Hsp90 inhibitors: Biosynthetic and antitumor evaluation of a non-geldanamycin polyketide

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Heat-shock protein 90 (Hsp90) is an ATP-dependent molecular chaperone that is required for stability and function of a large number of proteins, many of which are necessary for cancer cell growth and survival. Inhibition of Hsp90 causes dysfunction and degradation of these client proteins and therefore tumor inhibition. Many Hsp90 inhibitors, e.g. geldanamycin and radicicol, belong to a group of natural products called polyketides. These are natural products biosynthesized by microorganisms by the polymerization of acetyl and propionyl subunits. Early polyketide Hsp90 inhibitors suffer from chemical instability and narrow therapeutic windows. For example, geldanamycin was found to be hepatotoxic in several species and was not soluble in aqueous media. A geldanamycin derivative 17-AAG has improved therapeutic window over geldanamycin but the aqueous solubility of 17-AAG is still poor and poses a serious challenge for further development. A further derivative with increased water solubility 17-DMAG is now in clinical trials. We are therefore interested in developing water-soluble and non-geldanamycin based polyketides as potent Hsp90 inhibitors. We have identified a naturally occurring non-geldanamycin polyketide BC226 as an Hsp90 inhibitor. BC226 has slightly higher potency than geldanamycin in inhibiting yeast Hsp90 ATPase activity (IC₅₀ 2 μM vs 7 μM) and tighter binding to the protein than geldanamycin based on an ITC measurement (Kd 0.24 M^-1 vs 1.2 M^-1). BC226 inhibits tumor cell growth by degrading relevant client proteins of Hsp90. Inhibition of DU145 prostate cancer cell line by BC226 causes degradation of Craf1 and erbB2. BC226 also has potent in vivo antitumor activities. At 10 mg/kg, i.p., BC226 significantly (p<0.01; U-test) reduced tumor growth rates (minimum T/C value: 32%) in DU145 mouse xenograft. We have cloned and elucidated the gene cluster sequence for BC226 biosynthesis. Using this information, we have generated genetically engineered mutant strains which produce several novel polyketides. Furthermore, semi-synthetic derivatization of BC226 has generated novel derivatives with markedly increased water solubility and tolerability. We will present preclinical data on this series of novel polyketides, as well as the elucidation of the biosynthetic gene cluster responsible for producing the polyketide. Further exploration of the chemical space and structure-activity-relationship studies will also be presented.