Efficacy of triple combination therapy with zidovudine (ZDV) plus zalcitabine (ddC) plus lamivudine (3TC) versus double (ZDV+3TC) combination therapy in patients previously treated with ZDV+ddC

HIV nucleoside reverse transcriptase inhibitors

More than 40 years into the pandemic, HIV remains a global burden and as of now, there is no cure in sight. Fortunately, highly active antiretroviral therapy (HAART) has been developed to manage and suppress HIV infection. Combinations of two to three drugs targeting key viral proteins, including compounds inhibiting HIV reverse transcriptase (RT), have become the cornerstone of HIV treatment. This review discusses nucleoside reverse transcriptase inhibitors (NRTIs), including chain terminators, delayed chain terminators, nucleoside reverse transcriptase translocation inhibitors (NRTTIs), and nucleotide competing RT inhibitors (NcRTIs); focusing on their history, mechanism of action, resistance, and current clinical application, including long-acting regimens.

Finding a Role for Zalcitabine in the HAART era

Zalcitabine (ddC) is a nucleoside analogue reverse transcriptase inhibitor with demonstrated clinical benefit in combination use. More widespread use of zalcitabine has been limited by a number of factors including peripheral neuropathy and three times daily dosing. However, screening for the risk factors for peripheral neuropathy may enable a reduction in the incidence of neuropathy to below 10%. Additionally, new data on the use of zalcitabine twice daily suggest, based on the long intracellular half-life of the active triphosphate, that this is feasible. Additionally, while limited data exist for zalcitabine in true HAART combinations, data from small trials suggest a similar proportion of responders to standard HAART regimens.

Evolution of HIV drug Resistance in Zidovudine/Zalcitabine- and Zidovudine/ Didanosine-Experienced Patients Receiving Lamivudine-Containing Combination Therapy

Drug resistance patterns and their correlation to treatment response 24 weeks after addition of lamivudine to a previous zidovudine/zalcitabine or zidovudine/ didanosine combination treatment, or after substitution of lamivudine for the zalcitabine or didanosine in these combinations, were evaluated in 21 patients with CD4 counts between 50 and 300 cells/mm3. Clinical, immunological and virological outcomes were measured at baseline and at weeks 4, 12 and 24. Drug resistance was assessed using the recombinant virus assay and direct solid-phase sequencing of the reverse transcriptase gene. Viral load decreased by an average of 0.78 log RNA copies/ml at week 4, but returned to baseline at week 24 for most patients. CD4 cell counts increased in all treatment groups without reaching statistical significance. Resistance to lamivudine was readily acquired in all four treatment groups and was often associated with a partial reversal of phenotypic zidovudine resistance and a moderate increase in the 50% inhibitory concentrations for didanosine and zalcitabine. One patient receiving the zidovudine/didanosine combination treatment harboured multi-nucleoside analogue-resistant virus. A poor response to lamivudine tended to be related to the extent of pre-existing zidovudine resistance.

Significant Drug Interactions in the HIV Patient

Current treatment guidelines advocate the use of triple combination antiretroviral therapy for the management of human immunodeficiency virus (HIV) infection with one protease inhibitor (PI) or nonnucleoside reverse transcriptase inhibitor (NNRTI) plus two nucleoside reverse transcriptase inhibitors (NRTIs). Additionally, patients often receive medications for prophylaxis and treatment of opportunistic infections and comorbid illnesses. Polypharmacy with potent medications places the patient with HIV at significant risk of drug interactions. The purpose of this review is to assist clinicians regarding awareness and management of clinically significant drug interactions frequently encountered in the HIV population.

Pharmacodynamic and pharmacokinetic interactions are discussed in depth with several tables of significant interactions encountered in the patient with HIV. Certain combinations of NRTIs have antagonistic antiretroviral action (e.g., zidovudine plus stavudine), and thus should be avoided. All of the currently available PIs and delavirdine are cytochrome (CYP) 450 inhibitors and can cause significant inhibition of concomitant drug metabolism. Nevirapine is a CYP 450 inducer and can cause induction of drug metabolism, whereas efavirenz may cause inhibition or induction. Drug-herbal interactions are reviewed, including the decreased concentrations of indinavir following the administration of St. John’s wort. Additionally, the pharmacist’s role as an integral member of the HIV patient care team is discussed.

Synergistic Inhibition of Human Immunodeficiency Virus Type 1 Replication in Vitro by Two- and Three-Drug Combinations of Delavirdine, Lamivudine and Zidovudine

Delavirdine (DLV), a non-nucleoside human immunodeficiency virus type 1 (HIV-1) reverse transcriptase inhibitor, was evaluated in two- and three-drug combination regimens with lamivudine (3TC) and zidovudine (ZDV). The effect of continuous drug treatment on HIV-1JR-CSF replication in human peripheral blood mononuclear cells was measured by an ELISA for p24 core antigen. Drug synergy, estimated by the combination index method and the method of Pritchard & Shipman, was observed when DLV was combined with 3TC over a range of drug concentrations (DLV at 1, 3, 10, 30 and 100 nM; 3TC at 3, 10, 30, 100 and 300 nM). Two-drug combinations of ZDV and DLV at a 1: 3 ratio or ZDV and 3TC at a 1: 10 ratio were synergistic at greater than 75% inhibition levels. Three-drug combinations of ZDV, DLV and 3TC (ZDV at 0.3, 1, 3 and 10 nM; DLV at 1, 3, 10 and 30 nM; 3TC at 3, 10, 30 and 100 nM) at the ratio of 1: 3: 10 also yielded significant synergistic effects. None of the combinations studied showed significant additive or synergistic drug toxicity. These in vitro data suggest that DLV should be evaluated in two- and three-drug combinations with 3TC and ZDV in clinical trials.

Relevance of the organic cation transporters 1 and 2 for antiretroviral drug therapy in human immunodeficiency virus infection

Carrier-mediated transport across cell membranes is an important determinant of activity, resistance, and toxicity of chemotherapeutic agents including antiretroviral (ARV) drugs (ARDs). The organic cation transporters (OCTs) 1 and 2 have been implicated in the translocation of different cationic drugs but so far were insufficiently tested for interactions with ARDs. Here, we assessed among cationic drugs commonly used in human immunodeficiency virus (HIV) therapy inhibitors and substrates of OCTs, and analyzed the tissue distribution of OCTs and their expression in lymph nodes (LNs), the primary intracellular target of HIV and ARDs. Inhibitors were identified by measuring the attenuated uptake of the radiolabeled model substrate 1-methyl-4-phenylpyridinium into OCT-transfected human embryonic kidney-293 cells in the presence of ARDs. Substrates were identified by measuring OCT-specific intracellular accumulation using liquid chromatography/tandem mass spectrometry. Inhibitory drugs were (in order of increasing potency): nelfinavir < ritonavir < saquinavir < indinavir < trimethoprim < pentamidine, with consistently lower IC(50) values determined for OCT1. Substrates with highest transport efficacy (V(max)/K(m)) were lamivudine (OCT1, 8 microl/mg protein/min; OCT2, 4.4 microl/mg protein/min) and zalcitabine (OCT1, 4.1 microl/mg protein/min; OCT2, 2.6 microl/mg protein/min). Using quantitative real-time polymerase chain reaction, a marked expression level of OCT1 was detected in human samples of liver, ovary, prostate, and testis, and of OCT2 in kidney, colon, heart, skeletal muscle, and testis. Expression of OCTs in LNs was low in HIV-negative control individuals but dramatically increased in HIV-infected persons. These data suggest that drug interactions about the OCTs may be relevant for the ARV therapy, in particular by influencing drug accession to infected tissues and hepatic or renal elimination.

Long-Term Foscarnet Therapy Remodels Thymidine Analogue Mutations and Alters Resistance to Zidovudine and Lamivudine in HIV-1

Objective

To study the evolution of multi-drug-resistant HIV-1 in treatment-experienced patients receiving foscarnet (PFA) as part of salvage therapy and to investigate the virological consequences of emerging mutations.

Methods

Genotypic and phenotypic resistance tests were performed on plasma viruses from seven patients at baseline and during treatment with PFA. The phenotypic effects of mutations suspected to be associated with PFA resistance were evaluated by site-directed mutagenesis of wild-type or thymidine analogue mutations (TAM)-carrying pNL4–3. Reversion of single mutations was performed in a patient-derived recombinant clone.

Results

Baseline multi-drug-resistant isolates exhibited hypersusceptibility to PFA. In two patients who received >12 months of PFA treatment, a novel mutation pattern including K70G, V75T, K219R and L228R emerged. These viruses had 3–6-fold resistance to PFA, a 2–20-fold decrease in resistance to zidovudine compared to baseline, and 14–39-fold resistance to lamivudine, in the absence of M184V. In wild-type clones mutations K70G and V75T induced moderate PFA resistance. In the case of TAMs, combinations of ≥3 mutations (K70G+K219R+L228R±V75T) induced PFA resistance and decreased zidovudine resistance 3–13-fold. These mutants exhibited high-level lamivudine resistance (>20-fold) without mutation M184V. Reversion of K70G→R and K219R→E in a patient-derived clone confirmed the contribution of individual mutations and the negative association between PFA resistance and zidovudine resistance.

Conclusions

In the context of multiple TAMs, hypersusceptibility to PFA was observed and a novel pattern of resistance, including alternative amino acid substitutions at TAM loci, emerged. This mutational pattern was associated with decreases in zidovudine resistance and surprisingly high-level lamivudine resistance.

Cytokines involved in Toxoplasmic encephalitis

Reactivation of cerebral toxoplasmosis occurs in approximately 30% of acquired immune deficiency syndrome (AIDS) patients who are seropositive for Toxoplasma gondii and a change in the levels of cytokines during this relapse is observed. Several cytokines are able to initiate meningeal inflammation and may play a role in the immunopathogenesis of the disease. The induction of a type 1 inflammatory cytokine response is a key event in the initiation of immunity to T. gondii. Interleukin (IL)-10 production in infected brain facilitates the persistence of parasites by down-regulating the intracerebral immune response. The in vivo and in vitro models are very numerous and this may explain the heterogeneity of the results. The role of gamma interferon is important because it is the principal mediator inducing a host resistance against T. gondii. Several cytokines stimulating or decreasing interferon (IFN)-gamma levels are reported. The particular case of AIDS patients whose humoral response is altered, is studied.

Pharmacology of nucleoside and nucleotide reverse transcriptase inhibitor-induced mitochondrial toxicity

OBJECTIVE

This paper reviews the function of the mitochondria and the mechanisms by which nucleoside and nucleotide reverse transcriptase inhibitors (NRTIs) cause mitochondrial toxicity.

BACKGROUND

Highly active antiretroviral therapy (HAART) reduces rates of morbidity and mortality due to HIV disease. However, long-term treatment with these drugs may be associated with adverse effects. Nucleoside and nucleotide analogues are potent inhibitors of HIV reverse transcriptase and have become the cornerstone of HAART. Unfortunately, these drugs have also been shown to inhibit cellular polymerases, most notably mitochondrial DNA polymerase gamma.

RESULTS

Studies of the NRTIs in enzyme assays and cell cultures demonstrate the following hierarchy of mitochondrial DNA polymerase gamma inhibition: zalcitabine > didanosine > stavudine > lamivudine > zidovudine > abacavir. In vitro investigations have also documented impairment of the mitochondrial enzymes adenylate kinase and the adenosine diphosphate/adenosine triphosphate translocator. Inhibition of DNA polymerase gamma and other mitochondrial enzymes can gradually lead to mitochondrial dysfunction and cellular toxicity. The clinical manifestations of NRTI-induced mitochondrial toxicity resemble those of inherited mitochondrial diseases (ie, hepatic steatosis, lactic acidosis, myopathy, nephrotoxicity, peripheral neuropathy, and pancreatitis). Fat redistribution syndrome, or HIV-associated lipodystrophy, is another side effect attributed in part to NRTI therapy. The morphologic and metabolic complications of this syndrome are similar to those of the mitochondrial disorder known as multiple symmetric lipomatosis: suggesting that this too may be related to mitochondrial toxicity. The pathophysiology of less common adverse effects of nucleoside analogue therapy, such as diabetes, ototoxicity, and retinal lesions, may be related to mitochondrial dysfunction but have not been adequately studied.

CONCLUSION

NRTls can block both HIV reverse transcriptase and mitochondrial DNA polymerase gamma. Inhibition of the latter enzyme is the most likely cause of the adverse effects associated with these drugs.

A critical analysis of the pharmacology of AZT and its use in AIDS

The triphosphorylated form of the nucleoside analogue 3'-azido-3'-deoxythymidine (Zidovudine, AZT) is claimed to interrupt the HIV replication cycle by a selective inhibition of viral reverse transcriptase, thereby preventing the formation of new proviral DNA in permissive, uninfected cells. Given that initial HIV infection of an individual instigates abundant HIV replication from inception until death, and that the life of infected T-cells is only several days, the administration of AZT should lead both in vitro and in vivo (i) to decreased formation of proviral DNA; and thus (ii) to decreased frequencies of 'HIV isolation' (detection of p24 or reverse transcription or both) in stimulated cultures/cocultures of T-cells from seropositive individuals; (iii) to decreased synthesis of HIV p24 and RNA ('antigenaemia', 'plasma viraemia', 'viral load') ultimately resulting in low or absent levels of all three parameters; and (iv) to a perfect and direct correlation between all these parameters. A critical analysis of the presently available data shows that no such evidence exists, an outcome not unexpected given the pharmacological data on AZT. HIV experts all agree that only the triphosphorylated form of AZT (AZTTP) and not the unphosphorylated form administered to patients, nor its mono- or diphosphate, is the active agent. Furthermore, the mechanism of action is the ability of AZTTP to halt the formation of HIV-DNA (chain termination). However, although this claim was posited from the outset, AZT underwent clinical trials and was introduced as a specific anti-HIV drug many years before there were any data proving that the cells of patients are able to triphosphorylate the parent compound to a level considered sufficient for its putative pharmacological action. Notwithstanding, from the evidence published since 1991 it has become apparent that no such phosphorylation takes place and thus AZT cannot possess an anti-HIV effect. However, the scientific literature does elucidate: (i) a number of biochemical mechanisms which predicate the likelihood of widespread, serious toxicity from use of this drug; (ii) in vitro data proving that AZT has significant antibacterial and antiviral properties which confound interpretation of its effects when administered to patients. Based on all these data it is difficult if not impossible to explain why AZT was introduced and still remains the most widely recommended and used anti-HIV drug.

Comparison of the LiPA HIV-1 RT test, selective PCR and direct solid phase sequencing for the detection of HIV-1 drug resistance mutations

The performance to detect drug resistance mutations in the reverse transcriptase gene of HIV-1 was compared for direct solid phase sequencing, selective polymerase chain reaction (PCR) using the amplification refractory mutation system (ARMS) and the new line probe assay (LIPA) HIV-1 RT. The three tests were undertaken on 50 plasma samples from 25 treatment-experienced patients under combination therapy with dideoxynucleoside analogues. LiPA HIV-1 RT gave interpretable results in 80 to 96% of the samples depending on the codon of interest. In 2% of the samples a failure to amplify resulted in uninterpretable results for sequencing. ARMS gave no result in seven samples (14%). Overall, there was a 73 to 100% concordance between the three methods. In this study, LiPA HIV-1 RT proved to be an accurate and reliable alternative to DNA sequencing for the detection of drug resistance mutations in patient samples.

Mechanisms of clinical resistance by HIV-I variants to zidovudine and the paradox of reverse transcriptase sensitivity

Even with the development of novel nucleoside analog inhibitors, zidovudine (AZT or 3'-azido-3'-deoxythymidine) remains a potent and frequently prescribed antiretroviral therapy for HIV-positive individuals. Failure of AZT in monotherapy due to the emergence of drug-resistant virus has not excluded it from use in most combination therapies with other nucleoside analogs, non-nucleoside reverse transcriptase inhibitors and protease inhibitors. Thus, an understanding of the mechanism of AZT resistance could be the key in predicting the failure of many treatment strategies. In this review, the occurrence, characterization and ramification of AZT resistance in HIV-positive individuals will be discussed in the context of genotypic and phenotypic analyses of AZT-resistant viruses and reverse transcriptases. The mechanisms of resistance to AZT may be distinct from the mechanisms of resistance to other nucleoside analogs.