Solstice Oncology has raised $225 million in Series A financing to advance a next-generation immunotherapy that the biotech believes could expand the reach of cancer treatment.
The Boston-based company licensed porustobart, an Fc-enhanced antibody targeting cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), from China’s Harbour BioMed when Solstice was established in February. The agreement provided Harbour with an upfront package worth $105 million and gave Solstice rights to develop and market the therapy outside China. The biotech has already moved porustobart into Phase 2 testing in patients with stage II and III microsatellite stable (MSS) colon cancer.
According to Solstice, this group of patients has so far seen little to no benefit from immunotherapy.
The Series A was led by RA Capital Management, with participation from Forbion and Canaan Partners, among other investors. The proceeds will support a Phase 2 study evaluating porustobart in combination with Merck’s Keytruda, with enrollment expected to begin this year and results anticipated during the second half of 2027. Solstice also intends to pursue porustobart in another indication that it has not yet disclosed.
MSS colon cancer is considered a “cold” tumor because it typically generates a weak immune response and has shown limited sensitivity to most immunotherapies. Surgery and chemotherapy remain the standard treatment approach. Porustobart is intended to complement and broaden the activity of PD-1 inhibitors such as Keytruda by removing another immune-system checkpoint while also reducing regulatory T cells that suppress immune attacks against tumors.
Solstice CEO Caroline Loew said the company is seeking a synergistic effect by combining porustobart with Keytruda, particularly in MSS colon cancer, where existing immunotherapy options have produced limited responses.
Loew noted that anti-PD-L1 treatment alone has historically produced no meaningful response in this setting, while combining PD-L1 inhibition with a first-generation CTLA-4 antibody has resulted in response rates of up to about 5%. By contrast, she said the company has observed response rates of approximately 30% when its treatment is added, pointing to results from Harbour BioMed’s Phase 1b study. Patients receiving porustobart in that trial remained in remission for an average of nearly 8.5 months.
The CEO said the difference illustrates the potential value of simultaneously engaging multiple immune mechanisms rather than relying on a single pathway.
Porustobart also has a shorter half-life than currently available CTLA-4 antibodies. The drug is expected to remain in the body for roughly four to five days, compared with approximately two to three weeks for existing CTLA-4 therapies. Solstice believes that characteristic could allow greater flexibility in dosing and potentially limit both the frequency and duration of immune-related adverse events.
Loew described the program as an effort to unlock the broader potential of CTLA-4 inhibition and make it more capable of changing clinical practice. She said first-generation immunotherapies produced remarkable outcomes for some patients, while Solstice is attempting to extend those benefits to a larger population.
Beyond its second-generation CTLA-4 design, porustobart is being evaluated as a neoadjuvant treatment, meaning it would be given before surgery.
Solstice Oncology has launched with a major $225 million Series A financing to advance its next-generation immunotherapy program into Phase 2 clinical development. The financing was led by RA Capital Management, with participation from Canaan Partners, Forbion, and other investors.
The funding provides Solstice with significant resources to develop porustobart, a second-generation, Fc-enhanced CTLA-4 antibody designed to strengthen the immune system’s ability to attack cancer. Solstice is initially focusing on microsatellite-stable (MSS) stage II–III colon cancer, an area where immunotherapy has historically shown limited benefit.
Solstice Targets MSS Colon Cancer
Solstice plans to evaluate porustobart in combination with pembrolizumab in the neoadjuvant setting, meaning treatment will be given before surgery. The Phase 2 trial is expected to begin enrolling patients in early Q4 2026, with data anticipated in the second half of 2027.
