Belonging to three structural sub-classes each characterized by distinct linkers or central scaffold-substitutions, to explore different molecular rigidity patterns and to test related metabolic assumptions . All divalent compounds were fully profiled in vitro, and compared in terms of overall druglike properties. In particular, 9a displayed in vitro low nM affinity values for the BIR3 domains of XIAP, cIAP1 and cIAP2, but also for XIAP-BIR2BIR3; it also showed good cytotoxicity properties against a selected breast cancer cell line. Notably, due to its ionisable secondary amino groups, 9a is soluble in physiological buffer and could be administered in vivo; thus, it resulted as the most promising compound in our library, and was selected for early in vivo characterization . 9a displayed significant potency as a single agent in reducing the Ligustilide supplier development of solid tumours in mice injected subcutaneously with a human ovarian cancer cell line, and increased the median survival time of mice in a human ovarian ascites model . In this communication we present biochemical, biophysical and structural characterization of 9a in its complexes with XIAP-BIR3, XIAP-BIR2BIR3 and cIAP1-BIR3. In particular, we report data on compound 9a binding to different BIR domains through analytical gel filtration and small angle X-ray scattering . Moreover, we present the crystal structures of cIAP1-BIR3 and XIAP-BIR3 domains in the presence of 9a, describing the molecular details of divalent Smac-mimetic recognition. Taken together, all the experimental evidences here reported suggest that 9a is one of the most powerful divalent Smac-mimetics known to date; the structural analysis of its recognition patterns, here presented, is the basis for further optimization in terms of target affinity and bioavailability. To test the capability of inducing caspase activation and apoptosis, 581073-80-5 MDA-MB-231 cells were treated with 9a, or left untreated. 9a not only inhibited cell growth in the MDA-MB-231 cell line, but Western blot analysis showed activation of caspase-8, -3 and -9, and apoptosis . Moreover, Western blot analysis shows that 9a induces degradation of cIAP1 and of cIAP2 , already at 30 min post treatment. Binding and displacement assays based on fluorescent polarization were us
