Solstice Oncology raises $225M to advance next-generation CTLA-4 therapy

Solstice Oncology has emerged from stealth with a $225 million Series A, led by RA Capital Management, with participation from Canaan Partners, Forbion and other investors.

The Boston-based biotech is developing porustobart, a second-generation, Fc-enhanced CTLA-4 antibody licensed from Harbour BioMed. The deal reportedly included $105 million upfront and could reach $1.2 billion in total value.

Solstice plans to test porustobart with Merck’s pembrolizumab (Keytruda) in a Phase 2 neoadjuvant study for microsatellite-stable (MSS) stage II–III colon cancer—an area where immunotherapy has shown limited benefit.

The trial is expected to begin enrolling in early Q4 2026, with initial data anticipated in the second half of 2027. A second indication remains undisclosed.

Porustobart is designed to enhance immune-cell activity and deplete immunosuppressive regulatory T cells. Its shorter four-to-five-day half-life may offer more flexible dosing and potentially reduce the duration of immune-related adverse events.

The program is supported by earlier Harbour BioMed data:
in a Phase 2 study, porustobart plus tislelizumab produced a 30% objective response rate in a small subgroup of late-line MSS colorectal-cancer patients without liver metastases.