Development of drug resistance in HIV infected children with Treatment failure is a major impediment to selection of appropriate therapy. Studies done on Indian population indicate that prevalence of HIV resistance mutations is around 9.6% in treatment naïve population and up to 81-96% in treatment experienced patients with virological failure. The pediatric population in particular is not only at a higher risk of developing resistance (due to greater viral loads and difficulty in adherence and accurate dosing), but resistance proves a greater challenge as they require longer term therapy than adults. There are 2 types of resistance testing available predominantly.
Genotype tests :
They detect specific genetic mutations. They are based on amplification procedures and can detect mutations in plasma samples with more than 1000 copies/ml of HIV RNA. This involves the nucleotide sequencing of the relevant HIV genes, from which an amino acid sequence of the reverse transcriptase and protease enzymes is predicted. Genotypic testing is cheaper and more widely available compared to phenotypic testing.Phenotype tests :
Phenotypic assays measure the IC50 of the drug against the virus in vitro. Although, the sensitivity patterns of the virus tested can be determined, it may not detect minor species of resistant viruses.
In patients with HIV, genotypic resistance testing is preferred. Resistance testing should be performed by laboratories that have appropriate operator training, certification and proficiency assurance. Drugs for which the virus requires only one mutation to develop high level resistance (such as Lamivudine and Nevirapine) will in fact generate very rapid resistance when used as monotherapy. For other drugs such as AZT, ABC, TDF and most PIs, high-grade resistance requires the serial accumulation of multiple mutations and is thus slower to emerge. Other drugs such as ddI & d4T are associated only with low level of resistance as measured in phenotypic assays predicting decreased efficacy.
Nomenclature of resistance
The nomenclature used to describe mutations involves one letter followed by a number and followed by another letter e.g. M184V. In this case the first letter refers to a specific amino acid which is present in nonresistant viruses, and the second letter identifies the amino acid present in the resistant virus. The number refers to the position of that particular amino acid within either the protease gene or the reverse transcriptase gene. Thus, M184V signifies that the usual amino acid in drug sensitive viruses, methionine (M), at the 184th amino acid of the reverse transcriptase protein has been replaced by a V (valine), which leads to high level resistance to Lamivudine.
NRTI resistance pattern
There are four general patterns of resistance observed:
- M184V: This is the first reverse transcriptase mutation to appear with most regimens containing Lamivudine (3TC) or Emtricitabine (FTC). This results in high level resistance to 3TC and FTC but resultant increase in activity to Zidovudine (AZT), Stavudine (d4T) and Tenofovir (TDF). For this reason, it is common practice to continue 3TC or FTC despite resistance, but in such cases these drugs should not be counted as active components of an antiviral regimen.
- TAMS / NAMS: A collection of mutations typically associated with thymidine analogue use (i.e. ZDV, d4T), which include M41L, D67N, K70R, L210W, T215F/T and T219Q/E. These are variably termed TAMS (thymidine analogue associated mutations) or NAMS (nucleoside analogue associated mutations). TAMs lead to multi NRTI resistance with reduced susceptibility to all NRTIs.
- K65R, L74V: These mutations are mutually exclusive, and occur following treatment with Tenofovir (K65R), Abacavir (both) and Didanosine (both). The K65R mutation confers wider cross resistance to all the nucleoside analogues, other than Zidovudine, whereas the L74V only compromises Abacavir and Didanosine.
- Multinucleoside/nucleotide Resistance Complexes: This confers widespread resistance to nucleoside analogues.
NNRTI resistance pattern
By contrast to NRTIs, resistance to NNRTIs is straightforward. K103N mutations confers resistance to both NVP and EFV. In addition, there may be accumulation of multiple mutations.
PI resistance pattern
Several drugs in the PI groups have distinctive major resistance mutations. One strategy to avoid accumulations of multiple mutations is to use low dose ritonavir to “boost” other PIs. Protease inhibitor resistance mutations can be classified as major or minor mutations. The major mutations are known to specifically reduce activity of one or more drugs. By contrast, minor mutations by themselves do not appear to impact significantly on drug resistance. Virtually all research work on resistance has been undertaken on the subtype B virus. Some mutations classified as minor protease inhibitor resistance mutations in subtype B virus are already pre-existing as the wild type virus for other subtypes.
Conclusion
Antiretroviral treatment reduces illness and mortality among children living with HIV. With a gamut of antiretroviral drugs and multiple treatment regimens to select from, treatment of HIV infected children is complex and difficult. In addition, adherence issues, lifelong therapy and evolution of resistance indicate that therapy should be given under consultation from expert who deals with management of these children.
Reference
- Hirsch MS, Burn-Vezinet F, Clotet B et al. Antiretroviral Drug Resistance testing in adults infected with Human Immunodeficiency Virus Type-1: 2003 Recommendations of an International AIDS Society – USA Panel. Clin Infect Dis 2003;37:113-28.
- Shah I, Parikh S. HIV Genotype Resistance Testing In Antiretroviral (Art) Exposed Indian Children – A Need Of The Hour. Indian Journal of Pediatrics Accepted for Publication. 4th April 2012