Rebound Hypercalcemia After Denosumab Therapy in a Child With Cherubism

Background: Cherubism, caused by autosomal dominant mutations in the SH3BP2 gene, is characterized by increased bone resorption with development of bilateral fibro-osseous lesions limited to the maxilla and mandible. The SH3BP2 gene is thought to be involved in osteoclastogenesis. Affected children, while usually asymptomatic at birth, typically present at 2–5 years of age with cheek and jaw swelling with upward tilting of eyes due to expansion of fibrous tissues. Bone resorption and proliferation of lesions continues until puberty after which spontaneous regression occurs. RANKL is a cytokine expressed on the surface of osteoclast precursors and is responsible for inducing osteoclast differentiation. Denosumab is an anti-RANKL monoclonal antibody which prevents osteoclast maturation. However, it has a short half-life, and effects on bone turnover have been found to be rapidly reversible after drug discontinuation. The rebound increased bone turnover can lead to severe hypercalcemia. Clinical Case: A 4 year old boy with cherubism (c.1253C>G pathogenic variant in SH3BP2), after failing a 10-month trial of tacrolimus, was placed on monthly denosumab (2 mg/kg) for a total of 10 doses. During denosumab therapy, he received calcium and vitamin D to prevent hypocalcemia; these were stopped once denosumab was discontinued. He presented to the hospital 4 months after the final denosumab dose with polyuria, polydipsia, fatigue, nausea and abdominal pain. Work-up revealed serum Ca 15.3 mg/dL (N: 8.4–10.2), PTH <3 pg/mL (N: 24–86), 25-OH vitamin D 32 ng/mL (N: >19 ng/mL), 1,25-dihydroxyvitamin D 6.7 pg/mL (N: 19.9–79.3), and urine Ca/Cr 0.48. Renal ultrasound showed normal kidneys with a small amount of layering debris in the bladder. During hospitalization, he received IV fluids, 1 dose of furosemide, 3 doses of calcitonin, 24 hours of hydrocortisone, and a single 0.5 mg/kg dose of pamidronate. He was discharged 48 hours after the bisphosphonate with serum Ca 9.5 mg/dL. He returned with serum Ca 13.5 mg/dL 9 days after the pamidronate and was readmitted. He again received 4 doses of calcitonin and 1 dose of pamidronate (0.5 mg/kg). Calcium levels improved to 9.5 mg/dL at discharge but rose to 11.6 mg/dL a week later. He received a 0.05 mg/kg dose of zoledronate outpatient, with improvement in serum Ca to 10.1 mg/dL. A week later, he twisted his ankle, resulting in transverse impacted buckle fractures of his left distal tibia and fibula; no lytic or sclerotic lesions were noted on x-ray. His leg was immobilized by Orthopedics. Calcium levels remained within range (9.9 mg/dL) 7 months after the zoledronate. Conclusion: Rebound hypercalcemia can occur months after denosumab withdrawal, indicating the need for close monitoring of calcium levels in patients who receive this drug. The hypercalcemia appears to respond best to bisphosphonates, with a more sustained response to zoledrenate compared to pamidronate.