Prediction of nonresponse and theranostic imaging in MIBC being resistant to NACT within the Bladder BRIDGister

e16603 Background: Patients with muscle invasive urothelial carcinoma achieving pathological complete response upon neoadjuvant chemotherapy (NACT) have improved prognosis. Previously we did show that luminal tumors respond better to NACT, while FGFR1 expression is associated with NACT resistance (Ecke et al. 2022). Interestingly the expression of the radioligand targets CXCR4 and FAP is found in chemoresistant, stroma-associated tumors. The objective of this study was to prospectively validate the predictive value of molecular target typing from TUR biopsy samples and compare PET CT imaging by FDG and FAP radioligand imaging in selected patients after two cycles of NACT to justify subsequent adaptive trial concepts within the "Bladder BRIDGister." Methods: Formalin fixed paraffin embedded tissues (FFPE) from transurethral resections (TUR) before chemotherapy and cystectomy samples after NACT of 36 patients were retrospectively collected and 650 TURB samples were prospectively collected as part of the Bladder BRIDGister. RNA from FFPE tissues were extracted by commercial kits, relative gene expression of subtyping markers (KRT5, KRT20, FGFR1) and radioligand target genes (CXCR4, FAP) were analyzed by standardized RT-qPCR systems (STRATIFYER Molecular Pathology GmbH, Cologne). PET CT Imaging by FDG and FAP was performed after two cycles cisplatinum based NACT. Results: The neoadjuvant cohort consisted of 36 patients (median age 69, male 83%, female 17%) with 92% of patients being node negative. Hierarchical clustering revealed CXCR4 and FAP to be elevated in stromal rich, KRT5 & KRT20 negative tumors not responding to NACT. Elevated FAP mRNA expression was sig. associated with resistance to NACT (chi 2 4.314 p=0,0378). Combining elevated FAP and CXCR4 mRNA expression did identify 1/3 of the patients to be at high risk of NACT resistance (90%). Exemplarily one pT3 G3 patient was selected for PET CT imaging after two cycles that was predicted to be unresponsive to NACT by molecular subtyping. FDG PET CT revealed a hepatic metastatic lesion. In contrast FAP PET CT indicated multiple hepatic and a pancreatic metastatic lesion indicating tumor progression under NACT. Therapy was switched to MVAC with persistent non response. Then pembrolizumab monotherapy was administered due to PD-L1 positivity of the initial TURB (10% TPS /CPC 15). However there still was fulminant progression of the liver metastasis so that ICI therapy had to be stopped. Conclusions: Expression of the radioligand targets CXCR4 and FAP has been shown to be associated with aggressive stromal associated tumors and being resistant to NACT. This could be validated by selecting patients after two cycles of NACT for PET/CT imaging. Stratified FAP PET/CT turned out to be more sensitive than conventional FDG PET CT and may enable future theranostic treatment combinations in patients unresponsive to standard chemo- or immunotherapies.