Biomedical subjects

Terri Wrin

Publications and source records attributed to Terri Wrin.

At least 37 records · Page 2Linked to original sources

PubMed2004

Neutralization profiles of newly transmitted human immunodeficiency virus type 1 by monoclonal antibodies 2G12, 2F5, and 4E10.

As the AIDS epidemic continues unabated, the development of a human immunodeficiency virus (HIV) vaccine is critical. Ideally, an effective vaccine should elicit cell-mediated and neutralizing humoral immune responses. We have determined the in vitro susceptibility profile of sexually transmitted viruses from 91 patients with acute and early HIV-1 infection to three monoclonal antibodies, 2G12, 2F5, and 4E10. Using a recombinant virus assay to measure neutralization, we found all transmitted viruses were neutralized by 4E10, 80% were neutralized by 2F5, and only 37% were neutralized by 2G12. We propose that the induction of 4E10-like antibodies should be a priority in designing immunogens to prevent HIV-1 infection.

Antibodies, Monoclonal↗

PubMed2004

HIV evolution and escape.

Human immunodeficiency virus (HIV) exemplifies the principles of Darwinian evolution with a telescoped chronology. Because of its high mutation rate and remarkably high rates of replication, evolution can be appreciated over periods of days in contrast to the durations conceived of by Darwin. Certain selective pressures that drive the evolution of HIV include chemotherapy, anatomic compartmentalization and the immune response. Examples of these selective forces on HIV evolution are described.

AIDS Vaccines↗

PubMed2003

Rapid evolution of the neutralizing antibody response to HIV type 1 infection.

A recombinant virus assay was used to characterize in detail neutralizing antibody responses directed at circulating autologous HIV in plasma. Examining serial plasma specimens in a matrix format, most patients with primary HIV infection rapidly generated significant neutralizing antibody responses to early (0-39 months) autologous viruses, whereas responses to laboratory and heterologous primary strains were often lower and delayed. Plasma virus continually and rapidly evolved to escape neutralization, indicating that neutralizing antibody exerts a level of selective pressure that has been underappreciated based on earlier, less comprehensive characterizations. These data argue that neutralizing antibody responses account for the extensive variation in the envelope gene that is observed in the early months after primary HIV infection.

Acquired Immunodeficiency Syndrome↗

PubMed2003

Prolonged CD4+ cell/virus load discordance during treatment with protease inhibitor-based highly active antiretroviral therapy: immune response and viral control.

Mechanisms that underly discordant CD4+ cell/virus load (VL) responses in patients who receive highly active antiretroviral therapy (HAART) were studied in 30 human immunodeficiency virus (HIV)-positive patients, in 3 groups. Discordant responders maintained CD4+ cell levels >200/mm(3) with stable or increasing trend, despite sustained VLs of 500-5000 copies/mL, for >2 years. Treatment-success patients had CD4+ cell counts >200/mm(3) with stable or increasing trend and VLs <50 copies/mL, for >2 years. Treatment-failure patients initially responded to HAART, followed by decreasing CD4+ cell counts and increasing VLs. Interferon-gamma production to gag and noncytolytic CD8+ cell suppressive activity were greater in discordant responders. Cellular activation was greatest in patients with treatment failure. All discordant responders had non-syncytium-inducing (CCR5-tropic) viruses. Viruses from discordant responders and from patients with treatment failure had extensive resistance mutations; discordant responders had significantly lower viral replication capacities. These findings suggest that discordant responses may be related to enhanced HIV-directed immune responses, diminished cellular activation, decreased viral replication capacity, and preservation of non-syncytium-inducing virus strains.

Adult↗

PubMed2003

Persistence of drug-resistant HIV-1 after a structured treatment interruption and its impact on treatment response.

OBJECTIVE: Among treated patients with drug-resistant viremia, structured treatment interruptions often result in the re-emergence of drug-susceptible HIV-1. Theoretically, this may allow for a more durable response to salvage therapy. We therefore studied the long-term treatment outcome to antiretroviral therapy in a cohort of patients who had previously interrupted therapy, focusing on the determinants of treatment success versus failure. DESIGN: A prospective observational study of the response to antiretroviral therapy in patients resuming therapy after a treatment interruption. Virological and immunological studies were performed every month for 3 months and then every 3 months. RESULTS: Twenty-four patients underwent a structured treatment interruption and resumed therapy after a variable period of time (median 20 weeks). The median duration of treatment after the treatment interruption was 109 weeks. A transient virological response was observed in all patients who resumed a regimen containing no drug to which their pre-interruption virus was fully susceptible. Virus isolated during virological failure was genotypically and phenotypically identical to the pre-interruption virus, exhibited reduced replicative capacity, and replicated at levels similar to the pre-interruption baseline. In contrast, durable viral suppression (< 200 copies/ml) was observed in patients who initiated a regimen containing only one drug to which their pre-interruption virus was fully susceptible. Despite viral suppression, the pre-interruption drug-resistant virus population remained detectable in two patients. CONCLUSION: Although drug-resistant HIV-1 persists at low levels during and after the interruption of therapy, durable suppression of this virus population may be achieved with a combination regimen containing only one fully active agent.

Adult↗

PubMed2003

Identification of a transmission chain of HIV type 1 containing drug resistance-associated mutations.

We have investigated a potential transmission chain of HIV-1 with drug resistance-associated mutations between three individuals over a period of 5 years by use of cloning and sequencing of viral genes, and phenotypic characterization. Viruses containing reverse transcriptase drug resistance-associated mutations were transmitted sequentially between three homosexual men (A, B, and C), and persisted in one individual for at least 4 years, despite intermittent therapy and reduced viral replicative capacity compared with wild-type strains. Clonal analysis of the envelope gene from semen and blood virus showed that the virus transmitted to patient C was more closely related to virus from the semen than the blood of patient B. Our data suggest that HIV variants with drug resistance-associated mutations can persist following primary infection, despite intervening antiretroviral therapy, and subsequently sexually transmitted. We provide "proof of principle" that such mutations can therefore become "fixed" within the circulating virus pool.

Anti-HIV Agents↗

PubMed2003

Practical applications of viral fitness in clinical practice.

PURPOSE OF REVIEW: To review recent clinical data regarding viral fitness and its potential utility in clinical practice. Therapeutic failure of antiretroviral regimens is often due to the development of drug resistance. The viral loads achieved by these drug-resistant variants frequently remain suppressed below pretreatment levels. Furthermore, many patients with virologic failure continue to maintain stable CD4 counts and remain clinically well. One possible explanation for these observations is that the resistant virus that emerges during failure of highly active antiretroviral therapy is less fit than its wild-type counterpart as a result of the requirement to maintain resistance mutations that allow it to replicate in the presence of antiretroviral drugs. Since there are many patients with limited treatment options due to extensive resistance or drug toxicities, or both, there is intense interest in exploring the possibility that viral fitness can be exploited for clinical benefit. This review describes the assays that are used to measure fitness and replication capacity, and investigations of their utility in clinical practice. RECENT FINDINGS: A number of methods have been developed to measure viral fitness. Replication kinetic and competitive culture assays are accurate, but labour and time intensive. The availability of a rapid, single-cycle recombinant assay that measures viral replication capacity presents the possibility that assessments of viral fitness could be incorporated into clinical management. The replication capacity assay appears to correlate with more standard measures of viral fitness, and recently accumulated data in both wild-type and drug resistant viral populations suggest that replication capacity describes an intrinsic characteristic of HIV-1. A number of investigations of the clinical utility of fitness and replication capacity assays have established correlations between these measurements and virologic and immunologic outcomes. SUMMARY: Much progress has been made in the past year in our understanding of viral fitness and replication capacity. Assays that measure these viral characteristics have the potential to expand the range of clinical strategies employed against HIV-1. Further investigations are needed to firmly establish the clinical utility of these assays.

Carbamates↗

PubMed2003

A novel TaqMan real-time PCR assay to estimate ex vivo human immunodeficiency virus type 1 fitness in the era of multi-target (pol and env) antiretroviral therapy.

Despite numerous studies on human immunodeficiency virus type 1 (HIV-1) fitness, many key conceptual and technical questions are still unsolved. For example, the proper system to determine virus fitness of HIV-1 is still unknown. In this study, an assay was developed to estimate HIV-1 fitness based on growth competition experiments and TaqMan real-time PCR. This novel technique was compared with several methods (i.e. virus growth kinetics, growth competition/heteroduplex-tracking analysis and single-cycle replication capacity assay) in order to analyse the impact of various genomic regions and overall genetic background on virus fitness. HIV-1 primary isolates and three different sets of recombinant viruses [i.e. recombinant clones carrying protease (PR), reverse transcriptase (RT) or the 3' end of Gag, PR and RT (3'Gag/PR/RT), sequences amplified by PCR from the same primary isolates)] were evaluated. Here, it is demonstrated that, in spite of intrinsic differences, both growth competition/TaqMan and single-cycle replication assays detected a significant reduction in HIV-1 fitness as a consequence of drug-resistant mutations in pol. However, this new assay, based on HIV-1 isolates, may be useful to quantify replicative fitness in viruses from patients treated simultaneously with antiretroviral drugs targeting different genomic regions of HIV-1 (e.g. pol and env).

Anti-HIV Agents↗

PubMed2003

Infectivity and replication capacity of drug-resistant human immunodeficiency virus type 1 variants isolated during primary infection.

It is believed that replication capacity is an important determinant of human immunodeficiency virus type 1 (HIV-1) pathogenicity and transmissibility. To explore this, we conducted a comprehensive analysis of the replication properties of nine drug-resistant and nine drug-susceptible viral isolates derived from patients with primary HIV-1 infection. Viral isolates were tested for single-cycle infectivity in the GHOST cell line. The infectivity of isolates carrying resistance-associated mutations was significantly higher than that of drug-susceptible isolates. Additionally, the growth kinetics of these isolates were determined in CD4+ T lymphocytes. Drug-resistant isolates replicated as well as drug-susceptible viruses. Insertion of the resistance-conferring regions into an NL4-3-based molecular background resulted in chimeras that displayed a modest but significant reduction in replication capacity compared to the drug-susceptible chimeric viruses. Of note, two multidrug-resistant isolates and one protease inhibitor-resistant isolate displayed higher rates of infectivity and growth kinetics than the other drug-resistant or drug-susceptible isolates. These distinct replicative features, however, were not seen in the corresponding chimeras, indicating that changes within the C-terminal region of Gag as well as within the protease and reverse transcriptase genes contribute to but are not sufficient for the level of compensatory adaptation observed. These findings suggest that some drug-resistant viruses isolated during primary infection possess unique adaptive changes that allow for both high viral replication capacity and resistance to one or more classes of antiretroviral drugs. Further studies are needed to elucidate the precise regions that are essential for these characteristics.

Amino Acid Substitution↗

PubMed2003

Access of antibody molecules to the conserved coreceptor binding site on glycoprotein gp120 is sterically restricted on primary human immunodeficiency virus type 1.

Anti-human immunodeficiency virus type 1 (HIV-1) antibodies whose binding to gp120 is enhanced by CD4 binding (CD4i antibodies) are generally considered nonneutralizing for primary HIV-1 isolates. However, a novel CD4i-specific Fab fragment, X5, has recently been found to neutralize a wide range of primary isolates. To investigate the precise nature of the extraordinary neutralizing ability of Fab X5, we evaluated the abilities of different forms (immunoglobulin G [IgG], Fab, and single-chain Fv) of X5 and other CD4i monoclonal antibodies to neutralize a range of primary HIV-1 isolates. Our results show that, for a number of isolates, the size of the neutralizing agent is inversely correlated with its ability to neutralize. Thus, the poor ability of CD4i-specific antibodies to neutralize primary isolates is due, at least in part, to steric factors that limit antibody access to the gp120 epitopes. Studies of temperature-regulated neutralization or fusion-arrested intermediates suggest that the steric effects are important in limiting the binding of IgG to the viral envelope glycoproteins after HIV-1 has engaged CD4 on the target cell membrane. The results identify hurdles in using CD4i epitopes as targets for antibody-mediated neutralization in vaccine design but also indicate that the CD4i regions could be efficiently targeted by small molecule entry inhibitors.

Antibodies, Monoclonal↗

PubMed2003

Relationship between in vitro human immunodeficiency virus type 1 replication rate and virus load in plasma.

Although plasma human immunodeficiency virus type 1 (HIV-1) RNA concentration is a major determinant of the rate of HIV-1 disease progression, the reasons for variability in plasma virus loads among infected individuals are not fully understood. We conducted investigations with 15 HIV-1-infected individuals who were not receiving antiretroviral therapy to evaluate the hypothesis that HIV-1 replication rate in vitro is a significant determinant of plasma virus load. Virus could not be isolated from one subject. Two subjects were excluded because they had features previously associated with distinct plasma virus loads and altered rates of disease progression; one harbored a syncytium-inducing virus and the second was heterozygous for a 32-bp deletion from the CCR5 gene. HIV-1 replication rates were determined by culturing autologous virus isolates in phytohemagglutinin-treated peripheral blood mononuclear cells (PBMC) and determining the rate of p24 antigen production during the logarithmic phase of viral replication. The contribution of HIV-1 reverse transcriptase (RT) and protease (PR) alleles to replication capacity was assessed using recombinant viruses in a single-cycle infection assay. HIV-1 replication rates ranged from 0.15 to 0.76 log(10) pg/ml/day and were reproducible within the same donor PBMC (coefficient of variation +/- 4%). RT-PR replication capacity ranged from 14 to 95% of that of control virus and was linearly related to replication rate (r(2) = 0.53; P = 0.007). Plasma HIV-1 RNA concentration was linearly related to replication rate (r(2) = 0.71; P < 0.001) and RT-PR replication capacity (r(2) = 0.44; P = 0.019). These data suggest that different RT-PR alleles are important determinants of HIV-1 replication rates and that HIV-1 replication rate explains much of the variability in plasma virus load in chronic HIV-1 infection.

Adult↗

PubMed2002

Hypersusceptibility to non-nucleoside reverse transcriptase inhibitors in HIV-1: clinical, phenotypic and genotypic correlates.

OBJECTIVE: The routine use of phenotypic drug resistance testing in patient management has revealed that many HIV-1 strains possess significantly increased drug sensitivity, or 'hypersusceptibility' compared with wild-type viruses. This study describes hypersusceptibility to non-nucleoside reverse transcriptase inhibitors (NNRTI) and was designed to determine the prevalence of and viral characteristics associated with NNRTI hypersusceptibility in patient-derived viruses. METHODS: Retrospective analyses were performed on a large clinical laboratory dataset containing phenotypic drug susceptibility and genotypic sequence results from HIV-1 patient isolates. Genetically engineered viruses were used to confirm the role of certain nucleoside reverse transcriptase inhibitor (NRTI)-resistance mutations in NNRTI hypersusceptibility. RESULTS: Hypersusceptibility to delavirdine, efavirenz and nevirapine was detected in 10.7, 10.8 and 8.0% of more than 17,000 consecutive plasma samples submitted for phenotypic susceptibility testing. In analyses limited to a subset of viruses derived from patients with known treatment histories, NNRTI hypersusceptibility was observed significantly more frequently among viruses from NRTI experienced/NNRTI-naive patients compared with viruses from NRTI/NNRTI-naive patients. Significant inverse correlations between NRTI and NNRTI susceptibility exist among the viruses from NRTI-experienced patients. Analyses of viruses classified according to their NNRTI susceptibility identified 18 positions in reverse transcriptase where substitutions were significantly associated with NNRTI hypersusceptibility. CONCLUSIONS: NNRTI hypersusceptibility is common among patient HIV-1 isolates, especially in NRTI-resistant viruses. Genotypic correlates of hypersusceptibility are complex and not easily defined by a simple analysis of NRTI-associated resistance mutations. NNRTI hypersusceptibility may provide an explanation for the superior virologic response to NNRTI-containing salvage regimens observed in NRTI-experienced patients in several clinical trials.

Anti-HIV Agents↗

PubMed2002

Amprenavir-resistant HIV-1 exhibits lopinavir cross-resistance and reduced replication capacity.

OBJECTIVES: To evaluate protease inhibitor (PI) cross-resistance and reductions in replication capacity conferred by amprenavir-selected mutations. METHODS: HIV-1IIIB variants derived from passage in increasing concentrations of amprenavir were studied, as well as 3'Gag/protease recombinants derived from them. These strains progressively accumulated mutations at codons 10, 46, 47, 50 and 84 in the protease as well as a p1/p6 cleavage site mutation at codon 449 in Gag. Their susceptibility (IC50) to various PI and their corresponding replication capacities were evaluated by a single-cycle growth assay and compared with measures using competitive cultures and p24 antigen production. RESULTS: Amprenavir susceptibility decreased with increasing numbers of protease mutations. Changes in lopinavir susceptibility paralleled changes in amprenavir susceptibility. Certain amprenavir-selected mutants conferred greater than 10-fold cross-resistance to lopinavir, including PrL10F/M46I/I50V-GagL449F (19-fold) and PrL10F/M46I/I47V/I50V-GagL449F (31-fold). Moreover, one isolate with only two mutations in the protease (L10F/84V) and GagL449F displayed a 7.7-fold increase in lopinavir IC50. Low-level cross-resistance to ritonavir and nelfinavir was also observed. The replication capacity of viruses containing either I84V or I50V was at least 90% lower than the reference virus in the single-cycle assay. The order of relative replication capacity was wild-type > L10F > L10F/I84V > L10F/M46I/I50V > L10F/M46I/I47V/I50V. CONCLUSION: These results indicate that until more comprehensive genotype-phenotype correlations between amprenavir and lopinavir susceptibility are established, phenotypic testing may be preferable to genotyping to detect cross-resistance, and should be considered when switching patients from a failing amprenavir-containing regimen. This study also provides data on the concordance of replication capacity measurements generated using rapid single-cycle growth and competition assays.

Carbamates↗

PubMed2002

Selection of drug-resistant HIV-1 mutants in response to repeated structured treatment interruptions.

BACKGROUND: A new HIV-1 treatment strategy based on repeated structured treatment interruptions (STI) is currently being evaluated in clinical trials to determine whether immune cell-mediated control of viral replication can be stimulated by intermittent periods of viral replication. The potential for selection of drug-resistant quasi-species remains a major concern of such a treatment strategy. METHODS: Plasma and peripheral blood lymphocyte (PBL) samples from 12 patients who had three consecutive STIs were studied. Genotypic analysis was based on population and clonal sequencing. Drug susceptibility and their corresponding replication capacities were evaluated by a single-cycle growth assay. RESULTS: Consistent with a loss of phenotypic susceptibility to lamivudine, the M184V mutation was detected by genotypic analysis (direct and clonal sequencing) in plasma samples collected from two patients at the end of the second or third STI. Longitudinal analysis of patient samples revealed a step-wise increase in the M184V mutation in each patient virus population over successive STIs, despite the lower replicative capacity associated with this mutation in the absence of antiviral agents. CONCLUSION: Drug-resistant virus can rise to high frequencies in chronically HIV-1 infected individuals during consecutive STIs. Evolution of resistance is likely to be more important in patients with prior suboptimal therapies, particularly when few mutations are required for resistance. Maximum care should be taken in designing STI protocols that minimize development of drug-resistant mutations that may lead to treatment failure. Thus, drug-resistance testing may be useful before restarting treatment during STI studies.

Adult↗

PubMed2002

CD4+ T cell kinetics and activation in human immunodeficiency virus-infected patients who remain viremic despite long-term treatment with protease inhibitor-based therapy.

T cell dynamics were studied in human immunodeficiency virus-infected patients who continued using antiretroviral therapy despite detectable plasma viremia (RNA copies >2500 /mL). CD4(+) cell fractional replacement rates, measured by the deuterated glucose technique, were lower in treated patients with detectable viremia than in untreated patients and were similar to those in patients with undetectable viremia. Cell cycle and activation markers exhibited similar trends. For any level of viremia, CD4(+) cell fractional replacement rates were lower in patients with drug-resistant virus than in patients with wild-type virus, which suggests that the resistant variant was less virulent. Interruption of treatment in patients with drug-resistant viremia resulted in increased CD4(+) cell activation, increased CD4(+) cell turnover, and decreased CD4(+) cell counts. These data indicate that partial virus suppression reduces CD4(+) cell turnover and activation, thereby resulting in sustained CD4(+) cell gains, and that measurements of T cell dynamics may provide an in vivo marker of viral virulence.

Adult↗

PubMed2002

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

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

Adenosine Triphosphate↗

PubMed2002

Changes in human immunodeficiency virus type 1 Gag at positions L449 and P453 are linked to I50V protease mutants in vivo and cause reduction of sensitivity to amprenavir and improved viral fitness in vitro.

Human immunodeficiency virus type 1 (HIV-1) Gag protease cleavage sites (CS) undergo sequence changes during the development of resistance to several protease inhibitors (PIs). We have analyzed the association of sequence variation at the p7/p1 and p1/p6 CS in conjunction with amprenavir (APV)-specific protease mutations following PI combination therapy with APV. Querying a central resistance data repository resulted in the detection of significant associations (P < 0.001) between the presence of APV protease signature mutations and Gag L449F (p1/p6 LP1'F) and P453L (p1/p6 PP5'L) CS changes. In population-based sequence analyses the I50V mutant was invariably linked to either L449F or P453L. Clonal analysis revealed that both CS mutations were never present in the same genome. Sequential plasma samples from one patient revealed a transition from I50V M46L P453L viruses at early time points to I50V M46I L449F viruses in later samples. Various combinations of the protease and Gag mutations were introduced into the HXB2 laboratory strain of HIV-1. In both single- and multiple-cycle assay systems and in the context of I50V, the L449F and P453L changes consistently increased the 50% inhibitory concentration of APV, while the CS changes alone had no measurable effect on inhibitor sensitivity. The decreased in vitro fitness of the I50V mutant was only partially improved by addition of either CS change (I50V M46I L449F mutant replicative capacity approximately 16% of that of wild-type virus). Western blot analysis of Pr55 Gag precursor cleavage products from infected-cell cultures indicated accumulation of uncleaved Gag p1-p6 in all I50V viruses without coexisting CS changes. Purified I50V protease catalyzed cleavage of decapeptides incorporating the L449F or P453L change 10-fold and 22-fold more efficiently than cleavage of the wild-type substrate, respectively. HIV-1 protease CS changes are selected during PI therapy and can have effects on both viral fitness and phenotypic resistance to PIs.

Binding Sites↗

PubMed2002

Evolution of phenotypic drug susceptibility and viral replication capacity during long-term virologic failure of protease inhibitor therapy in human immunodeficiency virus-infected adults.

Continued use of antiretroviral therapy despite the emergence of drug-resistant human immunodeficiency virus (HIV) has been associated with the durable maintenance of plasma HIV RNA levels below pretherapy levels. The factors that may account for this partial control of viral replication were assessed in a longitudinal observational study of 20 HIV-infected adults who remained on a stable protease inhibitor-based regimen despite ongoing viral replication (plasma HIV RNA levels consistently >500 copies/ml). Longitudinal plasma samples (n = 248) were assayed for drug susceptibility and viral replication capacity (measured by using a single-cycle recombinant-virus assay). The initial treatment-mediated decrease in plasma viremia was directly proportional to the reduction in replicative capacity (P = 0.01). Early virologic rebound was associated the emergence of a virus population exhibiting increased protease inhibitor phenotypic resistance, while replicative capacity remained low. During long-term virologic failure, plasma HIV RNA levels often remained stable or increased slowly, while phenotypic resistance continued to increase and replicative capacity decreased slowly. The emergence of primary genotypic mutations within protease (particularly V82A, I84V, and L90M) was temporally associated with increasing phenotypic resistance and decreasing replicative capacity, while the emergence of secondary mutations within protease was associated with more-gradual changes in both phenotypic resistance and replicative capacity. We conclude that HIV may be constrained in its ability to become both highly resistant and highly fit and that this may contribute to the continued partial suppression of plasma HIV RNA levels that is observed in some patients with drug-resistant viremia.

Adult↗