In this randomized trial, the use of short-course HAART led to a significantly lower prevalence of NNRTI resistance compared with the ZDV/sdNVP regimen (P
= 0.007). To our knowledge, this is the first time the prevalence of resistance after these 2 regimens has been compared in a randomized trial. As ZDV/sdNVP is recommended in nonimmunosuppressed HIV-1–infected women to prevent MTCT per World Health Organization guidelines, and short-course HAART is under consideration,3,19
this study provides critical information when assessing the relative merits of these antiretroviral interventions.
In this study, detectable levels of resistance ranged from 0.8% to 14.2% of an individual’s virus population and were detectable by a sensitive allele-specific PCR assay but were not detectable by population-based sequencing at 3 months posttreatment. The discordance between the results from the different assays used reflects the difference in their sensitivity: the population-based sequencing assay only reliably detects mutations that comprise >20% of the virus population, whereas the K103N and Y181C allele-specific PCR assays detect resistance down to 0.2% and 2%, respectively.
Here we show no resistance detectable by sequencing despite the fact that we estimate to have sequenced a minimum of 10–50 copies in 92% of samples tested. In addition, independent PCR and sequencing reactions were repeated up to 3 times on 82% of these samples, and the results were identical (data not shown). The fact that none of the women in our study had levels of resistance detectable with a sequencing assay is somewhat surprising. Previous studies of both sdNVP alone and ZDV/sdNVP have shown that NNRTI resistance is detectable by sequencing in 15%–69% of women at 4–8 weeks postpartum.11
The lack of resistance detectable by sequencing in our study could result from the fact that the majority of women in our cohort were infected with subtype A (72%) or subtype D (18%) virus, and previous studies suggest that lower rates of NVP resistance occur in subtypes A and D compared with C.25
There is also some evidence that resistance is more common in women with low baseline CD4 counts and high baseline viral load, and our cohort excluded women with CD4 counts <200 cells per cubic millimeter.12,26
In addition, we sampled at a time point later than previous studies, and viral variants with resistance mutations have been shown to wane over time.26–30
Seventy-five percent of the women in the ZDV/sdNVP arm had viral variants with K103N or Y181C mutations detectable by our allele-specific PCR assay, whereas only 18% of women in the HAART arm had detectable levels of resistant virus (P
= 0.007, and ). The fact that the study design was a randomized trial minimizes the chances that the prevalence of baseline resistance differed in the 2 arms. The reduction in risk of resistance after HAART cessation compared with ZDV/sdNVP may derive from the fact that suppression of plasma HIV-1 RNA is consistently several log10
greater with HAART compared with ZDV/sdNVP.19
As a result, the levels of replicating virus during the period when drug levels wane immediately after treatment cessation is lower with HAART compared with ZDV/sdNVP. In addition, the use of 3TC in the HAART regimen, which has a longer half-life than ZDV, decreases the amount of time that NVP may be found as the only active drug in plasma after treatment cessation. The effect of a second drug during NVP cessation is supported by other studies that have shown that the addition of a “tail” of antiretrovirals after cessation of NVP-based treatments reduces the risk of resistance (reviewed in31
). Studies that included 3–7 days of ZDV plus 3TC after the use of sdNVP showed reduced rates of resistance compared with sdNVP alone.32,33
Therefore, it is unknown whether the addition of a “tail” concurrent with treatment cessation of ZDV/sdNVP would make the prevalence of resistance in the 2 regimens studied here more similar. Our data suggest that even without a “tail” added during HAART cessation, the presence of 3TC alone may substantially reduce NVP resistance. This may be important in cases when a tail is not applied, such as unexpected treatment cessation.
Previous studies suggest that the emergence of resistance to NVP after treatment to prevent MTCT may have consequences for women who later require long-term HAART.12,13
Therefore, it is important to determine whether resistance persists over time. Although data from studies of resistance using standard genotyping assays suggest that resistance to NVP wanes over time,26,34
studies that utilize more sensitive assays suggest that resistance can persist at low levels for more than 1 year postpartum.27,28,35
Our data confirm this suggesting that in women with detectable levels of resistance at 3 months postpartum, resistance can persist at 6 and 12 months posttreatment ( and ). Of note, variants with Y181C mutations, which are more common in this cohort than K103N mutations, persisted above detection at 12 months posttreatment in over 80% of the women, with little evidence that they declined during this period. Thus, Y181C may be of particular concern in terms of its effect on future treatment options.
The data presented here provides strong evidence that short-course HAART results in lower rates of antiretroviral resistance compared with the standard ZDV/sdNVP regimen. However, the relative effects of these regimens on transmission rates remain less clear. Preliminary data from ongoing studies in Kenya and Tanzania suggest that 6 months of HAART during breastfeeding reduces transmission rates from the expected 25%–48% without treatment to 5% by 6 months postpartum.3,5,6
A short course of ZDV/3TC plus sdNVP have a documented transmission rate of 6.6% at 6 months postpartum.36
Given the imprecision of comparing data from distinct cohorts, 2 randomized clinical trials are currently underway to more accurately determine whether taking HAART during breastfeeding reduces transmission compared with standard short-course regimens.3,7
When considering antiretroviral strategies to reduce breastfeeding transmission, data on transmission rates must be balanced with factors such as resistance, safety, feasibility, and adherence. These issues, and the optimal approaches to PMTCT, are complex and are discussed in more detail elsewhere.1,3,19
A previous study of our cohort showed that 5 of 26 women (19%) randomized to HAART prematurely discontinued treatment either due to adverse events or inconvenience,19
suggesting adherence can be low with HAART. The current study was limited to 17 women on HAART, and we acknowledge that this analysis could be biased toward a group of women with good adherence. The reduced risk of resistance with HAART (even without an added tail) suggests that the use of HAART, although breastfeeding may have fewer implications for later treatment options compared with the combination ZDV/sdNVP regimen, as NNRTI resistance after treatment prophylaxis has been implicated to reduce the effectiveness of later treatment with NNRTI-based HAART.12,13
Therefore, the data presented here should be considered as a benefit of HAART when balancing the safety, efficacy, and feasibility of different strategies currently being tested to reduce breastfeeding transmission. These data are encouraging for settings in which adding a tail of 3TC and ZDV during HAART cessation is not feasible. However, the fact that the use of HAART did not completely eliminate resistance to NVP emphasizes that, when possible, the addition of ZDV and 3TC during treatment cessation may be beneficial.