# Targeted Therapy in Metastatic Bladder Cancer: Present Status and Future Directions
## 8 Sampling and Trial Setup  
Novel anticancer therapies may target rare molecular alterations. The rare occurrence of these individual driver alterations presents challenges regarding enrolling enough patients with the same driver alterations and tumor type into clinical trials. Basket trials are designed to overcome this challenge by enrolling patients with different tumor types but similar molecular alterations. These trials are based on the assumption that patients with similar driver alterations but different tumor types will have comparable responses to targeted therapy [69]. The histological background of a tumor determines how a patient's cancer cells have evolved and continue to evolve during treatment. For example, patients with FGFR2/3 alterations are more likely to respond to erdafitinib if they have urothelial carcinoma or cholangiocarcinoma instead of other tumor types [70]. A major problem with basket trials is the accrual of enough patients with the same tumor type. This could lead to underpowered evaluations in tumor-type-specific subanalyses. As treatment responses can differ between different tumor types, there is a risk that basket trials could potentiallylead to early termination of treatment arms and failure to identify (small) patient populations in whom biomarker-guided treatments are effective. Umbrella trials do not have this limitation as only patients with a single tumor type are enrolled. In umbrella trials, patients with the same tumor type are screened for multiple molecular aberrations and subsequently randomized for either the targeted therapy or standard of care. Umbrella trials are flexible, which allows for the closing of ineffective treatment arms and adding treatments without writing a new protocol. The flexibility of umbrella trials could be embraced to efficiently evaluate the safety profiles of novel drugs. Nonetheless, the probability that there is a direct therapeutic benefit to participants is low for both trial designs [69]. Clinical trials that evaluate biomarker-guided targeted therapy can be hampered by several hurdles that need to be overcome. Trials require the accrual of enough patients to allow for subanalyses, acquired samples should be appropriate for biomarker evaluation, and patient samples should be accessible for analyses. Problems with patient accrual can be overcome by intensifying the collaboration in international consortia and designing umbrella trials that include treatment arms for patients without detected biomarkers. The accurate biomarker prediction of treatment outcomes requires biomarkers to be assessed using a patient's lethal tumor clones. Biomarker accuracy is therefore hampered by tumor heterogeneity. The mutagenic nature of urothelial cancer gives rise to different tumor subclones that continue to evolve during a patient's natural disease course and treatment [71; 72]. The mutational variation observed within a primary tumor appears to explain a minor fraction of the total genetic tumor heterogeneity observed between primary and metastatic lesions [73; 74]. The vast intratumor heterogeneity is explained by metastatic spread, which is an early event in urothelial cancer evolution and thus few mutations are shared between primary tumor sites and metastatic lesions [72]. Biomarker assessment on primary tumors might therefore not be optimal for predicting a targeted treatment response in patients with mUC. Moreover, the genomic heterogeneity is even greater in patients with multiple primary tumors, further complicating the use of primary tumor samples for targeted treatment allocation [74]. The sampling bias that arises from a primary tumor that does not correspond with metastatic lesions could particularly hamper patient selection for anti-FGFR3 therapy, as FGFR3 mutation frequencies decrease with progression from non-muscle invasive bladder cancer (NMIBC) to MIBC to lymph node metastasis [12]. To conclude, biomarkers that guide targeted therapy are ideally evaluated in patients' most lethal tumor clones.