An analogue of substance P with broad receptor antagonist activity

Pharmacological lineage analysis revealed the binding affinity of broad-spectrum substance P antagonists to receptors for gonadotropin-releasing peptide

A group of synthetic substance P (SP) antagonists, such as [Arg(6),D-Trp(7,9),N(Me)Phe(8)]-substance P(6-11) and [D-Arg(1),D-Phe(5),D-Trp(7,9),Leu(11)]-substance P, bind to a range of distinct G-protein-coupled receptor (GPCR) family members, including V1a vasopressin receptors, and they competitively inhibit agonist binding. This extended accessibility enabled us to identify a GPCR subset with a partially conserved binding site structure. By combining pharmacological data and amino acid sequence homology matrices, a pharmacological lineage of GPCRs that are sensitive to these two SP antagonists was constructed. We found that sensitivity to the SP antagonists was not limited to the Gq-protein-coupled V1a and V1b receptors; Gs-coupled V2 receptors and oxytocin receptors, which couple with both Gq and Gi, also demonstrated sensitivity. Unexpectedly, a dendrogram based on the amino acid sequences of 222 known GPCRs showed that a group of receptors sensitive to the SP antagonists are located in close proximity to vasopressin/oxytocin receptors. Gonadotropin-releasing peptide receptors, located near the vasopressin receptors in the dendrogram, were also sensitive to the SP analogs, whereas α1B adrenergic receptors, located more distantly from the vasopressin receptors, were not sensitive. Our finding suggests that pharmacological lineage analysis is useful in selecting subsets of candidate receptors that contain a conserved binding site for a ligand with broad-spectrum binding abilities. The knowledge that the binding site of the two broad-spectrum SP analogs partially overlaps with that of distinct peptide agonists is valuable for understanding the specificity/broadness of peptide ligands.

Drugs interacting with G protein alpha subunits: selectivity and perspectives

Extracellular signal molecules as diverse as hormones, neurotransmitters and photons use a signal transduction pathway involving a receptor, a G protein and effectors. Compounds that interact directly with G proteins can mimic the receptor-G protein interaction or can block the activation of G proteins by receptors. Several binding sites exist on the G alpha protein that may be exploited for the design of synthetic stimulatory or inhibitory ligands. The effector binding site is regulated by endogenous proteins and appears to be a target for selective exogenous ligands. The GTP binding site presents a large homology within the G protein families and therefore the nucleotide analogs might not be considered as a tool to discriminate between the G protein subclasses. In contrast, different experimental strategies have substantiated the specificity in the interaction between a receptor and a G protein, the receptor binding site of G proteins should be considered as potential drug targets. Drugs interfering with this site such as mastoparan and related peptides, GPAnt-2 and suramin, are lead compounds in the design of selective G protein antagonists. Benzalkonium chloride and methoctramine have agonist or antagonist properties, depending on G protein subtypes. Such compounds would be very useful to delineate the functions of G proteins and G protein-coupled receptors, to understand some side effects of drugs used in therapy and to develop new therapeutic agents.

Substance P-related inhibitors of mast cell exocytosis act on G-proteins or on the cell surface

[p-Glu5,D-Trp(7,9,10)]substance P-(5-11) inhibited mastoparan-stimulated GTPase activity in homogenized rat peritoneal mast cells and decreased histamine secretion induced by mastoparan from streptolysin O-permeabilized mast cells (IC50 of about 30 microM), but not from intact cells. In contrast, [D-Pro4,D-Trp(7,9,10)]substance P-(4-11) inhibited the secretion from intact cells (IC50 of about 10 microM) but had no effect on histamine secretion from permeabilized cells, suggesting that this peptide exerts its inhibitory effect on the plasma membrane, whereas [p-Glu5,D-Trp(7,9,10)]substance P-(5-11) interacts with G proteins. Pretreatment of mast cells with neuraminidase led to an inhibition of the secretory response to mastoparan and related triggers. This response was restored following cell permeabilization, demonstrating the role of the cell surface on the entry of mastoparan and related triggers and on their ability to reach G proteins sensitive to pertussis toxin and [p-Glu5,D-Trp(7,9,10)]substance P-(5-11).

Direct demonstration of three different states of the pancreatic cholecystokinin receptor

We used rat pancreatic acini as well as COS-7 cells transfected with the cloned pancreatic cholecystokinin (CCK) receptor and measured the abilities of CCK octapeptide (CCK-8) and L-364,718 (a CCK receptor antagonist) to inhibit binding of 125I-labeled CCK-8 (125I-CCK-8) and [3H]L-364,718. With pancreatic acini 125I-CCK-8 bound to two different states of the CCK receptor. The high-affinity state (1% of the receptors) had a Kd for CCK-8 of 985 pM and the low-affinity state (19% of the receptors) had a Kd for CCK-8 of 30 nM. [3H]L-364,718 bound to low-affinity receptors and to a previously unrecognized very-low-affinity state (80% of the receptors) having a Kd for CCK-8 of 13 microM. L-364,718 had the same affinity (Kd 3 nM) for each of the three different states of the CCK receptor. Similar measurements using transfected COS cells also identified three different states of the CCK receptor, with the very-low-affinity state being the most abundant. Thus, the ability of the CCK receptor to exist in three different states is an intrinsic property of the CCK receptor molecule itself.

Characterization of a new diphenylpyrazolidinone cholecystokinin antagonist in vitro isolated rat pancreatic acini

The effects of a newly developed diphenylpyrazolidinone cholecystokinin (CCK) antagonist LY219,057 were examined in the isolated rat pancreatic acini and compared with those of devazepide (previously designated L364,718 or MK-329). LY219,057 caused a concentration-dependent inhibition of 100 pM CCK octapeptide (CCK-8)-stimulated amylase release, with a half-maximal inhibition (ID50) at 287.5 +/- 28.4 nM and was 200 times less potent than devazepide (ID50 = 1.4 +/- 0.2 nM). The antagonism was competitive in nature because LY219,057 caused a parallel rightward shift of the dose-response curve for CCK-8-stimulated amylase secretion without altering the maximal increase. LY219,057 significantly inhibited amylase release in response to CCK-8 and cerulein but had no effect on amylase release stimulated by other receptor secretagogs or agent bypassing receptors. LY219,057, whether added at the beginning or 20 min after the CCK-8 stimulation, inhibited amylase release. This compound caused a residual inhibition of the action of CCK-8. Acini preincubated with 1.0 microM LY219,057 for 30 min at 37 degrees C were threefold less sensitive to CCK-8 than the acini preincubated without LY219,057. These results indicate that LY219,057 acts as a potent, competitive, and specific CCK receptor antagonist of the action of CCK on the exocrine pancreas.

Non-peptide ligands for peptide receptors

Considerable progress has been made in synthesizing peptide analogs with improved stability for probing function of peptide-receptor systems. However, since peptide ligands are usually unsuitable for development as potent orally active long-duration therapeutic agents, considerable research effort is being directed to the development of non-peptidal ligands. Roger Freidinger compares the lead compounds that have now been described in the opioid, CCK-gastrin and angiotensin II fields, and discusses the progress that has been made in other receptor fields. Most of the successes have been achieved through screening natural sources and synthetic collections. Rational design based on the natural peptide ligand has been more difficult, although ACE inhibitors have been effectively developed from a nonapeptide lead.