The data presented are unique in describing treatment outcomes and patterns of antiretroviral resistance in a large cohort within a relatively understudied population: HIV-infected infants and young children on ART in a resource-limited setting. The results are concerning in terms of the percentage of children who developed major antiretroviral resistance mutations within 52 weeks (78%) when not virologically suppressed and in the limitations that this resistance places on future ART options for these children.
In this cohort, 17% of children on PI-based ART did not achieve virologic suppression by 52 weeks, and 78% of these had major antiretroviral-associated resistance mutations at the time of virologic endpoint posttreatment. The majority of children with resistance mutations had the lamivudine-associated resistance mutation M184V, which does not significantly limit future ARV regimen options. However, 36% developed major PI-associated resistance mutations, all of whom had a V82A, a mutation known to be associated with RTV.41
The high proportion of children with major PI-associated resistance mutations differs from findings in adults and children treated with RTV-boosted PI-based ART, where the development of major PI-associated resistance mutations is uncommon.13,19,42–44
The prevalence of major PI-associated resistance mutations in this cohort is likely a result of RTV-based ART, as seen in earlier adult cohorts prior to the advent of RTV-boosted PI-based ART.45,46
Supporting this assertion, the children who received RTV-based regimens at any point were less likely to achieve virologic suppression and more likely to develop major PI-associated resistance mutations, in contrast to those exclusively treated with a LPV/r-based regimen. None of the children treated solely with LPV/r developed major PI mutations, confirming the findings in other cohorts that virologic failure on LPV/r or other ritonavir-boosted PI-based regimens does not usually lead to the development of major PI-associated resistance mutations.13,14,19,44
As only a small number of children treated exclusively with LPV/r-based therapy did not achieve virologic suppression in our cohort, further investigation is needed to determine the profile of resistance mutations that are selected in a group unsuccessfully treated with a LPV/r-based regimen.
Our findings strongly support avoidance of RTV-only based ART for children who are being treated for mycobacterial infections or who are less than 6 months of age. Known drug–drug interactions, particularly with antimycobacterial therapy, the poor palatability of ritonavir, and the inferior efficacy of RTV-only regimens to RTV-boosted PI-based regimens may all have contributed to these findings.5,22,47,48
Fortunately, appropriate dosing of LPV/r for younger children is now available.48
This was not available at the time the study was conducted. Superboosting of LPV/r is also now recommended in South Africa for children requiring mycobacterial infection cotreatment.47,49
Other findings that merit attention include the effect of adherence to ART on treatment outcome and the development of resistance. Poor adherence was strongly associated with lack of virologic suppression in the cohort. In contrast, among the subgroup of children without virologic suppression, rates of nonadherence were highest (64%) in the children who did not develop major PI mutations compared with 29% in those who did develop major PI mutations. This is likely caused by a lack of drug-selective pressure on the virus in children with poor adherence. The link between higher levels of adherence and increased development of resistance mutations is described in adult studies of RTV-boosted and unboosted PI-based ART. Several investigations have shown that adherence levels over 75–85% were associated with development of resistance mutations, and lower levels of adherence led to ART failure without resistance.45,46,50
The adherence measure used in this study likely distinguished between children with less than or equal to 75% true adherence and those with more than 75% adherence, but was not likely to distinguish children with 75–85% adherence and those with >85% adherence. As such, adherence was correlated with virologic suppression, but nonadherence did not predict resistance. However, this is a speculation, and further studies with more rigorous adherence monitoring would be necessary in this population to determine the true cause of this parodoxical finding.
The data presented here also demonstrate an association between pretreatment major resistance mutations, most of which are mutations affecting the viral reverse transcriptase, and the development of PI-associated resistance mutations on treatment. Interaction between resistance-associated mutations has been seen for NRTI-related mutations, particularly in the sequence of thymidine analogue resistance-associated mutations.51,52
In viral protease, minor mutations frequently emerge after major mutations arise, and may have variable effects on antiretroviral efficacy and viral fitness.36
To our knowledge, an interaction between NNRTI- or NRTI-related resistance mutations and the development of subsequent PI-related resistance mutations has not been described. The possibility that the presence of resistance mutations in one enzyme would predispose HIV to develop resistance mutations in another enzyme is a finding that merits further exploration. However, small numbers precluded valid multivariate investigation in our study and we cannot rule out that the association may be due to confounding. For example, maternal viral load is related both to the presence of NNRTI-related resistance mutations at baseline and with poor treatment outcomes in children, so it is also plausible that high maternal viral load may explain the association.53
The limitations to this analysis include that the fact that it was conducted at a single site and findings may not be applicable to other settings. The size of the cohort is relatively small, although for pediatric cohorts it represents one of the largest in which the development of resistance has been monitored over time using HIV-1 genotypes. Resistance mutation profiles were not available for 24% of the children at baseline and at the study endpoint for 26% of the children who did not achieve virologic suppression, but the results were not significantly affected by stratification of the analysis by availability of HIV-1 genotype testing. The analyses involving plasma HIV-1 RNA level were limited due to the inability of the assay to detect levels over 750,000 copies/ml, which led to the need for categorization of this variable.
Despite these limitations, the clinical implications of the patterns of resistance observed in this cohort are of particular concern. The children participating in this study were well-monitored at regular study visits; the study team provided adherence counseling to caregivers at each encounter, and all children had access to HIV-1 plasma RNA level monitoring that enabled researchers to rapidly identify poor outcomes. These conditions are not currently replicated in most resource-limited settings.
The international community has begun to recognize the importance of virologic monitoring and resistance surveillance in adults as a tool to prevent the emergence of drug resistance and achieve success on first- and second-line therapy.54
The data presented here suggest that even in the setting of frequent virologic monitoring and adherence counseling, resistance can develop rapidly on RTV-based regimens. This is of concern for HIV-infected children, for whom second-line treatment options are more limited, and issues of exposure to nevirapine and cotreatment for mycobacterial disease are more prevalent than in HIV-infected adults.4
There is an urgent need for further research to determine the outcomes of second-line ART in children, to provide expanded options for second-line ART regimens, and to better understand the development of resistance to LPV/r-based regimens.