Prevention of Uremic Bone Disease Using Calcimimetic Compounds

The discovery, characterization, and cloning of the calcium-sensing receptor (CaR) in 1993 was soon followed by the creation of a new type of drug, the calcimimetics—NPS R-568 and NPS R-467—which are small phenylalkylamine derivative compounds that act as CaR agonists and increase the sensitivity of the CaR to activation by extracellular calcium (Ca²⁺). As expected, these compounds turned out to have a significant effect on the Ca²⁺/parathyroid hormone (PTH) relationship, resulting in a dramatically greater suppression of the PTH level than would otherwise occur at the actual extracellular Ca²⁺ levels. Renal osteodystrophy (RO) due to secondary hyperparathyroidism (HPT) in chronic renal failure was an obvious target for studying the effects of NPS R-568. In a study on experimental animals, the results clearly showed that this first generation of calcimimetics, NPS R-568, had an acute dose-dependent and short-lived suppressive effect on PTH secretion from the parathyroid glands. A similar effect was found in patients with chronic renal failure and secondary HPT. At the same time, the calcimimetics induced a slight degree of hypocalcemia. Such a significant suppressive effect on PTH secretion would be expected to result in therapeutic potential for a preventive or therapeutic effect on the RO accompanying chronic uremia. Administration would probably be in close concert with present strategies, phosphate binders and vitamin D analogs. A wide distribution of CaRs have now been demonstrated in the body, and an important question is how calcimimetics will affect the function of different tissues and organs when used for long-term treatment or prevention of secondary HPT and RO. Although relatively few experimental and clinical investigations have been completed, they clearly confirm the suppressive effect of calcimimetics on PTH secretion. In rats with experimental chronic renal failure, a significant and beneficial effect on the prevention of RO has been demonstrated. The effect of calcimimetic compounds is presently being evaluated in humans. Besides induction of hypocalcemia, the adverse effects in these mainly short-term studies have been few. Future studies with calcimimetics will further define the physiology and pathophysiology of the CaR and the long-term benefit of calcimimetic compounds in patients with chronic renal failure.