Fenfluramine-induced serotonin release decreases [11C]AZ10419369 binding to 5-HT1B-receptors in the primate brain.

The need for positron emission tomography (PET)-radioligands that are sensitive to changes in endogenous serotonin (5-HT) levels in brain is recognized in experimental and clinical psychiatric research. We recently developed the novel PET radioligand [(11)C]AZ10419369 that is highly selective for the 5-HT(1B) receptor. In this PET-study in three cynomolgus monkeys, we examined the sensitivity of [(11)C]AZ10419369 to altered endogenous 5-HT levels. Fenfluramine-induced 5-HT release decreased radioligand binding in a dose-dependent fashion with a regional average of 27% after 1 mg/kg and 50% after 5 mg/kg. This preliminary study supports that [(11)C]AZ10419369 is sensitive to endogenous 5-HT levels in vivo and may serve as a tool to examine the pathophysiology and treatment of major psychiatric disorders.

CCK peptides with combined features of hexa- and tetrapeptide CCK-A agonists.

Selective CCK-A agonist activity has been reported to induce satiety in a variety of animals, including man, and thereby suggests a therapeutic role for CCK in the management of obesity. To date, three general classes of CCK-A agonists have been reported, the full-length, sulfated hepta- and hexapeptides, a series of tetrapeptides, and most recently a series of benzodiazepines. The SAR of the hexa- and tetrapeptide classes suggests that the Hpa(SO(3)H) and Tac groups may not interact at the CCK-A receptor in the same location. However, the C-terminal dipeptide part of the hexapeptides and tetrapeptides appear to interact at the CCK-A receptor in a similar manner. Compound 7 (Hpa-Nle-Gly-Trp-Lys(Tac)-Asp-MePhe-NH(2)) derived from combining the features of the hexapeptides and the tetrapeptides has subnanomolar affinity and 3500-fold selectivity for CCK-A receptors. Compound 7 administered intraperitoneally produces potent, long-lasting reduction in food intake in rats and a corresponding weight loss when administered over nine consecutive days.

ARL 15849: a selective CCK-A agonist with anorectic activity in the rat and dog.

Cholecystokinin octapeptide (CCK-8) and the peptide analog ARL 14294, formerly FPL 14294, [Hpa(SO3H)-Met-Gly-Trp-Met-Asp-N(Me)Phe-NH2], have been reported to induce satiety by interaction with the CCK-A receptor subtype. ARL 15849 [Hpa(SO3H)-Nle-Gly-Trp-Nle-N(Me)-Asp-Phe-NH2] is an improved ARL 14294 analog with enhanced CCK-A receptor selectivity, greater stability, and a longer duration of action. The affinity of ARL 15849 for the CCK-A receptor (Ki = 0.034 nM) is 6,600 fold greater than for the CCK-B receptor (Ki = 224 nM), whereas CCK-8 and ARL 14294 are nonselective. Although comparable in potency to contract isolated gallbladder and induce pancreatic phosphatidylinositol hydrolysis, ARL 15849 is 3- and 100-fold more potent than ARL 14294 and CCK-8, respectively, to inhibit 3-h feeding in rats. The duration of feeding inhibition was significantly longer for ARL 15849 (>5 h), compared to equipotent doses of ARL 14294 (3 h), and CCK-8 (1 h). Intranasal administration of ARL 15849 inhibits feeding in beagle dogs with a greater separation between doses that induce emesis and those that inhibit feeding. Therefore, ARL 15849 is a potent, selective, intranasally active anorectic agent which may be useful in the treatment of eating disorders.

Synthesis and biological evaluation of potent, selective, hexapeptide CCK-A agonist anorectic agents.

Cholecystokinin (CCK) is a 33-amino acid peptide with multiple functions in both the central nervous system (via CCK-B receptors) and the periphery (via CCK-A receptors). CCK mediation of satiety via the A-receptor subtype suggest a role for CCK in the management of obesity. The carboxy terminal octapeptide (CCK-8) is fully active in this regard, but is lacking in receptor selectivity, metabolic stability, and oral bioavailability. Inversion of the chirality of Asp7 in conjunction with N-methylation of Phe8 produces compound 5 which exhibits high affinity and 2100-fold selectivity for CCK-A receptors. Compound 6 (Hpa(SO3H)-Nle-Gly-Trp-Nle-MeAsp-Phe-NH2), derived from moving the N-methyl group from Phe to Asp, decreased CCK-B affinity substantially without affecting CCK-A affinity, giving a compound with 6600-fold selectivity for CCK-A receptors. These compounds inhibit food intake with nanomolar potency following intraperitoneal administration in fasted rats. In addition to greater potency, compound 6 produces weight loss in rats when administered over nine consecutive days. Intranasal administration of 6 potently inhibits feeding in beagle dogs. Compound 6 produces potent anorectic activity via the CCK-A receptor system.

2-(1-Naphthyloxy)ethylamines with enhanced affinity for human 5-HT1D beta (h5-HT1B) serotonin receptors.

Although the beta-adrenergic antagonist propranolol (1) binds at rodent 5-HT1B serotonin receptors, it displays low affinity (Ki > 10,000 nM) for its species homologue 5-HT1D beta (i.e., h5-HT1B) receptors. The structure of propranolol was systematically modified in an attempt to enhance its affinity for the latter population of receptors. Removal of the alkyl hydroxyl group, shortening of the O-alkyl chain from three to two methylene groups, and variation of the terminal amine substituent resulted in compounds, such as N-monomethyl-2-(1-naphthyloxy)-ethylamine (11; Ki = 26 nM), that display significantly higher h5-HT1B affinity than propranolol. Compound 11 was shown to bind equally well at human 5-HT1D alpha (h5-HT1D) receptors (Ki = 34 nM) and was further demonstrated to possess h5-HT1B agonist character in an adenylate cyclase assay. It would appear that such (aryloxy)alkylamines may represent a novel class of 5-HT1D receptor agonists.

Mechanistic investigation of the stimulus properties of 1-(3-trifluoromethylphenyl)piperazine.

Using a standard two-lever operant procedure with rats trained to discriminate 1-(3-trifluoromethylphenyl)piperazine (TFMPP) (0.5 mg/kg) from saline, tests of stimulus antagonism and stimulus generalization were performed to better understand the stimulus properties of this agent. The agents examined for ability to antagonize the TFMPP stimulus were prazosin, quipazine, zacopride, buspirone, 8-hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT), 1-(2-methoxyphenol)-4-[4-(2-phthalimido)butyl]-piperazine (NAN-190), haloperidol, and 1-(2-pyrimidinyl)piperazine (1-PP); only buspirone attenuated the response to TF-MPP. In separate experiments, the lowest nondisrupting dose of buspirone (1.2 mg/kg) caused a rightward shift of the TFMPP dose-response curve (TFMPP alone, ED50 = 0.19 mg/kg; TFMPP + buspirone, ED50 = 0.43 mg/kg). In addition, 3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one (CP 93, 129), 7-trifluoromethyl-4-(4-methyl-1-piperazinyl)pyrolo[1,2-a]quinox ali ne (CGS 12066B), 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), 3-chlorophenylbiguanide (mCPBG), NAN-190, nisoxetine, zacopride, 1-PP, (+)-N-allylnormetazocine ((+)-NANM), and N-methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane (MDMA) were analyzed in tests of stimulus generalization. The TFMPP stimulus generalized only to CGS 12066B (ED50 = 4.2 mg/kg) and (+)-NANM (ED50 = 8.8 mg/kg). Tests with DOI and MDMA resulted in partial generalization. Up to doses that disrupted behavior, all other agents had little effect on TFMPP-appropriate responding. The results of these and other published studies suggest roles for 5-hydroxytryptamine 1B (5-HT1B), 5-HT1C, and, possibly, sigma-receptors in the mediation of the TFMPP stimulus and indicate a lack of involvement of 5-HT1A, 5-HT2, dopaminergic, and adrenergic mechanisms in this behavior.

Analysis of the active conformation of angiotensin II: a comparison of AII and non-peptide AII antagonists.

Several molecular models of angiotensin II (AII) were constructed using information derived primarily from conformationally constrained AII analogues. In order to refine the original models, AII was compared using computer-aided molecular graphics with several of the recently reported non-peptide AII antagonists. As a first approximation, the imidazole, phenyl ring and acidic moiety (i.e., carboxyl, tetrazole) of the non-peptide AII antagonists were overlapped with the corresponding features of the peptide. AII and Dup 753 were overlapped using Multifit (SYBYL) and then both were optimized individually and refit; this procedure was repeated three times to generate a working model of AII. The less-potent non-peptide AII antagonists S 8308 and Exp 6155 could also be overlaid with the model of AII, albeit not as well as DuP 753. The working model of AII, which has a twisted U-shape, places the imidazole of His6 and the terminal carboxylate approximately 8.7 A apart.

N-(phthalimidoalkyl) derivatives of serotonergic agents: a common interaction at 5-HT1A serotonin binding sites?

Several classes of agents are known to bind at central 5-HT1A serotonin sites In order to challenge the hypothesis that these agents bind in a relatively similar manner (i.e., share common aryl and terminal amine sites), we prepared N-(phthalimidobutyl) derivatives of examples of several such agents. With regard to arylpiperazines, we had previously shown that introduction of this functionality at the terminal amine is tolerated by the receptor and normally results in a significant (greater than 10-fold) enhancement in affinity. The results of the present study show that this bulky functionality is also tolerated by the receptor when incorporated into examples of all other major classes of 5-HT1A agents (e.g., 2-aminotetralin, phenylalklamine, indolylalkylamine, and (aryloxy)alkylamine derivatives). The length of the alkyl chain that separates the terminal amine from the phthalimido group is of major importance, and a four-carbon chain appears optimal. Alteration of the length of this chain can have a significant influence on affinity; decreasing the chain length from four to three carbon atoms can reduce affinity by an order of magnitude, and further shortening can have an even more pronounced effect.

Design and synthesis of propranolol analogues as serotonergic agents.

Serotonin (5-HT) binds with nearly identical affinity at the various central 5-HT binding sites. Few agents bind with selectivity for 5-HT1A sites. The beta-adrenergic antagonist propranolol binds stereoselectively both at 5-HT1A and 5-HT1B sites (with a several-fold selectivity for the latter) and, whereas it is a 5-HT1A antagonist, it appears to be a 5-HT1B agonist. As such, it could serve as a lead compound for the development of new 5-HT1A and 5-HT1B agents. The purpose of the present study was to modify the structure of propranolol in such a manner so as to reduce its affinity for 5-HT1B and beta-adrenergic sites while, at the same time, retaining its affinity for 5-HT1A sites. Removal of the side-chain hydroxyl group of propranolol, and conversion of its secondary amine to a tertiary amine, reduced affinity for 5-HT1B and beta-adrenergic sites. In addition, shortening the side chain by one carbon atom resulted in compounds with affinity for hippocampal 5-HT1A sites comparable to that of racemic propranolol, but with a 30- to 500-fold lower affinity for 5-HT1B sites and a greater than 1000-fold lower affinity for beta-adrenergic sites. The results of these preliminary studies attest to the utility of this approach for the development of novel serotonergic agents.

Stimulus generalization of 1-(3-trifluoromethylphenyl)piperazine (TFMPP) to propranolol, pindolol, and mesulergine.

Using standard operant procedures with rats trained to discriminate the serotonin (5-HT) agonist 1-(3-trifluoromethylphenyl)piperazine (TFMPP) (0.5 mg/kg) from saline, tests of stimulus generalization and stimulus antagonism were conducted with propranolol, pindolol, and mesulergine. Neither propranolol nor mesulergine antagonized the TFMPP stimulus (pindolol was not evaluated as an antagonist). However, TFMPP-stimulus generalization occurred with all three agents. These results suggest that the TFMPP-stimulus may involve both a 5-HT1B and a 5-HT1C mechanism and further suggest that propranolol, pindolol, and mesulergine may be capable of acting as agonists at certain populations of serotonin receptors.