Virco, the developer of HIV resistance tests, has released data suggesting that levels of ddI and d4T resistance are being underestimated by current tests, and that reduced sensitivity to these drugs may be more of problem than previously thought.
Brendan Larder, Chairman of Virco UK, told a press conference earlier this week that his company had tested susceptibility to all antiretroviral drugs in more than 1,000 HIV isolates from untreated people in order to come up with a better picture of the normal range of drug susceptibility in wild-type viruses.
By establishing the average and then calculating two standard deviations above the mean (the point that can be considered above the normal range with 97.5% confidence), they were able to show the reduction in susceptibility for each drug that should be considered abnormal.
Up until now, all phenotypic assay have used a one size fits all method for assessing when a viral isolate has reduced susceptibility, or resistance, to a drug. The cut-off points are 4-10 fold reduced susceptibility (borderline resistance) and greater than 10 fold reduced susceptibility (full resistance). Anything less than fourfold reduced susceptibility has until now been described as sensitive to the drug.
The Virco investigation has established new cut-offs for employment in its Antivirogram assay. The cut-off point for NNRTIs has been adjusted upwards to 7-10 fold, while the cut-off point for ddI and d4T has been adjusted downwards, to 3.5 and 3.0-fold respectively.
When the new cut-off points were tested against 5,000 isolates from US patients failing therapy, the incidence of reduced sensitivity to most drugs was unchanged, with the exception of d4T, ddI and ddC, where levels appear to have been underestimated. Previously the Antivirogram would have assessed the incidence of D-drug resistance at around 5-9%, but this has risen to around 15%-19% using the new cut-offs.
In another piece of research presented as a late breaker at the conference, Brendan Larder described the process used to narrow down the pattern of genotypic mutations that contributes to reduced susceptibility to d4T. Using a random selection of 188 out of 2286 isolates with reduced d4T susceptibility in the company’s database, researchers investigated how well the presence of nine mutations known to be associated with reduced d4T susceptibility predicted resistance to the drug in that selection. This was then compared with the additional predictive value of many other NRTI mutations added to the original set of mutations, and the mutations which cropped up most frequently in relation to the greatest reductions in d4T susceptibility were gradually extracted from the database.
More than 26 mutations, including those commonly selected during the failure of AZT therapy such as 41L, 67N and 215Y, were implicated, and were more predictive of resistance when added to the classic d4T mutations than the nine d4T mutations alone.
The Virco Virtual Phenotyping database also predicts that any secondary mutation added to the AZT mutations 41L, 215Y and 210 will make HIV no longer susceptible to d4T, whereas ddI resistance requires the accumulation of a much larger number of thymidine analogue mutations.
The findings led Professor Joep Lange of the University of Amsterdam, speaking at a symposium sponsored by Gilead Sciences, to suggest that it may be preferable to start therapy with a non-thymidine analogue-containing regimen, thus avoiding both AZT and d4T.
“We should consider using the NRTIs with the highest potential for cross-resistance later rather than earlier – at the moment all our most popular first-line combinations permit cross-resistance between thymidine analogues. I think eventually there will be a move away from thymidine analogue regimens – that’s my prediction” he said.
Invited to name his preferred first line NRTI backbone, Prof. Lange declined to commit himself, but did note that the failure of 3TC-containing regimens such as Trizivir usually leads to the development of the M184V mutation rather than the classic thymidine analogue mutations associated with AZT and d4T. A combination of ddI and 3TC might have a similar effect, and the experimental once a day NRTI emtricitabine (FTC), which has a similar resistance profile to 3TC, is currently being tested in combination with ddI.