Solstice Oncology emerged from stealth with a hefty $225 million Series A, backed by RA Capital, Canaan Partners and Forbion, to pursue a next-generation take on CTLA-4 — one of cancer immunotherapy’s oldest targets.
Its lead drug, porustobart, is a CTLA-4 antibody licensed from China’s Harbour BioMed ($105M upfront; up to $1.2B in total value). CEO Caroline Loew (ex-Mural Oncology, BMS, Merck) leads the company.
What’s different
First-generation CTLA-4 drugs are effective but toxic, partly because they linger in the body (a ~15-day half-life). Porustobart is engineered with a much shorter 4–5 day half-life, which Solstice argues enables more flexible dosing and fewer side effects, while still boosting the depletion of regulatory T cells (immune-suppressing cells that shield tumors). It’s designed to pair with a PD-1 blocker.
Why it matters
The primary target is microsatellite-stable (MSS) colon cancer — so-called “cold” tumors that have stubbornly resisted immunotherapy. Solstice plans to test porustobart in the neoadjuvant setting (before other treatment) and enter mid-stage trials by year-end 2026. “The team has rapidly designed and advanced a Phase 2 trial to generate an early, credible read on efficacy,” said RA Capital’s Josh Resnick. As always, the thesis still has to survive the clinic.