Recent work has demonstrated the existence of a systemic interaction between didanosine (ddI) and tenofovir disoproxyl fumarate (TDF) that leads to a significant increase in plasma ddI levels when coadministered with TDF (40 to 50% increase). These two drugs are, respectively, nucleoside and nucleotide analogues of adenosine and efficiently inhibit the human immunodeficiency virus (HIV) reverse transcriptase when transformed to their triphosphate moieties in the intracellular (IC) medium (ddA-TP and TFV-DP, respectively). Since ddI and TDF partly share the same IC metabolic pathway leading to the active triphosphates, we investigated a putative IC interaction. We used high-performance liquid chromatography-tandem mass spectrometry techniques to determine ddA-TP and TFV-DP IC levels in HIV-infected patients cotreated with both drugs, in comparison with patients treated with just one of the two drugs. These measurements revealed no significant differences in IC levels of the corresponding triphosphates when ddI (250 mg, once a day [QD]) was coadministered with TDF (300 mg, QD) compared to ddI 400 mg (QD) administered without TDF, thus supporting the dose adaptation proposed for this combination. However, we observed that both ddA-TP and TFV-DP have very long IC half-lives, resulting in unusual IC pharmacokinetic profiles with no significant changes in triphosphate concentrations between two dosings. In the case of TFV-DP, this t 1/2 of elimination was roughly estimated to be 180 h (7.5 days). This characteristic is certainly interesting in terms of efficacy but could have some drawbacks in terms of virus resistance for patients discontinuing these drugs.Nucleoside reverse transcriptase inhibitors (NRTIs) were the first antiretroviral class shown to be effective against human immunodeficiency virus (HIV) infection and still play a critical role as major components of highly active antiretroviral therapy. The first generation of NRTIs comprised 2Ј,3Ј-dideoxynucleosides which have to undergo three phosphorylation reactions within cells and work both as competitive inhibitors and as chain terminators with regards to HIV reverse transcriptase.Closely related drugs, i.e., acyclic phosphonate analogues of nucleotides (NtRTIs), have recently been approved for clinical applications and are now given as part of combination antiretroviral regimens. This new strategy consists of directly providing the cell with the monophosphate analogue, thus bypassing the first phosphorylation step, which proved to be rate limiting for many NRTIs. The first approved drug of this category is tenofovir disoproxyl fumarate (TDF), which provides high intracellular (IC) levels of tenofovir diphosphate (TFV-DP, equivalent to NRTI triphosphate) with very low mitochondrial toxicity (7).However, the issue of drug-drug interactions between NRTIs and also between NRTIs and NtRTIs is of major concern and should be addressed with all the analytical and chemical tools available. NRTIs and NtRTIs might interact not only at the systemic level (absorption, ...

No abstract