Establishment of Drug‐Resistant HIV‐1 in Latent Reservoirs

The risk of vertical transmission of HIV from an infected mother to her infant is 15%–30% in nonbreastfeeding populations. Breastfeeding by an infected mother increases the risk of transmission by 5%–20%, to 20%–45% [1]. The risk of mother-to-child transmission has been virtually eliminated in the developed world by treatment of mothers with effective antiretroviral therapy (ART), use of elective cesarean section, and exclusive use of formula to feed infants [2]. However, in developing countries, where the majority of HIV-infected women live, most of these interventions may not be affordable, available, feasible, or safe [3] Several simplified strategies for the prevention of mother-to-child transmission of HIV have been evaluated [4]. One of these is a regimen in which a single dose of the nonnucleoside reverse-transcriptase inhibitor nevirapine is given to the mother at the onset of labor and a single dose is given to the infant ⩽72 h after birth. In a population in which the majority of the children were breastfed, this single-dose nevirapine regimen lowered the risk of mother-to-child transmission by 47% at 14–16 weeks of age and 41% at 18 months of age [5, 6]. Treatment with single-dose nevirapine was more effective than zidovudine (AZT) monotherapy in reducing mother-to-child transmission [5, 6] The simplicity and low cost of single-dose nevirapine made it a common regimen for preventing mother-to-child transmission in developing countries, and its widespread use has resulted in the exposure of large numbers of women to this regimen. Owing to the greater availability of more-effective regimens and the problem of resistant variants being selected by single-dose nevirapine, the use of single-dose nevirapine alone is decreasing. The following regimen is currently recommended by the World Health Organization to prevent mother-to-child transmission in resource-limited settings for women who are not yet eligible for treatment: antepartum AZT beginning at week 28 of pregnancy (or as soon as possible thereafter), intrapartum AZT and lamivudine (3TC) plus a single dose of nevirapine, and postpartum AZT and 3TC for 7 days in women and a single dose of nevirapine and AZT for 1 week in infants. However, the recommendations also state that, “[i]n settings that do not currently have the capacity to deliver the recommended prophylactic regimen to prevent mother-to-child transmission, it may be necessary—as an absolute minimum—to implement the single-dose (mother and infant) NVP regimen” [4, p. 15] Although nevirapine is a potent nonnucleoside reverse-transcriptase inhibitor of HIV-1, if it administered as monotherapy the drug rapidly selects for resistant variants. These resistant variants may contain the K103N, Y181C, and/or G190A mutations in reverse transcriptase and are often cross-resistant to other nonnucleoside reverse-transcriptase inhibitors. The K103N mutation appears to have little effect on the replicative capacity of HIV-1, allowing variants with this mutation to persist long after nevirapine therapy has been stopped [7–9 ]. Nevirapine-resistant variants were detected by standard genotyping methods in 15%–50% of mothers who received an single dose of nevirapine intrapartum, with the level of resistant virus in most women decreasing below the limit of detection of standard genotyping methods by 6 months [10, 11] Several studies have investigated the development and persistence of nevirapine-resistant HIV strains found in plasma after treatment with single-dose nevirapine, using more sensitive methods than standard sequencing. These studies have shown that nevirapine resistance can be detected 6–8 weeks after receipt of single-dose nevirapine in up to 87% of treated mothers and infants, and a higher risk for development of nevirapine resistance has been linked to HIV subtypes C and D [11–15 ]. One study revealed that addition of short-course (i.e., 4- or 7-day) treatment with AZT and 3TC reduced the selection of nevirapine-resistant variants after single-dose nevirapine from 75% to 27% in plasma samples from women receiving this treatment, as determined by a sensitive allele-specific polymerase chain reaction assay [16]. However, even this treatment, which contains compounds with different modes of action, does not prevent the selection of nevirapine-resistant HIV-1 strains These sensitive methods also showed that the prevalence of nevirapine resistance decreases over time [13]. Nevertheless, one study found that nevirapine resistance could be detected after >1 year in 23% of the women in whom detectable resistance initially developed [15]. Nevirapine-resistant HIV strains were also detected in 1 individual 5 years after treatment with single-dose nevirapine [17]. Furthermore, even if nevirapine-resistant HIV-1 strains in plasma decrease over time, many experts in the field speculate that these nevirapine-resistant strains are maintained in latent HIV reservoirs. However, until now, definitive proof has been lacking The report in this issue of the Journal by Wind-Rotolo et al. [18], who used a novel method to characterize the latent reservoir in the presence of ongoing viral replication, provides the strongest evidence yet that nevirapine-resistant HIV can persist in the latent reservoir of resting CD4+ T cells. The authors examined blood samples from women in Uganda and South Africa obtained ⩾6 months after treatment with single-dose nevirapine and found nevirapine resistance (due to mutations K103N and G190A) in stably integrated proviral DNA in resting CD4+ T cells from 4 (8%) of 50 evaluable women. Their research indicated that, even when nevirapine resistance was not identified in the plasma, it could be found in the latent reservoir. Furthermore, in 1 patient, up to 28% of the viruses released from the latent reservoir contained the K103N mutation. These important findings indicate that nevirapine resistance, selected during treatment with single-dose nevirapine, can be archived in the latent reservoir and perhaps reemerge later in life The article by Wind-Rotolo et al. [18] also informs us about the long-term persistence of nevirapine-resistant strains; latently infected cells have a half-life of ⩾44 months, even in the presence of ART [19, 20]. To date, 2 studies, somewhat surprisingly, have shown that repeated use of single-dose nevirapine during consecutive pregnancies has not been associated with decreased efficacy [21, 22]. In both studies, the median time between deliveries has been >20 months. In contrast, the risk for virological failure during ART initiated after single-dose nevirapine seems to be increased if ART is initiated ⩽6 months after receipt of nevirapine [23]. The longest follow-up time for evaluating the efficacy of ART after receipt of single-dose nevirapine has, to our knowledge, been 24 months [24]. The findings by Wind-Rotolo and colleagues, which demonstrate the latent and, therefore, long-term persistence of nevirapine-resistant HIV strains, indicate that future studies should focus on the long-term risk for virological failure during ART initiated after treatment with single-dose nevirapine