(P 96) TRACP-5B AS A BIOMARKER FOR LONG-TERM DENOSUMAB TREATMENT OF UNRESECTABLE GIANT CELL TUMOR OF BONE

Objective: Giant cell tumor of bone (GCTB) is a rare, locally aggressive osteoclastogenic stromal tumor of the bone. Surgery is the standard treatment but tumor involving the pelvis and spine is difficult to control by surgery alone and denosumab treatment is indicated. Long-term treatment of denosumab is promising but adverse event such as osteonecrosis of the jaw, atypical fracture and anemia have been reported. It is also reported that cessation of treatment is a risk of recurrence. Tartrate-resistant acid phosphatase 5b (TRACP-5b) is used as a biomarker of bone resorption and is also a marker of osteoclast number. TRACP-5b is also reported to be a biomarker of osteoclast-like giant cell of GCTB. The aim of this study was to analyze the level of TRACP-5b and adjust the interval of denosumab treatment.

Methods: Three patients with unresectable GTCB were treated with denosumab at 120 mg every 4 weeks, with a loading dose of 120 mg on days 8 and 15. Serum TRACP-5b was assessed at clinic visit. The interval of denosumab treatment was lengthened if serum TRACP-5b level was below normal.

Results: The three patients were 2 male and 1 female. Site of tumor was thoracic spine, pelvis and distal femur. Age at presentation was 20, 49 and 50 years. One was primary tumor and two were recurrent tumor. Follow-up period was 24, 55 and 60 months. All patients had serum TRACP-5b level exceeding 1500 mU/dL at presentation (normal range 170-590 for male and 120-420 for female). After initial treatment with denosumab, all patients were relieved of pain and the serum TRACP-5b level dropped (81-103). The interval of treatment was lengthened to every two month after 3-11 months, if TRACP-5b level remained below normal and image assessment showed good response. Furthermore, the interval was lengthened to 3 months for one patient and six months for one patient. One patient at two-month interval skipped a visit and had relapse at four months. All patients at last follow-up have no symptom and tumor regrowth. No adverse event from denosumab was observed.

Conclusion: The interval of long-term treatment of GCTB with denosumab can be monitored by serum TRACP-5b level.