Comparison of in vitro biological properties and mouse toxicities of three thymidine analogs active against human immunodeficiency virus

Repurposing nucleoside reverse transcriptase inhibitors (NRTIs) to slow aging

Repurposing drugs already approved in the clinic to be used off-label as geroprotectors, compounds that combat mechanisms of aging, are a promising way to rapidly reduce age-related disease incidence in society. Several recent studies have found that a class of drugs-nucleoside reverse transcriptase inhibitors (NRTIs)-originally developed as treatments for cancers and human immunodeficiency virus (HIV) infection, could be repurposed to slow the aging process. Interestingly, these studies propose complementary mechanisms that target multiple hallmarks of aging. At the molecular level, NRTIs repress LINE-1 elements, reducing DNA damage, benefiting the hallmark of aging of 'Genomic Instability'. At the organellar level, NRTIs inhibit mitochondrial translation, activate ATF-4, suppress cytosolic translation, and extend lifespan in worms in a manner related to the 'Loss of Proteostasis' hallmark of aging. Meanwhile, at the cellular level, NRTIs inhibit the P2X7-mediated activation of the inflammasome, reducing inflammation and improving the hallmark of aging of 'Altered Intercellular Communication'. Future development of NRTIs for human aging health will need to balance out toxic side effects with the beneficial effects, which may occur in part through hormesis.

Zidovudine in synergistic combination with fosfomycin: an in vitro and in vivo evaluation against multidrug-resistant Enterobacterales

Multidrug-resistant (MDR) Enterobacterales are a priority health issue with few treatment options. Recently, fosfomycin has been reconsidered for MDR bacterial infections. Zidovudine, licensed for the treatment of human immunodeficiency virus (HIV), has unexploited antibacterial properties and has been considered for drug repurposing. The aim of this study was to assess the effect of the combination of fosfomycin plus zidovudine against clinical MDR Enterobacterales isolates. Minimum inhibitory concentration (MIC) determination and checkerboard assays for 36 MDR Enterobacterales strains were performed. In addition, fosfomycin-resistant strains were evaluated using time-kill assay and in an in vivo Galleria mellonella infection model. Zidovudine and fosfomycin MICs ranged between 0.06 to >64 mg/L and 0.125 to >512 mg/L, respectively. A synergistic effect [fractional inhibitory concentration index (FICI) ≤0.5] was observed in 25 isolates and no antagonistic effect was observed in the remaining isolates. For 7 of 8 fosfomycin-resistant strains (MIC > 32 mg/L), zidovudine combination was able to restore fosfomycin susceptibility. These results were confirmed by time-kill assays. Fosfomycin + zidovudine presented greater larval survival (20-50%) than monotherapy. Synergistic activity was observed for fosfomycin + zidovudine in 69.4% of the tested strains. In vivo experiments confirmed the enhanced effectiveness of the combination. The zidovudine concentrations tested here can be reached in human serum using the actual licensed dosage, therefore this combination deserves further clinical investigation.

Comparison of Antiviral Effects of Zidovudine Phosphoramidate and Phosphorodiamidate Derivatives against HIV and ULV in vitro

A number of zidovudine phosphoramidate and phosphorodiamidate derivatives were prepared, including some previously unreported benzyl esterified amino acyl compounds. These were found to be active in vitro against the human immunodeficiency virus (HIV-1), and were tested subsequently in the S+L-tissue culture assay against urethane leukaemia virus (ULV), a murine leukaemia virus (MuLV). The fifteen compounds tested showed a similar range of activity against the two viruses. No active compounds were missed in the MuLV system which was usually more sensitive to antiviral effects. Five compounds showed some toxicity to the mouse cells only. We are using this system in parallel with HIV assays to identify those derivatives which will be tested subsequently against a murine retrovirus in vivo.

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.

Myristoylated derivatives of 2',3'-didehydro-2',3'-dideoxythymidine (stavudine) bi-functional prodrugs with potent anti-HIV-1 activity and low cytotoxicity

BACKGROUND

To improve in vitro antiviral activity and selectivity of stavudine (d4T), a range of its bi-functional prodrugs, 5'-O-myristoylated derivatives, have been synthesized.

METHODS

Stavudine 5'-O-myristoylated esters were synthesized using modified Parang's procedure. The cytotoxicity and anti-HIV activity was evaluated in the established MT-4 cell line. The level of p24 protein in culture medium was assayed, and EC50 and EC90 values were determined.

RESULTS

Excellent anti-HIV activity was obtained for stavudine derivatives 2',3'-didehydro-2',3'-dideoxy-5'-O-(11-thioethylundecanoyl) thymidine, 2',3'-didehydro-2',3'-dideoxy-5'-O-(12-thioethyldodecanoyl) thymidine and 5'-O-(12-azidododecanoyl)-2',3'-didehydro-2',3'-dideoxythymidine with C10 and C11 alkyl chains bearing thioethyl- and azido- substituents. These prodrugs were more potent than the parent stavudine, as is clear from their EC50 values: 2',3'-didehydro-2',3'-dideoxy-5'-O-(11-thioethylundecanoyl) thymidine (R=CO(CH2)10SC2H5, EC50 0.06 μM), 2',3'-didehydro-2',3'-dideoxy-5'-O-(12-thioethyldodecanoyl) thymidine (R=CO(CH2)11SC2H5, EC50 0.09 μM) and 5'-O-(12-azidododecanoyl)-2',3'-didehydro-2',3'-dideoxythymidine (R=CO(CH2)11N3, EC50 0.06 μM), while 50% cytotoxic concentration was >16.65 μM, >7.5 μM and >18.53 μM, respectively.

CONCLUSIONS

Overall data demonstrate that compounds 2',3'-didehydro-2',3'-dideoxy-5'-O-(11-thioethylundecanoyl) thymidine, 2',3'-didehydro-2',3'-dideoxy-5'-O-(12-thioethyldodecanoyl) thymidine and 5'-O-(12-azidododecanoyl)-2',3'-didehydro-2',3'-dideoxythymidine are very potent and selective anti-HIV agents and could be useful in treatment of HIV infections of the central nervous system.

Monitoring metabolic responses to chemotherapy in single cells and tumors using nanostructure-initiator mass spectrometry (NIMS) imaging

Background

Tissue imaging of treatment-induced metabolic changes is useful for optimizing cancer therapies, but commonly used methods require trade-offs between assay sensitivity and spatial resolution. Nanostructure-Initiator Mass Spectrometry imaging (NIMS) permits quantitative co-localization of drugs and treatment response biomarkers in cells and tissues with relatively high resolution. The present feasibility studies use NIMS to monitor phosphorylation of 3^′-deoxy-3^′-fluorothymidine (FLT) to FLT-MP in lymphoma cells and solid tumors as an indicator of drug exposure and pharmacodynamic responses.

Methods

NIMS analytical sensitivity and spatial resolution were examined in cultured Burkitt’s lymphoma cells treated briefly with Rapamycin or FLT. Sample aliquots were dispersed on NIMS surfaces for single cell imaging and metabolic profiling, or extracted in parallel for LC-MS/MS analysis. Docetaxel-induced changes in FLT metabolism were also monitored in tissues and tissue extracts from mice bearing drug-sensitive tumor xenografts. To correct for variations in FLT disposition, the ratio of FLT-MP to FLT was used as a measure of TK1 thymidine kinase activity in NIMS images. TK1 and tumor-specific luciferase were measured in adjacent tissue sections using immuno-fluorescence microscopy.

Results

NIMS and LC-MS/MS yielded consistent results. FLT, FLT-MP, and Rapamycin were readily detected at the single cell level using NIMS. Rapid changes in endogenous metabolism were detected in drug-treated cells, and rapid accumulation of FLT-MP was seen in most, but not all imaged cells. FLT-MP accumulation in xenograft tumors was shown to be sensitive to Docetaxel treatment, and TK1 immunoreactivity co-localized with tumor-specific antigens in xenograft tumors, supporting a role for xenograft-derived TK1 activity in tumor FLT metabolism.

Conclusions

NIMS is suitable for monitoring drug exposure and metabolite biotransformation with essentially single cell resolution, and provides new spatial and functional dimensions to studies of cancer metabolism without the need for radiotracers or tissue extraction. These findings should prove useful for in vitro and pre-clinical studies of cancer metabolism, and aid the optimization of metabolism-based cancer therapies and diagnostics.

Synthesis and biological evaluation of fatty acyl ester derivatives of 2',3'-didehydro-2',3'-dideoxythymidine

A number of 5'-O-fatty acyl derivatives of 2',3'-didehydro-2',3'-dideoxythymidine (stavudine, d4T) were synthesized and evaluated for anti-HIV activities against cell-free and cell-associated virus, cellular cytotoxicity, and cellular uptake studies. The conjugates were found to be more potent than d4T. Among these conjugates, 5'-O-12-azidododecanoyl derivative of d4T (2), displaying EC(50) = 3.1-22.4 μM, showed 4- to 9-fold higher activities than d4T against cell-free and cell-associated virus. Cellular uptake studies were conducted on CCRF-CEM cell line using 5(6)-carboxyfluorescein derivatives of d4T attached through β-alanine (9) or 12-aminododecanoic acid (10) as linkers. The fluorescein-substituted analog of d4T with long chain length (10) showed 12- to 15-fold higher cellular uptake profile than the corresponding analog with short chain length (9). These studies reveal that conjugation of fatty acids to d4T enhances the cellular uptake and anti-HIV activity of stavudine.

Aryloxy phosphoramidate triesters: a technology for delivering monophosphorylated nucleosides and sugars into cells

Prodrug technologies aimed at delivering nucleoside monophosphates into cells (protides) have proved to be effective in improving the therapeutic potential of antiviral and anticancer nucleosides. In these cases, the nucleoside monophosphates are delivered into the cell, where they may then be further converted (phosphorylated) to their active species. Herein, we describe one of these technologies developed in our laboratories, known as the phosphoramidate protide method. In this approach, the charges of the phosphate group are fully masked to provide efficient passive cell-membrane penetration. Upon entering the cell, the masking groups are enzymatically cleaved to release the phosphorylated biomolecule. The application of this technology to various therapeutic nucleosides has resulted in improved antiviral and anticancer activities, and in some cases it has transformed inactive nucleosides to active ones. Additionally, the phosphoramidate technology has also been applied to numerous antiviral nucleoside phosphonates, and has resulted in at least three phosphoramidate-based nucleotides progressing to clinical investigations. Furthermore, the phosphoramidate technology has been recently applied to sugars (mainly glucosamine) in order to improve their therapeutic potential. The development of the phosphoramidate technology, mechanism of action and the application of the technology to various monophosphorylated nucleosides and sugars will be reviewed.

Early nucleoside reverse transcriptase inhibitors for the treatment of HIV: a brief history of stavudine (D4T) and its comparison with other dideoxynucleosides

The occasion of this 25th anniversary issue encouraged us to reminisce about the important history of the discovery of the dideoxynucleoside analogues for the treatment of HIV/AIDS and to chronicle our thoughts about a particular exciting and rewarding period of our scientific careers. Following the identification of the anti-HIV activity of zidovudine (AZT), we participated in the urgent quest to discover optimal treatments of HIV infection and AIDS. A number of previously synthesized nucleoside analogues were comparatively evaluated, and stavudine (D4T) emerged as a promising candidate for development. Following clinical evaluation, D4T became a mainstay of the initial antiretroviral combination therapy, prolonging and saving numerous lives. It has only recently been supplanted by better-tolerated treatments. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, vol. 85, issue 1, 2010.

Cytometric analysis for drug-induced steatosis in HepG2 cells

Drugs are capable of inducing hepatic lipid accumulation. When fat accumulates, lipids are primarily stored as triglycerides which results in steatosis and provides substrates for lipid peroxidation. An in vitro multiparametric flow cytometry assay was performed in HepG2 cells by using fluorescent probes to analyze cell viability (propidium iodide, PI), lipid accumulation (BODIPY493/503), mitochondrial membrane potential (tetramethyl rhodamine methyl ester, TMRM) and reactive oxygen species generation (ROS) (2',7'-dihydrochlorofluorescein diacetate, DHCF-DA) as functional markers. All the measurements were restricted to live cells by gating the cells that excluded PI or those that exhibited the typical forward and side scatter features of live cells. The assay was qualified by analyzing a number of selected model drugs with a well documented induction of steatosis in vivo using different mechanisms as positive controls and several non-steatosic compounds as negative controls. For the cytometric screening assay, the concentrations tested were up to the corresponding IC(10) value determined by the MTT assay. Among the parameters analyzed, increased BODIPY fluorescence was the most sensitive and selective marker of drug-induced steatosis. However, a more consistent predictive approach was the combination of two endpoints: lipid accumulation and ROS generation. The assay correctly identified 100% of steatosis-positive and steatosis-negative compounds, and a high steatosis risk was predicted for amiodarone, doxycycline, tetracycline and valproate treatments at therapeutic doses. The results suggest that this cell-based assay may be a useful approach to identify the potential of drug candidates to induce steatosis.

Synthesis of (+/-)-4'-ethynyl and 4'-cyano carbocyclic analogues of stavudine (d4T)

The synthesis of (+/-)-4'-ethynyl (8) and 4'-cyano (9) carbocyclic analogues of the anti-HIV agent stavudine (5, d4T) is reported. The carbocyclic unit (16) was constructed from readily available beta-keto ester 10. The ethynyl or cyano group of 8 and 9 were prepared, after the introduction of thymine base to 16, by manipulation of the ester function. Evaluation of the anti-HIV activity of 8 and 9 was also carried out.

Synthesis, Structure−Activity Relationships, and Drug Resistance of β-d-3‘-Fluoro-2‘,3‘-Unsaturated Nucleosides as Anti-HIV Agents

Our recent studies demonstrated that d- and l-2'-fluoro-2',3'-unsaturated nucleosides (d- and l-2'-F-d4Ns) display moderate to potent antiviral activities against HIV-1 and HBV. As an extension of these findings, beta-d-3'-fluoro-2',3'-unsaturated nucleosides were synthesized as potential antiviral agents. The key intermediate (2S)-5-(1,3-dioxolan)-1-benzoyloxy-3,3-difluoropentan-2-ol 6 was prepared from 2,3-O-isopropylidene-d-glyceraldehyde 1, which was converted to 5-O-benzoxy-d-2-deoxy-3,3-difluoropentofuranosyl acetate 7 by the ring-closure reaction under acidic conditions. The acetate 7 was condensed with silylated purine and pyrimidine bases, which produced the alpha and beta isomers. The 3',3'-difluoro nucleosides were then treated with t-BuOK to give the desired 3'-fluoro-unsaturated nucleosides. We studied the structure-activity relationships of d-3'-fluoro-2',3'-unsaturated nucleosides against HIV-1 in human peripheral blood mononuclear cells, from which we found that the cytosine derivative 26 was the most potent among the synthesized compounds. To understand the mode of action and drug resistance profile, with particular regard to the role of fluorine, we performed the molecular modeling studies of the cytidine analogue d-3'F-d4C and found a good correlation between calculated relative binding energies and activity/resistance data. Our model also shows interactions of the 3'-fluorine and the 2',3' double bond, which can be correlated to the observed biological data. Differences between fluorine substitution at the 3' and 2' positions may account for the higher cross-resistance with lamivudine observed in the 2'-fluorinated series.

Simple entry to 3'-substituted analogues of anti-HIV agent stavudine based on an anionic O --> C stannyl migration

Reaction of 5'-O-protected derivatives of the anti-HIV agent stavudine (d4T) with LTMP was investigated with the aim to lithiate the vinylic hydrogens (H-3' and H-2'). When the lithiation of the 5'-O-tert-butyldiphenylsilyl derivative 5 was carried out in the presence of HMPA, an anionic silyl migration took place to give the 3'-C-silylated product 4a. The stannyl version of this reaction was found to be also possible, which has disclosed a highly simple entry to the d4T analogues variously substituted at the 3'-position by manipulating the 3'-C-stannyl d4T as a common intermediate.

Structure-activity relationships of 2'-fluoro-2',3'-unsaturated D-nucleosides as anti-HIV-1 agents

We studied the structure-activity relationships of a series of 2'-fluoro-2',3'-unsaturated D-nucleosides against HIV-1 in human peripheral blood mononuclear (PBM) cells. The target compounds 10-21 and 28-33 were prepared by N-glycosylation of the acetate 4, which was readily prepared from 2,3-O-isopropylidene-D-glyceraldehyde in five steps. Among the newly synthesized nucleosides, 2-amino-6-chloropurine (11), adenine (14), inosine (16), guanine (18), 2,6-diaminopurine (20), and 5-fluorocytosine (30) derivatives were found to exhibit interesting anti-HIV activities with EC(50) values of 4.3, 0.44, 1.0, 2.6, 3.0, and 0.82 microM, respectively. The implications for drug resistance of the titled nucleosides with respect to lamivudine-resistant variants (M184V) were also examined, and no significant cross-resistance with the variants was observed with the D-series.

Synthesis and antiviral properties of novel analogues of monophosphate and diphosphate bioactive forms of acyclovir

New analogues (compounds 6, 7 and 9) of the mono- (8) and diphosphate (10) bioactive forms of the antiherpes drug acyclovir are described. In compound 6, the monophosphate moiety of 8 was replaced by an aminosulfonyloxy group, while in compounds 7 and 9, a phosphonoacetamidoxy and an O-ethyl phosphonoacetamidoxy moiety are, respectively present instead of the diphosphate one of 10. None of the compounds synthesized proved to possess an appreciable activity on herpes simplex virus (HSV) or human immunodeficiency virus (HIV).

Human herpesvirus 8 open reading frame 21 is a thymidine and thymidylate kinase of narrow substrate specificity that efficiently phosphorylates zidovudine but not ganciclovir

Human herpesvirus 8 (HHV8) open reading frame (ORF) 21 is predicted to encode a protein similar to the thymidine kinase (TK) enzyme of other herpesviruses. Expressed in mammalian cells, ORF 21 was found to have low TK activity, based on poor growth in media containing hypoxanthine-aminopterin-thymidine (HAT) and low incorporation of [(3)H]thymidine into high-molecular-weight DNA. Kinetic analysis using HHV8 TK as a purified glutathione S-transferase (GST) fusion protein showed that the enzyme has a comparatively high K(m) for thymidine (dThd) of approximately 33.2 microM. Nearly 50% of the phosphorylated product of the reaction with dThd was thymidylate. This monophosphate kinase activity was more pronounced with 3'-azido-3'-deoxythymidine (AZT), in which 78% of the reaction product was AZT diphosphate. Thymidine analogs competitively inhibited dThd phosphorylation by HHV8 TK, while 2'-deoxyguanosine, 2'-deoxyadenosine, 2'-deoxycytidine, and corresponding analogs did not. Further competition experiments revealed that the nucleoside analog ganciclovir (GCV), at up to 1,000-fold molar excess, could not significantly inhibit dThd phosphorylation by the enzyme. In support of these data, 143B TK(-) cells expressing HHV8 TK phosphorylated GCV very poorly and were not susceptible to GCV toxicity compared to parental cells. Phosphorylation of [(3)H]GCV by a purified GST-HHV8 TK fusion protein was not detected by high-pressure liquid chromatography analysis. Structural features of HHV8 TK substrate recognition were investigated. Therapeutic implications of these findings are discussed.

Development and optimization of anti-HIV nucleoside analogs and prodrugs: A review of their cellular pharmacology, structure-activity relationships and pharmacokinetics

Significant improvements in antiviral therapy have been realized over the past 10 years. Numerous nucleoside analogs, as well as prodrugs of active compounds, have been synthesized and tested for anti-HIV activity. In addition to the five nucleoside analogs currently used clinically for the treatment of HIV infection, a broad spectrum of anti-HIV nucleoside analogs (including 2',3'-dideoxynucleoside analogs, oxathiolanyl 2',3'-dideoxynucleoside analogs, dioxolanyl 2',3'-dideoxynucleoside analogs, carbocyclic 2',3'-dideoxynucleoside analogs and acyclic nucleoside analogs) and their prodrugs (including ester prodrugs, phospholipid prodrugs, dihydropyridine prodrugs, pronucleotides and dinucleotide analogs), targeted at HIV reverse transcriptase, are reviewed with focus on structure-activity relationships, cellular pharmacology and pharmacokinetics. Several of these anti-viral agents show promise in the treatment of AIDS.

The presence of substituents on the aryl moiety of the aryl phosphoramidate derivative of d4T enhances anti-HIV efficacy in cell culture: A structure-activity relationship

New substituted-aryl phosphoramidate derivatives of the anti-HIV drug d4T were synthesized as membrane-soluble intracellular prodrugs for the free bioactive phosphate to establish relationship(s) between compound structure and in vitro antiviral activity. The majority of compounds demonstrated an elevation of in vitro potency relative to that of the parent nucleoside, and unlike d4T, all retained full activity in thymidine kinase-deficient cells. The compound bearing a p-chloro aryl group (8e) expressed nanomolar activity in vitro, a 14-fold increase in activity relative to that of the unsubstituted phosphoramidate (100-fold compared to d4T). An assay using pig liver esterase was used to establish the stability of the compounds to enzymatic degradation. While there was no apparent correlation between in vitro activity and half-life of enzymatic degradation, there was a close correlation between compound lipophilicity, determined by octanol/water partition coefficient, and in vitro potency. We suggest that substitutions made to the aryl moiety of the aryl phosphoramidate of d4T that result in enhancing lipophilicity may serve to increase the cellular uptake of the prodrug by passive diffusion, leading to the expression of antiviral potency at reduced prodrug concentrations.

Substrates of DNA polymerases with planar conformation of sugar: model of substrate transition state?

Several years ago, we published an hypothesis concerning conformation of the glycone moiety of different substrates in active centers of several DNA metabolizing enzymes (Nucleosides & Nucleotides 1993, 12, 649-670). This hypothesis prompted us to further study the subtle conformational changes on substrates of DNA polymerases. Data collected in our, as well as other laboratories, have been analyzed, and models of active centers of different DNA polymerases are discussed below. Based on the model of substrate requirements, we now can divide DNA polymerases into two distinguished classes.

Chemotherapeutic agents for human immunodeficiency virus infection: mechanism of action, pharmacokinetics, metabolism, and adverse reactions

Since the mid-1980s, four new nucleoside reverse transcriptase (RT) inhibitors (zalcitabine, didanosine, stavudine, and lamivudine), two nonnucleoside RT inhibitors (nevirapine and delavirdine), and four new protease inhibitors (saquinavir, ritonavir, indinavir, and nelfinavir) have been approved by the US Food and Drug Administration for the treatment of patients with acquired immunodeficiency syndrome. The driving force behind the development of these new agents has been the increasing need for more potent agents with reduced or modified toxicity profiles. The purpose of this article is to review the absorption, distribution, metabolism, elimination, toxicities, adverse reactions, and mechanism of action of the currently available drugs.

Stavudine:pharmacology,clinical use and future role

Stavudine is a nucleoside analogue reverse transcriptase inhibitor of HIV-1 and HIV-2 and demonstrates in vitro activity with an acceptable therapeutic index in a range of T-lymphocyte and haematopoietic precursor cell lines. It is additive or synergistic in vitro with a range of other antiretrovirals, including the proteinase inhibitor saquinavir, in two- and three-way combinations and is active against zidovudine (ZDV)-resistant virus. It exhibits excellent oral bioavailability, with cerebrospinal fluid (CSF)/plasma penetration. In clinical use, stavudine monotherapy exhibits similar antiretroviral activity to ZDV, and is of proven clinical benefit in ZDV-pre-treated patients. In combination with ddI and/or nelfinavir it results in more substantial and durable responses in immunological and virological markers than reported with either drug alone. Further data on stavudine in combination with other antiretrovirals are now awaited. Comparative trials in ZDV-experienced patients suggest a similar frequency of adverse events to that observed with ZDV. Peripheral neuropathy is the most common dose-limiting toxicity, with haematological and hepatic function disturbance being infrequent. Resistance to stavudine develops slowly in vitro and in vivo but may lead to co-resistance to ZDV or ddI. Stavudine will be used clinically as a combination agent both in initial therapy and in patients with prior ZDV experience.

Prenatal and postpartum pharmacokinetics of stavudine (2',3'-didehydro-3'-deoxythymidine) and didanosine (dideoxyinosine) in pigtailed macaques (Macaca nemestrina)

Stavudine (5 mg/kg of body weight; n = 7) or didanosine (3.2 mg/kg; n = 4) was administered as an intravenous bolus to pregnant pigtailed macaques (Macaca nemestrina) near term and 4 to 5 weeks postpartum. No significant differences were found between the prenatal and postpartum total plasma drug clearance, steady-state volume of distribution, terminal plasma drug half-life, mean body residence time, or recovery of unchanged drug in urine. These data indicate that pregnancy does not affect the pharmacokinetics of stavudine or didanosine in M. nemestrina.

Mutants of feline immunodeficiency virus resistant to 2',3'-dideoxy-2',3'-didehydrothymidine

We selected mutants of feline immunodeficiency virus (FIV) that are resistant to 2',3'-dideoxy-2',3'-didehydrothymidine (d4T). Two mutants were selected in cultured cells with a stepwise increase in d4T concentration, resulting in mutants able to replicate in 100 microM d4T. These mutants were three- to sixfold more resistant to d4T than wild-type FIV. They were also cross-resistant to 3'-azido-3'-deoxythymidine (AZT), 3'-fluoro-2',3'-dideoxythymidine, 2',3'-dideoxycytidine, 2',3'-dideoxyinosine, and 9-(2-phosphonylmethoxyethyl)adenine, and they were highly resistant to phosphonoformic acid (PFA). Plaque-purified mutants were isolated from each of the mutant populations. The mutant phenotype was stable, because both of the plaque-purified mutants remained d4T resistant even after three passages in the absence of d4T. One of the plaque-purified mutants, designated D4R-3c, was further characterized. Compared with wild-type reverse transcriptase (RT), RT purified from D4R-3c was 3-fold resistant to inhibition by the 5'-triphosphate of d4T, 10-fold resistant to inhibition by the 5'-triphosphate of AZT, and 6-fold resistant to PFA. D4R-3c had a single point mutation in the RT-encoding region of the pol gene at position 2474, resulting in a Val to Ile mutation at codon 47 of the FIV RT. The role of this mutation in d4T resistance was confirmed by site-directed mutagenesis.

Zidovudine does not affect transplacental transfer or systemic clearance of stavudine (2',3'-didehydro-3'-deoxythymidine) in the pigtailed macaque (Macaca nemestrina)

Stavudine (22 micrograms/min/kg of body weight) was infused alone (via the femoral vein) or simultaneously with zidovudine (66 micrograms/min/kg) to three near-term pregnant macaques. No significant differences were found between the mean steady-state plasma stavudine concentrations in the dam (Cssd) and fetus (Cssf), the stavudine concentration in the amniotic fluid (Cssa), and the ratios Cssf/Cssd and Cssa/Cssf when stavudine was infused alone or in combination with zidovudine. The data obtained indicate that zidovudine administration does not affect the transfer of stavudine across the placenta in Macaca nemestrina.

Stavudine: a review of its pharmacodynamic and pharmacokinetic properties and clinical potential in HIV infection

Stavudine is a nucleoside analogue which undergoes intracellular phosphorylation to its active metabolite, stavudine-5'-triphosphate. At clinically relevant concentrations, the active metabolite restricts HIV replication by inhibiting the inclusion of thymidine-5'-triphosphate into proviral DNA by HIV reverse transcriptase, and/or by causing DNA chain termination. Viral resistance to stavudine does not commonly develop during treatment. Where it has developed, up to a 12-fold increase in resistance has been observed in clinical isolates from patients treated with stavudine for long periods. Stavudine 40mg twice daily and zidovudine 200mg 3 times daily were compared in 822 patients at various stages of HIV infection who had previously received long term zidovudine therapy. Stavudine was superior for both primary and surrogate end-points including clinical progression, treatment failure, increase in CD4+ cell counts and bodyweight gain. In a larger study, stavudine 40mg twice daily provided greater benefit than stavudine 20mg twice daily in terms of weight gain, haematological findings and the number of hospitalisations in 11 784 patients intolerant of or resistant to, zidovudine and didanosine. Peripheral neuropathy is the major dose-limiting adverse event associated with stavudine therapy and occurred more frequently with stavudine than zidovudine. However, haematological adverse events were observed less frequently with stavudine than with zidovudine. Thus, stavudine is effective in alleviating signs and symptoms of HIV infection in patients intolerant of or no longer responding to, zidovudine or didanosine. It is also more effective than zidovudine in slowing disease progression in patients previously treated with zidovudine for long periods. The results of studies which will reveal the role of stavudine therapy in untreated patients and in combination with other anti-HIV agents are awaited with interest.

Aryl phosphoramidate derivatives of d4T have improved anti-HIV efficacy in tissue culture and may act by the generation of a novel intracellular metabolite

New phosphate derivatives of the anti-HIV nucleoside analogue d4T were prepared as potential membrane-soluble prodrugs of the bioactive free nucleotide. The enhanced antiviral potency and/or reduced cytotoxicity of the derivatives leads to an increase in selectivity relative to the parent nucleoside analogue. Moreover, the derivatives appear to bypass the dependence of the nucleoside on thymidine kinase-mediated activation, retaining full activity in thymidine kinase-deficient cells. This strongly suggests the successful intracellular delivery of free nucleotides by the masked phosphate triester prodrugs. This is further confirmed by studies using radiolabeled compound which clearly demonstrate the generation of d4T mono-, di- and triphosphates from the prodrug, even in thymidine kinase-deficient cells. Moreover, we herein report the generation of a new metabolite, a partially hydrolyzed phosphate diester, alaninyl d4T monophosphate. We suggest that at least part of the antiviral action of the prodrugs derives from the intracellular generation of such novel diesters which may add considerable weight to the suggested further preclinical development of the phosphate prodrugs.

Antiretroviral activity of stavudine (2',3'-didehydro-3'-deoxythymidine, D4T)

Stavudine, 2',3'-didehydro-3'-deoxythymidine (D4T), is a potent inhibitor of HIV-1 reverse transcriptase in vitro. In clinical studies, stavudine has excellent oral bioavailability in excess of 80%. The dose-limiting toxicity is peripheral neuropathy, which occurred in 15% of stavudine versus 6% of zidovudine-treated patients for 80 weeks in a randomized, blinded, phase III trial. Stavudine-treated groups have experienced significant increases in mean CD4 cell counts and decreases in both mean serum p24 antigen levels and infectious HIV titers in peripheral blood mononuclear cells. In subjects with prior zidovudine treatment, the duration of these responses is limited; CD4 counts and serum p24 antigen levels return to baseline after approximately 6 months. The effect of stavudine on clinical outcome and survival has not yet been established in comparative trials. Stavudine offers an additional therapeutic option to those individuals who are refractory to or intolerant of other available antiretrovirals.

The toxicity of azidothymidine (AZT) on human and animal cells in culture at concentrations used for antiviral therapy

AZT, a chain terminator of DNA synthesis originally developed for chemotherapy, is now prescribed as an anti-human immunodeficiency virus (HIV) drug at 500 to 1500 mg/person/day, which corresponds to 20 to 60 microM AZT. The human dosage is based on a study by the manufacturer of the drug and their collaborators, which reported in 1986 that the inhibitory dose for HIV replication was 0.05 to 0.5 microM AZT and that for human T-cells was 2000 to 20,000 times higher, i.e. 1000 microM AZT. This suggested that HIV could be safely inhibited in humans at 20 to 60 microM AZT. However, after the licensing of AZT as an anti-HIV drug, several independent studies reported 20- to 1000-fold lower inhibitory doses of AZT for human and animal cells than did the manufacturer's study, ranging from 1 to 50 microM. In accord with this, life threatening toxic effects were reported in humans treated with AZT at 20 to 60 microM. Therefore, we have re-examined the growth inhibitory doses of AZT for the human CEM T-cell line and several other human and animal cells. It was found that at 10 microM and 25 microM AZT, all cells are inhibited at least 50% after 6 to 12 days, and between 20 and 100% after 38 to 48 days. Unexpectedly, variants of all cell types emerged over time that were partially resistant to AZT. It is concluded that AZT, at the dosage prescribed as an anti-HIV drug, is highly toxic to human cells.

Measurement of the anti-HIV agent 2',3'-didehydro-2',3'-dideoxythymidine (D4T) by competitive ELISA

2',3'-didehydro-2',3'-dideoxythymidine (D4T) is a thymidine analogue with potent anti-HIV activity in vitro and is currently being investigated therapeutically in patients with advanced HIV infection. We describe a first one-step competitive ELISA method developed for D4T measurement. Anti-D4T rabbit antibodies were raised against a D4T hemisuccinate-bovine serum albumin immunogen. A D4T-hemisuccinate-horseradish peroxidase conjugate and a monoclonal anti-rabbit IgG antibody insolubilized onto a microtiter plate were used as a tracer and capture system, respectively. The method was capable of detecting 2 ng/ml of D4T in cell cultures and 20 ng/ml of D4T in plasma samples previously separated in microconcentrator devices. Cross-reactivity analysis showed that thymidine, D4T monophosphate, or azidothymidine, were weakly recognized by the ELISA and that thymine or other nucleosides were unreactive. The test was successfully used for the quantification of D4T in cell extracts from CEM or Molt 4 cell lines cultured with D4T and in the plasma of patients with advanced HIV infection, receiving D4T therapy. Moreover this ELISA could be used for the indirect quantification of D4T phosphorylated intracellular metabolites previously separated by reverse phase HPLC and hydrolyzed with alkaline phosphatase.

5-Chloro-2',3'-dideoxy-3'-fluorouridine (935U83), a selective anti-human immunodeficiency virus agent with an improved metabolic and toxicological profile

5-Chloro-2',3'-dideoxy-3'-fluorouridine (935U83) is a selective anti-human immunodeficiency virus (HIV) agent. When tested in phytohemagglutinin-stimulated normal human peripheral blood lymphocytes against fresh clinical isolates of HIV type 1 (HIV-1) obtained from patients naive to AZT (3'-azido-3'-deoxythymidine [zidovudine]), 935U83 inhibited virus growth with an average 50% inhibitory concentration (IC50) of 1.8 microM; corresponding IC50s were 0.10 microM for FLT (3'-deoxy-3'-fluorothymidine) and 0.23, 0.49, and 0.03 microM for the approved agents AZT, ddI (2',3'-dideoxyinosine), and ddC (2',3'-dideoxycytosine), respectively. Importantly, 935U83 retained activity against HIV strains that were resistant to AZT, ddI, or ddC. Of additional interest, we were unable to generate virus which was resistant to 935U83 by passaging either HXB2 (AZT-sensitive) or RTMC (AZT-resistant) strains in the presence of high concentrations of 935U83. The anabolic profile of 935U83 was similar to that of AZT, and 935U83 triphosphate was a potent inhibitor of HIV-1 reverse transcriptase. Pharmacokinetic evaluation showed good oral bioavailability (86% in mice and 60% in monkeys) and less extensive metabolism to the glucuronide relative to AZT. 935U83 showed low toxicity. In an in vitro assay for toxicity to a human erythrocyte progenitor, erythroid burst-forming unit (BFU-E), the IC50 for 935U83 (> 400 microM) was more than 1,000-fold those of FLT (0.07 microM) and AZT (0.30 microM). Mild reversible reductions in erythrocytes and associated parameters were seen in mice dosed orally with 2,000 mg of 935U83 per kg per day for 1 and 6 months. In monkeys dosed orally with up to 700 mg/kg/day for 1 and 6 months, the only possible treatment-related finding was cataracts in 1 of 12 animals given the intermediate dose of 225 mg/kg/day. At the highest doses in mice and monkeys, maximal concentrations in plasma were more than 100-fold the anti-HIV IC50s against clinical isolates. This safety profile in animals compares very favorably with that of any of the anti-HIV drugs approved to date and has led us to begin evaluation of 935U83 in patients with HIV infection.

Metabolism and mechanism of antiretroviral action of purine and pyrimidine derivatives

Unlike herpes viruses, human immunodeficiency virus and other retroviruses do not encode specific enzymes required for the metabolism of the purine or pyrimidine nucleotides to their corresponding 5'-triphosphates. Therefore, 2',3'-dideoxynucleosides and acyclic nucleoside phosphonates must be phosphorylated and metabolized by host cell kinases and other enzymes of purine and/or pyrimidine metabolism. Different animal species (or even different cell types within one animal species) may differ in the efficiency of conversion of these drugs to their antivirally active metabolite(s). Three 2',3'-dideoxynucleosides are officially licensed for clinical use [i.e., zidovudine (3'-azido-2',3'-dideoxythymidine, AZT), didanosine (2',3'-dideoxyinosine, DDI) and zalcitabine (2',3'-dideoxycytidine, DDC)]. A number of other 2',3'-dideoxynucleoside analogues [among them stavudine (2',3'-didehydro-2',3'-dideoxythymidine, D4T), 2',3'-dideoxy-3'-thiacytidine (3TC), 2',3'-dideoxy-5-fluoro-3'-thiacytidine (FTC) and the acyclic nucleoside phosphonate 9-(2-phosphonylmethoxyethyl)adenine (PMEA)] are currently under clinical investigation and are candidate compounds for eventual licensing as anti-AIDS drugs. The metabolic pathways, antimetabolic effects and mechanism of antiviral action of these nucleoside analogues will be discussed.

Prodrugs of 2',3'-didehydro-3'-deoxythymidine (D4T): synthesis, antiviral activity, and rapid pharmacokinetic evaluation

A series of 5'-derivatives and modified pyrimidine analogues of 2',3'-didehydro-3'-deoxythymidine (d4T, stavudine, 1) were synthesized to determine their potential as oral prodrugs of d4T. Utilizing a screen developed for the rapid evaluation of a variety of prodrugs in mice, it was determined that 5'-acetate 2 provided comparable plasma levels of d4T after oral administration of the prodrug to that when d4T was administered alone. The relative oral bioavailability of methoxy acetate 3 and cyclohexyl carbonate 5 was 79 and 41%, respectively. Dihydropyridine ester 6 did not provide detectable levels of d4T up to 1 h after oral administration of 6. Thiopyrimidines 8 and 9, as well as aminopyrimidine 10 also failed to provide measurable levels of d4T after oral administration. 5'-Derivatives 3, 5, and 6 showed similar activity to that of d4T against HIV and MuLV, as did 5'-benzoyl-4-thio derivative 8. However, the corresponding 4-thio 5'-alcohol 9 was inactive.

Prodrugs of 2',3'-didehydro-3'-deoxythymidine

Six ester prodrugs of 2',3'-didehydro-3'-deoxythymidine (D4T) were synthesized, and their physicochemical properties were evaluated. Marked differences were observed. All of the prodrugs were chemically stable within the pH range 2-7. Hydrolysis of these esters was observed in all cases for four rat enzyme systems (plasma, liver, duodenum, and kidney), with D4T being regenerated. D4T or the prodrug was administered orally to rats, and the plasma concentrations of D4T and a corresponding prodrug were measured. The half-life of D4T after intravenous administration was 35.9 min. The half-life calculated from the terminal phase and the maximum concentration in plasma following oral administration of D4T were 35.9 min and 48.4 microM, respectively. After oral prodrug administration (with water or olive oil as a solvent), though none of the prodrugs was detected in plasma except for 5'-hemisuccinyl D4T and 5'-hemiglutaryl D4T with olive oil as a solvent, retention time of plasma D4T concentration was extended and the elevated D4T concentration in plasma decreased.

Empirical and rational approaches for development of inhibitors of the human immunodeficiency virus--HIV-1

The human immunodeficiency virus, HIV-1, is generally accepted to be responsible for AIDS. It is imperative that all approaches, empirical and rational, be taken for development of a drug for therapy of this disease. These approaches are discussed, with emphasis on the direction being pursued in our laboratory. Empirically, we found 3'-deoxy-2',3'-didehydrothymidine, a compound first synthesized for potential anticancer activity by J. Horwitz in the 1960s, to be a potent inhibitor of HIV-1. It is now in Phase II/III clinical trials. We have also synthesized several 2,5'-anhydro pyrimidine nucleoside analogs, which have interesting chemical and biological properties. We have evaluated a natural product, gossypol and synthesized various derivatives for anti-HIV-1 activity, but none were appreciably more inhibitory than the parent compound. More recently, we have taken the rational approach and synthesized a boron-modified tetrapeptide, Ac-Thr-Leu-Asn-boro-Phe, which corresponds to the COOH-terminal of the Phe-Pro scissle bond of the gag/pol gene polyprotein product. Potent inhibition of the HIV-1 encoded protease was observed. These approaches and findings will be discussed.