Solstice Oncology emerged from stealth with a $225 million Series A to advance porustobart, a second-generation, Fc-enhanced CTLA-4 antibody licensed from Harbour BioMed. The company is pairing porustobart with pembrolizumab in a Phase II study for microsatellite-stable (MSS) stage II–III colon cancer. Enrollment is expected to begin early in Q4 2026, with data targeted for the second half of 2027. Solstice is building on earlier Harbour BioMed-led Phase II results in late-line MSS metastatic colorectal cancer without liver metastases, where porustobart plus tislelizumab showed a 30% objective response rate and an 8.4-month median duration of response. CEO Caroline Loew framed the strategy around earlier intervention in neoadjuvant settings to leverage an intact immune system. University of Leeds professor Jenny Seligmann emphasized pathologic response as a fast signal for longer-term benefit in MSS colon cancer.