[Vitamin D treatment and renal osteodystrophy: indications and modalities]. / Traitement vitaminique D et ostéodystrophies rénales: indications et modalités.

1. 1 alpha (OH) vitamin D3 derivatives have an inconstant long term inhibitory effect on PTH secretion. As a matter of fact they act by three mechanisms, one of these being antagonistic: 1) a direct inhibitory action on the prepro-PTH gene; 2) an indirect inhibitory action by increasing plasma calcium; 3) an indirect stimulatory action by increasing plasma phosphate. These two latter phenomena are due to the stimulation of the intestinal absorption of these ions as well as to an intrinsic osteolytic action which may override the inhibition of the bone release of these ions in relation with the decrease of the PTH plasma levels. 2. The use of 1 alpha (OH)D3 derivatives in patients on chronic dialysis is justified in about 30% of the patients on dialysis when in spite of native vitamin D repletion and adequate predialysis control of plasma calcium (2.5 +/- 2 mmol/l) and of plasma phosphate (1.4 - 1.7 mmol/l), the PTH plasma levels are 3 or 5 fold the upper limit of normal whether the patient is on hemodialysis or on CAPD. When hyperphosphatemia is > 1.7 mmol/l it is first necessary to correct it by the use of higher doses of alkaline calcium salts given with the meals as phosphate binder together with a negative perdialytic calcium balance induced by a lower dialysate calcium in order to avoid hypercalcemia. Control of hyperphosphatemia is indeed a necessary prerequisite for the long term PTH suppressive efficacy of 1 alpha OH vitamin D derivatives. 3. The use of 1 alpha(OH)D3 derivatives in the treatment of the predialysis uremic patients is even more limited because there is no additional mean to decrease the risk of hypercalcemia when oral calcium is used as phosphate binder because of the danger of aluminum and magnesium phosphate binders. Fortunately in the adult, oral calcium used as phosphate binder in association with phosphate restriction and correction of possible vitamin D depletion and acidosis is usually efficace to control hyperparathyroïdism without 1 alpha OH vitamin D3. This is not the case in the child to whom protein and phosphate restriction should not be prescribed because of its incompatibility with the Recommended Diet Allowance. Fortunately the high remodeling rate of his growing bones, decreases the risk of hypercalcemia due to the combination of CaCO3 and 1 alpha OH vitamin D3.

1-alpha-Hydroxyvitamin D3 derivatives in the treatment of renal bone diseases: justification and optimal modalities of administration.

The use of 1 alpha-hydroxyvitamin D3 [1 alpha(OH)D3] derivatives in a uremic patient is justified only in the treatment of hyperparathyroidism (i.e. when plasma intact parathyroid hormone - PTH - levels are above five or three times the upper limit of normal according to whether the patient is on continuous ambulatory peritoneal dialysis or on hemodialysis and between 0.5-1.5, 1-2 and 2-3 times the upper limit of normal for a creatinine clearance of, respectively, 30, between 30 and 10, or below 10 ml/min/1.73 m2). The following prerequisites have however to be satisfied: (1) a good vitamin D3 repletion should be secured by plasma 25(OH(D) levels of 20-30 ng/ml (if necessary by administration of native vitamin D or 25(OH)D3), and (2) phosphate retention (which is aggravated by the increased phosphate intestinal absorption induced by the 1 alpha (OH)D derivatives) and the consequent possible hyperphosphatemia should be prevented or corrected by the oral administration of alkaline salts of calcium given before the meals as phosphate binders without inducing hypercalcemia. These prerequisites explain the narrow therapeutical margin of 1 alpha (OH)D3 derivatives in uremic patients before dialysis (more so in the adult than in the child) and the possible broadening of this margin in the patients on dialysis by the use of low dialysate calcium concentrations (1.25-1.00 mmol/l) in order to prevent hypercalcemia by inducing a negative perdialytic calcium balance. Once hyperphosphatemia is prevented by oral calcium, 1 alpha (OH)D3 derivatives have the advantage to suppress the transcription of the prepro PTH gene by a mechanism independent of an increase in plasma calcium. Controlled randomized trials have not confirmed the claimed advantage in efficacy and safety of the parenteral versus the oral route nor of the intermittent versus the daily mode of their administration. The advantages of using the so called 'nonhypercalcemic hyperphosphatemic' vitamin D3 derivatives in combination with oral calcium over 1 alpha(OH)D3 derivatives in the treatment of uremic hyperparathyroidism are still waiting for clinical demonstration. Vitamin D derivatives have no place in the treatment of aluminic bone diseases which necessitate long term deferoxamine treatment and prevention of aluminum exposure by the dialysate and the phosphate binders. They are not indicated in the treatment of 'idiopathic' adynamic bone disease which is due to uremia per se combined with an excessive PTH suppression for the degree of renal failure. This low bone turnover pattern is associated with an increased risk of hypercalcemia and hyperphosphatemia and necessitates only a stimulation of PTH secretion by inducing a negative calcium balance with a lower dialysate calcium concentration or simply by discontinuing the oral calcium supplement in the uremic patient not yet dialyzed. In rare cases this pattern is due to a granulomatosis and is corrected by prednisone.