Abstract 729: Acquired resistance to Hsp90-inhibitors and cancer progression

Heat shock protein 90 (HSP90) is a molecular chaperone required for the stability and function of many proteins. The chaperoning of mutated and over-expressed oncoproteins by HSP90 enhances survival, growth and invasive potential of cancer cells. Many HSP90 inhibitors, including the benzoquinone ansamycin 17-allylamino-17-demethoxygeldanamycin (17-AAG), are currently in clinical evaluation. However the mechanisms and implications of acquired resistance to this class of drug remain largely unexplored. We have generated isogenic human breast cancer cell lines that are resistant to 17-AAG by continued culturing in the compound. High level of resistance was maintained in the 17-AAG resistant cells after cessation of treatment. Cross resistance to other benzoquinone ansamycins such as geldanamycin and 17-DMAG was observed, as well as to compounds structurally unrelated to the benzoquinones such as radicicol, VER50589 and CCT018159. Gene expression profiling and western blot analyses revealed that bone marrow stromal cell antigen 2 (BST2), previously linked to increased bone metastasis, is elevated significantly in the resistant cells. An inverse correlation between the expression of the enzyme NAD(P)H/quinone oxidoreductase 1 (NQO1) and resistance to 17-AAG was also observed. The resistant cells demonstrated significant increase in chemotactic migration and accelerated wound closure. This was coupled by a decrease in growth both in anchorage-dependent and -independent conditions. In vivo study using xenograft mouse model showed decreased mammary tumour formation by the resistant cells. Decreased metastasis of the resistant cells to the lungs was observed following intracardiac inoculation. However, x-ray analysis revealed that nude mice inoculated with resistant cells had enhanced hindlimb bone lesions compared to the parental group. In addition, 17-AAG was also shown to increase the formation of bone resorbing osteoclasts in vitro. Pharmacological inhibition of the transcription factor HSF1 using quercetin and KNK437 suppressed 17-AAG-induced stress response and 17-AAG-enhanced osteoclast formation. These results indicate that resistance to Hsp90 inhibition is accompanied by changes in cancer cell biology that leads to decreased primary tumour formation and lower metastatic tumour burden in vivo. However, severity of bone lesion generated by the resistant cells is greater. 17-AAG also enhances osteoclast formation through a mechanism dependent on HSF1-mediated stress response. Findings from this study furthers our understanding of Hsp90 inhibition on cancer progression and increases our understanding of the potential or lack of, clinical efficacy of Hsp90-directed therapies. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 729. doi:10.1158/1538-7445.AM2011-729