Different effects of bisphosphonate and estrogen therapy on free and peptide-bound bone cross-links excretion

Abstract

We have measured the free and peptide-bound type I collagen cross-link excretions in normal women and in patients with metabolic bone disease using the HPLC technique and immunoassays recognizing specifically the free or peptide-bound forms of pyridinoline (Pyr). After menopause, free deoxypyridinoline (free D-Pyr) excretion measured by HPLC without urine hydrolysis and expressed as a fraction of the total excretion was lower than in premenopausal women (45 ± 15% vs. 59 ± 12%, p > 0.005), whereas the fraction of free Pyr was not changed. In normal pre- and postmenopausal women (n = 43), the fraction of free D-Pyr was negatively correlated with bone turnover rate as assessed by the total urinary excretion of Pyr (r = –0.64, p > 0.001). In patients with a variety of metabolic bone diseases characterized by increased bone turnover (osteoporosis, Paget's disease, and hyperthyroidism), the fractions of free Pyr and free D-Pyr were significantly lower than in premenopausal controls (p > 0.001 for all diseases). After 3 days of intravenous (iv) treatment with the bisphosphonate pamidronate in patients with Paget's disease and osteoporosis, the urinary excretion of cross-linked peptides measured by high performance liquid chromatography (HPLC) or enzyme-linked immunoassay (ELISA) (NTX and CrossLaps) was markedly decreased (–52% and –85% for NTX, –71% and –93% for CrossLaps™ in Paget's disease and osteoporosis, respectively). The excretion of total cross-links was decreased to a lesser extent after treatment (–25% and –25% for total Pyr, –37% and –45% for total D-Pyr) and, surprisingly, free cross-links measured either by HPLC without urine hydrolysis or with an ELISA specific for free D-Pyr were unchanged after treatment. In contrast to bisphosphonate therapy, estrogen treatment of postmenopausal women decreased not only total and cross-linked peptides but also the free cross-link excretion that was reduced by about 30–40%. The different effects of bisphosphonate and estrogen therapy on the excretion of peptide-bound and free cross-link excretion were confirmed by gel filtration of the urine on Sephadex G 25 before and after treatment. In conclusion, we have shown that increased bone turnover in patients with metabolic bone disease could result in a larger increase of the urinary excretion of cross-linked peptides over the increase of free cross-links. Bisphosphonate therapy decreased markedly cross-linked peptides without significant change in free cross-link excretion contrasting with a decrease of both free and peptide-bound cross-links after estrogen therapy. These data suggest that these two antiresorptive therapies may affect differently the pattern of bone collagen degradation, an intriguing possibility that should be further investigated in vitro.