Background:  Despite the relative success of ART, emergence of drug-resistant HIV-1 variants continues to be the major cause for treatment failure. OBP-601 (4’-Ed4T) is a novel nucleoside analog with potent anti-HIV-1 activity and limited cellular toxicity, which has a unique in vitro resistance profile. Here we evaluated the ability of OBP-601 to inhibit the replication of multi-drug resistant viruses, and its potential use in combination therapy.

Methods:  A panel of 40 3’Gag/PR/RT-recombinant viruses were generated using polymerase chain reaction (PCR) products from clinical samples harboring selected sets of NRTI resistance mutations (e.g., NAM, TAM41, TAM67, M41l+V118I, K65R, L74V, and Q151M, plus K103N + M184V). These viruses were employed in drug susceptibility assays to test the activity of OBP-601 alone or in combination with zidovudine (AZT), stavudine (d4T), abacavir (ABC), tenofovir (TDF), lamivudine (3TC), efavirenz (EFV), nevirapine (NVP), lopinavir (LPV), atazanavir (ATV), and darunavir (DRV). Viral replication was quantified using luciferase expression and/or an in-house reverse transcriptase (RT) assay, and 50% (IC₅₀) inhibition concentration were determined. Antiviral isobolograms were calculated for the combination studies.

Results:  OBP-601 susceptibility of recombinant viruses carrying wild type 3’Gag/PR/RT sequences ranged from 0.76 to 5.80 µM in our in vitro system, slightly lower than the parental compound d4T (1.57 to 6.06 µM). The anti-HIV-1 activity of OBP-601 was reduced in most viruses carrying NAM (5- to 10-fold), TAM41 (0.3- to 4.3-fold), and TAM67 (1.6- to 7.8-fold) resistance mutations, together with K103N + M184V. Interestingly, viruses carrying the Q151M substitution were hypersusceptible to OBP-601 (0.1- to 0.2-fold), even in the presence of K65R (0.7- to 1.2-fold). More important, OBP-601 showed strong (AZT, EFV) to moderate (ABC, TDF, NVP) synergistic interaction with different antiretroviral drugs in wild type viruses. Similar results were obtained with 3TC, EFV, and ABC in viruses carrying M184V, K103N, and Q151M, respectively.

Conclusions:  This novel nucleoside analog has proven to be more potent and less toxic than d4T. Its unique resistance profile, effectiveness against drug-resistant viruses (e.g., carrying the Q151M complex), and the synergistic effect with other reverse transcriptase inhibitors (RTI) (in both wild type and drug-resistant viruses) warrant further investigation for its potential use in combination therapy against HIV-1.