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Serum Cathepsin K Concentrations Reflect Osteoclastic Activity in Women with Postmenopausal Osteoporosis and Patients with Paget's Disease by Christian Meier, Udo Meinhardt, Jerry R. Greenfield, James De Winter, Tuan V. Nguyen, Colin R. Dunstan, Markus J. Seibel

Introduction: Cathepsin K, a cysteine protease, plays an essential role in osteoclast-mediated collagen degradation. Recently, an immunoassay to quantify cathepsin K in serum has been developed. We assessed the usefulness of serum cathepsin K as a marker of bone turnover in cross-sectional and longitudinal studies of patients with metabolic bone disease.
Methods: The study cohort consisted of 40 healthy subjects, 21 women with postmenopausal osteoporosis [66.1±7.9 yrs] and 10 patients with Paget’s disease of bone [67.1±11.6 yrs]. All patients were started on oral or intravenous bisphosphonate treatment and were followed prospectively over 6 months. Circulating cathepsin K levels were determined by a specific sandwich enzyme immunoassay (Biomedica, Vienna, Austria). In addition, serum carb- oxyterminal cross-linked telopeptide of type I collagen (βCTX-I) and bone-specific alkaline phosphatase (BALP) were measured for comparison.
Results: When compared to healthy subjects, mean serum cathepsin K levels were significantly elevated in women with postmenopausal osteoporosis (3.1 ± 1.9 vs. 11.3 ± 13.1 pmol/L, p=0.01) and in patients with Paget’s disease of bone (6.2 ± 4.4 pmol/L, p=0.04). In postmenopausal osteoporotic women, both oral and intravenous bisphosphonate treatment resulted in a significant reduction in serum cathepsin K levels (p=0.03) with most of the effect occurring after one month (mean% change: -33%). In patients with mild Paget’s disease, serum cathepsin K levels decreased during bisphosphonate treatment.
Conclusions: Serum concentrations of cathepsin K appear to reflect osteoclastic activity in patients with postmenopausal osteoporosis and Paget’s disease of bone and may hold promise as a marker of osteoclast activity.

DOI: Clin. Lab. 2006;52:1-10