Transforming Early-Stage Colon Cancer Outcomes: Why We Invested in Solstice Oncology

Our best chance at offering a cure today for solid tumors that impact millions of people is to catch cancer early, remove it surgically, and supplement care with life-extending medicines. Treating the disease before surgery in the neo-adjuvant setting has shown tremendous promise for some of the most common solid tumors. Yet, most novel cancer medicines are often first approved in the last lines of metastatic disease when all other options have failed, and it takes decades to bring new medicines to the pre-metastatic setting, where the greatest survival impacts can be obtained.

Today, Canaan company Solstice Oncology emerged from stealth with a $225 million Series A financing from Canaan, RA Capital, Forbion and other investors. The company is advancing porustobart, a next-generation, Fc-enhanced CTLA-4 antibody, in combination with pembrolizumab across two clinical indications. Its first clinical study will be a Phase 2 trial in stage II-III microsatellite-stable, or MSS, colon cancer, which is expected to open for enrollment early in the fourth quarter of 2026.

Solstice is testing whether immunotherapy, medicines that help the immune system recognize and attack cancer, could be more effective when used earlier in the course of disease, while the cancer can still be removed through surgery.

An Old Assumption Came From Treating Cancer Late

Cancer is frightening, and what patients ultimately want is to know that it is gone. For people with localized colon cancer, surgery has traditionally offered the best chance of achieving that. Yet for some patients with stage II-III disease, microscopic cancer cells may already have moved beyond the primary tumor, and the cancer can return after an apparently successful procedure. Once MSS colon cancer has spread, outcomes can be poor, and treatment options become more limited.

MSS colon cancer represents the majority of colon cancer cases. It has also largely resisted immunotherapy. These tumors tend to remain hidden from the immune system, making it difficult for immune-based medicines to recognize and attack them.

Many of the first attempts to use immunotherapy in MSS colon cancer took place after the disease had already spread. When those trials produced disappointing results, the field concluded that this form of cancer simply did not respond to immunotherapy.

But cancer can change as it advances. A metastatic tumor, particularly one that has spread to the liver or other organs, can be biologically different from a localized tumor. A medicine's failure late in the disease does not necessarily tell us what it could accomplish earlier.

New evidence began to challenge that old assumption. Academic researchers testing short courses of CTLA-4 and PD-1 immunotherapy before surgery reported pathological responses in some patients with MSS colon cancer. Similarly, patients who had more limited metastatic disease outside of the liver were responding to anti-CTLA-4 antibodies where historical responses were almost non-existent. These signals gave us a reason to reconsider when to use these medicines, especially with the next-generation, optimized versions of the anti-CTLA4 compounds.

The Team: Moving at Startup Speed With Large-Pharma Quality

Recognizing the opportunity was only the beginning. Pursuing it requires a team that understands this class of medicines, can run a complex trial in patients whose cancer may still be curable, and can generate evidence strong enough to support much larger studies.

That team was foundational to our decision to invest. CEO Caroline Loew, Ph.D., has assembled leaders who helped develop CTLA-4 and PD-1 checkpoint inhibitors at Bristol Myers Squibb. The CMO, David Feltquate, a former colleague of Caroline's and Nils', brings experience running global trials and preparing new medicines for broad use, along with the ability to execute a complex neoadjuvant study within a small biotechnology company.

They are not learning about the CTLA-4 class of medicines as they build the company. They helped define it decades ago. They knew what they were looking for and how to execute rapidly for patients who are waiting.

That experience matters because a study conducted before surgery carries a different level of responsibility. These patients may already have a potentially curative option. An experimental treatment cannot unnecessarily delay surgery or compromise the patient's ability to undergo it. The company must coordinate oncologists, surgeons, pathologists, regulators and clinical sites while measuring results consistently.

Solstice's execution to date shows why this team is suited to the work. Since the company was founded in February 2026, it has licensed and transferred porustobart from Harbour BioMed, designed the clinical trial, filed and cleared an IND, and prepared its Phase 2 study to begin enrolling patients in early Q4.

This is a team of fewer than 25 people operating at startup speed with large-pharma quality. They understand that speed matters, but not at the expense of generating evidence physicians, regulators and patients can trust.

The team is also building beyond the first readout. If the study succeeds, Solstice will need to advance into larger trials and ultimately make the treatment available beyond the academic centers running the earliest studies. Its decisions today reflect that longer-term ambition.

Why Canaan Was Brought In

RA Capital and RAven formed and incubated Solstice around rights to porustobart secured from Harbour BioMed. Emily Minkow, an experienced company builder, investor and longtime friend of Julie's, first brought forward the opportunity. RA Capital and RAven invited Canaan to join the syndicate because of our complementary experience evaluating CTLA-4 biology, building oncology companies and helping medicines move through mid- and late-stage clinical development.

Nils was part of the team that translated foundational CTLA-4 biology into ipilimumab, or Yervoy, the first FDA-approved immune checkpoint inhibitor. That experience gave us the foundation to assess porustobart, its clinical data and the scientific rationale behind Solstice's development strategy. Julie has spent much of her career building oncology companies and developing innovative, bold clinical and commercial strategies to move products toward the market as fast as possible.

When RA Capital brought us the opportunity, we reviewed the molecule, patient data from a trial conducted by Harbour BioMed and the proposed trial alongside RA and Forbion. We saw three elements coming together: new human evidence that immunotherapy could work differently before surgery, a clinically active drug designed to improve upon the first generation of CTLA-4 antibodies both on efficacy and safety, and a team capable of rigorously pursuing the right studies to test the hypothesis.

The investment thesis was not the space, the medicine or the team in isolation. It was the convergence of all three.

Treating Before Surgery Creates a Different Opportunity

The period before surgery is not simply waiting time. It may be a distinct window in which the tumor is still present, the immune system is more intact, and treatment can begin before microscopic cancer cells establish themselves elsewhere.

Administering immunotherapy during this window could allow the primary tumor to act as a training ground for the immune system. The treatment is intended to help immune cells recognize the tumor and circulate throughout the body, where they may also find cancer cells too small to appear on a scan.

It also gives researchers a direct opportunity to learn. After a short course of treatment, the tumor is removed as planned. Pathologists can then examine the tissue and measure how much cancer remains, providing an early indication of whether the immune system responded.

Solstice is taking the signal first observed in small academic studies and testing it through a rigorous, industry-sponsored, multisite Phase 2 trial.

What Porustobart Is Designed to Do

Porustobart is an Fc-enhanced, heavy-chain-only CTLA-4 antibody being developed with pembrolizumab, a PD-1 inhibitor. CTLA-4 and PD-1 are different brakes that can limit an immune response to cancer. Blocking both may help the immune system start and sustain a stronger attack.

Pembrolizumab is intended to reactivate immune cells that have already recognized the cancer. Porustobart is designed to release a second brake while also reducing the number of regulatory T cells that can protect a tumor from immune attack.

Porustobart is also designed to remain in the body for less time than first-generation CTLA-4 antibodies. Its half-life is approximately four to five days, compared with roughly two to three weeks for existing CTLA-4 therapies. This may create more control over dosing and potentially reduce the frequency or duration of immune-related side effects. Clinical testing will determine whether those design features translate into better outcomes or tolerability.

The molecule is not starting from a scientific hypothesis alone. In Phase 1b studies conducted by Harbour BioMed, porustobart combined with a PD-1 inhibitor produced an objective response in seven of 23 late-line MSS colon cancer patients without liver metastases. The median duration of response was 8.4 months. Although early, these data provided evidence that the drug was active in patients and supported advancing directly into Phase 2.

The Core Question

Solstice's Phase 2 trial will evaluate porustobart with pembrolizumab before surgery in patients with stage II-III MSS colon cancer. The immediate question is whether a brief course of treatment can produce a meaningful response in the tumor without compromising the patient's path to surgery.

The core question is larger: can activating the immune system while the cancer is still localized help surgery remove the cancer and keep it from returning?

Solstice is also advancing porustobart in a second, currently undisclosed clinical indication, and the same early-treatment strategy may ultimately apply across multiple tumor types.

Solstice's success will not simply mean another immunotherapy reached the market. It will mean that a large group of people whose cancers have historically resisted immunotherapy may finally be able to benefit at the moment when the possibility of cure is greatest.

We are proud to partner with Caroline and the entire Solstice team, alongside RA Capital, Forbion and our fellow investors, as they pursue that goal.

Learn more about Solstice here: https://www.solsticeoncology.com/press-release?utm_source=linkedin&utm_medium=organic