Combining the rapid MTT formazan exocytosis assay and the MC65 protection assay led to the discovery of carbazole analogs as small molecule inhibitors of Aβ oligomer-induced cytotoxicity

The discovery of small molecule inhibitors of cytotoxicity induced by amyloid-β (Aβ) oligomers, either applied extracellularly or accumulated intraneuronally, is an important goal of drug development for Alzheimer's disease (AD), but has been limited by the lack of efficient screening methods. Here we describe our approach using two cell-based methods. The first method takes advantage of the unique ability of extracellularly applied Aβ oligomers to rapidly induce the exocytosis of formazan formed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). We employed a short protocol to quantify this toxicity, and quickly identified two novel inhibitors, code-named CP2 and A5, from two compound libraries. A second independent screen of the same libraries using our previously published MC65 protection assay, which identifies inhibitors of toxicity related to intracellular Aβ oligomers, also selected the same two leads, suggesting that both assays select for the same anti-Aβ oligomer properties displayed by these compounds. We further demonstrated that A5 attenuated the progressive aggregation of existing Aβ oligomers, reduced the level of intracellular Aβ oligomers, and prevented the Aβ oligomer-induced death of primary cortical neurons, effects similar to those demonstrated by CP2. Our results suggest that, when combined, the two methods would generate fewer false results and give a high likelihood of identifying leads that show promises in ameliorating Aβ oligomer-induced toxicities within both intraneuronal and extracellular sites. Both assays are simple, suitable for rapid screening of a large number of medicinal libraries, and amenable for automation.