Solstice Oncology launches with $225m Series A for CTLA-4 antibody
Solstice Oncology has launched from stealth with a $225 million Series A financing, one of the largest debut rounds for a clinical-stage immuno-oncology company in recent memory. The Boston-based company, founded in February 2026, is advancing porustobart, a second-generation Fc-enhanced CTLA-4 antibody licensed from Harbour BioMed, in the neoadjuvant treatment setting. The round was led by RA Capital Management, with Canaan Partners and Forbion among the syndicate.
Solstice's lead programme targets microsatellite-stable (MSS) clinical stage II-III colon cancer, a population that has historically failed to respond to immunotherapy. Its Phase 2 trial, evaluating porustobart in combination with pembrolizumab (Merck's PD-1 inhibitor), is expected to open for enrolment in the fourth quarter of 2026. The company anticipates data from the study in the second half of 2027. A second indication is in development but has not been disclosed.
The science behind porustobart
Porustobart is designed to attack tumour-mediated immunosuppression through two distinct mechanisms: CTLA-4 checkpoint blockade and depletion of regulatory T cells (Tregs) via antibody-dependent cellular cytotoxicity. Its engineered heavy-chain-only structure gives it a substantially shorter half-life than first-generation CTLA-4 antibodies such as ipilimumab, roughly four to five days compared with two to three weeks. Solstice says this shortened exposure profile could reduce the frequency and severity of immune-related adverse events, a well-documented limitation that constrained uptake of earlier CTLA-4 agents.
Phase 1/2 data generated by Harbour BioMed showed that porustobart combined with tislelizumab, a PD-1 inhibitor, produced a 30% objective response rate in 23 heavily pre-treated late-line MSS metastatic colorectal cancer patients without liver metastases, with a median duration of response of 8.4 months. The company describes the safety profile as supportive of later-stage development.
Chief executive Caroline Loew said the neoadjuvant setting offers the best opportunity to improve cure rates because "the tumour is still present, the immune system is still intact, and disease hasn't yet hardened its resistance to treatment." The rationale is mechanistically coherent: treating earlier, when immune architecture is less compromised, may allow a systemic anti-tumour response to address micrometastatic spread beyond the primary lesion.
Market context and competitive landscape
The neoadjuvant immuno-oncology space has grown considerably since pembrolizumab's approval in early-stage triple-negative breast cancer and nivolumab's success in resectable non-small cell lung cancer. MSS colorectal cancer, however, remains a conspicuous gap in the immunotherapy landscape. MSS tumours carry a low mutational burden and a cold immune microenvironment, making them largely refractory to PD-1 blockade alone. A number of companies and academic groups are pursuing combination strategies to convert these tumours into immunotherapy-responsive disease, and Solstice joins a small cohort betting that next-generation CTLA-4 agents are the key.
The size of the Series A reflects both the strength of the syndicate's conviction and the capital intensity of running combination immuno-oncology trials. RA Capital partner Josh Resnick described porustobart as a "de-risked molecule" given the existing Phase 1/2 clinical activity, though it should be noted that the prior data were generated in a metastatic late-line setting, which differs meaningfully from the curative-intent neoadjuvant context Solstice is now pursuing.
Pathologic complete response is expected to serve as the primary efficacy signal in the Phase 2 study, a surrogate endpoint that regulators have accepted as a basis for accelerated approval in other tumour types. Whether that precedent will extend to MSS colon cancer remains a question that the 2027 readout will begin to answer.