Different responses of biochemical markers of bone resorption to bisphosphonate therapy in Paget disease.

We examined the response of different biochemical markers of bone resorption to bisphosphonate therapy (400 mg of etidronate daily for 6 months) in mild Paget disease (n = 14). Urinary markers included hydroxyproline (OHP), total (T) and free (F) pyridinolines (Pyds) determined by HPLC, immunoreactive FPyds, immunoreactive TPyds, and the N- and C-terminal telopeptides of type I collage (NTx, CL). Serum measurements included tartrate-resistant acid phosphatase (TRAcP) and the C-terminal telopeptide of type I collagen (ICTP). ICTP and TRAcP showed a minimal response to therapy (% change at 6 months, -13.1 +/- 6.8 and -6.7 +/- 3.4, respectively). The response was greatest for urinary telopeptides (NTx and CL; % change -75.7 +/- 7.5 and -73.4 +/- 8.9, respectively). The response was somewhat greater for TPyds than for FPyds. We conclude that: (a) ICTP and TRAcP are unreliable indicators of changes in bone turnover; (b) oligopeptide-bound Pyds and telopeptide fragments of type I collagen in urine show a somewhat greater response to therapy than do FPyds and may be more sensitive indicators of bone resorption; and (c) as yet no evidence suggests that these markers are substantially better predictors of the clinical response to therapy than serum total alkaline phosphatase or urinary OHP. There are several problems with the interpretation of these measurements in Paget disease, and the clinical utility of these measurements remains uncertain.