A 2-year-old girl presents with failure to thrive, short stature (height <3rd percentile for age), and biochemical evidence of hypophosphatemic rickets (serum phosphate 2.0 mg/dL, normal 3.5–5.5). Renal function and 1,25-dihydroxyvitamin D levels are normal. Genetic testing confirms X-linked hypophosphatemic rickets (XLH). What is the drug of choice for long-term management to improve growth and prevent skeletal deformities?

Explanation

## Management of X-Linked Hypophosphatemic Rickets (XLH) ### Paradigm Shift: From Phosphate + Calcitriol to FGF23 Inhibition **Key Point:** Burosumab, a monoclonal antibody against fibroblast growth factor 23 (FGF23), is now the preferred first-line therapy for XLH in children. It addresses the underlying pathophysiology (FGF23-mediated phosphate wasting) rather than merely supplementing lost phosphate. **High-Yield:** Burosumab was approved by FDA (2018) and is now recommended by international guidelines (Endocrine Society 2019, European Society of Pediatric Endocrinology) as the drug of choice for XLH in growing children. It improves linear growth velocity, reduces rickets severity, and prevents dental abnormalities. ### Comparison of XLH Treatment Approaches | Treatment | Mechanism | Efficacy in Growth | Adherence | Adverse Effects | |---|---|---|---|---| | **Phosphate + Calcitriol** | Replaces lost phosphate; stimulates 1α-hydroxylase | Modest; linear growth 1–2 cm/year above baseline | Poor (4–6 doses/day) | Nephrocalcinosis, hyperparathyroidism, tertiary hyperparathyroidism | | **Burosumab (FGF23 inhibitor)** | **Blocks FGF23-mediated renal phosphate wasting** | **Superior; linear growth 3–4 cm/year** | **Excellent (IV every 2 weeks)** | **Hyperphosphatemia (manageable), mild headache** | | Cholecalciferol alone | Increases 1,25-diOH vitamin D | Inadequate; does not correct phosphate wasting | Good | Hypercalcemia, hypercalciuria | | Calcium supplementation | Raises serum calcium | Ineffective; does not address phosphate defect | Good | Hypercalcemia | **Clinical Pearl:** In XLH, FGF23 is constitutively elevated due to PHEX gene mutation. FGF23 suppresses renal 1α-hydroxylase and promotes renal phosphate wasting. Burosumab neutralizes FGF23, allowing phosphate reabsorption and normalization of 1,25-diOH vitamin D without exogenous calcitriol. ### Mechanism of Burosumab ```mermaid flowchart TD A[PHEX gene mutation]:::outcome --> B[↑ FGF23 production]:::outcome B --> C[Renal phosphate wasting]:::outcome C --> D[Hypophosphatemia]:::outcome D --> E[Rickets + Growth failure]:::urgent F[Burosumab]:::action --> G[Neutralizes FGF23]:::action G --> H[↓ Renal phosphate wasting]:::outcome H --> I[Phosphate reabsorption]:::outcome I --> J[Improved mineralization + Growth]:::outcome ``` **Mnemonic:** **FGF23-FIX** — **F**GF23 is the **F**oe in XLH; **I**nhibit it with **X** (burosumab). ### Dosing and Monitoring - **Burosumab:** 0.8 mg/kg IV infusion every 2 weeks (max 90 mg per dose) - **Titration:** Increase by 0.4 mg/kg every 4 weeks if serum phosphate remains <3.5 mg/dL - **Target:** Serum phosphate 3.5–5.5 mg/dL (normal range) - **Monitoring:** Serum phosphate, alkaline phosphatase, PTH, renal function every 4–12 weeks