Solstice Oncology emerged from stealth with a $225 million Series A to advance porustobart, a next-generation CTLA-4 antibody designed to extend immunotherapy into microsatellite-stable colorectal cancer. The funding follows a multibillion-dollar licensing deal with Harbour Biomed and includes a planned Phase II study of porustobart plus pembrolizumab in stage II–III MSS colon cancer in the neoadjuvant setting. A key development is the stated timeline for enrollment starting in Q4 2026 and an initial readout expected in the second half of 2027, positioning the company to produce early translational signals in a difficult-to-treat population. Sector relevance is high because CTLA-4 combinations continue to be explored as an option for tumors that do not respond to checkpoint inhibitors alone, and the launch reflects intensified competition in MSS CRC immuno-oncology.