Abstract 1841: MRTX849 inhibits P-gp and demonstrates CNS exposure in mouse models and cancer patients and demonstrates antitumor activity in intracranial mouse models of lung cancer brain metastasis

Abstract MRTX849 is a covalent, mutant selective, KRAS G12C inhibitor in development for cancer patients harboring this mutation. KRAS G12C is mutated in ~14% of lung adenocarcinoma and KRAS G12C inhibitors adagrasib and sotorasib have demonstrated clinical activity. While early data are encouraging, approximately one-third of KRAS-mutant non-small cell lung cancer (NSCLC) patients develop brain metastases and this remains a significant unmet medical need. P-glycoprotein 1 (P-gp)-mediated efflux is a major mechanism for the active transport of small molecules out of the CNS. MRTX849 is a P-gp inhibitor and inhibits its own efflux at plasma exposure levels achieved in humans at the 600 mg BID dose level resulting in achieving appreciable drug levels in cerebrospinal fluid (CSF). Total and free-fraction adjusted plasma concentrations of MRTX849 were 8.6 μM and 43 nM; respectively, 8 hours post administration of a clinically relevant oral dose of 200 mg/kg to mice. In addition, the CSF concentration, a measure of the free brain concentration, was 52 nM, which is above the cellular pERK1/2 IC50 value measured at 24 hours in cancer cell lines (~5 nM). Administration of 100 mg/kg MRTX849 to mice also resulted in CSF exposure above the cellular IC50 and a CSF/plasma (free-fraction adjusted) partition coefficient (Kp,uu) value of 0.4. To evaluate the tumor growth inhibition following oral administration of MRTX849 in an orthotopic model of lung cancer brain metastasis, immunocompromised mice were intracranially implanted with luciferase-labeled human NSCLC LU99 cells. Lower bioluminescence imaging (BLI)-based tumor flux was observed for the 100 mg/kg BID MRTX849-treated mice compared to vehicle and an 88% reduction in bioluminescence signal compared to baseline was observed suggesting strong tumor regression and consistent with significantly longer survival in treated mice. In addition, MRTX849 treated brain tumors demonstrated 85% reduced ERK phosphorylation. Similarly, treatment with MRTX849 resulted in potent tumor regression and significant survival extension in another intracranial KRAS G12C model of NSCLC using LU65 cells. CSF levels were determined at steady-state in two patients in the MRTX849-001 Phase1/2 clinical trial. The CSF/plasma (free fraction adjusted) Kp,uu value was 0.47 with patient CSF levels achieved consistent with CSF levels observed in responding mouse models (24-35 nM). These data demonstrate MRTX849 crosses the blood brain barrier in preclinical models and cancer patients. In addition, antitumor activity observed in mouse models of brain metastases provides rationale for exploring the utility of MRTX849 for the treatment of patients harboring KRAS G12C mutant lung cancer with brain metastases. Citation Format: Kazuhide Shimizu, Jill Hallin, Mohini Singh, Naema Nayyar, Matthew R. Strickland, Aaron C. Burns, Cornelius Cilliers, Lauren Hargis, Peter A. Olson, Matthew A. Marx, Priscilla K. Brastianos, Hiroaki Wakimoto, James G. Christensen. MRTX849 inhibits P-gp and demonstrates CNS exposure in mouse models and cancer patients and demonstrates antitumor activity in intracranial mouse models of lung cancer brain metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1841.