title: 19-substituted benzoquinone ansamycin heat shock protein-90 inhibitors: biological activity and decreased off-target toxicity.
abstract: The benzoquinone ansamycins (BQAs) are a valuable class of antitumor agents that serve as inhibitors of heat shock protein (Hsp)-90. However, clinical use of BQAs has resulted in off-target toxicities, including concerns of hepatotoxicity. Mechanisms underlying the toxicity of quinones include their ability to redox cycle and/or arylate cellular nucleophiles. We have therefore designed 19-substituted BQAs to prevent glutathione conjugation and nonspecific interactions with protein thiols to minimize off-target effects and reduce hepatotoxicity. 19-Phenyl- and 19-methyl-substituted versions of geldanamycin and its derivatives, 17-allylamino-17-demethoxygeldanamycin and 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin (17-DMAG), did not react with glutathione, whereas marked reactivity was observed using parent BQAs. Importantly, although 17-DMAG induced cell death in primary and cultured mouse hepatocytes, 19-phenyl and 19-methyl DMAG showed reduced toxicity, validating the overall approach. Furthermore, our data suggest that arylation reactions, rather than redox cycling, are a major mechanism contributing to BQA hepatotoxicity. 19-Phenyl BQAs inhibited purified Hsp90 in a
  quinone oxidoreductase 1 (NQO1)-dependent manner, demonstrating increased efficacy of the hydroquinone ansamycin relative to its parent quinone. Molecular modeling supported increased stability of the hydroquinone form of 19-phenyl-DMAG in the active site of human Hsp90. In human breast cancer cells, 19-phenyl BQAs induced growth inhibition also dependent upon metabolism via NQO1 with decreased expression of client proteins and compensatory induction of Hsp70. These data demonstrate that 19-substituted BQAs are unreactive with thiols, display reduced hepatotoxicity, and retain Hsp90 and growth-inhibitory activity in human breast cancer cells, although with diminished potency relative to parent BQAs.
authors.affiliation: Department of Pharmaceutical Sciences, Skaggs School of Pharmacy and Pharmaceutical Sciences, University of Colorado Anschutz Medical Campus, Aurora, Colorado (C.-H.C., D.A.D., D.S., Q.Y., D.S.B., C.J., D.R.); and School of Chemistry, University of Nottingham, Nottingham, United Kingdom (R.R.A.K., C.J.M.).
authors.forename: Chuan-Hsin
authors.initials: CH
authors.lastname: Chang
authors.forename: Derek A
authors.initials: DA
authors.lastname: Drechsel
authors.forename: Russell R A
authors.initials: RR
authors.lastname: Kitson
authors.forename: David
authors.initials: D
authors.lastname: Siegel
authors.forename: Qiang
authors.initials: Q
authors.lastname: You
authors.forename: Donald S
authors.initials: DS
authors.lastname: Backos
authors.forename: Cynthia
authors.initials: C
authors.lastname: Ju
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Moody
authors.forename: David
authors.initials: D
authors.lastname: Ross
mesh_terms: D000818:Animals; D014408:Biomarkers, Tumor / Q000378:metabolism; D001943:Breast Neoplasms / Q000378:metabolism / Q000473:pathology; D045744:Cell Line, Tumor; D002478:Cells, Cultured; D005978:Glutathione / Q000737:chemistry; D018841:HSP90 Heat-Shock Proteins / Q000037:antagonists & inhibitors*; D006801:Humans; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D017828:Rifabutin / Q000737:chemistry / Q000494:pharmacology*
chemical_list: D014408:Biomarkers, Tumor; D018841:HSP90 Heat-Shock Proteins; D017828:Rifabutin; D005978:Glutathione