A new immuno-oncology play raises $225M for a next-generation drug

An article from Dive Brief A new immuno-oncology play raises $225M for a next-generation drug Solstice Oncology is advancing a medicine that it believes is able to address the weaknesses of drugs aimed at CTLA-4, a popular but tricky cancer immunotherapy target. Published Sept. 9, 2026 Gwendolyn Wu Senior Reporter Share Copy link Email LinkedIn X/Twitter Facebook Print License Add us on Google Dive Brief: Solstice Oncology, a biotechnology startup developing new immunotherapies that go after a notoriously tricky cancer target, emerged from stealth Wednesday armed with $225 million in financing.

Its lead program is a CTLA-4 antibody called porustobart, which it’s taking the unusual step of testing in colon cancer patients before they’re exposed to other treatments. That drug is set to enter mid-stage testing by the end of the year, said Caroline Loew, the company’s CEO and a veteran executive of Mural Oncology, Bristol Myers Squibb and Merck & Co. Porustobart was licensed from Chinese drug developer Harbour Biomed in February, through a deal that carried a $105 million upfront payment and could be worth $1.2 billion in total.

The capital from Solstice’s Series A round came from a bevy of blue-chip venture investors such as RA Capital Management, Canaan Partners and Forbion. Dive Insight: In the early stages of developing cancer immunotherapies two decades ago, scientists zeroed in on two different “checkpoint” pathways tumors use to hide from the immune system: CTLA-4 and PD-1. Since then, though, their trajectories have diverged.

Drugs that block PD-1 have become dominant in cancer care, boosted by the success of medications like Merck & Co.’s Keytruda and Bristol Myers Squibb’s Opdivo. CTLA-4, conversely, ended up proving a slippery target even for experienced drugmakers. The first commercial drug to target CTLA-4, Bristol Myers’ Yervoy, gained approval in 2011 as a melanoma treatment.

It has since been cleared in multiple other settings, often alongside Opdivo, and is a multibillion-dollar seller. But Yervoy is also associated with sometimes severe immune-related side effects, and drugmakers have found medicines like it to be less powerful than PD-1 blockers. Still, some drugmakers have continued to hammer away at CTLA-4, believing that next-generation medicines might be able to perform better.

AstraZeneca, which sells a CTLA-4 inhibitor called Imjudo, is one of them. Even though a double-barreled drug that targets CTLA-4 and another protein recently failed a lung cancer study, AstraZeneca is continuing on with other trials. Solstice is the latest to take a stab at it.

Traditional immuno-oncology treatments have not been very successful in treating microsatellite-stable, or MSS, colon cancer, which makes up the majority of that patient population. They have what are called “cold” tumors, which have an easier time evading the immune system. Loew said the company is combining porustobart with a traditional PD-1 blocker, which has in early human testing shown it can provoke a durable response in patients who have undergone several other lines of cancer treatment.

Loew noted how Solstice’s drug is designed to increase the depletion of regulatory T cells, while the PD-1 blocker “can sustain” that activity. Its half-life is also shorter, she added, coming in around four to five days compared to the 15-day half-life of first-generation CTLA-4 drugs. That allows Solstice to develop “a very flexible dosing structure” and reduces the length and severity of side effects.

“You're juicing up the CTLA-4 that was there before,” Loew said. “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,” Josh Resnick, a partner at RA Capital, said in a statement. Add us on Google Share Copy link Email LinkedIn X/Twitter Facebook Print License Filed Under: Biotech