Solstice Oncology launches with $225M Series A to treat cancer earlier
What's the deal? Solstice OncologyDealroom has a profile for this one. Try Dealroom → launched on September 9, 2026 with a $225 million Series A led by RA Capital Management, with Canaan Partners, Forbion, and other investors joining. The Boston-based clinical-stage company is developing porustobart, an immuno-oncology therapy for cancer in the neoadjuvant setting — treating disease before it spreads.
What's the endgame? Porustobart is a second-generation, Fc-enhanced CTLA-4 antibody designed to block an immune checkpoint and deplete regulatory T cells that shield tumors. It is meant to be paired with anti-PD-(L)1 therapy, widening the immune response to reach disease beyond the primary tumor.
Why now? Solstice's Phase 2 trial, testing porustobart with pembrolizumab in microsatellite-stable stage II-III colon cancer, opens for enrollment early in the fourth quarter of 2026. The company, founded in February, has already filed and cleared an IND and expects data in the second half of 2027.
What could go wrong? MSS colon cancer has historically been unresponsive to immunotherapy. Solstice is betting on a newer class of Fc-enhanced CTLA-4 molecules to change that — data it says shows a "true clinical synergistic effect" with PD-(L)1 blockade.
Porustobart's engineered structure gives it a shorter half-life than earlier CTLA-4 antibodies — four to five days versus roughly two to three weeks — which the company believes could allow more flexible dosing and fewer immune-related side effects.
Chief executive officer Caroline Loew framed the strategy around timing. "We believe the greatest opportunity in oncology lies in treating disease earlier, in the neoadjuvant setting, when the tumor is still present, the immune system is still intact, and disease hasn't yet hardened its resistance to treatment," she said.
The signal: At $225 million, Solstice's raise sits in the 99th percentile of all Series A rounds ever recorded in its sector and country — a rare vote of confidence for a company only months old, and a sign investors are willing to write big early checks on immuno-oncology bets aimed at treating cancer sooner.
Read more: solsticeoncology.com
Image credit: Solstice Oncology
