Immune checkpoint blockade in locally advanced rectal cancer with deficient mismatch repair (dMMR): Retrospective multicenter experience.

e15628 Background: Total neoadjuvant therapy (TNT) with cytotoxic chemotherapy and chemoradiation followed by surgical resection is a standard treatment for locally advanced rectal cancer (LARC). In a limited number of LARC patients with deficient mismatch repair (dMMR), treatment with an immune checkpoint inhibitor (ICI) resulted in all patients achieving complete clinical response (cCR) (median follow-up of 29 months) such that radiotherapy and surgery were avoided. These and other data have resulted in the routine use of neoadjuvant ICI for dMMR LARC. Here we report a multicenter series of patients with dMMR LARC treated with neoadjuvant ICI. Methods: Adult patients (N = 21) with dMMR LARC who received neoadjuvant ICI at Mayo Clinic, MD Anderson Cancer Center, Duke University or University of Pittsburgh were identified from the electronic medical record (EMR) and relevant data were abstracted. All patients were staged with pelvic MRI; clinical response was evaluated by repeat pelvic MRI and sigmoidoscopy. Data were summarized using descriptive statistics. This study was approved by the Mayo Clinic IRB. Results: Patients with clinical stage I (n = 2), II (n = 3) or III (n = 15) dMMR LARC were treated with neoadjuvant PD-1 monotherapy (n = 20) or combination (n = 1, ipilimumab+ nivolumab). Median age was 46 years (IQR 39, 65), 15 (71%) were male, 10 (48%) had distal tumors, and 14 had Lynch Syndrome. Median duration of ICI treatment was 7 mos (IQR 6, 12); one patient remains on dostarlimab at data cut-off. Clinical complete response (cCR) was achieved in 11 (52%) patients, partial response (PR) in 7 (33%), stable disease (SD) in 1 (5%), and progressive disease (PD) in 2 (10%) patients. Median time from first dose of ICI to best clinical response was 6 mos (IQR 5, 8) months. All partial or non-responders received salvage TNT with (n = 6) or without (n = 3) surgery. At a median follow up of 19 mos (range 6 to 56), tumor regrowth occurred in 1 patient who was subsequently treated with TNT. Any grade immune-related adverse event occurred in 6 (29%) patients; one grade 3 hepatitis occurred in the patient on Ipi+Nivo (Table1). Conclusions: Neoadjuvant ICI treatment of dMMR LARC resulted in half of treated patients achieving cCR. Careful monitoring is necessary to identify non responders who will require salvage TNT. Patient characteristics and clinical response to ICI. ID Sex Age Stage ICI Response to ICI ID Sex Age Stage ICI Response to ICI 1 M 62 T3N+ Pembro cPR 12 M 37 T3N+ Pembro cCR 2 M 60 T2N0 Pembro cCR 13 M 41 T3N0 Pembro cCR 3 M 80 T2N0 Pembro cPD 14 M 46 T4N+ Pembro cPR 4 M 36 T3N+ Ipi+Nivo cCR 15 M 68 T3N+ Pembro cPR 5 M 57 T3N+ Dostarli cCR 16 M 77 T3N0 Pembro cPR 6 M 39 T3N+ Dostarli cPR 17 M 40 T4Nx Pembro cCR 7 F 44 T2N+ <jats:td colspan