No abstract

Targeted inhibition of the molecular chaperone Hsp90 results in the simultaneous blockade of multiple oncogenic signaling pathways and has, thus, emerged as an attractive strategy for the development of novel cancer therapeutics. Ganetespib (formerly known as STA-9090) is a unique resorcinolic triazolone inhibitor of Hsp90 that is currently in clinical trials for a number of human cancers. In the present study, we showed that ganetespib exhibits potent in vitro cytotoxicity in a range of solid and hematologic tumor cell lines, including those that express mutated kinases that confer resistance to small-molecule tyrosine kinase inhibitors. Ganetespib treatment rapidly induced the degradation of known Hsp90 client proteins, displayed superior potency to the ansamycin inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG), and exhibited sustained activity even with short exposure times. In vivo, ganetespib showed potent antitumor efficacy in solid and hematologic xenograft models of oncogene addiction, as evidenced by significant growth inhibition and/or regressions. Notably, evaluation of the microregional activity of ganetespib in tumor xenografts showed that ganetespib was efficiently distributed throughout tumor tissue, including hypoxic regions >150 mm from the microvasculature, to inhibit proliferation and induce apoptosis. Importantly, ganetespib showed no evidence of cardiac or liver toxicity. Taken together, this preclinical activity profile indicates that ganetespib may have broad application for a variety of human malignancies, and with select mechanistic and safety advantages over other first-and second-generation Hsp90 inhibitors. Mol Cancer Ther; 11(2); 475-84. Ó2011 AACR.

In human trials certain heat shock protein 90 (Hsp90) inhibitors, including 17-DMAG and NVP-AUY922, have caused visual disorders indicative of retinal dysfunction; others such as 17-AAG and ganetespib have not. To understand these safety profile differences we evaluated histopathological changes and exposure profiles of four Hsp90 inhibitors, with or without clinical reports of adverse ocular effects, using a rat retinal model. Retinal morphology, Hsp70 expression (a surrogate marker of Hsp90 inhibition), apoptotic induction and pharmacokinetic drug exposure analysis were examined in rats treated with the ansamycins 17-DMAG and 17-AAG, or with the second-generation compounds NVP-AUY922 and ganetespib. Both 17-DMAG and NVP-AUY922 induced strong yet restricted retinal Hsp70 up-regulation and promoted marked photoreceptor cell death 24h after the final dose. In contrast, neither 17-AAG nor ganetespib elicited photoreceptor injury. When the relationship between drug distribution and photoreceptor degeneration was examined, 17-DMAG and NVP-AUY922 showed substantial retinal accumulation, with high retina/plasma (R/P) ratios and slow elimination rates, such that 51% of 17-DMAG and 65% of NVP-AUY922 present at 30 min post-injection were retained in the retina 6h post-dose. For 17-AAG and ganetespib, retinal elimination was rapid (90% and 70% of drugs eliminated from the retina at 6h, respectively) which correlated with lower R/P ratios. These findings indicate that prolonged inhibition of Hsp90 activity in the eye results in photoreceptor cell death. Moreover, the results suggest that the retina/plasma exposure ratio and retinal elimination rate profiles of Hsp90 inhibitors, irrespective of their chemical class, may predict for ocular toxicity potential.

Activating mutations and/or overexpression of FGFR3 are common in bladder cancer, making FGFR3 an attractive therapeutic target in this disease. In addition, FGFR3 gene rearrangements have recently been described that define a unique subset of bladder tumors. Here, a selective HSP90 inhibitor, ganetespib, induced loss of FGFR3-TACC3 fusion protein expression and depletion of multiple oncogenic signaling proteins in RT112 bladder cells, resulting in potent cytotoxicity comparable with the pan-FGFR tyrosine kinase inhibitor BGJ398. However, in contrast to BGJ398, ganetespib exerted pleiotropic effects on additional mitogenic and survival pathways and could overcome the FGFR inhibitor-resistant phenotype of FGFR3 mutant-expressing 97-7 and MHG-U3 cells. Combinatorial benefit was observed when ganetespib was used with BGJ398 both in vitro and in vivo. Interestingly, two additional FGFR3 fusion-positive lines (RT4 and SW480) retained sensitivity to HSP90 inhibitor treatment by the ansamycins 17-AAG and 17-DMAG yet displayed intrinsic resistance to ganetespib or AUY922, both second-generation resorcinol-based compounds. Both cell lines, compared with RT112, expressed considerably higher levels of endogenous UGT1A enzyme; this phenotype resulted in a rapid glucuronidationdependent metabolism and subsequent efflux of ganetespib from SW780 cells, thus providing a mechanism to account for the lack of bioactivity.

The clinical benefits of chemotherapy are commonly offset by insufficient drug exposures, narrow safety margins, and/or systemic toxicities. Over recent decades, a number of conjugate-based targeting approaches designed to overcome these limitations have been explored. Here, we report on an innovative strategy that utilizes HSP90 inhibitor-drug conjugates (HDC) for directed tumor targeting of chemotherapeutic agents. STA-12-8666 is an HDC that comprises an HSP90 inhibitor fused to SN-38, the active metabolite of irinotecan. Mechanistic analyses in vitro established that high-affinity HSP90 binding conferred by the inhibitor backbone could be exploited for conjugate accumulation within tumor cells. In vivo modeling showed that the HSP90 inhibitor moiety was required for selective retention of STA-12-8666, and this enrichment promoted extended release of active SN-38 within the tumor compartment. Indeed, controlled intratumoral payload release by STA-12-8666 contributed to a broad therapeutic window, sustained biomarker activity, and remarkable degree of efficacy and durability of response in multiple cell line and patient-derived xenograft models. Overall, STA-12-8666 has been developed as a unique HDC agent that employs a distinct mechanism of targeted drug delivery to achieve potent and sustained antitumor effects. These findings identify STA-12-8666 as a promising new candidate for evaluation as novel anticancer therapeutic.