Bone and parathyroid inhibitory effects of S-2(3-aminopropylamino)ethylphosphorothioic acid. Studies in experimental animals and cultured bone cells.

S-2-(3-aminopropylamino)ethylphosphorothioic acid (WR 2721) is a radio- and chemoprotective agent which produces hypocalcemia in humans. Intravenous injection of 30 mg/kg WR 2721 in rats and 15 mg/kg in dogs lowers serum calcium by 19 and 25%, respectively. Hypocalcemia in dogs is associated with a fall in serum immunoreactive parathyroid hormone (PTH), which suggests that the mechanism of its hypocalcemic effect is acute hypoparathyroidism. Despite this effect on PTH, in eight chronically parathyroidectomized rats on a low phosphate diet, WR 2721 reduced serum calcium from 9.4 +/- 0.6 to 7.7 +/- 0.5 mg/dl (P less than 0.01) at 3 h. Furthermore, in dogs rendered hypercalcemic by continuous infusion of PTH, WR 2721 reduced serum calcium from 11.0 +/- 0.5 to 10.6 +/- 0.5 mg/dl (P less than 0.01). To determine whether WR 2721 causes hypocalcemia by enhancing the exit of calcium from the circulation or inhibiting its entry, the drug was infused 3 h after administration of 45Ca to rats. WR 2721 did not significantly increase the rate of disappearance of 45Ca from the circulation even though serum calcium fell by 19%. However, serum 45Ca specific activity was higher at 1.5 h (P less than 0.01) and 3 h (P less than 0.05) in rats given WR 2721 than in rats given vehicle alone, which suggests that WR 2721 blocks the entry of nonradioactive calcium into the circulation, presumably from bone. In incubations with fetal rat long bone labeled in utero with 45Ca, 10(-3) M WR 2721 inhibited PTH-stimulated, but not base-line release of 45Ca. Bone resorption by primary culture of chick osteoclasts was inhibited by WR 2721 at concentrations as low as 10(-4) M in the absence of hormonal stimulation. These studies suggest that WR 2721 lowers serum calcium predominantly by blocking calcium release from bone. This acute hypocalcemic effect is at least in part independent of its effect on the parathyroid glands, and is most likely a direct effect of the agent on bone resorption.