Solstice Oncology launches with $225M Series A to advance next-generation CTLA-4 antibody porustobart in neoadjuvant colon cancer
核心洞察
Solstice Oncology (搜索) emerged from stealth with $225 million in Series A financing led by RA Capital Management (搜索), joined by Canaan Partners and Forbion.
Its lead candidate porustobart, a second-generation Fc-enhanced CTLA-4 (搜索) antibody, will enter a Phase 2 trial in neoadjuvant MSS stage II-III colon cancer (搜索) by the fourth quarter of 2026.
In a prior Phase 2 study, porustobart combined with PD-1 inhibitor tislelizumab achieved a 30% objective response rate in late-line MSS metastatic colorectal cancer patients without liver metastases.
Solstice Oncology (搜索), a clinical-stage immuno-oncology company, emerged from stealth Wednesday with $225 million in Series A financing to advance porustobart, a second-generation, Fc-enhanced CTLA-4 (搜索) antibody, in the neoadjuvant treatment of colon cancer (搜索). The financing was led by RA Capital Management (搜索), with participation from Canaan Partners, Forbion and other investors.
The company's lead program targets a notoriously difficult checkpoint pathway. Porustobart is being advanced across two clinical indications, beginning with microsatellite-stable (MSS) clinical stage II-III colon cancer (搜索), the largest segment of colon cancer patients, which has yet to benefit from immunotherapy. A Phase 2 trial evaluating porustobart in combination with pembrolizumab in the neoadjuvant setting will open for enrollment early in the fourth quarter of 2026, with data anticipated in the second half of 2027.
A differentiated approach to a slippery target
CTLA-4 (搜索) and PD-1 were the two checkpoint pathways scientists first identified as mechanisms tumors use to evade the immune system. While PD-1 blockers such as Merck & Co.'s Keytruda and Bristol Myers Squibb's Opdivo have become dominant in cancer care, CTLA-4 has proven a more difficult target. The first commercial CTLA-4 drug, Bristol Myers' Yervoy, gained approval in 2011 for melanoma and has since been cleared in multiple settings, often alongside Opdivo, becoming a multibillion-dollar seller. However, Yervoy is associated with sometimes severe immune-related side effects, and CTLA-4 medicines have generally proven less powerful than PD-1 blockers.
Solstice is the latest company to pursue a next-generation CTLA-4 (搜索) approach. Caroline Loew, the company's CEO and a veteran executive of Mural Oncology, Bristol Myers Squibb and Merck & Co., said porustobart is designed to increase the depletion of regulatory T cells, while a PD-1 blocker "can sustain" that activity. The drug's half-life is also shorter—around four to five days compared to the 15-day half-life of first-generation CTLA-4 drugs—which Loew said allows for "a very flexible dosing structure" and reduces the length and severity of side effects.
"You're juicing up the CTLA-4 (搜索) that was there before," Loew said.
Clinical evidence in colorectal cancer
Traditional immuno-oncology treatments have not been very successful in treating MSS colon cancer (搜索), which makes up the majority of that patient population. These patients have what are called "cold" tumors, which have an easier time evading the immune system.
In a Phase 2 clinical study conducted by Harbour BioMed (搜索), porustobart in combination with tislelizumab, a PD-1 inhibitor, demonstrated a 30% objective response rate (7 of 23 patients) in late-line patients with MSS metastatic colorectal cancer without liver metastases, with responses showing encouraging durability. The median duration of response was 8.4 months, and porustobart demonstrated a favorable safety profile supporting its continued clinical development. These findings provide clinical support for further evaluating porustobart in combination with PD-(L)1 blockade and in earlier lines of treatment, including the neoadjuvant setting.
Rationale for the neoadjuvant setting
Solstice is built around the neoadjuvant treatment setting, the point in a patient's disease course where the company believes the opportunity to improve cure rates and long-term survival is greatest.
"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," said Loew. "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, which is where we see the greatest opportunity to improve cure rates and long-term survival."
Since founding the company in February 2026, Solstice has filed and cleared an investigational new drug (IND) application and is now advancing directly into a Phase 2 trial.
Origins and partnership
Porustobart was discovered and developed by Harbour BioMed (搜索), where it is known as HBM4003. In February 2026, Harbour BioMed entered into a license agreement and equity partnership with Solstice Oncology (搜索) for the exclusive development and commercialization of porustobart outside Greater China. The deal carried a $105 million upfront payment and could be worth $1.2 billion in total.
"The team has rapidly designed and advanced a Phase 2 trial to generate an early, credible read on efficacy, and the size of this financing and the strength of the syndicate reflect our conviction in the science, the strategy, and this team's ability to execute," said Josh Resnick, a partner at RA Capital, in a statement.
"As the originator of porustobart and a partner of Solstice, we look forward to supporting the continued development of this differentiated CTLA-4 (搜索) antibody and exploring its potential to benefit patients worldwide," said Dr. Jingsong Wang, Founder, Chairman and CEO of Harbour BioMed (搜索).