Hypocalcaemia and hypophosphataemia following denosumab and IV ferric carboxymaltose in an older patient with normal renal function

Background Severe hypocalcaemia and hypophosphataemia following the co-administration of denosumab and IV ferric carboxymaltose has previously been reported in the setting of chronic kidney disease and malignancy. Despite scarce evidence, there has been growing concern amongst clinicians of a possible drug interaction between denosumab and IV iron therapies. Although electrolyte abnormalities are well known side effects of these individual therapies, it is possible that co-administration of the agents can exaggerate these effects and result in a potentially life-threatening interaction. Aim We present a case of profound hypocalcaemia and hypophosphataemia following same day administration of denosumab and IV ferric carboxymaltose in a hospitalised older patient. Clinical details An older female with normal renal function, serum calcium, phosphate and 25-hydroxy-vitamin D levels, and no history of malignancy was given denosumab 60 mg SC for secondary fracture prophylaxis following an acute osteoporotic fracture. On the same day, she received ferric carboxymaltose 1 g IV for treatment of iron-deficiency anaemia. Outcomes Following a review of her pathology results during the inpatient hospital admission, profound hypocalcaemia and hypophosphataemia was identified 8 days after denosumab and ferric carboxymaltose administration. Despite supplementation, electrolyte abnormalities persisted for 12 days following administration. Conclusion Although the patient had no pre-existing risk factors for developing electrolyte disturbance, profound hypocalcaemia and hypophosphatemia following denosumab and IV ferric carboxymaltose administration occurred, suggesting a possible interaction between the two therapies. Caution should be exercised by clinicians when these agents are co-administered within a close timeframe.