CMS subtypes correlate with complete response in trial of neoadjuvant Galunisertib plus chemoradiation in rectal cancer

Improving response to neoadjuvant therapy for patients with locally advanced rectal cancer has the potential to improve organ preservation and disease-free survival. Knowing which patients may need therapeutic escalation or de-escalation from standard-of-care treatment remains an area of investigation. We previously reported the primary and secondary endpoints of our single-arm study combining transforming growth factor beta receptor inhibitor, Galunisertib, with neoadjuvant chemoradiation in patients with locally advanced rectal cancer. Here we analyze RNA sequencing data obtained from tissue biopsies at baseline and after 2 weeks of galunisertib. Differences in expression of pathways associated with MYC, inflammation, and epithelial-to-mesenchymal transition were observed between complete responders (CR) and <CR, with galunisertib upregulating MYC pathway expression in CR. Radiosensitivity and TGFβ response scores demonstrated limited ability to predict for response to galunisertib + chemoradiation. Typically treatment resistant consensus molecular subtype 4 (CMS4), characterized by TGFβ expression, was associated with response to galunisertib + chemoradiation. Differences in correlations between RNA based measures of cell composition and immunohistologic quantification of infiltrates and extracted MRI parameters were observed for CIBERSORT, MCPcounter, and Xcell methodologies. Based on these data, we hypothesize that the stromal radioresistant phenotype driven by TGFβ can be overcome by the addition of galunisertib to chemoradiation in rectal cancer.