Background Immunotherapy with immune checkpoint inhibitors (ICIs) is highly effective in microsatellite instability–high (MSI‐H) metastatic colorectal cancer (mCRC); however, specific predictive biomarkers are lacking. Patients and Methods Data and samples from 85 patients with MSI‐H mCRC treated with ICIs were gathered. Tumor infiltrating lymphocytes (TILs) and tumor mutational burden (TMB) were analyzed in an exploratory cohort of “super” responders and “clearly” refractory patients; TILs were then evaluated in the whole cohort of patients. Primary objectives were the correlation between the number of TILs and TMB and their role as biomarkers of ICI efficacy. Main endpoints included response rate (RR), progression‐free survival (PFS), and overall survival (OS). Results In the exploratory cohort, an increasing number of TILs correlated to higher TMB (Pearson's test, p = .0429). In the whole cohort, median number of TILs was 3.6 in responders compared with 1.8 in nonresponders (Mann‐Whitney test, p = .0448). RR was 70.6% in patients with high number of TILs (TILs‐H) compared with 42.9% in patients with low number of TILs (odds ratio = 3.20, p = .0291). Survival outcomes differed significantly in favor of TILs‐H (PFS: hazard ratio [HR] = 0.42, p = .0278; OS: HR = 0.41, p = .0463). Conclusion A significant correlation between higher TMB and increased number of TILs was shown. A significantly higher activity and better PFS and OS with ICI in MSI‐H mCRC were reported in cases with high number of TILs, thus supporting further studies of TIL count as predictive biomarker of ICI efficacy. Implications for Practice Microsatellite instability is the result of mismatch repair protein deficiency, caused by germline mutations or somatic modifications in mismatch repair genes. In metastatic colorectal cancer (mCRC), immunotherapy (with immune checkpoint inhibitors [ICIs]) demonstrated remarkable clinical benefit in microsatellite instability–high (MSI‐H) patients. ICI primary resistance has been observed in approximately 25% of patients with MSI‐H mCRC, underlining the need for predictive biomarkers. In this study, tumor mutational burden (TMB) and tumor infiltrating lymphocyte (TIL) analyses were performed in an exploratory cohort of patients with MSI‐H mCRC treated with ICIs, demonstrating a significant correlation between higher TMB and increased number of TILs. Results also demonstrated a significant correlation between high number of TILs and clinical responses and survival benefit in a large data set of patients with MSI‐H mCRC treated with ICI. TMB and TILs could represent predictive biomarkers of ICI efficacy in MSI‐H mCRC and should be incorporated in future trials testing checkpoint inhibitors in colorectal cancer.

BackgroundAnalysis of deficiency in DNA mismatch repair (dMMR) is currently considered a standard molecular test in all patients with colorectal cancer (CRC) for its implications in screening, prognosis and prediction of benefit from immune checkpoint inhibitors. While the molecular heterogeneity of CRC has been extensively studied in recent years, specific data on dMMR status are lacking, and its clinical consequences are unknown.Case presentationWe report the case of a metastatic CRC (mCRC) patient with immunohistochemical and molecular heterogeneity in dMMR/microsatellite instability status in the primary tumour. The patient was treated with nivolumab plus ipilimumab and achieved a deep and lasting response with clear clinical benefit. Whole-exome sequencing and RNA-seq data are reported to support the evidence for molecular heterogeneity. Re-biopsy at the time of progression ruled out the selection of MMR proficient clones as an escape mechanism. A large single-institution retrospective dataset was interrogated to further explore the real incidence of heterogeneity in its different presentations.ConclusionsThe present case supports the efficacy of immune checkpoint inhibition in mCRC with heterogeneity in MMR/microsatellite instability status. Clinical issues that may arise in these rare patients are discussed in detail.

Background Few real‐world series on the efficacy and safety of anti‐programmed cell death protein‐1(PD‐1)/programmed death ligand‐1(PD‐L1)–based therapy are available in molecularly unselected patients with poor performance status (PS) and specific types of advanced cancers, because such populations are typically excluded from clinical trials due to poor life expectancy and risk of toxicity. Materials and Methods This multicenter retrospective case series included patients with microsatellite instability (MSI)‐high metastatic cancers with Eastern Cooperative Oncology Group (ECOG) PS of 2 or 3 not related to comorbidities receiving anti‐PD‐1 with or without anti‐CTLA‐4 therapy after failure of at least one prior treatment line. Results We included 27 patients with six diverse tumor types: colorectal (n = 18), gastric (n = 5), biliary tract, pancreatic, small bowel, and endometrial cancers (n = 1 each). Baseline ECOG PS was 2 (74%) or 3 (26%). Overall response rate was 33%, with six partial and three complete responses. Median time to response was 3.1, months and median duration of response was 16.9 months. Median progression‐free survival was 3.4 months (95% CI: 2.3 to not evaluable), and 18‐month overall survival was 50.8% (95% confidence interval, 32.7–78.8). Baseline variables were not associated with survival outcomes. ECOG PS 1 was reached by 52% of patients in a median time of 6 weeks, and ECOG PS 0 was reached by 30% of patients in a median time of 10 weeks. Conclusion In a high proportion of patients with MSI‐high cancers and poor performance status related to end‐stage disease, salvage immunotherapy can induce potentially long‐lasting “Lazarus responses”. Immunotherapy decisions near the end‐of‐life should be carefully integrated with predictive biomarkers and with palliative care measures in the real‐world setting. Implications for Practice In this retrospective cohort study of 27 pretreated patients with microsatellite instability (MSI)‐high cancers and Eastern Cooperative Oncology Group performance status of 2 or 3 not related to comorbidities, PD‐1/PD‐L1‐based therapy induced a RECIST response in 33% of patients, with a median duration of 16.9 months, and an improvement of performance status in 52% of patients. MSI‐high status can be used in clinical practice as a tumor‐agnostic predictive biomarker to select critically ill patients with end‐stage cancers for salvage immunotherapy.