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# Solstice Oncology Launches With $225 Million to Bring Next-Generation CTLA-4 Into Early Colon Cancer
- URL: https://bioweek.com/solstice-oncology-launches-with-225-million-to-bring-next-generation-ctla-4-into-early-colon-cancer/
- Published: 2026-09-11T05:06:18.000Z
- Updated: 2026-09-11T05:06:18.000Z
- Description: Solstice Oncology launched with a $225 million Series A led by RA Capital to advance Harbour BioMed-licensed porustobart into neoadjuvant MSS colon cancer.
- Author: BioWeek
- Tags: Markets, Pipeline

Boston-based Solstice Oncology [announced](https://www.solsticeoncology.com/press-release?ref=bioweek.com) its launch on September 9, 2026, closing a $225 million Series A financing led by RA Capital Management. Canaan Partners, Forbion, and other healthcare investors joined the syndicate, with RA Capital partner Dr. Josh Resnick joining the board. The company is directing that capital toward porustobart, an Fc-enhanced CTLA-4 antibody targeting neoadjuvant microsatellite-stable colon cancer, a setting where checkpoint inhibitors have historically failed.

The leadership team brings direct experience in late-stage immuno-oncology. Chief executive Caroline Loew previously led Mural Oncology and Glympse Bio after senior R&D strategy roles at Bristol Myers Squibb and Merck. Chief medical officer David Feltquate brings more than 25 years of oncology development experience, including prior CMO roles at iTeos and Palleon and clinical development leadership on checkpoint inhibitors at Bristol Myers Squibb. Chief operating officer and general counsel Maiken Keson-Brookes served as chief legal officer at Mural Oncology alongside earlier legal leadership at Biogen and uniQure.

### A Harbour antibody with a shorter half-life

Porustobart, formerly designated HBM4003, is a fully human, heavy-chain-only monoclonal antibody against CTLA-4 generated on Harbour BioMed's HCAb platform. In February 2026, Solstice [licensed](https://www.prnewswire.com/news-releases/harbour-biomed-partner-solstice-oncology-announces-225-million-series-a-financing-to-advance-porustobart-a-neoadjuvant-immuno-oncology-therapy-302873689.html?ref=bioweek.com) exclusive rights outside Greater China from Harbour BioMed. Harbour disclosed more than $105 million in upfront consideration, including $50 million in cash, $5 million in near-term cash, and over $50 million in Solstice equity, alongside up to $1.1 billion in clinical, regulatory, and commercial milestones plus tiered royalties.

The molecule features a dual mechanism of action: CTLA-4 checkpoint blockade to activate effector T cells, paired with an engineered Fc domain driving antibody-dependent cellular cytotoxicity to deplete immunosuppressive regulatory T cells within the tumor microenvironment. Its heavy-chain-only architecture yields a systemic half-life of 4 to 5 days, compared with 15 to 21 days for first-generation CTLA-4 agents like ipilimumab. Because combination checkpoint blockade often causes protracted immune-related adverse events, rapid systemic clearance is designed to allow flexible dosing and reduce persistent toxicities.

### The neoadjuvant approach in cold tumors

Solstice is focusing on the neoadjuvant window, administering therapy before surgery while the primary tumor remains in place. An intact primary tumor functions as an in situ antigen reservoir, potentially expanding tumor-infiltrating lymphocytes and establishing systemic immune memory to eliminate micrometastatic disease prior to surgical resection.

The target indication is resectable clinical stage II-III microsatellite-stable (MSS) colon cancer. Colorectal cancer represents the second leading cause of cancer mortality worldwide. MSS tumors account for roughly 85% to 90% of all colorectal malignancies and have proven refractory to single-agent PD-1 and PD-L1 inhibitors, which show near-zero response rates outside the 10% to 15% microsatellite instability-high subgroup.

Prior clinical validation comes from advanced disease. In a Phase 1/2 trial conducted by Harbour BioMed in pre-treated metastatic MSS colorectal cancer without liver metastases, porustobart combined with the PD-1 inhibitor tislelizumab [reported](https://www.cancernetwork.com/view/porustobart-tislelizumab-elicits-responses-in-mss-metastatic-colorectal-cancer?ref=bioweek.com) a 30% objective response rate (7 of 23 patients) and an 8.4-month median duration of response. Across all 23 evaluable patients, the objective response rate was 34.8% with median progression-free survival of 4.2 months, accompanied by predominantly grade 1 and 2 adverse events and no fatal treatment-emergent toxicities.

### The 100-patient surgical timeline

Solstice's lead trial is [registered](https://clinicaltrials.gov/study/NCT07808151?ref=bioweek.com) as NCT07808151, an international Phase 2 study evaluating porustobart with pembrolizumab in 100 patients with resectable stage II-III MSS colon cancer across sites in the United States and Europe. Enrollment opens in late September 2026\. The primary endpoint measures pathologic response, including pathologic complete and major pathologic response rates, with topline results expected in the second half of 2027.

The strategic challenge lies in the timing. In operable stage II and III colon cancer, upfront surgery already offers curative intent. Introducing dual checkpoint blockade prior to resection introduces the risk of immune-mediated colitis or hepatitis that could delay surgery or complicate operative outcomes. Furthermore, establishing whether pathologic clearance reliably translates into prolonged disease-free survival in MSS tumors remains an open question that Solstice has committed its Series A capital to resolve.