Heat-shock proteins (Hsp) are ubiquitous molecules in the cell which increases their expression when exposed to elevated temperature or oxidative stress by reactive oxygen species. The main role of these proteins, particularly of the heat shock protein 90 (Hsp90) family is to maintain the native conformation of proteins in the cell. Since cancer cells possess many conformationally unstable proteins that are critical for their survival and malignancy, they tend to utilize the chaperone function of Hsp90 even more than healthy cells. For this reason, Hsp90 inhibitors represent potential antitumor drugs with researchers increasingly focusing on modulating protein function by interacting with the C-terminal domain of Hsp90. Therefore, the aim of this Master's thesis was to synthesize and optimize Hsp90 C-terminal inhibitors bearing the 2,6-diamino-4,5,6,7-tetrahydrobenzothiazole core.
Two libraries of compounds (A and B) were designed with the aim of investigating the structure-activity relationships (SAR) of these ligands. Library A focused primarily on optimizing the amine-bearing moiety attached to position C-2 of the tetrahydrobenzothiazole scaffold. By changing the length of the spacer and introducing chiral centers to the central scaffold we aimed primarily to determine which position of the cationic center will give the strongest inhibitory activity. In library B, on the other hand, the introduction of different aromatic systems to position C-6 of the 4,5,6,7-tetrahydrobenzothiazole moiety was to be compared in order to obtain information on the size of the most potent substituents and to allow additional interactions with the residues of the binding site.
Five final compounds were evaluated for their antiproliferative activity in MCF-7 breast cancer cell lines. Comparison of the IC50 values of compounds 12c and 12a confirmed that the aromatic system at position C-6 is a mandatory requirement for activity. Compound 12b showed that there is an additional possibility to form hydrogen bonds in the active site. Again, the bonding is significantly affected when the aromatic ring is substituted. At the C-2 position of the tetrahydrobenzothiazole ring, both the size and spatial orientation of the amine moiety must be considered.
Of all the compounds tested, compound 7f showed the lowest IC50 value of 17 μM, which is still too high for a potential antitumor agent. The cytotoxic evaluation of the remaining six synthesized compounds 7a-e and 7h would help to clarify the effects of adding an aromatic ring to the amine moiety bound at position C-2 and give a rough estimate of the optimal length of the spacer between the amine of interest and the tetrahydrobenzothiazole scaffold. With these results this Master's thesis could provide even more key information on the structure-activity relationship of 4,5,6,7-tetrahydrobenzothiazole Hsp90 C-terminal domain modulators.