Gary P. O'Neill

Characterization of the human cysteinyl leukotriene CysLT1 receptor

The cysteinyl leukotrienes—leukotriene C4(LTC4), leukotriene D4(LTD4) and leukotriene E4(LTE 4)—are important mediators of human bronchial asthma,. Pharmacological studies have determined that cysteinyl leukotrienes activate at least two receptors, designated CysLT1 and CysLT2 (refs 4,5,6). The CysLT1-selective antagonists, such as montelukast (Singulair), zafirlukast (Accolate) and pranlukast (Onon), are important in the treatment of asthma. Previous biochemical characterization of CysLT1 antagonists and the CysLT1 receptor has been in membrane preparations from tissues enriched for this receptor. Here we report the molecular and pharmacological characterization of the cloned human CysLT1 receptor. We describe the functional activation (calcium mobilization) of this receptor by LTD4 and LTC4, and competition for radiolabelled LTD4 binding to this receptor by the cysteinyl leukotrienes and three structurally distinct classes of CysLT1-receptor antagonists. We detected CysLT1-receptor messenger RNA in spleen, peripheral blood leukocytes and lung. In normal human lung, expression of the CysLT 1-receptor mRNA was confined to smooth muscle cells and tissue macrophages. Finally, we mapped the human CysLT1-receptor gene to the X chromosome.

Expression of mRNA for cyclooxygenase‐1 and cyclooxygenase‐2 in human tissues

The rate‐limiting step in the formation of prostanoids is the conversion of arachidonic acid to prostaglandin H2 by cyclooxygenase, also known as prostaglandin G/H synthase/cyclooxygenase. Two forms of cyclooxygenase have been characterized: a ubiquitously expressed form (COX‐1) and a recently described second form (COX‐2) inducible by various factors including mitogens, hormones, serum and cytokines. Here we quantitate by the reverse transcriptase‐polymerase chain reaction (RT‐PCR) the expression of COX‐1 and COX‐2 mRNA in human tissues including lung, uterus, testis, brain, pancreas, kidney, liver, thymus, prostate, mammary gland, stomach and small intestine. All tissues examined contained both COX‐1 and COX‐2 mRNA and could be grouped according to the level of COX mRNA expression. The highest levels of COX mRNAs were detected in the prostate where approximately equal levels of COX‐1 and COX‐2 transcripts were present. In the lung high levels of COX‐2 were observed whereas COX‐1 mRNA levels were about 2‐fold lower. An intermediate level of expression of both COX‐1 and COX‐2 mRNA was observed in the mammary gland, stomach, small intestine, and uterus. The lowest levels of COX‐1 and COX‐2 mRNA were observed in the testis, pancreas, kidney, liver, thymus, and brain.

Characterization of the Human Cysteinyl Leukotriene 2 Receptor

The contractile and inflammatory actions of the cysteinyl leukotrienes (CysLTs), LTC4, LTD4, and LTE4, are thought to be mediated through at least two distinct but related CysLT G protein-coupled receptors. The human CysLT1 receptor has been recently cloned and characterized. We describe here the cloning and characterization of the second cysteinyl leukotriene receptor, CysLT2, a 346-amino acid protein with 38% amino acid identity to the CysLT1 receptor. The recombinant human CysLT2 receptor was expressed in Xenopusoocytes and HEK293T cells and shown to couple to elevation of intracellular calcium when activated by LTC4, LTD4, or LTE4. Analyses of radiolabeled LTD4 binding to the recombinant CysLT2 receptor demonstrated high affinity binding and a rank order of potency for competition of LTC4 = LTD4 ≫ LTE4. In contrast to the dual CysLT1/CysLT2 antagonist, BAY u9773, the CysLT1 receptor-selective antagonists MK-571, montelukast (SingulairTM), zafirlukast (AccolateTM), and pranlukast (OnonTM) exhibited low potency in competition for LTD4 binding and as antagonists of CysLT2receptor signaling. CysLT2 receptor mRNA was detected in lung macrophages and airway smooth muscle, cardiac Purkinje cells, adrenal medulla cells, peripheral blood leukocytes, and brain, and the receptor gene was mapped to chromosome 13q14, a region linked to atopic asthma.

Discovery of a receptor related to the galanin receptors

We report the isolation of a cDNA clone named GPR54, which encodes a novel G protein‐coupled receptor (GPCR). A PCR search of rat brain cDNA retrieved a clone partially encoding a GPCR. In a library screening this clone was used to isolate a cDNA with an open reading frame (ORF) encoding a receptor of 396 amino acids long which shared significant identities in the transmembrane regions with rat galanin receptors GalR1 (45%), GalR3 (45%) and GalR2 (44%). Northern blot and in situ hybridization analyses revealed that GPR54 is expressed in brain regions (pons, midbrain, thalamus, hypothalamus, hippocampus, amygdala, cortex, frontal cortex, and striatum) as well as peripheral regions (liver and intestine). In COS cell expression of GPR54 no specific binding was observed for 125I‐galanin. A recent BLAST search with the rat GPR54 ORF nucleotide sequence recovered the human orthologue of GPR54 in a 3.5 Mb contig localized to chromosome 19p13.3.

The discovery of rofecoxib, [MK 966, VIOXX®, 4-(4′-methylsulfonylphenyl)-3-phenyl-2(5H)-furanone], an orally active cyclooxygenase-2 inhibitor

The development of a COX-2 inhibitor rofecoxib (MK 966, Vioxx) is described. It is essentially equipotent to indomethacin both in vitro and in vivo but without the ulcerogenic side effect due to COX-1 inhibition.

Biochemical and pharmacological profile of a tetrasubstituted furanone as a highly selective COX-2 inhibitor

DFU (5,5‐dimethyl‐3‐(3‐fluorophenyl)‐4‐(4‐methylsulphonyl)phenyl‐2(5H)‐furanone) was identified as a novel orally active and highly selective cyclo‐oxygenase‐2 (COX‐2) inhibitor. In CHO cells stably transfected with human COX isozymes, DFU inhibited the arachidonic acid‐dependent production of prostaglandin E2 (PGE2) with at least a 1,000 fold selectivity for COX‐2 (IC50=41±14 nM) over COX‐1 (IC50>50 μM). Indomethacin was a potent inhibitor of both COX‐1 (IC50=18±3 nM) and COX‐2 (IC50=26±6 nM) under the same assay conditions. The large increase in selectivity of DFU over indomethacin was also observed in COX‐1 mediated production of thromboxane B2 (TXB2) by Ca2+ ionophore‐challenged human platelets (IC50>50 μM and 4.1±1.7 nM, respectively). DFU caused a time‐dependent inhibition of purified recombinant human COX‐2 with a Ki value of 140±68 μM for the initial reversible binding to enzyme and a k2 value of 0.11±0.06 s−1 for the first order rate constant for formation of a tightly bound enzyme‐inhibitor complex. Comparable values of 62±26 μM and 0.06±0.01 s−1, respectively, were obtained for indomethacin. The enzyme‐inhibitor complex was found to have a 1 : 1 stoichiometry and to dissociate only very slowly (t1/2=1–3 h) with recovery of intact inhibitor and active enzyme. The time‐dependent inhibition by DFU was decreased by co‐incubation with arachidonic acid under non‐turnover conditions, consistent with reversible competitive inhibition at the COX active site. Inhibition of purified recombinant human COX‐1 by DFU was very weak and observed only at low concentrations of substrate (IC50=63±5 μM at 0.1 μM arachidonic acid). In contrast to COX‐2, inhibition was time‐independent and rapidly reversible. These data are consistent with a reversible competitive inhibition of COX‐1. DFU inhibited lipopolysaccharide (LPS)‐induced PGE2 production (COX‐2) in a human whole blood assay with a potency (IC50=0.28±0.04 μM) similar to indomethacin (IC50=0.68±0.17 μM). In contrast, DFU was at least 500 times less potent (IC50>97 μM) than indomethacin at inhibiting coagulation‐induced TXB2 production (COX‐1) (IC50=0.19±0.02 μM). In a sensitive assay with U937 cell microsomes at a low arachidonic acid concentration (0.1 μM), DFU inhibited COX‐1 with an IC50 value of 13±2 μM as compared to 20±1 nM for indomethacin. CGP 28238, etodolac and SC‐58125 were about 10 times more potent inhibitors of COX‐1 than DFU. The order of potency of various inhibitors was diclofenac>indomethacin∼naproxen>nimesulide∼ meloxicam∼piroxicam>NS‐398∼SC‐57666>SC‐58125>CGP 28238∼etodolac>L‐745,337>DFU. DFU inhibited dose‐dependently both the carrageenan‐induced rat paw oedema (ED50 of 1.1 mg kg−1 vs 2.0 mg kg−1 for indomethacin) and hyperalgesia (ED50 of 0.95 mg kg−1 vs 1.5 mg kg−1 for indomethacin). The compound was also effective at reversing LPS‐induced pyrexia in rats (ED50=0.76 mg kg−1 vs 1.1 mg kg−1 for indomethacin). In a sensitive model in which 51Cr faecal excretion was used to assess the integrity of the gastrointestinal tract in rats, no significant effect was detected after oral administration of DFU (100 mg kg−1, b.i.d.) for 5 days, whereas chromium leakage was observed with lower doses of diclofenac (3 mg kg−1), meloxicam (3 mg kg−1) or etodolac (10–30 mg kg−1). A 5 day administration of DFU in squirrel monkeys (100 mg kg−1) did not affect chromium leakage in contrast to diclofenac (1 mg kg−1) or naproxen (5 mg kg−1). The results indicate that COX‐1 inhibitory effects can be detected for all selective COX‐2 inhibitors tested by use of a sensitive assay at low substrate concentration. The novel inhibitor DFU shows the lowest inhibitory potency against COX‐1, a consistent high selectivity of inhibition of COX‐2 over COX‐1 (>300 fold) with enzyme, whole cell and whole blood assays, with no detectable loss of integrity of the gastrointestinal tract at doses >200 fold higher than efficacious doses in models of inflammation, pyresis and hyperalgesia. These results provide further evidence that prostanoids derived from COX‐1 activity are not important in acute inflammatory responses and that a high therapeutic index of anti‐inflammatory effect to gastropathy can be achieved with a selective COX‐2 inhibitor.

IDENTIFICATION OF A GABAB RECEPTOR SUBUNIT, GB2, REQUIRED FOR FUNCTIONAL GABAB RECEPTOR ACTIVITY

G protein-coupled receptors are commonly thought to bind their cognate ligands and elicit functional responses primarily as monomeric receptors. In studying the recombinant γ-aminobutyric acid, type B (GABAB) receptor (gb1a) and a GABAB-like orphan receptor (gb2), we observed that both receptors are functionally inactive when expressed individually in multiple heterologous systems. Characterization of the tissue distribution of each of the receptors by in situhybridization histochemistry in rat brain revealed co-localization of gb1 and gb2 transcripts in many brain regions, suggesting the hypothesis that gb1 and gb2 may interact in vivo. In three established functional systems (inwardly rectifying K+channel currents in Xenopus oocytes, melanophore pigment aggregation, and direct cAMP measurements in HEK-293 cells), GABA mediated a functional response in cells coexpressing gb1a and gb2 but not in cells expressing either receptor individually. This GABA activity could be blocked with the GABAB receptor antagonist CGP71872. In COS-7 cells coexpressing gb1a and gb2 receptors, co-immunoprecipitation of gb1a and gb2 receptors was demonstrated, indicating that gb1a and gb2 act as subunits in the formation of a functional GABAB receptor.

Molecular pharmacology of the human prostaglandin D2 receptor, CRTH2

The recombinant human prostaglandin D2 (PGD2) receptor, hCRTH2, has been expressed in HEK293(EBNA) and characterized with respect to radioligand binding and signal transduction properties. High and low affinity binding sites for PGD2 were identified in the CRTH2 receptor population by saturation analysis with respective equilibrium dissociation constants (KD) of 2.5 and 109 nM. This revealed that the affinity of PGD2 for CRTH2 is eight times less than its affinity for the DP receptor. Equilibrium competition binding assays revealed that of the compounds tested, only PGD2 and several related metabolites bound with high affinity to CRTH2 (Ki values ranging from 2.4 to 34.0 nM) with the following rank order of potency: PGD2>13,14‐dihydro‐15‐keto PGD2>15‐deoxy‐Δ12,14‐PGJ2>PGJ2>Δ12‐PGJ2>15(S)‐15 methyl‐PGD2. This is in sharp contrast with the rank order of potency obtained at DP : PGD2>PGJ2>Δ12‐PGJ2>15‐deoxy‐Δ12,14‐PGJ2 >>>13,14‐dihydro‐15‐keto‐PGD2. Functional studies demonstrated that PGD2 activation of recombinant CRTH2 results in decrease of intracellular cAMP in a pertussis toxin‐sensitive manner. Therefore, we showed that CRTH2 can functionally couple to the G‐protein Gαi/o. PGD2 and related metabolites were tested and their rank order of potency followed the results of the membrane binding assay. By Northern blot analysis, we showed that, besides haemopoietic cells, CRTH2 is expressed in many other tissues such as brain, heart, thymus, spleen and various tissues of the digestive system. In addition, in situ hybridization studies revealed that CRTH2 mRNA is expressed in human eosinophils. Finally, radioligand binding studies demonstrated that two eosinophilic cell lines, butyric acid‐differentiated HL‐60 and AML 14.3D10, also endogenously express CRTH2.

Molecular characterization and expression of cloned human galanin receptors GALR2 and GALR3.

Abstract: Galanin is a 29‐ or 30‐amino acid peptide with wide‐ranging effects on hormone release, feeding behavior, smooth muscle contractility, and somatosensory neuronal function. Three distinct galanin receptor (GALR) subtypes, designated GALR1, 2, and 3, have been cloned from the rat. We report here the cloning of the human GALR2 and GALR3 genes, an initial characterization of their pharmacology with respect to radioligand binding and signal transduction pathways, and a profile of their expression in brain and peripheral tissues. Human GALR2 and GALR3 show, respectively, 92 and 89% amino acid sequence identity with their rat homologues. Radioligand binding studies with 125I‐galanin show that recombinant human GALR2 binds with high affinity to human galanin (KD = 0.3 nM). Human GALR3 binds galanin with less affinity (IC50 of 12 nM for porcine galanin and 75 nM for human galanin). Human GALR2 was shown to couple to phospholipase C and elevation of intracellular calcium levels as assessed by aequorin luminescence in HEK‐293 cells and by Xenopus melanophore pigment aggregation and dispersion assays, in contrast to human GALR1 and human GALR3, which signal predominantly through inhibition of adenylate cyclase. GALR2 mRNA shows a wide distribution in the brain (mammillary nuclei, dentate gyrus, cingulate gyrus, and posterior hypothalamic, supraoptic, and arcuate nuclei), and restricted peripheral tissue distribution with highest mRNA levels detected in human small intestine. In comparison, whereas GALR3 mRNA was expressed in many areas of the rat brain, there was abundant expression in the primary olfactory cortex, olfactory tubercle, the islands of Calleja, the hippocampal CA regions of Ammons horn, and the dentate gyrus. GALR3 mRNA was highly expressed in human testis and was detectable in adrenal gland and pancreas. The genes for human GALR2 and 3 were localized to chromosomes 17q25 and 22q12.2–13.1, respectively.

Suppression of Niacin-induced Vasodilation with an Antagonist to Prostaglandin D2 Receptor Subtype 1

Niacin (nicotinic acid) reduces cardiovascular events in patients with dyslipidemia. However, symptoms associated with niacin-induced vasodilation (e.g., flushing) have limited its use. Laropiprant is a selective antagonist of the prostaglandin D(2) receptor subtype 1 (DP1), which may mediate niacin-induced vasodilation. The aim of this proof-of-concept study was to evaluate the effects of laropiprant (vs placebo) on niacin-induced cutaneous vasodilation. Coadministration of laropiprant 30, 100, and 300 mg with extended-release (ER) niacin significantly lowered flushing symptom scores (by approximately 50% or more) and also significantly reduced malar skin blood flow measured by laser Doppler perfusion imaging. Laropiprant was effective after multiple doses in reducing symptoms of flushing and attenuating the increased malar skin blood flow induced by ER niacin. In conclusion, the DP1 receptor antagonist laropiprant was effective in suppressing both subjective and objective manifestations of niacin-induced vasodilation.Niacin (nicotinic acid) reduces cardiovascular events in patients with dyslipidemia. However, symptoms associated with niacin‐induced vasodilation (e.g., flushing) have limited its use. Laropiprant is a selective antagonist of the prostaglandin D2 receptor subtype 1 (DP1), which may mediate niacin‐induced vasodilation. The aim of this proof‐of‐concept study was to evaluate the effects of laropiprant (vs placebo) on niacin‐induced cutaneous vasodilation. Coadministration of laropiprant 30, 100, and 300 mg with extended‐release (ER) niacin significantly lowered flushing symptom scores (by approximately 50% or more) and also significantly reduced malar skin blood flow measured by laser Doppler perfusion imaging. Laropiprant was effective after multiple doses in reducing symptoms of flushing and attenuating the increased malar skin blood flow induced by ER niacin. In conclusion, the DP1 receptor antagonist laropiprant was effective in suppressing both subjective and objective manifestations of niacin‐induced vasodilation.