Solstice Oncology raises a $225M Series A to rebuild CTLA-4 cancer immunotherapy

Solstice Oncology has raised $225 million in a Series A led by RA Capital Management, Canaan Partners and Forbion. The Boston company is developing next-generation CTLA-4 cancer immunotherapy.

Why the size is notable

A $225 million Series A is larger than the great majority of Series C rounds raised this year in software. In biotech the arithmetic is different - a company may need to fund years of preclinical and clinical work before there is anything resembling revenue - but even by biotech standards this is a large first round.

Rounds structured this way are usually a signal that the investors intend to fund the company through a specific clinical milestone in one go, rather than returning to the market between phases.

The target

CTLA-4 is one of the original validated checkpoint targets in cancer immunotherapy. The first generation of drugs against it established that the mechanism works, and also established its limits: efficacy came with a toxicity profile that constrained how widely the drugs could be used.

"Next-generation" in this context generally means attempting to keep the anti-tumour activity while reducing that toxicity. Solstice has not published detailed preclinical data publicly, so the specific approach is not yet assessable from the outside.

Sources

1. Tech Startups: Venture Capital & Startup Funding Roundup, September 9, 2026