SYNTHESIS OF WILLARDIINE AND 6-AZAWILLARDIINE ANALOGS - PHARMACOLOGICAL CHARACTERIZATION ON CLONED HOMOMERIC HUMAN AMPA AND KAINATE RECEPTOR SUBTYPES

Both willardiine and azawillardiine analogs (18-28) have been reported to be potent and selective agonists for either AMPA or kainate recept ors. We report here the novel synthesis and pharmacological characteri zation of a range of willardiine (18-23) and 6-azawillardiine (24-28) analogs on cells individually expressing human homomeric hGluR1, hGluR 2, hGluR4, or hGluR5 receptors. Reaction of the sodium salts of substi tuted uracils (7-12) or 6-azauracils (13-16) with (S)-3-[(tert-butoxyc arbonyl)amino]oxetan-2-one (17) in dry DMF, subsequent deprotection in TFA, and purification by ion-exchange chromatography gave mainly the willardiine analog in which alkylation took place on N1 of the uracil ring. We have investigated the subtype selectivity of these compounds by examining their binding affinity for homomeric hGluR1, -2, -4, or - 5 (and hGluR6 in the case of 5-iodowillardiine (22)). From this study we have demonstrated that 22 has high affinity for hGluR5 and, compare d to kainate, displays excellent selectivity for this receptor over bo th the AMPA receptor subtypes and the homomeric kainate receptor, hGlu R6. 5-Fluorowillardiine (19) has higher affinity than AMPA for both ho momeric hGluR1 and hGluR2 and compared to AMPA displays greater select ivity for AMPA receptor subtypes over the kainate receptor, hGluR5. So me structural features required for optimal activity at homomeric AMPA or kainate receptor subtypes have also been identified. It would appe ar that quite large lipophilic substituents at the 5-position of the u racil ring not only are accommodated by hGluR5 receptors but also lead to enhanced affinity for these receptors. In contrast to this, for op timal binding affinity to hGluR1, -2, or -4, smaller, electron-withdra wing substituents are required. For optimal activity at hGluR4 recepto rs a 6-azasubstituted willardiine is favored. The subtype-selective co mpounds described here are likely to be useful tools to probe the dist ribution and the physiological roles of the various glutamate receptor subunits in the central nervous system.