Abstract 762: Ganetespib resistance inKRASmutant NSCLC is mediated through bypassing the G2/M checkpoint and reactivating the PI3K/MTOR pathway

Proceedings: AACR 106th Annual Meeting 2015; April 18-22, 2015; Philadelphia, PA One third of all malignancies and approximately 25% of non-small cell lung cancer (NSCLC) patients have KRAS mutations, which in turn activate several growth regulatory signaling pathways including RAF/MEK/ERK and PI3K/AKT/MTOR. Unfortunately, there are no current therapies targeting this critical oncogene. Heat shock protein 90 (HSP90) is a molecular chaperone required for the stability of hundreds of ‘client’ proteins, many of which are effectors of KRAS. Unfortunately, limited efficacy was observed in early clinical studies of single agent HSP90 inhibitors (HSP90i) in KRAS mutant NSCLC. Here, we examined the mechanism(s) of acquired resistance to ganetespib in KRAS mutant NSCLC to develop rationale combinations with ganetespib. Ganetespib, a Phase 3 HSP90i, was potently cytotoxic in a panel of KRAS mutant NSCLC cell lines and promoted decreased expression and activity of both RAF/MEK/ERK and PI3K/AKT/MTOR pathway members. Contrary to prior reports with other HSP90i, we found that ganetespib dependent growth inhibition was independent of its ability to stabilize p53 or induce apoptosis. We derived three KRAS mutant NSCLC ganetespib resistant (GR) cell lines to identify the resistance mechanism(s) and reveal targetable pathways to overcome this resistance. First, we demonstrated that A549-GR cells are cross-resistant to a first generation HSP90i, 17-AAG, suggesting that altered metabolism of ganetespib is unlikely to explain this resistance. Moreover, the ganetespib induced G2/M checkpoint arrest observed in A549 parental cells was significantly diminished in A549-GR cells. These results suggest that bypass of this checkpoint may be an important part of the resistance mechanism. Furthermore, we demonstrated that A549-GR cells were cross-resistant to docetaxel, an antimicrotubule agent. In addition, expression and activity of the PI3K/AKT/mTOR pathway members were significantly increased suggesting that reactivation of PI3K/AKT/mTOR pathway may be responsible for the observed resistance. To test this hypothesis, we treated parental and GR cells with a dual PI3K/mTOR inhibitor, BEZ235. Remarkably, A549-GR cells were more sensitive to BEZ235 compared to the parental ones suggesting that the acquired ganetespib resistance lead to increased dependence on the PI3K/mTOR pathway. Interestingly, a key activator of the PI3K/mTOR pathway, p90 ribosomal S6 kinase (RSK) was strikingly increased in the GR cells. Furthermore, A549-GR cells showed increased sensitivity to two highly specific RSK inhibitors, BI-D1870 and SL0101. These data suggests that the combination of inhibitors for HSP90 and PI3K/mTOR may prevent ganetespib resistance and/or help overcome the resistance after single agent treatment, providing the preclinical rationale for our planned Phase I/II trial of the combination of ganetespib and a dual PI3K/MTOR inhibitor in KRAS mutant NSCLC. Citation Format: Suman Chatterjee, Timothy F. Burns. Ganetespib resistance in KRAS mutant NSCLC is mediated through bypassing the G2/M checkpoint and reactivating the PI3K/MTOR pathway. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 762. doi:10.1158/1538-7445.AM2015-762