Phase II single-arm, single-center clinical trial of all-trans-retinoic acid, bevacizumab, and atezolizumab in refractory microsatellite stable colorectal cancer.

TPS324 Background: A critical clinical challenge in microsatellite stable (MSS) metastatic colorectal (mCRC) patients is to identify strategies to overcome lack of response to immune checkpoint inhibitors. An immunosuppressive tumor microenvironment comprising myeloid-derived suppressor cells (MDSC), endothelial cells, regulatory T cells, tumor associated macrophages, and cancer associated fibroblasts promotes immune evasion and resistance to these agents. MDSCs are bone marrow derived myeloid cells that suppress T-cell function and promote tumor growth. Among all cancers, mCRC patients have one of the highest frequency of MDSC (CD11b+, CD33+, CD14+ HLA-DRneg) in blood (1). It was demonstrated that altering the MDSC% in the tumor microenvironment by ATRA and anti-VEGF therapy (bevacizumab) can enhance the effects of immune checkpoint inhibitors (2-4). The hypothesis of the current clinical trial is that the combination of ATRA, bevacizumab and atezolizumab will lead to a decrease in MDSC population in tumor microenvironment leading to a clinically meaningful improvement in response rates among refractory MSS mCRC patients. Methods: This is a single-arm, open-label, phase 2 clinical trial combining ATRA, bevacizumab, and atezolizumab in refractory MSS mCRC patients. It will enroll a total of 21 patients over 24 months at UT Southwestern Medical Center that are MSS by PCR or NGS testing or are proficient in immunohistochemical expression of all four mismatch repair enzymes (MLH1, MSH2, MSH6, PMS2). ATRA will be administered orally at 45 mg/m2/day in 2 divided doses on days 1-7 and repeated every 14 days; atezolizumab will be given intravenously on day 1 at 840 mg dose every 14 days, and bevacizumab will be administered intravenously on day 1 at 10 mg/kg dose every 14 days. The first six patients enrolled on this study will contribute to the safety lead-in phase of this study. The primary outcome is to assess the overall response rate by RECIST v1.1. Secondary outcomes include assessment of disease control rate and frequency of adverse events using CTCAE v5.0. Exploratory outcomes include assessment of PFS and OS and collecting blood and tissue samples at defined timelines to study the changes in the MDSC population among responders and non-responders (NCT05999812). 1. Kobayashi, M. et. al., Clin Cancer Res, 2019. 2. Mirza, N., et al., Cancer Res, 2006. 3. Tobin, R.P., et al., Int Immunopharmacol, 2018. 4. Tobin, R.P., et. al., Clin Can Res, 2023. Clinical trial information: NCT05999812 .