DOXERCALCIFEROL IS A SYNTHETIC VITAMIN D2 ANALOG THAT UNDERGOES METABOLIC ACTIVATION IN VIVO TO FORM 1?,25-DIHYDROXYVITAMIN D2 (1?,25-(OH)2D2), A NATURALLY OCCURRING, BIOLOGICALLY ACTIVE FORM OF VITAMIN D2. CALCITRIOL (1?,25-(OH)2D3) AND 1?,25-(OH)2D2 REGULATE BLOOD CALCIUM AT LEVELS REQUIRED FOR ESSENTIAL BODY FUNCTIONS. SPECIFICALLY, THE BIOLOGICALLY ACTIVE VITAMIN D METABOLITES CONTROL THE INTESTINAL ABSORPTION OF DIETARY CALCIUM, THE TUBULAR REABSORPTION OF CALCIUM BY THE KIDNEY AND, IN CONJUNCTION WITH PARATHYROID HORMONE (PTH), THE MOBILIZATION OF CALCIUM FROM THE SKELETON. THEY ACT DIRECTLY ON BONE CELLS (OSTEOBLASTS) TO STIMULATE SKELETAL GROWTH, AND ON THE PARATHYROID GLANDS TO SUPPRESS PTH SYNTHESIS AND SECRETION. THESE FUNCTIONS ARE MEDIATED BY THE INTERACTION OF THESE BIOLOGICALLY ACTIVE METABOLITES WITH SPECIFIC RECEPTOR PROTEINS IN THE VARIOUS TARGET TISSUES. IN PATIENTS WITH CHRONIC KIDNEY DISEASE (CKD), DEFICIENT PRODUCTION OF BIOLOGICALLY ACTIVE VITAMIN D METABOLITES (DUE TO LACK OF OR INSUFFICIENT 25-HYDROXYVITAMIN D-1-ALPHA-HYDROXYLASE ACTIVITY) LEADS TO SECONDARY HYPERPARATHYROIDISM, WHICH CONTRIBUTES TO THE DEVELOPMENT OF METABOLIC BONE DISEASE.