Solstice Oncology exited stealth with a $225 million series A to advance porustobart, a next-generation CTLA-4 antibody intended to expand immunotherapy reach into microsatellite-stable colon cancer. The financing, led by RA Capital Management and joined by Canaan Partners and Forbion among others, is anchored on a phase II study planned with porustobart plus pembrolizumab in stage II–III MSS colon cancer. The company said enrollment is expected to begin in the fourth quarter of 2026, with an initial readout targeted for the second half of 2027—positioning CTLA-4 strategy as a potential counter to limited checkpoint responsiveness in MSS disease.