Mechanisms of nucleoside analog antiviral activity and resistance during human immunodeficiency virus reverse transcription

Antiretroviral Therapy during Long-term Surgical Care: 'Exploring Difficult Cases in HIV Clinics' of the Korean Society for AIDS Conference in 2023

With advancements in antiretroviral therapy (ART), the average lifespan of people with human immunodeficiency virus (HIV) is increasing, as is the number of older adults with HIV. Accordingly, the number of patients with HIV who undergo surgery or require critical care for various reasons is increasing. Since the prognosis of people with HIV depends on the continuous and effective maintenance of ART, there is a need to consider effectively maintaining ART in people with HIV in these conditions. This case involved a 55-year-old patient with well-controlled HIV who received ART and presented to the emergency room with acute abdominal pain. He was diagnosed with extensive bowel infarction and panperitonitis and received critical care in the intensive care unit, including mechanical ventilation and continuous renal replacement therapy. The patient was administered enteral nutrition via a nasogastric tube. The patient subsequently underwent extensive small bowel resection and developed short bowel syndrome. The patient maintained ART during that period. A literature review related to the use of ART under these conditions is included in this study. This case was discussed at the [Exploring Difficult Cases in HIV Clinics] of the Korean Society for AIDS Conference held in 2023.

Lipophilic Nucleoside Triphosphate Prodrugs of Anti-HIV Active Nucleoside Analogs as Potential Antiviral Compounds

Nucleoside analogs require three phosphorylation steps catalyzed by cellular kinases to give their triphosphorylated metabolites. Herein, the synthesis of two types of triphosphate prodrugs of different nucleoside analogs is disclosed. Triphosphates comprising: i) a γ-phosphate or γ-phosphonate bearing a bioreversible acyloxybenzyl group and a long alkyl group and ii) γ-dialkyl phosphate/phosphonate modified nucleoside triphosphate analogs. Almost selective conversion of the former TriPPPro-compounds into the corresponding γ-alkylated nucleoside triphosphate derivatives is demonstrated in CEM/0 cell extracts that proved to be stable toward further hydrolysis. The latter γ-dialkylated triphosphate derivatives lead to the slow formation of the corresponding NDPs. Both types of TriPPPro-compounds are highly potent in wild-type CEM/0 cells and more importantly, they exhibit even better activities against HIV-2 replication in CEM/TK- cell cultures. A finding of major importance is that, in primer extension assays, γ-phosphate-modified-NTPs, γ-mono-alkylated-triphosphates, and NDPs prove to be substrates for HIV-RT but not for cellular DNA-polymerases α,γ.

Nucleoside Analogs and Nucleoside Precursors as Drugs in the Fight against SARS-CoV-2 and Other Coronaviruses

Coronaviruses (CoVs) are positive-sense RNA enveloped viruses, members of the family Coronaviridae, that cause infections in a broad range of mammals including humans. Several CoV species lead to mild upper respiratory infections typically associated with common colds. However, three human CoV (HCoV) species: Severe Acute Respiratory Syndrome (SARS)-CoV-1, Middle East Respiratory Syndrome (MERS)-CoV, and SARS-CoV-2, are responsible for severe respiratory diseases at the origin of two recent epidemics (SARS and MERS), and of the current COronaVIrus Disease 19 (COVID-19), respectively. The easily transmissible SARS-CoV-2, emerging at the end of 2019 in China, spread rapidly worldwide, leading the World Health Organization (WHO) to declare COVID-19 a pandemic. While the world waits for mass vaccination, there is an urgent need for effective drugs as short-term weapons to combat the SARS-CoV-2 infection. In this context, the drug repurposing approach is a strategy able to guarantee positive results rapidly. In this regard, it is well known that several nucleoside-mimicking analogs and nucleoside precursors may inhibit the growth of viruses providing effective therapies for several viral diseases, including HCoV infections. Therefore, this review will focus on synthetic nucleosides and nucleoside precursors active against different HCoV species, paying great attention to SARS-CoV-2. This work covers progress made in anti-CoV therapy with nucleoside derivatives and provides insight into their main mechanisms of action.

T-705 induces lethal mutagenesis in Ebola and Marburg populations in macaques

Nucleoside analogues (NA) disrupt RNA viral RNA-dependent RNA polymerase (RdRP) function and fidelity for multiple viral families. The mechanism of action (MOA) of T-705 has been attributed alternatively or concurrently to chain termination and lethal mutagenesis depending on the viral species during in vitro studies. In this study, we evaluated the effect of T-705 on the viral population in non-human primates (NHPs) after challenge with Ebola virus (EBOV) or Marburg virus (MARV) to identify the predominant in vivo MOA. We used common virological assays in conjunction with deep sequencing to characterize T-705 effects. T-705 exhibited antiviral activity that was associated with a reduction in specific infectivity and an accumulation of low frequency nucleotide variants in plasma samples collected day 7 post infection. Stranded analysis of deep sequencing data to identify chain termination demonstrated no change in the transcriptional gradient in negative stranded viral reads and minimal changes in positive stranded viral reads in T-705 treated animals, questioning as a MOA in vivo. These findings indicate that lethal mutagenesis is a MOA of T-705 that may serve as an indication of therapeutic activity of NAs for evaluation in clinical settings. This study expands our understanding of MOAs of these compounds for the Filovirus family and provides further evidence that lethal mutagenesis could be a preponderant MOA for this class of therapeutic compounds.

Structural and functional roles of dynamically correlated residues in thymidylate kinase

Thymidylate kinase is an important enzyme in DNA synthesis. It catalyzes the conversion of thymidine monophosphate to thymidine diphosphate, with ATP as the preferred phosphoryl donor, in the presence of Mg²⁺. In this study, the dynamics of the active site and the communication paths between the substrates, ATP and TMP, are reported for thymidylate kinase from Thermus thermophilus. Conformational changes upon ligand binding and the path for communication between the substrates and the protein are important in understanding the catalytic mechanism of the enzyme. High-resolution X-ray crystal structures of thymidylate kinase in apo and ligand-bound states were solved. This is the first report of structures of binary and ternary complexes of thymidylate kinase with its natural substrates ATP and ATP-TMP, respectively. Distinct conformations of the active-site residues, the P-loop and the LID region observed in the apo and ligand-bound structures revealed that their concerted motion is required for the binding and proper positioning of the substrate TMP. Structural analyses provide an insight into the mode of substrate binding at the active site. The residues involved in communication between the substrates were identified through network analysis using molecular-dynamics simulations. The residues identified showed high sequence conservation across species. Biochemical analyses show that mutations of these residues either resulted in a loss of activity or affected the thermal stability of the protein. Further, molecular-dynamics analyses of mutants suggest that the proper positioning of TMP is important for catalysis. These data also provide an insight into the phosphoryl-transfer mechanism.

Inhibitory Effect of Penciclovir-Triphosphate on Duck Hepatitis B Virus Reverse Transcription

The aim of this study was to investigate the mechanism of inhibition of hepatitis B virus replication by penciclovir-triphosphate, the active metabolite of famciclovir. A recently developed in vitro translation assay for the expression of an enzymatically active duck hepatitis B virus (DHBV) reverse transcriptase was used to assess the inhibitory activity of penciclovir-triphosphate (PCV-TP) in comparison with other guanosine analogue triphosphates. Acyclovir-triphosphate (ACV-TP), the chiral triphosphates of penciclovir (PCV), ( R)-PCV-TP and ( S)-PCV-TP, and carbocyclic 2′-deoxyguanosine-TP (CDG-TP) did inhibit reproducibly minus strand DNA synthesis to different extents. CDG-TP was the most potent inhibitor of dGTP incorporation. The inhibitory effect of these compounds against the incorporation of the first nucleotide of minus strand DNA, dGMP, was similar to that observed with DNA chain elongation. 2′,3′-dideoxyguanosine-TP (ddG-TP), ACV-TP and both ( R) and ( S)-PCV-TP inhibited the incorporation of the next nucleotides in the short DNA primer, whereas CDG-TP did not. These results demonstrate that PCV-TP inhibits hepadnavirus reverse transcription by inhibiting the synthesis of the short DNA primer. The data obtained with the inhibition of the enzymatic activity of the DHBV polymerase provides a new insight into the mechanism of action of penciclovir-triphosphate on HBV replication.

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

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

Isothiazole Derivatives as Novel HIV Replication Inhibitors

A series of 3,4,5-trisubstituted isothiazoles has been screened against HIV-1 (IIIB) and HIV-2 (ROD) at sub-toxic concentrations in acutely infected MT-4 cells. Among the tested compounds, only 3-mercapto-5-phenyl-4-isothiazolecarbonitrile was found to inhibit the replication of HIV-1 (IIIB) and HIV-2 (ROD) at 50% effective concentrations (EC50) of 7.8 and 9.7 μg/ml, respectively. The presence of a thioalkyl chain or dialkylamino function in the 3-position caused a loss of anti-HIV activity. New 4-cyano-5-phenylisothiazoles with other substituents in the 3-position have also been synthesized and studied as potential anti-HIV agents. Our results have demonstrated that 5-phenyl-3-(4-cyano-5-phenylisothiazol-3-yl) disulphanyl-4-isothiazolecarbonitrile and S-(4-cyano-5-phenylisothiazol-3-yl)- O-ethyl thiocarbonate are effective against both HIV-1 (IIIB) (EC50=13.6 and 15.2 μg/ml, respectively) and HIV-2 (ROD) (EC50=17.4 and 13.4 μg/ml, respectively).

Novel acyclic phosphonylated 1,2,3-triazolonucleosides with an acetamidomethyl linker: synthesis and biological activity

A new series of 4-substituted [(1,2,3-triazol-1-yl)acetamido]methylphosphonates as acyclic nucleotide analogs were synthesized from diethyl (2-chloroacetamido)methylphosphonate via azidation followed by 1,3-dipolar cycloaddition with selected alkynes derived from natural nucleobases or their mimetics. All compounds were tested for their antiviral activities against DNA and RNA viruses as well as for cytostatic activity or cytotoxicity. Among all tested compounds, [(1,2,3-triazol-1-yl)acetamido]methylphosphonate 6e substituted with the N(3)-Bz-benzuracil moiety showed activity against the vesicular stomatitis virus (EC50 = 45 µM) in HeLa cell cultures.

A new DNA building block, 4'-selenothymidine: synthesis and modification to 4'-seleno-AZT as a potential anti-HIV agent

The first synthesis of 4'-selenothymidine (1), a novel DNA building block, and 4'-seleno-AZT (2) was accomplished from 2-deoxy-d-ribose via stereoselective formation of 2-deoxy-4-seleno-d-furanose 17 and a Pummerer-type base condensation as key steps. 4'-Selenothymidine (1) was discovered to adopt the same 2'-endo/3'-exo conformation as thymidine, which is unusual in that 4'-selenouridine has the opposite conformation to that of uridine.

Synthesis and anti-HIV activity of [ddN]-[ddN] dimers and benzimidazole nucleoside dimers

In an attempt to combine the HIV-inhibitory capacity of different 2',3'-dideoxynucleoside (ddN) analogs, we have designed and synthesized several dimers of [AZT]-[AZT] and [AZT]-[d4T]. In addition, we also synthesized the dimers of 1-(1H-benzimidazol-1-yl)-1-deoxy-beta-D-ribofuranose. The in vitro anti-HIV activity of these compounds on a pseudotype virus, pNL4-3.Luc.R-E-, in the 293T cells has been determined. Among these compounds, 2,2'-(propane-1,3-diyl)bis[1-(beta-D-ribofuranosyl)-1H-benzimidazole] showed the highest anti-HIV activity with similar effect as AZT.

Potent cross-reactive immune response against the wild-type and drug-resistant forms of HIV reverse transcriptase after the chimeric gene immunization

HIV reverse transcriptase (RT) can be considered as a target and an instrument of immunotherapy aimed at limiting the emergence and spread of drug-resistant HIV. The chimeric genes coding for the wild-type and multi-drug-resistant RT (RT1.14) fused to lysosome-associated membrane protein 1 (LAMP-1) were injected intramuscularly into BALB/c mice. The immune response was assessed by ELISpot, cytokine ELISA intracellular IFN-gamma staining, and antibody ELISA. The genes for RT- and RT1.14-LAMP fusions (RT-LAMP and RT1.14-LAMP) were immunogenic generating a mixed Th1/Th2-profile of immune response, while the wild-type RT gene induced only weak immune response. Specific secretion of Th1-cytokines increased with increasing level of RT modification: RT Collapse

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MESH Headings

• AIDS Vaccines/genetics
• AIDS Vaccines/immunology
• Animals
• Antibody Formation
• Cross Reactions
• Cytokines/immunology
• Drug Resistance, Viral/immunology
• HIV Antibodies/blood
• HIV Antibodies/immunology
• HIV Infections/immunology
• HIV Infections/prevention & control
• HIV Reverse Transcriptase/genetics
• HIV Reverse Transcriptase/immunology
• HIV-1/enzymology
• HIV-1/immunology
• HeLa Cells
• Humans
• Immunity, Cellular
• Immunoglobulin A/blood
• Immunoglobulin A/immunology
• Immunoglobulin G/blood
• Immunoglobulin G/immunology
• Lymphocyte Activation
• Lysosomal Membrane Proteins/immunology
• Mice
• Mice, Inbred BALB C
• Th1 Cells/immunology
• Th2 Cells/immunology
• Vaccines, DNA/genetics
• Vaccines, DNA/immunology

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Grants Collapse

Affiliation(s)

Elizaveta Starodubova Swedish Institute for Infectious Disease Control, Nobelsvägen 18, 17182 Stockholm, Sweden.
Andreas Boberg
Alexander Ivanov
Oleg Latyshev
Natalia Petrakova
Yulia Kuzmenko
Marina Litvina
Alexander Chernousov
Sergey Kochetkov
Vadim Karpov
Britta Wahren
Maria G Isaguliants

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Tetra-chloro-(bis-(3,5-dimethylpyrazolyl)methane)gold(III) chloride: An HIV-1 reverse transcriptase and protease inhibitor

The title compound ([3,5-Me(2)bpzaH(2)][AuCl(4)]Cl, 1) (Me(2)bpza=bis(3,5-dimethylpyrazolyl)acetic acid), was prepared by reacting H[AuCl(4)] with 3,5-Me(2)bpza; and spectroscopically and structurally characterized. In the solid state structure of 1, the pyrazolyl ligand is doubly protonated to form two strong charge assisted hydrogen bonds of the type N(+)Hcdots, three dots, centeredCl(-) with the single chloride anion whilst the [AuCl(4)](-) anion remains discrete. The anti-HIV-1 activity of 1 was determined by a colorimetric direct enzyme reverse transcriptase (RT) assay and a fluorogenic protease (PR) assay. Compound 1 significantly (p<0.05) inhibited RT over a concentration range of 5-250muM and inhibited HIV-1 protease at 100muM. Compound 1 inhibited two very important HIV-1 enzymes (RT and PR) in direct enzyme assays and therefore warrants further evaluation.

Vaccination against drug resistance in HIV infection

HIV-1 resistance to currently employed antiretroviral drugs and drug-associated adverse reactions and toxicity point to a need for additional measures to control HIV-1 replication in HIV-infected patients. The immune system of HIV-infected individuals mount an immune response against the regions harboring drug-resistance mutations, sometimes stronger than that against the parental wild-type sequences. A potent cross-reactive immune response against drug-resistant pol proteins can suppress the replication of drug-escaping HIV. This suggests the possibility for a vaccination against existing and anticipated drug-resistant HIV variants. If successful, therapeutic vaccines against drug resistance would ease the therapeutic modalities and limit the spread of drug-resistant HIV. A better understanding of the complex interactions between patterns of drug-resistance mutations, immune responses against these mutations and their antigen presentation by particular human lymphocyte antigen alleles could help to tailor these vaccines after new drugs/new mutations. In this review, we describe the developments in the field of immunization against mutations conferring drug resistance and evaluate their prospects for human vaccination.

Solid-supported diphosphitylating and triphosphitylating reagents for nucleoside modification

This unit describes procedures for synthesis of diphosphitylating and triphosphitylating reagents. The synthesized reagents are first immobilized on appropriate polymer-bound linkers. Rigid and sterically hindered polymer-bound diphosphitylating and triphosphitylating reagents are then reacted selectively with the 5'-hydroxyl group of nucleosides in the presence of excess nucleosides. Typical oxidation with tert-butyl hydroperoxide, deprotection, and final cleavage of the products from the resins using a trifluoroacetic acid cocktail afford various nucleoside 5'-O-diphosphate and nucleoside 5'-O-triphosphate analogs. The use of the diphosphitylating and polymer-bound diphosphitylating reagents in preparation of oligodeoxynucleotides containing diphosphodiester internucleotide bridges is also described. This solid-phase strategy allows for the synthesis of the phosphorylated compounds without the need for nucleoside phosphate precursors, protected nucleosides, or purification of intermediates.

Synthesis of AZT analogues: 7-(3-azido-2-hydroxypropyl)-, 7-(3-amino-2-hydroxypropyl)-,7-(3-triazolyl-2-hydroxypropyl)theophylline

Nucleophilic displacement of the tosyloxy group in 7-(2-hydroxy-3-p-toluenesulfonyloxypropyl)theophylline (1) with azide anion afforded 7-(3-azido-2-hydroxypropyl)theophylline (2). Reduction of the 3-azido group in 2 with Ph3P/Py/NH4OH afforded the 3-amino derivative 4, alternatively obtained by regioselective amination of 7-(2,3-epoxypropyl)theophylline (3). Selective acetylation of 4 gave the N-acetyl derivative 5. 1,3-Dipolar cycloaddition of the azide group in 2 with N1-propargyl thymine (6) afforded the regioisomeric triazole 7.

Virus fitness: concept, quantification, and application to HIV population dynamics

Viral fitness has been broadly studied during the past three decades, mainly to test evolutionary models and population theories difficult to analyze and interpret with more complex organisms. More recent studies, however, are focused in the role of fitness on viral transmission, pathogenesis, and drug resistance. Here, we used human immunodeficiency virus (HIV) as one of the most relevant models to evaluate the importance of viral quasispecies and fitness in HIV evolution, population dynamics, disease progression, and potential clinical implications.

Monitoring processed, mature Human Immunodeficiency Virus type 1 particles immediately following treatment with a protease inhibitor-containing treatment regimen

Protease inhibitors (PIs) block HIV-1 maturation into an infectious virus particle by inhibiting the protease processing of gag and gag-pol precursor proteins. We have used a simple anti-HIV-1 p24 Western blot to monitor the processing of p55gag precursor into the mature p24 capsid immediately following the first dosage of a PI-containing treatment regimen. Evidence of PI activity was observed in plasma virus as early as 72 hours post treatment-initiation and was predictive of plasma viral RNA decrease at 4 weeks.

Emerging treatments in chronic hepatitis B

Although the management of chronic hepatitis B has improved over the last decade, none of the available therapeutic agents, IFN-alpha, lamivudine and adefovir dipivoxil, can achieve sustained off-therapy responses in most cases. Therefore, several newer, mainly antiviral and immunomodulatory agents, are being evaluated. Pegylated IFN-alpha(2a) has been shown to be more effective than lamivudine or standard IFN-alpha monotherapy in achieving post-therapy biochemical and virological responses, and is expected to be licensed soon for the treatment of chronic hepatitis B. Newer antiviral agents, such as entecavir and telbivudine, appear to be quite effective initially, but their sustained off-therapy response rates remain unknown. The preliminary data of monotherapies with immunomodulatory agents, or of combination therapies, have been rather disappointing. Long-term maintenance treatment with antiviral agent(s) with good safety and tolerability profiles and low resistance rates appears to be the most realistic future therapeutic option for most chronic hepatitis B patients.

Stereoselective synthesis of 9-β-d-arabianofuranosyl guanine and 2-amino-9-(β-d-arabianofuranosyl)purine

9-beta-d-Arabianofuranosyl guanine (6) and 2-amino-9-(beta-d-arabianofuranosyl)purine (8) were prepared from 2-amino-6-chloro-9-(2,3,5-triphenylmethoxyl-beta-d-arabianofuranosyl)purine (4), a key intermediate which was stereoselectively prepared from 2,3,5-triphenylmethoxyl-d-arabianofuranose and 2-amino-6-chloro-purine. The yield of the intermediate was obviously improved and only beta-isomer was formed by using the activated molecular sieve as environmental friendly catalyst, overcoming the defect that a 1:1 mixture of alpha- and beta-isomers was formed, which was difficult to separate, when toxic mercury cyanide was previously used as catalyst.

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 evaluation of a novel synthetic phosphocholine lipid-AZT conjugate that double-targets wild-type and drug resistant variants of HIV

INK-20, a synthetic phosphocholine lipid-AZT conjugate, was evaluated for antiviral activity against wild-type HIV-1, a matched pair of pre-AZT and post-AZT and multidrug resistant clinical isolates. In addition, it was tested for activity against viruses resistant to nucleoside (AZT, 3TC) and nonnucleoside (nevirapine) reverse transcriptase and protease (saquinavir) inhibitors using the syncytial plaque reduction assay for infectious virus multiplication. The EC50 values were 0.004, and 0.005 microM against wild-type HIV-1 for INK-20 and AZT, respectively. INK-20 showed little or no cytotoxicity when assayed in CEM-SS cells and four other cell types including PBMC. This resulted in a selective index of > 25,000 and > 20,000 for INK-20 and AZT, respectively. When tested against a matched pair of pre-AZT and post-AZT clinical isolates, the EC50 values were 0.01 and 0.03 microM for INK-20 and 0.0005 and 0.33 microM for AZT, respectively. INK-20 had moderate to good activity against two other AZT resistant variants and very good activity against a multi-drug resistant clinical isolate compared to marked resistance of these viruses to AZT alone. INK-20 retained significant activity against viruses resistant to 3TC, nevirapine, and saquinavir. The synthetic phosphocholine lipid-AZT conjugate INK-20 represents a novel class of anti-HIV compounds, which may provide new strategies for the treatment of HIV drug-resistant variants.

Purifying Selection Masks the Mutational Flexibility of HIV-1 Reverse Transcriptase

DNA and RNA polymerases share a core architecture composed of three structurally conserved motifs: A, B, and C. Although the amino acid sequences of these motifs are highly conserved between closely related organisms, variation across broader evolutionary distances suggests that only a few residues in each motif are indispensable for polymerase function. To test this, we constructed libraries of human immunodeficiency virus type-1 (HIV-1) containing random single amino acid replacements in motif B of reverse transcriptase (RT), and we used selection in culture to assess RT function. Despite the nearly absolute constancy of motif B in vivo, virus replicating in culture tolerated a range of conservative and nonconservative substitutions at 10 of the 11 amino acid positions examined. These included residues that are invariant across all retroviral subfamilies and highly conversed in diverse retroelements. Several mutants retained wild type infectivity, and serial passage experiments revealed replacements that were neutral or even beneficial to viral fitness. In addition, a number of the selected variants exhibited altered susceptibility to the nucleoside analog inhibitors AZT and 3TC. Taken together, these data indicate that HIV-1 tolerates a range of substitutions at conserved RT residues and that selection against slightly deleterious mutations (purifying selection) in vivo masks a large repertoire of viable phenotypic variants. This mutational flexibility likely contributes to HIV-1 evolution in response to changing selection pressures in infected individuals.

Characterization of a subtype D human immunodeficiency virus type 1 isolate that was obtained from an untreated individual and that is highly resistant to nonnucleoside reverse transcriptase inhibitors

Human immunodeficiency virus type 1 (HIV-1) isolates derived from HIV-infected, treatment-naive Ugandan infants were propagated and tested for sensitivity to antiretroviral (ARV) drugs. Although most subtype A and D isolates displayed inhibition profiles similar to those of subtype B strains, a subtype D isolate identified as D14-UG displayed high-level resistance to nevirapine in peripheral blood mononuclear cell cultures (>2,000-fold) and in MT4 cell cultures ( approximately 800-fold) but weaker resistance to delavirdine ( approximately 13-fold) and efavirenz ( approximately 8-fold) in MT4 cell cultures. To investigate the possible mechanism for this resistance to nonnucleoside reverse transcriptase (RT) inhibitors (NNRTIs), the RT coding region in pol was sequenced and compared to the consensus RT sequence of NNRTI-resistant and NNRTI-sensitive subtype A, B, and D HIV-1 isolates. D14-UG did not contain the classic amino acid substitutions conferring NNRTI resistance (e.g., Y181C, K103N, and G190A) but did have some putative sites associated with drug resistance, I135L, T139V, and V245T. Wild-type and mutated protease-RT genes from D14-UG and an NNRTI-sensitive subtype D isolate from Uganda (D13-UG) were cloned into pNL4-3 to produce recombinant viruses and to determine the effects of the mutations on susceptibility to ARV drugs, specifically, NNRTIs. The results showed that I135L and/or V245T mutations can confer high-level resistance to nevirapine and delavirdine as well as low level cross-resistance to efavirenz. Finally, ex vivo fitness analyses suggested that NNRTI-resistant sites 135L and 245T in wild-type isolate D14-UG may reduce RT fitness but do not have an impact on the fitness of the primary HIV-1 isolate.

Reverse transcriptase-based DNA vaccines against drug-resistant HIV-1 tested in a mouse model

Drug resistance is becoming a problem in the treatment of the human immunodeficiency virus type one (HIV-1). To obtain therapeutic DNA vaccines that would target multiple drug-resistance (DR) mutations, we cloned genes for DR HIV-1 reverse transcriptase (RT) and codon-optimized synthetic genes encoding clusters of human CTL epitopes located at the sites of DR-mutations (RT minigenes) and antibody and CTL-epitope tags. Expression of RT genes/minigenes in eukaryotic cells was confirmed by Western blotting and immunofluoresence staining with RT- or tag-specific antibodies. Immunization of mice with DR-RT gene induced no RT-specific antibodies. Immunization of HLA-A(*)0201-transgenic mice with RT minigenes induced RT-specific cellular responses detected by interferon-gamma secretion. This documents first steps in creating therapeutic vaccine against drug-resistant HIV strains.

HIV resistance to nevirapine and other non-nucleoside reverse transcriptase inhibitors

Nevirapine and other members of the non-nucleoside reverse transcriptase inhibitor (NNRTI) family of anti-HIV-1 drugs are essential components of antiretroviral treatment regimens. Unfortunately, drug resistance has become an important issue with respect to all therapeutic targets in HIV-1. This paper summarizes current knowledge about the mutations in the reverse transcriptase gene of HIV-1 that are responsible for drug resistance and the mechanisms whereby drug resistance develops.

Once-Daily Administration of Antiretrovirals

Adherence to therapy is of critical importance for the long-term success of the treatment of HIV infection. Once-daily administration of antiretroviral agents is appealing, as it may increase patient's adherence. The pharmaceutical industry is making huge efforts to develop drugs or combinations of drugs with pharmacokinetic properties allowing once-daily administration. The major pharmacokinetic requirement for once-daily administration is that the intracellular concentration of the antiretroviral or its active metabolite remains above the minimal concentration that can inhibit viral replication during the entire 24-hour period. Soon, all three major classes of antiretroviral agents will be available as once-daily formulations. However, only a few clinical trials have yet assessed the efficacy and safety of truly once-daily antiretroviral combinations. Preliminary results from these small pilot studies suggest that once-daily administration of antiretrovirals is a feasible approach. Large comparative trials are needed before the real benefits of such a strategy can be fully assessed.

Time-dependent changes in HIV nucleoside analogue phosphorylation and the effect of hydroxyurea

BACKGROUND

Nucleoside analogues are activated to their triphosphates, which compete with endogenous deoxynucleoside triphosphate (dNTP) pools to inhibit HIV reverse transcriptase. Hydroxyurea has been administered with nucleoside analogues to modulate intracellular dNTP pools and thus the ratio of drug triphosphate:endogenous triphosphate.

OBJECTIVES

To examine changes in drug activation over time and investigate the effects of hydroxyurea on intracellular phosphorylation of antiretroviral nucleoside analogues.

PATIENTS

A total of 229 HIV-infected individuals receiving abacavir, lamivudine and zidovudine were randomly assigned to receive or not nevirapine and hydroxyurea. Twenty-four patients were recruited to an observational substudy measuring intracellular drug triphosphate and dNTP concentrations at 0, 2, 6, 12, 24 and 48 weeks.

METHODS

Drugs were extracted from isolated peripheral blood mononuclear cells before analysis of endogenous dNTP and drug triphosphates by primer extension assays.

RESULTS

Twenty-two out of 24 patients were followed to completion of the substudy. Hydroxyurea had no demonstrable effect on endogenous dNTP or drug triphosphate levels at any timepoint. However, the ratio of zidovudine triphosphate to endogenous deoxythymidine triphosphate was significantly increased with hydroxyurea. A significant decrease in lamivudine triphosphate (3TCTP) and the 3TCTP:endogenous deoxycytidine triphosphate ratio was seen over 48 weeks. In five patients who failed therapy in the first 24 weeks, significantly reduced 3TCTP was seen.

CONCLUSION

Hydroxyurea does not affect measurable pools of endogenous nucleosides in vivo. Decreased lamivudine phosphorylation over time may provide a novel pharmacological explanation for the mechanism of resistance to this drug.

Targeting HIV: antiretroviral therapy and development of drug resistance

Inhibitors of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) and protease (PR) are widely used in the clinical treatment of AIDS. However, the emergence of drug-resistant variants of HIV-1 severely limits the long-term effectiveness of these drugs. In this review, the molecular basis of resistance to RT and PR inhibitors will be addressed, focusing on the mutations that confer resistance to nucleoside and non-nucleoside drugs. The emergence of multidrug-resistant viruses results from the introduction of potent antiretroviral therapy and involves the combined effects of different drug-resistance mutations. The optimization of current antiretroviral drug regimens and the development of new drugs are challenging issues in HIV chemotherapy.

Polymer-supported reagents for methylphosphorylation and phosphorylation of carbohydrates

Two polymer-supported reagents for methylphosphorylation and phosphorylation of carbohydrates have been developed. p-Hydroxybenzyl alcohol and beta-mercaptoethanol were immobilized on cross-linked divinylbenzene-polystyrene copolymer and conjugated with methyl N,N-diisopropylchlorophosphoramidite. Carbohydrates were reacted with polymer-bound phosphitylating reagents. Further oxidation, with or without the methoxy group deprotection, and cleavage yielded methylphosphorylated or phosphorylated carbohydrates, respectively.

Nucleoside analogues for chronic hepatitis B: antiviral efficacy and viral resistance

Nucleoside analogues have been recently introduced in the management of chronic hepatitis B virus (HBV) infection. They mainly act by inhibition of HBV polymerase activity resulting in decrease of viral replication. They are administered orally, and most of them have an excellent tolerance and safety profile. Lamivudine is the only nucleoside analogue licensed for chronic hepatitis B. It has potent activity against HBV, and a 12-month course achieves clearance of hepatitis B e antigen (HBeAg) in 20-30% of HBeAg-positive patients and both biochemical and virological remission in more than 65-70% of HBeAg-negative chronic hepatitis B patients. Famciclovir and ganciclovir are less effective, whereas other nucleoside or nucleotide analogues, such as adefovir, entecavir, and emtricitabine, are currently under evaluation. Prolonged effective antiviral therapy is required for eradication of chronic HBV infection, but long-term treatment with nucleoside analogues has been found to be associated with progressively increasing rates of viral resistance because of emergence of resistant HBV mutant strains. Virological breakthroughs usually develop after the first 6 months of lamivudine monotherapy, and their rate ranges between 15% and 30% at 12 months and exceeds 50% after 3 yr of therapy. Resistant HBV mutant strains harbor point mutations in the HBV polymerase gene and predominantly in the well-conserved YMDD motif. Although resistant HBV strains may have impaired replication capacity compared with the wild HBV, their clinical significance has not been completely clarified yet. No significant biochemical or clinical event may develop in some cases, whereas severe biochemical breakthroughs with or without deterioration of liver function may develop in others. To date, there is no proven effective therapy for the resistant HBV mutant strains, although adefovir and entecavir seem to be interesting candidates.

Molecular mechanism of DApd/DXG against zidovudine- and lamivudine- drug resistant mutants: a molecular modelling approach

In order to understand molecular mechanism of antiviral drug resistance of HIV-1 reverse transcriptase (RT) as well as potent antiviral activity of 2,6-diaminopurine dioxolane (DAPD) [prodrug of (-)-beta-D-dioxolane guanine (DXG)] against drug-resistant RTs, molecular modelling studies of three structurally distinct nucleoside RT inhibitor (NRTI)-triphosphates (TP) [zidovudine (AZT)-TP, lamivudine (3TC)-TP and DXG-TP] complexed with the wild-type (WT) and mutated RT were conducted. The computational analyses indicated that the antiviral activity and the calculated relative binding energy of the RT inhibitor triphosphates can be correlated, and the minimized structures gave information on the molecular mechanism of drug resistance conferred by mutations. The interactions between the NRTI-TP and adjacent amino acid residues (Lys65, Lys70, Arg72, Tyr115 and/or Gln151) played important roles in stabilizing the enzyme-inhibitor complex. Particularly, Arg72 was found to stabilize the dioxolane and oxathiolane sugar moiety through hydrogen bonding, which was responsible for favourable binding affinity of DXG-TP to AZT- as well as 3TC-resistant mutants. The conformational changes in these amino acid residues caused by mutation always affected the changes in the tertiary structures of enzyme-inhibitor complexes through either closing or opening the gap between the fingers and palm domains. The enzyme-inhibitor complexes with good binding affinity showed tight binding modes by closing the gap between the two domains, whereas weak inhibitors gave open and loose complexes.

Neuroimmune and neurovirological aspects of human immunodeficiency virus infection

Like most lentiviruses, HIV-1 causes both immune suppression and neurological disease. Neurological disease may occur at any stage of HIV infection but is most apparent with severe immune suppression. Cognitive impairment, reflected strikingly by HIV-associated dementia, has attracted intense interest since the outset of the HIV epidemic, and understanding of its pathogenesis has been spurred on by the emergence of several hypotheses outlining potential pathogenic mechanisms. The release of inflammatory molecules by HIV-infected microglia and macrophages and the concurrent neuronal damage play central roles in the conceptualization of HIV neuropathogenesis. Many inflammatory molecules appear to contribute to the pathogenic cascade and their individual roles remain undefined. At the same time, the abundance of virus in the brain and the type or strain of virus found in the brain may also be important codeterminants of neurological disease, as shown for other neurotropic viruses. Coreceptor use by HIV found in the brain appears to closely mirror what has been reported in systemic macrophages. The impact of HAART on viral genotype and phenotype found in the brain, and its relationship to clinical disease, remain uncertain. Several interesting animal models have been developed, using other lentiviruses, transgenic animals, and HIV-infected SCID mice, that may prove useful in future pathogenesis and therapeutic studies. Despite the progress in the understanding of HIV neuropathogenesis, many questions remain unanswered.

Anti-HIV type 1 activity of 3'-fluoro-3'-deoxythymidine for several different multidrug-resistant mutants

The objective of this work was to test the antiviral activity of a potent nucleoside reverse transcriptase inhibitor, 3'-fluoro-3'-deoxythymidine (FLT), on both a wild-type human immunodeficiency virus (HIV-1) isolate and multidrug-resistant HIV-1 patient isolates. Drug-resistant viral isolates were selected on the basis of four different categories of well-characterized and representative multidrug-resistant mutants. The isolates included three variants containing 151M alone or in combination; three variants containing 215Y and 41L, 67N, 184V, 210W, and 219N in combination; two insertion mutant viruses (69 + EA and 69 + SA); and two deletion mutant viruses (del67NG and del67GS), the latter two groups both also containing other significant mutations. The activity of FLT and AZT against these isolates was determined by drug susceptibility assays and by measuring viral antigen p24 by ELISA. The cytotoxicity of FLT and AZT was assessed in PHA-stimulated PBMCs. Development of resistant mutants under FLT pressure was attempted by passaging HIV-1 isolates in SupT1 cells and stepwise increasing the concentration of FLT. The multidrug-resistant mutant HIV-1 isolates exhibited 7-fold to >100-fold increased resistance to AZT, but showed IC(50) values for FLT of 0.0014-0.0168 microM, which were lower than or similar to that of wild type (0.0075 microM). The cellular cytotoxicities of FLT and AZT fell into a similar range in PBMCs. The development of HIV mutants resistant to FLT appeared to be slower than for other RT inhibitors. HIV isolates with mutations resulting in multidrug resistance had no evidence of resistance to FLT. FLT may be useful in salvage therapies for patients harboring resistant strains and a reassessment of its therapeutic potential seems required.

Antiretroviral Resistance: Mechanisms, Detection and Clinical Implications

Widespread use of antiretroviral agents and the epidemic of human immunodeficiency virus (HIV) strains resistant to these drugs have raised a lot of issues including the biology and clinical implications of HIV resistance, reliability of resistant assays and their role in clinical practice. In May 2000, the International AIDS panel endorsed and recommended the use of antiretroviral drug resistance testing in patients. Considerable data on HIV drug resistance testing that strongly suggest that utility of these assays may be of great value have been published and presented at major meetings. Although the genotypic and phenotypic assays are available for antiretroviral drug resistance testing, the testing has certain limitations. The role of these resistance assays is not clearly defined in clinical practice. Prospective studies are needed to define the long-term benefits of these assays. HIV drug resistance testing in the near future may become an important tool and standard of practice for patients infected with HIV. Clinicians caring for HIV-positive patients should be familiar with the antiretroviral drug resistant assays.

Relative replication fitness of a high-level 3'-azido-3'-deoxythymidine-resistant variant of human immunodeficiency virus type 1 possessing an amino acid deletion at codon 67 and a novel substitution (Thr-->Gly) at codon 69

The combination of an amino acid deletion at codon 67 (delta 67) and Thr-to-Gly change at codon 69 (T69G) in the reverse transcriptase (RT) of human immunodeficiency virus type 1 (HIV-1) is associated with high-level resistance to multiple RT inhibitors. To determine the relative contributions of the delta 67 and T69G mutations on viral fitness, we performed a series of studies of HIV replication using recombinant variants. A high-level 3'-azido-3'-deoxythymidine (AZT)-resistant variant containing delta 67 plus T69G/K70R/L74I/K103N/T215F/K219Q in RT replicated as efficiently as wild-type virus (Wt). In contrast, the construct without delta 67 exhibited impaired replication (23% of growth of Wt). A competitive fitness study failed to reveal any differences in replication rates between the delta 67+T69G/K70R/L74I/K103N/T215F/+ ++K219Q mutant and Wt. Evaluation of proviral DNA sequences over a 3-year period in a patient harboring the multiresistant HIV revealed that the T69G mutation emerged in the context of a D67N/K70R/T215F/K219Q mutant backbone prior to appearance of the delta 67 deletion. To assess the impact of this stepwise accumulation of mutations on viral replication, a series of recombinant variants was constructed and analyzed for replication competence. The T69G mutation was found to confer 2',3'-dideoxyinosine resistance at the expense of fitness. Subsequently, the development of the delta 67 deletion led to a virus with improved replication and high-level AZT resistance.

Genetic diversity of protease and reverse transcriptase sequences in non-subtype-B human immunodeficiency virus type 1 strains: evidence of many minor drug resistance mutations in treatment-naive patients

Most human immunodeficiency virus (HIV) drug susceptibility studies have involved subtype B strains. Little information on the impact of viral diversity on natural susceptibility to antiretroviral drugs has been reported. However, the prevalence of non-subtype-B (non-B) HIV type 1 (HIV-1) strains continues to increase in industrialized countries, and antiretroviral treatments have recently become available in certain developing countries where non-B subtypes predominate. We sequenced the protease and reverse transcriptase (RT) genes of 142 HIV-1 isolates from antiretroviral-naive patients: 4 belonged to group O and 138 belonged to group M (9 subtype A, 13 subtype B, 2 subtype C, 5 subtype D, 2 subtype F1, 9 subtype F2, 4 subtype G, 5 subtype J, 2 subtype K, 3 subtype CRF01-AE, 67 subtype CRF02-AG, and 17 unclassified isolates). No major mutations associated with resistance to nucleoside reverse transcriptase inhibitors (NRTIs) or protease inhibitors were detected. Major mutations linked to resistance to non-NRTI agents were detected in all group O isolates (A98G and Y181C) and in one subtype J virus (V108I). In contrast, many accessory mutations were found, especially in the protease gene. Only 5.6% of the 142 strains, all belonging to subtype B or D, had no mutations in the protease gene. Sixty percent had one mutation, 22.5% had two mutations, 9.8% had three mutations, and 2.1% (all group O strains) had four mutations. In order of decreasing frequency, the following mutations were identified in the protease gene: M36I (86.6%), L10I/V (26%), L63P (12.6%), K20M/R (11.2%), V77I (5.6%), A71V (2.8%), L33F (0.7%), and M46I (0.7%). R211K, an accessory mutation associated with NRTI resistance, was also observed in 43.6% of the samples. Phenotypic and clinical studies are now required to determine whether multidrug-resistant viruses emerge more rapidly during antiretroviral therapy when minor resistance-conferring mutations are present before treatment initiation.

Synthesis and antiviral evaluation of the beta-L-enantiomers of some thymine 3'-deoxypentofuranonucleoside derivatives

3'-Deoxy-beta-L-erythro- (3), 3'-deoxy-beta-L-threo- (6), 2'-fluoro- (7) and 2'-azido-2',3'-dideoxy-beta-L-erythro- (10) pentofuranonucleoside derivatives of thymine have been synthesized and their antiviral properties examined. All these derivatives were stereospecifically prepared by glycosylation of thymine with a suitable peracylated 3-deoxy-L-erythro-pentofuranose sugar (1), followed by appropriate chemical modifications. The prepared compounds were tested for their activity against HIV, but they did not show an antiviral effect.

Towards the selection of phosphorothioate aptamers optimizing in vitro selection steps with phosphorothioate nucleotides

The high affinity of a given nucleic acid for a protein ligand can be used to isolate specific inhibitors of enzymes involved in pathological situations. The latter property is the basis of the SELEX (systematic evolution of ligands by exponential enrichment) technique. Recently, several potent nucleic acids inhibitors of HIV-1 replication have been isolated using the SELEX approach. However, phosphodiester oligodeoxynucleotides (PO-ODNs) were not used as antiviral agents because of their sensitivity to nucleases. Our goal in this work was to explore the possibility of selecting, from a fully substituted phosphorothioate library, oligonucleotides having both a strong affinity for HIV-1 reverse transcriptase (RT) and nuclease resistance. HIV-1 RT initiates in vivo reverse transcription from the 3' end of a host tRNALys. Although phosphorothioate ODNs (PS-ODNs) have been claimed to bind unspecifically to proteins, we have shown previously that an ODN corresponding to the acceptor stem of tRNALys was able to inhibit specifically HIV-1 replication in HIV-1 infected cells, without showing cytotoxicity up to 10 microM. As the SELEX strategy requires 'in vitro' transcription and reverse transcription of the selected DNA, we have assayed the available PS precursors as a model system by using PS-dNTPs and rNTPs. We have also developed an experimental procedure to optimize the incorporation of four PS-dNTPs during the PCR step of the SELEX approach. In the course of this work, we have showed that the PS-dGTP is a strong inhibitor of thermostable DNA polymerases as well as of HIV-1 RT.

Wild-type and YMDD mutant murine leukemia virus reverse transcriptases are resistant to 2',3'-dideoxy-3'-thiacytidine

The antiretroviral nucleoside analog 2',3'-dideoxy-3'-thiacytidine (3TC) is a potent inhibitor of wild-type human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT). A methionine-to-valine or methionine-to-isoleucine substitution at residue 184 in the HIV-1 YMDD motif, which is located at the RT active site, leads to a high level of resistance to 3TC. We sought to determine whether 3TC can inhibit the replication of wild-type murine leukemia virus (MLV), which contains V223 at the YVDD active site motif of the MLV RT, and of the V223M, V223I, V223A, and V223S mutant RTs. Surprisingly, the wild type and all four of the V223 mutants of MLV RT were highly resistant to 3TC. These results indicate that determinants outside the YVDD motif of MLV RT confer a high level of resistance to 3TC. Therefore, structural differences among similar RTs might result in widely divergent sensitivities to antiretroviral nucleoside analogs.

Clinical utility of testing human immunodeficiency virus for drug resistance

Human immunodeficiency virus (HIV) type 1 drug-resistance testing is quickly moving from the research laboratory to the clinic as data defining its utility as a prognostic indicator of response to therapy become available. In July 1998, a panel of the International AIDS Society-USA did not recommend the widespread application of resistance testing, but by May 2000 this panel endorsed and recommended the incorporation of resistance testing in patient-care management. Considerable data supporting the use of drug-resistance testing have now been published or presented at international conferences. These data strongly suggest that drug-resistance testing is of considerable value in many clinical settings. Prospective trials of resistance testing as a clinical management tool are still ongoing, and the long-term benefits still need to be evaluated. Nevertheless, early results from several studies showed a significantly better virological response when treatment regimens were based on resistance-testing data, rather than on the standard of care. HIV drug-resistance testing is also useful as a tool for new antiretroviral drug design and development, as well as for monitoring the spread of primary HIV drug resistance.

Variable sensitivity of CCR5-tropic human immunodeficiency virus type 1 isolates to inhibition by RANTES analogs

Aminooxypentane (AOP)-RANTES efficiently and specifically blocks entry of non-syncytium-inducing (NSI), CCR5-tropic (R5) human immunodeficiency virus type 1 (HIV-1) into host cells. Inhibition appears to be mediated by increased intracellular retention of the CCR5 coreceptor- AOP-RANTES complex and/or competitive binding of AOP-RANTES with NSI R5 HIV-1 isolates for CCR5. Although AOP-RANTES and other beta-chemokine analogs are potent inhibitors, the extreme heterogeneity of the HIV-1 envelope glycoproteins (gp120 and gp41) and variable coreceptor usage may affect the susceptibility of variant HIV-1 strains to these drugs. Using the same peripheral blood mononuclear cells (PBMC) with all isolates, we observed a significant variation in AOP-RANTES inhibition of 13 primary NSI R5 isolates; 50% inhibitory concentrations (IC(50)) ranged from 0.04 nM with HIV-1(A-92RW009) to 1.3 nM with HIV-1(B-BaL). Experiments performed on the same isolate (HIV-1(B-BaL)) with PBMC from different donors revealed no isolate-specific variation in AOP-RANTES IC(50) values but did show a considerable difference in virus replication efficiency. Exclusive entry via the CCR5 coreceptor by these NSI R5 isolates suggests that variable inhibition by AOP-RANTES is not due to alternative coreceptor usage but rather differential CCR5 binding. Analysis of the envelope V3 loop sequence linked a threonine or arginine at position 319 (numbering based on the HXB2 genome) with AOP-RANTES resistance. With the exception of one isolate, A319 was associated with increased sensitivity to AOP-RANTES inhibition. Distribution of AOP-RANTES IC(50) values with these isolates has promoted ongoing screens for new CCR5 agonists that show broad inhibition of HIV-1 variants.