Background: Treatment options are limited for pts with NMIBC and MIBC who experience disease recurrence or who are ineligible for or refuse standard of care. Erda, an oral selective pan-FGFR tyrosine kinase inhibitor, is approved in adults with locally advanced or metastatic urothelial cancer with select FGFR3/2 alterations (alt) who have progressed during or after ≥1 line of platinum-containing chemotherapy. FGFRalt are among the most common oncogenic drivers detected in NMIBC and MIBC, and are more prevalent in NMIBC. TAR-210 is an intravesical drug delivery system designed to provide local, continuous release of erda within the bladder, thus limiting systemic toxicity. This study evaluates the safety, pharmacokinetics (PK), and efficacy of TAR-210 in pts with NMIBC or MIBC with select FGFRalt. Methods: Open-label, multicenter phase 1 study of TAR-210 in pts with recurrent NMIBC or MIBC (NCT05316155). Eligible pts are aged ≥18 yrs with adequate organ function and tumors with select FGFRalt. A flexible molecular eligibility strategy is used to allow for local or central fresh/archival tissue-based FGFR testing by next-generation sequencing (NGS) or PCR, or urine cell-free DNA NGS testing. Four cohorts will be enrolled: pts with recurrent, bacillus Calmette-Guerin (BCG)-experienced papillary-only high-risk (HR) NMIBC (high-grade Ta/T1) refusing or ineligible for radical cystectomy (RC) (Cohort 1) or scheduled for RC (Cohort 2); pts with recurrent, intermediate-risk NMIBC (Ta/T1) with a history of low-grade disease (Cohort 3); pts with cT2-T3a MIBC scheduled for RC refusing or ineligible for neoadjuvant cisplatin (Cohort 4). Pts in Cohorts 1 and 2 will have TURBT with resection of all visible disease prior to dosing, whereas pts in Cohort 3 must have visible disease prior to dosing. The primary end point is safety (adverse events, including dose-limiting toxicity). Secondary end points include PK, recurrence-free survival (Cohorts 1 and 2), complete response (CR) rate and duration of CR (Cohort 3), and pathologic CR rate, pT0 rate, and rate of downstaging to <pT2 (Cohort 4). Dose escalation (Part 1; n≈12, Cohorts 1 and 3 only) will be followed by dose expansion (Part 2; n≈50-80). Cohorts 1 and 3: response assessment will be after a 3-mo dosing cycle; pts with CR may receive ≤3 additional 3-mo dosing cycles if no recurrence, progression, or unacceptable toxicity. Cohorts 2 and 4: response assessment will be at RC after 8 wks of dosing. Follow-up disease surveillance (cystoscopy, urine cytology, imaging) will be every 3 mos to end of Yr 2 and every 6 mos in Yr 3 in Cohorts 1 and 3 and at 3 mos post-RC in Cohorts 2 and 4. Four pts were enrolled since April 2022 (1 in Cohort 1, 3 in Cohort 3); enrollment for Cohorts 2 and 4 is planned soon. Clinical trial information: NCT05316155.