Background: Certain somatic mutations are thought to promote immune evasion and resistance to immunotherapy. PIK3CA was identified in an in vivo genomic screen for mechanisms of resistance to anti-PD1 therapy. MC38 cells (murine colon adenocarcinoma) were engineered to express a library of human cancer-associated mutations from TCGA. Resultant tumors in vivo were exposed to immune pressure with anti-PD1 therapy. Cells that proliferated were then analyzed for mutations that impart immune resistance. Multiple activating mutations in PIK3CA conferred resistance to anti-PD1 therapy. Coadministered PI3K inhibition reversed this resistance. Multiple studies have shown the impact of the phosphatidylinositol 3-kinase (PI3K) pathway on the tumor microenvironment, and 20% of colorectal cancer (CRC) tumors have an activating mutation of PI3K. Methods: A multi-center, open-label, phase I/II study with the combination copanlisib and nivolumab, a PD1 inhibitor, in relapsed/refractory solid tumors with expansions in relapsed/refractory microsatellite-stable (MSS) CRC was developed. Copanlisib is an inhibitor of PI3K and exhibits its most potent inhibitory effect on the isoforms PI3Kα and PI3Kδ. The first phase seeks to determine the maximum tolerated dose (MTD) of copanlisib with fixed dose nivolumab of 480 mg given every 4 weeks. Following determination of the MTD the second phase seeks to determine the 6-month objective response rate of the combination in MSS CRC patients and contains two cohorts 1) PIK3CA wildtype, 2) PIK3CA mutated. The study is planned with 21 evaluable subjects per cohort and allows early termination for lack of efficacy. Tumor assessments will be made using RECIST 1.1. Patients will have a pre-treatment biopsy followed by nivolumab on Day 1 of each 4 week cycle and copanlisib on Day 1, 8 and 15. A second biopsy will occur after six weeks. Eligibility criteria includes completed NGS for PI3K status, and patients must have received at least 2 prior lines of standard therapy. Patients can not have received a prior checkpoint inhibitor or PI3K inhibitor. Secondary and exploratory objectives, in addition to survival and safety outcomes, include exploring immune cell subsets in the local tumor microenvironment and in the peripheral circulation, as well as investigating immune activation and suppressive pathways through RNA expression and additional NGS techniques. The clinical study was activated in January 2019 (NCT03711058). Clinical trial information: NCT03711058.