Xilio Therapeutics Reports Preliminary 26% Objective Response Rate in Phase 2 Trial of Vilastobart Combined with Atezolizumab for Metastatic MSS Colorectal Cancer

Xilio Therapeutics announced Phase 2 trial results for vilastobart combined with atezolizumab in metastatic microsatellite stable colorectal cancer (MSS CRC) patients, revealing a 26% objective response rate in heavily pre-treated patients without liver metastases. The combination showed deep and durable responses, lasting up to 37 weeks, alongside reductions in tumor biomarkers and improved clinical symptoms, with a favorable safety profile and low incidence of immune-related adverse events. Xilio is proceeding with further development, including higher dosage levels, and is seeking partnership opportunities to accelerate the program; additional data is expected in the first half of 2026.

Analysis

Xilio Therapeutics (XLO) has presented updated Phase 2 clinical trial data for vilastobart in combination with atezolizumab for metastatic microsatellite stable colorectal cancer (MSS CRC), revealing a preliminary objective response rate (ORR) of 26% in heavily pre-treated patients without liver metastases. These responses were reported as deep and durable, lasting up to 37 weeks, and were accompanied by significant reductions in tumor biomarkers and improvements in clinical symptoms. Crucially, the combination therapy demonstrated a favorable safety profile, with a low incidence of immune-related adverse events, notably only 7% colitis, which is a key differentiator for anti-CTLA-4 agents. The data, presented at the 2025 ASCO Annual Meeting, addresses a significant unmet medical need in MSS CRC, a tumor type generally resistant to immunotherapy. While the 26% ORR is observed in a challenging late-line patient population, its ultimate clinical significance will be further assessed with ongoing development; Xilio is enrolling approximately 10 patients in a higher dose cohort (150 mg Q6W), with additional data anticipated in the first half of 2026. The company is actively seeking partnerships to advance vilastobart, a critical step given its reliance on successful trial outcomes and its financial runway extending to Q1 2026. Insider activity shows a notable $1.83 million purchase of XLO stock by Gilead Sciences, Inc. in the past six months, offering a positive signal amidst mixed institutional sentiment, which saw significant Q1 2025 exits by funds like Rock Springs Capital Management LP and FIL Ltd, contrasting with increased holdings by Vanguard Group Inc. and Morgan Stanley.

Key Decisions for Investors

• Investors should weigh the promising, albeit preliminary, Phase 2 efficacy and safety data for vilastobart against the inherent risks of early-stage biotech development and the company's stated need for partnerships or funding beyond Q1 2026.
• Monitor upcoming catalysts, particularly the additional clinical data from the higher dose cohort expected in the first half of 2026 and any announcements regarding strategic partnerships for vilastobart, as these could significantly impact XLO's valuation.
• Consider the conflicting signals from institutional investors, with notable fund exits alongside a significant insider purchase from Gilead Sciences, suggesting a need for careful due diligence on the long-term viability and competitive positioning of vilastobart.