Effect of immunotherapy on the survival outcomes in tumor mutational burden-high (TMB-H) microsatellite stable (MSS) metastatic colorectal cancer (mCRC): A single-institution experience.

Abstract

239 Background: The benefit of immunotherapy for patients with mCRC with high tumor mutational burden (TMB-H) has been widely debated. In 2020, the FDA approved the use of pembrolizumab for the treatment of patients with TMB-H unresectable or metastatic solid tumors, with TMB-H defined as ≥10 mutations/Mb on a commercial tissue-based assay, based on KEYNOTE 158 results. However, the clinical value of applying this universal cut off to mCRC needs further investigation. Methods: We queried Moffitt Cancer Center (MCC) databases for patients with MSS mCRC, harboring TMB-H (tested with tissue and/or liquid biopsies) who received immunotherapy between January 2018 and December 2021. Patients with incomplete records were excluded. Clinical data were extracted by trained staff from electronic medical records. Objective response rate was measured by using clinical assessment from chart review. Results: We identified 40 patients with TMB-H MSS mCRC 13 of whom received immune checkpoint inhibitor therapy. Female patients represented 31% (n=4) of the 13 treated patients. Median age for the patients was 60 years (range 34-71. Thirty-eight percent (n=5) of the primary tumors originated in the right side. Thirty-one percent (n=4) presented with stage 4 CRC at diagnosis. Histopathology was adenocarcinoma in 92% (n=12) and one had neuroendocrine differentiation. Tumors were well-differentiated 8% (n=1), moderately differentiated 46% (n=6), or poorly differentiated 23% (n=3). Four patients (31%) had POLE/ POLD-1 mutations. The objective response rate (ORR) was 31% (4/13) with responses limited to tumors only with POLE/ POLD-1 mutations (100%, 4/4). The median progression free survival (mPFS) was 3.8 months for the overall cohort and 28.5 months for patients with POLE/POLD-1 mutated tumors. Agent specific analysis showed mPFS for patients treated with pembrolizumab was 3.5 months (mean 5.2; range 0.6-15.7), and 3.4 months (mean 12.1; range 0.5-12.1) for patients treated with nivolumab. Five patients received immunotherapy in combination with regorafenib. The mPFS for these patients was 3.4 months (mean 3.8; range 0.5-8.0). Of five patients with TMB>30 (38%), 4 (80%) had POLE/POLD-1 mutated tumors. One case without POLE/POLD-1 mutations, but with a TMB>30 did not experience any response. Conclusions: Although TMB-H demonstrated therapeutic significance in the KEYNOTE 158 study, the utility of 10 mutations/MB as a universal cutoff warrants additional evaluation. Here we report that a TMB-H cutoff value of ≥ 10 for patients with MSS CRC was not associated with clinically meaningful response to immunotherapy, but patients with MSS CRC with POLE/ POLD-1 mutations may be more likely to benefit from immunotherapy.