Xilio Therapeutics Announces Late-Breaking Phase 2 Data for Vilastobart in Patients with MSS mCRC and High Plasma Tumor Mutational Burden at Society for Immunotherapy of Cancer (SITC) 40th Annual Meeting

40% ORR in heavily pre-treated patients with MSS mCRC without liver metastases and with high plasma tumor mutational burden (TMB)

Estimate 55% of patients with MSS CRC have high plasma TMB, representing a meaningful population with high unmet need

Company to host conference call and webcast on Monday, November 10, 2025, at 4:30 p.m. ET with leading cancer experts to review the data

WALTHAM, Mass., Nov. 07, 2025 (GLOBE NEWSWIRE) -- Xilio Therapeutics, Inc. (Nasdaq: XLO), a clinical-stage biotechnology company discovering and developing tumor-activated immuno-oncology therapies for people living with cancer, today announced new data from its ongoing Phase 2 clinical trial evaluating vilastobart, a tumor-activated, Fc-enhanced anti-CTLA-4, in combination with atezolizumab (Tecentriq®) in patients with microsatellite stable (MSS) metastatic colorectal cancer (mCRC). A 40% objective response rate (ORR) was demonstrated in heavily pre-treated patients with MSS mCRC without liver metastases and with high plasma TMB (≥10 mutations/Mb), as well as a statistically significant correlation between plasma TMB status and response. These new clinical data are being presented in a late-breaking poster presentation (Abstract # 1315) today at the Society for Immunotherapy of Cancer (SITC) 40th Annual Meeting, taking place from November 5-9, 2025, in National Harbor, Maryland.

“These compelling new Phase 2 data for vilastobart in combination with atezolizumab demonstrated a 40% objective response rate in heavily pre-pretreated patients with MSS mCRC without liver metastases and with high plasma TMB, signifying an important advance in our understanding of response to novel immunotherapy in MSS mCRC,” said Katarina Luptakova, M.D., chief medical officer of Xilio. “In addition, these data showed a statistically significant correlation between plasma-based TMB and response. We estimate that approximately 55% of patients with MSS CRC have high plasma TMB, representing a meaningful patient population who could ultimately benefit from combination treatment with vilastobart and a substantially greater prevalence of patients with high TMB than previously estimated using traditional tissue-based assays.”

“These new data highlight the potential to use plasma TMB as a predictive biomarker and identify patients with MSS mCRC who may benefit from treatment with vilastobart, a tumor-activated anti-CTLA-4, in combination with a PD-(L)1,” said Diwakar Davar, M.D., Associate Professor of Medicine, Clinical Director of the Melanoma Program and Medical Oncologist/Hematologist at the UPMC Hillman Cancer Center. “This biomarker guided approach, utilizing a feasible and reproducible biomarker, together with vilastobart’s differentiated safety profile, continue to support the promising opportunity for vilastobart in combination with PD-(L)1 or PD1-VEGF in MSS mCRC, as well as other tumor types.”

Promising Opportunity for Plasma-Based TMB as a Biomarker Predictive of Response to Combination Treatment with Vilastobart in Patients with MSS mCRC

Tissue-based TMB has demonstrated potential as a biomarker predictive of response to immune checkpoint inhibitors in many tumor types. However, tissue-based TMB assays have not shown predictive utility in MSS mCRC historically, and these tissue-based assays may underestimate the mutational burden in patients. In particular, tissue-based assays are limited to evaluating mutations in a single-site biopsy specimen, which is typically taken once at the time of diagnosis, and cannot account for tumor heterogeneity or changes in TMB over time.

Newer plasma-based assays show potential to better assess TMB status and be used as a biomarker predictive of response in patients with MSS mCRC. These plasma-based assays are more sensitive than traditional tissue-based assays in that plasma-based TMB assays provide a comprehensive assessment of mutational load and account for tumor heterogeneity. In addition, plasma can be readily obtained from blood samples throughout the course of treatment, accounting for changes in TMB over time without the need for invasive procedures.

Approximately 55% of non-MSI-H CRC patients were plasma TMB high (>10 mutations/Mb) based on an analysis of the GuardantINFORM real-world clinical-genomic database in approximately 8,000 patients who received the Guardant360 Liquid (Infinity) assay and who had non-MSI-H disease and a reportable TMB result. In contrast, historical data for tissue-based TMB assays have suggested