A Case Report of Ruxolitinib Induced Hypocalcemia: A Stochastic or Deterministic Effect?

Ruxolitinib is a novel selective JAK 1/2 inhibitor approved for the treatment of myelofibrosis (MF) and polycythemia vera (PCV). Hypocalcemia associated with ruxolitinib has not been reported in early trials or in literature. A 65 year female with history of CKD stage 3, PCV since 1989, papillary thyroid carcinoma, total thyroidectomy and hypoparathyroidism since 1996 presented with complaints of severe myalgia, fatigue, paresthesia and critical hypocalcemia. Her corrected S. Ca 5.8 mg /dl (8.9-10.1 mg/dL) and Ionised Ca 2.9 mg/dl (4.5 - 5.4 mg/dL). Other labs showed stable creatinine 1.5 mg/dL, intact PTH 29 pg/ml(14-72pg/ml) and 25,OH vitamin D level 35ng/ml (30-100ng/ml). She was recently started on ruxolitinib 4 months ago for PCV due to worsening thrombocytosis. Since the start of ruxolitinib she had these symptoms. Follow up labs showed rapid fall in blood counts because of which ruxolitinib was stopped after 2 months. She continued to have symptoms even after stopping the drug. Her symptoms were attributed to very low S. Ca level. She was treated with multiple IV / oral calcium doses and initiated on calcitriol once her phosphorus dropped below 5.5mg/dl. The Ca level corrected and was discharged on Ca carbonate, Ca acetate and calcitriol