Background: Regimens with two nucleoside analogue reverse transcriptase inhibitors (NRTI) plus tenofovir DF have been associated with a high failure rate when administered as first line therapy. Little is known about patients with undetectable viral loads who are switched to these regimens.
Methods: A post-hoc review of the virological outcomes at 24 weeks of patients who switched from a successful (< 50 copies/ml) highly active antiretroviral therapy regimen to a tenofovir plus two NRTI combination.
Results: Fifty-five patients started a two NRTI plus tenofovir regimen mostly because of previous toxicity/intolerance of the original drugs (74%). After 24 weeks, only 17 patients (31%) remained virologically suppressed. Patients with a regimen including a didanosine plus tenofovir-based regimen had significantly poorer outcomes than those on other combinations (success rate 5 versus 47.1%, P = 0.001). In contrast, patients on a regimen including zidovudine plus tenofovir showed a trend towards a better outcome (75 versus 27%, P = 0.083). Multivariate analysis confirmed the combination of didanosine plus tenofovir as the only variable associated with a higher rate of failure (odds ratio 17.7; 95% confidence interval 2.1-147; P = 0.007). Patients with previous reverse transcriptase mutations presented virological failure in all cases. At failure a new pattern, including the K65R mutation with M184V or thymidine analogue mutations, was observed.
Conclusions: Even in patients with suppressed viraemia, a two NRTI plus tenofovir regimen is associated with a high virological failure rate, but significant variations are found depending on the nucleosides included.