Xilio Therapeutics (XLO) Study Update summary

Study background and unmet need

• Vilastobart (XTX-101), a tumor-activated Fc-enhanced anti-CTLA-4, is being studied in combination with atezolizumab for metastatic microsatellite stable colorectal cancer (MSS-CRC), a population with limited treatment options and poor response to immunotherapy.

• MSS-CRC is highly resistant to immunotherapy, with standard treatments offering median progression-free survival of 6–9 months and objective response rates of 0–3% for approved IO agents.

• The study targets patients with advanced, heavily pretreated disease, including those with and without liver metastases.

Study design and patient population

• Phase 2 trial enrolled 40 U.S. patients with metastatic MSS-CRC, most having received three or more prior lines of chemotherapy.

• Patients received vilastobart 100 mg every six weeks plus atezolizumab 1,200 mg every three weeks.

• 18 patients were evaluable for response at data cutoff; 11 without liver metastases, 7 with liver metastases.

Efficacy results

• Vilastobart plus atezolizumab showed a 27% preliminary response rate in heavily pre-treated metastatic MSS-CRC patients without liver metastases, with three partial responses (two confirmed, one pending) and ongoing treatment at data cutoff.

• Disease control rate was 55% in patients without liver metastases and 14% in those with liver metastases.

• One patient with peritoneal metastasis had a 24% decrease in target lesion size and normalized CEA.

• Tumor reductions were rapid, often seen at first assessment (nine weeks), and responses deepened over time.

• Responses were associated with decreases in CEA, ctDNA, and improved clinical symptoms.