Serum 1,25-dihydroxyvitamin D may be related inversely to disease activity in breast cancer patients with bone metastases

1,25-dihydroxyvitamin D (1,25-(OH)2D) stimulates differentiation and controls proliferation in breast cancer cells. The role of endogenous 1,25-(OH)2D and its relation to PTH related protein (PTHrP) during the progression of breast cancer is not known; we therefore investigated these hormones in two studies. In a cross-sectional study of patients with breast cancer at different stages of disease, serum 1,25-(OH)2D levels (mean ± se) were highest in early disease (102 ± 3.7 pmol/L), fell in normocalemic patients with bone metastases (52 ± 5.3 pmol/L; P < 0.01), and were lowest in hypercalcemic patients (33 ± 5.6 pmol/L; P< 0.001). PTHrP was detectable in the serum of only one normocalcemic patient with progressive metastases but was present in 11 of the 12 hypercalcemic patients, thus PTHrP did not stimulate 1,25-(OH)2D synthesis. In a 6-month longitudinal study of normocalcemic patients with bone metastases undergoing hormonal therapy, serum 1,25-(OH)2D concentrations fell in patients whose disease progre...