*4.1.1. Tau aggregation inhibitors*

The direct inhibition of tau aggregation was the major therapeutic strategy being developed and had entered the clinical trials [151]. The development of tau aggregation inhibitor was initiated in the mid-1990s. The first platform to screen the drugs was reported in 1996 with the discovery of phenothiazine, a relatively potent tau aggregation inhibitor in vitro [152]. In this platform, recombinant PHF core tau fragment was incubated in wells, by reciprocal treating the wells with recombinant full-length tau, the protease-resistant tau aggregation could form [152]. By incubating the wells with compounds, the goal was to identify inhibitors that could effectively disrupt tau aggregation through the high-throughput assay, and phenothiazine showed a strong potency [152]. Unfortunately, phenothiazine was found no efficacy in clinical trials, and it was blamed for its poor absorption and was difficult to be transported into the brain [153]. Years later, a renewed platform was designed [153]. In this platform, fibroblasts overexpressing a cocktail of different tau isoforms were incubated in wells. This setting could yield tau aggregations inside the cells, and compounds were tested to compare the labeled tau immunofluorescence as the readout [153]. Although this platform was a lower throughput, it overcame the shortages of the first platform in which cytotoxicity was unknown [153, 154]. With this platform, TRx0237 was later selected to be the lead candidate and went into clinical trials [151]. Unfortunately, the drug did not work in phase 3 as it failed to slow cognitive decline in AD patients [155].

Biochemically, the inhibitors could be categorized into two types, covalently and no-covalently tau aggregation inhibitors [156]. However, although they are called "tau" inhibitors, these chemicals are most likely inhibitors to other protein aggregations, and their selectivity is highly questionable [151]. Therefore, a highly selective with high-affinity tau inhibitor is still waiting to be discovered [151, 156].
