Viral evolution in response to the broad-based retroviral protease inhibitor TL-3

The AutoDock suite at 30

The AutoDock suite provides a comprehensive toolset for computational ligand docking and drug design and development. The suite builds on 30 years of methods development, including empirical free energy force fields, docking engines, methods for site prediction, and interactive tools for visualization and analysis. Specialized tools are available for challenging systems, including covalent inhibitors, peptides, compounds with macrocycles, systems where ordered hydration plays a key role, and systems with substantial receptor flexibility. All methods in the AutoDock suite are freely available for use and reuse, which has engendered the continued growth of a diverse community of primary users and third-party developers.

Selection of Drug-Resistant Feline Immunodeficiency Virus (FIV) Encoding FIV/HIV Chimeric Protease in the Presence of HIV-Specific Protease Inhibitors

An infectious chimeric feline immunodeficiency virus (FIV)/HIV strain carrying six HIV-like protease (PR) mutations (I37V/N55M/V59I/I98S/Q99V/P100N) was subjected to selection in culture against the PR inhibitor lopinavir (LPV), darunavir (DRV), or TL-3. LPV selection resulted in the sequential emergence of V99A (strain S-1X), I59V (strain S-2X), and I108V (strain S-3X) mutations, followed by V37I (strain S-4X). Mutant PRs were analyzed in vitro , and an isogenic virus producing each mutant PR was analyzed in culture for LPV sensitivity, yielding results consistent with the original selection. The 50% inhibitory concentrations (IC 50 s) for S-1X, S-2X, S-3X, and S-4X were 95, 643, 627, and 1,543 nM, respectively. The primary resistance mutations, V99 82 A, I59 50 V, and V37 32 I, are consistent with the resistance pattern developed by HIV-1 under similar selection conditions. While resistance to LPV emerged readily, similar PR mutations causing resistance to either DRV or TL-3 failed to emerge after passage for more than a year. However, a G37D mutation in the nucleocapsid (NC) was observed in both selections and an isogenic G37D mutant replicated in the presence of 100 nM DRV or TL-3, whereas parental chimeric FIV could not. An additional mutation, L92V, near the PR active site in the folded structure recently emerged during TL-3 selection. The L92V mutant PR exhibited an IC 50 of 50 nM, compared to 35 nM for 6s-98S PR, and processed the NC-p2 junction more efficiently, consistent with increased viral fitness. These findings emphasize the role of mutations outside the active site of PR in increasing viral resistance to active-site inhibitors and suggest additional targets for inhibitor development.

Small molecule regulation of protein conformation by binding in the Flap of HIV protease

The fragment indole-6-carboxylic acid (1F1), previously identified as a flap site binder in a fragment-based screen against HIV protease (PR), has been cocrystallized with pepstatin-inhibited PR and with apo-PR. Another fragment, 3-indolepropionic acid (1F1-N), predicted by AutoDock calculations and confirmed in a novel inhibition of nucleation crystallization assay, exploits the same interactions in the flap site in two crystal structures. Both 1F1 and 1F1-N bind to the closed form of apo-PR and to pepstatin:PR. In solution, 1F1 and 1F1-N raise the Tm of apo-PR by 3.5-5 °C as assayed by differential scanning fluorimetry (DSF) and show equivalent low-micromolar binding constants to both apo-PR and pepstatin:PR, assayed by backscattering interferometry (BSI). The observed signal intensities in BSI are greater for each fragment upon binding to apo-PR than to pepstatin-bound PR, consistent with greater conformational change in the former binding event. Together, these data indicate that fragment binding in the flap site favors a closed conformation of HIV PR.

A drug discovery platform: a simplified immunoassay for analyzing HIV protease activity

Although numerous methods for the determination of HIV protease (HIV-PR) activity have been described, new high-throughput assays are required for clinical and pharmaceutical applications due to the occurrence of resistant strains. In this study, a simple enzymatic immunoassay to identify HIV-PR activity was developed based on a Ni(2+)-immobilized His(6)-Matrix-Capsid substrate (H(6)MA-CA) is cleaved by HIV protease-His(6) (HIV-PRH(6)) which removes the CA domain and exposes the free C terminus of MA. Following this cleavage, two monoclonal antibodies specific for either the free C-terminal MA or CA epitope are used to quantify the proteolytic activity using a standard ELISA-based system. Specificity for detection of the HIV-PRH(6) activity was confirmed with addition of protease inhibitor (PI), lopinavir. In addition, the assay was able to detect an HIV-PR variant activity indicating that this assay is capable of assessing viral mutation affect HIV-PR activity. The efficacy of commercially available PIs and their 50% inhibitory concentration (IC(50)) were determined. This assay provides a high-throughput method for both validating the efficiency of new drugs in vitro and facilitating the discovery of new PIs. In addition, it could serve as a method for examining the influence of various mutations in HIV-PRs isolated from drug-resistant strains.

Evaluation of different antiretroviral drug protocols on naturally infected feline immunodeficiency virus (FIV) cats in the late phase of the asymptomatic stage of infection

The aim of this study was to evaluate the efficacy of the antiretrovirals: Zidovudine (ZDV) alone; ZDV + Recombinant Human Interferon-α (rHuIFN-α); ZDV + Lamivudine (3TC) and ZDV + valproic acid (Valp) on naturally feline immunodeficiency virus (FIV)-infected cats, in the late phase of the asymptomatic stage of infection. The follow-up was performed over one year, through clinical evaluation and the determination of viral loads and CD4+/CD8+ ratios. Neurological signs were studied by visual and auditory evoked potentials (VEP, AEP) and the responses were abnormal in 80% of the FIV-infected cats. After one year, an improvement in VEP and AEP was observed in the ZDV + Valp group and a worsening in the group receiving ZDV + rHuIFN-α. The CD4+/CD8+ ratio showed a significant increase (both intra and inter-groups) only in ZDV and ZDV + 3TC, between their pre-treatment and one year values, as well as among the other groups. Viral load only showed a significant decrease in ZDV and ZDV + 3TC groups, when comparing the values at one year of treatment vs. pre-treatment values and when the different groups were compared. In addition, the viral load decrease was significantly more pronounced in the ZDV + 3TC vs. ZDV group. We conclude that ZDV and ZDV + 3TC produce significant reductions in viral load and stimulate a recovery of the CD4+/CD8+ ratio, compared with the other protocols. It is clear that the addition of 3TC resulted in a greater reduction in viral load than use of ZDV as a single drug. Therefore, the combination ZDV + 3TC could be more effective than the sole use of ZDV.

Structural basis for drug and substrate specificity exhibited by FIV encoding a chimeric FIV/HIV protease

A chimeric feline immunodeficiency virus (FIV) protease (PR) has been engineered that supports infectivity but confers sensitivity to the human immunodeficiency virus (HIV) PR inhibitors darunavir (DRV) and lopinavir (LPV). The 6s-98S PR has five replacements mimicking homologous residues in HIV PR and a sixth which mutated from Pro to Ser during selection. Crystal structures of the 6s-98S FIV PR chimera with DRV and LPV bound have been determined at 1.7 and 1.8 Å resolution, respectively. The structures reveal the role of a flexible 90s loop and residue 98 in supporting Gag processing and infectivity and the roles of residue 37 in the active site and residues 55, 57 and 59 in the flap in conferring the ability to specifically recognize HIV PR drugs. Specifically, Ile37Val preserves tertiary structure but prevents steric clashes with DRV and LPV. Asn55Met and Val59Ile induce a distinct kink in the flap and a new hydrogen bond to DRV. Ile98Pro→Ser and Pro100Asn increase 90s loop flexibility, Gln99Val contributes hydrophobic contacts to DRV and LPV, and Pro100Asn forms compensatory hydrogen bonds. The chimeric PR exhibits a comparable number of hydrogen bonds, electrostatic interactions and hydrophobic contacts with DRV and LPV as in the corresponding HIV PR complexes, consistent with IC(50) values in the nanomolar range.

A cleavage enzyme-cytometric bead array provides biochemical profiling of resistance mutations in HIV-1 Gag and protease

Most protease-substrate assays rely on short, synthetic peptide substrates consisting of native or modified cleavage sequences. These assays are inadequate for interrogating the contribution of native substrate structure distal to a cleavage site that influences enzymatic cleavage or for inhibitor screening of native substrates. Recent evidence from HIV-1 isolates obtained from individuals resistant to protease inhibitors has demonstrated that mutations distal to or surrounding the protease cleavage sites in the Gag substrate contribute to inhibitor resistance. We have developed a protease-substrate cleavage assay, termed the cleavage enzyme- cytometric bead array (CE-CBA), which relies on native domains of the Gag substrate containing embedded cleavage sites. The Gag substrate is expressed as a fluorescent reporter fusion protein, and substrate cleavage can be followed through the loss of fluorescence utilizing flow cytometry. The CE-CBA allows precise determination of alterations in protease catalytic efficiency (k(cat)/K(M)) imparted by protease inhibitor resistance mutations in protease and/or gag in cleavage or noncleavage site locations in the Gag substrate. We show that the CE-CBA platform can identify HIV-1 protease present in cellular extractions and facilitates the identification of small molecule inhibitors of protease or its substrate Gag. Moreover, the CE-CBA can be readily adapted to any enzyme-substrate pair and can be utilized to rapidly provide assessment of catalytic efficiency as well as systematically screen for inhibitors of enzymatic processing of substrate.

Identification of broad-based HIV-1 protease inhibitors from combinatorial libraries

Clinically approved inhibitors of the HIV-1 protease function via a competitive mechanism. A particular vulnerability of competitive inhibitors is their sensitivity to increases in substrate concentration, as may occur during virion assembly, budding and processing into a mature infectious viral particle. Advances in chemical synthesis have led to the development of new high-diversity chemical libraries using rapid in-solution syntheses. These libraries have been shown previously to be effective at disrupting protein-protein and protein-nucleic acid interfaces. We have screened 44000 compounds from such a library to identify inhibitors of the HIV-1 protease. One compound was identified that inhibits wild-type protease, as well as a drug-resistant protease with six mutations. Moreover, analysis of this compound suggests an allosteric non-competitive mechanism of inhibition and may represent a starting point for an additional strategy for anti-retroviral therapy.

Generation of infectious feline immunodeficiency virus (FIV) encoding FIV/human immunodeficiency virus chimeric protease

Feline immunodeficiency virus (FIV) and human immunodeficiency virus type 1 (HIV-1) proteases (PRs) share only 23% amino acid identity and exhibit distinct specificities yet have very similar 3-dimensional structures. Chimeric PRs in which HIV residues were substituted in structurally equivalent positions in FIV PR were prepared in order to study the molecular basis of PR specificity. Previous in vitro analyses showed that such substitutions dramatically altered the inhibitor specificity of mutant PRs but changed the rate and specificity of Gag cleavage so that chimeric FIVs were not infectious. Chimeric PRs encoding combinations of the I37V, N55M, M56I, V59I, L97T, I98P, Q99V, and P100N mutations were cloned into FIV Gag-Pol, and those constructs that best approximated the temporal cleavage pattern generated by wild-type FIV PR, while maintaining HIV-like inhibitor specificity, were selected. Two mutations, M56I and L97T, were intolerant to change and caused inefficient cleavage at NC-p2. However, a mutant PR with six substitutions (I37V, N55M, V59I, I98P, Q99V, and P100N) was selected and placed in the context of full-length FIV-34TF10. This virus, termed YCL6, had low-level infectivity ex vivo, and after passage, progeny that exhibited a higher growth rate emerged. The residue at the position of one of the six mutations, I98P, further mutated on passage to either P98H or P98S. Both PRs were sensitive to the HIV-1 PR inhibitors lopinavir (LPV) and darunavir (DRV), as well as to the broad-based inhibitor TL-3, with 50% inhibitory concentrations (IC(50)) of 30 to 40 nM, consistent with ex vivo results obtained using mutant FIVs. The chimeras offer an infectivity system with which to screen compounds for potential as broad-based PR inhibitors, define structural parameters that dictate specificity, and investigate pathways for drug resistance development.

Fragment-based screen against HIV protease

We have employed a fragment-based screen against wild-type (NL4-3) HIV protease (PR) using the Active Sight fragment library and X-ray crystallography. The experiments reveal two new binding sites for small molecules. PR was co-crystallized with fragments, or crystals were soaked in fragment solutions, using five crystal forms, and 378 data sets were collected to 2.3-1.3 A resolution. Fragment binding induces a distinct conformation and specific crystal form of TL-3 inhibited PR during co-crystallization. One fragment, 2-methylcyclohexanol, binds in the 'exo site' adjacent to the Gly(16)Gly(17)Gln(18)loop where the amide of Gly(17)is a specific hydrogen bond donor, and hydrophobic contacts occur with the side chains of Lys(14)and Leu(63). Another fragment, indole-6-carboxylic acid, binds on the 'outside/top of the flap' via hydrophobic contacts with Trp(42), Pro(44), Met(46), and Lys(55), a hydrogen bond with Val(56), and a salt-bridge with Arg(57). 2-acetyl-benzothiophene also binds at this site. This study is the first fragment-based crystallographic screen against HIV PR, and the first time that fragments were screened against an inhibitor-bound drug target to search for compounds that both bind to novel sites and stabilize the inhibited conformation of the target.

A copper(I)-catalyzed 1,2,3-triazole azide-alkyne click compound is a potent inhibitor of a multidrug-resistant HIV-1 protease variant

Treatment with HIV-1 protease inhibitors, a component of highly active antiretroviral therapy (HAART), often results in viral resistance. Structural and biochemical characterization of a 6X protease mutant arising from in vitro selection with compound 1, a C 2-symmetric diol protease inhibitor, has been previously described. We now show that compound 2, a copper(I)-catalyzed 1,2,3-triazole derived compound previously shown to be potently effective against wild-type protease (IC 50 = 6.0 nM), has low nM activity (IC 50 = 15.7 nM) against the multidrug-resistant 6X protease mutant. Compound 2 displays similar efficacy against wild-type and 6X HIV-1 in viral replication assays. While structural studies of compound 1 bound to wild type and mutant proteases revealed a progressive change in binding mode in the mutants, the 1.3 A resolution 6X protease-compound 2 crystal structure reveals nearly identical interactions for 2 as in the wild-type protease complex with very little change in compound 2 or protease conformation.

FIV as a Model for HIV: An Overview

Animal models for human immunodeficiency virus (HIV) infection play a key role in understanding the pathogenesis of AIDS and the development of therapeutic agents and vaccines. As the only lentivirus that causes an immunodeficiency resembling that of HIV infection, in its natural host, feline immunodeficiency virus (FIV) has been a unique and powerful model for AIDS research. FIV was first described in 1987 by Niels Pedersen and co-workers as the causative agent for a fatal immunodeficiency syndrome observed in cats housed in a cattery in Petaluma, California. Since this landmark observation, multiple studies have shown that natural and experimental infection of cats with biological isolates of FIV produces an AIDS syndrome very similar in pathogenesis to that observed for human AIDS. FIV infection induces an acute viremia associated with Tcell alterations including depressed CD4 :CD8 T-cell ratios and CD4 T-cell depletion, peripheral lymphadenopathy, and neutropenia. In later stages of FIV infection, the host suffers from chronic persistent infections that are typically self-limiting in an immunocompetent host, as well as opportunistic infections, chronic diarrhea and wasting, blood dyscracias, significant CD4 T-cell depletion, neurologic disorders, and B-cell lymphomas. Importantly, chronic FIV infection induces a progressive lymphoid and CD4 T-cell depletion in the infected cat. The primary mode of natural FIV transmission appears to be blood-borne facilitated by fighting and biting. However, experimental infection through transmucosal routes (rectal and vaginal mucosa and perinatal) have been well documented for specific FIV isolates. Accordingly, FIV disease pathogenesis exhibits striking similarities to that described for HIV-1 infection.

A look inside HIV resistance through retroviral protease interaction maps

Retroviruses affect a large number of species, from fish and birds to mammals and humans, with global socioeconomic negative impacts. Here the authors report and experimentally validate a novel approach for the analysis of the molecular networks that are involved in the recognition of substrates by retroviral proteases. Using multivariate analysis of the sequence-based physiochemical descriptions of 61 retroviral proteases comprising wild-type proteases, natural mutants, and drug-resistant forms of proteases from nine different viral species in relation to their ability to cleave 299 substrates, the authors mapped the physicochemical properties and cross-dependencies of the amino acids of the proteases and their substrates, which revealed a complex molecular interaction network of substrate recognition and cleavage. The approach allowed a detailed analysis of the molecular-chemical mechanisms involved in substrate cleavage by retroviral proteases.

Crystal structure of an FIV/HIV chimeric protease complexed with the broad-based inhibitor, TL-3

We have obtained the 1.7 Å crystal structure of FIV protease (PR) in which 12 critical residues around the active site have been substituted with the structurally equivalent residues of HIV PR (12X FIV PR). The chimeric PR was crystallized in complex with the broad-based inhibitor TL-3, which inhibits wild type FIV and HIV PRs, as well as 12X FIV PR and several drug-resistant HIV mutants [1-4]. Biochemical analyses have demonstrated that TL-3 inhibits these PRs in the order HIV PR > 12X FIV PR > FIV PR, with K_i values of 1.5 nM, 10 nM, and 41 nM, respectively [2-4]. Comparison of the crystal structures of the TL-3 complexes of 12X FIV and wild-typeFIV PR revealed theformation of additinal van der Waals interactions between the enzyme inhibitor in the mutant PR. The 12X FIV PR retained the hydrogen bonding interactions between residues in the flap regions and active site involving the enzyme and the TL-3 inhibitor in comparison to both FIV PR and HIV PR. However, the flap regions of the 12X FIV PR more closely resemble those of HIV PR, having gained several stabilizing intra-flap interactions not present in wild type FIV PR. These findings offer a structural explanation for the observed inhibitor/substrate binding properties of the chimeric PR.

Altered gag polyprotein cleavage specificity of feline immunodeficiency virus/human immunodeficiency virus mutant proteases as demonstrated in a cell-based expression system

We have used feline immunodeficiency virus (FIV) protease (PR) as a mutational system to study the molecular basis of substrate-inhibitor specificity for lentivirus PRs, with a focus on human immunodeficiency virus type 1 (HIV-1) PR. Our previous mutagenesis studies demonstrated that discrete substitutions in the active site of FIV PR with structurally equivalent residues of HIV-1 PR dramatically altered the specificity of the mutant PRs in in vitro analyses. Here, we have expanded these studies to analyze the specificity changes in each mutant FIV PR expressed in the context of the natural Gag-Pol polyprotein ex vivo. Expression mutants were prepared in which 4 to 12 HIV-1-equivalent substitutions were made in FIV PR, and cleavage of each Gag-Pol polyprotein was then assessed in pseudovirions from transduced cells. The findings demonstrated that, as with in vitro analyses, inhibitor specificities of the mutants showed increased HIV-1 PR character when analyzed against the natural substrate. In addition, all of the mutant PRs still processed the FIV polyprotein but the apparent order of processing was altered relative to that observed with wild-type FIV PR. Given the importance of the order in which Gag-Pol is processed, these findings likely explain the failure to produce infectious FIVs bearing these mutations.

Use of a novel assay based on intact recombinant viruses expressing green (EGFP) or red (DsRed2) fluorescent proteins to examine the contribution of pol and env genes to overall HIV-1 replicative fitness

Multiple studies have described a reduction in the replicative fitness of HIV-1 isolates harboring mutations that confer resistance to antiretroviral drugs. Contradictory results, however, have been obtained depending on the methodology used in each study (Quinones-Mateu, M.E., Arts, E.J., 2002. Fitness of drug resistant HIV-I: methodology and clinical implications. Drug Resist. Update 5, 224-233), affecting our understanding of the potential relationship of viral replicative fitness with HIV-1 disease. It has been demonstrated previously that both pol and env genes play a major role in HIV-1 replicative fitness of clinical isolates. Therefore, measuring clinically relevant replicative fitness using recombinant viruses where a single mutation and/or viral gene have been introduced does not seem like a reasonable approach in this era of multi-target antiretroviral therapy. A novel method was developed to measure HIV-1 replicative fitness based on recombinant viruses expressing the enhanced green fluorescent (EGFP) or the Discosoma sp. red fluorescent (DsRed2) proteins in a HIV-1NL4-3 backbone. Contrary to previous designs to analyze HIV-1 fitness, these replication competent viruses were created in an intact viral genetic background (without deleting or affecting the expression of any viral gene). This new system was used to evaluate the contribution of drug-resistance mutations in the pol and env genes to overall viral replicative fitness (in the presence and absence of drug pressure) using direct growth competition experiments. Mutations in pol showed a stronger effect on HIV-1 replicative fitness than mutations in the env gene associated with resistance to enfuvirtide, corroborating the plasticity of the later gene to accept mutations and the sensibility of the protease and reverse transcriptase enzymes to drug-associated primary mutations. In conclusion, a new protocol was used to measure HIV-1 replicative fitness in either the presence or absence of antiretroviral drugs, which may be used as a high-throughput assay to help us understand the clinical significance of viral fitness.

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.

Structural insights into the mechanisms of drug resistance in HIV-1 protease NL4-3

The development of resistance to anti-retroviral drugs targeted against HIV is an increasing clinical problem in the treatment of HIV-1-infected individuals. Many patients develop drug-resistant strains of the virus after treatment with inhibitor cocktails (HAART therapy), which include multiple protease inhibitors. Therefore, it is imperative that we understand the mechanisms by which the viral proteins, in particular HIV-1 protease, develop resistance. We have determined the three-dimensional structure of HIV-1 protease NL4-3 in complex with the potent protease inhibitor TL-3 at 2.0 A resolution. We have also obtained the crystal structures of three mutant forms of NL4-3 protease containing one (V82A), three (V82A, M46I, F53L) and six (V82A, M46I, F53L, V77I, L24I, L63P) point mutations in complex with TL-3. The three protease mutants arose sequentially under ex vivo selective pressure in the presence of TL-3, and exhibit fourfold, 11-fold, and 30-fold resistance to TL-3, respectively. This series of protease crystal structures offers insights into the biochemical and structural mechanisms by which the enzyme can overcome inhibition by TL-3 while recovering some of its native catalytic activity.

Assessment of FIV-C infection of cats as a function of treatment with the protease inhibitor, TL-3

Background

The protease inhibitor, TL-3, demonstrated broad efficacy in vitro against FIV, HIV and SIV (simian immunodeficiency virus), and exhibited very strong protective effects on early neurologic alterations in the CNS of FIV-PPR infected cats. In this study, we analyzed TL-3 efficacy using a highly pathogenic FIV-C isolate, which causes a severe acute phase immunodeficiency syndrome, with high early mortality rates.

Results

Twenty cats were infected with uncloned FIV-C and half were treated with TL-3 while the other half were left untreated. Two uninfected cats were used as controls. The general health and the immunological and virological status of the animals was monitored for eight weeks following infection. All infected animals became viremic independent of TL-3 treatment and seven of 20 FIV-C infected animals developed severe immunodepletive disease in conjunction with significantly (p ≤ 0.05) higher viral RNA loads as compared to asymptomatic animals. A marked and progressive increase in CD8⁺ T lymphocytes in animals surviving acute phase infection was noted, which was not evident in symptomatic animals (p ≤ 0.05). Average viral loads were lower in TL-3 treated animals and of the 6 animals requiring euthanasia, four were from the untreated cohort. At eight weeks post infection, half of the TL-3 treated animals and only one of six untreated animals had viral loads below detection limits. Analysis of protease genes in TL-3 treated animals with higher than average viral loads revealed sequence variations relative to wild type protease. In particular, one mutant, D105G, imparted 5-fold resistance against TL-3 relative to wild type protease.

Conclusions

The findings indicate that the protease inhibitor, TL-3, when administered orally as a monotherapy, did not prevent viremia in cats infected with high dose FIV-C. However, the modest lowering of viral loads with TL-3 treatment, the greater survival rate in symptomatic animals of the treated cohort, and the lower average viral load in TL-3 treated animals at eight weeks post infection is indicative of a therapeutic effect of the compound on virus infection.

Resolution and prevention of feline immunodeficiency virus-induced neurological deficits by treatment with the protease inhibitor TL-3

In vivo tests were performed to assess the influence of the protease inhibitor TL-3 on feline immunodeficiency virus (FIV)-induced central nervous system (CNS) deficits. Twenty cats were divided into four groups of five animals each. Group 1 received no treatment, group 2 received TL-3 only, group 3 received FIV strain PPR (FIV-PPR) only, and group 4 received FIV-PPR and TL-3. Animals were monitored for immunological and virological status, along with measurements of brain stem auditory evoked potential (BAEP) changes. Groups 1 and 2 remained FIV negative, and groups 3 and 4 became virus positive and seroconverted by 3 to 5 weeks postinoculation. No adverse effects were noted with TL-3 only. The average peak viral load for the virus-only group 3 animals was 1.32 x 10(6) RNA copies/ml, compared to 6.9 x 10(4) copies/ml for TL-3-treated group 4 cats. Group 3 (virus-only) cats exhibited marked progressive delays in BAEPs starting at 2 weeks post virus exposure, which is typical of infection with FIV-PPR. In contrast, TL-3-treated cats of group 4 exhibited BAEPs similar to those of control and drug-only cats. At 97 days postinfection, treatments were switched; i.e., group 4 animals were taken off TL-3 and group 3 animals were treated with TL-3. BAEPs in group 3 animals returned to control levels, while BAEPs in group 4 animals remained at control levels. After 70 days on TL-3, group 3 was removed from the drug treatment regimen. Delays in BAEPs immediately increased to levels observed prior to TL-3 treatment. The findings show that early TL-3 treatment can effectively eliminate FIV-induced changes in the CNS. Furthermore, TL-3 can counteract FIV effects on the CNS of infected cats, although continued treatment is required to maintain unimpaired CNS function.

Fitness of drug resistant HIV-1: methodology and clinical implications

Recent studies of human immunodeficiency virus type 1 (HIV-1) fitness have examined the potential relationship with plasma viral load, drug resistance, and disease progression. For example, treatment of HIV-1 infected individuals with antiretroviral drugs may result in the selection and emergence of inhibitor-resistant variants with reduced replicative capacity. However, it is still unclear whether in vitro HIV-1 fitness has any direct relationship to in vivo disease progression or treatment success. A related question is which in vitro assay of viral fitness is the most appropriate for comparison with in vivo HIV-1 fitness. Characterization of the relative viral fitness of drug-resistant HIV-1 strains may lead to a better understanding of whether or not less fit viruses pose a clinical benefit to the patient.