Mechanism of action of 1-beta-D-2,6-diaminopurine dioxolane, a prodrug of the human immunodeficiency virus type 1 inhibitor 1-beta-D-dioxolane guanosine

H(N3)dap (Hdap = 2,6-Diaminopurine) Recognition by Cu2(EGTA): Structure, Physical Properties, and Density Functional Theory Calculations of [Cu4(μ-EGTA)2(μ-H(N3)dap)2(H2O)2]·7H2O

Reactions in water between the Cu2(µ-EGTA) chelate (EGTA = ethylene-bis(oxyethyleneimino)tetraacetate(4-) ion) and Hdap in molar ratios 1:1 and 1:2 yield only blue crystals of the ternary compound [Cu4(μ-EGTA)2(μ-H(N3)dap)2(H2O)2]·7H2O (1), which has been studied via single-crystal X-ray diffraction and various physical methods (thermal stability, spectral and magnetic properties), as well as DFT theoretical calculations. In the crystal, uncoordinated water is disordered. The tetranuclear complex molecule also has some irrelevant disorder in an EGTA-ethylene moiety. In the complex molecule, both bridging organic molecules act as binucleating ligands. There are two distorted five- and two six-coordinated Cu(II) centers. Each half of EGTA acts as a tripodal tetradentate Cu(II) chelator, with a mer-NO2 + O(ether, distal) conformation. Hdap exhibits the tautomer H(N3)dap, with the dissociable H-atom on its less basic N-heterocyclic atom. These features favor the efficient cooperation between Cu-N7 or Cu-N9 bonds with appropriate O-EGTA atoms, as N6-H···O or N3-H···O interligand interactions, respectively. The bridging role of both organics determines the tetranuclear dimensionality of the complex. In this crystal, such molecules associate in zig-zag chains built by alternating π-π interactions between the five- or six-atom rings of Hdap ligands of adjacent molecules. DFT theoretical calculations (using two different theoretical models and characterized by the quantum theory of "atoms in molecules") reveal the importance of these π-π interactions between Hdap ligands, as well as those corresponding to the referred hydrogen bonds in the contributed tetranuclear molecule.

Advance of structural modification of nucleosides scaffold

With Remdesivir being approved by FDA as a drug for the treatment of Corona Virus Disease 2019 (COVID-19), nucleoside drugs have once again received widespread attention in the medical community. Herein, we summarized modification of traditional nucleoside framework (sugar + base), traizole nucleosides, nucleoside analogues assembled by other drugs, macromolecule-modified nucleosides, and their bioactivity rules. 2′-“Ara”-substituted by –F or –CN group, and 3′-“ara” substituted by acetylenyl group can greatly influence their anti-tumor activities. Dideoxy dehydrogenation of 2′,3′-sites can enhance antiviral efficiencies. Acyclic nucleosides and L-type nucleosides mainly represented antiviral capabilities. 5-F Substituted uracil analogues exihibit anti-tumor effects, and the substrates substituted by –I, –CF₃, bromovinyl group usually show antiviral activities. The sugar coupled with 1-N of triazolid usually displays anti-tumor efficiencies, while the sugar coupled with 2-N of triazolid mainly represents antiviral activities. The nucleoside analogues assembled by cholesterol, polyethylene glycol, fatty acid and phospholipid would improve their bioavailabilities and bioactivities, or reduce their toxicities.

Highlights of the 30th International Conference on Antiviral Research

The 30th International Conference on Antiviral Research (ICAR) was held in Atlanta, GA, USA from May 18 to 21, 2017. This report provides an account of award lectures, invited keynote addresses and oral presentations during the meeting. The 2017 Gertrude Elion Memorial Lecture Award by Michael Sofia highlighted one of the most important accomplishments in recent drug discovery in antiviral research, the identification of the hepatitis C virus direct-acting antiviral sofosbuvir and new alternatives to combat hepatitis B virus (HBV) infection. The Antonín Holý Lecture Award by David Chu on medicinal chemistry provided an overview of early developments of nucleoside analogs for the treatment of HIV and varicella zoster virus infection and how this knowledge serves to develop new drugs targeting HBV. Priscilla Yang gave the first ISAR Women in Science lecture. She reported on pharmacological validation of new antiviral targets for dengue, Zika and other flaviviruses. The William Prusoff Young Investigator Lecture Award by Maaike Everts described the Alabama Drug Discovery Alliance and the Antiviral Drug Discovery and Development Consortium, and how they are helping to accelerate the development of new antivirals. The 30th ICAR was a success in promoting new discoveries in antiviral drug development and research. The 31st ICAR will be held in Porto, Portugal, June 11–15, 2018.

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The 30th ICAR was held in Atlanta, Georgia, USA from May 18–21, 2017.

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This article summarizes presentations by ISAR award recipients, principal invited lectures and keynote addresses.

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Mini-symposium topics included antiviral immunity and emerging viral infections.

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Other sessions and topics included hepatitis and retroviruses, respiratory viruses, DNA viruses and medicinal chemistry.

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The 31st ICAR will be held in Porto, Portugal, June 11–15, 2018.

Synthesis, structural characterization and biological evaluation of 4'-C-methyl- and phenyl-dioxolane pyrimidine and purine nucleosides

Nucleoside analogues play an important role in antiviral, antibacterial and antineoplastic chemotherapy. Herein we report the synthesis, structural characterization and biological activity of some 4'-C -methyl- and -phenyl dioxolane-based nucleosides. In particular, α and β anomers of all natural nucleosides were obtained and characterized by NMR, HR-MS and X-ray crystallography. The compounds were tested for antimicrobial activity against some representative human pathogenic fungi, bacteria and viruses. Antitumor activity was evaluated in a large variety of human cancer cell-lines. Although most of the compounds showed non-significant activity, 23α weakly inhibited HIV-1 multiplication. Moreover, 22α and 32α demonstrated a residual antineoplastic activity, interestingly linked to the unnatural α configuration. These results may provide structural insights for the design of active antiviral and antitumor agents.

HIV-1 Reverse Transcriptase Still Remains a New Drug Target: Structure, Function, Classical Inhibitors, and New Inhibitors with Innovative Mechanisms of Actions

During the retrotranscription process, characteristic of all retroviruses, the viral ssRNA genome is converted into integration-competent dsDNA. This process is accomplished by the virus-coded reverse transcriptase (RT) protein, which is a primary target in the current treatments for HIV-1 infection. In particular, in the approved therapeutic regimens two classes of drugs target RT, namely, nucleoside RT inhibitors (NRTIs) and nonnucleoside RT inhibitors (NNRTIs). Both classes inhibit the RT-associated polymerase activity: the NRTIs compete with the natural dNTP substrate and act as chain terminators, while the NNRTIs bind to an allosteric pocket and inhibit polymerization noncompetitively. In addition to these two classes, other RT inhibitors (RTIs) that target RT by distinct mechanisms have been identified and are currently under development. These include translocation-defective RTIs, delayed chain terminators RTIs, lethal mutagenesis RTIs, dinucleotide tetraphosphates, nucleotide-competing RTIs, pyrophosphate analogs, RT-associated RNase H function inhibitors, and dual activities inhibitors. This paper describes the HIV-1 RT function and molecular structure, illustrates the currently approved RTIs, and focuses on the mechanisms of action of the newer classes of RTIs.

HIV-1 reverse transcriptase inhibitors: beyond classic nucleosides and non-nucleosides

Reverse transcriptase (RT) of HIV-1 remains an important target in current treatments of HIV-1 infection. Clinically available inhibitors of HIV-1 RT include nucleoside analog RT inhibitors and non-nucleoside RT inhibitors. Nucleoside analog RT inhibitors compete with the natural dNTP substrate and act as chain terminators, while non-nucleoside RT inhibitors bind to an allosteric pocket, inhibiting polymerization noncompetitively. In addition to these two classes of approved drugs, there are a number of RT inhibitors that target the enzyme in different ways. These include nonobligate chain terminators, nucleotide-competing RT inhibitors, pyrophosphate analogs and compounds that inhibit the RT-associated RNase H activity. Here, we review the mechanisms of action associated with these compounds and discuss opportunities and challenges in drug discovery and development efforts.

Lack of pharmacokinetic interaction between amdoxovir and reduced- and standard-dose zidovudine in HIV-1-infected individuals

Amdoxovir (AMDX) inhibits HIV-1 containing the M184V/I mutation and is rapidly absorbed and deaminated to its active metabolite, beta-D-dioxolane guanosine (DXG). DXG is synergistic with zidovudine (ZDV) in HIV-1-infected primary human lymphocytes. A recent in silico pharmacokinetic (PK)/enzyme kinetic study suggested that ZDV at 200 mg twice a day (b.i.d.) may reduce toxicity without compromising efficacy relative to the standard 300-mg b.i.d. dose. Therefore, an intense PK clinical study was conducted using AMDX/placebo, with or without ZDV, in 24 subjects randomized to receive oral AMDX at 500 mg b.i.d., AMDX at 500 mg plus ZDV at 200 or 300 mg b.i.d., or ZDV at 200 or 300 mg b.i.d. for 10 days. Full plasma PK profiles were collected on days 1 and 10, and complete urine sampling was performed on day 9. Plasma and urine concentrations of AMDX, DXG, ZDV, and ZDV-5'-O-glucuronide (GZDV) were measured using a validated liquid chromatography-tandem mass spectrometry method. Data were analyzed using noncompartmental methods, and multiple comparisons were performed on the log-transformed parameters, at steady state. Coadministration of AMDX with ZDV did not significantly change either of the plasma PK parameters or percent recovery in the urine of AMDX, DXG, or ZDV/GZDV. Larger studies with AMDX/ZDV, with a longer duration, are warranted.

Nucleoside and nucleotide HIV reverse transcriptase inhibitors: 25 years after zidovudine

Twenty-five years ago, nucleoside analog 3'-azidothymidine (AZT) was shown to efficiently block the replication of HIV in cell culture. Subsequent studies demonstrated that AZT acts via the selective inhibition of HIV reverse transcriptase (RT) by its triphosphate metabolite. These discoveries have established the first class of antiretroviral agents: nucleoside and nucleotide reverse transcriptase inhibitors (NRTIs). Over the years that followed, NRTIs evolved into the main component of antiretroviral drug combinations that are now used for the treatment of all populations of HIV infected patients. A total of thirteen NRTI drug products are now available for clinical application: eight individual NRTIs, four fixed-dose combinations of two or three NRTIs, and one complete fixed-dose regimen containing two NRTIs and one non-nucleoside RT inhibitor. Multiple NRTIs or their prodrugs are in various stages of clinical development and new potent NRTIs are still being identified through drug discovery efforts. This article will review basic principles of the in vitro and in vivo pharmacology of NRTIs, discuss their clinical use including limitations associated with long-term NRTI therapy, and describe newly identified NRTIs with promising pharmacological profiles highlighting those in the development pipeline. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, volume 85, issue 1, 2010.

Simultaneous quantification of 9-(beta-D-1,3-dioxolan-4-yl)guanine, Amdoxovir and Zidovudine in human plasma by liquid chromatography-tandem mass spectrometric assay

A sensitive method was developed and validated for simultaneous measurement of an investigational antiviral nucleoside, Amdoxovir (DAPD), its deaminated metabolite 9-(beta-D-1,3-dioxolan-4-yl)guanine (DXG), and Zidovudine (ZDV) in human plasma. This method employed high-performance liquid chromatography-tandem mass spectrometry with electrospray ionization. DXG and DAPD separation with sufficient resolution was necessary since they differ in only one mass to charge ratio, which increases the risk of overlapping MS/MS signals. However, the new method was observed to have functional sensitivity and specificity without interference. Samples were purified by ultrafiltration after protein precipitation with methanol. The total run time was 29 min. A linear calibration range from 2 to 3000 ng mL(-1) and 2 to 5000 ng mL(-1) was achieved for DAPD and DXG, and ZDV, respectively. Precisions and accuracies were both +/-15% (+/-20% for the lower limit of quantification) and recoveries were higher than 90%. Matrix effects/ion suppressions were also investigated. The analytes were chemically stable under all relevant conditions and the method was successfully applied for the analysis of plasma samples from HIV-infected persons treated with combinations of DAPD and ZDV.

5'-O-Aliphatic and amino acid ester prodrugs of (-)-beta-D-(2R,4R)-dioxolane-thymine (DOT): synthesis, anti-HIV activity, cytotoxicity and stability studies

A series of (-)-beta-D-(2R,4R)-dioxolane-thymine-5'-O-aliphatic acid esters as well as amino acid esters were synthesized as prodrugs of (-)-beta-D-(2R,4R)-dioxolane-thymine (DOT). The compounds were evaluated for anti-HIV activity against HIV-1(LAI) in human peripheral blood mononuclear (PBM) cells as well as for their cytotoxicity in PBM, CEM and Vero cells. Improved anti-HIV potency in vitro was observed for the compound 2-4 (5'-O-aliphatic acid esters) without increase in cytotoxicity in comparison to the parent drug. Chemical and enzymatic hydrolysis of the prodrugs was also studied, in which the prodrugs exhibited good chemical stability with the half-lives from 3 h to 54 h at pH 2.0 and 7.4 phosphate buffer. However, the prodrugs were relatively labile to porcine esterase with the half-lives from 12.3 to 48.0 min.

Development of an optimized dose for coformulation of zidovudine with drugs that select for the K65R mutation using a population pharmacokinetic and enzyme kinetic simulation model

In vitro selection studies and data from large genotype databases from clinical studies have demonstrated that tenofovir disoproxil fumarate and abacavir sulfate select for the K65R mutation in the human immunodeficiency virus type 1 polymerase region. Furthermore, other novel non-thymine nucleoside reverse transcriptase (RT) inhibitors also select for this mutation in vitro. Studies performed in vitro and in humans suggest that viruses containing the K65R mutation remained susceptible to zidovudine (ZDV) and other thymine nucleoside antiretroviral agents. Therefore, ZDV could be coformulated with these agents as a "resistance repellent" agent for the K65R mutation. The approved ZDV oral dose is 300 mg twice a day (b.i.d.) and is commonly associated with bone marrow toxicity thought to be secondary to ZDV-5'-monophosphate (ZDV-MP) accumulation. A simulation study was performed in silico to optimize the ZDV dose for b.i.d. administration with K65R-selecting antiretroviral agents in virtual subjects using the population pharmacokinetic and cellular enzyme kinetic parameters of ZDV. These simulations predicted that a reduction in the ZDV dose from 300 to 200 mg b.i.d. should produce similar amounts of ZDV-5'-triphosphate (ZDV-TP) associated with antiviral efficacy (>97% overlap) and reduced plasma ZDV and cellular amounts of ZDV-MP associated with toxicity. The simulations also predicted reduced peak and trough amounts of cellular ZDV-TP after treatment with 600 mg ZDV once a day (q.d.) rather than 300 or 200 mg ZDV b.i.d., indicating that q.d. dosing with ZDV should be avoided. These in silico predictions suggest that 200 mg ZDV b.i.d. is an efficacious and safe dose that could delay the emergence of the K65R mutation.

Development of a novel mouse model to evaluate drug candidates against hepatitis B virus

Woodchuck hepatitis virus (WHV)-infected woodchucks have been used for preclinical development of drugs against hepatitis B virus (HBV). However, there is no simple in vivo model to evaluate small amounts of compounds against HBV. To develop such a model, HepAD38 cells, in which HBV replication is regulated by tetracycline (tet), were grown as subcutaneous tumours in nude mice. Mice developing viraemia were then left untreated or given tet in the drinking water. In some of the mice given tet, it was removed and the mice were injected intraperitoneally with phosphate buffer saline (PBS), lamivudine (3TC), clevudine (CLV) or tenofovir dipivoxil fumarate (TDF). Virus DNA titres were measured by real-time PCR during and after drug treatment. In water-fed and PBS-injected mice, virus titres reached approximately 10(9) copies/ml serum within 35 days of HepAD38 injection, whereas in tet-treated mice, virus titres remained at 10(4)-10(5) copies/ml. HBV DNA levels were suppressed by 3TC, TDF and CLV, with the latter two drugs showing more sustained virus suppression compared with 3TC. Combination therapy with CLV plus TDF was much more effective than either drug alone in suppressing virus titre for at least 3 weeks after the end of treatment. There was no demonstrable toxicity to HepAD38 cells in drug-treated mice. Hence, a robust tet-controlled system for HBV replication in vivo was demonstrated, validated with monotherapies against HBV and shown to be useful in assessing combination therapy. This system will be useful for preclinical assessment of small amounts of single or multiple compounds against HBV in vivo.

Antiviral effect of orally administered (-)-beta-D-2-aminopurine dioxolane in woodchucks with chronic woodchuck hepatitis virus infection

(-)-beta-D-2-Aminopurine dioxolane (APD) is a nucleoside prodrug that is efficiently converted to 9-(beta-D-1,3-dioxolan-4-yl)guanine (DXG). DXG has antiviral activity in vitro against hepatitis B virus (HBV) but limited aqueous solubility, making it difficult to administer orally to HBV-infected individuals. APD is more water soluble than DXG and represents a promising prodrug for the delivery of DXG. A placebo-controlled, dose-ranging efficacy and pharmacokinetic study was conducted with woodchucks that were chronically infected with woodchuck hepatitis virus (WHV). APD was efficiently converted to DXG after oral and intravenous administrations of APD, with serum concentrations of DXG being higher following oral administration than following intravenous administration, suggestive of a considerable first-pass intestinal and/or hepatic metabolism. APD administered orally at 1, 3, 10, and 30 mg/kg of body weight per day for 4 weeks produced a dose-dependent antiviral response. Doses of 3 and 10 mg/kg/day reduced serum WHV viremia by 0.4 and 0.7 log(10) copies/ml, respectively. The 30-mg/kg/day dose resulted in a more pronounced, statistically significant decline in serum WHV viremia of 1.9 log(10) copies/ml and was associated with a 1.5-fold reduction in hepatic WHV DNA. Individual woodchucks within the highest APD dose group that had declines in serum WHV surface antigen levels, WHV viremia, and hepatic WHV DNA also had reductions in hepatic WHV RNA. There was a prompt recrudescence of WHV viremia following drug withdrawal. Therefore, oral administration of APD for 4 weeks was safe in the woodchuck model of chronic HBV infection, and the effect on serum WHV viremia was dose dependent.

Pharmacokinetics of the anti-human immunodeficiency virus agent 1-(beta-D-dioxolane)thymine in rhesus monkeys

Beta-D-dioxolane-thymine (D-DOT) has potent and selective in vitro activity against several clinically important resistant human immunodeficiency virus (HIV) mutants and is in advanced preclinical development. Therefore, the single-dose intravenous and oral pharmacokinetics of D-DOT were studied with three rhesus monkeys. The pharmacokinetic profiles of D-DOT in serum and urine were adequately described by a two-compartment open pharmacokinetic model. D-DOT was rapidly and almost completely absorbed (absorption rate constant = 2.7 h(-1); fraction of oral dose absorbed = 0.82 to 1.06). The average serum beta half-life was 2.16 h. The average central and steady-state volumes of distributions were 0.52 and 1.02 liter/kg of body weight, respectively, and the average systemic and renal clearance values were 0.36 liter/h/kg and 0.18 liter/h/kg. Four or eight percent of administered D-DOT was eliminated in the urine as glucuronide within 8 h after intravenous or oral administration, respectively. D-DOT reached levels in the cerebrospinal fluid in excess of 10 to 20 times the median effective concentration for wild-type HIV and resistant mutants. The potent antiretroviral activity of D-DOT against a lamivudine- and zidovudine-resistant HIV-1 mutant, together with an excellent pharmacokinetic profile for rhesus monkeys, suggest that further development is warranted.

Interaction of 2'-deoxyguanosine triphosphate analogue inhibitors of HIV reverse transcriptase with human mitochondrial DNA polymerase gamma

Mitochondrial toxicity is a limiting factor in the use of some nucleoside reverse transcriptase inhibitors of HIV. To further understand the impact of structural features on the incorporation and exonuclease removal of nucleoside monophosphate (MP) analogues by human mitochondrial DNA polymerase (pol gamma), transient kinetic studies were done with analogues of 2'-deoxyguanosine triphosphate. The kinetic parameters for the incorporation and removal of carbovir (CBV)-MP, dioxolane guanosine (DXG)-MP and 2',3'-dideoxy-2',3'-didehydroguanosine (d4G)-MP were studied with pol gamma holoenzyme. The importance of the ribose oxygen in incorporation by pol gamma was illustrated by an approximate 3,000-fold decrease in the incorporation efficiency of an analogue lacking the ribose oxygen (CBV-TP) relative to those containing a ribose oxygen (DXG-TP and d4G-TP). As a result, a comparison with previous data for the incorporation by HIV reverse transcriptase showed CBV-TP to be approximately 800-8,000-fold more selective for its antiviral target over pol gamma relative to the other guanosine analogues. However, DXG-TP and d4G-TP were found to be much more selective than previously reported values for mitochondrial toxic nucleoside analogues. Structural modelling based on sequence homology with other polymerase A family members suggests that an interaction between the ribose oxygen and arginine 853 in pol gamma may play a critical role in causing this differential incorporation. Exonuclease removal of a chain-terminating CBV-MP was also found to be more efficient by pol gamma. These results help to further elucidate the structure activity relationships for pol gamma and should aid in the design of more selective antiviral agents.

Biochemical studies on the mechanism of human immunodeficiency virus type 1 reverse transcriptase resistance to 1-(beta-D-dioxolane)thymine triphosphate

A large panel of drug-resistant mutants of human immunodeficiency virus type 1 reverse transcriptase (RT) was used to study the mechanisms of resistance to 1-(beta-d-dioxolane)thymine triphosphate (DOT-TP) and other nucleotide analogs. RT containing thymidine analog-associated mutations (TAM) or RT with a T69S-SG insertion in combination with TAM removed 3'-azido-3'-deoxythymidine-5'-monophosphate or tenofovir more efficiently than DOT-monophosphate from chain-terminated DNA primer/template through ATP-mediated pyrophosphorolysis. For non-ATP-dependent discrimination toward DOT-TP, high levels of resistance were found for RT bearing the Q151M mutation with family mutations, while RT bearing only the M184V or the Y115F mutation conferred no resistance to DOT-TP. A lower degree of resistance to DOT-TP than to tenofovir diphosphate or carbovir-TP was found for RT containing the K65R mutation. In the present studies, 1-(beta-d-dioxolane)guanine triphosphate, another nucleotide with a dioxolane sugar moiety, showed a resistance profile similar to that of DOT-TP. The results suggest that DOT, compared with other approved nucleoside analogs, is overall more resilient to mutations such as TAM, M184V, and K65R, which are commonly found in viruses derived from subjects failing multinucleoside therapy.

Synthesis and anti-HIV activity of (-)-beta-D-(2R,4R)-1,3-dioxolane-2,6-diamino purine (DAPD) (amdoxovir) and (-)-beta-D-(2R,4R)-1,3-dioxolane guanosine (DXG) prodrugs

Prodrugs of (-)-beta-D-(2R,4R)-1,3-dioxolane-2,6-diamino purine (DAPD), organic salts of DAPD, 5'-L-valyl DAPD and N-1 substituted (-)-beta-D-(2R,4R)-1,3-dioxolane guanosine (DXG) have been synthesized with the objective of finding molecules which might be superior to DAPD and DXG in solubility as well as pharmacologic profiles. Synthesized prodrugs were evaluated for anti-HIV activity against HIV-1(LAI) in primary human lymphocytes (PBM cells) as well as their cytotoxicity in PBM, CEM and Vero cells. DAPD prodrugs, modified at the C6 position of the purine ring, demonstrated several folds of enhanced anti-HIV activity in comparison to the parent compound DAPD without increasing the toxicity. The presence of alkyl amino groups at the C6 position of the purine ring increased the antiviral potency several folds, and the most potent compound (-)-beta-D-(2R,4R)-1,3-dioxolane-2-amino-6-aminoethyl purine (8) was 17 times more potent than that of DAPD. 5'-L-Valyl DAPD 20 and organic acid salts 21-24 also exhibited enhanced anti-HIV activity in comparison to DAPD, while DXG prodrugs 16 and 17 exhibited lower potency than that of DXG or DAPD.

HIV-1 reverse transcriptase inhibitors

Reverse transcriptase (RT) is one of the three enzymes encoded by the human immunodeficiency virus type 1 (HIV-1), the etiological agent of AIDS. Together with protease inhibitors, drugs inhibiting the RNA- and DNA-dependant DNA polymerase activity of RT are the major components of highly active antiretroviral therapy (HAART), which has dramatically reduced mortality and morbidity of people living with HIV-1/AIDS in developed countries. In this study, we focus on RT inhibitors approved by the US Food and Drugs Administration (FDA) or in phases II and III clinical trials. RT inhibitors belong to two main classes acting by distinct mechanisms. Nucleoside RT inhibitors (NRTIs) lack a 3' hydroxyl group on their ribose or ribose mimic moiety and thus act as chain terminators. Non-NRTIs bind into a hydrophobic pocket close to the polymerase active site and inhibit the chemical step of the polymerization reaction. For each class of inhibitors, we review the mechanism of action, the resistance mechanisms selected by the virus, and the side effects of the drugs. We also discuss the main perspectives for the development of new RT inhibitors.

Current developments in HIV chemotherapy

HIV infection is the leading cause of death worldwide and despite major advances in treatment, more new cases were diagnosed in 2004 than any previous year. Current treatment regimens are based on the use of two or more drugs from two or more classes of inhibitors termed highly active antiretroviral therapy (HAART). Although HAART is capable of suppressing viral loads to undetectable levels, problems of toxicity, patient adherence, and particularly the emergence of drug-resistant viruses continues to spur the development of new chemotherapeutics to combat HIV. Clinical candidates from the four existing classes of inhibitors are presented in this review along with lead compounds against new viral targets, with special emphasis on HIV integrase.

Emerging reverse transcriptase inhibitors for the treatment of HIV infection in adults

A combination of three or more antiretroviral drugs, commonly called 'highly active antiretroviral therapy' (HAART), has become the standard-of-care treatment for HIV-infected patients in the developed world. There are now 21 licensed anti-HIV drugs to choose from when starting a HAART regimen. The currently approved antiretroviral drugs fall into four categories: nucleoside/nucleotide reverse transcriptase inhibitors, non-nucleoside reverse transcriptase inhibitors, protease inhibitors and fusion inhibitors. Novel compounds currently in preclinical or clinical development are either focusing on new viral proteins or the same specific viral elements targeted by the available drugs. When developing new anti-HIV drugs of an already existing class, focus should be held on maximising potency, minimising toxicity, diminishing the risk for resistance development and producing effective drugs for patients who already have resistance to currently available drugs. In addition, pill burden should be ideally reduced to once-daily dosing, thereby enhancing a patient's adherence and reducing treatment costs. The present review focuses on emerging drugs to inhibit the reverse transcriptase of HIV.

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.

Pharmacology and pharmacokinetics of the antiviral agent beta-D-2',3'-dideoxy-3'-oxa-5-fluorocytidine in cells and rhesus monkeys

Beta-D-2',3'-dideoxy-3'-oxa-5-fluorocytidine (D-FDOC) is an effective inhibitor of human immunodeficiency virus 1 (HIV-1) and HIV-2, simian immunodeficiency virus, and hepatitis B virus (HBV) in vitro. The purpose of this study was to evaluate the intracellular metabolism of d-FDOC in human hepatoma (HepG2), human T-cell lymphoma (CEM), and primary human peripheral blood mononuclear (PBM) cells by using tritiated compound. By 24 h, the levels of D-FDOC-triphosphate (D-FDOC-TP) were 2.8 +/- 0.4, 6.7 +/- 2.3, and 2.0 +/- 0.1 pmol/10(6) cells in HepG2, CEM, and primary human PBM cells, respectively. Intracellular D-FDOC-TP concentrations remained greater than the 50% inhibitory concentration for HIV-1 reverse transcriptase for up to 24 h after removal of the drug from cell cultures. In addition to d-FDOC-monophosphate (D-FDOC-MP), -diphosphate (D-FDOC-DP), and -TP, D-FDOC-DP-ethanolamine and d-FDOC-DP-choline were detected in all cell extracts as major intracellular metabolites. D-FDOC was not a substrate for Escherichia coli thymidine phosphorylase. No toxicity was observed in mice given D-FDOC intraperitoneally for 6 days up to a dose of 100 mg/kg per day. Pharmacokinetic studies in rhesus monkeys indicated that D-FDOC has a t(1/2) of 2.1 h in plasma and an oral bioavailability of 38%. The nucleoside was excreted unchanged primary in the urine, and no metabolites were detected in plasma or urine. These results suggest that further safety and pharmacological studies are warranted to assess the potential of this nucleoside for the treatment of HIV- and HBV-infected individuals.

Mechanism of anti-human immunodeficiency virus activity of beta-D-6-cyclopropylamino-2',3'-didehydro-2',3'-dideoxyguanosine

To better understand the importance of the oxygen in the ribose ring of planar unsaturated nucleoside analogs that target human immunodeficiency virus (HIV), a 6-cyclopropyl-substituted prodrug of 2',3'-didehydro-2',3'-dideoxyguanosine (cyclo-d4G) was synthesized, and its cellular metabolism, antiviral activity, and pharmacokinetic behavior were studied. Cyclo-d4G had selective anti-HIV activity in primary blood mononuclear cells (PBMCs), effectively inhibiting the LAI strain of HIV-1 by 50% at 1.1 +/- 0.1 microM while showing 50% inhibition of cell viability at 84.5 microM. The antiviral activity in PBMCs was not markedly affected by mutations of methionine to valine at position 184 or by thymidine-associated mutations in the viral reverse transcriptase. Mutations of leucine 74 to valine and of lysine 65 to arginine had mild to moderate resistance (as high as fivefold). Studies to delineate the mechanism of cellular metabolism and activation of cyclo-d4G showed reduced potency in inhibiting viral replication in the presence of the adenosine/adenylate deaminase inhibitor 2'-deoxycoformycin, implying that the antiviral activity is due to its metabolism to the 2'-dGTP analog d4GTP. Intracellular formation of sugar catabolites illustrates the chemical and potentially enzymatic instability of the glycosidic linkage in d4G. Further studies suggest that cyclo-d4G has a novel intracellular phosphorylation pathway. Cyclo-d4G had a lower potential to cause mitochondrial toxicity than 2',3'-dideoxycytidine and 2',3'-didehydro-3'-deoxythymidine in neuronal cells. Also, cyclo-d4G had advantageous synergism with many currently used anti-HIV drugs. Poor oral bioavailability observed in rhesus monkeys may be due to the labile glycosidic bond, and special formulation may be necessary for oral delivery.

Anti-HIV Activity of (−)-(2R,4R)-1- (2-Hydroxymethyl-1,3-dioxolan-4-yl)- thymine against Drug-Resistant HIV-1 Mutants and Studies of Its Molecular Mechanism

(-)-(2R,4R)-1-(2-Hydroxymethyl-1,3-dioxolan-4-yl)thymine (DOT) is the first thymidine kinase-activated nucleoside that is significantly active against all of the clinically significant NRTI-resistant HIV-1 mutants, including AZT (D67N/K70R/T215Y/K219Q), Tenofovir (K65R), and Lamivudine (M184V). To understand the molecular mechanism of drug resistance and the antiviral activity of DOT against drug-resistant RTs, molecular modeling studies of DOT-TP complexed with the wild-type (WT) and mutated RT were conducted. The key reason for this interesting antiviral activity profile is the presence of a dioxolane ring.

Emerging anti-HIV drugs

There are now exactly 20 anti-HIV drugs licenced (approved) for clinical use, and > 30 anti-HIV compounds under (pre)clinical development. The licensed anti-HIV drugs fall into five categories: nucleoside reverse transcriptase inhibitors (NRTIs: zidovudine, didanosine, zalcitabine, stavudine, lamivudine, abacavir and emtricitabine); nucleotide reverse transcriptase inhibitors (NtRTIs: tenofovir disoproxil fumarate); non-nucleoside reverse transcriptase inhibitors (NNRTIs: nevirapine, delavirdine and efavirenz); protease inhibitors (PIs: saquinavir, indinavir, ritonavir, nelfinavir, amprenavir, lopinavir, atazanavir and fosamprenavir); and fusion inhibitors (FIs: enfuvirtide). The compounds that are currently under clinical (Phase I, II or III) or preclinical investigation are either targeted at the same specific viral proteins as the licensed compounds (i.e., reverse transcriptase [NRTIs: PSI-5004, (-)-dOTC, DPC-817, elvucitabine, alovudine, MIV-210, amdoxovir, DOT; NNRTIs: thiocarboxanilide, UC-781, capravirine, dapivirine, etravirine, rilpivirine], protease [PIs: tipranavir, TMC-114]) or other specific viral proteins (i.e., gp120: cyanovirin N; attachment inhibitors: AIs, such as BMS-488043; integrase: L-870,812, PDPV-165; capsid proteins: PA-457, alpha-HCG); or cellular proteins (CD4 downmodulators: CADAs; CXCR4 antagonists: AMD-070, CS-3955; CCR5 antagonists: TAK-220, SCH-D, AK-602, UK-427857). Combination therapy is likely to remain the gold standard for the treatment of AIDS so as to maximise potency, minimise toxicity and diminish the risk for resistance development. Ideally, pill burden should be reduced to once-daily dosing so as to optimise the patient's compliance and reduce the treatment costs.

Synthesis of enantiomerically pure D-FDOC, an anti-HIV agent

The beta-D-enantiomer of FDOC (2',3'-dideoxy-5-fluoro-oxacytidine) exhibits potent anti-HIV-1 activity. It was obtained in optically pure form by employing a tandem kinetic resolution/chiral salt crystallization protocol. In addition, conditions were developed that allowed the unwanted butyrate ester of the L-enantiomer of FDOC to be racemized. This material could then be recycled in future resolutions.

In vitro combination of amdoxovir and the inosine monophosphate dehydrogenase inhibitors mycophenolic acid and ribavirin demonstrates potent activity against wild-type and drug-resistant variants of human immunodeficiency virus type 1

Amdoxovir [(-)-beta-d-2,6-diaminopurine dioxolane (DAPD)] is a nucleoside analogue reverse transcriptase inhibitor of human immunodeficiency virus type 1 (HIV-1) replication. DAPD is deaminated by adenosine deaminase to the guanosine analogue dioxolane guanosine (DXG), which is subsequently phosphorylated to the corresponding 5' triphosphate (DXG-TP). DXG-TP competes with the natural substrate dGTP for binding to the enzyme-nucleic acid complex. Mycophenolic acid (MPA) and ribavirin (RBV), inhibitors of inosine monophosphate dehydrogenase (IMPDH), inhibit the de novo synthesis of guanine nucleotides, including dGTP. Reducing the intracellular levels of dGTP would be expected to augment the antiviral activity of analogues of deoxyguanosine. In this study we examined the effect of MPA and RBV on the anti-HIV activity of DAPD and DXG. When tested against wild-type virus, both MPA and RBV decreased the 50% effective concentration (EC(50)) for DXG by at least 10-fold. In contrast, both MPA and RBV increase the EC(50) value for zidovudine. MPA and RBV completely reversed the resistance to DXG observed with HIV isolates containing mutations which confer partial resistance to DAPD and DXG. Similarly, when tested against a mutant virus fully resistant to inhibition by DAPD (K65R/Q151M), MPA and RBV reduced the EC(50) for DAPD to within twofold of that for the wild type. The combination of MPA or RBV with DAPD or DXG did not result in increased cytotoxicity or reduced levels of mitochondrial DNA when tested at physiologically relevant concentrations. These studies suggest a potential role for the use of IMPDH inhibitors in combination therapy with amdoxovir in the treatment of HIV.

Synthesis and biological evaluation of 5R- and 5S-methyl substituted d- and l-configuration 1,3-dioxolane nucleoside analogs

1,3-Dioxolane and 1,3-oxathiolane nucleoside analogs play an important role in anti-viral and anti-neoplastic chemotherapy. We report here the synthesis of 2-hydroxymethyl-5-methyl-1,3-dioxolanylpurine nucleosides from 4-acetoxy-2-(benzyloxymethyl)-5-methyldioxolane. Dioxolanes of alpha-D-, beta-D-, alpha-L-, and beta-L-configuration were prepared, that included 5-methyl derivatives of both 5R and 5S configuration. Molecular mechanics calculations indicate that the 5S and 5R diastereoisomeric 1,3-dioxolanes possess distinct conformational bias, suggesting that methyl substitution may alter the conformational preference of 1,3-dioxolanes. The ability of the 1,3-dioxolanes to inhibit HCV RNA replication was evaluated in a cell-based, subgenomic replicon assay. In addition, activity against vaccinia and HIV was evaluated in cell-based assays. The 2-hydroxymethyl-5-methyl-1,3-dioxolanes were found to be inactive.

Anabolism of amdoxovir: phosphorylation of dioxolane guanosine and its 5'-phosphates by mammalian phosphotransferases

Amdoxovir [(-)-beta-D-2,6-diaminopurine dioxolane, DAPD], the prodrug of dioxolane guanosine (DXG), is currently in Phase I/II clinical development for the treatment of HIV-1 infection. In this study, we examined the phosphorylation pathway of DXG using 15 purified enzymes from human (8), animal (6), and yeast (1) sources, including deoxyguanosine kinase (dGK), deoxycytidine kinase (dCK), high Km 5'-nucleotidase (5'-NT), guanylate (GMP) kinase, nucleoside monophosphate (NMP) kinase, adenylate (AMP) kinase, nucleoside diphosphate (NDP) kinase, 3-phosphoglycerate (3-PG) kinase, creatine kinase, and pyruvate kinase. In addition, the metabolism of 14C-labeled DXG was studied in CEM cells. DXG was not phosphorylated by human dCK, and was a poor substrate for human dGK with a high Km (7 mM). Human 5'-NT phosphorylated DXG with relatively high efficiency (4.2% of deoxyguanosine). DXG-MP was a substrate for porcine brain GMP kinase with a substrate specificity that was 1% of dGMP. DXG-DP was phosphorylated by all of the enzymes tested, including NDP kinase, 3-PG kinase, creatine kinase, and pyruvate kinase. The BB-isoform of human creatine kinase showed the highest relative substrate specificity (47% of dGDP) for DXG-DP. In CEM cells incubated with 5 microM DXG for 24 h, 0.015 pmole/10(6) cells (approximately 7.5 nM) of DXG-TP was detected as the primary metabolite. Our study demonstrated that 5'-nucleotidase, GMP kinase, creatine kinase, and NDP kinase could be responsible for the activation of DXG in vivo.

Favorable interactions between enfuvirtide and 1-beta-D-2,6-diaminopurine dioxolane in vitro

We evaluated the in vitro anti-human immunodeficiency virus type 1 (HIV-1) interactions between 1-beta-D-2,6-diaminopurine dioxolane (DAPD) and enfuvirtide (T-20) against clinical isolates sensitive and resistant to reverse transcriptase and protease inhibitors. Interactions between T-20 and DAPD were synergistic to nearly additive, with combination index values ranging from 0.53 to 1.06 at 95% inhibitory concentrations. These studies suggest that a combination of T-20 and DAPD might be useful in the treatment of antiretroviral drug-experienced patients.

The role of 2',3'-unsaturation on the antiviral activity of anti-HIV nucleosides against 3TC-resistant mutant (M184V)

Molecular modeling studies show that the 2',3'-double bond of the sugar moiety of various 2',3'-unsaturated nucleosides interacts with the aromatic moiety of Tyr115 of HIV-1 reverse transcriptase (RT) by hydrophobic pi-pi interaction. In 3TC-resistant mutant (M184V) RT, 2'-fluoro-2',3'-unsaturated nucleosides with a bulky 4'-substituent experience significant steric hindrance with the side chain of Val184.

Structural determinants in human DNA polymerase gamma account for mitochondrial toxicity from nucleoside analogs

Although antiviral nucleoside analog therapy successfully delays progression of HIV infection to AIDS, these drugs cause unwelcome side-effects by inducing mitochondrial toxicity. We and others have demonstrated that the mitochondrial polymerase, DNA polymerase gamma (pol gamma), participates in mitochondrial toxicity by incorporating these chain-terminating antiviral nucleotide analogs into DNA. Here, we explore the role of three highly conserved amino acid residues in the active site of human pol gamma that modulate selection of nucleotide analogs as substrates for incorporation. Sequence alignments, crystal structures and mutagenesis studies of family A DNA polymerases led us to change Tyr951 and Tyr955 in polymerase motif B to Phe and Ala, and Glu895 in polymerase motif A was changed to Ala. The mutant polymerases were tested for their ability to incorporate natural nucleotides and the five antiviral nucleoside analogs currently approved for antiviral therapy: AZT, ddC, D4T, 3TC and carbovir. Steady-state kinetic analysis of the pol gamma derivatives with the normal and antiviral nucleotides demonstrated that Tyr951 is largely responsible for the ability of pol gamma to incorporate dideoxynucleotides and D4T-MP. Mutation of Tyr951 to Phe renders the enzyme resistant to dideoxynucleotides and D4T-TP without compromising the activity of the polymerase. Alteration of Glu895 and Tyr955 to Ala had the largest effect on overall polymerase activity with normal nucleotides, producing dramatic increases in K(m(dNTP)) and large decreases in k(cat). Mutation of Tyr955 in pol gamma causes the degenerative disease progressive external ophthalmoplegia in humans, and we show that this residue partially accounts for the ability of pol gamma to incorporate D4T-MP and carbovir. Alteration of Glu895 to Ala slightly increased discrimination against dideoxynucleotides and D4T-TP. The mechanisms by which pol gamma selects certain nucleotide analogs are discussed.

Dioxolane guanosine 5'-triphosphate, an alternative substrate inhibitor of wild-type and mutant HIV-1 reverse transcriptase. Steady state and pre-steady state kinetic analyses

The frequency of human immunodeficiency virus, type 1 (HIV-1) mutations in response to antiviral therapy and resulting drug resistance is of major concern. Amdoxovir ((-)-beta-D-2,6-diaminopurine dioxolane), the prodrug of dioxolane guanosine (DXG), is currently in phase I/II clinical development for the treatment of HIV-1 infection. In vitro, HIV-1 mutants resistant to 3'-azido-3'-deoxythymidine (M41L/D67N/K70R/T215Y/K219Q) and (-)beta-L-2',3'-dideoxy-3'-thiacytidine (3TC) (M184V) remain sensitive to DXG. HIV-1 with the reverse transcriptase mutations K65R, L74V, and/or Q151M were less sensitive to DXG, whereas the mutation K103N re-sensitized the virus to the inhibitory effect of DXG. In order to understand these observations at the enzyme level, we investigated the inhibition of the HIV-1 reverse transcriptase-catalyzed viral DNA synthesis by dioxolane guanosine 5'-triphosphate (DXG-TP), 3'-azido-3'-deoxythymidine-TP, and 3TC-TP by using steady state kinetic analysis and the incorporation of DXG-5'-monophosphate by using pre-steady state kinetic analysis. This mechanistic study provided detailed information on the amdoxovir-related drug resistance at a molecular level. Overall, the enzymatic data correlated well with the antiviral data obtained from cell culture experiments and further supported the use of amdoxovir for the treatment of nucleoside reverse transcriptase inhibitor-experienced patients.

Pleiotropic mechanisms of ribavirin antiviral activities

Renewed interest in the mechanism of action of ribavirin results from its synergistic enhancement of interferon therapy and the need to develop more efficacious agents to treat hepatitis C virus infection. Since the discovery of ribavirin over 30 years ago by scientists at ICN Pharmaceuticals, many mechanisms of action for ribavirin have been proposed. These include inhibition of host inosine monophosphate dehydrogenase by ribavirin monophosphate, inhibition of viral capping enzymes, inhibition of viral RNA synthesis by ribavirin triphosphate, lethal mutagenesis of viral RNA genomes resulting from promiscuous incorporation of ribavirin triphosphate by the viral RNA polymerase, and modulation of the host immune responses. In this article, we will briefly review the evidence for these mechanisms, emphasizing recent findings. In addition, we will discuss strategies for development of nucleoside analogs that may replace ribavirin in the future.

New antiretroviral drugs

Despite the availability of 16 antiretroviral drugs approved for the treatment of HIV infection, current combination regimens present challenges. Newer antiretroviral drugs are needed to improve convenience, reduce toxicity and, of particular importance, to provide antiretroviral activity against viral strains resistant to the currently available antiretroviral agents. Candidate drugs with novel properties are in development in the two currently available drug classes: HIV reverse transcriptase inhibitors (nucleoside analogs, non-nucleoside analogs and nucleotide analogs) and HIV protease inhibitors (PI). Investigational nucleoside analog reverse transcriptase inhibitors (nRTI) include emtricitabine (FTC) and amdoxovir (DAPD), and investigational non-nucleoside reverse transcriptase inhibitors (NNRTI) include DPC 083 and TMC 125. New protease inhibitors under investigation include atazanavir (BMS-232 632), tipranavir, and TMC 114. In addition, newer agents with novel mechanisms of action such as HIV entry inhibitors (that inhibit the three steps of HIV entry: CD4 attachment, chemokine receptor binding and membrane fusion) and HIV integrase inhibitors are under investigation. Investigational entry inhibitors include PRO 542 (a CD4 attachment inhibitor), Schering C (a chemokine receptor inhibitor), enfuvirtide (T-20) and T-1249, inhibitors of membrane fusion. Investigational HIV integrase inhibitors include S-1360. Continued progress in the treatment of HIV disease will result from the development of new antiretroviral drugs.

Activation and deactivation of a broad-spectrum antiviral drug by a single enzyme: adenosine deaminase catalyzes two consecutive deamination reactions

Ribavirin is an approved broad-spectrum antiviral drug. A liver-targeting prodrug of ribavirin, viramidine, is in clinical trial in an attempt to provide a better therapeutic index. The conversion of viramidine to ribavirin, and of ribavirin to an inactive metabolite through adenosine deaminase, is reported. Kinetic analysis indicates that adenosine deaminase is likely involved in activation of viramidine in vivo, and the process is highly pH sensitive. The differential activities of two consecutive deamination reactions are kinetically studied and interpreted based on adenosine deaminase structural information. A comprehensive understanding of the viramidine and ribavirin deamination mechanism should help in designing better nucleoside therapeutics in the future.

Enzymatic assay for measurement of intracellular DXG triphosphate concentrations in peripheral blood mononuclear cells from human immunodeficiency virus type 1-infected patients

DXG ([2R-cis]-2-amino-1,9-dihydro-9-[2-[hydroxymethyl]-1,3-dioxolan-4-yl]-6H-purin-6-one) and its prodrug DAPD ([2R-cis]-4-[2,6-diamino-9H-purin-9-yl]-1,3-dioxolane-2-methanol; amdoxovir) are novel 2',3'-dideoxynucleosides (ddNs) displaying activity against human immunodeficiency virus type 1 (HIV-1). In this paper, we describe the development of an enzymatic assay for determining the intracellular active metabolite of DXG and DAPD, DXG triphosphate (DXGTP), in peripheral blood mononuclear cells (PBMCs) from HIV-infected patients. The assay involves inhibition of HIV reverse transcriptase (RT), which normally incorporates radiolabeled deoxynucleoside triphosphates (dNTPs) into a synthetic template primer. DXGTP (0.6 pmol) inhibited control product formation with or without a preincubation step. Inhibition was greatest when the template primer was most diluted. DAPDTP inhibited control product formation only at very high levels (50 pmol) and when a preincubation procedure was used. However, reduced template primer stability in assays using preincubation steps, coupled with potential interference by DAPDTP, led to the current assay method for DXGTP being performed without preincubation. Standard DXGTP inhibition curves were constructed. The presence of PBMC extracts or endogenous dGTP did not interfere with the DXGTP assay. Intracellular DXGTP and dGTP concentrations were determined in PBMCs from HIV-infected patients receiving oral DAPD (500 mg b.i.d.). Peak concentrations of DXGTP were obtained 8 h after dosing and were measurable through 48 h postdose. Levels of endogenous dGTP were also determined over 48 h. No direct relationship was observed between concentrations of DXGTP and dGTP. Quantification of DXGTP concentrations in PBMCs from patients receiving a clinically relevant dose of DAPD is possible with this enzymatic assay.

New anti-HIV agents and targets

Virtually all the compounds that are currently used or are subject of advanced clinical trials for the treatment of HIV infections, belong to one of the following classes: (i) nucleoside reverse transcriptase inhibitors (NRTIs): i.e., zidovudine, didanosine, zalcitabine, stavudine, lamivudine, abacavir, emtricitabine and nucleotide reverse transcriptase inhibitors (NtRTIs) (i.e., tenofovir disoproxil fumarate); (ii) non-nucleoside reverse transcriptase inhibitors (NNRTIs): i.e., nevirapine, delavirdine, efavirenz, emivirine; and (iii) protease inhibitors (PIs): i.e., saquinavir, ritonavir, indinavir, nelfinavir, amprenavir, and lopinavir. In addition to the reverse transcriptase and protease reaction, various other events in the HIV replicative cycle can be considered as potential targets for chemotherapeutic intervention: (i) viral adsorption, through binding to the viral envelope glycoprotein gp120 (polysulfates, polysulfonates, polycarboxylates, polyoxometalates, polynucleotides, and negatively charged albumins); (ii) viral entry, through blockade of the viral coreceptors CXCR4 (i.e., bicyclam (AMD3100) derivatives) and CCR5 (i.e., TAK-779 derivatives); (iii) virus-cell fusion, through binding to the viral envelope glycoprotein gp41 (T-20, T-1249); (iv) viral assembly and disassembly, through NCp7 zinc finger-targeted agents [2,2'-dithiobisbenzamides (DIBAs), azadicarbonamide (ADA)]; (v) proviral DNA integration, through integrase inhibitors such as 4-aryl-2,4-dioxobutanoic acid derivatives; (vi) viral mRNA transcription, through inhibitors of the transcription (transactivation) process (flavopiridol, fluoroquinolones). Also, various new NRTIs, NNRTIs, and PIs have been developed that possess, respectively: (i) improved metabolic characteristics (i.e., phosphoramidate and cyclosaligenyl pronucleotides by-passing the first phosphorylation step of the NRTIs), (ii) increased activity ["second" or "third" generation NNRTIs ( i.e., TMC-125, DPC-083)] against those HIV strains that are resistant to the "first" generation NNRTIs, or (iii), as in the case of PIs, a different, modified peptidic (i.e., azapeptidic (atazanavir)) or non-peptidic scaffold (i.e., cyclic urea (mozenavir), 4-hydroxy-2-pyrone (tipranavir)). Non-peptidic PIs may be expected to inhibit HIV mutant strains that have become resistant to peptidomimetic PIs.

Molecular mechanisms of resistance to human immunodeficiency virus type 1 with reverse transcriptase mutations K65R and K65R+M184V and their effects on enzyme function and viral replication capacity

Human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) resistance mutations K65R and M184V result in changes in susceptibility to several nucleoside and nucleotide RT inhibitors. K65R-containing viruses showed decreases in susceptibility to tenofovir, didanosine (ddI), abacavir, and (-)-beta-D-dioxolane guanosine (DXG; the active metabolite of amdoxovir) but appeared to be fully susceptible to zidovudine and stavudine in vitro. Viruses containing the K65R and M184V mutations showed further decreases in susceptibility to ddI and abacavir but increased susceptibility to tenofovir compared to the susceptibilities of viruses with the K65R mutation. Enzymatic and viral replication analyses were undertaken to elucidate the mechanisms of altered drug susceptibilities and potential fitness defects for the K65R and K65R+M184V mutants. The relative inhibitory capacities (K(i)/K(m)) of the active metabolites of tenofovir, ddI, and DXG were increased for the RT containing the K65R mutation compared to that for the wild-type RT, but the relative inhibitory capacity of abacavir was only minimally increased. For the mutant viruses with the K65R and M184V mutations, the increase in tenofovir susceptibility compared to that of the mutants with K65R correlated with a decrease in the tenofovir inhibitory capacity that was mediated primarily by an increased K(m) of dATP. The decrease in susceptibility to ddI by mutants with the K65R and M184V mutations correlated with an increase in the inhibitory capacity mediated by an increased K(i). ATP-mediated removal of carbovir as well as small increases in the inhibitory capacity of carbovir appear to contribute to the resistance of mutants with the K65R mutation and the mutants with the K65R and M184V mutations to abacavir. Finally, both the HIV-1 K65R mutant and, more notably, the HIV-1 K65R+M184V double mutant showed reduced replication capacities and reduced RT processivities in vitro, consistent with a potential fitness defect in vivo and the low prevalence of the K65R mutation among isolates from antiretroviral agent-experienced patients.

New developments in anti-HIV chemotherapy

Virtually all the compounds that are currently used, or are subject of advanced clinical trials, for the treatment of human immunodeficiency virus (HIV) infections, belong to one of the following classes: (i) nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs): i.e. zidovudine (AZT), didanosine (ddI), zalcitabine (ddC), stavudine (d4T), lamivudine (3TC), abacavir (ABC), emtricitabine [(-)FTC], tenofovir disoproxil fumarate; (ii) non-nucleoside reverse transcriptase inhibitors (NNRTIs): i.e. nevirapine, delavirdine, efavirenz, emivirine; and (iii) protease inhibitors (PIs): i.e. saquinavir, ritonavir, indinavir, nelfinavir, amprenavir and lopinavir. In addition to the reverse transcriptase (RT) and protease reaction, various other events in the HIV replicative cycle can be considered as potential targets for chemotherapeutic intervention: (i) viral adsorption, through binding to the viral envelope glycoprotein gp120 (polysulfates, polysulfonates, polycarboxylates, polyoxometalates, polynucleotides, and negatively charged albumins); (ii) viral entry, through blockade of the viral coreceptors CXCR4 [bicyclam (AMD3100) derivatives] and CCR5 (TAK-779 derivatives); (iii) virus-cell fusion, through binding to the viral envelope glycoprotein gp41 (T-20, T-1249); (iv) viral assembly and disassembly, through NCp7 zinc finger-targeted agents [2,2'-dithiobisbenzamides (DIBAs), azadicarbonamide (ADA)]; (v) proviral DNA integration, through integrase inhibitors such as 4-aryl-2,4-dioxobutanoic acid derivatives; (vi) viral mRNA transcription, through inhibitors of the transcription (transactivation) process (flavopiridol, fluoroquinolones). Also, various new NRTIs, NNRTIs and PIs have been developed that possess, respectively: (i) improved metabolic characteristics (i.e. phosphoramidate and cyclosaligenyl pronucleotides by-passing the first phosphorylation step of the NRTIs), (ii) increased activity ["second" or "third" generation NNRTIs (i.e. TMC-125, DPC-083)] against those HIV strains that are resistant to the "first" generation NNRTIs, or (iii) as in the case of PIs, a different, nonpeptidic scaffold [i.e. cyclic urea (mozenavir), 4-hydroxy-2-pyrone (tipranavir)]. Nonpeptidic PIs may be expected to inhibit HIV mutant strains that have become resistant to peptidomimetic PIs. Given the multitude of molecular targets with which anti-HIV agents can interact, one should be cautious in extrapolating the mode of action of these agents from cell-free enzymatic assays to intact cells. Two examples in point are L-chicoric acid and the nonapeptoid CGP64222, which were initially described as an integrase inhibitor or Tat antagonist, respectively, but later shown to primarily act as virus adsorption/entry inhibitors, the latter through blockade of CXCR4.

New single-isomer compounds on the horizon

In 1992, the Food and Drug Administration (FDA) issued new guidelines governing stereoisomerism in new-drug development. The guidelines strongly encourage the development of single isomers and discourage stereoisomeric (eg, racemic) mixtures. As a result, most new chiral drugs are being developed as single enantiomers (ie, single isomers). There are three mechanisms for the identification and development of new single-isomer drugs: chiral switches (CS), chiral metashifts (CM), and new single-isomer chemical entities (NSICEs). In a CS, one of the two enantiomers of an established racemate is developed as a new drug, with the expectation that the single-isomer form has advantages over the racemic parent in terms of efficacy and/or adverse effects. Many new CS drugs are in development, eg, (S)-oxybutynin for urinary incontinence and escitalopram for depression. In a CM, a chiral metabolite of a drug is developed, in single-isomer form, as an agent with advantages over the parent. Among the current CM drugs in development are (+)-norcisapride (safer GI prokinetic agent than the racemic parent cisapride) and (S)-desmethylzopiclone (antianxiety agent, metabolite of the sedative-hypnotic zopiclone). Many NSICEs are in development, eg, rosuvastatin as an antihypercholesterolemic, posaconazole as an antifungal, sitafloxacin as a fluoroquinolone antibacterial, pregabalin as an anticonvulsant, abarelix as an antineoplastic, etc. As in the development of any new drug, not every single-isomer candidate will reach the clinic, but there is no doubt that the move to single-isomer agents is an important step forward in the search for better and safer drugs.