Use of the nucleotide analogue tenofovir (VireadTM) and the nucleoside analogue ddI (didanosine, VidexTM) together is not recommended, particularly in patients with a low CD4 cell count and high viral load, European clinicians are being warned by a “Dear Doctor” letter issued by the drugs’ manufacturers, Gilead Sciences and Bristol Myers Squibb (BMS). The letter also warns that in circumstances where the two drugs have to be used in conjunction, patients should be carefully monitored for the efficacy of the combination and ddI-related side-effects.
The letter was issued after a high rate of virological failure and resistance was observed in several clinical trials in which tenofovir and ddI were provided in combination with an NNRTI in patients who had never taken anti-HIV drugs before and had a high viral load and low CD4 cell count. Similar findings were reported in studies in studies looking at the safety and efficacy of tenofovir and ddI in combination with another nucleoside analogue.
Gilead and BMS also caution doctors that they cannot rule out that the use of tenofovir and ddI together might not have similar results in antiretroviral experienced patients, or individuals taking the two drugs with a protease inhibitor. The drug companies therefore warn “co-administration of didanosine and tenofovir DF is not recommended within any antiretroviral treatment regimen unless judged strictly necessary.”
Pharmacokinetic studies have in addition shown that the use of tenofovir and ddI together can result in a 40-60% increase in levels of ddI, which may result in an increased risk of ddI-related side-effects. Rare instances of pancreatitis and lactic acidosis, some of which have been fatal, have been reported.
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