Immunotherapy for localized MSI-H colorectal cancer: Characterizing endoscopic and imaging response

3610 Background: Early data suggests that immunotherapy (IO) for localized colorectal cancer (CRC) is associated with high rates of pathologic complete response (pCR) and durable remission. This study aims to characterize endoscopic, imaging, and pathological outcomes among patients who received IO for localized CRC. Methods: This was a single-institution retrospective analysis of patients with MSI-H CRC who received at least one cycle of anti-PD-1 therapy for localized (stage I-III) MSI-H CRC. Patient and tumor characteristics were obtained from the clinical record. Endoscopy reports were assessed using standardized criteria to determine response and imaging was assessed using RECIST v1.1 where applicable. Endoscopic complete response (endoCR) was characterized by flat, white scar and/or telangiectasia without residual ulcer or nodularity. Categorical variables were compared using Fisher’s exact test. Results: 37 patients who received a median of 8 cycles of pembrolizumab (n = 36) or nivolumab (n = 1) for localized CRC were identified, of which 16 (43%) had a rectal primary. 10 patients (27%) had mucinous features on pathology. The best endoscopic response was complete response (CR) in 13 of 30 (43%) patients, which was achieved after a median of 6 cycles. The rate of endoCR after ≤ 4 cycles of IO was similar to the rate of endoCR after > 4 cycles (29% vs 50%, p = 0.284). The best imaging response was CR in 6 of 32 (19%) of patients. The rate of CR on imaging after ≤ 4 cycles and > 4 cycles of IO was 6% and 36% respectively (p = 0.064). In cases where imaging and endoscopy were available (n = 24), discrepancy in best response was noted in 14 (58%) cases. The most common discrepancy was partial response on imaging when endoCR was noted (n = 6 cases). 16 patients proceeded to surgical resection (after a median of 8 cycles) of which 11 (69%) had pCR despite non-CR on either endoscopy (n = 7) or imaging (n = 9). The endoscopic reports indicating residual disease prior to surgery included findings such as ulceration, scarring, strictures, and an obstructing mass. Of 15 patients who did not proceed to surgery and had follow-up more than 6 months after completion of IO, 13 (87%) had no evidence of progression at a median follow-up of 19.3 months from IO start. Conclusions: Discrepancies between endoscopic, imaging, and pathological outcomes were frequent, indicating a need for improved clinical response criteria. Nevertheless, immunotherapy was associated with deep and durable responses in most patients with localized MSI-H CRC.