LAUSR

e15046 Background: Immune checkpoint inhibition (ICKi) for mCRC with deficient mismatch repair (dMMR) or microsatellite instability high (MSI-H) demonstrates a high clinical activity rate that appears durable. The impact of immunotherapy on pathological tumor response in dMMR/MSI-H CRC is unknown. Our aim was to assess the pathological response and clinical outcomes following surgical resection in dMMR/MSI-H CRC patients who undergo surgical resection following immunotherapy. Methods: All dMMR/MSI-H mCRC patients treated with anti-PD1/L1 based ICKi between November 2016 and December 2018 at MD Anderson Cancer Center, Houston, Texas and Saint-Antoine Hospital, Paris, France who then underwent surgical resection were included in this retrospective analysis. MSI-H status was determined by ICH in 4 patients and by PCR and ICH in 8 patients. The primary outcome was pathological response in resected surgical specimens following at least 1 cycle of ICKi. Complete pathological response was defined as the absence of residual cancer cells in the surgical specimen. Results: 12 patients (median age at diagnosis 45 years (range 30-70); 10 germline mutations,2 sporadic) of 111 who received ICKi (11%) underwent surgical resection of primary tumor only (n = 5), metastases only (n = 6) or both primary tumor and metastases (n = 1). 42% of patients received PD1 combined with CTLA4 iCKi while 58% received PD1 monotherapy. The median time from ICKi introduction to surgery was 12 months (range 2-28). Prior to surgery 1 patient had complete radiographic response while 11 patients had residual tumor on imaging. 9 surgeries were done with curative intent and 3 with palliative intent. 11 patients achieved pCR while 1 achieved near pCR with rare viable tumor remaining (prior ICKi duration was 2 months). Median follow-up from surgery is 7 months. No patients have progressed following surgical resection. Conclusions: Complete pathological response is achievable with ICKi for dMMR/MSI-H mCRC. For these patients this data supports the curative potential of anti-PD1 based therapy and suggests that residual radiographic tumor may not require surgical resection following response to anti-PD1 based therapy.