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

Visualizing nucleic acid metabolism using non-natural nucleosides and nucleotide analogs

Nucleosides and their corresponding mono-, di-, and triphosphates play important roles in maintaining cellular homeostasis. In addition, perturbations in this homeostasis can result in dysfunctional cellular processes that cause pathological conditions such as cancer and autoimmune diseases. This review article discusses contemporary research areas applying nucleoside analogs to probe the mechanistic details underlying the complexities of nucleoside metabolism at the molecular and cellular levels. The first area describes classic and contemporary approaches used to quantify the activity of nucleoside transporters, an important class of membrane proteins that mediate the influx and efflux of nucleosides and nucleobases. A focal point of this section is describing how biophotonic nucleosides are replacing conventional assays employing radiolabeled substrates to study the mechanism of these proteins. The second section describes approaches to understand the utilization of nucleoside triphosphates by cellular DNA polymerases during DNA synthesis. Emphasis here is placed on describing how novel nucleoside analogs such as 5-ethynyl-2'-deoxyuridine are being used to quantify DNA synthesis during normal replication as well as during the replication of damaged DNA. In both sections, seminal research articles relevant to these areas are described to highlight how these novel probes are improving our understanding of these biological processes. This article is part of a Special Issue entitled: Physiological Enzymology and Protein Functions.

Neurotoxic effects of AZT on developing and adult neurogenesis

Azidothymidine (AZT) is a synthetic, chain-terminating nucleoside analog used to treat HIV-1 infection. While AZT is not actively transported across the blood brain barrier, it does accumulate at high levels in cerebrospinal fluid, and subsequently diffuses into the overlying parenchyma. Due to the close anatomical proximity of the neurogenic niches to the ventricular system, we hypothesize that diffusion from CSF exposes neural stem/progenitor cells and their progeny to biologically relevant levels of AZT sufficient to perturb normal cell functions. We employed in vitro and in vivo models of mouse neurogenesis in order to assess the effects of AZT on developing and adult neurogenesis. Using in vitro assays we show that AZT reduces the population expansion potential of neural stem/progenitor cells by inducing senescence. Additionally, in a model of in vitro neurogenesis AZT severely attenuates neuroblast production. These effects are mirrored in vivo by clinically-relevant animal models. We show that in utero AZT exposure perturbs both population expansion and neurogenesis among neural stem/progenitor cells. Additionally, a short-term AZT regimen in adult mice suppresses subependymal zone neurogenesis. These data reveal novel negative effects of AZT on neural stem cell biology. Given that the sequelae of HIV infection often include neurologic deficits—subsumed under AIDS Dementia Complex (Brew, 1999)—it is important to determine to what extent AZT negatively affects neurological function in ways that contribute to, or exacerbate, ADC in order to avoid attributing iatrogenic drug effects to the underlying disease process, and thereby skewing the risk/benefit analysis of AZT therapy.

Fully-spliced HIV-1 RNAs are reverse transcribed with similar efficiencies as the genomic RNA in virions and cells, but more efficiently in AZT-treated cells

We have shown previously that HIV actively and selectively packages the spliced HIV RNAs into progeny virions. In the present study, by using a RT-QPCR and QPCR strategies, we show that spliced viral RNAs are present in infectious particles and consequently participate, along with the unspliced genomic RNA, to some of the early steps of infection such as the reverse transcription step. This work provides the first quantitative data on reverse transcription of the fully spliced viral RNAs, also called the early transcripts, in target cells but also inside virions. The latter results were obtained by measuring the natural endogenous reverse transcription activity directly on intact HIV-1 particles. Our study reveals that spliced HIV RNAs are reverse transcribed as efficiently as the genomic RNA, both in cells and virions. Interestingly, we also show that reverse transcription of spliced RNAs is 56-fold less sensitive to the inhibitor AZT than reverse transcription of the genomic RNA. Therefore, the selection mediated by inhibitors of reverse transcription used to treat patients could lead to increased representativeness of spliced forms of HIV, thus favoring recombination between the HIV DNA species and facilitating HIV recovery.

Pathogenesis of osteopenia/osteoporosis induced by highly active anti-retroviral therapy for AIDS

The advent of highly active anti-retroviral therapy (HAART) has dramatically decreased the rate of AIDS-related mortality and significantly extended the life span of patients with AIDS. A variety of metabolic side effects are associated with these therapies, one of which is metabolic bone disease. A higher prevalence of osteopenia and osteoporosis in HIV-infected patients receiving anti-retroviral therapy than in patients not on therapy has now been reported in several studies. Several factors have been demonstrated to influence HIV-associated decreases in bone mineral density (BMD), including administration of nucleoside reverse transcriptase inhibitors (NRTIs). In this article, discussion will focus on the molecular pathogenesis and treatment of HAART-associated osteopenia and osteoporosis.

Effectiveness of Short-Term and Long-Term Zidovudine Prophylaxis on Detection of HIV-1 Subtype E in Human Placenta and Vertical Transmission

Antiretroviral treatment with zidovudine (ZDV) from the 14th week until the end of pregnancy has markedly reduced the vertical transmission rate of HIV-1 in Europe and North America. A shorter duration of treatment has reduced this rate in Africa and Southeast Asia to a lesser degree. In Southeast Asia, subtype E is the major subtype rather than subtype B as in Western countries. The goals of this study were to determine the optimal duration of ZDV prophylaxis for subtype E and to confirm its effectiveness at the histologic level. Fifty pregnant women seropositive for HIV-1 subtype E were given ZDV prophylaxis consisting of 300 mg administered twice daily, switching to 300 mg administered every 3 hours from the onset of labor until delivery. Twenty-seven received "short-term" ZDV lasting 14 to 35 days before delivery, whereas the other 23 received "long-term" ZDV lasting 62 to 92 days. The effectiveness of ZDV prophylaxis was assessed by detection of HIV-1 in the placenta using in situ polymerase chain reaction (PCR). All babies in this study were tested up to one year of age. Three were not positive until after one month of age, but one was positive as a neonate. Four neonates were positive for HIV-1 as detected by PCR on peripheral blood, including one in the neonatal period. All cases were from the short-term prophylaxis group. Decidual glandular epithelial cells were the only cell type in the placenta that expressed HIV proviral DNA under ZDV prophylaxis. Sixty-seven percent of placentas in the short-term ZDV group showed more than occasional positive cells compared with 22% in the group receiving long-term ZDV prophylaxis (P < 0.02). This is first study to compare the effectiveness of short-term and long-term ZDV prophylaxis with respect to the presence of HIV in the placenta. Our study shows that longer (at least 60 days) prophylaxis is more effective in reducing HIV expression in the placenta and is associated with reduced transmission to neonates.