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CGP 4718 A (4-[5-chloro-benzofuranyl-2-]-1-methylpiperidine HCl) was found to inhibit MAO A preferentially in vitro in a competitive manner. Assessment of its in vivo effects by an ex vivo approach showed it to be a relatively weak, reversible inhibitor of MAO A. There were also effects on MAO B but they were inferior by a factor of about 10. The onset of the inhibitory effects in rat liver and brain was rapid, being maximal in about 1 h following administration of CGP 4718 A p.o. The inhibition was of relatively short duration with the effects being undetectable 24 h after treatment. CGP 4718 A also inhibited the reuptake of serotonin (5-HT) in synaptosomes in vitro and ex vivo. Evidence for 5-HT uptake inhibition was also found by using the H 75/12 depletor model. Its in vitro and in vivo potency as a 5-HT uptake inhibitor was approximately the same as that of imipramine. The effects on MAO A and on 5-HT uptake occurred over a similar dose range (above 10 mg/kg p.o.) and also had a similar time course. No evidence for inhibitory effects on noradrenaline uptake was found in vivo.

CGP 11305 A [4-(5-methoxy-7-bromo-benzofuranyl-2)piperidine HCl), a reversible, selective inhibitor of MAO A, increased the levels of rat brain noradrenaline, dopamine, and serotonin dose-dependently and to approximately the same extent. Concomitantly, it lowered the levels of their metabolites, MHPG-SO4, HVA, DOPAC, and 5-HIAA. When compared with the irreversible MAO A inhibitor clorgyline, the effects of CGP 11305 A were of much shorter duration. Moreover, the increases of noradrenaline and serotonin and the decreases of their metabolites MHPG-SO4 and 5-HIAA were smaller after CGP 11305 A than after clorgyline in equieffective doses for MAO A inhibition. CGP 11305 A decreased the synthesis of catecholamines and serotonin less markedly than clorgyline. This is probably due to the reversibility of the interaction of the compound with MAO A. In contrast to CGP 11305 A, clorgyline increased the level of dopamine less than those of noradrenaline and serotonin. This is explained by assuming that dopamine synthesis is particularly sensitive to end product inhibition. CGP 11305 A also exhibited some inhibitory properties on the uptake of serotonin and, to a lesser degree, of noradrenaline in vitro and in vivo. Compared with MAO inhibition, however, uptake inhibition required 30-100 times higher doses.

Ifoxetine (CGP 15210 G; (+/-)-bis-[cis-3-hydroxy-4-(2,3-dimethyl-phenoxy)]-piperidine sulfate) prevented the depletion of serotonin (5-HT) induced by H 75/12 and p-chloromethamphetamine in the rat brain, and that caused by endogenously released dopamine after the combined administration of haloperidol and amfonelic acid in the rat striatum. These effects are typically caused by compounds that inhibit 5-HT reuptake. Unexpectedly, ifoxetine only weakly inhibited the uptake of radiolabelled 5-HT into rat brain synaptosomes in vitro or ex vivo, the human thrombocytes in vitro or into rat thrombocytes after pretreatment. The following, among the possible explanations for this apparent discrepancy, were considered and regarded as unlikely: the involvement of an active metabolite; the possibility that ifoxetine accumulates in the brain to an extent sufficient to cause in vivo uptake inhibition; a pharmacokinetic interaction with the depleting agents. The possibility that the depletor tests give false positives was also considered. However, ifoxetine lowered brain 5-hydroxyindoleacetic acid and reduced the accumulation of 5-hydroxytryptophan after central decarboxylase inhibition. This suggests that it also interferes with 5-HT metabolism in the absence of depleting agents, which means that it interacts in some way with serotonergic transmission. Ifoxetine displayed weak or no interactions with 5-HT1, 5-HT2, alpha 1-, alpha 2- and beta-noradrenoceptors, histamine H1, muscarinic acetylcholine, opiate, GABA A, and benzodiazepine receptors in vitro, and with dopamine and 5-HT2 receptors in vivo. It did not antagonize the noradrenaline (NA) depletion induced by H 77/77 in rat brain and only weakly interfered with the uptake of i.v. injected radiolabelled NA into the rat heart. This suggests that its interaction with the 5-HT system is specific. Due to its atypical properties, among which the rather weak potentiation of the neurological effects of 5-hydroxytryptophan is also important, ifoxetine may exhibit a therapeutic and/or side-effect profile which differs from that of classical 5-HT uptake inhibitors.

The effects of two new compounds, 1-(1-methylamino-2-hydroxy-3-propyl)-dibenzo[b,e]bicyclo[2,2,2]octadiene-HCl (C 49802-B-Ba) and 4-(5,6-dimethyl-2-benzofuranyl) piperidine HCl (CGP 6085 A), on noradrenaline (NA) and serotonin (5-HT) uptake were investigated in different test systems, CGP 6085 A is a very potent and selective inhibitor of 5-HT uptake in rat brain (ED50 1-4 mg/kg p.o., depending on test system). Doses up to 1000 mg/kg p.o. did not inhibit NA uptake. C 49802-B-Ba is a potent and selective inhibitor of NA uptake in rat brain (ED50 5-10 mg/kg p.o. depending on test system) and heart (ED50 1.5 mg/kg p.o.). At 300 mg/kg p.o., this compound caused no inhibition of 5-HT uptake.