Stability of the Hsp90 inhibitor 17AAG hydroquinone and prevention of metal-catalyzed oxidation

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Hsp90 is a chaperone protein which promotes the folding and stability of a number of oncogenic proteins and therefore many oncogenic signals can be blocked simultaneously by inhibiting Hsp90. 17-(allylamino)-17-demethoxygeldanamycin (17AAG) is a benzoquinone ansamycin Hsp90 inhibitor and has promising anticancer activity in vitro, in animal models and in clinical trials. 17AAG has poor water-solubility which is a potential problem for clinical formulation. The hydroquinone derivative of 17AAG, 17AAGH₂, is considerably more water soluble and previously we demonstrated that 17AAGH₂ was a more potent Hsp90 inhibitor than its parent quinone. However, 17AAGH₂ can be oxidized back to 17AAG under aerobic conditions so we tested the relative stability of 17AAGH₂ and the effect of different metal ions and metal chelators on the oxidation of 17AAGH₂. We found that copper could accelerate 17AAGH₂ oxidation while copper chelators such as penicillamine could inhibit oxidation. Human serum albumin (HSA) has copper-binding ability and we also found that HSA could prevent 17AAGH₂ oxidation. Sequestration of 17AAGH₂ via non-covalent association with HSA could also explain the slower rate of oxidation of 17AAGH₂ but we found that although 17AAG could associate with HSA, association was not observed with 17AAGH₂. In summary, our data demonstrates that copper chelators can prevent 17AAGH₂ oxidation and suggests that HSA prevents 17AAGH₂ oxidation via a copper chelation rather than a sequestration mechanism. Agents that prevent oxidation may be useful in clinical formulations of 17AAGH₂. (Supported by National Institute of Health grant R01 CA51210).