Pharmacokinetics and metabolism of racemic 2',3'-dideoxy-5-fluoro-3'-thiacytidine in rhesus monkeys

Synthesis of Fluorinated Nucleosides/Nucleotides and Their Antiviral Properties

The FDA has approved several drugs based on the fluorinated nucleoside pharmacophore, and numerous drugs are currently in clinical trials. Fluorine-containing nucleos(t)ides offer significant antiviral and anticancer activity. The insertion of a fluorine atom, either in the base or sugar of nucleos(t)ides, alters its electronic and steric parameters and transforms the lipophilicity, pharmacodynamic, and pharmacokinetic properties of these moieties. The fluorine atom restricts the oxidative metabolism of drugs and provides enzymatic metabolic stability towards the glycosidic bond of the nucleos(t)ide. The incorporation of fluorine also demonstrates additional hydrogen bonding interactions in receptors with enhanced biological profiles. The present article discusses the synthetic methodology and antiviral activities of FDA-approved drugs and ongoing fluoro-containing nucleos(t)ide drug candidates in clinical trials.

The best backbone for HIV prevention, treatment, and elimination: Emtricitabine+tenofovir

The advent of antiretroviral combination therapy has significantly impacted the HIV/AIDS epidemic. No longer a death sentence, HIV infection can be controlled and suppressed using cocktail therapies that contain two or more small molecule drugs. This review aims to highlight the discovery, development, and impact of one such molecule, namely, emtricitabine (FTC, emtriva), which is one of the most successful drugs in the fight against HIV/AIDS and has been taken by over 94% of individuals infected with HIV in the USA. We also pay tribute to Dr. John C. Martin, former CEO and Chairman of Gilead Sciences, who unexpectedly passed away in 2021. A true visionary, he was instrumental in delivering FTC, as part of combination therapy with TDF (tenofovir, viread) to the global stage. As the fight to eradicate HIV marches on, we honor Dr. Martin's legacy of collaboration, achievement, and hope.

Approved Antiviral Drugs over the Past 50 Years

Since the first antiviral drug, idoxuridine, was approved in 1963, 90 antiviral drugs categorized into 13 functional groups have been formally approved for the treatment of the following 9 human infectious diseases: (i) HIV infections (protease inhibitors, integrase inhibitors, entry inhibitors, nucleoside reverse transcriptase inhibitors, nonnucleoside reverse transcriptase inhibitors, and acyclic nucleoside phosphonate analogues), (ii) hepatitis B virus (HBV) infections (lamivudine, interferons, nucleoside analogues, and acyclic nucleoside phosphonate analogues), (iii) hepatitis C virus (HCV) infections (ribavirin, interferons, NS3/4A protease inhibitors, NS5A inhibitors, and NS5B polymerase inhibitors), (iv) herpesvirus infections (5-substituted 2'-deoxyuridine analogues, entry inhibitors, nucleoside analogues, pyrophosphate analogues, and acyclic guanosine analogues), (v) influenza virus infections (ribavirin, matrix 2 protein inhibitors, RNA polymerase inhibitors, and neuraminidase inhibitors), (vi) human cytomegalovirus infections (acyclic guanosine analogues, acyclic nucleoside phosphonate analogues, pyrophosphate analogues, and oligonucleotides), (vii) varicella-zoster virus infections (acyclic guanosine analogues, nucleoside analogues, 5-substituted 2'-deoxyuridine analogues, and antibodies), (viii) respiratory syncytial virus infections (ribavirin and antibodies), and (ix) external anogenital warts caused by human papillomavirus infections (imiquimod, sinecatechins, and podofilox). Here, we present for the first time a comprehensive overview of antiviral drugs approved over the past 50 years, shedding light on the development of effective antiviral treatments against current and emerging infectious diseases worldwide.

Absolute Configuration of the Antiviral Agent (−)-cis-5-Fluoro-1-[2-Hydroxymethyl)-1,3-Oxathiolan-5-yl]Cytosine

The structure and absolute configuration of (−)- cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine (FTC), has been determined by X-ray crystallographic analysis. The results confirm that the L-isomer of the nucleoside analogue is the most active enantiomer and that the correct absolute configuration of (−)-FTC is 5-fluoro-(2′R,5′S)-(−)-1-[2-hydroxymethyl)oxathiolan-5-yl]-fluorocytosine. The two molecules in the asymmetric unit show conformations that combine conformational features of two other classes of potent antiviral nucleosides. Both oxathiolane rings have the 3′-sulphur atom in nearly perfect S3′- exo envelope conformations, similar to what is observed for 3′-azido-3′-deoxythymidine (AZT) and 2′,3′-dideoxycytidine. One of the two molecules has a glycosylic link conformation in which the base is eclipsed with the C5′-O1′ bond. This mimics the high- anti conformation that has been observed in the structures of several 2′,3′-didehydro-2′,3′-dideoxypyrimidine nucleosides but is inaccessible for saturated pyrimidine nucleosides. However, the observed conformations cannot be superimposed adequately with other active antiviral nucleosides to suggest a common ‘active site’ conformation.

Comparative Pharmacokinetics of Racivir®, (±)-β-2′,3′-Dideoxy-5-Fluoro-3′-Thiacytidine in Rats, Rabbits, Dogs, Monkeys and HIV-Infected Humans

Racivir® is a 50:50 racemic mixture of the (–)- and (+)-β-enantiomers of 2′-deoxy-3′-thia-5-fluorocytosine (FTC), which is being developed for the treatment of HIV and hepatitis B virus (HBV). The (+)-enantiomer of FTC is approximately 10–20-fold less potent than (–)-FTC, but it selects for a different HIV mutation in human lymphocytes. Plasma concentrations from a group of 54 rats, 12 pregnant rabbits and 60 dogs enrolled in large toxicity studies using a wide variety of oral doses, were compared using non-compartment pharmacokinetic modelling versus dose, treatment duration, species and gender. The pharmacokinetics of Racivir® were also compared with those of a previously published pharmacokinetic study in rhesus monkeys and with data from HIV-infected human male volunteers. The (+)-FTC, but not the (–)-enantiomer, can be deaminated to the non-toxic inactive metabolite (+)-FTU. Therefore, the plasma exposure to (+)-FTU was also determined. The order of relative plasma exposure to (+)-FTU was rhesus monkeys > humans > pregnant rabbits > dogs > rats. Allometric scaling was performed to relate systemic clearance/fraction of drug absorbed (Cl/F) and terminal phase volume of distribution (Vβ/F) versus species body weights. No individual animal species mimicked the Cl/F values in humans. However, allometric scaling using a combination of rats, pregnant rabbits and monkeys predicted the mean human Cl/F value better than a combination of rats and rabbits only (within 0.24 and SD of mean vs 0.81 SD of the observed mean value). Similarly, human Vβ/F values were best predicted using a combination of rat and monkey data (within 0.64 SD of mean value). Species demonstrating greater deamination to (+)-FTU tended to have greater than predicted Cl/F values. The Cmax values of dogs were the closest to humans, but were statistically different. This study highlights the importance of selecting animal species that demonstrate similar cytidine deaminase activity to humans when performing preclinical dosing studies on Racivir® other antiviral agents that are substrates for mammalian cytidine deaminases.

Cyclodextrin assisted enantiomeric recognition of emtricitabine by 19F NMR spectroscopy

A ¹⁹F NMR spectroscopic method using cyclodextrins as chiral recognition agents was developed for the quantitative determination of emtricitabine enantiomers.

Therapy of HIV Infection: Current Approaches and Prospects

The human immunodeficiency virus type 1 (HIV-1) is the causative agent of one of the most dangerous human diseases - the acquired immune deficiency syndrome (AIDS). Over the past 30 years since the discovery of HIV-1, a number of antiviral drugs have been developed to suppress various stages of the HIV-1 life cycle. This approach has enables the suppression of virus replication in the body, which significantly prolongs the life of HIV patients. The main downside of the method is the development of viral resistance to many anti-HIV drugs, which requires the creation of new drugs effective against drug-resistant viral forms. Currently, several fundamentally new approaches to HIV-1 treatment are under development, including the use of neutralizing antibodies, genome editing, and blocking an integrated latent provirus. This review describes a traditional approach involving HIV-1 inhibitors as well as the prospects of other treatment options.

Antiretroviral bioanalysis methods of tissues and body biofluids

Research in the many areas of HIV treatment, eradication and prevention has necessitated measurement of antiretroviral (ARV) concentrations in nontraditional specimen types. To determine the knowledgebase of critical details for accurate bioanalysis, a review of the literature was performed and summarized. Bioanalytical assays for 31 ARVs, including metabolites, were identified in 205 publications measuring various tissues and biofluids. 18 and 30% of tissue or biofluid methods, respectively, analyzed more than one specimen type; 35-37% of the tissue or biofluid methods quantitated more than one ARV. 20 and 76% of tissue or biofluid methods, respectively, were used for the analysis of human specimens. HPLC methods with UV detection predominated, but chronologically MS detection began to surpass. 40% of the assays provided complete intra- and inter-assay validation data, but only 9% of publications provided any stability data with even less for the prevalent ARV in treatments.

Growth-associated protein-43 and ephrin B3 induction in the brain of adult SIV-infected rhesus macaques

Understanding the mechanisms of neuronal regeneration and repair in the adult central nervous system is a vital area of research. Using a rhesus lentiviral encephalitis model, we sought to determine whether recovery of neuronal metabolism after injury coincides with the induction of two important markers of synaptodendritic repair: growth-associated protein-43 (GAP-43) and ephrin B3. We examined whether the improvement of neuronal metabolism with combined anti-retroviral therapy (cART) after simian immunodeficiency virus (SIV) infection in rhesus macaques involved induction of GAP-43, also known as neuromodulin, and ephrin B3, both implicated in axonal pathfinding during neurodevelopment and regulation of synapse formation, neuronal plasticity, and repair in adult brain. We utilized magnetic resonance spectroscopy to demonstrate improved neuronal metabolism in vivo in adult SIV-infected cART animals compared to untreated and uninfected controls. We then assessed levels of GAP-43, ephrin B3, and synaptophysin, a pre-synaptic marker, in three brain regions important for cognitive function, cortex, hippocampus, and putamen, by quantitative real-time RT-PCR and immunohistochemistry. Here we demonstrate that (1) GAP-43 mRNA and protein are induced with SIV infection, (2) GAP-43 protein is higher in the hippocampus outer molecular layer in SIV-infected animals that received cART compared to those that did not, and (3) activated microglia and infiltrating SIV-infected macrophages express abundant ephrin B3, an important axonal guidance molecule. We propose a model whereby SIV infection triggers events that lead to induction of GAP-43 and ephrin B3, and that short-term cART results in increased magnitude of repair mechanisms especially in the hippocampus, a region known for high levels of adult plasticity.

Impact of short-term combined antiretroviral therapy on brain virus burden in simian immunodeficiency virus-infected and CD8+ lymphocyte-depleted rhesus macaques

Antiretroviral drugs suppress virus burden in the cerebrospinal fluid of HIV-infected individuals; however, the direct effect of antiretrovirals on virus replication in brain parenchyma is poorly understood. We investigated the effect of short-term combined antiretroviral therapy (CART) on brain virus burden in rhesus monkeys using the CD8-depletion model of accelerated simian immunodeficiency virus (SIV) encephalitis. Four monkeys received CART (consisting of the nonpenetrating agents PMPA and RCV) for four weeks, beginning 28 days after SIV inoculation. Lower virus burdens were measured by real-time RT-PCR in four of four regions of brain from monkeys that received CART as compared with four SIV-infected, untreated controls; however, the difference was only significant for the frontal cortex (P < 0.05). In contrast, significantly lower virus burdens were measured in plasma and four of five lymphoid compartments from animals that received CART. Surprisingly, despite normalization of neuronal function in treated animals, the numbers of activated macrophages/microglia and the magnitude of TNF-alpha mRNA expression in brain were similar between treated animals and controls. These results suggest that short-term therapy with antiretrovirals that fail to penetrate the blood-cerebrospinal fluid barrier can reduce brain virus burden provided systemic virus burden is suppressed; however, longer treatment may be required to completely resolve encephalitic lesions and microglial activation, which may reflect the longer half-life of the principal target cells of HIV/SIV in the brain (macrophages) versus lymphoid tissues (T lymphocytes).

Pharmacokinetics of the antiviral agent beta-D-2'-deoxy-2'-fluoro-2'-C-methylcytidine in rhesus monkeys

beta-D-2'-Deoxy-2'-fluoro-2'-C-methylcytidine (PSI-6130) is an effective inhibitor of hepatitis C virus (HCV) replication in vitro. The purpose of this study was to evaluate the single-dose pharmacokinetics of PSIota-6130 in rhesus monkeys following intravenous (i.v.) and oral administration. Noncompartmental analysis of the serum data obtained following oral and i.v. administration was performed. Pharmacokinetic studies with rhesus monkeys indicated slow and incomplete absorption with a mean absorption time (MAT) of 4.6 h and an oral bioavailability of 24.0% +/- 14.3% (mean +/- standard deviation), with comparable mean apparent half-lives following i.v. (4.54 +/- 3.98 h) and oral (5.64 +/- 1.13 h) administrations. The average percentages of the total dose recovered unchanged and in deaminated form in the urine were 32.9% +/- 12.6% and 18.9% +/- 6.6% (i.v.) and 6.0% +/- 3.9% and 3.9% +/- 1.0% (oral), respectively. The total bioavailability, taking into account the parent drug and its deaminated metabolite 2'-deoxy-2'-fluoro-2'-C-methyluridine (PSI-6206), was 64% +/- 26%. PSI-6130 was present in the cerebrospinal fluid after oral and i.v. dosing. However, no deamination of radiolabeled PSI-6130 was detected after 8 h of incubation in monkey and human whole blood. An N(4)-modified prodrug of PSI-6130 (PSI-6419) was orally administered to monkeys, but it failed to improve the oral bioavailability of PSI-6130. Further studies are warranted to improve the oral bioavailability and reduce the deamination of PSI-6130 in order to explore the potential of this drug for the treatment of HCV-infected individuals.

Pharmacology of current and promising nucleosides for the treatment of human immunodeficiency viruses

Nucleoside antiretroviral agents are chiral small molecules that have distinct advantages compared to other classes including long intracellular half-lives, low protein binding, sustained antiviral response when a dose is missed, and ease of chemical manufacture. They mimic natural nucleosides and target a unique but complex viral polymerase that is essential for viral replication. They remain the cornerstone of highly active antiretroviral therapy (HAART) and are usually combined with non-nucleoside reverse [corrected] transcriptase and protease inhibitors to provide powerful antiviral responses to prevent or delay the emergence of drug-resistant human immunodeficiency virus (HIV). The pharmacological and virological properties of a selected group of nucleoside analogs are described. Some of the newer nucleoside analogs have a high genetic barrier to resistance development. The lessons learned are that each nucleoside analog should be treated as a unique molecule since any structural modification, including a change in the enantiomeric form, can affect metabolism, pharmacokinetics, efficacy, toxicity and resistance profile.

New insights into the neuroimmunity of SIV infection by magnetic resonance spectroscopy

(1)H magnetic resonance spectroscopy (MRS) was employed to noninvasively monitor neuronal injury in eight rhesus macaques infected with simian immunodeficiency virus (SIV), whose immune system was compromised by CD8 T lymphocyte depletion and treated with highly active antiretroviral therapy (HAART). SIV infection and CD8 depletion resulted in a rapid decline in cerebral N-acetylaspartate (NAA) levels, a sensitive marker of neuronal health. Within 3 months of SIV infection and CD8 depletion, four animals developed AIDS and severe SIV encephalitis. The other four macaques underwent daily doses of HAART beginning 4 weeks after infection/CD8 depletion. HAART involved drugs that do not penetrate the central nervous system (CNS) including 9-[2(R)-(phosphonomethoxy)propyl]adenine and a racemic mixture of D: -L: -enantiomers of 2',3'-dideoxy-5-fluoro-3'thiacytidine. HAART resulted in reversal of NAA/Cr decline after 4 weeks of therapy, and no virus or encephalitis was found in brain samples analyzed. These results indicate that the CNS injury in AIDS is entirely dependent on events involving the peripheral immune system mediated by trafficking of SIV-infected monocytes into the brain. The rapid decline in NAA/Cr with SIV infection/CD8 depletion and its rapid recovery with HAART suggest that: (1) infected monocyte turnover in the CNS is rapid, occurring in days to weeks; (2) there are endogenous mechanisms that reverse neuronal injury; and (3) a threshold level of infected monocytes/macrophages in the CNS is required to overcome the neuronal recovery processes. These observations explain the clinical success of antiretroviral therapy in reducing the incidence of HIV-associated dementia and minor cognitive/motor disorder and suggest novel targets for drug development.

Emtricitabine, a new antiretroviral agent with activity against HIV and hepatitis B virus

Emtricitabine (FTC) is a new nucleoside agent that has activity against both human immunodeficiency virus (HIV) and hepatitis B virus. It is very similar to lamivudine (3TC) with respect to its activity, convenience, and safety and resistance profile. Indeed, with the exception of the longer intracellular half-life of triphosphate FTC, there is little to distinguish between the 2 drugs. Clinical trials comparing FTC with 3TC as part of a triple-drug regimen have demonstrated their equivalence, whereas a study comparing activity of FTC with that of stavudine demonstrated FTC's superiority. In clinical practice, the choice of 3TC versus FTC will most likely be made in the context of drugs coformulated with them. Although FTC is not formally approved for use in patients coinfected with HIV and hepatitis B virus, it is often a preferred choice for such patients when combined with tenofovir, which also has anti-hepatitis B virus activity. Recent treatment guidelines for the treatment of HIV infection by both the International AIDS Society-USA and US Department of Health and Human Services have placed FTC in combination with tenofovir, didanosine, or zidovudine in the preferred category of nucleoside backbone regimens for patients receiving antiretroviral therapy.

The central nervous system is a viral reservoir in simian immunodeficiency virus--infected macaques on combined antiretroviral therapy: a model for human immunodeficiency virus patients on highly active antiretroviral therapy

This study used a simian immunodeficiency virus (SIV)-macaque model to determine whether virus persists in the central nervous system (CNS) of human immunodeficiency virus (HIV)-infected individuals in which plasma viral load has been suppressed by highly active antiretroviral therapy. SIV-infected macaques were treated with two reverse transcriptase inhibitors: PMPA (q- R-(2-phosphonomethoxypropyl)adenine)which does not cross the blood-brain barrier, and FTC (beta-2('),3(')-dideoxy-3 thia-5-fluorocytidine), which does. Viral DNA and RNA were quantitated in the brain after 6 months of suppression of virus replication in blood and cerebrospinal fluid (CSF). Viral DNA was detected in brain from all macaques, including those in which peripheral viral replication had been suppressed either by antiretroviral therapy or host immune responses. Significant neurological lesions were observed only in one untreated macaque that had active virus replication in the CNS. Expression of the inflammatory markers, major histocmopatibility complex (MHC) II and CD68 was significantly lower in macaques treated with PMPA/FTC. Thus, although antiretroviral treatment may suppress virus replication in the periphery and the brain and reduce CNS inflammation, viral DNA persists in the brain despite treatment. This suggests that the brain may serve as a long-term viral reservoir in HIV-infected individuals treated with antiretroviral drugs that suppress virus replication.

Safety, pharmacokinetics, and efficacy of (+/-)-beta-2',3'-dideoxy-5-fluoro-3'-thiacytidine with efavirenz and stavudine in antiretroviral-naïve human immunodeficiency virus-infected patients

Racivir [RCV; (+/-)-beta-2',3'-dideoxy-5-fluoro-3'-thiacytidine], a 50:50 racemic mixture of the two beta nucleoside enantiomers, is currently in development for the treatment of human immunodeficiency virus type 1 (HIV-1) infections. RCV was administered once a day orally for 14 days at doses of 200, 400, or 600 mg in combination with stavudine and efavirenz to HIV-1-infected treatment-naïve male volunteers in a phase Ib/IIa study. Six volunteers at each dose were monitored for a total of 35 days for tolerance, pharmacokinetics, and plasma HIV RNA levels. RCV in combination with stavudine and efavirenz was well tolerated at all doses tested. Pharmacokinetic parameters were dose proportional, and the maximum concentration of drug in serum at all doses exceeded the 90% effective concentration for wild-type HIV-1. Viral loads dropped as expected in all dosage groups, with mean reductions from 1.13 to 1.42 log10 by day 4 and 2.02 to 2.43 log10 by day 14. HIV RNA levels remained suppressed for more than 2 weeks in the absence of any additional therapy, with mean viral loads ranging from 2.1 to 2.6 log10 below baseline through day 28. By day 35, HIV RNA levels began to increase but still remained >1 log10 below baseline levels.

Magnetic resonance spectroscopy reveals that activated monocytes contribute to neuronal injury in SIV neuroAIDS

Difficulties in understanding the mechanisms of HIV neuropathogenesis include the inability to study dynamic processes of infection, cumulative effects of the virus, and contributing host immune responses. We used H magnetic resonance spectroscopy and studied monocyte activation and progression of CNS neuronal injury in a CD8 lymphocyte depletion model of neuroAIDS in SIV-infected rhesus macaque monkeys. We found early, consistent neuronal injury coincident with viremia and SIV infection/activation of monocyte subsets and sought to define the role of plasma virus and monocytes in contributing to CNS disease. Antiretroviral therapy with essentially non-CNS-penetrating agents resulted in slightly decreased levels of plasma virus, a significant reduction in the number of activated and infected monocytes, and rapid, near-complete reversal of neuronal injury. Robust macrophage accumulation and productive virus replication were found in brains of infected and CD8 lymphocyte-depleted animals, but no detectable virus and few scattered infiltrating macrophages were observed in CD8 lymphocyte-depleted animals compared with animals not receiving antiretroviruses that were sacrificed at the same time after infection. These results underscore the role of activated monocytes and monocyte infection outside of the brain in driving CNS disease.

Polysaccharide-based chiral phase under polar organic mode of elution in the determination of the enantiomeric purity of emtricitabine an anti-HIV analogue nucleoside

This work reports the separation of FTC enantiomers using an amylose tris[(S)-1-phenylethylcarbamate] coated onto APS-Nucleosil (7 microm particle size, 500 A pore size, 20% w/w, 15 x 0.46 cm ID) chiral column under polar organic elution mode. Good enantioselectivity (alpha=1.9) with excellent enantioresolution (R(S)=3.3) was achieved by the use of methanol with 0.02% of triethylamine acetate as mobile phase. The method allows the accurate determination of as low as 0.2% of each enantiomer as an impurity. The validated method proved to be reliable and sensitive for the quantification of both enantiomers as impurity in different batches of emtricitabine and beta-D-(+)-FTC.

Novel role of 3-phosphoglycerate kinase, a glycolytic enzyme, in the activation of L-nucleoside analogs, a new class of anticancer and antiviral agents

l-Nucleoside analogs are a new class of clinically active antiviral and anticancer agents. The phosphorylation of these analogs from diphosphate to triphosphate metabolites is crucial for their biological action. We studied the role of 3-phosphoglycerate kinase, a glycolytic enzyme, in the metabolism of l-nucleoside analogs, using small interfering RNAs to down-regulate the amount of this enzyme in HelaS3 and 2.2.15 cells, chosen as models for studying the impact of the enzyme on the anticancer and antihepatitis B virus activities of these analogs. Decrease in the expression of 3-phosphoglycerate kinase led to a corresponding decrease in the formation of the triphosphate metabolites of l-nucleoside analogs (but not d-nucleoside analogs), resulting in detrimental effects on their activity. The enzyme is important for generating as well as maintaining the steady state levels of l-nucleotides in the cells, thereby playing a key role in the activity of l-nucleoside analogs against human immunodeficiency virus, hepatitis B virus, and cancer. This study also indicates a structure-based distinction in the metabolism of l- and d-nucleoside analogs, disputing the classic notion that nucleoside diphosphate kinases are responsible for the phosphorylation of all classes of nucleoside analog diphosphates.

Resting CD4+ T lymphocytes but not thymocytes provide a latent viral reservoir in a simian immunodeficiency virus-Macaca nemestrina model of human immunodeficiency virus type 1-infected patients on highly active antiretroviral therapy

Despite suppression of viremia in patients on highly active antiretroviral therapy (HAART), human immunodeficiency virus type 1 persists in a latent reservoir in the resting memory CD4(+) T lymphocytes and possibly in other reservoirs. To better understand the mechanisms of viral persistence, we established a simian immunodeficiency virus (SIV)-macaque model to mimic the clinical situation of patients on suppressive HAART and developed assays to detect latently infected cells in the SIV-macaque system. In this model, treatment of SIV-infected pig-tailed macaques (Macaca nemestrina) with the combination of 9-R-(2-phosphonomethoxypropyl)adenine (PMPA; tenofovir) and beta-2',3'-dideoxy-3'-thia-5-fluorocytidine (FTC) suppressed the levels of plasma virus to below the limit of detection (100 copies of viral RNA per ml). In treated animals, levels of viremia remained close to or below the limit of detection for up to 6 months except for an isolated "blip" of detectable viremia in each animal. Latent virus was measured in blood, spleen, lymph nodes, and thymus by several different methods. Replication-competent virus was recovered after activation of a 99.5% pure population of resting CD4(+) T lymphocytes from a lymph node of a treated animal. Integrated SIV DNA was detected in resting CD4(+) T cells from spleen, peripheral blood, and various lymph nodes including those draining the gut, the head, and the limbs. In contrast to the wide distribution of latently infected cells in peripheral lymphoid tissues, neither replication-competent virus nor integrated SIV DNA was detected in thymocytes, suggesting that thymocytes are not a major reservoir for virus in pig-tailed macaques. The results provide the first evidence for a latent viral reservoir for SIV in macaques and the most extensive survey of the distribution of latently infected cells in the host.

Effect of oral administration of emtricitabine on woodchuck hepatitis virus replication in chronically infected woodchucks

Emtricitabine [(-)FTC] [(-)-beta-2', 3'-dideoxy-5-fluoro-3'-thiacytidine] has been shown to be an effective inhibitor of hepatitis B virus (HBV) in cell culture, with a potency and selectivity that are essentially identical to those of lamivudine. The antiviral activity of oral administration of (-)FTC against WHV replication in chronically infected woodchucks, an established and predictive model for antiviral therapy against HBV, was examined in a placebo-controlled study. (-)FTC significantly reduced viremia and intrahepatic WHV replication in a dose-dependent manner that was comparable to the antiviral activity of lamivudine observed in previous studies conducted by our laboratories. No effect on the levels of hepatic WHV RNA or the levels of woodchuck hepatitis surface antigen or anti-woodchuck hepatitis surface and core antibodies in the serum of the treated animals was observed. No evidence of drug-related toxicity was observed in any of the animals treated.

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.

Pharmacokinetics of beta-L-2',3'-dideoxy-5-fluorocytidine in rhesus monkeys

beta-L-2',3'-Dideoxy-5-fluorocytidine (beta-L-FddC), a novel cytidine analog with an unnatural beta-L sugar configuration, has been demonstrated by our group and others to exhibit highly selective in vitro activity against human immunodeficiency virus types 1 and 2 and hepatitis B virus. This encouraging in vitro antiviral activity prompted us to assess its pharmacokinetics in rhesus monkeys. Three monkeys were administered an intravenous dose of [3H] beta-L-FddC at 5 mg/kg of body weight. Following a 3-month washout period, an equivalent oral dose was administered. Plasma and urine samples were collected at various times for up to 24 h after dosing, and drug levels were quantitated by high-pressure liquid chromatography. Pharmacokinetic parameters were obtained on the basis of a two-compartment open model with a first-order elimination from the central compartment. After intravenous administration, the mean peak concentration in plasma (Cmax) was 29.8 +/- 10.5 microM. Total clearance, steady-state volume of distribution, terminal-phase plasma half-life (t1/2 beta), and mean residence time were 0.7 +/- 0.1 liters/h/kg, 1.3 +/- 0.1 liters/kg, 1.8 +/- 0.2 h, and 1.9 +/- 0.2 h, respectively. Approximately 47% +/- 16% of the intravenously administered radioactivity was recovered in the urine as the unchanged drug with no apparent metabolites. beta-L-FddC exhibited a Cmax of 3.2 microM after oral administration, with a time to peak drug concentration of approximately 1.5 h and a t1/2 of 2.2 h. One monkey in the oral administration arm of the study had a significant delay in the absorption of the aqueous administered dose. The absolute bioavailability of orally administered beta-L-FddC ranged from 56 to 66%.

Intracellular metabolism of 2',3'-dideoxynucleosides in duck hepatocyte primary cultures

The intracellular fate of the potent duck hepatitis B virus (DHBV) inhibitor 2,6-diaminopurine 2',3'-dideoxyriboside (ddDAPR), its deamination product 2',3'-dideoxyguanosine (ddG), and the less effective DHBV-inhibitor 2',3'-dideoxycytidine (ddC) was investigated in duck hepatocyte primary cultures. After a 1-min exposure of [3H]ddDAPR to duck blood, 95% of the compound was converted to ddG. Similarly, [3H]ddDAPR was converted rapidly to ddG in duck hepatocyte primary cultures, with ddG exhibiting resistance to further catabolism. The major pathway of ddG utilization in these cells was phosphorylation, yielding a concentration of 2.1 and 1.9 microM total ddG nucleotides after 5 and 26 hr, respectively, of exposure to 4 microM ddG. Removal of exogenous ddG led to a rapid (T1/2 = 1.6 hr) decrease in the total intracellular ddG nucleotide pools. Duck hepatocytes treated with 4 microM ddC exhibited a time-dependent accumulation of ddC nucleotides, culminating in a maximum intracellular total ddC nucleotide concentration of 1.4 microM after 24-26 hr. The intracellular total ddC nucleotide level decreased with a T1/2 of 4.4 hr following the removal of exogenous ddC. The formation of ddC nucleotides was reduced in the presence of excess 2'-dideoxycytidine implicating deoxycytidine kinase in the initial step of ddC phosphorylation. A 25-fold excess of 2'-deoxycytidine had no effect on ddG phosphorylation in duck hepatocytes. However, a 92% inhibition of ddG nucleotide formation occurred in duck hepatocytes treated for 5 hr with 4 microM [3H]dG + 100 microM adenosine in the presence of the adenosine deaminase inhibitor 2'-deoxycoformycin, suggesting that, in these cells, adenosine kinase is involved in the ddG phosphorylation process.

Antiviral therapy for human immunodeficiency virus infections

Depending on the stage of their intervention with the viral replicative cycle, human immunodeficiency virus inhibitors could be divided into the following groups: (i) adsorption inhibitors (i.e., CD4 constructs, polysulfates, polysulfonates, polycarboxylates, and polyoxometalates), (ii) fusion inhibitors (i.e., plant lectins, succinylated or aconitylated albumins, and betulinic acid derivatives), (iii) uncoating inhibitors (i.e., bicyclams), (iv) reverse transcription inhibitors acting either competitively with the substrate binding site (i.e., dideoxynucleoside analogs and acyclic nucleoside phosphonates) or allosterically with a nonsubstrate binding site (i.e., non-nucleoside reverse transcriptase inhibitors), (v) integration inhibitors, (vi) DNA replication inhibitors, (vii) transcription inhibitors (i.e., antisense oligodeoxynucleotides and Tat antagonists), (viii) translation inhibitors (i.e., antisense oligodeoxynucleotides and ribozymes), (ix) maturation inhibitors (i.e., protease inhibitors, myristoylation inhibitors, and glycosylation inhibitors), and finally, (x) budding (assembly/release) inhibitors. Current knowledge, including the therapeutic potential, of these various inhibitors is discussed. In view of their potential clinical the utility, the problem of virus-drug resistance and possible strategies to circumvent this problem are also addressed.

Pharmacokinetics, oral bioavailability, and metabolism in mice and cynomolgus monkeys of (2'R,5'S-)-cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl] cytosine, an agent active against human immunodeficiency virus and human hepatitis B virus

(2'R,5'S-)-cis-5-Fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl] cytosine (524W91) is a nucleoside analog with potent anti-human immunodeficiency virus and anti-human hepatitis B virus activities in vitro. The pharmacokinetics and bioavailability of 524W91 after oral dosing were studied in mice dosed with 10, 100, and 600 mg of 524W91 per kg of body weight by the oral and intravenous routes. Cynomolgus monkeys were dosed with 10 and 80 mg of 524W91 per kg. In both species, the clearance of 524W91 was rapid, via the kidney, and was independent of dose. In monkeys, the total body clearance of 10 mg of 524W91 per kg was 0.7 +/- 0.1 liter/h/kg, and the volume of distribution at steady state was 0.8 +/- 0.02 liter/kg. The terminal elimination half-life was 1.0 +/- 0.2 h. The absolute bioavailability after oral dosing was 63% +/- 4% at 10 mg/kg. Concentrations of 524W91 in the cerebrospinal fluid were 4% +/- 0.7% of the corresponding levels in plasma. In mice, the total clearance of 10 mg of 524W91 per kg was 2.3 liters/kg/h, and the volume of distribution at steady state was 0.9 liter/kg. Absolute bioavailability in mice after oral dosing was 96% at a dose of 10 mg/kg. The metabolism of orally administered [6-3H]524W91 was studied in cynomolgus monkeys at a dose of 80 mg/kg and in mice at a dose of 120 mg/kg. Monkeys excreted 41% +/- 6% of the radioactive dose in the 0- to 72-h urine, 33% +/- 10% in the feces, and 10% +/- 7% in the cage wash. Unchanged 524W91 was 64% of the total radiolabeled drug recovered in the urine. The glucuronide was a minor urinary metabolite. 5-Fluorouracil was not detected (less than 0.02% of the dose). Mice dosed orally with 120 mg of [6-3H]524W91 per kg excreted 67% +/- 7% of the radiolable in the )- to 48-h urine. Small amounts of the 3' -sulfoxide and glucuronide metabolites were observed in the urine, but 5-fluorouracil was not detected. Good bioavailability after oral dosing and resistance to metabolism recommend 524W91 for further preclinical evaluation.

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.

Pharmacokinetics of 2',3'-dideoxy-5-fluoro-3'-thiacytidine in rats

Although several drugs have shown clinical anti-human immunodeficiency virus activity, reduced activity with long-term use and toxicity make new agents with high therapeutic indices desirable. Racemic cis-2',3'-dideoxy-5-fluoro-3'-thiacytidine (FTC) is a new synthetic nucleoside analogue that is usually potent against human immunodeficiency virus types 1 and 2 and hepatitis B virus in vitro. The purpose of this study was to characterize the preclinical pharmacokinetics of FTC in rats. Rats were administered 10, 50, and 100 mg of FTC per kg of body weight intravenously. Concentrations of FTC in plasma and urine were determined by HPLC. Pharmacokinetic parameters were generated by area/moment analysis. Plasma FTC concentrations declined rapidly in a biexponential fashion, with a terminal half-life of approximately 2 h. The area under the plasma FTC concentration-time curve increased proportionally with increasing dose, and there were no statistically significant differences in pharmacokinetic parameters among the three doses. Thus, the disposition of FTC was independent of dose over the range of 10-100 mg/kg. Since the disposition of FTC was linear, pharmacokinetic parameters were averaged for the three doses. The average total clearance of FTC was 1.91 +/- 0.32 L/h/kg (mean +/- SD), the average renal clearance was 1.08 +/- 0.26 L/h/kg, and the average nonrenal clearance was 0.83 +/- 0.27 L/h/kg. Approximately 55% of the dose of FTC was recovered as unchanged drug in the urine. The steady-state volume of distribution of FTC averaged 2.17 +/- 0.59 L/kg.

Characterization of human immunodeficiency viruses resistant to oxathiolane-cytosine nucleosides

The (-) enantiomers of 2',3'-dideoxy-5-fluoro-3'-thiacytidine [(-)-FTC] and 2',3'-dideoxy-3'-thiacytidine [(-)-BCH-189] were recently shown to inhibit selectively human immunodeficiency viruses (HIV) and hepatitis B virus in vitro. In the current study, the potential for HIV type 1 (HIV-1) resistance to these compounds was evaluated by serial passage of the virus in human peripheral blood mononuclear cells and MT-2 cells in the presence of increasing drug concentrations. Highly drug-resistant HIV-1 variants dominated the replicating virus population after two or more cycles of infection. The resistant variants were cross-resistant to (-)-FTC, (-)-BCH-189, and their (+) congeners but remained susceptible to 2',3'-dideoxycytidine, 3'-azido-3'-deoxythymidine, 3'-fluoro-3'-deoxythymidine, 2',3'-dideoxyinosine, phosphonoformate, the TIBO compound R82150, and the bis(heteroaryl)piperazine derivative U-87201E. Reverse transcriptase derived from drug-resistant viral particles was 15- to 50-fold less susceptible to the 5'-triphosphates of FTC and BCH-189 compared with enzyme from parental drug-susceptible virus. DNA sequence analysis of the reverse transcriptase gene amplified from resistant viruses consistently identified mutations at codon 184 from Met (ATG) to Val (GTG or GTA) or Ile (ATA). Sequence analysis of amplified reverse transcriptase from a patient who had received (-)-BCH-189 therapy for 4 months demonstrated a mixture of the Met-184-to-Val (GTG) mutation and the parental genotype, indicating that the Met-184 mutation can occur in vivo.