Abstract A002: KRAS inhibitors as sensitizing agents in treatment-refractory rectal cancer

Abstract Background: KRAS, one of the three human RAS isoforms, is the most extensively studied and most frequently mutated among known cancer-related genes. KRAS mutations (KRASmut) are key drivers of cancer progression and treatment resistance. KRAS mutations have been previously undruggable and contribute to worse oncologic outcomes, tumor growth, and resistance to anti-EGFR therapies. Recently, many KRAS-targeted inhibitors have been approved by the FDA or are in clinical trials. We hypothesized that patient-specific treatment guided by allele-specific selection would both improve tumor kill and more effectively sensitize resistant rectal cancer (RC) tumors to standard therapy. Methods: We examined a large clinical RC dataset to examine the most common KRAS mutations in RC (n=783) along with recurrence risk according to KRAS alteration (n=115). Further, we employed our RC biorepository of RC patient-derived organoids (PDOs) as a platform to test responses to KRAS inhibitors, including Sotorasib, a KRASG12C inhibitor; MRTX-1133, a KRASG12D inhibitor; and BI-2865, a Pan-KRAS inhibitor. We further tested these inhibitors, using patient-specific 5-FU-based therapy and radiotherapy, applied as combination treatments in 11 RC PDOs. All experiments were done in biological triplicate. MSK-IMPACT was used to assess KRAS mutation frequencies. Kaplan-Meier estimates were used to compare survival outcomes (overall survival and disease-free survival (OS and DFS)). P-values <0.05 were considered significant. Results: KRAS alterations occur in 43% of patients and are associated with worse OS and DFS. The most common KRAS mutations in RC are G12D, G12V, G13D and G12C, representing 26%, 20%, 18%, and 6% respectively. A146T and G12S, along with others, account for the remaining 30%. IC50 values for each inhibitor were derived for each RC PDO, and response curves were created. In KRASG12Dmut RC PDOs, use of MRTX-1133 exhibited lower IC50 values (40-70 nM), compared to RC PDOs without the KRASG12D mutation, where IC50 values were 20-200 times higher (p<0.05). Further, we noted a significant allele-specific reduction in IC50 values using Sotorasib in KRASG12C RC PDOs (60 nM-6 µM). In contrast, in RC PDOs lacking the KRASG12C mutation, we noted significantly higher IC50 values (7 to 800-fold). In KRASG12D, interestingly, BI-2865 demonstrated a broad range of efficacy across the RC PDO cohort, with IC50s from 1.5 μM to 50 μM. The KRASG12D RC PDOs were further treated with MRTX-1133 combined with 5-FU or radiotherapy. The data showed synergistic effects using MRTX-1133 and either 5-FU or radiotherapy, indicating improved efficacy compared to monotherapy. Conclusions: KRAS alterations occur in ∼40% of RC patients, and the most common alteration is KRASG12D. Preclinical data from rectal cancer PDO models indicate that combining allele-specific KRAS inhibition with 5-FU-based therapy may help overcome KRAS-driven drug resistance and thereby improve tumor response and outcomes for patients with rectal cancer. Citation Format: Min Jung Kim, Chao Wu, Wini V. Zambare, Aron Bercz, Michael Del Latto, Sara Flowers, Michael G. White, X Steven Chen, Philip B. Paty, Xiling Shen, Scott Kopetz, Paul B. Romesser, J Joshua. Smith. KRAS inhibitors as sensitizing agents in treatment-refractory rectal cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: RAS Oncogenesis and Therapeutics; 2026 Mar 5-8; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(5_Suppl_1):Abstract nr A002.