Abstract 5225: Temporal inhibition of ERK is sufficient for tumor growth inhibition in KRAS-mutant or BRAF-mutant tumors

The combination of BRAF and MEK inhibitors have been clinically studied, and three different combinations (dabrafenib plus trametinib, vemurafenib plus cobimetinib and encorafenib plus binimetinib) are approved by the FDA for treatment of melanoma patients with BRAF V600E mutation. Published reports indicate that each combination has a different efficacy and toxicity profile. It is believed that the toxicity profile of some of these drugs is linked to their pharmacokinetic (PK) profile leading to prolonged pharmacodynamics (PD) or target inhibition. ERK inhibitors have been discovered recently to overcome the acquired resistance to BRAF plus MEK inhibitors in BRAF-mutant melanoma and for combination therapies in RAS/MAPK altered cancers including KRAS-mutant cancers. Several ERK inhibitors have entered clinical development as monotherapy or in combination. However, it is not clear what the minimum time on target is required for efficacy without causing significant toxicity, and whether that would differentiate the therapeutic index of one inhibitor from another. Therefore, we have conducted PK-PD-efficacy relationship studies of different MEK and ERK inhibitors with different PK profiles in mice in BRAF-mutant or KRAS-mutant xenograft models to determine the minimum time on target required for efficacy. We have tested the PK-PD-efficacy of MEK inhibitors trametinib and cobimetinib, as well as ERK inhibitors BVD-523, GDC-0994 and LY3214996 in BRAF V600E or KRAS mutant xenograft models including A375 BRAF V600E melanoma, Colo205 BRAF V600E colorectal carcinoma and HCT116 KRAS G13D colorectal carcinoma. Additionally, an infusion study with LY3214996 was conducted in rats implanted with HCT116 colorectal carcinoma tumors, where drug exposure was controlled for a specified time to get ≥50% pRSK1 inhibition. Our data suggests that ≥50% inhibition of pRSK1 for 8h is required for significant efficacy of ERK inhibitor LY3214996, which is similar to the data presented for GDC-0994 (Robarge K et al, Abstract number DDT02-03, AACR Annual Meeting 2014). Overall assessment of the data in different models suggested that ≥50% target inhibition for 8-16h is enough for significant tumor growth inhibition and regressions by an ERK inhibitor in BRAF-mutant or KRAS-mutant tumors without any obvious toxicity as measured by body weight loss or mortality. LY3214996 is in Phase 1 clinical development and the preliminary PK profile in patients looks as predicted from preclinical data. Therefore, we believe that the PK profile of LY3214996 offers flexibility with dose and schedule to balance efficacy and safety, and to achieve a better therapeutic index in combination therapy for RAS/MAPK pathway altered cancers. Citation Format: Shripad V. Bhagwat, Weihua Shen, Baohui Zhao, Shufen Cai, Lisa Kindler, William T. McMillen, Eunice Yuen, Denis McCann, Jason Manro, Andrew J. Dropsey, Melinda D. Willard, Sajan Joseph, Sheng-Bin Peng. Temporal inhibition of ERK is sufficient for tumor growth inhibition in KRAS-mutant or BRAF-mutant tumors [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5225.