Activity Of At13387, A Novel, Non-Ansamycin Inhibitor Of Heat Shock Protein 90, Against Gastrointestinal Stromal Tumors (Gist)

105 Background: AT13387 is a second-generation potent, novel non-ansamycin HSP90 inhibitor (Kd 0.71nM). The majority of GIST tumors are characterized by activating mutations of c-KIT, an HSP90 client protein. Secondary resistance mutations within c-KIT limit clinical responses to TKIs. The dependence of c-KIT and its mutated forms on HSP90 suggests that HSP90 inhibition may be a valuable treatment option for imatinib-sensitive and resistant clones. In vitro, AT13387 inhibited the proliferation of imatinib-sensitive (GIST882, GIST-T1) and imatinib-resistant (GIST430, GIST48) cell lines. In vivo, AT13387 demonstrated anti-tumor activity in the imatinib-sensitive (GIST-PSW) and imatinib-resistant (GIST430) xenograft models. Induction of HSP70, depletion of phospho-c-KIT and inhibition of c-KIT signaling were observed in both models. Combination treatment of imatinib and AT13387 in the GIST430 model was well tolerated and significantly enhanced tumor growth inhibition over either monotherapies. Methods: In a completed phase I study, AT13387 was administered IV over 1 hour twice weekly or weekly of a 28-day cycle in a standard 3+3 dose-escalation design. The primary endpoint was to determine the MTD; secondary endpoints included PK, PD, safety and tolerability. Results: The PK exposures were dose-dependent and linear. AT13387 was well tolerated on both schedules. DLTs included primarily G2 AEs of GI toxicities, fatigue and infusion site reactions. The once weekly RP2D was determined to be 260 mg/m 2 . HSP70 induction was 2–7 fold at higher doses. A total of 7 GIST subjects were enrolled. An objective and durable PR was observed in one subject and 2 SDs at 8, 7 and 11 months, respectively. The PR subject demonstrated molecular resistance to kinase inhibitor treatment in the c-KIT gene prior to initiation of AT13387 therapy in two resected lesions by harboring the same activating c-KIT deletion in exon 11 and two separate TKI resistance mutations in exon 17. Conclusions: Overall, these results suggest AT13387 is a promising agent in GIST, including TKI-resistant c-Kit positive GIST. AT13387 is currently being evaluated in combination with imatinib in an ongoing phase I/II study. Clinical trial information: NCT01294202.