Development of radamide analogs as Grp94 inhibitors.

Hsp90 isoform-selective inhibition is highly desired as it can potentially avoid the toxic side-effects of pan-inhibition. The current study developed selective inhibitors of one such isoform, Grp94, predicated on the chimeric and pan-Hsp90 inhibitor, radamide (RDA). Replacement of the quinone moiety of RDA with a phenyl ring (2) was found to be better suited for Grp94 inhibition as it can fully interact with a unique hydrophobic pocket present in Grp94. An extensive SAR for this scaffold showed that substitutions at the 2- and 4-positions (8 and 27, respectively) manifested excellent Grp94 affinity and selectivity. Introduction of heteroatoms into the ring also proved beneficial, with a 2-pyridine derivative (38) exhibiting the highest Grp94 affinity (K(d)=820 nM). Subsequent cell-based assays showed that these Grp94 inhibitors inhibit migration of the metastatic breast cancer cell line, MDA-MB-231, as well as exhibit an anti-proliferative affect against the multiple myeloma cell line, RPMI 8226.