Abstract 5707: Preclinical evaluation of KRLS-017, a potent, highly selective and reversible CDK7 inhibitor with broad antitumor effect in preclinical models, in preparation for a Phase 1 clinical trial in advanced solid tumor malignancies

Abstract KRLS-017 is a potent, selective, and reversible inhibitor of Cyclin Dependent Kinase 7 (CDK7). Its molecular formula is C24H35N5O2Si and its molecular weight is 453. 65 Daltons. KRLS-017 is being developed for the treatment of patients with advanced solid tumor malignancies. To differentiate KRLS-017 from other CDK7 inhibitors, we created a reversible inhibitor to allow for treatment schedule optimization and maximization of therapeutic index in clinic. High selectivity of KRLS-017 for CDK7 was confirmed using a panel of 313 recombinant human kinases. The IC50 of KRLS-017 against recombinant human CDK7 was determined to be 16 nM, and no other kinases in the panel showed IC50 values less than 300 nM. Antiproliferative activity of KRLS-017 was investigated in vitro using a broad panel of human tumor cell lines. KRLS-017 showed potent antiproliferative effect across a wide variety of both solid and hematologic tumor types, producing GI50 values 200 nM in 258/302 human tumorcell lines tested. Antitumor activity of KRLS-017 was investigated in vivo using multiple mouse xenograft tumormodels at dose levels ranging from 25 mg/kg to 200 mg/kg using both continuous (daily) andintermittent dosing schedules. Statistically significant tumor growth inhibition was seen at doselevels of 50 mg/kg and higher, with tumor regression observed at 100 mg/kg and 200 mg/kg. Better in vivo tolerability at efficacious doses was observed using intermittent dosing schedules in mouse xenograft models, which led to adoption of an intermittent schedule in the proposed first-in-human phase 1 trial. KRLS-017 is orally bioavailable in mice, rats, and dogs and plasma drug exposure increased in a proportional manner with dose. Comparing first dose and 28th dose in the dog, minimal plasma accumulation was observed, suggesting once daily dosing in humans would be initially tested. Repeat dose 28 day GLP toxicology studies in rat and dog showed adverse effects in highly proliferative bone marrow and gastrointestinal tissues which is consistent with the known mechanism of action for CDK7 and is considered to be monitorable and manageable in the setting of oncology. The clinical development plan for KRLS-017 includes an initial Phase 1 dose escalation study (KRLS-017-101) in patients with refractory solid tumor malignancies to assess pharmacokinetics/pharmacodynamics and to determine an optimal dose level and treatment schedule to test in expansion cohorts. Given the potential of CDK7 inhibitors in the treatment of hormone receptor positive breast cancer, combination work with other agents (such as selective estrogen receptor degraders) is on-going. Citation Format: Alison L. Hannah, Steve Ritland, Thomas Steele, Steven Smith, Ashwin Ram, BT Slingsby, Kapil Dhingra, James Chrisman, Rajesh Dua, Yasunori Tokunaga, Shigeyuki Kono, Yasuhiro Aga. Preclinical evaluation of KRLS-017, a potent, highly selective and reversible CDK7 inhibitor with broad antitumor effect in preclinical models, in preparation for a Phase 1 clinical trial in advanced solid tumor malignancies abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 5707.