Solstice Oncology Raises $225M for Earlier Cancer Treatment

Solstice Oncology is starting with a timing question that has been hiding inside immuno-oncology for years: can a potent immune therapy do more when it is given before surgery, while the tumor is still present and the immune system has a target to recognize?

The Boston-based biotech launched on September 9 with a $225 million Series A to pursue that question. The round was led by RA Capital Management, with participation from Canaan Partners, Forbion, and other investors. Solstice says the capital will fund Phase 2 development of porustobart, a next-generation CTLA-4 antibody, in resectable cancers.

That is a large first institutional round, but its size makes more sense when viewed against the work ahead. Solstice is not financing a discovery-stage hypothesis. It is taking a licensed, clinical-stage molecule into a 100-patient study across multiple countries, preparing a second indication, and building the operating team required to produce a meaningful clinical readout.

A company formed around the neoadjuvant window

Solstice was founded in February 2026 and is led by President and CEO Caroline J. Loew, PhD. Its strategy centers on neoadjuvant treatment, which means treating patients before surgery rather than waiting until cancer has progressed or spread.

The premise is biologically appealing but still needs clinical proof. Before surgery, the primary tumor remains in place and can provide antigens that help the immune system recognize cancer. Patients may also have stronger immune function and lower tumor burden than those enrolled in late-line metastatic studies. Solstice believes those conditions may give porustobart a better opportunity to generate a durable systemic response.

The company's first study will test porustobart with pembrolizumab in patients with stage II or III mismatch repair-proficient or microsatellite-stable colon cancer. These tumors have historically responded poorly to checkpoint inhibitors. The Phase 2 trial record lists an estimated enrollment of 100 patients and sites in the United States and Europe. As of September 9, the study was not yet recruiting, with a planned start in late September 2026.

What porustobart is designed to change

Porustobart is a fully human, Fc-enhanced anti-CTLA-4 antibody originally developed by Harbour BioMed. CTLA-4 blockade is an established approach in cancer immunotherapy, but first-generation medicines can carry substantial immune-related toxicity and long systemic exposure.

Solstice says porustobart was engineered to combine checkpoint blockade with stronger depletion of regulatory T cells inside tumors. The company also points to a shorter half-life of roughly four to five days, compared with about 15 days for first-generation CTLA-4 antibodies. The intended advantage is a potent immune effect with more manageable exposure, although that benefit still has to be demonstrated in the planned studies.

The program already has a clinical signal behind it. Harbour BioMed reported a 30% objective response rate, or seven responses among 23 patients, in a Phase 1 study of porustobart plus a PD-1 antibody in late-line microsatellite-stable metastatic colorectal cancer without liver metastases. The reported median duration of response was 8.4 months. Those results are encouraging for a difficult-to-treat population, but they came from a small, selected cohort and do not establish how the drug will perform before surgery.

The financing also funds a development handoff

Solstice holds rights to porustobart outside Greater China through a February 2026 license and equity agreement with Harbour BioMed. Under that agreement, Harbour BioMed disclosed more than $105 million in upfront consideration, including $50 million in cash, $5 million in near-term cash, and more than $50 million in Solstice equity. It could also receive up to roughly $1.1 billion in development, regulatory, and commercial milestones, plus royalties.

A March 2026 Form D filing showed approximately $121.3 million sold at that point in an offering that began on February 20. That filing appears to capture an earlier close within the Series A, not a separate round. The reported financing total remains $225 million.

The syndicate reflects both the capital intensity and the risk profile. RA Capital, Canaan, and Forbion are experienced life sciences investors, and representatives from all three firms sit on the company's board. RA Capital partner Josh Resnick told BioPharma Dive that the financing was designed to give Solstice enough capital and organizational depth to pursue a broad clinical plan.

Leadership built for clinical execution

Loew brings nearly three decades of biopharma experience, most recently as CEO of Mural Oncology. She previously led Glympse Bio and held strategy and planning roles at Bristol Myers Squibb after earlier work at Merck.

Chief Medical Officer David Feltquate, MD, PhD, has more than 25 years of oncology and drug development experience. His prior roles include chief medical officer positions at iTeos Therapeutics and Palleon Pharmaceuticals, hematology development leadership at Novartis, and clinical development work on checkpoint inhibitors at Bristol Myers Squibb.

Chief Operating Officer and General Counsel Maiken Keson-Brookes, LLB, LLM, previously served as chief legal officer of Mural Oncology and held senior legal roles at Rubius Therapeutics, Synlogic, uniQure, Forum Pharmaceuticals, and Biogen. The three executives give Solstice experience across company building, clinical development, and transactions, which matters when a young company begins with an international trial and a major licensing agreement.

What the next clinical readout needs to answer

Solstice expects to begin enrollment in early fourth-quarter 2026 and anticipates data in the second half of 2027. The study's primary measures include pathologic response after treatment and safety. Investigators will also assess immune activity and longer-term outcomes.

The central question is not whether CTLA-4 can activate an immune response. That has been established. The question is whether porustobart's design and the earlier treatment setting can produce enough benefit to justify using a powerful immune therapy in patients whose tumors are still operable.

The $225 million gives Solstice room to test that thesis at a meaningful scale. The Phase 2 results will determine whether the company's unusually large launch round bought more than time. They will show whether changing when CTLA-4 is used can change what it delivers.