Mechanisms of HIV-1 drug resistance

Molecular Epidemiology and Trends in HIV-1 Transmitted Drug Resistance in Mozambique 1999–2018

HIV drug resistance (HIVDR) can become a public health concern, especially in low- and middle-income countries where genotypic testing for people initiating antiretroviral therapy (ART) is not available. For first-line regimens to remain effective, levels of transmitted drug resistance (TDR) need to be monitored over time. To determine the temporal trends of TDR in Mozambique, a search for studies in PubMed and sequences in GenBank was performed. Only studies covering the pol region that described HIVDR and genetic diversity from treatment naïve patients were included. A dataset from seven published studies and one novel unpublished study conducted between 1999 and 2018 were included. The Calibrated Population Resistance tool (CPR) and REGA HIV-1 Subtyping Tool version 3 for sequences pooled by sampling year were used to determine resistance mutations and subtypes, respectively. The prevalence of HIVDR amongst treatment-naïve individuals increased over time, reaching 14.4% in 2018. The increase was most prominent for non-nucleoside reverse transcriptase inhibitors (NNRTIs), reaching 12.7% in 2018. Subtype C was predominant in all regions, but a higher genetic variability (19% non-subtype C) was observed in the north region of Mozambique. These findings confirm a higher diversity of HIV in the north of the country and an increased prevalence of NNRTI resistance among treatment naïve individuals over time.

[HIV drug resistance: past and current trends]

HIV infection is incurable, but effective antiretroviral therapy (ART) makes it possible to achieve an undetectable viral load (VL), to preserve the function of the immune system and to prevent the patient's health. Due to the constant increase in the use of ART and the high variability of HIV, especially in patients receiving so-called suboptimal therapy for various reasons, the incidence of drug resistance (DR) is increasing. In turn, the presence of DR in an HIV-infected patient affects the effectiveness of therapy, which leads to a limited choice and an increase in the cost of treatment regimens, disease progression and, consequently, an increased risk of death, as well as transmission of infection to partners. The main problems of drug resistance, its types and causes, as well as factors associated with its development are considered. The main drug resistance mutations for each of the drug classes are described.

Host-HIV-1 Interactome: A Quest for Novel Therapeutic Intervention

The complex nature and structure of the human immunodeficiency virus has rendered the cure for HIV infections elusive. The advances in antiretroviral treatment regimes and the development of highly advanced anti-retroviral therapy, which primarily targets the HIV enzymes, have dramatically changed the face of the HIV epidemic worldwide. Despite this remarkable progress, patients treated with these drugs often witness inadequate efficacy, compound toxicity and non-HIV complications. Considering the limited inventory of druggable HIV proteins and their susceptibility to develop drug resistance, recent attempts are focussed on targeting HIV-host interactomes that are essential for viral reproduction. Noticeably, unlike other viruses, HIV subverts the host nuclear pore complex to enter into and exit through the nucleus. Emerging evidence suggests a crucial role of interactions between HIV-1 proteins and host nucleoporins that underlie the import of the pre-integration complex into the nucleus and export of viral RNAs into the cytoplasm during viral replication. Nevertheless, the interaction of HIV-1 with nucleoporins has been poorly described and the role of nucleoporins during nucleocytoplasmic transport of HIV-1 still remains unclear. In this review, we highlight the advances and challenges in developing a more effective antiviral arsenal by exploring critical host-HIV interactions with a special focus on nuclear pore complex (NPC) and nucleoporins.

In silico design, synthesis, and screening of novel deoxyhypusine synthase inhibitors targeting HIV-1 replication

The human enzyme deoxyhypusine synthase (DHS) is an important host cell factor that participates in the post-translational hypusine modification of eukaryotic initiation factor 5A (eIF-5A). Hypusine-modified eIF-5A plays a role in a number of diseases, including HIV infection/AIDS. Thus, DHS represents a novel and attractive drug target. So far, four crystal structures are available, and various substances have been tested for inhibition of human DHS. Among these inhibitors, N-1-guanyl-1,7-diaminoheptane (GC7) has been co-crystallized in the active site of DHS. However, despite its potency, GC7 is not selective enough to be used in drug applications. Therefore, new compounds that target DHS are needed. Herein we report the in silico design, chemical synthesis, and biological evaluation of new DHS inhibitors. One of these inhibitors showed dose-dependent inhibition of DHS in vitro, as well as suppression of HIV replication in cell cultures. Furthermore, the compound exhibited no cytotoxic effects at active concentrations. Thus, this designed compound demonstrated proof of principle and represents a promising starting point for the development of new drug candidates to specifically interfere with DHS activity.

A novel immunodominant CD8+ T cell response restricted by a common HLA-C allele targets a conserved region of Gag HIV-1 clade CRF01_AE infected Thais

Background

CD8+ T cell responses play an important role in the control of HIV-1. The extensive sequence diversity of HIV-1 represents a critical hurdle to developing an effective HIV-1 vaccine, and it is likely that regional-specific vaccine strains will be required to overcome the diversity of the different HIV-1 clades distributed world-wide. Unfortunately, little is known about the CD8+ T cell responses against CRF01_AE, which is responsible for the majority of infections in Southeast Asia.

Methodology/Principal Findings

To identify dominant CD8+ T cell responses recognized in HIV-1 clade CRF01_AE infected subjects we drew upon data from an immunological screen of 100 HIV-1 clade CRF01_AE infected subjects using IFN-gamma ELISpot to characterize a novel immunodominant CD8+ T cell response in HIV-1 Gag restricted by HLA-Cw*0102 (p24, ₂₇₇YSPVSILDI₂₈₅, YI9). Over 75% of Cw*0102+ve subjects targeted this epitope, representing the strongest response in more than a third of these individuals. This novel CD8 epitope was located in a highly conserved region of HIV-1 Gag known to contain immunodominant CD8 epitopes, which are restricted by HLA-B*57 and -B*27 in clade B infection. Nonetheless, viral escape in this epitope was frequently observed in Cw*0102+ve subjects, suggestive of strong selection pressure being exerted by this common CD8+ T cell response.

Conclusions/Significance

As HLA-Cw*0102 is frequently expressed in the Thai population (allelic frequency of 16.8%), this immunodominant Cw*0102-restricted Gag epitope may represent an attractive candidate for vaccines specific to CRF01_AE and may help facilitate further studies of immunopathogenesis in this understudied HIV-1 clade.

5-Modified-2'-dU and 2'-dC as mutagenic anti HIV-1 proliferation agents: synthesis and activity

With the goal of limiting HIV-1 proliferation by increasing the mutation rate of the viral genome, we synthesized a series of pyrimidine nucleoside analogues modified in position 5 of the aglycone moiety but unmodified on the sugar part. The synthetic strategies allow us to prepare the targeted compounds directly from commercially available nucleosides. All compounds were tested for their ability to reduce HIV-1 proliferation in cell culture. Two of them (5-hydroxymethyl-2'-dU (1c) and 5-hydroxymethyl-2'-dC (2c)) displayed a moderate antiviral activity in single passage experiments. The same two compounds plus two additional ones (5-carbamoyl-2'-dU (1a) and 5-carbamoylmethyl-2'-dU (1b)) were potent inhibitors of HIV-1 RT activity in serial passage assays, in which they induced a progressive loss of HIV-1 replication. In addition, viruses collected after seven passages in the presence of 1c and 2c replicated very poorly after withdrawal of these compounds, consistent with the accumulation of deleterious mutations in the HIV-1 genome.

The spread, treatment, and prevention of HIV-1: evolution of a global pandemic

The most up-to-date estimates demonstrate very heterogeneous spread of HIV-1, and more than 30 million people are now living with HIV-1 infection, most of them in sub-Saharan Africa. The efficiency of transmission of HIV-1 depends primarily on the concentration of the virus in the infectious host. Although treatment with antiviral agents has proven a very effective way to improve the health and survival of infected individuals, as we discuss here, the epidemic will continue to grow unless greatly improved prevention strategies can be developed and implemented. No prophylactic vaccine is on the horizon. However, several behavioral and structural strategies have made a difference--male circumcision provides substantial protection from sexually transmitted diseases, including HIV-1, and the application of antiretroviral agents for prevention holds great promise.

Escape of HIV-1 from a small molecule CCR5 inhibitor is not associated with a fitness loss

Fitness is a parameter used to quantify how well an organism adapts to its environment; in the present study, fitness is a measure of how well strains of human immunodeficiency virus type 1 (HIV-1) replicate in tissue culture. When HIV-1 develops resistance in vitro or in vivo to antiretroviral drugs such as reverse transcriptase or protease inhibitors, its fitness is often impaired. Here, we have investigated whether the development of resistance in vitro to a small molecule CCR5 inhibitor, AD101, has an associated fitness cost. To do this, we developed a growth-competition assay involving dual infections with molecularly cloned viruses that are essentially isogenic outside the env genes under study. Real-time TaqMan quantitative PCR (QPCR) was used to quantify each competing virus individually via probes specific to different, phenotypically silent target sequences engineered within their vif genes. Head-to-head competition assays of env clones derived from the AD101 escape mutant isolate, the inhibitor-sensitive parental virus, and a passage control virus showed that AD101 resistance was not associated with a fitness loss. This observation is consistent with the retention of the resistant phenotype when the escape mutant was cultured for a total of 20 passages in the absence of the selecting compound. Amino acid substitutions in the V3 region of gp120 that confer complete AD101 resistance cause a fitness loss when introduced into an AD101-sensitive, parental clone; however, in the resistant isolate, changes elsewhere in env that occurred prior to the substitutions within V3 appear to compensate for the adverse effect of the V3 changes on replicative capacity. These in vitro studies may have implications for the development and management of resistance to other CCR5 inhibitors that are being evaluated clinically for the treatment of HIV-1 infection.

Molecular tongs containing amino acid mimetic fragments: new inhibitors of wild-type and mutated HIV-1 protease dimerization

We have designed, synthesized, and evaluated the inhibitory activity and metabolic stability of new peptidomimetic molecular tongs based on a naphthalene scaffold for inhibiting HIV-1 protease dimerization. Peptidomimetic motifs were inserted into one peptidic strand to make it resistant to proteolysis. The peptidic character of the molecular tongs can be decreased without changing the way they inhibit dimerization. Mutated HIV-1 proteases are also vulnerable to dimerization inhibitors, and the multimutated protease ANAM-11 is twice as sensitive to the inhibitor compared to wild-type protease. Thus, the metabolic stability of antidimeric molecular tongs can be increased without compromising their ability to inhibit wild-type and mutated HIV-1 proteases in vitro.

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.

New reporter cell line to evaluate the sequential emergence of multiple human cytomegalovirus mutations during in vitro drug exposure

We developed a new reporter cell line for human cytomegalovirus (HCMV) drug susceptibility testing. This cell line was obtained by incorporating the luciferase reporter gene under the control of an HCMV-specific promoter into the genome of astrocytoma cells (U373MG). We then used our reporter cell line to evaluate phenotypic changes conferred by the sequential emergence of HCMV UL54 and UL97 mutations following long-term drug exposure. The laboratory strain AD169 was passaged in the presence of increasing concentrations of ganciclovir (one viral line) or foscarnet (two viral lines). Resistant viruses were plaque purified at five different concentrations of ganciclovir and at three different concentrations of foscarnet. In addition to the previously described M460I and L595S UL97 mutations and the L545S and V812L UL54 mutations, exposition to ganciclovir (up to 3,000 microM) resulted in the selection of two unreported UL54 mutations (P829S and D879G). Passages in the presence of foscarnet (up to 3,000 microM) resulted in the selection of seven not previously described UL54 mutations (K500N, T552N, S585A, N757K, L802V, L926V, and L957F) in addition to the N408D mutation that has been associated with ganciclovir and cidofovir resistance. Long-term exposure of HCMV to either ganciclovir or foscarnet ultimately resulted in the selection of multiple UL54 mutations that conferred high levels of resistance to all approved HCMV DNA polymerase inhibitors, i.e., ganciclovir, cidofovir, and foscarnet. Emergence of each viral mutation conferred stepwise increases in drug 50% inhibitory concentrations that could be objectively measured with the new reporter cell assay.

Crystal structure of dihydrofolate reductase from Plasmodium vivax: pyrimethamine displacement linked with mutation-induced resistance

Pyrimethamine (Pyr) targets dihydrofolate reductase of Plasmodium vivax (PvDHFR) as well as other malarial parasites, but its use as antimalarial is hampered by the widespread high resistance. Comparison of the crystal structures of PvDHFR from wild-type and the Pyr-resistant (SP21, Ser-58 --> Arg + Ser-117 --> Asn) strain as complexes with NADPH and Pyr or its analog lacking p-Cl (Pyr20) clearly shows that the steric conflict arising from the side chain of Asn-117 in the mutant enzyme, accompanied by the loss of binding to Ser-120, is mainly responsible for the reduction in binding of Pyr. Pyr20 still effectively inhibits both the wild-type and SP21 proteins, and the x-ray structures of these complexes show how Pyr20 fits into both active sites without steric strain. These structural insights suggest a general approach for developing new generations of antimalarial DHFR inhibitors that, by only occupying substrate space of the active site, would retain binding affinity with the mutant enzymes.

Generation and properties of a human immunodeficiency virus type 1 isolate resistant to the small molecule CCR5 inhibitor, SCH-417690 (SCH-D)

We describe the generation of two genetically related human immunodeficiency virus type 1 (HIV-1) isolates highly (>20,000-fold) resistant to the small molecule CCR5 inhibitor, SCH-417690 (formerly SCH-D). Both viruses were cross-resistant to other small molecules targeting entry via CCR5, but they were inhibited by some MAbs against the same coreceptor on primary CD4+ T-cells. The resistant isolates remained sensitive to inhibitors of other stages of virus entry, and to replication inhibitors acting post-entry. Neither escape mutant could replicate detectably in peripheral blood mononuclear cells (PBMC) from two donors homozygous for the CCR5-Delta32 allele and both were insensitive to the CXCR4-specific inhibitor, AMD3100. Hence, the SCH-D escape mutants retained the R5 phenotype. One of the resistant isolates was, however, capable of replication in U87.CD4.CXCR4 cells and, after expansion in those cells, was sensitive to AMD3100 in primary CD4+ T-cells. Hence, some X4 variants may be present in this escape mutant swarm. A notable observation was that the SCH-D escape mutants were also cross-resistant to PSC-RANTES and AOP-RANTES, chemokine derivatives that are reported to down-regulate cell surface CCR5 almost completely. However, the extent to which CCR5 is down-regulated was dependent upon the detection MAb. Hence, the escape mutants may be using a CCR5 configuration that is only detected by some anti-CCR5 MAbs. Finally, two SCH-D-resistant clonal viruses revealed no amino acid changes in the gp120 V3 region relative to the parental viruses, in marked contrast to clones resistant to the AD101 small molecule CCR5 inhibitor that possess 4 such sequence changes. Several sequence changes elsewhere in gp120 (V2, C3 and V4) were present in the SCH-D-resistant clones. Their influence on the resistant phenotype remains to be determined.

V3: HIV's switch-hitter

The third variable region, V3, of the gp120 surface envelope glycoprotein is an approximately 35-residue-long, frequently glycosylated, highly variable, disulfide-bonded structure that has a major influence on HIV-1 tropism. Thus the sequence of V3, directly or indirectly, can determine which coreceptor (CCR5 or CXCR4) is used to trigger the fusion potential of the Env complex, and hence which cells the virus can infect. V3 also influences HIV-1's sensitivity to, and ability to escape from, entry inhibitors that are being developed as antiviral drugs. For some strains, V3 is a prominent target for HIV-1 neutralizing antibodies (NAbs); indeed, for many years it was considered to be the "principal neutralization determinant" (PND). Some efforts to use V3 as a vaccine target continue to this day, despite disappointing progress over more than a decade. Recent findings on the structure, function, antigenicity, and immunogenicity of V3 cast new doubts on the value of this vaccine approach. Here, we review recent advances in the understanding of V3 as a determinant of viral tropism, and discuss how this new knowledge may inform the development of HIV-1 drugs and vaccines.

Challenges for HIV vaccine dissemination and clinical trial recruitment: if we build it, will they come?

HIV vaccine availability does not guarantee uptake. Given suboptimal uptake of highly efficacious and already accessible vaccines in the United States, low vaccine coverage in the developing world, and the expectation that initial HIV vaccines will be only partially efficacious, the public health community will face formidable challenges in disseminating U.S. Food and Drug Administration (FDA)-approved HIV vaccines. HIV/AIDS stigma, fear of vaccine- induced HIV infection, social side effects of testing HIV-positive, and mistrust of government and research present additional obstacles to HIV vaccine dissemination. Increased risk behaviors because of HIV vaccine availability can undermine the effectiveness of partially efficacious vaccines in reducing HIV incidence. HIV vaccine efficacy trials also face significant challenges in recruitment of sufficient volunteers and possible increases in risk behaviors due to trial participation. Planning and designing interventions to facilitate successful recruitment for large-scale phase 3 efficacy trials is a vital step towards U.S. FDA-approved HIV vaccines. Rather than despair in the face of momentous HIV vaccine dissemination challenges, or presume unrealistically that vaccine uptake will ensue automatically and that risk behavior increases will not occur, let us deem the estimated 10-year window to an approved HIV vaccine as an opportunity to investigate and confront these challenges. A consumer research agenda founded on social marketing principles is needed to facilitate the design of empirically-based interventions tailored to the unique needs and preferences of specific segments of consumers. Social marketing interventions may increase future HIV vaccine uptake and clinical trial participation, and mitigate increases in HIV risk behaviors.

New Constrained “Molecular Tongs” Designed To Dissociate HIV-1 Protease Dimer

New "molecular tongs" based on naphthalene and quinoline scaffolds linked to two peptidic strands were synthesized. They were designed to prevent dimerization of HIV-1 protease by targeting the antiparallel beta-sheet involving N- and C-termini of each monomer. Compared to "molecular tongs" previously described (Bouras, A.; Boggetto, N.; Benatalah, Z.; de Rosny, E.; Sicsic, S.; Reboux-Ravaud, M. J. Med. Chem. 1999, 42, 957-962), two main different structural features were introduced: positively charged quinoline as a new scaffold and two peptidic strands displaying different sequences. Seventeen new "molecular tongs" with dipeptidic or tripeptidic strands were synthesized. These molecules were assayed on HIV-1 protease using the Zhang kinetic technique. Eleven molecules behaved as pure dimerization inhibitors, mostly at the submicromolar range. Compared to a naphthalene scaffold, the quinoline one was shown in several cases to favor dimerization inhibition. The simplified hydrophobic Val-Leu-Val-OMe strand was confirmed as particularly favorable. The C-terminal analogue strand Thr-Leu-Asn-OMe was shown to be the best one for inducing dimerization inhibition (K(id) of 80 nM for compound 30). The mechanism of inhibition was ascertained using ANS binding and gel filtration. Experimental results are in agreement with the dissociation of the HIV-1 protease dimeric form in the presence of the synthesized molecular tongs.

Genetic and phenotypic analyses of human immunodeficiency virus type 1 escape from a small-molecule CCR5 inhibitor

We have described previously the generation of an escape variant of human immunodeficiency virus type 1 (HIV-1), under the selection pressure of AD101, a small molecule inhibitor that binds the CCR5 coreceptor (A. Trkola, S. E. Kuhmann, J. M. Strizki, E. Maxwell, T. Ketas, T. Morgan, P. Pugach, S. X. L. Wojcik, J. Tagat, A. Palani, S. Shapiro, J. W. Clader, S. McCombie, G. R. Reyes, B. M. Baroudy, and J. P. Moore, Proc. Natl. Acad. Sci. USA 99:395-400, 2002). The escape mutant, CC101.19, continued to use CCR5 for entry, but it was at least 20,000-fold more resistant to AD101 than the parental virus, CC1/85. We have now cloned the env genes from the the parental and escape mutant isolates and made chimeric infectious molecular clones that fully recapitulate the phenotypes of the corresponding isolates. Sequence analysis of the evolution of the escape mutants suggested that the most relevant changes were likely to be in the V3 loop of the gp120 glycoprotein. We therefore made a series of mutant viruses and found that full AD101 resistance was conferred by four amino acid changes in V3. Each change individually caused partial resistance when they were introduced into the V3 loop of a CC1/85 clone, but their impact was dependent on the gp120 context in which they were made. We assume that these amino acid changes alter how the HIV-1 Env complex interacts with CCR5. Perhaps unexpectedly, given the complete dependence of the escape mutant on CCR5 for entry, monomeric gp120 proteins expressed from clones of the fully resistant isolate failed to bind to CCR5 on the surface of L1.2-CCR5 cells under conditions where gp120 proteins from the parental virus and a partially AD101-resistant virus bound strongly. Hence, the full impact of the V3 substitutions may only be apparent at the level of the native Env complex.

Molecular biology of the human immunodeficiency virus: current and future targets for intervention

Inhibitors of the reverse transcriptase and protease enzymes of the human immunodeficiency virus form the backbone of current antiretroviral therapy. However, other therapeutic targets have been identified in studies of interactions between HIV and its target cells. This review presents an overview of HIV replication, emphasizing recently recognized cellular and viral molecules that may be exploited in future pharmacological approaches to prevent and treat infection with HIV.

Lack of persistent drug-resistant mutations evaluated within and between treatment interruptions in chronically HIV-1-infected patients

OBJECTIVE

To determine the effect of treatment interruptions (TI) on the evolution and persistence of drug-resistant viruses in chronically HIV-1-infected suppressed patients.

METHODS

The emergence of viral resistance to combination antiretroviral therapy was monitored in 11 suppressed chronically HIV-1-infected patients undergoing from one up to four sequential TI (a total of 25 TI), by genotyping of the virus for known mutations in the genes for protease and reverse transcriptase. Resistance assays were performed at the first viral rebound > 100 copies/ml.

RESULTS

All subjects achieved resuppression of HIV-1 under the same antiretroviral therapy, regardless of the number of TI. Five of eleven patients showed no development of resistance. In the remaining six patients, the following patterns of mutations associated with viral resistance were found: one mutation (K70R), which was observed in one patient during the 1st TI and persisted during follow-up; two mutations (L90M, M184V), which were observed in four patients during the 1st TI and were intermittently present or lost following extended TI, treatment reinitiation and/or during subsequent TI; and evolution of two mutations (M184V, K219E) observed in two patients. These two mutations were not present during the 1st TI and were subsequently lost following therapy reinitiation or during the next TI.

CONCLUSIONS

Detection of drug resistance during TI by virus genotyping assays does not predict failure to resuppress after antiretroviral therapy reinitiation nor persistence of a resistant viral population during extended interruptions or subsequent TI.

Dimerization Inhibitors of HIV-1 Protease Based on a Bicyclic Guanidinium Subunit

Original inhibitors of HIV-1 protease based on a chiral bicyclic guanidinium scaffold linked to short peptidic mimics of the terminal protease sequences and to a lipophilic group were designed. These inhibitors prevent dimerization of the native protease by an interfacial structure at the highly conserved antiparallel beta-strand involving both the N and C termini that substantially account for dimerization. The preorganized guanidinium spacer introduces additional electrostatic hydrogen-bonding interactions with the C-terminal Phe-99 carboxylate. Lipophilic residues linked to side chains and the guanidinium scaffold are essential for dimerization inhibition as ascertained by Zhang kinetics (4, K(id) = 290 nM; 6 or 6', K(id) = 150 nM; 8, K(id) = 400 nM) combined with a circular dichroism study on the enzyme thermal stability. Remarkably, less hydrophobic compounds result in mixed dimerization (1a and 3) or active site inhibitors (5). Removal of the guanidinium hydrophobic groups leads to less active or inactive ligands.

Antiviral activities and cellular toxicities of modified 2',3'-dideoxy-2',3'-didehydrocytidine analogues

The antiviral efficacies and cytotoxicities of 2',3'- and 4'-substituted 2',3'-didehydro-2',3'-dideoxycytidine analogs were evaluated. All compounds were tested (i) against a wild-type human immunodeficiency virus type 1 (HIV-1) isolate (strain xxBRU) and lamivudine-resistant HIV-1 isolates, (ii) for their abilities to inhibit hepatitis B virus (HBV) production in the inducible HepAD38 cell line, and (iii) for their abilities to inhibit bovine viral diarrhea virus (BVDV) production in acutely infected Madin-Darby bovine kidney cells. Some compounds demonstrated potent antiviral activities against the wild-type HIV-1 strain (range of 90% effective concentrations [EC(90)s], 0.14 to 5.2 micro M), but marked increases in EC(90)s were noted when the compounds were tested against the lamivudine-resistant HIV-1 strain (range of EC(90)s, 53 to >100 micro M). The beta-L-enantiomers of both classes of compounds were more potent than the corresponding beta-D-enantiomers. None of the compounds showed antiviral activity in the assay that determined their abilities to inhibit BVDV, while two compounds inhibited HBV production in HepAD38 cells (EC(90), 0.25 micro M). The compounds were essentially noncytotoxic in human peripheral blood mononuclear cells and HepG2 cells. No effect on mitochondrial DNA levels was observed after a 7-day incubation with the nucleoside analogs at 10 micro M. These studies demonstrate that (i) modification of the sugar ring of cytosine nucleoside analogs with a 4'-thia instead of an oxygen results in compounds with the ability to potently inhibit wild-type HIV-1 but with reduced potency against lamivudine-resistant virus and (ii) the antiviral activity of beta-D-2',3'-didehydro-2',3'-dideoxy-5-fluorocytidine against wild-type HIV-1 (EC(90), 0.08 micro M) and lamivudine-resistant HIV-1 (EC(90) = 0.15 micro M) is markedly reduced by introduction of a 3'-fluorine in the sugar (EC(90)s of compound 2a, 37.5 and 494 micro M, respectively).