Summary: Hsp90 is a molecular chaperone that is responsible for the conformational maturation of signaling proteins associated with all ten hallmarks of cancer, making it a promising target for the treatment of cancer, as multiple signaling nodes can be simultaneously derailed as a consequence of Hsp90 inhibition. Moreover, researchers have shown that Hsp90 inhibitors accumulate in tumors with high differential selectivity, making Hsp90 a highly sought after target for cancer. Unfortunately, clinical trials with 17 small molecule inhibitors have led to multiple detriments that have significantly dampened enthusiasm for Hsp90 inhibitors, as increased levels of Hsp90 were observed in the clinic, which led to dose-escalating toxicities among other concerns. Consequently, Hsp90 remains a desirable target for the development of cancer chemotherapeutics, but new approaches to inhibit the protein machinery are needed that do not induce Hsp90 levels. Through a number of seminal studies, it has been shown that inhibitors of the Hsp90 C-terminal domain can segregate Hsp90 inhibition from induction of Hsp90 levels, and therefore, we propose in this application to optimize these compounds and to perform a number of pre-IND studies on the best molecules in an effort to move them toward clinical evaluation.