Abstract B14: Hsp90-inhibitor drug conjugate STA-12-8666 demonstrates complete tumor regression in preclinical models of pediatric sarcoma

Long-term survival in patients with metastatic relapsed or recurrent Ewing sarcoma (ES) and rhabdomyosarcoma (RMS) is dismal. Encouraging responses to irinotecan, a topoisomerase 1 (Top1) inhibitor, have been seen in these patients; however, limitations in irinotecan bioavailability, including low conversion rate to the active metabolite (SN-38) and high excretion rate of the inactive form, hinder efficacy. HSP90 is widely expressed in cancer cells, and HSP90 inhibitors (HSP90i) have favorable pharmacokinetics for anticancer use, as they remain in tumors for longer periods of time and at higher steady-state levels compared to normal tissue. This property makes them ideal intracellular delivery vehicles for chemotherapeutic drugs, allowing for high tumor exposure and low systemic toxicity. STA-12-8666 (Synta Pharmaceuticals) is an HSP90i drug conjugate (HDC) consisting of a weak HSP90i attached to SN-38 through a cleavable chemical linker. The purpose of this study was to test this HDC in xenograft models of pediatric sarcoma and to investigate its mechanism of action. To test therapeutic efficacy of this HDC, female SCID mice underwent orthotopic injection of ES or RMS cells from established cell lines or PDX tissue. When tumors reached a desired size, mice were randomized and then treated weekly with HDC, vehicle, ganetespib (a highly potent HSP90i), high dose irinotecan, protracted dose irinotecan, or irinotecan plus ganetespib. Tumors were measured twice per week, and mice were weighed weekly to determine drug tolerability. Tumors were harvested at midpoints and at study endpoint for biology studies. Activity of pharmacodynamic (PD) markers was investigated in tumor tissue. In xenograft models of ES and RMS, treatment with HDC produced superior antitumor efficacy compared to the other arms. When initial treatment began in mice with palpable tumors (between 100 and 500 mm3 (ES) or 50 and 90 mm3 (RMS)), all tumors underwent complete regression after 2 doses of HDC, with total tumor eradication in all ES mice and several RMS mice. In the RMS group, 6/8 mice relapsed by 23 weeks, and all 6 of those responded to retreatment with HDC. When initial treatment was delayed until tumors reached between 800 mm3 and 1000 mm3, complete regressions were again achieved in ES after 2 doses and RMS after 4 doses. Compared with high dose weekly irinotecan, which also induced tumor regression, mice treated with HDC had longer and more persistent remissions. A dose response effect was seen in HDC with cures noted in mice with ES at the 100- and 150-mg/kg doses and longest remissions noted in RMS in the 150-mg/kg group. Tolerability of the HDC was excellent with no toxicity-related deaths or weight loss in any treated mice. Studies using PDX models are ongoing and will be reported. Activity of γH2AX in tumor samples was explored as a PD marker of Top1 inhibitor activity. Mice bearing ES were treated with a single dose of vehicle, irinotecan, or HDC and one mouse per group was sacrificed at serial intervals between 6 hours and 10 days post-treatment. Expression of γH2AX in irinotecan mice began to wane between days 1 and 3, whereas in HDC mice, it was still detectable at day 7, suggesting that HDC results in more persistent inhibition of topoisomerase 1 compared to irinotecan. To look at the potential role of HSP90 inhibition in this HDC, HSP70 activity was investigated as a marker of HSP90 inhibition in samples from the dose finding experiment. In ES, no HSP70 was detected in samples from mice treated with irinotecan or HDC, suggesting the primary mechanism of action is via SN-38. In RMS, HSP70 was slightly induced in mice treated with higher doses of the HDC, perhaps contributing to the higher relapse rate in this model. Preclinical data suggest that this HDC may be a promising anticancer agent for ES and RMS patients. Citation Format: Christine M. Heske, Arnulfo Mendoza, Choh Yeung, David A. Proia, Len Neckers, Lee J. Helman. Hsp90-inhibitor drug conjugate STA-12-8666 demonstrates complete tumor regression in preclinical models of pediatric sarcoma. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Pediatric Cancer Research: From Mechanisms and Models to Treatment and Survivorship; 2015 Nov 9-12; Fort Lauderdale, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(5 Suppl):Abstract nr B14.