Design and synthesis of novel alpha(1)(a) adrenoceptor-selective antagonists. 3. Approaches to eliminate opioid agonist metabolites by using substituted phenylpiperazine side chains.

Dihydropyrimidinones, such as 1, represent a novel class of alpha(1a) adrenoceptor antagonists with potential for the treatment of benign prostatic hyperplasia (BPH) (see part 1 of this series). Analysis of the metabolites of 1 revealed that 4-methoxycarbonyl-4-phenylpiperidine is formed as the major metabolite and is an agonist at the mu-opioid receptor. To circumvent any potential liability resulting from the metabolite, we decided to identify alternate templates devoid of agonist activity at the mu-opioid receptor to replace the 4-methoxycarbonyl-4-phenylpiperidine moiety. The present study describes the synthesis and SAR of dihydropyrimidinones linked to substituted 4-phenylpiperazine containing side chains. Compound (+)-38 was identified as a lead compound with a binding and functional profile comparable to that of 1. The putative metabolite 2-carboxamidophenylpiperazine has negligible affinity for the mu-opioid receptor.

In vitro and in vivo evaluation of dihydropyrimidinone C-5 amides as potent and selective alpha(1A) receptor antagonists for the treatment of benign prostatic hyperplasia.

alpha(1) Adrenergic receptors mediate both vascular and lower urinary tract tone, and alpha(1) receptor antagonists such as terazosin (1b) are used to treat both hypertension and benign prostatic hyperplasia (BPH). Recently, three different subtypes of this receptor have been identified, with the alpha(1A) receptor being most prevalent in lower urinary tract tissue. This paper explores 4-aryldihydropyrimidinones attached to an aminopropyl-4-arylpiperidine via a C-5 amide as selective alpha(1A) receptor subtype antagonists. In receptor binding assays, these types of compounds generally display K(i) values for the alpha(1a) receptor subtype <1 nM while being greater than 100-fold selective versus the alpha(1b) and alpha(1d) receptor subtypes. Many of these compounds were also evaluated in vivo and found to be more potent than terazosin in both a rat model of prostate tone and a dog model of intra-urethral pressure without significantly affecting blood pressure. While many of the compounds tested displayed poor pharmacokinetics, compound 48 was found to have adequate bioavailability (>20%) and half-life (>6 h) in both rats and dogs. Due to its selectivity for the alpha(1a) over the alpha(1b) and alpha(1d) receptors as well as its favorable pharmacokinetic profile, 48 has the potential to relieve the symptoms of BPH without eliciting effects on the cardiovascular system.

Design and synthesis of novel alpha(1)(a) adrenoceptor-selective antagonists. 1. Structure-activity relationship in dihydropyrimidinones.

Dihydropyrimidinones such as compound 12 exhibited high binding affinity and subtype selectivity for the cloned human alpha(1a) receptor. Systematic modifications of 12 led to identification of highly potent and subtype-selective compounds such as (+)-30 and (+)-103, with high binding affinity (K(i) = 0.2 nM) for alpha(1a) receptor and greater than 1500-fold selectivity over alpha(1b) and alpha(1d) adrenoceptors. The compounds were found to be functional antagonists in human, rat, and dog prostate tissues. Compound (+)-103 exhibited excellent selectively to inhibit intraurethral pressure (IUP) as compared to lowering diastolic blood pressure (DBP) in mongrel dogs (K(b)(DBP)/K(b)(IUP) = 40) suggesting uroselectivity for alpha(1a)-selective compounds.

Bradykinin stimulation of phosphoinositide hydrolysis in guinea-pig ileum longitudinal muscle.

1. Bradykinin (BK)-induced contraction of ileal smooth muscle is assumed to be due to phosphoinositide hydrolysis but this has never been reported. We have investigated whether BK receptors are linked to this transduction mechanism in guinea-pig ileum longitudinal muscle and determined whether these receptors are equivalent to those labelled in [3H]-BK binding assays. 2. In membranes prepared from longitudinal muscle, [3H]-BK bound to a single class of sites with high affinity. Characterization of the binding with BK analogues indicated that the radioligand selectivity labelled a B2 type receptor. 3. BK significantly elevated tissue levels of [3H]-inositol phosphates in longitudinal muscle slices preincubated with [3H]-myo-inositol. The agonists potencies of BK, Lys-BK, Met-Lys-BK, Tyr5-BK and Tyr8-BK were in agreement with their relative potencies in the binding assay. The B1 receptor agonist des-Arg9-BK, did not stimulate inositol phosphate production. The response to BK was blocked by known B2 receptor antagonists but not by the B1 antagonist des-Arg9, Leu8-BK. 4. BK-induced phosphoinositide hydrolysis was unaffected by exposure of muscle slices to either atropine or indomethacin. 5. The results indicate that the B2 receptors linked to phosphoinositide turnover in ileal longitudinal muscle exhibit properties similar to those involved in contractile responses. Also, the receptor mediating the phosphoinositide response is likely to be that labelled in the [3H]-BK binding studies.

Characterization of solubilized bradykinin B2 receptors from smooth muscle and mucosa of guinea pig ileum.

Bradykinin (BK) B2 receptors in guinea pig ileum were characterized in both membrane and soluble form. [3H]BK bound to a single class of sites with almost identical affinities in membranes prepared from the longitudinal muscle, circular muscle and mucosal layers of the ileum. The pharmacology of the binding in the distinct layers was indistinguishable. The detergent 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate (CHAPS) maximally solubilized nearly 80% of membrane binding activity in a very stable conformation. In soluble preparations, [3H]BK labeled a single class of sites but with about 10-fold lower affinity. The affinities of BK analogs in competition studies were similarly reduced. There was no difference in the pharmacology of the binding in soluble receptors prepared from the different layers of the ileum. The results show that the ileum is a good source of solubilized B2 receptors and that the receptors in the smooth muscle and the mucosa are very similar.

Inhibition of N-methyl-D-aspartate evoked sodium flux by MK-801.

The inhibition of N-methyl-D-aspartate (NMDA) stimulated 22Na+ efflux from rat hippocampal slices was studied using competitive and non-competitive receptor antagonists. There was a good correlation between the abilities of the competitive antagonists to block NMDA evoked 22Na+ efflux and their potencies as inhibitors of L-[3H]glutamate binding. The recently reported novel NMDA receptor antagonist, (+)-5-methyl-16,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) was shown to non-competitively inhibit NMDA stimulated 22Na+ efflux with an IC50 value of 0.4 microM. Relatively high (10 microM) concentrations of MK-801 had no effects on quisqualic acid, alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA), or kainic acid stimulated efflux. However, MK-801 was able to block 22Na+ efflux induced by ibotenic acid and L-homocysteic acid, amino acids that act as NMDA receptor agonists. MK-801, (-)-MK-801, and non-competitive NMDA receptor antagonists of the arylcyclohexylamine and dioxolane classes inhibited NMDA stimulated 22Na+ efflux with potencies that reflected their abilities to compete for [3H]MK-801 binding sites in rat cortical membranes. These results indicate the utility of the 22Na+ efflux assay in studying the properties of NMDA receptors and confirm the nature and selectivity of the inhibition of NMDA receptor linked ion channel activation by MK-801.

Polyamine and ifenprodil interactions with the NMDA receptor's glycine site.

Ifenprodil partially inhibited [3H]glycine binding to the N-methyl-D-aspartate (NMDA) receptor in rat telencephalic membranes. The polyamine spermine significantly enhanced [3H]glycine binding and decreased ifenprodils inhibitory potency. However, ifenprodil was unable to completely reverse the stimulation of binding produced by spermine. Also, ifenprodil was found to reduce the maximum degree to which spermine enhanced binding but had no effect on the polyamine's potency. These results show that ifenprodil and spermine do not competitively interact with respect to their abilities to modify [3H]glycine binding to the NMDA receptor complex.

NMDA-induced hippocampal [3H]norepinephrine release is modulated by glycine.

Kynurenic acid (KYN) non-competitively inhibited N-methyl-D-aspartate (NMDA)-induced [3H]norepinephrine ([3H]NE) release from rat hippocampal slices. At 100 microM KYN, the effect on release was primarily on the maximal obtainable response to NMDA. Glycine was able to completely block the inhibitory effects of 100 microM KYN on NMDA-evoked release. This ability to prevent KYN inhibition of release was shared by other amino acids with the following order of potency: glycine greater than D-serine greater than D-alanine much greater than L-serine greater than or equal to L-alanine. Neither isomer of valine or threonine was able to reverse KYN inhibition of NMDA-induced release. These potencies agreed with the relative abilities of these amino acids to displace strychnine-insensitive [3H]glycine binding to rat brain membranes. Glycine and D-serine had no effect on the inhibition of NMDA-stimulated [3H]NE release produced by D-2-amino-5-phosphonovaleric acid, MK-801 or Mg2+. Also, neither amino acid modified KYN inhibition of kainic acid-induced release. These data demonstrate that the glycine regulatory site associated with the NMDA receptor can be demonstrated in whole brain slices by using an antagonist to attenuate the influences of endogenous glycine.

Bradykinin B1 receptors in rabbit aorta smooth muscle cells in culture.

Kinin B1 receptors on rabbit aorta smooth muscle cells in culture were investigated. [3H]Des-Arg10-kallidin labeled a single site in cells at early passage with an equilibrium dissociation constant of 258 pM and a maximal binding density of approximately 680 sites/cell. Treatment of the same cells for 18 h with epidermal growth factor increased the binding density over 6-fold without affecting the ligand's affinity. At latter passages, the density of binding sites was found to increase and the growth factor had a much less pronounced effect. The rank order of potencies for agonist inhibition of binding (des-Arg10-kallidin > des-Arg9-BK = kallidin > bradykinin) was consistent with the specific labeling of a B1 receptor. Also, [3H]des-Arg10-kallidin binding was potently inhibited by the B1 receptor antagonist des-Arg9[Leu8]bradykinin but not by the B2 receptor antagonist Hoe 140. The agonists were found to stimulate phosphoinositide hydrolysis in the smooth muscle cells with an order of potencies that reflected their binding assay activities. Des-Arg9[Leu8] BK blocked the des-Arg10-kallidin response with a potency consistent with its known B1 receptor activity while Hoe 140 was inactive. These results demonstrate the presence of inducible B1 receptors on rabbit aorta smooth muscle cells in culture that couple to phospholipase C activation. These cells should be useful in future studies of the mechanisms and factors involved in the regulation of expression of the B1 receptor.

Bradykinin agonist activity of a novel, potent oxytocin antagonist.

From a series of potent cyclic hexapeptide oxytocin (OT) antagonists, a compound that exhibited significant bradykinin (BK) agonist activity was identified. L-366,811 (cyclo[L-proline-D-tryptophan-L-isoleucine-D-pipecolic acid-L-piperazine-2-carboxylic acid-N-Me-D-phenylalanine]) stimulated phosphatidylinositol (PI) turnover in rat uterine slices in vitro (approximately EC50, 2 microM) with a maximal effect (15-fold increase over basal) greater than that obtained for either BK or OT. L-366,811 also elicited dose-related contractions of the isolated rat uterus, producing measurable effects at 100 nM. Several other equally potent OT antagonists from the cyclic hexapeptide structural class were either less potent or inactive as activators of uterine PI turnover or contractility. The stimulatory effects of L-366,811 on uterine PI turnover and contractions were blocked by BK antagonists but not by an arginine vasopressin (AVP)/OT antagonist. In radioligand binding studies, L-366,811 exhibited moderate affinity (IC50, 360 nM) for the [3H]BK binding site in rat uterus, consistent with its potency in the functional models. These results indicate that L-366,811 exhibits BK agonist activity in rat uterus in vitro.

Glycine modulation of NMDA-evoked release of [3H]acetylcholine and [3H]dopamine from rat striatal slices.

Superfused slices of rat striatum were used to investigate whether N-methyl-D-aspartate (NMDA) receptors mediating [3H]acetylcholine (ACh) and [3H]dopamine (DA) release are modulated by glycine. Kynurenic acid, a compound possessing antagonist activity at the receptor's glycine modulatory site, dose-dependently blocked NMDA release of both transmitters. The inhibition was reversed by glycine, D-serine, D-alanine, D-cysteine, L-serine and L-alanine. The relative potencies of these amino acids was in agreement with their previously reported abilities to inhibit strychnine-insensitive [3H]glycine binding and enhance [3H]MK-801 binding to the NMDA receptor complex. These data indicate that glycine modulatory sites are linked to NMDA receptors involved in striatal [3H]ACh and [3H]DA release.

Interaction of xylamine with peripheral sympathetic neurons.

Xylamine (XYL) administered to intact rats caused a 70-80% reduction in norepinephrine (NE) uptake by the vas deferens but had little or no effect on NE content in that tissue. The vas deferens accumulates 3H-XYL in vitro by a desmethylimipramine (DMI)-sensitive mechanism. Vasa deferentia from 6-hydroxydopamine (6OHDA) pretreated animals exhibited a 80% reduction in both NE content and XYL uptake activity. These results indicate that XYL is taken up by sympathetic nerve terminals and can reduce NE uptake activity without depleting terminals of neurotransmitter.

5-HT-1a and 5-HT-1b selectivity of two phenylpiperazine derivatives: evidence for 5-HT-1b heterogeneity.

The ability of m-trifluoromethylphenylpiperazine (TFMPP) and an N-substituted derivative, LY 165163 (p-NH2-PE-TFMPP), to discriminate 5-HT-1 binding sites labelled by [3H]5-HT is compared in rat corpus striatum and rat cortex. TFMPP displays at least a 30 fold selectivity for the 5-HT-1b subtype. Furthermore, TFMPP reveals heterogeneity within the 5-HT-1b binding sites. TFMPP displays a Kd of 6 nM for approximately two-thirds of the 5-HT-1b binding sites and a Kd of 273 nM for the remaining one third of the 5-HT-1b sites. p-NH2-PE-TFMPP, on the other hand, discriminates the 5-HT-1 sites in a manner similar to spiperone, displaying a 110 fold selectivity for the 5-HT-1a sites. p-NH2-PE-TFMPP displays a Kd of about 3 nM for the 5-HT-1a sites. p-NH2-PE-TFMPP does not discriminate subtypes within the 5-HT-1b binding sites. The significance of the selectivity of these two compounds as well as structurally related compounds is discussed.

Effects of 2-substituted-4-phenylquinolines on uptake of serotonin and norepinephrine by isolated brain synaptosomes.

In this present communication, the in vitro inhibition of the uptake of [3H]-L-norepinephrine ([3H] NE) and [3H]-Serotonin ([3H] 5-HT) by eleven synthesized 2-substituted-4-phenyl quinolines were studied using rat brain synaptosomal preparations. Compounds with an open side chain were relatively weak inhibitors of the synaptosomal uptake of [3H] NE and [3H] 5HT. Compounds having a distance of three atoms between the terminal basic nitrogen of the side chain and the quinoline ring were better inhibitors of serotonin uptake than those compounds having a four-atom distance. The replacement of the side chain with a piperazine ring produced compounds which were more potent and selective inhibitors of the uptake of either [3H] 5-HT or [3H] NE. Further structure-activity relationships are also discussed.

Barbiturate and benzodiazepine modulation of GABA receptor binding and function.

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) acts primarily on receptors that increase chloride permeability in postsynaptic neurons. These receptors are defined by sensitivity to the agonist muscimol and the antagonist bicuculline, and are also subject to indirect allosteric inhibition by picrotoxin-like convulsants and enhancement by the clinically important drugs, the benzodiazepines and the barbiturates. All of these drugs modulate GABA-receptor regulated chloride channels at the cellular level assayed by electrophysiological or radioactive ion tracer techniques. Specific receptor sites for GABA, benzodiazepines, picrotoxin/convulsants, and barbiturates can be assayed in vitro by radioactive ligand binding. Mutual chloride-dependent allosteric interactions between the four receptor sites indicate that they are all coupled in the same membrane macromolecular complex. Indirect effects of barbiturates on the other three binding sites define a pharmacologically specific, stereospecific receptor. All of the activities can be solubilized in the mild detergent 3-[(3-cholamidopropyl)-dimethylammonio]propane sulfonate (CHAPS) and co-purify as a single protein complex.

[125I]PIP HOE 140, a high affinity radioligand for bradykinin B2 receptors.

A high affinity radioligand for bradykinin B2 receptors was prepared by coupling an activated ester of [125I]4-iodobenzoic acid to the amino terminus nitrogen of the potent B2 antagonist HOE 140. The ligand, [125I]para-iodophenyl HOE 140 ([125I]PIP HOE 140), bound to a homogeneous set of sites in guinea pig ileal membranes with an equilibrium dissociation constant of 15 pM and a maximal binding density of 193 fmole/mg protein. Competition studies with a number of BK-related peptides indicated that the ligand specifically labeled B2 receptors in the preparation. The results suggest that [125I]PIP HOE 140 will be a useful tool for future studies of B2 receptors.

(+)-3-[123I]Iodo-MK-801: synthesis and characterization of binding to the N-methyl-D-aspartate receptor complex.

Synthetic methods have been established for preparing high specific activity (+)-3-[123I]Iodo-MK-801 in high radiochemical yield. The binding of the radiotracer to rat cortical membranes has been examined to assess its potential use as an in vivo imaging agent for the N-methyl-D-aspartate (NMDA) receptor-ion channel complex. Under the conditions of the assay, specific (+)-3-[123I]Iodo-MK-801 binding to membrane homogenates represented greater than 95% of the total binding. Several structurally distinct, noncompetitive NMDA receptor antagonists inhibited binding with potencies in accordance with their reported inhibitory activity at the receptor complex. The concentration of (+/-)-3-Iodo-MK-801 required to inhibit 50% of (+)-3-[123I]Iodo-MK-801 binding (IC50) was 3.4 nM when using a low ionic strength assay buffer and 5.5 nM in a physiological buffer. In a thoroughly washed membrane preparation, (+)-3-[123I]Iodo-MK-801 binding was enhanced by L-glutamate and glycine at concentrations known to activate the NMDA receptor. The results indicate that (+)-3-[123I]Iodo-MK-801 specifically labels the NMDA receptor complex in rat brain membranes and the retention of high affinity under near physiological assay conditions suggests that it may be useful as a SPECT imaging agent for the receptor in vivo.

Cloning and pharmacological characterization of bradykinin receptors.

A human B2 bradykinin receptor cDNA was cloned from the lung fibroblast cell line, CCD-Lu. This clone was utilized to isolate a genomic clone of a mouse B2 bradykinin receptor. Both clones encode a protein that has the predicted characteristics of a seven transmembrane domain G-protein-coupled receptor. The DNA sequence of these two clones is 84% identical in the putative coding region. The clones have been heterologously expressed in a mammalian cell line lacking endogenous bradykinin receptors, COS-7, and a comparative analysis of their pharmacology was done. Both clones exhibit properties characteristic of the B2 bradykinin receptor, binding bradykinin with high affinity (KD = 0.1-0.2 nM) and binding des-Arg9 bradykinin with a very low affinity (IC50 > 5 microM). Interestingly, the mouse B2 bradykinin receptor has a 60-80 fold higher affinity than the human B2 bradykinin receptor for the peptide antagonists D-Arg0[Hyp3,Thi5,8,D-Phe7]bradykinin and D-Arg0[Hyp3,D-Phe7]bradykinin.