Background: Erdafitinib (ERDA), a pan-FGFR kinase inhibitor, was US FDA approved for adults with mUC with susceptible FGFR3/2 alterations (FGFRa) who progressed on ≥ 1 line of prior platinum-based chemotherapy based on primary results of the BLC2001 trial (NCT02365597). At final analysis (median 2-y follow-up), ERDA showed median OS of 11.3 mo and a manageable safety profile. Some FGFRi toxicities are distinct from those of other small-molecule TKIs. Proactive AE management can avoid treatment discontinuation, ensuring maximum benefit. We report the frequency and management of TEAEs of interest (central serous retinopathy [CSR], hyperphosphatemia [“on-target” FGFRi class effect], stomatitis, and skin and nail toxicities) for the optimal schedule of ERDA from the final analysis of BLC2001. Methods: The open-label, phase II BLC2001 study enrolled pts with measurable mUC, prespecified FGFRa, and progression during/after ≥ 1 line of prior chemotherapy or ≤ 12 mos of (neo)adjuvant chemotherapy or who were cisplatin ineligible, chemo naive. Optimal dose schedule in the study was 8 mg/d continuous ERDA in 28-d cycles with uptitration to 9 mg/d (ERDA 8 mg/d UpT) if prespecified serum phosphate level was not reached and no significant TEAEs occurred. ERDA 8 mg/d UpT safety results as of Aug 9, 2019 (final analysis) are summarized here. AEs were graded using NCI CTCAE v4.0. Results: Median follow-up for 101 pts treated with ERDA 8 mg/d UpT was 24.0 mos; median treatment duration was 5.4 mos. All pts had ≥ 1 TEAE. Hyperphosphatemia, stomatitis, nail disorders, skin disorders, and CSR TEAEs occurred in 78%, 59%, 59%, 55%, and 27% of pts, respectively (few were grade [gr] 3; none were gr ≥ 4; Table). TEAEs were mostly managed with concomitant treatment and dose modifications. As of data cutoff, hyperphosphatemia had resolved in 74/79 (94%) pts; stomatitis in 44/60 (73%); nail and skin TEAEs in 26/60 (43%) and 25/55 (45%), respectively; and CSR in 17/27 (63%). Most unresolved TEAEs were gr 1–2. No treatment-related deaths occurred. Conclusions: ERDA had measurable benefit in pts with advanced UC with FGFRa. As with other targeted therapies, exposure to ERDA is associated with a pattern of AEs. The most common and FGFRi class effect TEAEs were generally reversible and managed by supportive care and dose modification. Clinical trial information: NCT02365597. Research Sponsor: Janssen Research & Development, LLC

^a TEAEs leading to dose interruption followed by reduction reported as reduction.