Pharmacology and therapeutic use of vitamin D and its analogues

The term vitamin D is generally used to describe a number of chemically related compounds with common antirachitic properties, but which have differences in the rapidity of their action, the way they are produced in the body, and the conditions under which their results are optimal. Ergocalciferol, cholecalciferol, 25-hydroxycholecalciferol (calcifediol), dihydrotachysterol, 1 alpha-hydroxycholecalciferol (alfacalcidol), and 1,25-dihydroxycholecalciferol (calcitriol) are currently the most commonly used vitamin D metabolites. In man, cholecalciferol produced on the skin and the fraction obtained from the diet in the gastrointestinal tract are converted in the liver to 25-hydroxycholecalciferol and then in the kidney to 1,25-dihydroxycholecalciferol. The demonstration of these metabolic pathways has helped to elucidate the aetiology of such conditions a hepatobiliary osteodystrophy, drug-induced anticonvulsant osteomalacia, the hypocalcaemia of hypoparathyroidism and above all azotaemic osteodystrophy. In the therapy of azotaemic osteodystrophy, the period of 'vitamin D resistance' when large doses of vitamin D2 and D3 had to be used is now over, and these patients can be efficiently and successfully treated with almost physiological doses of 1 alpha-hydroxycholecalciferol and 1,25-dihydroxycholecalciferol. Attention to diet, calcium supplements and oral phosphate binders are also important. During repetitive haemodialysis, the above principles still hold true, but in some of these patients an osteomalacic syndrome resistant to 1,25-dihydroxycholecalciferol has been recognised. These patients readily become hypercalcaemic when given 1,25-dihydroxycholecalciferol and their fractures and osteomalacia do not improve. Aluminium intoxication, possibly related to the use of impure dialysis fluid, is currently thought to be the most likely explanation of this dialysis osteomalacic syndrome.