15th HIV Resistance Workshop: tenofovir resistance in Africans may be quicker, more frequent

New antiretrovirals with novel mutational profiles allow physicians and patients to sequence drugs without overlapping resistance – that’s the theory. A somewhat disconcerting report from Botswana conducted by a Botswana-Harvard-McGill coalition and reported by Florence Doualla-Bell from McGill University in Montreal lent further credence to the view that some drug resistance mutations emerge at different frequencies and speeds in different viral subtypes. In particular the findings have serious implications for the eventual use of tenofovir in countries with subtype C HIV epidemics.

Much of what is known until now of K65R associated with tenofovir (TDF) resistance, has come from patients with clade B viruses. In this recent study, patients were infected with subtype C, the predominant profile in Botswana. Samples were assessed from 23 patients experiencing virologic failure with prior experience of regimens containing ddI and d4T in first or second line therapy. Seven of these patients had developed K65R following exposure to ddI and d4T with a median exposure of 8 months (range 4-18 months). Four patients developed K65R whilst on ddI/d4T therapy.

Interestingly, three patients harboured only K65R whilst the remaining four developed F116Y, S68G and the multi-drug resistance (MDR) complex Q151M. Two of these patients that had experienced ddI/d4T as well as AZT/3TC, subsequently failed with K65R and M184V. A group of patients did not develop K65R, and the authors explained that this was due to treatment with AZT and 3TC; these patients went on to develop the alternate thymidine analogue mutations (M41L, D67N, K70R, T215Y/F, 219E/Q).

This research confirms that K65R may be more prone to occur in subtype C infections. These findings are of concern since they imply choices for second line strategies may be limited in subtype C infections in patients with prior ddI/d4T exposure. However, this situation may change as patients in Botswana now use AZT/3TC rather than ddI/d4T as a nucleoside backbone in first-line therapy.

A mechanistic explanation for this phenomenon in subtype C viruses was provided by Mark Wainberg’s team at McGill. Sequence analysis from a number of subtypes including B, C, A, CRF 1 (AE), G and HIV-2 revealed that subtype C viruses possess unique polymorphisms in reverse transcriptase at codons 64 (AAG – AAA), 65 (AAA – AAG) and codon 66 (AAA – AAG) that are absent in other viral subtypes. Under TDF pressure in tissue culture, this apparently results in the early appearance of K65R by week 12 (AAG – AGG) compared to no K65R emergence after 52-74 weeks in subtype B.

One virus from each of clades A, AG and G developed K65R by 30-33 weeks and three HIV-2 viruses showed K65R at 27-29 weeks. In subtype AE, K65R appeared as late as 55-73 weeks in culture. These subtype differences did not vary according to subtype for M184V with 3TC pressure (weeks 8-14) although K65R was slower to present in subtype C patients with M184V resistance.

Replicative capacity (RC) was also assessed but the mechanism of resistance evolution rather than replicative competence was suggested as a reason for the differences with no significant differences in RC. This research suggests that TDF-based treatments should be monitored carefully in settings with a preponderance of C viruses, and it may also have implications for TDF-based pre-exposure and post-exposure prophylaxis, as well as any potential use of tenofovir as prophylaxis in infants exposed to HIV perinatally.