Drug Metabolism Determines Resistance of Colorectal Cancer to Resorcinol-Based HSP90 Inhibitors

Tumor cells are characterized by intrinsic proteotoxic stress and the accumulation of mutated proteins. Therefore, malignant cells depend on the activity of heat shock protein 90 (HSP90) much more than normal cells to maintain a functionally intact proteome. Thus, the inhibition of HSP90 by small molecules is currently being evaluated as a new approach to cancer therapy. One of the most promising drugs of this class is ganetespib, which is currently in intensive clinical trials. We analyzed the susceptibility of colorectal cancer (CRC) cell lines to ganetespib and recorded large differences in the cell line-specific drug concentrations required for 50% growth inhibition (IC50). Two groups of cancer cells became apparent; ganetespib-sensitive and -resistant cell lines with a difference in IC50 values up to 70–fold (36 – 2,500 nM). Therefore, the aim of this study was to elucidate the molecular determinants that govern the response of CRC cells to ganetespib treatment. A statistical correlation of the IC50 values of CRC cell lines with their transcriptomes revealed that the expression of UDP-glucuronosyltransferase 1A (UGT1A) correlates strongly with resistance to ganetespib. UGT1A is involved in the metabolism of a variety of drugs, and it has been previously reported that high expression levels of this enzyme in cancer cells are responsible for the resistance to some established chemotherapeutic drugs. Knockdown of UGT1A in ganetespib-resistant cells and overexpression of UGT1A in ganetespib-sensitive cells confirmed the causal connection of increased ganetespib tolerance to the expression of UGT1A. The protective effect of UGT1A was also observed by immunoblot analysis of HSP90 clients, as they were not degraded in sensitive cell lines despite the presence of ganetespib when UGT1A was overexpressed. A similar resistance in cells with increased UGT1A expression was also observed for another, structurally related HSP90 inhibitor, NVP-AUY922. However, HSP90 inhibitors from other classes do not seem to be subject to glucuronidation-induced resistance mechanisms. We hypothesize that glucuronidation by UGT1A takes place at the resorcinol moiety of ganetespib and NVP-AUY922. The metabolites of this glucuronidation process, the glucuronides of ganetespib and NVP-AUY922, were detected by mass spectrometry in a collaborating laboratory to support this hypothesis. In summary, we show that the biological activities of ganetespib and NVP-AUY922, two resorcinolic HSP90 inhibitors, are impaired by UGT1A-catalyzed glucuronidation in CRC cell lines. Notably, the UGT1A expression levels of primary CRC tumor samples have been found to be in the same range as in the CRC cell lines. Therefore, the occurrence of resistance to ganetespib and NVP-AUY922 in clinical applications is a conceivable scenario. We suggest that the expression of UGT1A can be used as a drug-related biomarker in cancer to ensure the activity of resorcinolic HSP90 inhibitors. These findings are of pivotal importance for the clinical application of these drugs in cancers which have the potential to express UGT1A