Xilio Therapeutics presented the first detailed Phase 2 results for its vilastobart combination therapy, showing a 40 % objective response rate in heavily pre‑treated metastatic colorectal cancer patients who lack liver metastases and have high plasma tumor‑mutational burden (TMB). The data were disclosed during a conference call and webcast held on November 10, 2025, and represent the most robust evidence yet that vilastobart can activate the immune system in a population that has historically responded poorly to checkpoint inhibition.
The study’s biomarker focus is significant because Xilio estimates that roughly 55 % of microsatellite stable (MSS) colorectal cancer patients exhibit high plasma TMB, a figure that is substantially higher than the 20–30 % range typically identified by tissue‑based assays. By validating plasma TMB as a predictive marker, the company can broaden the addressable market for vilastobart, potentially moving the drug into a cohort that currently has limited therapeutic options.
Safety data from the trial were encouraging. Most treatment‑related adverse events were Grade 1 or 2, and the incidence of immune‑mediated adverse events such as colitis was only 7 %. This safety profile is differentiated from other anti‑CTLA‑4 combinations, which often report higher rates of severe colitis and other immune‑mediated toxicities. The lower toxicity burden could improve patient adherence and support a favorable benefit‑risk assessment for regulators and payers.
Management emphasized the strategic implications of the findings. Chief Medical Officer Katarina Luptakova noted that the 40 % response rate “signifies an important advance in our understanding of response to novel immunotherapy in MSS mCRC” and highlighted the statistically significant correlation between plasma‑based TMB and clinical benefit. President and CEO Ruth du Moulin added that the data “clarify the clinical trajectory of our tumor‑activated immuno‑oncology platform and strengthen investor confidence in our strategy to target hard‑to‑treat cancers with improved safety profiles.” Xilio is actively seeking partners to accelerate vilastobart’s development and is also advancing other pipeline candidates, including efarindodekin alfa and masked T‑cell engagers, to diversify its therapeutic portfolio.
The call was attended by analysts and institutional investors, marking the first comprehensive update on vilastobart since Xilio’s last public briefing. While the fact‑check report does not provide specific market‑price movements, it notes that the data were viewed favorably by the investment community, reflecting the strong clinical signal and the company’s differentiated safety profile.
In summary, Xilio’s Phase 2 results demonstrate that vilastobart, when combined with atezolizumab, can achieve a meaningful response rate in a difficult‑to‑treat MSS mCRC population and that high plasma TMB is a viable biomarker for patient selection. The safety profile, coupled with the expanded patient cohort, positions Xilio to pursue strategic partnerships and potentially bring a novel therapy to a large, underserved market.
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