Pharmacological characterization of PD 118717, a putative piperazinyl benzopyranone dopamine autoreceptor agonist

PD 118717 (7-[3-[4-(2-pyrimidinyl)-1-piperazinyl]-propoxy]-2H-1- benzopyran-2-one sulfate) proved to be a dopamine (DA) D-2 autoreceptor agonist in biochemical and electrophysiological studies in rats and to exhibit an antipsychotic-like profile in behavioral tests in rodents and monkeys. In vitro binding studies indicated that PD 118717 bound selectively to DA D-2 vs. D-1 receptors and exhibited agonist binding properties (biphasic inhibitory curves and GTP shift) similar to DA. It also had significant affinity for serotonin-(5-HT)1A but not 5-HT1B and 5-HT2 receptors. PD 118717 was active in antagonizing the tau-butyrolactone-induced accumulation of dopa in rat striatum and mesolimbic regions. PD 118717 also depressed the firing of DA neurons in substantia nigra pars compacta of rats. In both of the latter tests the effects of PD 118717 were reversed by haloperidol. PD 118717 decreased brain DA metabolism, decreased DA utilization, decreased accumulation of dopa after inhibition of L-aromatic amino acid decarboxylase, stimulated serum corticosterone and inhibited stimulated serum prolactin levels. PD 118717 did not alter striatal acetylcholine levels; nor did it induce locomotor stimulation or stereotypy in normal animals, suggesting a lack of postsynaptic DA stimulation of normosensitive DA receptors. In tests designed to reveal even weak postsynaptic DA agonist effects, PD 118717 stimulated locomotor activity in 6-hydroxydopamine-lesioned animals and relatively higher doses induced a low degree of stereotyped behavior when combined with the DA D-1 agonist SKF 38393. PD 118717 decreased the accumulation of 5-hydroxytryptophan in brain, an effect probably due to an agonist action at 5-HT1A receptors. PD 118717 decreased spontaneous locomotor activity in rodents, antagonized amphetamine-stimulated hyperactivity in mice and inhibited Sidman avoidance in monkeys, effects seen with antipsychotic agents. Unlike DA antagonist antipsychotics, PD 118717 did not induce extrapyramidal dysfunction in monkeys. PD 118717 displayed behavioral activity after p.o. dosing and its effects did not show tolerance on repeated dosing. In conclusion, PD 118717 has the profile of a DA autoreceptor agonist in neurochemical and neurophysiological tests and produces effects suggestive of antipsychotic efficacy without neurological side effect liability in preclinical behavioral tests.