Monitoring denosumab therapy using the calcium isotope marker (CIM) technology.

Denosumab provides a well-established therapy for osteoporosis. Dual-energy X-ray absorptiometry (DXA) and bone turnover markers (BTMs) track changes in bone mineral density (BMD) and turnover. Calcium (Ca) Isotope Markers (CIM), which measure naturally occurring variations in stable Ca isotope ratios in serum (CIM_serum) and urine (CIM_urine), offer a potentially more sensitive and individualized approach for monitoring bone health and therapy responsiveness. In this pilot study, 13 postmenopausal women with DXA-confirmed osteoporosis were initiated on denosumab. Over 24 weeks, serial measurements of CIM_serum, CIM_urine, BMD, BTMs, and parathyroid hormone (PTH) were obtained. CIM thresholds distinguishing net bone Ca uptake from net bone Ca efflux were applied. Baseline CIM values, adjusted for Ca supplement intake (average CIM_serum: -1.09±0.15 ‰ and CIM_urine: 0.00±0.22 ‰), indicated net bone Ca loss. After 60 mg denosumab injection, all patients showed substantial increases in CIM_serum and CIM_urine values by ∼ +0.4 ‰ after one week. Peak values were reached in week 4 (CIM_serum: ∼ -0.8 ‰) to 8 (CIM_serum: ∼ -0.7 ‰). CIM_serum and CIM_urine values correlated positively to PTH. Some patients´ CIM values quickly rose above threshold levels, while others showed more modest or transient changes. Although DXA indicated an increase in the lumbar and hip T-score values after 24 weeks, DXA primarily confirmed group-level gains. BTMs did not reflect individual variations in CIM response. While further validation in larger, controlled cohorts is warranted, these findings highlight CIM's potential to enhance osteoporosis management through personalized treatment monitoring.