Solstice Oncology launched on September 9 with $225 million in Series A financing and a plan to treat cancer earlier. The clinical-stage biotech is developing porustobart, an experimental antibody it intends to combine with another immunotherapy before patients undergo surgery for colon cancer.
The approach is called neoadjuvant treatment: giving medicine before the operation rather than waiting until afterward. Solstice is targeting microsatellite-stable colon cancer, a form that has generally been difficult to treat with immunotherapy. The proposed clinical workflow is to administer the drug combination while the tumor is still present and assess its effect as patients proceed through treatment.
Porustobart targets CTLA-4, one of the immune system's control mechanisms. Solstice says the engineered antibody is designed both to release that brake and to reduce regulatory immune cells that shield tumors. A companion PD-1 medicine targets a different control mechanism. The intended result is a broader attack on cancer, but that is a treatment hypothesis—not proof that the combination improves outcomes before surgery.
BioPharma Dive describes why this is a difficult development path: earlier CTLA-4 drugs established the target's potential but also exposed serious immune-related side effects. Solstice is testing a shorter-lasting antibody that it believes could make dosing more flexible. Its lead medicine was licensed from Harbour BioMed, making the new company's work one of clinical development rather than the sale of an existing approved product.
RA Capital Management led the financing, joined by Canaan Partners, Forbion and other investors. Solstice says its Phase 2 colon-cancer study is expected to open for enrollment early in the fourth quarter of 2026, with data anticipated in the second half of 2027. These are forward-looking milestones, not completed results.
The opportunity is to make an immune-based treatment useful for patients while surgery remains an option. The essential question is whether the combination can produce meaningful benefit with tolerability appropriate for that earlier setting. Porustobart remains investigational and has not been approved by a regulator; favorable findings in a different, previously treated patient group do not settle that question.