Solstice Oncology emerged from stealth with a $225 million Series A designed to advance porustobart, its next-generation CTLA-4 antibody built for microsatellite-stable (MSS) colon cancer. The company said its CTLA-4 asset was licensed from Harbour Biomed, and the funding will support a phase II program testing porustobart plus pembrolizumab in stage II-III MSS colorectal cancer in the neoadjuvant setting. The financing—led by RA Capital Management and joined by Canaan Partners and Forbion—positions Solstice to generate early efficacy and biomarker signals in a difficult immune landscape where CTLA-4 has historically struggled to overcome resistance. Enrollment is expected to begin in the fourth quarter of 2026, with an initial readout targeted for the second half of 2027. By pairing CTLA-4 with PD-1 blockade, Solstice is aligning its development plan with current immunotherapy combination strategies while focusing on a patient population that has been challenging for checkpoint monotherapy approaches. The launch also underscores investor appetite for next-gen immune checkpoints targeting MSS tumors. Separately, the company’s debut follows an earlier Harbour Biomed licensing milestone, reinforcing how cross-border IP deals are feeding new oncology startups and pipelines.