Solstice Oncology Launches With $225M to Advance Next-Generation CTLA-4 Antibody Porustobart

Solstice Oncology launched with a $225 million Series A financing led by RA Capital Management, joined by Canaan Partners, Forbion, and other investors.¹ The Boston-based, clinical-stage immuno-oncology company is built around treating cancer in the neoadjuvant setting — before surgery, while the tumor is still present and the immune system remains intact — rather than waiting until later, more advanced stages of disease.¹ Solstice's lead candidate, porustobart, is a second-generation, Fc-enhanced cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) antibody licensed from China's Harbour BioMed, where it is also known as HBM4003.² The company is advancing porustobart across two clinical indications: a phase 2 trial combining it with pembrolizumab (Keytruda) in the neoadjuvant setting for microsatellite-stable (MSS) clinical stage II-III colon cancer, set to open for enrollment in the fourth quarter of 2026, and a second, undisclosed indication.¹

Caroline Loew, Ph.D., chief executive officer of Solstice Oncology, said, "We believe the greatest opportunity in oncology lies in treating disease earlier, in the neoadjuvant setting, when the tumor is still present, the immune system is still intact, and disease hasn't yet hardened its resistance to treatment. Treating patients early with an immuno-oncology combination therapy like the one we are evaluating in our phase 2 trial could allow the immune response to act systemically, reaching micrometastatic disease well beyond the primary tumor."³

How does porustobart differ from existing CTLA-4 antibodies?

CTLA-4 is an immune checkpoint that, when activated on T cells, dampens the immune response; blocking it releases that brake and allows for a stronger antitumor immune response.⁴ Existing CTLA-4 antibodies, including Bristol Myers Squibb's Yervoy (ipilimumab) and AstraZeneca's Imjudo (tremelimumab), have established the mechanism's clinical value but carry a narrow therapeutic window, with immune-related toxicities limiting how they can be dosed.² Porustobart is engineered with an Fc-enhanced design intended to extend its half-life relative to existing CTLA-4 antibodies, which Solstice says could allow more flexible dosing regimens.²

Why target microsatellite-stable colon cancer specifically?

Immune checkpoint inhibitors have transformed outcomes in microsatellite instability-high colorectal cancer, but MSS colorectal tumors — which make up the large majority of colorectal cancer cases — have historically responded poorly to checkpoint blockade, reflecting an immunologically "cold" tumor microenvironment with lower mutational burden and fewer neoantigens for the immune system to recognize.⁵ Solstice's phase 2 trial specifically targets clinical stage II-III MSS colon cancer, a population the company describes as not yet having benefited from immunotherapy.¹

That kind of precision-targeting logic echoes a broader theme in oncology drug development. Alexander Shoushtari, M.D., melanoma oncologist and cellular therapist at Memorial Sloan Kettering Cancer Center, told BioPharm International in an interview discussing five-year Kimmtrak data in uveal melanoma — another cancer type historically considered immunologically resistant — "I think what this trial has really shown is that you have to get more precise as a scientist, and you have to be more aware as a physician that different tools now exist to manipulate the body's immune system to fight cancer."⁶

Jingsong Wang, M.D., Ph.D., founder, chairman, and chief executive officer of Harbour BioMed, said, "As the originator of porustobart and a partner of Solstice, we look forward to supporting the continued development of this differentiated CTLA-4 antibody and exploring its potential to benefit patients worldwide."²

What happens next?

Solstice's phase 2 trial combining porustobart with Keytruda in neoadjuvant MSS colon cancer is expected to open for enrollment in the fourth quarter of 2026, with data anticipated in the second half of 2027.² The company has not disclosed details of its second, undisclosed clinical indication.

References

1. Solstice Oncology. Solstice Oncology Launches with $225 Million Series A Financing to Advance Porustobart, a Neoadjuvant Immuno-Oncology Therapy. Press release. Published September 9, 2026. Accessed September 10, 2026.
2. Maddox W. Solstice Rises With $225M to Develop Next-Gen CTLA-4 Drug From China. Fierce Biotech. Published September 9, 2026. Accessed September 10, 2026.
3. Harbour BioMed Therapeutics. Harbour BioMed Partner, Solstice Oncology, Announces $225 Million Series A Financing to Advance Porustobart, a Neoadjuvant Immuno-Oncology Therapy. Press release. Published September 9, 2026. Accessed September 10, 2026.
4. Passardi A, Canale M, Valgiusti M, Ulivi P. Immune Checkpoints as a Target for Colorectal Cancer Treatment. Int J Mol Sci. 2017;18(6):1324. doi:10.3390/ijms18061324.
5. Gandini A, Puglisi S, Pirrone C, et al. The Role of Immunotherapy in Microsatellites Stable Metastatic Colorectal Cancer: State of the Art and Future Perspectives. Front Oncol. 2023;13:1161048. doi:10.3389/fonc.2023.1161048.
6. Shoushtari A, Schoenthaler E. Kimmtrak Data Challenge Long-Held Assumptions About Immunotherapy in Uveal Melanoma. BioPharm International. Published May 29, 2026. Accessed September 10, 2026.