[Micro-autoradiography: in vivo nuclear receptor binding of OCT]

Maxacalcitol, 1alpha,25-dihydroxy-22-oxavitamin D(3) (OCT), is a new synthetic analogue of 1alpha,25 (OH)(2)vitamin D(3) (1alpha,25 (OH)(2)D(3)), to be used for the treatment of secondary hyperparathyroidism. The side effect of hyper-calcemia can be prevented by short plasma half life of OCT, while there is a possibility of short retention in the target site. Micro-autoradiography is a powerful method to demonstrate the direct cellular distribution of drugs, especially nuclear receptor-binding materials as vitamin D. This study was performed to compare the time-course of receptor binding in the parathyroid chief cells between OCT and 1alpha,25 (OH)(2)D(3) after intravenous injection of [26-(3)H] OCT or [26, 27-methyl-(3)H] 1alpha,25 (OH)(2)D(3) to mice. Nuclear receptor binding of (3)H-OCT appeared equal to or higher than that of (3)H-1alpha,25 (OH)(2)D(3) while the plasma concentration of (3)H-OCT was much lower than that of (3)H-1alpha,25 (OH)(2)D(3).