Characterizing a Novel Small Molecule Inhibitor of HIV-1 Env-Mediated Entry, MF275

Abstract

Human Immunodeficiency Virus-1 (HIV-1) entry into cells is mediated by the envelope (Env) trimer of gp120 and gp41 heterodimers. Sequential binding to the target cell receptors, CD4 and CCR5 or CXCR4, triggers the metastable Env to undergo entry-related conformational changes. PF-68742 was recently identified as a small molecule that inhibits infection of a subset of HIV-1 strains. We found that of the four PF-68742 diastereomers, only one, MF275, inhibited the infection of Cf2Th-CD4-CCR5 and Cf2Th-CD4-CXCR4 cells by some HIV-1 strains. Unexpectedly, MF275 activated the infection of Cf2Th-CCR5 cells by several HIV-1 strains resistant to the compound’s inhibitory effects in Cf2Th-CD4-CCR5 cells. In both cases, the strain susceptibility profiles were unique from those of other entry inhibitors. Washout of MF275 prior to addition of the virus mixture to target cells was able to abrogate its inhibitory effect in Cf2Th-CD4-CCR5 cells, whereas abrogation of the MF275 activating effect in Cf2Th-CCR5 cells was strain-specific. Sensitivity to other entry inhibitors in the presence of MF275 indicated that MF275-activated virus entry requires State 1-to-State 2 transitions, CCR5 binding, and 6-helix bundle formation and can be enhanced by pre-activation by a CD4 mimetic, whereas MF275 inhibition occurs regardless of CD4 mimetic pre-activation. In contrast to CD4 mimetic compounds, MF275 binding and activity did not depend upon availability of the gp120 Phe43 cavity, a known target of some other HIV-1 entry inhibitors. MF275 inhibition did not demonstrate a preference for State 1 versus State 2 Env conformations, but MF275 activation was abrogated by gp120 inner domain and gp41 ectodomain changes that favor the State 1 Env conformation. Changes in the gp120 C5 and gp41 fusion peptide (FP) and disulfide loop (DSL) regions conferred resistance to MF275 inhibition but not MF275 activation or CD4 mimetic activity. Finally, MF275 induced distinct conformational changes (as determined by a panel of broadly-neutralizing antibodies) in an Env susceptible to inhibition versus an Env susceptible to activation, with the former demonstrating conformational changes similar but not identical to those induced by CD4 or CD4 mimetics. Alterations in the gp120 C5, gp41 FP, and gp41 DSL desensitized Envs to broadly-neutralizing antibodies recognizing the FP and gp120-gp41 interface and abrogated the MF275-induced conformational changes associated with inhibition, but did not affect activation. Overall, MF275 is unique among HIV-1 entry inhibitors, mediating strain-specific inhibition or activation by two distinct mechanisms. We propose that MF275 binds and activates susceptible Envs in a manner similar to that of the CD4 receptor and CD4 mimetics; in addition, the Envs of some HIV-1 strains are susceptible to inhibition of a post-receptor binding step in virus entry.