The relationship between mismatch repair (MMR) proteins status and the response to conventional neoadjuvant therapy in colorectal cancer (CRC).

e15627 Background: MMR proteins status is not routinely detected in CRC patients. Neoadjuvant use of immune checkpoint inhibitors (ICIs) in deficient mismatch repair (dMMR) CRC patients can achieve a pathological complete response (pCR) rate of 60-100%. Conversely, dMMR/MSI-H patients may exhibit a poor response to conventional neoadjuvant therapy, although supporting evidence is limited. Methods: We retrospectively collected clinical data from patients who underwent neoadjuvant chemoradiotherapy or chemotherapy at Fudan University Shanghai Cancer Center from 2008 to 2021, excluding those who received neoadjuvant ICIs. MMR proteins were detected using immunohistochemistry on surgical samples or preoperative colonoscopy biopsy samples, and some patients underwent genetic testing to evaluate their microsatellite status. This study included M0 CRC patients with complete tumor regression grading (TRG) assessment and MMR/MSI status. Responses were assessed using TRG evaluation (AJCC-TRG system) on the primary tumor. Results: Among 1005 patients, the median age was 57 years (range: 19-95), with 643 males (64.0%) and 362 females (36.0%). Seventy-two (7.2%) had right colon cancer, 80 (8.0%) had left colon cancer, and 853 (84.9%) had rectal cancer. Fifty-five (5.5%) had dMMR, and 950 (94.5%) had proficient mismatch repair (pMMR). In dMMR patients, defective proteins included hMLH1 and PMS2 combined in 22 patients, hMSH6 in 14 patients, PMS2 in 9 patients, hMSH6 and hMSH2 combined in 6 patients, hMLH1 in 2 patients, hMLH1 and hMSH6 combined in 1 patient, and hMSH6, PMS2, and hMSH2 combined in 1 patient. Compared to pMMR patients, dMMR patients showed either no response or poor response to conventional neoadjuvant therapy, with no patients exhibiting TRG = 0/1. One hundred and eleven (11.7%) pMMR patients showed a good response (TRG = 0/1). There was no statistically significant difference in overall survival between the two groups of patients (HR, 1.547; 95% CI: 0.9221-2.596; P = 0.172). Conclusions: Although our study is retrospective and some patients with pCR lacked MMR expression information, we can still conclude that dMMR patients have a poor response to conventional neoadjuvant therapy, and neoadjuvant ICIs treatment may be a better choice for dMMR patients. We recommend routine immunohistochemistry of MMR proteins in preoperative colonoscopies to guide the development of neoadjuvant strategies. [Table: see text]