Nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs) efficiently suppress HIV and serve as backbone of antiretroviral therapies. However, new therapeutics are needed for continued control of HIV infection. 4?- Ethynyl-2-fluoro-2?-deoxyadenosine (EFdA) is an NRTI with exceptional potency, stability, and unique mechanism of action against HIV, which has been licensed by Merck. EFdA (also known as MK-8591 or Islatavir) has been successfully used in Phase I and recently introduced in Phase II clinical trials. EFdA has generated ?compelling early results for both treatment and prevention? in patients, tested at remarkably low doses (up to >10,000-fold lower doses than current NRTI drugs) aiming at once-weekly oral and once-yearly slow-release dosing, unprecedented modalities for HIV therapies. Hence, this work is directly relevant to NIH guidelines for high priority AIDS funding (NOT-OD-15-137) for ?next generation therapies?that are longer acting.? We have shown that a) EFdA has high potency in vitro, in cell culture (EC50=50 pM in PBMCs), mice and non- human primate animal models. Although EFdA retains the 3?-OH, it blocks HIV reverse transcriptase (RT) in vitro, primarily as an immediate/obligate and at times delayed chain terminator due to difficulty of EFdA- terminated viral DNA to translocate. Thus, EFdA is termed a nucleotide reverse transcriptase translocation inhibitor (NRTTI). However, the inhibition mechanism of EFdA in HIV-infected cells (primary or cell lines) remains unknown. Toward that end, Co-I Malim has established an innovative assay that enables study of the EFdA antiviral mechanism in HIV-infected cells. Additionally, in vitro passage experiments have identified mutations that impart EFdA resistance through two paradigm-shifting dual mechanisms or resistance: decreased incorporation/enhanced excision and decreased incorporation/enhanced translocation. Data on an EFdA analog suggest efficient inhibition of EFdA-resistant HIV, although the mechanism of this phenomenon is not understood. Notably, treatment with key new generation NNRTI doravirine (DOR) leads to resistance mutations that impart hypersusceptibility to EFdA, paving the way for EFdA/DOR combination therapies. We hypothesize that the structural attributes of EFdA and its analogs impact interactions with diverse RTs (wild-type, drug- resistant, from diverse clades, viruses), leading to clinically important differences in efficiency of inhibition, resistance, hypersusceptibility, and biochemical mechanism of action. This hypothesis will be tested through three specific aims, which endeavor to answer the above questions using a combination of novel sequencing, biochemical, structural, and virological approaches. This work will help optimize combination therapies that reduce drug burden and have exceptional long-acting potential, addressing adherence challenges of chronic HIV treatment.

Public Health Relevance

4?-ethynyl-2-fluoro-2?-deoxyadenosine (EFdA, also known as MK-8591) is an extremely potent inhibitor of HIV, including drug-resistant viral strains, and blocks the virus by a different mechanism than all currently approved anti-HIV drugs. This project will determine the EFdA mechanism of action in the context of HIV infection in cells, and will also study the factors that affect development of high-level HIV resistance to EFdA, why certain HIV viruses that are resistant to other approved antivirals are hypersusceptible to EFdA, and address EFdA?s efficacy and resistance profile in various HIV subtypes. These studies will provide important insights on the design of combination therapies of EFdA with other approved antivirals leading to optimal treatments for HIV-infected patients around the world.

National Institute of Health (NIH)
National Institute of Allergy and Infectious Diseases (NIAID)
Method to Extend Research in Time (MERIT) Award (R37)
Project #
Application #
Study Section
HIV Molecular Virology, Cell Biology, and Drug Development Study Section (HVCD)
Program Officer
Sanders, Brigitte E
Project Start
Project End
Budget Start
Budget End
Support Year
Fiscal Year
Total Cost
Indirect Cost
Emory University
Schools of Medicine
United States
Zip Code