Cascade Transformation of the Ansamycin Benzoquinone Core into Benzoxazole Influencing Anticancer Activity and Selectivity.

The metal-free cascade transformation of geldanamycin benzoquinone core is proposed at relatively mild conditions. This approach yields new benzoxazole ansamycin antibiotics and enables their functionalization in an atom-economic manner, irrespective of the type of amine used. The analysis of the heterocyclization course reveals the dependence of its rate on the nature of the para-substituent within the benzylamine moiety (EDG/EWG) and the strength of the base. The reduction of the ansamycin core enables an increase in anticancer potency and selectivity.

Modifications of geldanamycin via CuAAC altering affinity to chaperone protein Hsp90 and cytotoxicity.

Functionalization of alkyne (1) and azide (2) derivatives of geldanamycin (GDM) via dipolar cycloaddition CuAAC yielded 35 new congeners (3-37) with C(17)-triazole arms bearing caps of different nature (basic vs. acidic, hydrophilic vs. hydrophobic). Confrontation of biological data (anticancer activity vs. toxicity in normal cells) with lipophilicity (clogP), dissociation constants (Kd) of complexes with Hsp90 and binding modes to Hsp90 revealed SAR in specific subgroups of GDM derivatives. The most potent GDM congeners 14-16, bearing C(17)-triazole-benzyl-halogen arms exhibited the most optimal clogP values of 2.7-3.1 at favourable binding to Hsp90 (KdHsp90 at µM level). The anticancer activity of 14-16 (IC50 = 0.23-0.41 µM) is higher than those of GDM (IC50 = 0.58-0.64 µM) and actinomycin D (ActD, IC50 = 0.62-0.71 µM) in SKBR-3, SKOV-3 and PC-3 cell lines, with a comparable cytotoxicity in healthy cells. The relationship between structure and attractive anticancer potency (IC50 = 0.53-0.74 µM) is also observed for congeners with C(17)-triazole-saccharide or C(17)-triazole-unsaturated arms. In the former, the absolute configuration at C(4) (ᴅ-glucose vs. ᴅ-galactose) whereas in the latter the length of the unsaturated arm influences the cytotoxic effects due to different binding strength (Kd, ΔE) and modes with Hsp90. Among all triazole congeners of GDM that are biologically attractive and exhibit lower toxicity in normal cells than GDM and ActD, the derivative 22, bearing the C(17)-triazole-cinnamyl arm, shows the lowest Kd (Hsp90), optimal clogP = 2.82, the best pro-apoptotic properties in SKBR-3 and SKOV-3 and the best selectivity indices (SI). For the most potent GDM derivatives with C(17)-triazole arm, the docking studies have suggested the importance of the intermolecular stabilization between the arm and the D57 or Y61 of Hsp90.