Background: Progress in developing new effective therapies in advanced and relapsing urothelial cancer has been stagnant in the last few decades and a paradigm shift is desperately needed. Aurora kinase-A overexpression has been previously described in bladder cancer and spindle checkpoint dysregulation is a common feature of human UC. Alisertib (Millennium Inc.) is an orally available, selective small molecule inhibitor of Aurora A kinase. Single agent and combination treatment of MLN8237 with either paclitaxel (TXL) or gemcitabine synergistically reduced UC cell viability compared with either drug alone. Hence, sequential application of MLN8237 and TXL warrants clinical investigation. Phase 1 trials of both single agent and the combination with TXL defined the recommended doses for phase 2 trials. Methods: A multistep approach will be adopted for this Phase 2 trial. A single-group run-in phase will be conducted first with Alisertib 50 mg orally BID for 7 days, followed by 14d rest until disease progression. In case of activity, a confirmatory randomized (1:1) trial of weekly TXL plus either Alisertib or Placebo will follow, incorporating efficacy and futility boundaries for early stopping. In a single-blind design, TXL will be given on days 1,8,15 q4wks at the dose of 60 mg/m2 with alisertib and 80 mg/m2 with placebo. Alisertib dose will be 40 mg BID days 1-3, 8-10 and 15-17, q4wks. In the single-arm phase, primary endpoint (EP) will be RECIST 1.1 response-rate. 20 pts will be accrued, ≥3 responses will be required (10% type I and 20% type II error costraints). An accrual of 110 pts is foreseen in the randomized phase. Primary EP: progression-free survival (PFS), assuming an improvement in PFS from a median of 2.5 months (H₀) to a median of 4.5 months (H₁) (44% hazard rate reduction, 10% drop out rate). Eligibility will include diagnosis of metastatic UC and failure of 1-2 CT regimens (single-arm) or 1 prior CT only (randomized phase). A relapse within 6 months of a peri-operative CT will be counted as 1 line. Computed tomography and PET will be done every 2 cycles (2 months). Additional pharmacodynamic and translational analyses are planned on pre- post- blood and tissue samples. Clinical trial information: 2014-000557-36.