Abstract LB165: A novel PDX-based 2-stage screening platform can identify active drug combinations against KRAS-mutated colorectal cancer

Abstract Introduction: Targeting RASmut colorectal cancers (CRC) remains challenging and targeting has focused on the RAS-MAPK cascade. MEK inhibitors for KRASmut tumors were not successful in vivo or in clinic as they slowed tumor growth, but did not cause regression at clinically achievable doses, however, potential for synergy has been shown. In vivo modeling commonly guides decisions to advance to the clinic but these studies are heterogeneously conducted, with rare use of statistically justified sample sizes or prespecified alternate hypotheses. The purpose of this study was to develop a robust PDX screening platform for pre-clinical evaluation of combination strategies, and pilot it with MEK inhibitor combinations. Methods: The screening model was based on Simon's two-stage design, with alternate hypothesis of 30% response rate (null hypothesis of 5% response rate). Stage 1 involved testing drug combinations against 5 KRASmut PDX models. Drug combinations that demonstrated efficacy in Stage 1 (≥10% decrease in tumor volume at Day 22 post-treatment) were tested in Stage 2 against an additional 13 PDX models. The null hypothesis was rejected if ≥3/18 PDX models showed response. This design yields a type I error rate of 0.05 and power of 0.8. Twenty RAS-MAPK cascade inhibitor-based combinations were tested. RP2D drug doses were used. Results: In Stage 1, MEKi plus CDK4/6i combination demonstrated efficacy in 3/5 models. This effect was sustained in Stage 2 (9/13 models with ≥10% decrease in tumor volume) for a total of 12/18. MEKi/BETi was next best (total 4/18 models). MEKi/PARPi and MEKi/ERKi - 3/18. Two combinations (MEKi/TKi and MEKi/AKTi) passed Stage 1, but failed in Stage 2 (2/18 total). Other combinations did not meet efficacy threshold in Stage 1 and were deemed inactive. In an expansion study of various RASmut PDX models (6 codon 12/13 KRASmut, 5 codon 61/146 RASmut and 4 RASwt), trametinib/palbociclib differentially attenuated tumor growth relative to untreated controls and caused tumor regression in all tested PDX models. Protein expression analysis (RPPA) demonstrated increases in cell cycle regulators (Cyclin-B1, FOXM1, PLK1) in non-responders, and higher pMEK and FOXO3 protein levels in responders. Combination treatment resulted in decreased expression of MAPK/ERK and CDK4/6 pathway-related proteins. mRNA expression analysis (GSEA) revealed that MEKi-resistant PDX models were enriched in cell cycle, ERK, mTORC1, ER stress and DDR signaling pathways. Primary drug combination toxicity (weight loss) was most pronounced with trametinib/palbociclib compared to tram or palbo alone. Using in vivo dose/response matrix screening, we maintained >90% efficacy while significantly reducing toxicity (decreasing weight loss from >15% to <10%) by decreasing palbociclib dose and maintaining tram dose intensity. Conclusions: The two-stage screening design demonstrated efficacy, time and cost-efficiency in drug combination screening. Efficacy of MEKi/CDK4/6i, MEKi/PARPi and MEKi/BETi combinations in RASmut CRCs was confirmed. These combinations are now being explored in prospective clinical trials. Citation Format: Alexey Sorokin, Preeti Kanikarla Marie, Fengqin Gao, Zhensheng Liu, David Menter, Scott Kopetz. A novel PDX-based 2-stage screening platform can identify active drug combinations against KRAS-mutated colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr LB165.