In our study of HIV infected women who took ART for pMTCT, we observed relatively high rates of new HIV drug-resistance mutations. While these women all had ART induced suppression of viral replication, resistance was detected when plasma HIV RNA was >400c/mL during gestation or in viral rebound specimens when ART was stopped between 2 and 24 months postpartum. In this small study, resistance rates did not differ in women randomized to NVP- versus NFV-ART. However, the pattern of resistance differed, with significantly greater selection of resistance to lamivudine (M184V) in women randomly assigned to NFV, and selection of nevirapine resistance (K103N, Y181C and/or G190A) in women randomized to NVP.
The high rate of resistance in our study is consistent with rates reported in a larger observational study of NFV-ART, which detected M184V in 29% by consensus sequencing and 52% by allele-specific PCR.5
Mutations encoding M184I/V have been reported across studies of pregnant and postpartum women.5,12,13
However, the clinical significance of M184V on later ART has not been evaluated.
When ART is discontinued unequal drug pressure can occur due to variation in the pharmacokinetics of each ARV. The longer half-life of NVP14
compared to 3TC15
likely protected participants randomized to NVP-ART from selection of M184V. Instead, when NVP pressure was unopposed NNRTI mutations were preferentially selected. Resistance to NVP and 3TC can occur rapidly because the single base changes that confer high-level resistance likely exist in the viral population due to the permissive nature of HIV polymerase, and when ARV concentrations are low or not complemented by other ARV these mutants are rapidly selected. In contrast, two or more mutations are required for high-level resistance to ZDV and to most PI, except NFV, and the likelihood for two or more specific mutations occurring randomly in the same virus is exceedingly low,8
providing a greater genetic barrier to the selection of drug resistant virus.
Several factors, in addition to stopping ART postpartum, could have contributed to the high rates of ARV-resistance detected in this study. First, women who stopped ART postpartum may not have been committed to treatment, and may have had poorer adherence during gestation.16
Our adherence data showed that half the women in the study reported decreased adherence during gestation, but resistant viruses were selected in women who reportedly adhered to ART during gestation. Second, variable pharmacokinetics of NFV in pregnant women were evident in the women we studied, consistent with previously published studies of both the “older” and “newer” NFV formulations that observed lower and variable third trimester NFV concentrations17–22
compared to postpartum values. Low NFV concentrations 17
along with diminished concentrations of the active form of ZDV in a subset of infected cells,23
would allow selection of 3TC resistance (i.e. low NFV trough concentrations plus low triphosphorylation of ZDV in cells would result in unopposed 3TC pressure and favor the selection of M184V).
The more frequent detection of resistance mutations by OLA compared to consensus sequencing indicates that mutants were at low concentrations in the population.10
Cell-free virions in the plasma are known to turn-over rapidly, with resistance fading from the plasma within weeks of stopping therapy.24,25
The detection of mutants at high concentration in the PBMC and plasma of one participant at the time of study enrollment is consistent with transmitted drug-resistance.26
Our study has several important limitations. The study population was small, and while a significant difference was detected in the selection of M184V, the small group sizes precluded an analysis of differences in the selection of resistance to other ARV. The increased detection of resistance in the NFV compared to NVP arm when testing plasma by OLA may be biased by the lack of paired specimens for all time-points tested. The absence of ARV concentrations, and information concerning ART adherence practices did not allow us to evaluate the potential mechanisms leading to viral rebound and selection of resistance during pregnancy. Importantly, we were not able to determine whether the resistance first detected in the postpartum specimens of certain women was due to stopping ART or if these women had low ARV concentration throughout gestation due to suboptimal pharmacokinetics or non-adherence. In addition, our study did not evaluate the clinical significance of the resistance on the efficacy of future ART.
Many HIV-infected pregnant women do not qualify for ART based on symptoms or CD4 cell counts. Recently, the WHO revised its pMTCT Guidelines to favor ART regardless of maternal CD4.27
This may increase episodic use of ART during gestation. Comparative studies are needed to select the safest ART regimen for episodic use in pregnancy. Ideally, these studies would compare various gestational ART regimens on the efficacy of subsequent ART. As drug resistant viruses are rarely detected at “virologic failure” of ritonavir-boosted PI regimens,28
either PI-based regimens, or ARV “tails” 29,30
may diminish the selection of drug-resistant mutants. Given that the detection of mutants in association with ARV given to pMTCT ARV diminish the efficacy of later ART,31–33
pre-ART resistance testing could help guide health care workers select ARV with a high likelihood of suppressing viral replication.
In summary, a high rate of drug resistant HIV was detected in women taking NFV- or NVP-based-ART for pMTCT who stopped ART within two years postpartum. 3TC-resistance was selected at a higher rate in women randomized to NFV-ART, which suggests suboptimal NFV pharmacokinetics in pregnancy. As more programs utilize ART for pMTCT, studies such as the NIH-sponsored PROMISE trial are important to inform us on the risks of stopping compared to continuing ART postpartum. Additionally, studies are needed to define the risks of discontinuing various gestational ART regimens on the clinical outcome of ART subsequently administered to treat the mother’s HIV-associated immunodeficiency.