Abstract PO1-12-02: Impact of Discontinuation of Denosumab on Bone Health in Breast Cancer Patients

Abstract Denosumab (Dmab), an inhibitor of osteoclast activity and proliferation is approved for treatment of women on aromatase inhibitor (AI) therapy for early breast cancer (60 mg Q 6-months) and metastatic disease (120 mg Q 4-weeks) to protect against bone loss and prevention of skeletal-related events (SREs). 60 mg given twice a year has shown fracture reduction, especially in women on aromatase inhibitor therapy, and recent data supports benefits in disease free survival, bone-metastasis free-survival and overall survival with Dmab in early breast cancer. It appears superior in reduction of SREs versus zoledronic acid (ZA) in established skeletal metastatic disease; thus, a preferred choice in the oncology setting. In the osteoporosis setting, withdrawal of Dmab therapy leads to a “rebound” increase of bone resorption, rapid bone loss and potential vertebral fractures. Increased risk of spontaneous and multiple vertebral fractures has been seen in this patient population. To prevent this, after discontinuation of Dmab, the addition of another anti-resorptive such as ZA is initiated. Dmab withdrawal events are uncommonly reported in patients with metastatic skeletal disease due to generally poor prognosis. Dmab given as 120 mg/month in this setting may need to be curtailed for a variety of indications including hypocalcemia, major dental procedures, osteonecrosis of the jaw or atypical femoral fracture. In the oncologic setting, it is unclear if discontinuation of Dmab will lead to rebound bone resorption or increased incidence of fractures. We describe the case is of a 32-year-old woman with a history of T2N3M1, oligometastatic breast cancer (solitary bone metastases at L1, biopsy-proven) that was ER/PR/HER2 positive. She was treated with neoadjuvant trastuzumab, pertuzumab, taxotere and carboplatin followed by bilateral mastectomy, and radiation to chest wall and L1. She received total estrogen blockade initially with GnRH-agonist and exemestane and ultimately bilateral salpingo-oophorectomy plus exemestane. She continues pertuzumab and trastuzumab (80 treatments to date) for the past 5 years and denosumab 120 mg/month for 14 treatments and then every 3 months, 20 total treatments. Development of hip pain (regular runner) and concern for atypical fracture vs metastatic disease led to bone scan and PET/CT. Metastatic disease was ruled out and stress reaction was diagnosed. Dmab therapy was discontinued with monitoring of her metabolic bone turnover. 1 year after her last Dmab injection, bone turnover markers began to increase and ultimately demonstrated a very significant “rebound phenomenon.” Subsequently, a single dose of 120 mg denosumab decreased bone markers to previously low levels which remain low on routine monitoring, table below. In future, ZA will be initiated and continued on a yearly basis. Dmab has replaced ZA in managing bone health in breast cancer patients. Many oncologists use this without the assistance of bone health endocrinologists, who are an integral part of our cancer center. Bone modifying agents are often discontinued without consideration of the negative impact this may have. As this is a major survivorship issue, we document this case with literature review, to raise awareness for oncologists prescribing these medications. Citation Format: Susan Tannenbaum, Alvaro Alvarez Soto, Pamela Taxel. Impact of Discontinuation of Denosumab on Bone Health in Breast Cancer Patients abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-12-02.