Background:  BMS-488043 is a small-molecule HIV-1 attachment inhibitor with demonstrated antiviral efficacy in HIV-1-infected subjects.

Methods: gp120/sCD4 binding, [³H]-BMS-488043/gp120 binding, virion/sCD4 binding, and sCD4-induced exposure of gp41 HR1 assays and phenotypic and genotypic analysis of virus variants selected against several attachment inhibitor analogs.

Results:  Mechanism of action studies showed that attachment inhibitors inhibited the binding of 10 distinct HIV envelope gp120 proteins to sCD4. [³H]-attachment inhibitor bound readily to gp120, and this binding was inhibited by sCD4. Attachment inhibitors could not displace sCD4 from a gp120/CD4 complex and binding of attachment inhibitors and CD4 were mutually exclusive. gp120, labeled with a photoaffinity analog, did not form a ternary complex with sCD4. BMS-488043 inhibited virion envelope trimers from binding to sCD4-IgG, and inhibition decreased with increasing sCD4-IgG concentrations. BMS-488043 also repressed the sCD4-induced exposure of the gp41 groove in virion, but only when added prior to sCD4. Furthermore, binding of attachment inhibitors induced conformational changes within gp120 as demonstrated by circular dichroism spectral shifts, thrombin accessibility of the V3 loop and gp120 recognition by conformationally dependent monoclonal antibodies CD4i and CD4BS. Collectively, the results indicate that binding of attachment inhibitors likely alters envelope conformation, with subsequent disruption of required gp120-CD4 interactions. Resistance mapping confirmed the binding site of BMS-488043 and showed that selected substitutions span the CD4 binding pocket and other regions of the envelope. Finally, 6 gp120 variants with CD4 binding pocket substitutions were severely defective in compound binding.

Conclusions:  The data are supportive of a mechanism by which attachment inhibitors bind to gp120 and cause conformational alterations, which disrupt the binding of HIV-1 gp120 to cellular CD4 receptors. Attachment inhibitor binding can also affect other conformation-dependent envelope functions.

Keywords: Envelope inhibitor; Mechanism; Conformational changes