152P Biomarkers of neoadjuvant chemoradiotherapy response in locally advanced rectal cancer

Neoadjuvant chemoradiotherapy (nCRT) is the standard initial treatment for locally advanced rectal cancer (LARC) to reduce tumor size before surgery. Although some patients achieve complete pathological response and become eligible for organ-preserving strategies, predicting nCRT response remains challenging. The intratumoral genetic heterogeneity (ITGH) has emerged as a potential biomarker in LARC patients. Tumor-infiltrating lymphocytes (TILs) enriched with cytotoxic CD8+ T-cells indicate better prognosis and treatment response. An immunohistochemistry-based score, estimating CD3+ and CD8+ enrichment in LARC TILs, is proposed as a predictive test. Elements from anti-tumoral immune response could serve as potential predictive biomarkers. We analyzed 80 primary LARC tumors (18 responders, nCRT-R, versus nonresponders 62, nCRT-NR) through WXS and TCRseq. We evaluated nCRT response, tumor mutation burden (TMB), neoantigen load (NAL), HLA polymorphism, somatic mutation, TCR diversity/clonality, ITGH levels, and DNA mismatch repair deficiency. We identified six significantly mutated genes (KRAS, APC, TP53, FBXW7, ARID1A, SLITRK6) (p<0.05; FDR<=0.1). Somatic mutations in driver genes, 23 dMMR genes, and COSMIC somatic signatures S6, S15, S20, or S26 did not show any association with nCRT response (p>0.5; Fisher Test). We measured ITGH through MATH score, but no significant difference between nCRT-R and NR (p=0.8; Wilcoxon test) was observed. TMB and NAL were significantly higher in nCRT-NR (p<0.05). HLA somatic mutation did not have a significant impact on antigen presentation and treatment response (8 mutations in 8 patients; Pearson Chi-squared, p=0.76). HLA polymorphism was not relevant (Pearson Chi-squared, p=0.64). Finally, nCRT response was not associated with TCR Shannon diversity or clonality (Wilcoxon test, p=0.25 and p=0.48, respectively); however, we could not quantify CD8+ T-cells. Gene driver signature, ITGH, or dMMR mutations/signatures were not predictive biomarkers of nCRT response; however, preliminary findings suggest a subset of highly immunogenic nCRT-NR LARCs patients could benefit from immunotherapy.