FGFR3 protein expression and gene mutation in primary and metastatic urothelial carcinoma (UC) tumors.

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4577 Background: FGFR3 protein expression may represent a valid therapeutic target in metastatic UC. The prevalence of both mutation and overexpression is unknown in metastatic UC. Methods: Tissue microarrays of formalin fixed paraffin-embedded urothelial carcinomas (UC) were stained for FGFR3 by immunohistochemistry (IHC) [primary (n=250); metastatic (n=31); of which (n=14) were paired]. FGFR3 immunostaining was scored as negative or positive based on previously reported scoring systems. FGFR3 mutation in primary tumors was assessed by iPlex and confirmed by hME sequencing (n=141) or Affymetrix OncoScan FFPE Express 2.0 (primary: n=17; metastases n=31). Results: FGFR3 IHC positivity was present in 48% of metastases (95% CI=32-65%) and 26% of primary tumors, (95%=CI 21-32%), though strong staining was rare (<1%). Paired primary and metastatic tumors were both negative in 50% of cases, with 14% positive only in the metastasis, 14% positive only in the primary tumor, and 21% positive in both. If the primary tumor showed staining, 71% of the metastases showed staining. FGFR3 IHC staining did not impact overall survival (p=0.8). FGFR3 mutations were observed in 9.6% of metastatic tumors (95% CI=3.3-25%), compared to 3.5% of primary tumors (95% CI=1.5%-8%). Co-occurrence of mutation and FGFR3 DNA copy number gain was observed in one specimen. Conclusions: FGFR3 IHC staining is present 26 % of primary tumors of patients who go on to develop metastatic disease, and nearly half of metastatic tumor sites. FGFR3 mutation frequency in primary and metastatic tumor specimens is low. Further investigation of the frequency of FGFR3 protein expression in metastases is needed. The presence of FGFR3 protein by IHC staining in primary and metastatic specimens suggests that FGFR3 may represent a therapeutic target even in the absence of mutation. Further functional studies are needed.