Boston-based Solstice Oncology launched with a USD 225 million Series A financing to advance porustobart (HBM4003), an Fc-enhanced CTLA-4 antibody that previously showed clinical activity in microsatellite-stable (MSS) metastatic colorectal cancer, into neoadjuvant development beginning with stage II-III MSS colon cancer.
RA Capital Management led the financing, with participation from Canaan Partners, Forbion, and other investors. Solstice was founded in February 2026, when it licensed ex-Greater China rights to porustobart from HBM Holdings Limited (HKEX: 02142), formerly Harbour BioMed, in exchange for USD 55 million in upfront and near-term cash payments and more than USD 50 million in Solstice equity. HBM is also eligible for development and commercial milestones. Solstice said the Series A will support development of porustobart across two indications.
Porustobart is a fully human, heavy-chain-only CTLA-4 antibody engineered to combine checkpoint blockade with Fc-mediated depletion of regulatory T cells in the tumor microenvironment. The antibody has a reported half-life of approximately four to five days, substantially shorter than first-generation CTLA-4 antibodies, which Solstice said could allow more flexible dosing and potentially limit the duration of immune-related adverse events.
The asset already has clinical evidence in colorectal cancer. In an open-label Phase II study conducted by HBM and reported in October 2025, porustobart combined with the PD-1 inhibitor tislelizumab produced a 34.8% objective response rate and 60.9% disease control rate among 23 evaluable patients with heavily pretreated MSS metastatic colorectal cancer without liver metastases. HBM subsequently reported a median progression-free survival of 4.2 months and 12-month overall survival rate of 84%. The findings provided the basis for evaluating the CTLA-4/PD-1 combination earlier in the disease course.