The K65R mutation in HIV-1 reverse transcriptase: genetic barriers, resistance profile and clinical implications

Reverse Transcriptase Genotypes in Pediatric Patients Failing Initial Antiretroviral Therapy in Gaborone, Botswana

Background

Limited data are available on patterns of resistance mutations in pediatric patients in southern Africa, where HIV-1 subtype C (HIV-1C) predominates.

Methods

Retrospective chart review of pediatric patients. Nucleoside reverse transcriptase inhibitor (NRTI)- and nonnucleoside reverse transcriptase inhibitor (NNRTI)-associated resistance mutations quantified from population-based sequencing genotypic resistance assay results taken at time of first-line antiretroviral therapy (ART) failure (first-line ART = stavudine [d4T] or zidovudine [ZDV] + lamivudine [3TC] + nevirapine [NVP] or efavirenz [EFV]).

Results

Total number of patients with resistance assays analyzed is 45. Nucleoside reverse transcriptase inhibitor-associated mutation frequencies noted were M184V (n = 41; 91.1%); thymidine analogue mutations (TAMs; n = 20; 44.4%); >1 TAM (n = 9; 20%); TAM-2 pathway (n = 10; 22.2%); TAM-1 pathway (n = 7; 15.6%); TAM-1 and TAM-2 pathways (n = 3; 6.7%); K65R (n = 2; 4.4%); Q151M (n = 1; 2.2%); and L74V (n = 0; 0%). Nonnucleoside reverse transcriptase inhibitor-associated mutation frequencies noted were associated with notable resistance to either/both NVP and EFV (n = 40; 88.9%); K103N (n = 15; 33.3%); ≥1 mutations associated with etravirine (ETR) failure (K101E, Y181C, and G190A; n =20; 44.4%); and ≥2 notable NNRTI mutations (n = 12; 26.7%).

Conclusions

In this cohort, low-genetic barrier mutations were common, as were TAMs, including more than 1 TAM. Mutations compromising nonthymidine analogue backbones were rare, suggesting that it is likely that children who fail first-line NRTI backbones containing d4T or ZDV/3TC would still respond to abacavir (ABC), didanosine (ddI), and, for adolescents, tenofovir (TDF). Our data support the empiric continuation of 3TC in second-line regimens.

Immune-mediated attenuation of HIV-1

Immune escape mutations selected by human leukocyte antigen class I-restricted CD8(+) cytotoxic T lymphocytes (CTLs) can result in biologically and clinically relevant costs to HIV-1 replicative fitness. This phenomenon may be exploited to design an HIV-1 vaccine capable of stimulating effective CTL responses against highly conserved, mutationally constrained viral regions, where immune escape could occur only at substantial functional costs. Such a vaccine might 'channel' HIV-1 evolution towards a less-fit state, thus lowering viral load set points, attenuating the infection course and potentially reducing the risk of transmission. A major barrier to this approach, however, is the accumulation of immune escape variants at the population level, possibly leading to the loss of immunogenic CTL epitopes and diminished vaccine-induced cellular immune responses as the epidemic progresses. Here, we review the evidence supporting CTL-driven replicative defects in HIV-1 and consider the implications of this work for CTL-based vaccines designed to attenuate the infection course.

Interactions of tenofovir, Lamivudine, abacavir and Didanosine in primary human cells

Certain triple nucleoside/tide reverse transcriptase inhibitor (NRTI) regimens containing tenofovir (TDF) have been associated with rapid early treatment failure. The mechanism is unknown, but may be at the level of drug transport. We measured the lipophilicity of the drugs [³H]-lamivudine (3TC), -didanosine (ddI), -TDF and -ABC. Peripheral blood mononuclear cells (PBMCs) were used to evaluate drug–drug interactions at the level of drug transport. PBMCs were measured for the expression of P-glycoprotein (P-gp), multidrug resistance-associated protein-1 (MRP-1) and breast cancer resistance protein (BCRP) by flow cytometry. The rank order of the lipophilicity of the drugs were ABC⋙3TC≥ddI>TDF. The accumulation of [³H]-3TC, -ddI and -TDF were temperature sensitive (suggesting facilitated transport), in contrast to [³H]-ABC. ABC reduced the accumulation of [³H]-3TC, and cell fractionation experiments suggested this was mainly in membrane-bound [³H]-3TC. ABC/TDF and ABC/ddI increased the accumulation of [³H]-3TC and 3TC/TDF also increased the accumulation of [³H]-TDF. In contrast, none of the NRTI/NtRTI incubations (alone or in combination) altered the accumulation of [³H]-ABC and -ddI. PBMC expression of P-gp, MRP1 and BCRP were detected, but none correlated with the accumulation of the drugs. The high failure rates seen with TDF, ABC and 3TC are not fully explained by an interaction at transporter level.

Oral preexposure anti-HIV prophylaxis for high-risk U.S. populations: current considerations in light of new findings

This article reviews the status of current research evaluating oral preexposure prophylaxis (PrEP) for prevention of HIV infection in high-risk populations. In animal model studies, the use of antiretrovirals has been shown to be effective in preventing HIV acquisition. Early-phase PrEP studies have established safety in humans. Currently, more than 20,000 men and women will soon be enrolled in studies of oral or topical chemoprophylaxis, testing a variety of drug delivery methods including tenofovir disoproxil fumarate (TDF) gel applied vaginally or rectally, as well as oral PrEP using TDF by itself or coformulated with emtricitabine (FTC). The largest global PrEP trial in men who have sex with men (MSM), known as iPrEx has demonstrated that oral chemoprophylaxis can decrease HIV incidence in this population. Although TDF/FTC PrEP was generally well tolerated, side effects such as nausea, as well as mild and reversible renal abnormalities were increased among the men who received active medication, suggesting that PrEP users will need ongoing PrEP clinical monitoring. The prophylactic benefits of TDF/FTC were substantially attenuated by nonadherence, indicating that effective PrEP implementation programs will need to focus on this behavioral variable, in addition to safer sex counseling. This article considers biological, policy, and practical implications of large-scale oral PrEP implementation.

Role of HIV Subtype Diversity in the Development of Resistance to Antiviral Drugs

Despite the fact that over 90% of HIV-1 infected people worldwide harbor non-subtype B variants of HIV-1, knowledge of resistance mutations in non-B HIV-1 and their clinical relevance is limited. Due to historical delays in access to antiretroviral therapy (ART) on a worldwide basis, the vast majority of reports on drug resistance deal with subtype B infections in developed countries. However, both enzymatic and virological data support the concept that naturally occurring polymorphisms among different nonB subtypes can affect HIV-1 susceptibility to antiretroviral drugs (ARVs), the magnitude of resistance conferred by major mutations, and the propensity to acquire some resistance mutations. Tools need to be optimized to assure accurate measurements of drug susceptibility of non-B subtypes. Furthermore, there is a need to recognize that each subtype may have a distinct resistance profile and that differences in resistance pathways may also impact on cross-resistance and the selection of second-line regimens. It will be essential to pay attention to newer drug combinations in well designed long-term longitudinal studies involving patients infected by viruses of different subtypes.