Chemotherapy-Induced Electrolyte Abnormalities

Recognition and management of hypomagnesemia, hypokalemia, hypophosphatemia, hyperphosphatemia, and other electrolyte disturbances caused by cancer therapy

Key Points

• Cisplatin causes profound renal tubular magnesium wasting — hypomagnesemia affects 90% of patients on cisplatin-based regimens and can persist for months to years after therapy ends.
• Hypomagnesemia drives secondary hypokalemia and hypocalcemia that are refractory to potassium and calcium replacement until magnesium is repleted first.
• Cetuximab and other EGFR inhibitors cause hypomagnesemia through a different mechanism — blockade of EGFR-mediated magnesium reabsorption in the distal tubule.
• Ifosfamide causes Fanconi syndrome — a proximal tubular toxicity causing urinary wasting of phosphate, amino acids, glucose, uric acid, and bicarbonate simultaneously.
• Hypophosphatemia from ferric carboxymaltose (IV iron) infusions is a distinctive late effect caused by FGF-23 elevation; it is self-limiting but can be severe in patients receiving repeated doses.
• FGFR inhibitors (erdafitinib, pemigatinib, futibatinib) cause hyperphosphatemia by blocking renal FGF-23 signaling — phosphate is retained rather than excreted. Hyperphosphatemia serves as a pharmacodynamic marker of FGFR target engagement and is managed with dietary restriction and phosphate binders rather than always prompting dose reduction.
• Routine electrolyte monitoring (magnesium, potassium, phosphate) before each cycle is essential for cisplatin, carboplatin, and ifosfamide-based regimens.

Cisplatin-Induced Hypomagnesemia

Hypomagnesemia (serum magnesium <1.7 mg/dL) is the most common and most clinically significant electrolyte abnormality associated with platinum-based chemotherapy. Cisplatin causes direct injury to the thick ascending limb of the loop of Henle and the distal convoluted tubule — the nephron segments responsible for 70% of magnesium reabsorption. This tubular damage impairs magnesium reabsorption, resulting in urinary magnesium wasting that can persist indefinitely even after cisplatin is discontinued. **Incidence and severity:** - Hypomagnesemia affects up to 90% of patients on cisplatin and…

Hypokalemia in Cancer Patients

Hypokalemia (serum potassium <3.5 mEq/L) is common in cancer patients through multiple overlapping mechanisms. **Causes in oncology:** *Chemotherapy-induced:* - Cisplatin and carboplatin: renal tubular damage causes potassium wasting independent of (and compounded by) magnesium depletion - Ifosfamide: Fanconi syndrome (see below) — potassium lost in urine along with other tubular solutes - High-dose chemotherapy conditioning (cyclophosphamide, busulfan, carmustine): severe mucositis-induced vomiting causes hypokalemia from GI losses - Cisplatin-induced vomiting: even with antiemetic…

Ifosfamide-Induced Fanconi Syndrome and Hypophosphatemia

Ifosfamide causes proximal renal tubular toxicity — Fanconi syndrome — in which the proximal tubule fails to reabsorb multiple solutes normally filtered at the glomerulus. **Clinical features of ifosfamide-induced Fanconi syndrome:** - **Hypophosphatemia**: renal phosphate wasting (low serum phosphate, high urine phosphate) - **Hypokalemia**: renal potassium wasting - **Non-anion gap metabolic acidosis** (renal tubular acidosis type II — bicarbonaturia) - **Glycosuria** without hyperglycemia (glucose lost despite normal blood glucose) - **Aminoaciduria**: amino acids spilled in urine -…

FGFR Inhibitor-Induced Hyperphosphatemia

Fibroblast growth factor receptor (FGFR) inhibitors represent an increasingly used class of targeted agents in oncology, and hyperphosphatemia is their most distinctive and class-defining electrolyte toxicity. **Approved FGFR inhibitors and their oncology indications:** - **Erdafitinib (Balversa)**: pan-FGFR1–4 inhibitor; approved for FGFR2/3-altered locally advanced or metastatic urothelial carcinoma. Hyperphosphatemia occurs in ~77% of patients, including grade 3 in ~30%. - **Pemigatinib (Pemazyre)**: selective FGFR1/2/3 inhibitor; approved for FGFR2 fusion-positive or…

Monitoring and Prevention Framework

**Recommended electrolyte monitoring by regimen:** *Cisplatin-based regimens (GU, lung, head and neck, GYN cancers):* - Serum magnesium, potassium, creatinine, sodium: before each cycle - Mid-cycle (day 7–10): magnesium + creatinine (especially with cumulative cisplatin doses) - After cycle 3+ (cumulative effect increases risk): more frequent monitoring - Prophylactic magnesium: oral magnesium oxide 400–800 mg twice daily starting cycle 1; some centers add 2 g IV magnesium sulfate with each cisplatin infusion *Ifosfamide-based regimens:* - Serum phosphate, potassium, bicarbonate, glucose:…