Antagonizing the parathyroid calcium receptor stimulates parathyroid hormone secretion and bone formation in osteopenic rats

Abstract

Parathyroid hormone (PTH) is an effective bone anabolic agent, but it must be administered parenterally. An orally active anabolic agent would provide a valuable alternative for treating osteoporosis. NPS 2143 is a novel, selective antagonist (a "calcilytic") of the parathyroid cell Ca(2+) receptor. Daily oral administration of NPS 2143 to osteopenic ovariectomized (OVX) rats caused a sustained increase in plasma PTH levels, provoking a dramatic increase in bone turnover but no net change in bone mineral density. Concurrent oral administration of NPS 2143 and subcutaneous infusion of 17beta-estradiol also resulted in increased bone turnover. However, the antiresorptive action of estrogen decreased the extent of bone resorption stimulated by the elevated PTH levels, leading to an increase in bone mass compared with OVX controls or to either treatment alone. Despite the sustained stimulation to the parathyroid gland, parathyroid cells did not undergo hyperplasia. These data demonstrate that an increase in endogenous PTH secretion, induced by antagonism of the parathyroid cell Ca(2+) receptor with a small molecule, leads to a dramatic increase in bone turnover, and they suggest a novel approach to the treatment of osteoporosis.

Figure 1

Structure of NPS 2143.

Figure 2

Effect of daily administration of NPS 2143 (100 μmol/kg, per os) or rat PTH(1–34) (5 μg/kg, subcutaneously) on bone mineral density (BMD) in the proximal tibia of osteopenic OVX rats. Values shown as mean ± SE; n = 10–14 per group. ^ASignificantly different from sham-vehicle; P < 0.05. ^BSignificantly different from OVX-vehicle; P < 0.05. ^CSignificantly different from OVX–NPS 2143; P < 0.05.

Figure 3

Plasma levels of PTH after the administration of NPS 2143 (100 μmol/kg, per os) or rat PTH(1–34) (5 μg/kg, subcutaneously) to OVX rats. Both endogenous and exogenous PTH levels are of the 1-34 peptide. Values are mean ± SE; n = 3–4 per timepoint.

Figure 4

Plasma levels of NPS 2143 after its oral administration (100 μmol/kg) to rats. Values are mean ± SE; n = 3.

Figure 5

Histomorphometric analysis of cancellous bone in the proximal tibial metaphysis from osteopenic OVX rats treated daily for 8 weeks with NPS 2143 (100 μmol/kg, per os) or rat PTH(1–34) (5 μg/kg, subcutaneously). See Methods for abbreviations. Values are mean ± SE; n = 10–14 per group. ^ASignificantly different from sham-vehicle; P < 0.05. ^BSignificantly different from OVX-vehicle; P < 0.05. ^CSignificantly different from OVX–NPS 2143; P < 0.05.

Figure 6

Sections of proximal tibia stained with von Kossa stain from OVX rats with established osteopenia, showing the anabolic effects on cancellous bone of daily oral administration for 5 weeks of NPS 2143 (100 μmol/kg) when given in combination with continuous subcutaneous infusion of 17β-estradiol. Shown are a sham-operated rat given vehicle (a), an OVX rat given vehicle (b), an OVX rat given 17β-estradiol alone (c), and an OVX rat treated with NPS 2143 and 17β-estradiol (d).

Figure 7

Histomorphometric analysis of cancellous bone in the proximal tibial metaphysis from osteopenic OVX rats treated daily for 5 weeks with 17β-estradiol (continuous subcutaneous infusion), NPS 2143 (100 μmol/kg, per os.), or NPS 2143 and 17β-estradiol. See Methods for abbreviations. Values are mean ± SE, n = 9–10 per group. ^ASignificantly different from sham-vehicle; P < 0.05. ^BSignificantly different from OVX-vehicle; P < 0.05. ^CSignificantly different from OVX-estradiol; P < 0.05. ^DSignificantly different from OVX–NPS 2143; P < 0.05.

Figure 8

Effects of daily treatment of osteopenic OVX rats for 5 weeks with 17β-estradiol (continuous subcutaneous infusion), NPS 2143 (100 μmol/kg, per os), or NPS 2143 and 17β-estradiol on BMD in the distal femur. Values are mean ± SE; n = 9–10 per group. ^BSignificantly different from OVX-vehicle; P < 0.05. ^CSignificantly different from OVX–NPS 2143; P < 0.05.

Figure 9

Absence of changes in cell proliferation in the parathyroid gland of osteopenic OVX rats treated daily for 5 weeks with 17β-estradiol (continuous subcutaneous infusion), NPS 2143 (100 μmol/kg, per os), or NPS 2143 and 17β-estradiol. Values are mean ± SE; n = 4–5 per group; P = 0.26.

Figure 10

NPS 2143 does not affect basal or PTH-stimulated cyclic AMP formation or alkaline phosphatase activity in TF274 osteoblastic cells. Values are mean ± SE of experiments performed in triplicate.