Sara Iannuzzi
Freie Universität Berlin/Robert Koch Institut
Objectives: The two nucleoside reverse transcriptase inhibitors (NRTI) Tenofovir disoproxil fumarate (TDF) and emtricitabine (FTC) are used in combinationfor treatment of HIV-1 infection and in pre-exposure prophylaxis. In-vitro studies have proposed that TDF and FTC act synergistically within an HIV infected cell. However, it is unclear whether and which direct drug-drug interactions mediate the apparent synergy.
The goal of this work was to refine a mechanistic model for the molecular mechanism of action (MMOA) of nucleoside analogues in order to analyse whether putative direct interactions may account for the in-vitro observed synergistic effects.
Methods: A previously defined model [1] for the molecular mechanism of action (MMOA) was implemented. The model explicitly considers reverse transcriptase (RT)-mediated polymerization of nascent viral DNA and the effect of activated NRTI-triphosphates. Mechanistically, the activated NRTI-triphosphates (NRTI-TPs) interfere with polymerization by competing with endogenous deoxynucleotide triphosphates (dNTPs) for incorporation into viral DNA. Upon integration of the NRTI-TP into the extending primer, the lack of a hydroxyl group impairs incorporation of the next incoming nucleotide, resulting in a halt of the polymerization process. This state can be reversed by excision of the NRTI-TP from the primer. By using in-vitro measurable microkinetic parameters as input, the model can be used to compute the inhibition of reverse transcription (𝜀) by NRTIs, and from there the inhibition of cell infection (𝜂), in the presence of different concentrations of single drug as well as drug combination.
Two extended models were generated on the basis of putative synergistic mechanisms from in-vitro studies [2, 3], namely the alteration of dNTP pools and/or the formation of a dead-end complex. The models were used to assess – for realistic parameter ranges – if the proposed hypotheses could be mechanistic explainations of the observed levels of synergy between the drugs. Altogether, three models (dNTP alterations, DEC formation and dNTP alterations + DEC formation) with mechanism-specific modifications with were generated and tested against the unmodified one. To assess the combined effect of FTC and TDF, common synergy metrics were applied.
Results: Our predictions with the MMOA model, as well as available experimental data indicate that the direct interaction of FTC-TP and TFV-DP is mainly mediated by a depletion of dNTP pools, at clinically relevant concentrations.Interactions at the level of DEC formation play a minor role for this drug combination at physiologically meaningful drug concentrations. However, experimental data suggests that DEC formation seems to be greatly facilitated when NNR- TIs are added to the TDF/FTC backbone.
Conclusions: The refined model output (η) can be used as a drug efficacy parameter. It can be integrated in other routines, providing a link between the pharmacokinetic and pharmacodynamic of combination therapy.
References:
[1] Duwal, S. and M. von Kleist, Top-down and bottom- up modeling in system pharmacology to understand clinical efficacy: An example with NRTIs of HIV-1. Eur J Pharm Sci, 2016. 94: p. 72-83.
[2] Feng, J.Y., et al.,Thetriplecombinationoftenofovir, emtricitabine and efavirenz shows synergistic anti- HIV-1 activity in vitro: a mechanism of action study. Retrovirology, 2009. 6: p. 44
[3] Chen, X., et al., Analysis of the Endogenous De- oxynucleoside Triphosphate Pool in HIV-Positive and -Negative Individuals Receiving Tenofovir- Emtricitabine. Antimicrob Agents Chemother, 2016. 60(9): p. 5387-92.
Reference: PAGE 30 (2022) Abstr 9955 [www.page-meeting.org/?abstract=9955]
Poster: Drug/Disease Modelling - Infection