One of the key pathological features of Alzheimer's disease is the aggregation of tau protein. We are therefore searching for compounds capable of inhibiting this reaction. On the basis of an initial screen of 200000 compounds [Pickhardt, M., Gazova, Z., von Bergen, M., Khlistunova, I., Wang, Y., Hascher, A., Mandelkow, E. M., Biernat, J., and Mandelkow, E. (2005) Anthraquinones inhibit tau aggregation and dissolve Alzheimer's paired helical filaments in vitro and in cells, J. Biol. Chem. 280, 3628−3635], we performed an in silico screen and predicted a new phenylthiazolyl-hydrazide (PTH) compound as a possible hit [Larbig, G., Pickhardt, M., Lloyd, D. G., Schmidt, B., and Mandelkow, E. (2007) Screening for inhibitors of tau protein aggregation into Alzheimer paired helical filaments:  A ligand based approach results in successful scaffold hopping. Curr. Alzheimer Res. 4 (3), 315−323.]. Synthesis of this compound showed that it was indeed active in terms of inhibiting de novo tau aggregation and disassembling preformed aggregates (IC₅₀ = 7.7 μM and DC₅₀ = 10.8 μM). We have now synthesized 49 similar structures and identified the core of the PTHs to be crucial for activity, thus representing a lead structure. Analysis of the binding epitope by saturation transfer difference NMR shows strong interactions between the tau protein and the ligand in the aromatic regions of the inhibitor. By chemical variation of the core, we improved the inhibitory potency five-fold. The compounds showed a low toxicity as judged by an N2A cell model of tau aggregation and lend themselves for further development.