Discovery of novel peptidic dopamine transporter ligands by screening a positional scanning combinatorial hexapeptide library

The acute reinforcing effects of cocaine are thought by some to result from cocaine binding to the dopamine (DA) transporter, which inhibits DA uptake and increases synaptic DA levels in the mesolimbic system. Other data sugg est that neurotransmitters other than DA contribute to cocaine reinforcemen t and addiction. These considerations illustrate the need to have additiona l research tools with which to test the "DA hypothesis." One strategy is to identify drugs which bind to the DA transporter (DAT ligands) but which do not inhibit DA uptake as effectively as cocaine. The purpose of the presen t study was to identify members of a novel structural class of DAT ligands and to characterize their interactions at the DA transporter. A positional scanning hexapeptide D-amino acid library was screened for inhibition of [I -125]RTI-55 binding to rat caudate DA transporters. Based on the results, 1 2 peptides were synthesized. All 12 peptides inhibited [I-125]RTI-55 bindin g to DA transporters with IC50 values, which ranged from 1.8 mu M to 12 mu M The two most potent peptides (TPI-669-1 and TPI-669-4) were prepared in l arger quantities and were characterized further for activity at the DAT and 5-HT transporter. Both peptides inhibited DA and 5-HT uptake and transport er binding with IC50/K-i values in the low micromolar range. In vivo microd ialysis studies demonstrated that both peptides increase extracellular DA a nd 5-HT in the nucleus accumbens of rats. These data demonstrate that pepti des can function as inhibitors of biogenic amine transport. Future work wil l focus on developing more potent and selective peptides. Published 1999 Wi ley-Liss, Inc.dagger.