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M. El-Tanani

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    MO12 - Prognostic and Predictive Biomarkers III (ID 96)

    • Event: WCLC 2013
    • Type: Mini Oral Abstract Session
    • Track: Medical Oncology
    • Presentations: 1
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      MO12.01 - Novel Mechanisms of Sensitivity and Acquired Resistance to HSP90 inhibition by Ganetespib (ID 2739)

      10:30 - 12:00  |  Author(s): M. El-Tanani

      • Abstract
      • Presentation
      • Slides

      HSP90 is a promising anti-cancer target. Inhibition by the Hsp90 inhibitor ganetespib has shown promising activity with improved survival in patients with metastatic lung adenocarcinoma, and it is now being evaluated in malignant pleural mesothelioma. However, the mechanisms underlying resistance are currently unknown. The aims of this study were to establish the role for mitochondrial apoptosis in mediating the anti-cancer activity of ganetespib, and to also identify mechanisms of acquired resistance to support personalised therapy.

      We conducted a functional genetic screen to determine the role of the proapoptotic BAX/BAK proteins using double knockout mouse embryonic fibroblasts (MEFs) shRNA and siRNA. Focused RNAi targeting BH3-only proteins, Caspase 8 and MCL1 was conducted in MSTO-211H, H460 and H23 cell lines. Apoptosis was measured by a Caspase3 activity assay and data were validated by western blot and SubG1 population analysis. Prosurvival Bcl2 family regulation was evaluated by western blot, and MCL1 transcriptional suppression monitored by real-time quantitative PCR and luciferase reporter assay. MCL1 amplification was quantified by genomic real-time PCR. Ganetespib resistant cells were generated by increasing drug exposure. Hsp90 ATP-binding site and Caspase8 were sequenced in both parental and resistant cell lines.

      Ganetespib required a functional mitochondrial pathway for induction of apoptosis. Interrogation of pro-apoptotic BH3-only proteins revealed a co-operation between BID, BIK and PUMA. Caspase8 activates BID and, when silenced, protected cells from ganetespib. MCL1 was transcriptionally suppressed by ganetespib, and when Mcl-1 downregulation was achieved by siRNA, it was sufficient to induce BID/BIK-dependent apoptosis in MCL1-dependent cells. We observed that MCL1 addicted cells were also more sensitive to ganetespib than non-addicted. In addition, amplification of MCL1 was detected only in ganetespib sensitive cell lines. Ectopic MCL1 was not sufficient to rescue from ganetespib-induced apoptosis. To better understand mechanisms of resistance, we established ganetespib-resistant cell lines. Resistant cells did not select for HSP90 mutations, and these cells conserved on-target suppression of PI3K/AKT, MAPK signalling, upregulation of HSP70, and MCL1 downregulation. However addiction to MCL1 was lost as was block of Caspase8 activation with consequent cross-resistance to TRAIL. PCR of Caspase8 cDNA revealed an acquired structural alteration in the 3’-untranslated region.

      Here we show that HSP90 inhibition requires engagement of the mitochondrial apoptosis pathway, and involves cooperation of multiple BH3-only proteins with parallel suppression of MCL1. Interestingly, ganetespib may exploit tumour dependence on MCL1; this may be clinically relevant given that MCL1 (1q21.2) amplification correlates with dependence and its gene copy number alteration is one of the most frequent across cancers. Acquired resistance involves selection for loss of dependence on MCL1, and a block in Caspase8 signalling which lies upstream of BID. Failure of ectopic MCL1 overexpression to rescue is indicative of redundant death signalling by ganetespib. Clinical significance of core apoptosis gene expression will be explored and presented in a correlative analysis of the 9090-06 ganetespib monotherapy clinical trial in NSCLC.

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