# Targeted Therapy in Metastatic Bladder Cancer: Present Status and Future Directions
## 3 Pharmacological Inhibition of Fibroblast Growth Factor Receptor Signaling  
RTKs and their downstream signaling pathway members are frequently mutated in treatment-naive MIBC [8; 9; 10]. Whole-exome sequencing of 131 MIBC patients identified molecular alterations in the RTK/MAPK pathway in 45% of cases and the PI3K/AKT/mTOR pathway in 42% of cases [8; 9; 10]. Both MAPK and PI3K/AKT/mTOR pathways can be activated by the same RTKs, as illustrated in Figure 1[11]. RTKs contain an extracellular ligand-binding domain, a transmembrane domain, and an intracellular tyrosine-kinase domain. Ligand binding causes RTKs to dimerize and become activated. Constitutively activated RTK signaling can occur in the presence of missense mutations, gene fusions, and increased RTK protein expression through altered gene expression or gene amplifications. RTKs involved in UC are primarily members of the FGFR family (FGFR1, FGFR2, FGFR3, and FGFR4) and members of the ErbB receptor family [8; 9; 10]. Approximately 11-12% of MIBC patients undergoing a cystectomy have activated FGFR3 mutations [8; 12]. Of these mutations, 95% are in the extracellular or transmembrane domain and result  
Figure 1: Therapeutic targets in muscle-invasive bladder cancer (MIBC). Schematic overview of (RTK)/MAPK (blue) and (RTK)/PI3K/AKT/mTOR (yellow) pathway alterations that are frequently detected in MIBC patients. Gene alteration frequencies are indicated as percentages under the corresponding proteins. Therapeutic agents that target particular or multiple RTKs are shown above their corresponding targets. Therapeutic agents targeting PIK3 CA or mTOR (orange) are depicted next to their targets. in an amino acid changing into a cysteine. This causes the formation of disulfide bridges between two receptor molecules and the constitutive activation of the receptors [13]. FGFR3-activating mutations were detected in 8/27 superficial tumor areas from patients with MIBC, yet only 4 of these 8 mutations were also present in the muscle-invasive front [12]. In addition, 5% of the lymph node metastases contained FGFR3 mutations matching those detected in the primary tumor [12]. Future studies should include multiple samples from papillary (superficial) and invasive tumor areas to confirm initial findings that imply a decline in FGFR3 mutation frequencies as tumors progress. Nonetheless, the corresponding FGFR3 mutations between primary MIBC tumors and lymph node metastases provide a rationale for systemic FGFR3-directed therapy. In addition to FGFR3 mutations, 30-40% of patients with MIBC have FGFR3 overexpression [14; 15]. However, FGFR3 overexpression has no effect on clinical outcome in MIBC patients treated with radical cystectomy [16]. FGFR3 gene amplifications (3%) and FGFR3 gene fusions (2%) are rare events in MIBC [8; 9; 10]. FGFR1 amplifications are the most frequently detected FGFR1 alterations and are reported in 7% of UC patients [17]. In addition to gene amplifications, alternative splicing of FGFR1 is associated with increased tumor stage and grade [18]. Interestingly, FGFR1 overexpression induced epithelial-to-mesenchymal transition (EMT) in UC cell lines [19; 20]. FGFR2 gene fusions have been described in 2-3% of BC patients [17]. Thus, molecular alterations in FGFR1, FGFR2, and FGFR3 have been described as cancer driver genes in mUC. Patient tumors can simultaneously harbor multiple FGFR alterations [8; 9; 10]. In addition, tumor cells can upregulate one FGFR in response to inhibition of another. The presence of multiple synchronous FGFR-alterations and resistance mechanisms provide a rationale for combined inhibition of FGFR1, FGFR2, and FGFR3. Recently, several therapeutic agents targeting multiple FGFRs have been investigated (Table 1). Infigratinib (BGJ)-398) is an FGFR1-3 inhibitor with response rates of 40-50% in metastatic upper tract urothelial carcinoma (UTUC) patients with FGFR3 mutations or fusions [21; 22]. Adjuvant treatment of MIBC/UTUC with infigratinib is currently under investigation in the phase III PROOF 302 trial (NCT04197986).