Abstract 983: Preclinical characterization of PRT1419, a potent, selective and orally available inhibitor of MCL1

MCL1 is a member of the anti-apoptotic BCL2 family of proteins and plays a critical role in maintaining cellular homeostasis and promoting cell survival. It is frequently amplified in cancer and increased expression of MCL1 is associated with a higher grade and poor prognosis in multiple tumor types. Importantly, MCL1 has been implicated in mediating resistance to chemotherapy as well as targeted therapies, including the BCL2 inhibitor, venetoclax. Here, we describe the in vitro and in vivo activity of PRT1419, a potent and selective inhibitor of human MCL1, that can induce tumor cell death by apoptosis. PRT1419 inhibits the binding of MCL1 to its physiological ligand, BIM, with low nanomolar potency. PRT1419 also demonstrated >200-fold selectivity against other BCL2 family members, including BCL2 and BCL-XL. In vitro, PRT1419 treatment resulted in robust activation of apoptotic markers such as cleaved caspase-3 in a concentration-dependent manner in several cancer cell lines. Consistent with its pro-apoptotic effects, PRT1419 treatment led to robust inhibition of cell proliferation in a concentration-dependent manner in a panel of cancer cell lines. Cell lines representing hematologic cancers as well as a subset of breast and non-small cell lung cancer lines were sensitive to PRT1419, and this response was associated with a significantly higher MCL1/BCL-XL mRNA ratio. Also, PRT1419 treatment resulted in potent, concentration-dependent cytotoxic activity ex vivo in patient-derived xenograft (PDX) models of various subtypes of human sarcoma, breast and esophageal cancer. PRT1419 demonstrated good oral bioavailability and favorable pharmacokinetic properties in vivo. In subcutaneous cell-line derived xenograft (CDX) models of multiple myeloma, acute myeloid leukemia (AML) and diffuse large B-cell lymphoma, oral administration of PRT1419 demonstrated potent anti-tumor activity with complete tumor regressions observed at tolerable doses. This response correlated with a dose-dependent induction of cleaved caspase-3 and cleaved-PARP in tumor tissue. Significant in vivo activity, including complete responses, was also observed in PDX models of lymphoma. In preclinical models of solid tumors, PRT1419 demonstrated significant tumor growth inhibition in a PDX model of human soft tissue sarcoma and a CDX model of breast cancer. PRT1419 was also tested in combination with other approved targeted therapies in vitro and in vivo. In AML, combining PRT1419 with a BCL2 inhibitor revealed a synergistic interaction in cell lines, ex vivo PDX models as well as a CDX model in vivo. Further, PRT1419 demonstrated synergistic activity with tyrosine kinase inhibitors to inhibit the proliferation of breast, melanoma, and non-small cell lung cancer cell lines. PRT1419 is currently under evaluation in a Phase I clinical trial in patients with relapsed/refractory hematologic malignancies (NCT04543305). Citation Format: Neha Bhagwat, Alexander Grego, William Gowen-MacDonald, Min Wang, Miles Cowart, Xiaowei Wu, Jincong Zhuo, Andrew Combs, Bruce Ruggeri, Peggy Scherle, Kris Vaddi. Preclinical characterization of PRT1419, a potent, selective and orally available inhibitor of MCL1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 983.