Abstract 1261: Assessing intratumoral genetic heterogeneity in locally advanced rectal cancer

Locally Advanced Rectal Cancer (LARC) is typically treated with neoadjuvant chemoradiotherapy (nCRT), aimed at reducing tumor size to facilitate surgical resection. Patients achieving a complete clinical response (cCR) may be eligible for organ preservation strategies, such as the Watch-and-Wait protocol. However, clinical and radiological evaluations lack precision in predicting treatment response, with tumor regrowth occurring in 30% of cases within two years. We previously demonstrated that rectal tumors present high ITGH and that this heterogeneity increases after nCRT in patients who do not achieve a complete clinical or pathological response. Therefore, ITGH has emerged as a potential biomarker for assessing nCRT response, and we herein evaluate the association between distinct ITGH estimates and nCRT response using local and public genomic datasets. We analyzed a local cohort of 71 LARC patients who underwent long-course nCRT with 5FU-based chemotherapy or Capecitabine (SP cohort). Pre-treatment tumor biopsies were subjected to whole exome sequencing (WES), and patients were classified into two groups: nCRT-R (N=16), nCRT-NR (N=55). Public WES, mRNAseq, and clinical data from 81 patients from Chatila et al. (2022, MSKCC cohort) were also used to ITGH estimates using MATH score (in WES) and DEPTH2 (in mRNAseq). The treatment response was evaluated based on cCR or pathological complete response (pCR) in patients treated with 5FU, Capecitabine or Cisplatin, receiving nCRT alone or combined with induction/consolidation. Low ITGH values correlated with nCRT response in the SP cohort (median MATH score of 28.89 versus 50.97 in nCRT-R and NR, respectively; p≤0.05, Wilcoxon test). We used ROC curve analysis to determine the best MATH score threshold (26.34, sensitivity=90%, specificity=50%, AUC=0.67). Multivariate analysis with the clinical-radiological features showed a marginal association between MATH-low with nCRT-R (OR:16.05; p=0.003). We observed opposite tendencies for the MSKCC cohort (higher ITGH in nCRT-R): median MATH score of 38.65 (nCRT-R, N=18) versus 33.51 (nCRT-NR, N=63), but the difference was not significant (p=0.2536, Wilcoxon test). Considering MATH score limitations (e.g., number of somatic mutations, WES coverage) we also inferred ITGH from expression data for a subset of these patients. Still, we observed similar, non-significant results: median ITGH of 0.667 versus 0.652 for nCRT-R (N=18) and NR (N=54), respectively (p=0.503, Wilcoxon test). The results from our cohort suggest that ITGH remains a potential biomarker of response. The conflicting data observed may simply reflect the limited statistical power of the analyses. We believe it is necessary to increase the sample size and stratify response groups based on treatment regimens. Therefore, we consider it important to disclose these findings to encourage collaboration and facilitate the expansion of our case series. Citation Format: Vandeclecio Lira da Silva, Ezequias Santos Torres, Ramon Torreglosa Do Carmo, Franciele Hinterholz Knebel, Fabiana Bettoni, Guilherme Pagin São-Julião, Bruna Vailati, Bruna Vailati, Leonardo Corbi, Natalia Mariana Felicio, Angelita Habr Gama, Anamaria Camargo, Cibele Masotti, Rodrigo Oliva Perez. Assessing intratumoral genetic heterogeneity in locally advanced rectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1261.