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  • Title: Binding of highly concentrated maxacalcitol to the nuclear vitamin D receptors of parathyroid cells.
    Author: Shiizaki K, Hayakawa N, Imazeki I, Hatamura I, Okada T, Negi S, Sakaguchi T, Shigematsu T, Akizawa T.
    Journal: Nephrol Dial Transplant; 2007 Apr; 22(4):1078-86. PubMed ID: 17234669.
    Abstract:
    BACKGROUND: Injection of maxacalcitol (OCT) directly into the parathyroid gland (PTG) is a clinically safe and effective treatment for advanced secondary hyperparathyroidism (A-SHPT) resistant to conventional medical treatment. In the present study, the degree of nuclear localization of directly injected OCT in parathyroid cells (PTC) was investigated by microautoradiography (mARG) in a model of A-SHPT. METHODS: The 5/6 nephrectomized Sprague-Dawley rats were fed a high-phosphate and low-calcium diet for 8 weeks and consequently the level of vitamin D receptor (VDR) in their PTC severely decreased. The bilateral PTG were surgically exposed and only the left gland were directly injected with 3H-OCT (DI-3H-OCT). The time course of the changes in both radioactivity and localization of 3H-OCT in the bilateral glands was analysed using a bioimaging analyser system and mARG, respectively. A very high dose of unlabelled calcitriol was administered intravenously (IV-1,25D3) prior to DI-3H-OCT, as a competitive study. RESULTS: Peak radioactivity levels in the directly injected and intact PTG occured immediately and 1 h, respectively, after DI-3H-OCT, and the difference was about 50-fold higher in the treated gland. The of mARG showed a marked concentration of silver grains in the nuclei of PTC in the gland treated with DI-3H-OCT and that concentration was significantly suppressed by IV-1,25D3. CONCLUSIONS: Direct injection of OCT into the PTG enables the administration of the highly concentrated drug for specific binding to nuclear vitamin D binding sites, including VDR of PTC, which markedly suppresses the parathyroid hormone, improves the response to calcium and vitamin D and induces apoptosis in PTC.
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