Inhibition of Hsp90 with Synthetic Macrolactones: Synthesis and Structural and Biological Evaluation of Ring and Conformational Analogs of Radicicol

Abstract

A series of benzo-macrolactones of varying ring size and conformation has been prepared by chemical synthesis and evaluated by structural and biological techniques. Thus, 12- to 16-membered lactones were obtained by concise routes, involving ring-closing metathesis as a key step. In enzyme assays, the 13-, 15-, and 16-membered analogs are good inhibitors, suggesting that they can adopt the required conformation to fit in the ATP-binding site. This was confirmed by cocrystallization of 13-, 14-, and 15-membered lactones with the N-terminal domain of yeast Hsp90, showing that they bind similarly to the "natural" 14-membered radicicol. The most active compounds in the ATPase assays also showed the greatest growth-inhibitory potency in HCT116 human colon cancer cells and the established molecular signature of Hsp90 inhibition, i.e., depletion of client proteins with upregulation of Hsp70.

Synthesis of macrolactam analogues of radicicol and their binding to heat shock protein Hsp90

“…show that there is a significant enthalpic penalty compared to radicicol, although the binding of the macrolactams was found to be superior to the macrolactone analogues we have previously described 33 Nevertheless the data suggest that the interaction of the epoxide plays a significant role in the interaction with the protein. Electron density contoured at 1.5σ for compound 27b and 27d was lacking for carbon atoms 4 to 9 of the macrocycle ring.…”

“…Additionally, the epoxide has been replaced with a thiirane, 32 a cyclic carbonate, 32 a diol 32 and a carbon chain (both saturated and unsaturated). 33 Whilst the former three are significantly detrimental to the potency, a carbon chain is tolerated with only a two-fold drop in performance. 33 The replacement of the dienone with more stable functionalities has also been investigated.…”

“…33 Whilst the former three are significantly detrimental to the potency, a carbon chain is tolerated with only a two-fold drop in performance. 33 The replacement of the dienone with more stable functionalities has also been investigated. Moody and Shinonaga have developed radicicol analogues with a C11 ketone and a either a saturated carbon chain from C7-C10 or an enone (4 and 5, respectively).…”

“…Moody and Shinonaga have developed radicicol analogues with a C11 ketone and a either a saturated carbon chain from C7-C10 or an enone (4 and 5, respectively). [32][33][34] Additionally, Danishefsky has replaced the dienone of cycloproparadicicol with a triazole 6 35 and we have also incorporated a triazole into the macrocycle (compound 7) to investigate the benefits of additional H-bonding potential. 36 Unfortunately, it appears that the conformational constraints of RALs is important for binding to the Hsp90 N-terminal domain and, as such, all of the analogues showed a significant drop in activity compared to radicicol itself, although potencies commensurate with other important Hsp90 inhibitors were achievable.…”

Discovery and validation of small-molecule heat-shock protein 90 inhibitors through multimodality molecular imaging in living subjects

“…Geldanamycin-based Hsp90 inhibitors (8), purine scaffold-(9-16), pyrazole scaffold- (17), and radicicol-based Hsp90 inhibitors (18,19), and other compounds are currently in preclinical and/or phase I/II clinical trials (20)(21)(22). Despite the diversity of Hsp90 inhibitors reported, inhibitors with better clinical therapeutic efficacy and reduced toxicity are needed.…”