Extracellular matrix-derived liquid biopsy for predicting response in patients with biliary tract cancer treated with nivolumab with or without ipilimumab combined with stereotactic body radiotherapy: Insights from the CheckPAC trial.

3043 Background: Biliary tract cancer (BTC) is a rare malignancy with limited treatment options and poor prognosis. Although combined chemotherapy and immune checkpoint inhibitors are first-line treatment for advanced BTC, durable response is uncommon. Current immunotherapy biomarkers, such as PD-L1 expression, have limited utility and treatment response biomarkers are urgently needed. BTC is characterized by a fibrotic extracellular matrix, where widespread tumor fibrosis has been linked to immunosuppression and treatment resistance. In this study, we used Nordic ProteinFingerPrint Technology to evaluate the pharmacodynamic and predictive value of the FDA supported fibrosis biomarker NordicPRO-C3 (type III collagen pro-peptide) and the T-cell infiltration biomarker NordicC4G (granzyme B degraded type IV collagen) in patients with metastatic BTC treated with nivolumab with or without ipilimumab plus stereotactic body radiotherapy (SBRT). Methods: PRO-C3 (NordicPRO-C3) and C4G (NordicC4G) were measured at baseline and 60 days after treatment initiation in serum from 61 patients with BTC from the CheckPAC trial (NCT02866383). Patients received SBRT combined with nivolumab (n = 19) or nivolumab/ipilimumab (n = 42). The primary endpoint was clinical benefit rate (CBR; CR+PR+SD vs. PD) assessed by RECIST 1.1. Biomarker levels at baseline and biomarker changes during therapy were evaluated relative to response using Fisher’s exact test; overall survival (OS) was assessed using uni- and multivariate Cox regression. Results: CBR was 0% in patients with high baseline PRO-C3 (243 ng/mL) versus 31% in patients with low PRO-C3 (p < 0.05). High PRO-C3 was associated with poor OS after adjusting for CA19-9, performance status, and modified Glasgow Prognostic Score (HR = 3.1, 95% CI 1.5-6.6, p = 0.004). Longitudinally, C4G increased significantly at day 60 in patients with clinical benefit (p < 0.001). CBR was 44% in patients with increased C4G versus 0% in those with decreased C4G (p = 0.007), and increased C4G was associated with improved OS (p = 0.01). All patients with clinical benefit had both increased C4G and low PRO-C3 at day 60. Among patients without clinical benefit, patients with increased C4G and low PRO-C3 and showed a trend towards improved OS (Median OS 8.3 months vs. 3.6 months, p = 0.08). Conclusions: High pre-treatment tumor fibrosis, quantified by elevated serum PRO-C3, is associated with poor response and OS in patients with BTC treated with SBRT plus immunotherapy. Changes in PRO-C3 and the T-cell infiltration biomarker C4G reflect pharmacodynamic changes, with increased C4G linked to improved outcome, particularly in patients with low PRO-C3. Together, these biomarkers may help to predict treatment response and guide clinical decision-making in BTC.