Calcilytic compounds: potent and selective Ca2+ receptor antagonists that stimulate secretion of parathyroid hormone.

Despite the discovery of many ions and molecules that activate the Ca2+ receptor, there are no known ligands that block this receptor. Reported here are the pharmacodynamic properties of a small molecule, NPS 2143, which acts as an antagonist at the Ca2+ receptor. This compound blocked (IC50 of 43 nM) increases in cytoplasmic Ca2+ concentrations [Ca2+]i elicited by activating the Ca2+ receptor in HEK 293 cells expressing the human Ca2+ receptor. NPS 2143, even when tested at much higher concentrations (3 microM), did not affect the activity of a number of other G protein-coupled receptors, including those most structurally homologous to the Ca2+ receptor. NPS 2143 stimulated parathyroid hormone (PTH) secretion from bovine parathyroid cells (EC50 of 41 nM) over a range of extracellular Ca2+ concentrations and reversed the effects of the calcimimetic compound NPS R-467 on [Ca2+]i and on secretion of PTH. When infused intravenously in normal rats, NPS 2143 caused a rapid and large increase in plasma levels of PTH. Ca2+ receptor antagonists are termed calcilytics and NPS 2143 is the first substance (either atomic or molecular) shown to possess such activity. The pharmacodynamic properties of NPS 2143 together with the recently demonstrated effects of this compound on bone formation support the view that orally active calcilytic compounds might provide a novel anabolic therapy for osteoporosis.

Chiral synthesis and pharmacological evaluation of NPS 1407: a potent, stereoselective NMDA receptor antagonist.

The stereoselective synthesis and biological activity of NPS 1407 (4a), (S)-(-)-3-amino-1,1-bis(3-fluorophenyl)butane, a potent, stereoselective antagonist of the NMDA receptor, are described. The racemate (4) was found to be active at the NMDA receptor in an in vitro assay, prompting the synthesis of the individual stereoisomers. The S isomer (4a) was found to be 12 times more potent than the R isomer (4b). Compound 4a demonstrated in vivo pharmacological activity in neuroprotection and anticonvulsant assays.

NPS 1506, a moderate affinity uncompetitive NMDA receptor antagonist: preclinical summary and clinical experience.

NPS Pharmaceuticals, Inc. (NPS) has synthesized a series of open-channel blockers with varying potencies at the NMDA receptor. NPS 1506 (Fig. 1) is a moderate affinity antagonist that inhibits NMDA/glycine-induced increases in cytosolic calcium in cultured rat cerebellar granule cells (IC50 = 476nM) and displaces the binding of [3H]MK-801 to rat cortical membranes (IC50 = 664nM).

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

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.

Synthesis, biological activity, and absolute stereochemical assignment of NPS 1392: a potent and stereoselective NMDA receptor antagonist.

The synthesis, biological activity, and single crystal X-ray structure of NPS 1392, (R)-(-)-3,3-bis(3-fluorophenyl)-2-methylpropan-1-amine (3a), a potent, stereoselective antagonist of the NMDA receptor, are described. The NMDA receptor selectively bound the levo isomer (3a) over its enantiomer (3b), which prompted a rigorous absolute configuration assignment. NPS 1392 has the R configuration based on the single-crystal X-ray diffraction analysis of the hydroiodide salt of NPS 1392. This compound is a potential neuroprotective agent for use in the treatment of ischemic stroke.

NPS 1506, a novel NMDA receptor antagonist and neuroprotectant. Review of preclinical and clinical studies.

NPS 1506 is a moderate affinity, uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist. NPS 1506 is neuroprotective in rodent models of ischemic stroke, hemorrhagic stroke, and head trauma, with a 2-hr window of opportunity. Neuroprotectant doses of NPS 1506 ranged from approximately 0.1-1.0 mg/kg, with peak plasma concentrations ranging from 8-80 ng/mL. Even at doses producing behavioral toxicity, NPS 1506 did not elicit MK-801-like behaviors, did not generalize to phencyclidine (PCP), and did not elicit neuronal vacuolization. In a Phase I study, intravenous (i.v.) doses of NPS 1506 from 5-100 mg were well tolerated and provided plasma concentrations in excess of those required for neuroprotection in rodents. Adverse events at the 100-mg dose included mild dizziness and lightheadedness, and mild to moderate ataxia. Neither PCP-like psychotomimetic effects nor cardiovascular effects were noted. The long plasma half-life of NPS 1506 (approximately 60 hr) suggests that a single i.v. dose will provide prolonged neuroprotection in humans.

Calcimimetics with potent and selective activity on the parathyroid calcium receptor.

Parathyroid hormone (PTH) secretion is regulated by a cell surface Ca2+ receptor that detects small changes in the level of plasma Ca2+. Because this G protein-coupled receptor conceivably provides a distinct molecular target for drugs useful in treating bone and mineral-related disorders, we sought to design small organic molecules that act on the Ca2+ receptor. We discovered that certain phenylalkylamine compounds, typified by NPS R-568 and its deschloro derivative NPS R-467, increased the concentration of cytoplasmic Ca2+ ([Ca2+]i) in bovine parathyroid cells and inhibited PTH secretion at nanomolar concentrations. These effects were stereoselective and the R enantiomers were 10- to 100-fold more potent than the S enantiomers. NPS R-568 potentiated the effects of extracellular Ca2+ on [Ca2+]i and PTH secretion but was without effect in the absence of extracellular Ca2+. Both compounds shifted the concentration-response curves for extracellular Ca2+ to the left. Presumably, these compounds act as positive allosteric modulators to increase the sensitivity of the Ca2+ receptor to activation by extracellular Ca2+. Both NPS R-467 and NPS R-568 increased [Ca2+]i in HEK 293 cells expressing the human parathyroid Ca2+ receptor but were without effect in wild-type HEK 293 cells. Neither compound affected the cytoplasmic Ca2+ responses elicited by several other G protein-coupled receptors in HEK 293 cells or in bovine parathyroid cells. Significantly, these compounds did not affect responses elicited by the homologous metabotropic glutamate receptors, mGluR1a, mGluR2, or mGluR8. These compounds therefore act selectively on the Ca2+ receptor. Compounds that mimic or potentiate the effects of extracellular Ca2+ at the Ca2+ receptor are termed calcimimetics. The discovery of calcimimetic compounds with potent and selective activity enables a pharmacological approach to regulating plasma levels of PTH. Calcimimetic compounds could conceivably provide a specific medical therapy for primary hyperparathyroidism.

Novel insecticidal peptides from Tegenaria agrestis spider venom may have a direct effect on the insect central nervous system.

Fractionation of venom from an agelenid spider, Tegenaria agrestis, resulted in the isolation of a family of three peptides with potent insecticidal activity. These peptide toxins, TaITX-1, -2, and -3, whose sequences were revealed from cloned cDNAs, each consist of 50 amino acid residues, six of which are cysteines. They appear to be amidated at their C-termini and exhibit greater than 90% sequence identity. Unlike other reported spider toxins, the TaI toxins are processed from precursors containing no propeptide sequences. In lepidopteran larvae and corn rootworm beetles, the insecticidal Tegenaria toxins caused an unusual excitatory symptomatology with 50% paralytic doses ranging from 0.23 to 2.6 nmol/g. In a series of electrophysiological experiments performed in house fly larvae, these toxins caused an elevated rate of firing from central nervous system neurons. No significant effects were found when any peripheral sensory or motor systems were examined. Thus, it appears that the TaI toxins may act in a fashion not previously reported for insecticidal peptide toxins; they may act directly on the insect central nervous system.

Inhibition of human pancreatic elastase 2 by peptide chloromethyl ketones.

The inactivation of human pancreatic elastase 2 (EC 3.4.21.11) by a series of peptide chloromethyl ketones has been investigated. Among a series of compounds with the structure X-Ala-Ala-Pro-Y-CH2Cl (where X=acetyl-, succinyl-, methylsuccinyl-, or H-), the kinetic parametrs for inhibition of elastas 2 depend markedly on the amino acid (Y) in the P1 position. Succinyl-Ala-Ala-Pro-Leu-CH2Cl was found to be an extremely effective inhibitor of human elastase 2, qith a first-order rate constant for covalent bond formation (k3) of 0.033s-1 and a dissociation constant, Ki, for the enzyme inhibitor complex of 7.4 . 10(-7) M. The second-order rate constant k3/Ki for inhibition of elastase 2 by the analogous compound containing a free amino group in place of the succinyl moiety is 150 times lower than that found for the succinyl or acetyl derivative, suggesting that the presence of a positive charge at this position reduces the proper binding of the inhibitor to the enzyme.