with the antiviral therapy.

CONCLUSION

Cutaneous hypersensitivity reactions to probenecid may be seen more frequently with the increasing use of cidofovir in AIDS patients. Our pre-treatment protocol is one therapeutic alternative to be considered in order to continue with probenecid.

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.

Anticytomegaloviral activity and safety of cidofovir in patients with human immunodeficiency virus infection and cytomegalovirus viruria

Cidofovir (HPMPC; (S)-1-[3-hydroxy-2-(phosphonylmethoxy)propyl]cytosine) is a nucleotide analog with activity against human cytomegalovirus (CMV). A phase I/II dose escalation trial was conducted with asymptomatic human immunodeficiency virus (HIV)-infected patients with CMV viruria to determine its pharmacokinetics, maximally tolerated dose, and preliminary antiviral activity against CMV. Qualitative CMV blood and urine cultures were monitored weekly to assess anti-CMV activity. Twenty-one HIV-infected persons with CD4 counts from 0 to 389 cells per microliters (median, 39) were enrolled in six dose-ranging groups. The first five groups enrolled four patients each to receive cidofovir infusions either weekly or biweekly for 4 weeks or every 3 weeks for 12 weeks. The sixth group enrolled one patient who received infusions of 5 mg/kg of body weight every other week. Patients receiving 0.5 or 1.5 mg/kg twice weekly experienced no serious toxicity. The first two patients who received 5 mg/kg twice weekly developed glycosuria and 2+ proteinuria. Subsequent patients received concomitant probenecid to attempt to ameliorate renal toxicity. Seventeen patients experienced proteinuria on one or more occasions; 6 of them experienced at least 2+ proteinuria. Four patients did not complete the study as planned because of renal toxicity. Positive CMV urine cultures reverted to negative in 2 of 8 patients receiving doses of < or = 1.5 mg/kg twice weekly and 11 of 13 patients receiving higher doses. Cidofovir has in vivo anti-CMV activity demonstrated by prolonged clearing of CMV viruria, although this observation is tempered by the fact that clearance of viremia could not be demonstrated. The dose-limiting toxicity is renal; however, concurrent administration of probenecid may be protective. The maximally tolerated weekly intravenous dose with probenecid is approximately 5 mg/kg. Efficacy trials with CMV disease will define the therapeutic utility and optimal dosing interval for cidofovir.

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.

Zidovudine. An update of its pharmacodynamic and pharmacokinetic properties, and therapeutic efficacy

Zidovudine remains the mainstay in the treatment of patients infected with human immunodeficiency virus (HIV). The drug delays disease progression to acquired immunodeficiency syndrome (AIDS) and to AIDS-related complex (ARC), reduces opportunistic infections, and increases survival in patients with advanced HIV infection. There is evidence to suggest that zidovudine also delays disease progression in patients with mild symptomatic disease. Although one study has shown zidovudine to have no significant beneficial effects on survival or disease progression in patients with asymptomatic HIV infection, several other studies have shown zidovudine to delay disease progression in this patient group. Results from related ongoing studies are awaited with interest. Zidovudine reduces the incidence of AIDS dementia complex (ADC) and appears to prolong survival in these patients, and improves other neurological complications of HIV infection. The drug also appears to enhance the efficacy of interferon-alpha in patients with Kaposi's sarcoma. Although zidovudine is widely used as postexposure prophylaxis following accidental exposure to HIV, its efficacy in preventing seroconversion is unclear. Whether zidovudine prevents vertical transmission also remains to be determined. The overall efficacy of zidovudine in the treatment of children with HIV infection appears similar to that in adults despite more rapid disease progression in younger patients. Zidovudine-resistant isolates can emerge as early as after 2 months' therapy, and primary infection with zidovudine-resistant strains has been documented. Both zidovudine resistance and the syncytium-inducing HIV phenotype appear to be associated with poor clinical outcome. However, zidovudine resistance may revert on drug withdrawal or switching to an alternative therapy. Zidovudine-associated haematotoxicity may be dose-limiting. Nonhaematological adverse events associated with zidovudine therapy are generally mild and usually resolve spontaneously. Dosages of approximately 500 to 600 mg/day appear to be at least as effective as dosages of 1200 to 1500 mg/day and are better tolerated in patients with less advanced disease. However, optimal dosage are unclear. Despite beneficial effects, zidovudine monotherapy is not curative. There is evidence to suggest that the concomitant administration of zidovudine with didanosine or zalcitabine is effective in patients with HIV disease progression despite receiving zidovudine monotherapy, and there is some evidence that concomitant zidovudine plus didanosine therapy is more effective than alternating monotherapy. However, results from studies of combination therapy in asymptomatic patients, and from comparative combination therapy studies are awaited. Cotherapy with agents that augment haematopoiesis allows the continuation of therapeutic zidovudine dosages.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of 3'-azido-3'-deoxythymidine catabolism by probenecid and acetaminophen in freshly isolated rat hepatocytes

Metabolic studies of 3'-azido-3'-deoxythymidine (AZT) in humans have demonstrated that this compound is primarily eliminated as a 5'-O-glucuronide, 3'-azido-3'-deoxy-5'-beta-D-glucopyranuronosylthymidine (GAZT), accounting for approximately 80% of the administered dose. Recently, we characterized the complete catabolic pathway of AZT in freshly isolated rat hepatocytes in suspension, demonstrating extensive formation of three catabolites, including GAZT, 3'-amino-3'-deoxythymidine (AMT), and 3'-amino-3'-deoxy-5'-beta-D-glucopyranuronosylthymidine (GAMT). The present study evaluated the effects of probenecid (PROB) and acetaminophen (ACET), two agents which are also metabolized by UDP-glucuronyltransferase, on the metabolism and transmembrane distribution of AZT in rat hepatocytes. Pre-exposure of cells to 350 microM PROB 30 min prior to the addition of 10 microM [3H]AZT decreased intracellular GAZT levels by approximately 10-fold. Interestingly, AMT formation was enhanced approximately 1.5-fold in the presence of PROB, probably resulting from increased AZT availability. In contract, pre-exposure to 50 microM ACET 30 min prior to addition of 10 microM [3H]AZT did not substantially alter AZT glucuronidation. Additionally, decreased AZT catabolism by PROB did not contribute to the formation of 5'-phosphorylated derivatives of AZT. Agents which undergo glucuronidation may thus not necessarily affect AZT conversion to GAZT, and their potential interactions should be investigated using in vitro systems prior to co-administration with AZT.

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.