Discovery and pre-clinical profile of SNX-5422: an orally active Hsp90 inhibitor in phase 1 trials for solid and hematological tumors

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Background A chemoproteomics-based affinity screen was used to discover a novel series of Hsp90 inhibitors, exemplified by SNX-7081, that are distinct from all other structural classes. This methodology, along with a series of high content cell-based screens, was used for optimization studies leading to the identification of SNX-5422, an orally active Hsp90 inhibitor. Methods Early hit exploration employed x-ray crystallography, cancer cell proliferation assays, and cell-based assays for client proteins (her2, p-erk1/2) or downstream markers (p-S6, Hsp70) to advance the structure-activity relationships. Final optimization focused on PK profiling in mouse and rat as well as demonstration of oral anti-tumor activity in one or more mouse xenograft models. Results The binding mode for this series was established through x-ray analysis of a co-crystal of SNX-1231 with the N-terminal domain of hsp90. Structure-guided synthesis led to a portfolio of high potency Hsp90 inhibitors including SNX-2112. This analog potently blocked cancer cell proliferation with IC50 values of 0.9 - 114nM across more than 20 solid and hematological tumor cell lines. SNX-2112 was significantly more potent (10 to 500-fold) than 17-AAG across a series of hematological tumor lines with an average IC50 value of 4nM. SNX-2112 induced client protein degradation at concentrations (BT474 EC50; her2 = 21 nM, p-erk = 38nM) comparable to those needed to inhibit cell proliferation (IC50 = 37nM) and induce apoptosis (BT474; EC50 = 11nM). SNX-5422, a simple prodrug of SNX-2112, provided improved oral bioavailability and was selected as a development candidate. Consistent and rapid conversion of SNX-5422 to SNX-2112 was observed in mice, rats, dogs such that the only species observed in plasma following oral dosing is SNX-2112. Bioavailability of SNX-2112 when SNX-5422 is administered orally is 56% and 65% in rats and dogs, respectively, with a half-life of 5h and 11h. SNX-2112 is highly selective as it did not show significant activity across a diverse group of enzymes and receptors (CEREP 74 panel) at 1μM, nor did it significantly inhibit a panel of P450 enzymes up to10μM. SNX-5422, dosed orally on a variety of schedules, inhibited tumor growth (10 models), and in some cases induced tumor regression as a single agent (BT474). In a xenograft survival model of leukemia (MV411) in the mouse, SNX-5422, dosed orally at 40 mg/kg 3x/week, resulted in 100% survival compared to a 30% survival rate for the vehicle group when assessed 10 days after cessation of dosing. Conclusions SNX-5422 represents the first clinical candidate from a highly potent, selective and orally active series of Hsp90 inhibitors. Phase 1 studies are ongoing in both solid and hematological tumor patients.