Studies of the Biogenic Amine Transporters 15. Identification of Novel Allosteric Dopamine Transporter Ligands with Nanomolar Potency s

Novel allosteric modulators of the dopamine transporter (DAT) have been identified. We have shown previously that SRI-9804 [N-(diphenylmethyl)-2-phenyl-4-quinazolinamine], SRI-20040 [N-(2,2-diphenylethyl)-2-phenyl-4-quinazolinamine], and SRI-20041 [N-(3,3-diphenylpropyl)-2-phenyl-4-quinazolinamine] partially inhibit [I]RTI-55 ([I]3b-(49-iodophenyl)tropan-2b-carboxylic acid methyl ester) binding and [H]dopamine ([H]DA) uptake, slow the dissociation rate of [I]RTI-55 from the DAT, and allosterically modulate d-amphetamine–induced, DAT-mediated DA release. We synthesized and evaluated the activity of .500 analogs of these ligands and report here on 36 selected compounds. Using synaptosomes prepared from rat caudate, we conducted [H]DA uptake inhibition assays, DAT binding assays with [H]WIN35428 ([H]2b-carbomethoxy-3b-(4-fluorophenyl)tropane), and DATmediated release assays with either [H]MPP ([H]1-methyl-4phenylpyridinium) or [H]DA. We observed three groups of [H]DA uptake inhibitors: 1) full-efficacy agents with a one-site fit, 2) fullefficacy agents with a two-site fit, and 3) partial-efficacy agents with a one-site fit—the focus of further studies. These agents partially inhibited DA, serotonin, and norepinephrine uptake, yet were much less potent at inhibiting [H]WIN35428 binding to the DAT. For example, SRI-29574 [N-(2,2-diphenylethyl)-2-(imidazo[1,2-a] pyridin-6-yl)quinazolin-4-amine] partially inhibited DAT uptake, with an IC505 2.36 0.4 nM, without affecting binding to the DAT. These agents did not alter DAT-mediated release of [H]MPP in the absence or presence of 100 nM d-amphetamine. SRI-29574 had no significant effect on the d-amphetamine EC50 orEmax value for DATmediated release of [H]MPP. These studies demonstrate the existence of potent DAT ligands that partially block [H]DA uptake, without affecting DAT binding or d-amphetamine–induced [H]MPP release. These compounds may prove to be useful probes of biogenic amine transporter function as well as novel therapeutics.