Refractory Hypocalcaemia Induced by Denosumab and Nab-Paclitaxel in a Patient with Metastatic Breast Cancer

ABSTRACT Background/Objective Hypocalcaemia is a recognised complication of denosumab, yet additive effects from concurrent chemotherapeutic agents are rarely described. We present a case of refractory hypocalcaemia due to combined denosumab and nab-paclitaxel in a young woman with metastatic breast cancer. Case Report A 35-year-old woman with metastatic breast carcinoma and spinal metastases was started on palliative Chemotherapy with weekly nab-paclitaxel and denosumab. She had normal renal function and normal baseline calcium and vitamin D levels. Within days of denosumab administration, she developed symptomatic hypocalcaemia requiring intravenous calcium replacement. Despite aggressive oral and IV calcium and vitamin D supplementation, her calcium levels repeatedly declined after each cycle of nab-paclitaxel. Other causes of hypocalcaemia were excluded. The pattern suggested denosumab as the initial precipitant, with nab-paclitaxel contributing to the persistence and recurrence of hypocalcaemia. Discussion Denosumab induces hypocalcaemia by inhibiting osteoclastic bone resorption, while nab-paclitaxel has been associated with broader electrolyte disturbances, including calcium depletion. In this patient, concurrent exposure likely produced an additive effect, preventing normalisation of calcium despite preserved renal function. The case underscores the need for awareness of potential synergistic toxicity when combining agents with similar metabolic adverse effects. Conclusion Concurrent use of denosumab and nab-paclitaxel may significantly increase the risk and duration of hypocalcaemia. Close biochemical monitoring, early endocrine-oncology multidisciplinary involvement, and timely treatment adjustments are critical to prevent complications.