The pharmacology of the novel and selective sigma ligand, PD 144418.

The pharmacology of PD 144418 (1-propyl-5-(3-p-tolyl-isoxazol-5-yl)-1,2,3,6-tetrahydropyridine) was characterized using neurochemical, biochemical and behavioral techniques. For sigma (sigma 1 and sigma 2 respectively) sites, PD 144418 affinities were determined using whole guinea pig brain membranes with [3H](+)-pentazocine and neuroblastoma x glioma cell membranes using [3H]1,3,di-O-tolylguanidine (DTG) in the presence of 200 nM (+)-pentazocine. PD 144418 exhibited an affinity for sigma 1 of 0.08 nM (Ki) versus a K1 of 1377 nM for sigma 2 site. Additional receptor binding studies indicated that PD 144418 lacked affinity for dopaminergic, adrenergic, muscarinic and a variety of other receptors. In vitro studies indicated that PD 144418 reversed the N-methyl-D-aspartate (NMDA)-induced increase in cyclic GMP (cGMP) in rat cerebellar slices without affecting the basal levels, suggesting that sigma 1 sites may be important in the regulation of glutamine-induced actions. PD 144418 potentiated the decrease in 5-hydroxytryptophan caused by haloperidol in the mesolimbic region, but by itself had no effect in 5-hydroxytrypamine (5-HT) and dopamine (DA) synthesis. Behaviorally, similar to other sigma ligands, PD 144418 antagonized mescaline-induced scratching at doses that did not alter spontaneous motor activity. This action is suggestive of potential antipsychotic property. It exhibited no anxiolytic and antidepressant properties in the models used. These results show that PD 144418 is a very selective sigma 1 agent, devoid of any significant affinity for other receptors and that sigma 1 site may modulate actions in the CNS.

Agonist properties of a stable hexapeptide analog of neurotensin, N alpha MeArg-Lys-Pro-Trp-tLeu-Leu (NT1).

The major signal transduction pathway for neurotensin (NT) receptors is the G-protein-dependent stimulation of phospholipase C, leading to the mobilization of intracellular free Ca2+ ([Ca2+]i) and the stimulation of cyclic GMP. We investigated the functional actions of an analog of NT(8-13), N alpha MeArg-Lys-Pro-Trp-tLeu-Leu (NT1), and other NT related analogs by quantitative measurement of the cytosolic free Ca2+ concentration in HT-29 (human colonic adenocarcinoma) cells using the Ca(2+)-sensitive dye fura-2/AM and by effects on cyclic GMP levels in rat cerebellar slices. The NT receptor binding affinities for these analogs to HT-29 cell membranes and newborn (10-day-old) mouse brain membranes were also investigated. Data obtained from HT-29 cell and mouse brain membrane preparations showed saturable single high-affinity sites and binding densities (Bmax) of 130.2 and 87.5 fmol/mg protein, respectively. The respective KD values were 0.47 and 0.39 nM, and the Hill coefficients were 0.99 and 0.92. The low-affinity levocabastine-sensitive site was not present (K1 > 10,000) in either membrane preparation. Although the correlation of binding between HT-29 cell membranes and mouse brain membranes was quite significant (r = 0.92), some of the reference agents had lower binding affinities in the HT-29 cell membranes. The metabolically stable compound NT1 plus other NT analogs and related peptides [NT, NT(8-13), xenopsin, neuromedin N, NT(9-13), kinetensin and (D-Trp11)-NT] increased intracellular Ca2+ levels in HT-29 cells, indicating NT receptor agonist properties. The effect of NT1 in mobilizing [Ca2+]i blocked by SR 48692, a non-peptide NT antagonist. Receptor binding affinities of NT analogs to HT-29 cell membranes were positively correlated with potencies for mobilizing intracellular calcium in the same cells. In addition, NT1 increased cyclic GMP levels in rat cerebellar slices, confirming the latter findings of its NT agonist action. These results substantiate the in vitro NT agonist properties of the hexapeptide NT analog NT1.

Differential effects of the nonpeptide neurotensin antagonist, SR 48692, on the pharmacological effects of neurotensin agonists.

In in vitro studies, SR 48692, a nonpeptide neurotensin receptor antagonist, inhibited the binding of [3H] or [125I]neurotensin to membrane preparations from 10-day-old mouse brains and from HT-29 cells with Ki values of 3.9 and 8.6 nM, respectively. SR 48692 also antagonized the neurotensin-induced mobilization of intracellular calcium in HT-29 cells, in agreement with previous findings. In rat cerebellar slices SR 48692 blocked the increase in cyclic GMP levels evoked by neurotensin in a dose-dependent manner. In vivo, SR 48692 antagonized the increase in rat brain mesolimbic dopamine turnover induced by the systemically active neurotensin peptide, EI [(N-Me)Arg-Lys-Pro-Trp-tert-Leu-Leu]. No effects on dopamine turnover of either EI or SR 48692 were observed in the striatum. SR 48692 did not antagonize the EI-induced decreases in mouse body temperature and spontaneous locomotor activity (LMA) or the decreases in LMA induced by ICV-administered neurotensin. Although other explanations are possible, these findings support the hypothesis that a subtype of the NT receptor may mediate the locomotor and hypothermic actions of this peptide and that it is different from the NT receptor that is involved in dopamine turnover.

Studies of the active conformation of a novel series of benzamide dopamine D2 agonists.

Analogs of dopamine D2 agonist 11 were prepared in which a rigid trans decalin ring system was used to mimic various conformations of 11. The four rigid analogs where compared for their ability to bind to the DA D2 receptor and to inhibit forskolin-stimulated cAMP formation, a measure of DA agonist activity. Of the four rigid analogs of compound 11, only compound 12b had significant activity in both assays. Molecular modeling studies of 12a-d showed each had a single conformation with regard to the distance between the benzamide aryl-centroid and the 4-nitrogen atom of the pyridylpiperazine. Compound 12b was shown to have a greater distance between these functionalities (11.8 A) as compared to the other isomers (9.8-10.4 A). The distance between these two functionalities in 12b was similar to that of a conformer of 11 which has an extended conformation. This suggest that 11 is likely in an extended conformation when bound to the DA D2 receptor.

The discovery and structure-activity relationships of 1,2,3,6-tetrahydro-4-phenyl-1-[(arylcyclohexenyl)alkyl]pyridines. Dopamine autoreceptor agonists and potential antipsychotic agents.

A novel dopamine (DA) autoreceptor agonist, 1,2,3,6-tetrahydro-4-phenyl-1- [(3-phenyl-3-cyclohexen-1-yl)methyl]pyridine (14), was identified. The structure-activity relationships surrounding this compound were studied by synthesis of analogues and evaluation of their dopaminergic activity. The cyclohexene substitution pattern was varied along with the length of the chain connecting the 1,2,3,6-tetrahydro-4-phenylpyridine to the cyclohexene. Compound 14, having the 1,3-substitution pattern and a single methylene chain, was the most potent. The 1,2,3,6-tetrahydro-4-phenylpyridine could be replaced by other aryl-cyclic amines with a slight loss in activity. The phenyl group on the cyclohexene ring could be para substituted; electron-donating groups were better tolerated than electron-withdrawing groups. Finally, the enantiomers of 14 were resolved via the 1,1'-binaphthyl-2,2'-diyl hydrogen phosphate salts. Although both isomers were partial DA agonists, the (+)-enantiomer had higher intrinsic activity than the (-)-enantiomer. Syntheses were developed that allowed rapid preparation of analogues. An X-ray crystal structure determination of an intermediate identified the (+)-isomer of 14 as having R configuration. This compound, designated CI-1007 (PD 143188), was found to have antipsychotic-like activity in behavioral tests; in particular, it was orally active in the conditioned avoidance test in squirrel monkeys with an ED50 of 0.6 mg/kg. The overall profile suggests that (R)-(+)-14 may be a clinically useful antipsychotic agent.

Pharmacologic/pharmacokinetic evaluation of emesis induced by analogs of RSU 1069 and its control by antiemetic agents.

RB 6145, the ring-opened analog of RSU 1069, and PD 130908, the desoxy ring-opened analog of RSU 1069, were compared to RSU 1069 for their emetic potential in dogs. When RB 6145 and PD 130908 were administered intravenously at doses ranging from 20% to 50% of the mouse equivalent maximum tolerated dose (MTD), both analogs were less emetic than RSU 1069 on a molar basis. Furthermore, the 5HT3 antagonist ondansetron prevented emesis at doses as high as 75% of the MTD. The reactivity, and hence the emetic liability of these compounds, is thought to be mediated by formation of the corresponding aziridine intermediate. In mouse plasma, both analogs rapidly converted to two products, the reactive aziridine and a stable oxiazolidinone species formed upon reaction with bicarbonate in the blood. A positive correlation exists between the amounts of aziridine formed by these analogs and their emetic potential.

Dopamine autoreceptor agonists as potential antipsychotics. 2. (Aminoalkoxy)-4H-1-benzopyran-4-ones.

The synthesis and pharmacological properties of a novel type of [(arylpiperazinyl)alkoxy]-4H-1-benzopyran-4-ones with dopaminergic activity are described. The nature of the arylpiperazine (AP) moiety determines the dopamine (DA) agonist/antagonist character of this series of compounds; when the aryl portion of the AP is unsubstituted the compounds appear to be DA autoreceptor agonists while substituted aryl groups seem to impart DA antagonist activity. A heterocyclic piperazine, 7-[3-[4-(2-pyridinyl)-1-piperazinyl]propoxy]-4H-1-benzopyran-4-one (31, PD 119819) has been identified as an extremely selective DA autoreceptor agonist in tests that include [3H]haloperidol binding, inhibition of spontaneous locomotor activity, inhibition of brain DA synthesis, inhibition of brain DA neuronal firing, stereotypy assessment, and reversal of 6-hydroxydopamine (6-OHDA) induced akinesia in rats. In addition, 31 possesses good oral activity in the Sidman avoidance test in squirrel monkeys, a predictor of clinical antipsychotic efficacy. In another primate model, 31 has been found to lack the liability for extrapyramidal side effects observed with currently available antipsychotic drugs.

Synthesis and dopamine agonist properties of (+-)-trans-3,4,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano [4,3-b]-1,4-oxazin-9-ol and its enantiomers.

The dopamine agonist profile of (+-)-trans-3,4,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano [4,3-b]-1,4-oxazin-9-ol (16a) and its enantiomers (16b-c) was examined. Racemic 16a exhibited moderate affinity for the dopamine (DA) D2 receptor labeled with the DA antagonist ligand [3H]haloperidol and moderate in vivo activity; it attenuated gamma-butyrolactone-stimulated DA synthesis, decreased DA neuronal firing of substantia nigra DA neurons, and inhibited exploratory locomotor activity in rats, a profile consistent with a DA autoreceptor agonist mechanism of action. The (+)-enantiomer 16b possessed greater DA receptor affinity with the agonist ligand [3H]-N-propylnorapomorphine than with the antagonist ligand. In rats it potently inhibited DA synthesis and neuronal firing and also inhibited exploratory locomotion. The (-)-enantiomer, on the other hand, did not have significant activity in any of these tests. This profile indicates that like many other rigid DA agonists, the dopaminergic activity resides in one enantiomer, in this case the (+)-enantiomer 16b. On the basis of single-crystal X-ray analysis of a key intermediate, the absolute configuration of 16b was found to be 4aR, 10bR.

Comparison of the behavioral effects of adenosine agonists and dopamine antagonists in mice.

The adenosine agonists 5'-N-ethylcarboxamideadenosine (NECA), 2-chloroadenosine (2-CLA), N6-cyclohexyladenosine (CHA), N6-cyclopentyladenosine (CPA), 2-(phenylamino)adenosine (CV-1808) and R and S isomers of N6-phenylisopropyladenosine (R-PIA and S-PIA) decreased spontaneous locomotor activity in mice and, except for CPA, did so at doses that did not impair motor coordination, a profile shared by dopamine antagonists. CV-1808, the only agent with higher affinity for A2 as compared with A1 adenosine receptors, displayed the largest separation between locomotor inhibitory and ataxic potency. Like dopamine antagonists, NECA and CV-1808 also decreased hyperactivity caused by d--amphetamine at doses that did not cause ataxia whereas A1-selective adenosine agonists reduced amphetamine's effects only at ataxic doses. Unlike dopamine antagonists, adenosine agonists inhibited apomorphine-induced cage climbing only at doses that caused ataxia. Involvement of central adenosine receptors in these effects was suggested by the significant correlation obtained between potency for locomotor inhibition after IP and ICV administration. Affinity for A1 but not A2 adenosine receptors was significantly correlated with potency for inducing ataxia. These results suggest that the behavioral profile of adenosine agonists in mice is related to their affinity for A1 and A2 adenosine receptors and indicate that adenosine agonists produce certain behavioral effects that are similar to those seen with dopamine antagonists.

6- and 8-hydroxy-3,4-dihydro-3-(dipropylamino)-2H-1-benzopyrans. Dopamine agonists with autoreceptor selectivity.

The dopamine agonist profiles of 3,4-dihydro-3-(3-dipropylamino)-2H-1-benzopyran-6- and -8-ol (4 and 5, respectively) were examined. Both 4 and 5 exhibited greater relative affinity for receptors labeled with the dopamine agonist ligand [3H]propylnorapomorphine than for those labeled with the dopamine antagonist ligand [3H]haloperidol. Both compounds attenuated the stimulation of brain dopamine synthesis caused by gamma-butyrolactone (GBL) and decreased the firing rate of substantia nigra dopamine neurons in rats. This profile of activity, together with the ability of the dopamine antagonist haloperidol to reverse the inhibition of dopamine neuronal firing, indicate that both compounds are brain dopamine agonists.

Ribose-modified adenosine analogues as adenosine receptor agonists.

Analogues of the potent adenosine receptor agonist (R)-N-(1-methyl-2-phenylethyl)adenosine (R-PIA), modified at N9, were prepared and evaluated for adenosine A1 and A2 receptor binding and in vivo central nervous system and cardiovascular effects. The modifications at N9 include deoxy sugars, 5'-substituted-5'-deoxyriboses, non-ribose sugars, sugar ring homologues, and acyclic sugar analogues. Most of the derivatives have poor affinity for adenosine receptors. Only minor modifications at C5' and C3' maintain potent binding. In general, those derivatives exhibiting in vivo behavioral or cardiovascular effects also have the highest affinity for adenosine receptors.