540. Investigating the Mechanism of a Unique Human Immunodeficiency Virus-1 (HIV-1) Entry Inhibitor, MF275

BACKGROUND: HIV-1 entry into cells is mediated by sequential binding of target cell CD4 and CCR5 or CXCR4 to the metastable envelope (Env) trimer of gp120-gp41 heterodimers (Figure 1). We determined that MF275, a single diastereomer of the small molecule entry inhibitor PF-68742, is necessary and sufficient to inhibit entry of a subset of HIV-1 strains. We investigated the mechanism of MF275. METHODS: Recombinant luciferase-expressing HIV-1 pseudotyped by wild-type or mutant HIV-1 Envs was incubated with MF275, other entry inhibitors, and/or antibodies. The virus-inhibitor mixture was added to CD4+ CCR5+ or CD4− CCR5+ target cells and luciferase activity measured. RESULTS: Unlike other entry inhibitors, MF275 not only reversibly inhibited the infection of CD4+ CCR5+ cells by some HIV-1 strains, but also irreversibly enhanced the infection of CD4− CCR5+ cells by others. In both cases, the strain susceptibility profiles were unique from those of CD4-mimetics, BMS-378806, and maraviroc. Furthermore, MF275 activity was not affected by mutations conferring resistance to other entry inhibitors and vice versa. In line with its activating activity, MF275 sensitized susceptible Envs to neutralization by a variety of broadly neutralizing antibodies against different epitopes. Changes in the gp120 C5 and gp41 FP regions conferred resistance to MF275 inhibition but not activation. Furthermore, sensitivity to other entry inhibitors in the presence of MF275 indicated that inhibition and activation target different conformational intermediates along the entry pathway, with the former targeting the prehairpin intermediate. CONCLUSION: MF275 is unique among HIV-1 entry inhibitors. Depending on the conformation of the target Env, which appears related to the gp120-gp41 interface, MF275 mediates inhibition or activation via distinct mechanisms (Figure 2). Further characterization of the MF275 mechanisms and binding site/s will advance understanding of the HIV-1 entry pathway as well as assist optimization of its clinical utility as an antiretroviral in multi-class drug resistance and potentially as an adjunct to vaccines. [Image: see text] [Image: see text] DISCLOSURES: All authors: No reported disclosures.