SUN-303 Management Strategies for Hypercalcemia in Breast Cancer with Bone Metastases: A Focus on Glucocorticoids and Cinacalcet

Abstract L.T. Wu: None. F. Kaniski: None. A. Rajpal: None. Background: Hypercalcemia of malignancy (HCM) is seen in 20-30% of patients with advanced cancer and is a sign of poor prognosis. Two common mechanisms are elevated PTH-related protein (PTH-rp), called humoral hypercalcemia of malignancy (HHM), and increased bone resorption due to osteolytic metastases. Pharmacologic management of HHM and osteolytic metastases mainly involves antiresorptive agents such as bisphosphonates, denosumab, and calcitonin. Less commonly, HCM may be caused by elevated 1,25-dihydroxyvitamin D, which is typically seen in lymphomas and responds well to glucocorticoids. We present a rare case of HCM in a patient with breast cancer and bone metastases, who had increased 1,25-dihydroxyvitamin D levels along with elevated PTH-rp. Furthermore, we discuss the use of glucocorticoids and cinacalcet in the management of HCM. Clinical Case: A 61-year-old woman with breast cancer with bone metastases, who had three recent admissions for severe hypercalcemia, was readmitted for syncope secondary to second degree type II heart block in the setting of severe hypercalcemia of 14.9 mg/dL. Prior work up showed low PTH of 10 pg/mL and undetectable 25-hydroxyvitamin D, thus hypercalcemia was presumed to be due to HHM and bone metastases. She had received multiple courses of zoledronic acid, denosumab, and chemotherapy with no response. Her hypercalcemia only temporarily responded to intravenous (IV) fluids. Since the improvements did not last long, she had to be readmitted multiple times. Similarly, during this admission, the patient could not be safely weaned off continuous IV fluids without recurrent hypercalcemia. Further workup showed elevated PTH-rp of 58 pg/mL and 1,25-dihydroxyvitamin D of 234 pg/mL. The patient was started on dexamethasone and cinacalcet, and thereafter was successfully weaned off continuous IV fluids, and eventually achieved normocalcemia of 9.6 mg/dL. Subsequent trials off cinacalcet led to recurrent hypercalcemia and attempts to stop dexamethasone also led to the same outcome. Conclusion: This case demonstrates the unusual occurrence of elevated 1,25-dihydroxyvitamin D in the setting of solid tumor, thereby suggesting the utility of glucocorticoids as a treatment option for HCM resistant to usual therapy. It also shows the effectiveness of cinacalcet in managing non-PTH-mediated HCM, which possibly works via increased renal clearance of calcium and could be used as an additional effective therapeutic option in these patients. Sunday, June 2, 2024