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The Vitamin D Analog, KH1060, Is Rapidly Degraded Both in Vivo and in Vitro via Several Pathways: Principal Metabolites Generated Retain Significant Biological Activity¹

Departments of Biochemistry (F.J.D., G.J.) and Medicine (G.J.), Queen’s University, Kingston, Ontario, Canada K7L 3N6; Molecular Endocrinology Group (G.R.W.), Department of Medicine, Imperial College School of Medicine, Hammersmith Hospital, London, United Kingdom W12 0NN; Leo Pharmaceutical Products (A.-M.K., J.L.N., E.B., M.J.C.), DK-2750 Ballerup, Denmark; and the Department of Clinical Biochemistry, St. Bartholomew’s and the Royal London School of Medicine and Dentistry, University of London (H.L.J.M.), London, United Kingdom E1 2AD

Address all correspondence and requests for reprints to: Dr. Glenville Jones, Department of Biochemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6.

Vitamin D analogs are valuable drugs with established and potential uses in hyperproliferative disorders. Lexacalcitol (KH1060) is over 100 times more active than 1,25-dihydroxyvitamin D₃ [1,25-(OH)₂D₃], as judged by in vitro antiproliferative and cell differentiating assays. The underlying biochemical reasons for the increased biological activity of KH1060 are unknown, but are thought to include 1) metabolic considerations in addition to explanations based upon 2) enhanced stability of KH1060-liganded transcriptional complexes. In this study we explored the in vivo and in vitro metabolism of KH1060. We established by physicochemical techniques the existence of multiple side-chain hydroxylated metabolites of KH1060, including 24-, 24a-, 26-, and 26a-hydroxylated derivatives as well as side-chain truncated forms. KH1060 metabolism could be blocked by the cytochrome P450 inhibitor, ketoconazole. KH1060 was not an effective competitor of C24 oxidation of 1,25-(OH)₂D₃. Certain hydroxylated metabolites of KH1060 retained significant biological activity in vitamin D-dependent reporter gene systems (chloramphenicol acetyltransferase). Likewise, those metabolites accumulating in the target cell culture models in metabolism studies, particularly 24a-hydroxy-KH1060 and 26-hydroxy-KH1060, retained biological activities superior to those of 1,25-(OH)₂D₃ in native gene expression systems in vitamin D target cells (osteopontin and P450cc24). We conclude that KH1060 is rapidly metabolized by a variety of cytochrome P450-mediated enzyme systems to products, many of which retain significant biological activity in vitamin D-dependent assay systems. These results provide an explanation for the considerable biological activity advantage displayed by KH1060 compared with 1,25-(OH)₂D₃ in various in vitro assay systems.

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