After unblinding of the data set, seven of 18 (38.9%) patients with virologic failure and zero of 75 patients with virologic success showed NNRTI RAMs in the pre-HAART sample (one G190A; six K103N). In three cases, one with G190A and two with K103N, the RAMs were strongly positive by real-time PCR (G190Ahigh, K103Nhigh) and were also detected by bulk genotyping. In the other four cases, K103N was present at low frequency and was only detected by real-time PCR (K103Nlow). The mutations K65R, Y181C, and M184V were not detected in any of the samples. In four patients without RAMs in the pre-HAART sample, no RAMs were detected in an earlier sample collected at the time of HIV diagnosis. None of the patients with resistance showed additional RAMs in the RT products amplified from the primary RT-PCR or the mutation-positive amplicons derived from the real-time PCR.
Patients with pre-HAART resistance were all males diagnosed HIV-positive in 1998-2007 and infected with subtype B (n = 6) or C (n = 1) (Table 2). Their CD4 count at the time of resistance testing ranged between 184 and 400 cells/mm3. The detection of bulk pre-HAART resistance in these patients was significantly associated with virologic failure (three of 18 versus zero of 75; P = 0.006). A significant association was also found between detection of pre-HAART K103Nlow and virologic failure (four of 18 versus zero of 75; P = 0.001). Combining bulk and low-frequency resistance increased the strength of the association (seven of 18 versus zero of 75; P < 0.0001). Logistic regression analysis was not performed as a result of the lack of baseline resistance in the control group.
Resistance Profiles at Virologic Failure
At Week 24, the seven patients with primary virologic failure showed a median VL of 3.6 (range, 2.3-4.5) log10 copies/mL; they either changed treatment immediately or continued the same regimen for up to 53 weeks without achieving virologic suppression. The 11 patients with secondary virologic failure experienced confirmed VL rebound after a median of 36 (range, 14-48) weeks with a median VL of 3.2 (range, 2.4-4.8) log10 copies/mL. At the time of failure, bulk genotyping yielded a result in 16 of 18 patients, but failed in two patients with a VL of less than 1000 copies/mL (Table 2). The patient with G190Ahigh, the two patients with K103Nhigh, and two of four patients with K103Nlow showed a bulk genotype that included the pre-HAART mutations plus additional RT mutations. One other patient with K103Nlow showed K103R when the VL was 2.1 log10 copies/mL. The K103R mutation, which does not confer NNRTI resistance in isolation, was also detected in the pre-HAART sample by bulk genotyping. This patient maintained a persistent low-level VL up to Week 53 while receiving tenofovir, emtricitabine, and efavirenz without emergence of K103N or other mutations by bulk genotyping. The fourth patient with K103Nlow showed V106M+F227L plus additional RT mutations at Week 24. Among the 11 patients without TDR, nine had a genotype at failure and four showed RAMs consistent with the treatment regimen (Table 2).
NNRTI-based regimens are very effective in suppressing HIV-1 replication but are vulnerable to rapid loss of activity in the presence of drug resistance.22 The K103N mutation, which confers high-level resistance to nevirapine and efavirenz, is highly prevalent in NNRTI-experienced patients and is commonly found by bulk genotyping in patients with TDR.4 In this study, low-frequency K103N mutants were as prevalent as bulk-detectable variants in samples collected from drug-naïve patients before starting HAART. This finding is noteworthy given that most patients had been diagnosed in the years immediately after the introduction of NNRTIs in clinical practice. Low-frequency K103N was detected in patients diagnosed as early as 1998. Although the dates of HIV seroconversion were not known, the patients had established infection at the time of diagnosis and generally low CD4 counts at the time of resistance testing. Taken together, these observations indicate that transmission of NNRTI resistance started early after the introduction of NNRTIs in clinical practice. They also demonstrate that although K103N mutants can persist for several years after transmission, possibly reflecting preserved viral fitness, their frequency may decline over time and eventually fall below the detection threshold of bulk genotypic assays. Despite being present at low frequency, however, the mutants retain a persistent negative impact on virologic responses to nevirapine- or efavirenz-based HAART.
Interestingly, there was no evidence of low-frequency K65R, Y181C, M184V, and G190A in this population. It should be acknowledged that we did not screen for all clinically relevant mutations (eg, Y181V or G190S). However, the mutations we targeted are those found most commonly in treatment-experienced patients. Furthermore, we detected single NNRTI-resistant mutants with no evidence of additional, linked RAMs in the RT products amplified from the primary RT-PCR and the mutation-positive amplicons derived from the real-time PCR.
As described previously,19 we used qualitative real-time PCR point-mutation assays (with direct sequencing of resistance mutation-specific PCR products) that are able to detect mutants at levels as low as 0.001% to 0.2% when testing mixtures of cloned virus sequences.19 However, screening of archived wild-type virus samples from the preantiretroviral drug era (1982-1985) allowed us to measure background reactivity and to define mutation-specific interpretation cutoffs ranging between 0.3% and 0.9%.19 Applying these cutoffs gives us confidence in our ability to specifically detect TDR rather than naturally occurring variants, increasing specificity. The finding that K103N impacts on virologic responses to NNRTI-based HAART when present at a frequency above the interpretative cutoff of 0.9% identifies a clinically relevant breakpoint. Larger studies are required to refine the quantitative relationship between frequency of the mutant and magnitude of the impact on virologic responses.
Using the same methodology, it was recently reported that detection of low-frequency K103N, Y181C, or M184V significantly reduced virologic responses in drug-naïve patients starting efavirenz in combination with lamivudine plus abacavir or zidovudine.20 Although we were unable to quantify the relationship between TDR and virologic failure as a result of lack of baseline RAMs in the control subjects, taken together, the data indicate a significant impact of both high- and low-frequency TDR mutants on virologic outcomes of first-line HAART. Other studies found no association between detection of low-frequency resistant mutants in drug-naïve patients and either viral load decline between 1 and 6 months of therapy7 or virologic failure.21 Low numbers of patients with TDR might confound analyses of treatment outcomes involving low-frequency resistant mutants. An additional potential confounder is the sensitivity of the PCR assay, which can have a cutoff well below 0.1%, thus blurring the demarcation between natural sequence variation and TDR. This is an important distinction when considering that TDR would seed resistance in a high proportion of the viruses archived. It is plausible that the clinical significance of resistant mutants is at least partly related to the frequency expressed within the quasispecies. Consequently, the time elapsed between resistance testing and HAART initiation may play a confounding role, because the effect on responses may decline with decaying prevalence of the mutants.15,23 The type of resistance mutations detected is likely to further influence outcomes. It is interesting that studies reporting a high prevalence of low-frequency M184V generally failed to find a significant impact of low-frequency mutants on virologic responses.7,21 Although this observation may reflect the reduced fitness and hypersusceptibility effects of M184V,24 available studies are insufficient to draw firm conclusions as a result of inclusion of patients showing a very low frequency of M184V and receiving first-line therapy with ritonavir-boosted protease inhibitors. The composition and genetic barrier of the first-line regimen can be proposed as a key confounder, because it may be anticipated that low-frequency mutations in RT have a low overall impact on virologic responses to regimens with a high genetic barrier to resistance.25
Under selective drug pressure, low-frequency mutants would be expected to acquire a replication advantage and become detectable by bulk genotyping. We found a fair association between the pre-HAART detection of low-frequency K103N and the failure genotype. In one case, bulk genotyping at very low viral load may have biased the detection of mutants, whereas in one other patient, the dominant mutations may have evolved during 10 weeks of virologic failure.26 Prospective studies are required to investigate the kinetics of transmitted resistant mutants in the course of treatment failure.
Our study is limited by the small number of observations and studies focusing on very recent infections with potentially greater chances of TDR might allow for stronger analyses. In addition, pre-HAART viral load in patients who experienced virologic failure tended to be higher than in patients who maintained suppression, albeit the difference did not reach statistical significance. It is possible that the higher viral load in the failures increased the likelihood of detecting low-frequency mutants. Nonetheless, our data demonstrate that real-time PCR doubles the rates of NNRTI TDR in this U.K. cohort, provide evidence of a significant impact of NNRTI TDR on responses to first-line NNRTI-based HAART, and give support to the controversial concept that transmitted NNRTI-resistant mutants affect virologic outcomes even when present at low frequencies within the quasispecies.20 Because current U.K. guidelines recommend NNRTI-based regimens as the favored first-line combination,27 larger studies should be designed to confirm the benefit of introducing sensitive screening methods for detecting transmitted NNRTI resistance in routine practice. Based on our observation that patients acquire single NNRTI-resistant mutants, the impact of low-frequency variants on responses to second generation NNRTIs should be investigated.
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Keywords:© 2009 Lippincott Williams & Wilkins, Inc.
transmitted drug resistance; nevirapine; efavirenz; low-frequency mutants.