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 and most validated targets.

A quick history of the target

CTLA-4 is a molecular “brake” on the immune system. Blocking it — as the pioneering drug ipilimumab did over a decade ago — helped launch the entire field of checkpoint-inhibitor immunotherapy, which has transformed cancers like melanoma. But first-generation CTLA-4 drugs are also notoriously toxic, because unleashing the immune system everywhere can cause it to attack healthy tissue. That toxicity has limited how CTLA-4 drugs can be dosed and combined ever since.

What Solstice is doing differently

Its lead drug, porustobart, is a CTLA-4 antibody licensed from China’s Harbour BioMed ($105M upfront; up to $1.2B in total value). The key engineering tweak is a much shorter half-life — about 4–5 days versus roughly 15 days for first-generation drugs. A shorter half-life means the drug clears the body faster, which Solstice argues allows more flexible dosing and fewer side effects, while still doing the useful work of depleting regulatory T cells — immune-suppressing cells that tumors exploit to hide. It is designed to be paired with a PD-1 blocker, another checkpoint drug, for a one-two punch.

The target: “cold” colon cancer

Solstice is aiming at microsatellite-stable (MSS) colon cancer — the large majority of colorectal cancers, and a notoriously “cold” tumor type that has stubbornly resisted immunotherapy. Cold tumors are ones the immune system largely ignores; the hope is that a better-tolerated CTLA-4 drug can “heat them up” enough for immunotherapy to work. The company plans to test porustobart in the neoadjuvant setting (before surgery or other treatment) and to enter mid-stage trials by year-end 2026.

Why it matters — and the caveat

CEO Caroline Loew (formerly of Mural Oncology, Bristol Myers Squibb and Merck) brings deep industry experience, and $225 million is a large first round, signaling investor conviction. “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. Still, the graveyard of oncology is full of clever mechanisms that failed in patients, and cracking cold colon tumors has defeated many before. The thesis is compelling; the clinic will be the judge. Business and clinical news, not investment or medical advice.