• Perform monthly ophthalmological exams including assessment of visual acuity, slit lamp exam, fundoscopy, and optical coherence tomography during first 4 months of therapy and every 3 months afterward, and urgently for visual symptoms. Monitor for central serous retinopathy (CSR)/retinal pigment epithelial detachment. If any CSR occurs, hold erdafitinib and perform an ophthalmic evaluation within 2 wk: If improving within 14 days, restart at current dose. If not improving within 14 days, hold erdafitinib until improving; once improving, may resume at next lower dose level. On restarting erdafitinib, monitor for recurrence every 1 to 2 wk for a month. If recurs or has not improved after 4 wk of holding erdafitinib, consider permanent discontinuation. All patients should receive dry eye prophylaxis with ocular demulcents as needed.

Lab Test Considerations:

• Verify negative pregnancy test before starting therapy.
  • Patient selection is based on presence of susceptible FGFR genetic alterations in tumor specimens as detected by an FDA-approved companion diagnostic. Information on FDA-approved tests for the detection of FGFR3 genetic alterations in urothelial cancer is available at http://www.fda.gov/CompanionDiagnostics.
  • Assess serum phosphate levels 14–21 days after starting therapy. Restrict phosphate intake to 600–800 mg daily in all patients. If serum phosphate <6.99 mg/dL and no ocular disorders or ≥Grade 2 adverse reactions, continue at current dose. Monitor phosphate levels monthly for hyperphosphatemia. If serum phosphate 7–8.99 mg/dL , continue erdafitinib at same dose. Start phosphate binder with food until phosphate level is <7 mg/dL. Reduce dose if serum phosphate remains ≥7 mg/dL for a period of 2 mo or if clinically necessary. If serum phosphate 9.0–10 mg/dL, hold erdafitinib with weekly reassessments until level <5.5 mg/dL (or baseline). Start phosphate binder with food until phosphate level is <7 mg/dL. Reduce dose if serum phosphate remains ≥7 mg/dL for a period of 2 mo or if clinically necessary. Then restart at same dose level. Dose reduction may be used for hyperphosphatemia lasting >1 wk. If serum phosphate >10 mg/dL, hold erdafitinib with weekly reassessments until level returns to <7 mg/dL. Then may restart at first reduced dose level. If hyperphosphatemia (≥10 mg/dL) for >2 wk, discontinue erdafitinib permanently. Medical management of symptoms as clinically relevant.