Identification of Novel HSP90α/β Isoform Selective Inhibitors Using Structure-Based Drug Design. Demonstration of Potential Utility in Treating CNS Disorders such as Huntington’s Disease

Abstract

A structure-based drug design strategy was used to optimize a novel benzolactam series of HSP90α/β inhibitors to achieve >1000-fold selectivity versus the HSP90 endoplasmic reticulum and mitochondrial isoforms (GRP94 and TRAP1, respectively). Selective HSP90α/β inhibitors were found to be equipotent to pan-HSP90 inhibitors in promoting the clearance of mutant huntingtin protein (mHtt) in vitro, however with less cellular toxicity. Improved tolerability profiles may enable the use of HSP90α/β selective inhibitors in treating chronic neurodegenerative indications such as Huntington's disease (HD). A potent, selective, orally available HSP90α/β inhibitor was identified (compound 31) that crosses the blood-brain barrier. Compound 31 demonstrated proof of concept by successfully reducing brain Htt levels following oral dosing in rats.

Selective targeting of the stress chaperome as a therapeutic strategy

“…While these ligands bind with comparable affinity to the four HSP90 paralogs [85], conformational differences in the HSP90s have recently enabled the discovery of ligands selective for individual paralogs (Figure 2B,C) [32, 86, 87]. Such chemical tools with a high degree of selectivity for HSP90α/β [86, 88] and GRP94 [32] may pave the way for a better understanding of the biological role of these paralogs in specific disease contexts, as they enable parsing out their function in the context of an un-engineered human cell [32].…”

“…Such chemical tools with a high degree of selectivity for HSP90α/β [86, 88] and GRP94 [32] may pave the way for a better understanding of the biological role of these paralogs in specific disease contexts, as they enable parsing out their function in the context of an un-engineered human cell [32]. They may also enable the identification of disease states where inhibition of a specific HSP90 paralog may be more therapeutically advantageous than inhibition of all HSP90s.…”

“…For example, this can manifest at the local level, such as capturing changes in the structure of the protein by rearrangements of the protein in a particular domain, or more major in nature such as reflected in the major conformations noted for the HSP90s and HSP70s. Ligand-induced or captured local changes are numerous and best understood for the HSP90s and HSP70s [32, 85, 86, 100, 101]. For example, the N-terminal domain pan-HSP90 inhibitors insert themselves into the same pocket (Figure 2A).…”

Proteostasis in Huntington’s Disease: Disease Mechanisms and Therapeutic Opportunities

“…17-DMAG and 17-AAG, less toxic geldanamycin derivatives with more favourable pharmacokinetic properties, were also shown to induce expression of molecular chaperones and inhibit polyQ expanded HTT aggregation in cells and Drosophila models of HD respectively 118,120 as well as to cross the blood-brain barrier in Alzheimers models 121 . HSP90 isoformspecific inhibitors were shown to have improved tolerability compared to pan-HSP90 inhibitors and were also shown to be orally available, cross the blood-brain barrier and reduce HTT levels in rat brains 122 . Celastrol, a natural product anti-inflammatory agent, binds the C-terminal domain of HSP90…”

Hsp90 as a therapeutic target in endocrinology: current evidence

“…Thus, an 8-residue amino acid sequence present within the N-terminal binding pocket of Hsp90α and β, which differs in the first two amino acids of the corresponding sequence in both GRP94 and TRAP1, contributed to the stability of chemotype-induced conformations of the Hsp90α and β isoforms, resulting in a higher level of Hsp90α/β selectivity compared with GRP94 and TRAP1 (.1,000-fold). 74,115,116 Similarly, insertion of several purine-scaffold class chemical compounds into a new allosteric pocket, arising from the conformational flexibility of GRP94, led to the development of the purine-based ligands, PU-H54 and PU-WS13, that were .100-fold more selective for GRP94 over Hsp90α/β and TRAP1. 74,117 Another GRP94-specific ligand, NECA …”