Blood and urine liquid biopsy for molecular monitoring in HER2-positive muscle-invasive bladder cancer under bladder-preserving therapy.

847 Background: While trimodal therapy is the current standard for organ preservation in muscle-invasive bladder cancer (MIBC), its clinical efficacy remains limited, and there is a critical need for noninvasive biomarkers to support real-time treatment decisions. This study explores a novel bladder-sparing approach combining immunotherapy-based neoadjuvant treatment with radiotherapy in patients with locally advanced MIBC. To enable dynamic monitoring of treatment response, a next-generation sequencing (NGS)-based liquid biopsy assay was utilized to assess molecular changes in both urine and blood. Methods: In this exploratory analysis, 29 patients from two Phase 2 trials (HOPE02, HOPE03) received neoadjuvant tislelizumab combined with gemcitabine plus cisplatin/carboplatin (HOPE02) or disitamab vedotin (HOPE03), followed by bladder-sparing radiotherapy based on treatment response. Tumor response was assessed via imaging and cystoscopic biopsy. Longitudinal blood and urine samples were collected at key time points for ctDNA and utDNA analysis using the PredicineCARE NGS assay to detect somatic alterations and monitor tumor fractions. Results: A total of 29 patients had eligible blood samples and 28 had eligible urine samples for molecular analysis, among whom 23 had paired blood and urine samples available from baseline. Urine samples yielded a higher number of somatic mutations compared to blood. At baseline, 276 mutations were detected in urine and 61 in blood, with 40 overlapping mutations identified across both sample types. The most frequently mutated genes in urine were TP53 (57%), TERT (43%), ARID1A (35%), KDM6A (35%), and RB1 (30%). In blood, the most prevalent mutations were observed in TP53 (39%), PIK3CA (13%), ARID1A (9%), ATM (9%), and BRCA2 (9%). At baseline, tumor fraction levels derived from the NGS assay in urine were significantly associated with pathological T stage (p = 0.0202) and HER2 expression levels determined by immunohistochemistry (p = 0.0263). In blood samples, TP53 mutations were significantly correlated with higher Ki67 expression (p = 0.0157) and larger baseline tumor size (p = 0.0108). Longitudinal assessment of tumor fraction dynamics identified five molecularly unresponsive patients via urine analysis and seven via blood analysis. Extended clinical follow-up is required to validate the association between these molecular findings and definitive clinical outcomes. Conclusions: Our study demonstrates the feasibility of a bladder-preserving strategy involving neoadjuvant immunotherapy combinations and radiotherapy in MIBC patients. NGS-based liquid biopsy using both urine and blood samples enabled dynamic monitoring of treatment response and showed strong correlations with clinical risk factors, underscoring its potential as a noninvasive tool to guide personalized therapy. Clinical trial information: ChiCTR2100045213.