An in silico molecular modeling approach of halolactone derivatives as potential inhibitors for human immunodeficiency virus type-1 reverse transcriptase enzyme.

Acquired Immune Deficiency Syndrome (AIDS) is an infectious disease caused by Human Immunodeficiency Virus (HIV) infection and its replication requires the Reverse Transcriptase (RT) enzyme. RT plays a key role in the HIV life cycle, making it one of the most important targets for designing new drugs. Thus, in order to increase therapeutic options against AIDS, halolactone derivatives (D-halolactone) that have been showed as potential non-nucleoside inhibitors of the RT enzyme were studied. In the present work, a series of D-halolactone were investigated by molecular modeling studies, combining Three-dimensional Quantitative Structure-Activity Relationship (3 D-QSAR), molecular docking and Molecular Dynamics (MD) techniques, to understand the molecular characteristics that promote biological activity. The internal and external validation parameters indicated that the 3 D-QSAR model has good predictive capacity and statistical significance. Contour maps provided useful information on the structural characteristics of compounds for anti-HIV-1 activity. The docking results showed that D-halolactone present good complementarity by the RT allosteric site. In MD simulations it was observed that the formation of enzyme-ligand complexes were favorable, and from the free energy decomposition it was found that Leu100, Val106, Tyr181, Try188, and Trp229 are key residues for stabilization in the enzymatic site. Thus, the results showed that the proposed models can be used to design promising HIV-1 RT inhibitors. Communicated by Ramaswamy H. Sarma.

Design, synthesis, in vitro and in silico studies of novel 4-oxoquinoline ribonucleoside derivatives as HIV-1 reverse transcriptase inhibitors.

Human immunodeficiency virus type 1 (HIV-1) is a public health problem that affects over 38 million people worldwide. Although there are highly active antiretroviral therapies, emergence of antiviral resistant strains is a problem which leads to almost a million death annually. Thus, the development of new drugs is necessary. The viral enzyme reverse transcriptase (RT) represents a validated therapeutic target. Because the oxoquinolinic scaffold has substantial biological activities, including antiretroviral, a new series of 4-oxoquinoline ribonucleoside derivatives obtained by molecular hybridization were studied here. All synthesized compounds were tested against human immunodeficiency virus type 1 reverse transcriptase (HIV-1 RT), and 9a and 9d displayed the highest antiviral activities, with IC50 values of 1.4 and 1.6 µM, respectively. These compounds were less cytotoxic than AZT and showed CC50 values of 1486 and 1394 µM, respectively. Molecular docking studies showed that the most active compounds bound to the allosteric site of the enzyme, suggesting a low susceptibility to the development of antiviral resistance. In silico pharmacokinetic and toxicological evaluations reinforced the potential of the active compounds as anti-HIV candidates for further exploration. Overall, this work showed that compounds 9a and 9d are promising scaffold for future anti-HIV-1 RT drug design.

Design, synthesis and biological evaluation of quinoxaline compounds as anti-HIV agents targeting reverse transcriptase enzyme.

Infection by human immunodeficiency virus still represents a continuous serious concern and a global threat to human health. Due to appearance of multi-resistant virus strains and the serious adverse side effects of the antiretroviral therapy administered, there is an urgent need for the development of new treatment agents, more active, less toxic and with increased tolerability to mutations. Quinoxaline derivatives are an emergent class of heterocyclic compounds with a wide spectrum of biological activities and therapeutic applications. These types of compounds have also shown high potency in the inhibition of HIV reverse transcriptase and HIV replication in cell culture. For these reasons we propose, in this work, the design, synthesis and biological evaluation of quinoxaline derivatives targeting HIV reverse transcriptase enzyme. For this, we first carried out a structure-based development of target-specific compound virtual chemical library of quinoxaline derivatives. The rational construction of the virtual chemical library was based on previously assigned pharmacophore features. This library was processed by a virtual screening protocol employing molecular docking and 3D-QSAR. Twenty-five quinoxaline compounds were selected for synthesis in the basis of their docking and 3D-QSAR scores and chemical synthetic simplicity. They were evaluated as inhibitors of the recombinant wild-type reverse transcriptase enzyme. Finally, the anti-HIV activity and cytotoxicity of the synthesized quinoxaline compounds with highest reverse transcriptase inhibitory capabilities was evaluated. This simple screening strategy led to the discovery of two selective and potent quinoxaline reverse transcriptase inhibitors with high selectivity index.

New Efavirenz Derivatives and 1,2,3-Triazolyl-phosphonates as Inhibitors of Reverse Transcriptase of HIV-1.

BACKGROUND: According to the World Health Organization (WHO), the fight against Acquired Immunodeficiency Syndrome (AIDS) is still one of the most significant challenges facing humanity. Worldwide, it is estimated that 36.7 million people are infected by the Human Immunodeficiency Virus (HIV). Despite the variety of available drugs, the search for new enzymatic inhibitors of HIV is still important due to the presence of adverse effects and the development of resistant strains. Therefore, the present study aimed to design, synthesize, and biologically evaluate novel inhibitors of HIV Reverse Transcriptase (RT). MATERIALS AND METHODS: These compounds were obtained in two series, and compounds in both series contain a 1,2,3-triazole ring in their structures. The compounds in the first series are Efavirenz (EFV) analogues with the N-1 position substituted by another important fragment as described in the medicinal chemistry literature on anti-HIV drugs. The second series has a phosphonate chain similar to that in the structure of Tenofovir Disoproxil Fumarate (TDF). RESULTS AND CONCLUSION: The results of the biological evaluation showed that all compounds presented high RT inhibition values and lower or comparable inhibitory concentrations (the concentration needed to reduce the enzymatic activity by 50%, IC50 values, 0.8-1.9 µM). Among the compounds in the first series, the three with the lowest IC50 values had values between 0.8-0.9 µM, and of those in the second series, the most potent had an IC50 value of 1.1 µM; compounds in both series were equipotent to TDF (1.2 µM). Thus, the new compounds could be considered lead compounds for the development of new antiretroviral compounds.

The role of the glycosyl moiety of myricetin derivatives in anti-HIV-1 activity in vitro.

BACKGROUND: Plant extracts are sources of valuable compounds with biological activity, especially for the anti-proliferative activity against pathogens or tumor cells. Myricetin is a flavonoid found in several plants that has been described as an inhibitor of Human immunodeficiency virus type 1 (HIV-1) through its action against the HIV reverse transcriptase, but myricetin derivatives have not been fully studied. The aim of this study was to evaluate the anti-HIV-1 activity of glycosylated metabolites obtained from Marcetia taxifolia and derived from myricetin: myricetin rhamnoside and myricetin 3-(6-rhamnosylgalactoside). METHODS: Compounds were obtained from organic extracts by maceration of aerial parts of M. taxifolia. All biological assays were performed in the MT4 cell line. Antiviral activity was measured as inhibition of p24 and reverse transcriptase with a fluorescent assay. RESULTS: Both flavonoids have antiviral activity in vitro, with an EC50 of 120 µM for myricetin 3-rhamnoside (MR) and 45 µM for myricetin 3-(6-rhamnosylgalactoside) (MRG), both significantly lower than the EC50 of myricetin (230 µM). Although both compounds inhibited the reverse transcriptase activity, with an IC50 of 10.6 µM for MR and 13.8 µM for MRG, myricetin was the most potent, with an IC50 of 7.6 µM, and an inhibition greater than 80%. Molecular docking approach showed correlation between the free energy of binding with the assays of enzyme inhibition. CONCLUSIONS: The results suggest that glycosylated moiety might enhance the anti-HIV-1 activity of myricetin, probably by favoring the internalization of the flavonoid into the cell. The inhibition of the HIV-1 reverse transcriptase is likely responsible for the antiviral activity.

Efavirenz a nonnucleoside reverse transcriptase inhibitor of first-generation: Approaches based on its medicinal chemistry.

Acquired immunodeficiency syndrome (AIDS) is a disease caused by human immunodeficiency virus (HIV) that affects individuals on all continents. In 1987, the antiretroviral therapy began increasing survival rates and improving the quality of life for patients. Efavirenz (EFV) is a drug widely used in the treatment of HIV-AIDS since 1998. Belonging to a class of nonnucleoside reverse transcriptase inhibitors (NNRTI), it directly blocks the action of the enzyme and consequently the multiplication of the virus. Although EFV has provided excellent results in reducing viral load, cases of resistance associated with adverse effects have led to the search to find new analogs of this drug. Although many researchers are involved in this quest, curiously there is still no clinical substitute for EFV. To develop a second-generation version of EFV, it is essential understand the structure-activity relationships of the derivative compounds. Thus, the aims of the present review are to compare EFV and its derivatives using medicinal chemistry and to describe the main synthetic routes.

Computational drug design strategies applied to the modelling of human immunodeficiency virus-1 reverse transcriptase inhibitors.

Reverse transcriptase (RT) is a multifunctional enzyme in the human immunodeficiency virus (HIV)-1 life cycle and represents a primary target for drug discovery efforts against HIV-1 infection. Two classes of RT inhibitors, the nucleoside RT inhibitors (NRTIs) and the nonnucleoside transcriptase inhibitors are prominently used in the highly active antiretroviral therapy in combination with other anti-HIV drugs. However, the rapid emergence of drug-resistant viral strains has limited the successful rate of the anti-HIV agents. Computational methods are a significant part of the drug design process and indispensable to study drug resistance. In this review, recent advances in computer-aided drug design for the rational design of new compounds against HIV-1 RT using methods such as molecular docking, molecular dynamics, free energy calculations, quantitative structure-activity relationships, pharmacophore modelling and absorption, distribution, metabolism, excretion and toxicity prediction are discussed. Successful applications of these methodologies are also highlighted.

Computational drug design strategies applied to the modelling of human immunodeficiency virus-1 reverse transcriptase inhibitors

Reverse transcriptase (RT) is a multifunctional enzyme in the human immunodeficiency virus (HIV)-1 life cycle and represents a primary target for drug discovery efforts against HIV-1 infection. Two classes of RT inhibitors, the nucleoside RT inhibitors (NRTIs) and the nonnucleoside transcriptase inhibitors are prominently used in the highly active antiretroviral therapy in combination with other anti-HIV drugs. However, the rapid emergence of drug-resistant viral strains has limited the successful rate of the anti-HIV agents. Computational methods are a significant part of the drug design process and indispensable to study drug resistance. In this review, recent advances in computer-aided drug design for the rational design of new compounds against HIV-1 RT using methods such as molecular docking, molecular dynamics, free energy calculations, quantitative structure-activity relationships, pharmacophore modelling and absorption, distribution, metabolism, excretion and toxicity prediction are discussed. Successful applications of these methodologies are also highlighted.

Natural Plant Alkaloid (Emetine) Inhibits HIV-1 Replication by Interfering with Reverse Transcriptase Activity.

Ipecac alkaloids are secondary metabolites produced in the medicinal plant Psychotria ipecacuanha. Emetine is the main alkaloid of ipecac and one of the active compounds in syrup of Ipecac with emetic property. Here we evaluated emetine's potential as an antiviral agent against Human Immunodeficiency Virus. We performed in vitro Reverse Transcriptase (RT) Assay and Natural Endogenous Reverse Transcriptase Activity Assay (NERT) to evaluate HIV RT inhibition. Emetine molecular docking on HIV-1 RT was also analyzed. Phenotypic assays were performed in non-lymphocytic and in Peripheral Blood Mononuclear Cells (PBMC) with HIV-1 wild-type and HIV-harboring RT-resistant mutation to Nucleoside Reverse Transcriptase Inhibitors (M184V). Our results showed that HIV-1 RT was blocked in the presence of emetine in both models: in vitro reactions with isolated HIV-1 RT and intravirion, measured by NERT. Emetine revealed a strong potential of inhibiting HIV-1 replication in both cellular models, reaching 80% of reduction in HIV-1 infection, with low cytotoxic effect. Emetine also blocked HIV-1 infection of RT M184V mutant. These results suggest that emetine is able to penetrate in intact HIV particles, and bind and block reverse transcription reaction, suggesting that it can be used as anti-HIV microbicide. Taken together, our findings provide additional pharmacological information on the potential therapeutic effects of emetine.

Mechanism of allosteric inhibition of HIV-1 reverse transcriptase revealed by single-molecule and ensemble fluorescence.

Non-nucleoside reverse transcriptase (RT) inhibitors (NNRTIs) are routinely used to treat HIV-1 infection, yet their mechanism of action remains unclear despite intensive investigation. In this study, we developed complementary single-molecule fluorescence and ensemble fluorescence anisotropy approaches to discover how NNRTIs modulate the intra-molecular conformational changes and inter-molecular dynamics of RT-template/primer (T/P) and RT-T/P-dNTP complexes. We found that NNRTI binding to RT induces opening of the fingers and thumb subdomains, which increases the dynamic sliding motion of the enzyme on the T/P and reduces dNTP binding affinity. Further, efavirenz promotes formation of the E138-K101 salt bridge between the p51 and p66 subunits of RT, which contributes to opening of the thumb/fingers subdomains. Engineering a more polar salt bridge between p51 and p66 resulted in even greater increases in the thumb/fingers opening, RT sliding, dNTP binding disruption and in vitro and in vivo RT inhibition than were observed with wild-type RT. We also observed that K103N, a clinically relevant NNRTI resistance mutation, does not prevent binding between efavirenz and RT-T/P but instead allows formation of a stable and productive RT-T/P-dNTP complex, possibly through disruption of the E138-K101 salt bridge. Collectively, these data describe unique structure-activity-resistance relationships that could be exploited for drug development.

Molecular docking studies of marine diterpenes as inhibitors of wild-type and mutants HIV-1 reverse transcriptase.

AIDS is a pandemic responsible for more than 35 million deaths. The emergence of resistant mutations due to drug use is the biggest cause of treatment failure. Marine organisms are sources of different molecules, some of which offer promising HIV-1 reverse transcriptase (RT) inhibitory activity, such as the diterpenes dolabelladienotriol (THD, IC50 = 16.5 µM), (6R)-6-hydroxydichotoma-3,14-diene-1,17-dial (HDD, IC50 = 10 µM) and (6R)-6-acetoxydichotoma-3,14-diene-1,17-dial (ADD, IC50 = 35 µM), isolated from a brown algae of the genus Dictyota, showing low toxicity. In this work, we evaluated the structure-activity relationship (SAR) of THD, HDD and ADD as anti HIV-1 RT, using a molecular modeling approach. The analyses of stereoelectronic parameters revealed a direct relationship between activity and HOMO (Highest Occupied Molecular Orbital)-LUMO (Lowest Unoccupied Molecular Orbital) gap (E(LUMO)-E(HOMO)), where antiviral profile increases with larger HOMO-LUMO gap values. We also performed molecular docking studies of THD into HIV-1 RT wild-type and 12 different mutants, which showed a seahorse conformation, hydrophobic interactions and hydrogen bonds with important residues of the binding pocket. Based on in vitro experiments and docking studies, we demonstrated that mutations have little influence in positioning and interactions of THD. Following a rational drug design, we suggest a modification of THD to improve its biological activity.

Synthesis, biological evaluation and molecular modeling of 4,6-diarylpyrimidines and diarylbenzenes as novel non-nucleosides HIV-1 reverse transcriptase inhibitors.

A series of novel 4,6-diarylpyrimidines (4,6-DAPY) and diarylbenzenes (DABE) compounds were synthesized and evaluated as inhibitors of human immunodeficiency virus type-1 (HIV-1). Among them, the most potent HIV-1 inhibitors were 8b, 8d, 14b and 18 (EC(50) = 0.049, 0.381, 0.599 and 0.398 µM, respectively), with HIV-1 inhibitory activity improved or similar to nevirapine (NVP, EC(50) = 0.097 µM) and delavirdine (DEV, EC(50) = 0.55 µM). The other compounds displayed moderate activity (8c, EC(50) = 5.25 µM) or were inactive (8a and 14a) against HIV-1 replication. Molecular modeling studies were performed with the synthesized compounds in complex with the wild-type reverse transcriptase (RT). A correlation was found between the anti-HIV activity and the electrostatic energy of interaction with Lys101 residue. These findings enrich the SAR of these Non-Nucleoside Reverse Transcriptase Inhibitors (NNRTIs) families.

Safety and efficacy of GSK2248761, a next-generation nonnucleoside reverse transcriptase inhibitor, in treatment-naive HIV-1-infected subjects.

GSK2248761 is a novel, once-daily (QD), next-generation nonnucleoside reverse transcriptase inhibitor (NNRTI) with activity against efavirenz-resistant strains. Two phase I/IIa, double-blind, randomized, placebo-controlled studies investigated the antiviral activity, safety, and pharmacokinetics (PK) of several doses of GSK2248761 monotherapy in treatment-naive HIV-infected subjects. In the initial study, 10 subjects (8 active and 2 placebo) per dose received sequentially descending GSK2248761 monotherapy regimens of 800, 400, 200, and 100 mg QD for 7 days. Because a dose-response relationship was not identified, a second study examined a lower, 30-mg QD dose in 8 subjects (6 active and 2 placebo). Adverse events, viral load (VL), PK, and reverse transcriptase mutations were assessed and combined for analysis. Treatment with GSK2248761 for 7 days was well tolerated with no serious adverse events or discontinuations. The mean VL reductions from baseline on day 8 were 0.97, 1.87, 1.84, 1.81, and 1.78 log(10) copies/ml for GSK2248761 doses of 30, 100, 200, 400, and 800 mg QD, respectively. GSK2248761 PK (maximum drug concentration in serum [C(max)], area under the plasma concentration-time curve from 0 h to the end of the dosing interval [AUC(0-τ)], and concentration at the end of the dosing interval [C(τ)]) increased proportionally over the dose range of 30 to 800 mg QD. The relationship between short-term VL change and GSK2248761 PK was best described by a maximum-effect (E(max)) model using C(τ) (E(max) = 2.0; 50% effective concentration [EC(50)] = 36.9 ng/ml). No NNRTI resistance mutations emerged during the study. GSK2248761 at 100 to 800 mg QD for 7 days was well tolerated, demonstrated potent antiviral activity in treatment-naive HIV-infected subjects, and had favorable PK and resistance profiles. GSK2248761 is no longer in clinical development.

Inhibition of HIV-1 reverse transcriptase, toxicological and chemical profile of Calophyllum brasiliense extracts from Chiapas, Mexico.

Calophyllum species are sources of calanolides, which inhibit human immunodeficiency virus type 1 reverse transcriptase (HIV-1 RT). The hexane extract of the leaves from C. brasiliense collected in Soconusco, State of Chiapas, Mexico, analyzed by HPLC showed to contain apetalic acid, calanolides B, and C. It showed potent anti-HIV-1 RT inhibition (IC(50)=20.2 µg/ml), but was not toxic in mice (LD(50)=1.99 g/kg). The histological study of the mice treated at the highest dose revealed no alteration on hepatocytes, and an increase in the number of spleen megakaryocytes. These results suggest this extract is suitable to continue studies for developing a phytodrug against HIV-1.

Lack of high-level resistance mutations in HIV type 1 BF recombinant strains circulating in northeast Brazil.

Abstract The genetic variability and the prevalence of drug resistance-associated mutations (DRAM) of HIV-1 isolates from 50 women and 8 children from Feira de Santana, Bahia, Brazil were investigated. DNA samples were obtained and pol sequences were generated by PCR and direct sequencing. Phylogenetic analysis showed that 39 (67.2%) samples were subtype B, four (6.9%) F, one (1.7%) C, and 14 (24.1%) BF recombinants. Four different BF recombination patterns were detected. Twelve (20.7%) samples shared the same breakpoint within the reverse transcriptase (RT) sequence. Fifty-five (94.8%) isolates showed several resistance-associated mutations in the RT and the protease (PR) genes. Ten (17.2%) isolates presented mutations associated with a high level of resistance: nine (15.5%) to nucleoside RT inhibitors (NRTI), four (6.9%) to nonnucleoside RT inhibitors (NNRTI), and three (5.2%) to PR inhibitors (PIs). Subtype B-infected patients had, on average, 0.5 high-level DRAM per sequence while no mutations were observed in BF recombinants, although the two groups were under ARV for a similar period of time. Our data indicate the predominance of the subtype B, followed by BF recombinants in this population, and the dissemination of a recombinant strain in Bahia, which could be related to adaptive advantages of these variants over the predominant subtype B.

Oral colonization by Candida species in AIDS pediatric patients.

The aim of this study was to assess the prevalence of factors associated with oral colonization by Candida spp. in pediatric patients with AIDS. The sample comprised of 117 children. Clinical status, medicines in use, and laboratory findings were obtained from hospital records; sociodemographic data were given by relatives. A dental examination assessed the prevalence of dental caries. The prevalence of oral colonization by Candida was 62%. Only seven children presented clinical manifestation of oral candidosis despite their high viral load index and low-for-age CD4 count. Candida colonization was directly associated with frequent use of antibiotics (prevalence ratio [PR] = 1.44), sulfa drugs (PR = 1.23), alteration in the oral mucosa (PR = 1.55), and untreated dental caries (PR = 1.93). It was inversely associated with the use of antiretroviral therapies (PR = 0.65). Candida albicans was the most frequently detected species (80%); phenotypic tests did not detect C. dubliniensis strains. This study observed a low prevalence of Candida-related oral lesions in these patients, which is compatible with the hypothesis that antiretroviral medicines may have contributed to reducing oral manifestations from Candida infection. The high prevalence of Candida colonization in HIV+/AIDS children with untreated dental caries reinforces the importance of oral health care in interdisciplinary health units that assist these patients.

Rational approaches for the design of effective human immunodeficiency virus type 1 nonnucleoside reverse transcriptase inhibitors.

The binding of several classes of nonnucleoside reverse transcriptase inhibitors (NNRTIs) to wild-type (wtRT) and K103N mutant (mRT) human immunodeficiency virus type 1 (HIV-1) reverse transcriptase is studied by molecular dynamics and energy decomposition techniques. The imidoylthiourea (ITU), diaryltriazine (DATA), and diarylpyrimidine (DAPY) NNRTIs studied maintain the hydrogen bond with Lys101 during the 3 ns molecular dynamics trajectories. When bound to mRT, all the DAPYs studied establish hydrogen bonds with Glu138; among these, those of the potent inhibitors TMC120 and TMC125 are water-mediated. The molecular interactions of the NNRTIs in the binding pocket are correlated to the drugs' potency. Quantitative free energy analyses show a linear relationship between the van der Waals energetic component and the potency against wtRT. The molecular basis of the interaction between NNRTIs and RT presented here provide quantitative approaches for the design of novel effective anti-HIV drugs.

[New and "old" antiretroviral drugs in Pediatrics: new doses, formulations and associations]. / Nuevos y viejos anti-retrovirales en pediatría: nuevas dosis, presentaciones y asociaciones.

Of the 25 antiretroviral drugs available in the market, only 16 are allowed for prescription in the pediatric patients. The antiretroviral, pertaining to the first three families, used for two decades, remain valid and are important components of antiretroviral therapy in naive children. We describe doses, presentations and current associations for these drugs in children, and also discuss new co-formulations that will reduce the number of doses, improve tolerance and therefore achieve better adherence of pediatric patients.

Nuevos y viejos anti-retrovirales en pediatría: Nuevas dosis, presentaciones y asociaciones / New and “old” antiretroviral drugs in Pediatrics: New doses, formulations and associations

De los 25 anti-retrovirales disponibles en el mercado, sólo 16 están autorizados en la edad pediátrica. Los antiretrovirales, pertenecientes a las tres primeras familias, usados desde hace dos décadas, continúan vigentes y son parte importante de la terapia anti-retroviral en niños naïve. Se describen las dosis, presentaciones y asociaciones actuales de estos fármacos en la edad pediátrica y además se comentan las nuevas co-formulaciones que permitirán disminuir el número de dosis, mejorar la tolerancia y por lo tanto conseguir mejor adherencia de los pacientes pediátricos.

Of the 25 antiretroviral drugs available in the market, only 16 are allowed for prescription in the pediatric patients. The antiretroviral, pertaining to the first three families, used for two decades, remain valid and are important components of antiretroviral therapy in naive children. We describe doses, presentations and current associations for these drugs in children, and also discuss new co-formulations that will reduce the number of doses, improve tolerance and therefore achieve better adherence of pediatric patients.

Resistance profile of the new nucleoside reverse transcriptase inhibitor apricitabine.

Apricitabine is a novel deoxycytidine nucleoside reverse transcriptase inhibitor (NRTI) currently in clinical development for the treatment of HIV infection. Apricitabine shows antiviral activity in vitro against HIV-1 strains and clinical isolates with mutations in the reverse transcriptase that confer resistance to other NRTIs, including M184V, thymidine analogue mutations (TAMs), nucleoside-associated mutations such as L74V and certain mutations at codon 69. Apricitabine has shown activity in treatment-experienced HIV-1-infected patients with NRTI resistance (with M184V and up to five TAMs) as well as in treatment-naive patients. Resistance to apricitabine is slow to develop in vitro and there has been little evidence of development of resistance to apricitabine in clinical use thus far, including patients receiving apricitabine for up to 48 weeks. The resistance profile of apricitabine suggests there is a low potential for cross-resistance with the currently available NRTIs and, thus, apricitabine may provide a treatment option for treatment-experienced HIV-1-infected patients with resistance to other NRTIs. In particular, the activity of apricitabine in the presence of the M184V mutation, which confers high-level resistance to lamivudine and emtricitabine, lends it to being used as a replacement for deoxycytidine analogues in patients who have failed treatment with lamivudine or emtricitabine.