Cetuximab & Panitumumab
Erbitux · Vectibix · Anti-EGFR antibody
TRPM6 magnesium wasting.
LYTE
MildOpen →
FGFR inhibitor
Erdafitinib
Balversa · Erda
The first FGFR inhibitor in urothelial cancer — on-target FGFR1 blockade drives hyperphosphatemia.
ModerateFGFR inhibitor · approved 2019
FGFR2/3-altered locally advanced or metastatic urothelial carcinoma
Signature kidney injury
Electrolyte Wasting
Representative incidence73%
Hyperphosphatemia is the most common, on-target class adverse event — reported in the large majority of treated patients (~73-78% across studies) and used as a pharmacodynamic marker for protocol-driven dose up-titration. Grade >=3 hyperphosphatemia is much less frequent (~2%).
Source: Nishina et al., Invest New Drugs 2017
Mechanism of kidney injury
FGFR1 inhibition disrupts the FGF23-Klotho-FGFR1 axis that normally suppresses proximal-tubular phosphate reabsorption (by down-regulating the NaPi-2a/2c cotransporters) and inhibits 1-alpha-hydroxylase. Blocking this signaling increases tubular phosphate reabsorption and raises serum phosphate; sustained hyperphosphatemia with a high calcium-phosphate product risks soft-tissue and vascular calcification (including calcinosis cutis) and nephrocalcinosis.
Clinical presentation
Asymptomatic hyperphosphatemia detected on routine labs, often within the first 2-3 weeks; may be accompanied by an elevated calcium-phosphate product. Severe/persistent cases risk metastatic mineral deposition; non-renal effects (stomatitis, nail and skin changes, central serous retinopathy) are common.
Onset
Early — typically within the first 1-2 cycles, used to guide pharmacodynamic up-titration.
Reversibility
Reversible
Anticancer mechanism
Oral pan-FGFR (FGFR1-4) tyrosine kinase inhibitor; blocks oncogenic FGFR signaling in FGFR-altered tumors. Approved for FGFR2/3-altered locally advanced or metastatic urothelial carcinoma.
Management
Per label: if phosphate exceeds ~5.5 mg/dL, restrict dietary phosphate and start an oral phosphate binder (e.g. sevelamer); for higher thresholds (~7-10 mg/dL or symptoms) interrupt and dose-reduce, and discontinue for persistent levels >10 mg/dL despite intervention. Up-titrate the dose at day 14-21 only if phosphate is <5.5 mg/dL and no significant toxicity.
Risk factors
- Higher dose / up-titration to 9 mg
- Pre-existing CKD
- High dietary phosphate or phosphate supplements
Prevention
- Serum phosphate monitoring with protocol-based dose titration
- Low-phosphate diet (~600-800 mg/day) when phosphate rises
- Avoid phosphate-containing supplements/laxatives
Note · Hyperphosphatemia is an expected on-target effect (FGFR1) and a dosing biomarker — not idiosyncratic toxicity.
Clinical depth
Renal dose adjustment
No starting-dose change for mild-moderate renal impairment; severe impairment and dialysis are not well studied (use with caution). The principal 'dose adjustment' is phosphate-guided titration/interruption rather than a CrCl-based rule.
Dialyzability & ESKD dosing
Highly protein-bound oral small molecule; not expected to be appreciably dialyzed. No validated ESKD dosing — phosphate handling is also altered in ESKD, complicating the pharmacodynamic phosphate biomarker.
Differential diagnosis
FGFR-inhibitor hyperphosphatemia (on-target, early, isolated, dose-related) vs hyperphosphatemia of CKD/ESKD, tumor lysis (accompanied by hyperuricemia/hyperkalemia and AKI), or exogenous phosphate load. The temporal link to dosing and use as a PD marker are characteristic.
Monitoring
- Serum phosphate at baseline and every 2-3 weeks early (then monthly) to drive dose titration
- Serum calcium and calcium-phosphate product
- Ophthalmologic exams for central serous retinopathy; routine creatinine
Key trials & series
- BLC2001 (registrational FGFR-altered urothelial carcinoma)
- THOR (erdafitinib vs chemotherapy/pembrolizumab in urothelial carcinoma)
- RAGNAR (tumor-agnostic FGFR-altered solid tumors)
- Siefker-Radtke 2023 BLC2001 TEAE-management analysis
Clinical pearls
- Hyperphosphatemia here is an expected on-target FGFR1 effect and a dosing biomarker — not idiosyncratic toxicity — and is actually used to confirm adequate target engagement before up-titration.
- Manage with diet and binders first; reserve dose interruption/reduction for higher thresholds.
- Sustained high calcium-phosphate product can cause calcinosis cutis and nephrocalcinosis — keep the product down, not just the phosphate.
Where it strikes
Nephron segments
Proximal Tubule
Bulk reabsorption + drug uptake (OCT2, OATs)
Injury signatures
Electrolyte Wasting
Beyond the kidney
Class-level context for the major non-renal toxicities of fgfr inhibitors.
Ophthalmic
Keratopathy, uveitis, retinopathy
- Central serous retinopathy
Dermatologic
Rash, HFS, SJS/TEN, vitiligo
- Nail/skin changes, hand-foot syndrome
Evidence
7 peer-reviewed references. Citation metadata via PubMed / NLM.
LandmarkFGFR-targeted therapeutics: clinical activity, mechanisms of resistance and new directions.Katoh M et al. · Nat Rev Clin Oncol 2024 · PMID 38424198Authoritative review attributing FGFR-inhibitor hyperphosphatemia to off-target/on-target FGFR1 inhibition of renal phosphate handling.PMIDSafety, pharmacokinetic, and pharmacodynamics of erdafitinib, a pan-fibroblast growth factor receptor (FGFR) tyrosine kinase inhibitor, in patients with advanced or refractory solid tumors.Nishina T et al. · Invest New Drugs 2017 · PMID 28965185Phase 1 data: hyperphosphatemia in ~73.7% as the most common treatment-emergent event and PD marker.PMIDManagement of Fibroblast Growth Factor Inhibitor Treatment-emergent Adverse Events of Interest in Patients with Locally Advanced or Metastatic Urothelial Carcinoma.Siefker-Radtke AO et al. · Eur Urol Open Sci 2023 · PMID 37101768BLC2001 analysis detailing hyperphosphatemia (78%) and phosphate-guided dose-modification management.PMIDErdafitinib in patients with advanced solid tumours with FGFR alterations (RAGNAR): an international, single-arm, phase 2 study.Pant S et al. · Lancet Oncol 2023 · PMID 37541273Tumor-agnostic phase 2 trial reporting hyperphosphatemia among key adverse events.PMIDSafety and efficacy of the pan-FGFR inhibitor erdafitinib in advanced urothelial carcinoma and other solid tumors: A systematic review and meta-analysis.Zheng X et al. · Front Oncol 2023 · PMID 36776367Meta-analysis confirming hyperphosphatemia as the most common all-grade adverse event.PMIDCalcinosis cutis dermatologic toxicity associated with fibroblast growth factor receptor inhibitor for the treatment of Wilms tumor.Arudra K et al. · J Cutan Pathol 2018 · PMID 30021048Illustrates ectopic calcification (calcinosis cutis) from sustained FGFR-inhibitor hyperphosphatemia; supports monitoring phosphate, calcium, FGF23.PMIDCurrent Trends in Anti-Cancer Molecular Targeted Therapies: Renal Complications and Their Histological Features.Tonooka A et al. · J Nippon Med Sch 2021 · PMID 34840210Onconephrology review covering FGFR-inhibitor electrolyte and tubular effects.