Abstract
Benzoquinone ansamycins (BQAs) were the first class of Hsp90 inhibitors developed but members of this class, particularly geldanamycin, demonstrated hepatic toxicity. One major mechanism of toxicity of the BQA class is manifested via the electrophilic properties of the quinone and arylation of cellular nucleophiles at the 19-position of the ansamycin ring. Using selective halogenation and palladium-catalyzed coupling, we have synthesized a number of novel 19-substituted BQAs in the geldanamycin, 17-AAG and 17-DMAG series as a means to prevent arylation of cellular nucleophiles and have validated this approach using model thiols including N-acetylcysteine and glutathione. 19-Substituted BQAs did not react with thiols at the 19-position while marked reactivity could be detected using their parent quinones. 19-Substituted BQAs were tested for their ability to inhibit recombinant yeast Hsp90 and 19-substitution did not block the capacity of these novel molecules to inhibit the ATPase activity of the Hsp90 chaperone. This result was confirmed by molecular modeling of 19-substituted derivatives in the active site of human Hsp90 demonstrating that 19-substitution did not block entry of the molecule into the active site. The addition of NAD(P)H:quinone oxidoreductase 1 (NQO1) potentiated inhibition of recombinant yeast Hsp90 by 19-substituted BQAs confirming our previous data demonstrating increased inhibitory efficacy of the hydroquinone ansamycin relative to its parent quinone. Cellular effects of 19-substituted BQAs were examined in MDA468 breast cancer cells and the isogenic MDA468/NQ16 cell line which over-expresses NQO1. Growth inhibitory effects were observed using 19-substituted BQAs and were potentiated by the presence of NQO1 in the MDA468/NQ16 line. Hsp90 inhibition in MDA468 and MDA468/NQ16 cells was confirmed using decreases in the Hsp90 client protein Raf1 and a compensatory increase in Hsp70 as biomarkers. In summary, these data demonstrate that 19-substituted BQAs do not react with thiols at the 19-position but retain their Hsp90 inhibitory capacity using purified enzyme and in cells suggesting that they should undergo further translational evaluation as therapeutic candidates (Supported by CA51210 and the Parkinsons Disease Society UK).
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B102.
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