Solstice Oncology Launches With $225 Million Series A Financing to Advance Second-Generation Fc-Enhanced CTLA-4 Antibody Porustobart Into Neoadjuvant Colon Cancer Phase 2
Solstice Oncology, a Boston-based clinical-stage immuno-oncology company founded February 2026, announced Wednesday September 9, 2026 its formal launch and closing of an oversubscribed $225 million Series A financing to advance porustobart, a second-generation Fc-enhanced CTLA-4 monoclonal antibody licensed from Harbour BioMed. The Series A was led by RA Capital Management with participation from Canaan Partners, Forbion, and other investors. Porustobart is heading into a Phase 2 neoadjuvant trial combining with pembrolizumab (Merck's Keytruda) for microsatellite-stable clinical stage II-III colon cancer, targeting enrollment opening Q4 2026, plus a second undisclosed indication. Fc-enhancement is designed to improve the antibody's ability to deplete regulatory T-cells and enhance immunogenicity relative to first-generation CTLA-4 inhibitors like ipilimumab (Bristol Myers Squibb's Yervoy). The financing continues a Q3 2026 immuno-oncology financing wave and demonstrates continued investor appetite for differentiated checkpoint programs despite the maturity of the anti-PD-(L)1 class.