Effect of isolated V118I mutation in reverse transcriptase on response to first-line antiretroviral therapy

Although the V118I mutation in reverse transcriptase causes a reduced incorporation of zidovudine and lamivudine into transcribed viral DNA, it also decreases ATP-mediated pyrophosphorylysis. In this manner, its overall impact on enzymatic activity appears to be neutral when assessed in vitro. Nevertheless, V118I remains identified as a resistance mutation by a number of commonly utilised resistance mutation algorithms. A large clinical database was queried to identify antiretroviral-naive patients with V118I as the sole resistance associated mutation. Each index patient was matched to five control patients with no major mutations who were prescribed an identical first-line regimen. The two groups were compared in terms of virological and immunological response up to 24 months after initiation of first-line antiretroviral therapy in an intent-to-treat analysis. The V118I mutation was detected as the sole major reverse transcriptase mutation in 35 (1.8%) samples. Twenty-five of these patients initiated antiretroviral therapy (between March 1997 and October 2002) and could therefore be included in the analysis of therapeutic response. Twenty of 25 patients were prescribed zidovudine and/or lamivudine. The index and control (n = 125) groups were well balanced with respect to demographic and clinical characteristics. There was no statistically significant difference in the average reduction in HIV RNA viral load (global P = 0.9) or in the average increase in CD4 cell count (global P = 0.5) compared to baseline. Our study suggests that V118I should be excluded from mutation lists used for clinical epidemiological studies of transmitted drug resistance. Furthermore, the presence of V118I as the sole nucleoside reverse transcriptase inhibitor mutation should not be over-interpreted when deciding on therapeutic options.

The V118I mutation in reverse transcriptase (RT) is frequently observed in patients failing antiretroviral therapy (ART), and has been associated with regimens containing zidovudine/didanosine ^[1], zidovudine/lamivudine ^[2], stavudine/didanosine and abacavir ^[3]. However, it is usually observed in the presence of other thymidine analog mutations (TAMs), particularly the pattern involving T215Y rather than T215F; that is the TAM-1 pathway ^[4]. It is very rarely present as the sole RT mutation in patients with viral rebound on therapy.

The in vitro selection of V118I by zidovidine has been documented ^[5]. However, although it causes a reduced incorporation of zidovudine and lamivudine into transcribed viral DNA, it also decreases ATP-mediated pyrophosphorylysis ^[6]. In this manner, its overall impact on enzymatic activity appears to be neutral when assessed in vitro.

Nevertheless, V118I remains identified as a resistance mutation by a number of commonly utilised resistance mutation algorithms, which identify it as potentially contributing to nucleoside reverse transcriptase inhibitor (NRTI) resistance (http://hivdb.stanford.edu/). In addition, it is often included in epidemiological studies of prevalence of resistance in treated and untreated individuals, despite the fact that it is present as a polymorphism in 2-3% of subtype B viruses from untreated individuals ^[7]. There are no published data available with respect to whether V118I impacts unfavourably on the clinical response to therapy, either overall or to regimens containing specific NRTIs.

We searched the Chelsea and Westminster Hospital database to identify those patients with a pretherapy resistance test that revealed V118I as the only major mutation in RT or protease. To assess the impact of the mutation on virological and immunological response to first-line therapy, we matched each index patient to five control patients who: (i) had no major mutations in either the RT or protease genes; (ii) were prescribed an identical first-line regimen; and (iii) matched most closely on date of ART initiation. The reason for choosing five control patients per index case is the minimal statistical gain achieved beyond this value ^[8].

Baseline CD4 count and HIV-1 RNA viral load were defined as the last values (provided within 90 days) prior to starting ART. Changes in CD4 count and viral load from baseline were assessed at 6, 12, 18, and 24 months using the closest measurement to each time point, within a window of ± 3 months. Regression models of parameter change included baseline value and the interval between the date of measurement and the target time point, as well as the presence/absence of the V118I mutation. Predicted changes were standardized to baseline values of 200 CD4 cells/mm³ and 100 000 HIV RNA copies/ml. Treatment switches were not taken into account in the analysis of virological and immunological response because censoring at treatment switch ('on treatment' analysis) would have biased the analysis in favour of good responders. Viral load values were log₁₀-transformed prior to analysis, and normal 'interval' regression was used to account for values below the lower limit of the assay ^[9]. The main assays used were Chiron 3.0 (64% of measurements) (Chiron Corporation, Emeryville, California, USA) and Roche 1.5 (33%) (Roche Diagnostics, Mannheim, Germany). The lower limit of assay sensitivity ranged from 50-400 HIV RNA copies/ml.

The V118I mutation was detected in 70 (3.4%) of 2042 pretherapy samples, and as the sole major RT mutation in 35 (1.8%) samples. Ten of these patients had not yet initiated ART, leaving 25 patients who could be included in the analysis of therapeutic response. Their first-line ART regimens were initiated between March 1997 and October 2002. In 22 of the 25 patients, the resistance test was performed retrospectively (i.e. after the onset of therapy), although it was not predicated by the outcome of therapy.

The NRTI combinations in the first-line regimen were, in descending order of frequency: ZDV+3TC (n = 13), ddI+D4T (n = 5), D4T+3TC (n = 3), ZDV+3TC+ABC+TDF (n = 2), ZDV+3TC+ABC (n = 1), and 3TC+ABC (n = 1). Thirteen patients were prescribed a non-nucleoside reverse transcriptase inhibitor (NNRTI) (12 efavirenz, one nevaripine), six a protease inhibitor, and one patient both an NNRTI and protease inhibitor; dual and triple NRTI therapy was used in two and three patients respectively. The matching method used (see Patients and Methods) ensured that the distribution of regimens received was identical in the control group of 125 patients.

The two groups were well balanced in terms of sex distribution and exposure group (Table 1). On average, the control group was slightly younger compared to the index cases (mean difference 4 years) and had a higher proportion of Black patients. Baseline CD4 count and HIV RNA levels were similarly distributed in the two groups, although many patients initiated therapy at CD4 count levels below that recommended by treatment guidelines (overall median 166 cells/mm³), mainly as a result of late diagnoses of HIV infection. There was no evidence of a difference in the durability of the first-line regimen; by 24 months, an estimated 60% of index cases and 50% of controls had switched one of more of the drugs originally prescribed.

Table 1

The median interval between ART initiation and last available laboratory measurement was 40 months (interquartile range = 23-59 months) for the index cases and 47 months (interquartile range = 28-64 months) for the controls. The mean reduction in viral load (range 3.5-4.1 log₁₀ copies/ml) was highly similar in the two groups (global P = 0.9), as was the proportion of patients with an undetectable viral load (range 67-86%) (Fig. 1). The patterns of immunological response were also similar, with average CD4 count increases of approximately 150 cells/mm³ by 12 months and 200 cells/mm³ by 24 months (global P = 0.5). The lack of an effect of the V118I mutation was also observed in a secondary analysis limited to patients who received AZT+3TC (data not shown).

Fig. 1

We report the virological response to viruses containing the V118I mutation, in the absence of other classical NRTI mutations. This mutation is often classified as contributing to resistance. However, it is also a polymorphic position, with the isoleucine present in 2-3% of subtype B viruses in the absence of drug selective pressure ^[7]. It is therefore important to identify the clinical correlates of this mutation to guide clinical practice. For example, should zidovudine and lamuvidine be avoided in a drug-naïve patient infected with a V118I-containing virus?

We demonstrate that the presence of V118I as a polymorphism in drug-naïve individuals does not compromise the virological and immunological response to NRTI-based first-line therapy, over an extended period (24 months). This does not necessarily imply that the mutation has no adverse impact on susceptibility. Rather, we show that any such resistance is not apparent in the context of commonly used antiretroviral drug combinations. Indeed, most patients in our study were treated with zidovudine and lamuvidine, drugs to which V118I, in combination with other mutations, has previously been identified as conferring some degree of resistance in vitro. As this was a relatively small study, the possibility of missing a moderate effect of V118I cannot be excluded.

In summary, our data suggest that NRTI-based combination therapy is not compromised when used as first-line therapy in patients with viruses containing the V118I mutation. On this basis, we suggest that V118I be excluded from mutation lists used for clinical epidemiological studies of transmitted drug resistance. In addition, care should be taken not to overinterpret the presence of V118I if it is the sole NRTI mutation when deciding on therapeutic options.

Acknowledgements

The results were previously presented in part at the XIII International HIV Drug Resistance Workshop, June 2004, Tenerife, Spain.