Evaluation of Combinations of 4′-Ethynyl-2-Fluoro-2′-Deoxyadenosine with Clinically Used Antiretroviral Drugs

Abstract

i Drug combination studies of 4=-ethynyl-2-fluoro-2=-deoxyadenosine (EFdA) with FDA-approved drugs were evaluated by two different methods, MacSynergy II and CalcuSyn. Most of the combinations, including the combination of the two adenosine analogs EFdA and tenofovir, were essentially additive, without substantial antagonism or synergism. The combination of EFdA and rilpivirine showed apparent synergism. These studies provide information that may be useful for the design of EFdA combination regimens for initial and salvage therapy assessment. Combination antiretroviral therapies provide durable viral suppression and constitute the standard of care for HIV infection (http://www.aidsinfo.nih.gov/guidelines/) (1). For example, the combination of tenofovir disoproxil fumarate (TDF) and emtricitabine (FTC) (Truvada) is one of the preferred regimens for treatment of HIV-1 infection (2, 3). In vitro studies have shown that tenofovir (TFV) and FTC have synergistic antiretroviral activity (4, 5).The investigational nucleoside reverse transcriptase inhibitor (NRTI), 4=-ethynyl-2-fluoro-2=-deoxyadenosine (EFdA) is presently under preclinical evaluation. Unlike other NRTIs used in the treatment of HIV infection, EFdA retains the 3=-hydroxyl moiety. It also contains a 2-fluoro group on the adenine base and a 4=-ethynyl group on the deoxyribose ring. Although EFdA is an adenosine analog, its activation is initiated by phosphorylation by 2=-deoxycytidine kinase (dCK), and the drug is highly resistant to degradation by adenosine deaminase (ADA) (6). EFdA shows exceptional antiretroviral activity in vitro (6-8) and in vivo (9, 10) as well as a favorable cross-resistance profile with current reverse transcriptase inhibitors (RTIs) used in the clinic.Studies on potential interactions between EFdA and other antiretroviral drugs can provide information that could be useful in the development of combinatorial therapeutic strategies. The present study evaluates anti-HIV efficacy in combinations of EFdA with representative FDA-approved RTIs in vitro.We first determined the antiviral potencies of five NRTIs (zidovudine [AZT], lamivudine [3TC], FTC, TDF, and EFdA) and three nonnucleoside RTIs (NNRTIs; efavirenz [EFV], etravirine [ETR], and rilpivirine [RPV]) against HIV-1 NL4-3 in order to obtain an optimal range of drug concentrations for use in combination assay analyses. As previously demonstrated (6, 7), EFdA inhibited HIV-1 replication several orders of magnitude more efficiently than other currently approved NRTIs (Table 1). In the same cell-based assays, we evaluated antiretroviral activity of EFdA in combination with the FDA-approved RTIs. To obtain more comprehensive evaluations of drug combinations and to reduce analysis bias, we use two algorithms provided by the software packages MacSynergy II (version 1.0; Ann Arbor, MI) and CalcuSyn (Biosoft, Ferguson, MO), which are based on the Bliss independence model (11, 12) and the median effect principle (13), respectively. Quantitative differences in data analyses by the two algorit...

4′-Ethynyl-2-fluoro-2′-deoxyadenosine (EFdA) Inhibits HIV-1 Reverse Transcriptase with Multiple Mechanisms

“…Previously, we have shown that a series of 4Ј-substituted analogs with a 3Ј-OH possess anti-HIV activity (9 -11). EFdA is the most potent of this series and, to our knowledge, the most potent NRTI with subnanomolar EC 50 both in T-cells and peripheral blood mononuclear cells (10,11,16). The strong potency of EFdA is the result of its unique mechanism of action as well as its resistance to degradation by adenosine deaminase (15).…”

“…We have shown that EFdA is resistant to degradation by adenosine deaminase, providing an enhanced half-life to the compound compared with other NRTIs (11,15). EFdA is very effective not only against wild-type HIV but also against drug-resistant strains and has displayed synergistic antiviral effects when combined with other inhibitors such as the recently approved second generation non-NRTI rilpivirine (16). We have also shown that EFdA is significantly better than tenofovir (TFV), zidovudine, and emtricitabine in blocking simian immunodeficiency virus replication in monkey peripheral blood mononuclear cells and that it was highly effective at treating simian immunodeficiency virus infection and AIDS symptoms in simian immunodeficiency virus-infected macaques with advanced AIDS with no apparent signs of toxicity (17).…”

SAMHD1 Has Differential Impact on the Efficacies of HIV Nucleoside Reverse Transcriptase Inhibitors

“…They generally lack a 3=-OH group and thus act as chain terminators upon incorporation into viral DNA by reverse transcriptase (RT) (26)(27)(28)(29). However, 4=-ethynyl-2-fluoro-2=-deoxyadenosine (EFdA) retains a 3=-OH group, acts primarily by blocking RT translocation following incorporation of EFdA monophosphate (MP) into the template-primer, and has picomolar antiviral potency (30)(31)(32)(33)(34)(35)(36)(37). NRTIs are administered as nucleosides and are phosphorylated to their active forms by cellular kinases (38).…”

Structural basis of HIV inhibition by translocation-defective RT inhibitor 4′-ethynyl-2-fluoro-2′-deoxyadenosine (EFdA)

“…EFdA is effective against wild-type and clinical drug-resistant HIV strains (6), and acts synergistically with the second-generation nonnucleoside RT inhibitor rilpivirine (13). EFdA blocks simian immunodeficiency virus replication in vitro in macaque PBMCs better than tenofovir (TFV), zidovudine (AZT), and emtricitabine (FTC) (14).…”