Purpose: Immune checkpoint inhibitors (ICIs) appeared active in single-arm trials for patients with chemoresistant metastatic colorectal cancer (mCRC) harboring microsatellite instability (MSI). Given the paucity of randomised controlled trials (RCTs) in this setting, we evaluated the effect size of ICIs using intra-patients comparison and ARCAD database as historical controls., Patients and Methods: Individual-patient data from NIPICOL and CheckMate 142 phase II trials that evaluated a combination of ICIs for MSI mCRC patients (N = 176) and from five non-ICI mCRC historical RCTs in second-line or latter (N = 4026) were analyzed. Firstly, promising of ICIs was identified using intra-patient comparison based on growth modulation index (GMI) defined the ratio of progression-free survivals (PFS) on ICIs and previous line of therapy. Survival outcomes of ICIs-treated patients were then compared with those matched non-ICIs treated from ARCAD database historical RCTs., Results: Among ICIs-treated patients, median PFS on ICIs was 32.66 (range 0.10-74.25) versus 4.07 months (range 0.7-49.87) on prior therapy, resulting on median GMI of 4.97 (range 0.07-59.51; hazard-ratio (HR)= 0.16 (95 %CI=0.11-0.22, P < 0.001)). Compared to matched non-ICI patients, in third-line, median overall survival (OS) was not reached with ICIs versus 3.52 months with placebo (HR=0.20, 95 %CI=0.10-0.41, P < 0.001), and 6.51 months with active drugs (HR=0.30, 95 %CI=0.15-0.60, P = 0.001). In second-line, median OS was not reached with ICIs versus 11.7 months with chemotherapy+placebo (HR=0.12, 95 %CI=0.07-0.22, P < 0.001), and 16.3 months with chemotherapy+targeted therapy (HR=0.10, 95 %CI=0.05-0.19, P < 0.001)., Conclusion: ICIs demonstrates high effect size for MSI mCRC patients in second-line and later. This work might be useful as an example of methodology to avoid RCTs when benefit from experimental therapy is likely to be high., Competing Interests: Declaration of Competing Interest Romain Cohen reports honoraria from Astra-Zeneca, Bristol-Myers Squibb, Exeliom Biosciences, Enterome Bioscience, MSD Oncology, Mylan Medical, Pierre Fabre, Servier and non-financial support from Amgen, Bristol-Myers Squibb, Mylan Medical and Servier. Benoist Chibaudel reports consulting and advisory role from Amgen, Bayer, Beigine, Biocartis, Lilly, Merck, MSD, Pfizer, Pierre Fabre, Revolution Medicines, Roche, SeqOne, Sanofi, Servier, Takeda. Josep Tabernero reports personal financial interest in form of scientific consultancy role for Array Biopharma, AstraZeneca, Bayer, Boehringer Ingelheim, Cardiff Oncology, Chugai, Daiichi Sankyo, F. Hoffmann-La Roche Ltd, Genentech Inc, HalioDX SAS, Hutchison MediPharma International, Ikena Oncology, Inspirna Inc, IQVIA, Lilly, Menarini, Merck Serono, Merus, MSD, Mirati, Neophore, Novartis, Ona Therapeutics, Orion Biotechnology, Peptomyc, Pfizer, Pierre Fabre, Samsung Bioepis, Sanofi, Scandion Oncology, Scorpion Therapeutics, Seattle Genetics, Servier, Sotio Biotech, Taiho, Tessa Therapeutics, TheraMyc and Tolremo Therapeutics. Stocks: Oniria Therapeutics and also educational collaboration with Imedex/HMP, Medscape Education, MJH Life Sciences, PeerView Institute for Medical Education and Physicians Education Resource (PER). Qian Shi reports consulting/advisory role from Yiviva Inc, Boehringer Ingelheim Pharmaceuticals, Inc, Regeneron Pharmaceuticals, Inc., Hoosier Cancer Research Network, Kronos Bio, and Mirati Therapeutics Inc; Honorarium/speaker role from Chugai Pharmaceutical Co., Ltd (to myself), research funds from Celgene/BMS, Roche/Genentech, Janssen, Novartis (to institution). Thierry Andre reports attending advisory board meetings and receiving consulting fees from Abbvie, Astellas, Aptitude Health, Bristol Myers Squibb, Gritstone Oncology, Gilead, GlaxoSmithKline, Merck & Co. Inc., Nordic Oncology, Seagen, Servier, Takeda and Transgène and honoraria from Bristol Myers Squibb, GlaxoSmithKline, Merck & Co. Inc., Merck Serono, Pierre Fabre, Roche, Sanofi, Seagen and Servier; and support for meetings from Merck & Co. Inc. and Servier. Hideaki Bando reports research fund from Ono pharmaceutical, and honoraria from Ono pharmaceutical, Taiho pharmaceutical, Eli Lilly Japan. Michael J. Overman reports Consulting: Merck, Pfizer, Array, Gritstone, 3DMed, Nouscom, Atreca, Bayer, Simcere and Research Funding: Takeda, Roche, Lilly, Merck, Medimmune, BMS, Nouscom, Phanes. John Zalcberg reports leadership from ICON Group, Lipotek, PRAXIS; Stock from Biomarin, Ophthea, Amarin, Concert Pharmaceuticals, Frequency Therapeutics, Gilead, Madrigal Pharmaceuticals, UniQure, Zogenix, Orphazyme, Moderna Therapeutics, TWST, Novavax, Teladoc; Honoraria from Gilead Sciences, MSD Oncology, Viatris; Consulting & Advisory Role from Merck Sharp & Dohme, Specialized Therapeutics, CEND, Deciphera, REVOLUTION MEDICINE, FivePHusion, Genorbio, 1Global, Novatech, Alloplex Biotherapeutics Inc, Oncology Republic; Research Funding from BMS, AstraZeneca, Pfizer, IQvia, Mylan, Ipsen, Eisai, Medtronic, MSD Oncology, Servier; Travel from MSD Oncology, ICON Group, PRAXIS. Eric Van Cutsem repors participation to advisory boards for Abbvie, ALX, Amgen, Array, Astellas, Astrazeneca, Bayer, Beigene, Boehringer Ingelheim, Bristol-Myers Squibb, Daiichi, GSK, Incyte, Ipsen, Lilly, Merck Sharp & Dohme, Merck KGaA, Mirati, Novartis, Nordic, Pierre Fabre, Pfizer, Roche, Seattle Genetics, Servier, Takeda, Terumo, Taiho, Zymeworks; research grants from Amgen, Bayer, Boehringer Ingelheim, Bristol-Myers Squibb, Ipsen, Lilly, Merck Sharp & Dohme, Merck KGaA, Novartis, Roche, Servier paid to his institution. Takayuki Yoshino reports honoraria from Chugai Pharma, Takeda Pharma, Merck, Bayer Yakuhin, Ono Pharmaceutical and MSD K.K; consulting fee from Sumitomo Corp.; research grant from Amgen, Chugai, Daiichi Sankyo, Eisai, FALCO Biosystems, Genomedia Inc., Molecular Health, MSD, Nippon Boehringer Ingelheim, Ono, Pfizer, Roche Diagnostics, Sanofi, Sysmex and Taiho. Richard Adams reports Speaker fees/travel from Servier, Takeda, Amgen; Consultancy from Takeda, Seagen, Bayer; Institutional research funding from AZ, Jazz Pharma, MSD. Toshihiro Misumi reports Honoraria from Chugai, AstraZeneca, and Miyarisan. All remaining authors have declared no conflicts of interest., (Copyright © 2024 Elsevier Ltd. All rights reserved.)