IPI-504, a novel HSP90 inhibitor is effective in EGFR T790M and non-T790M gefitinib-resistant lung cancer cell lines.

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Background: Acquired resistance to gefitinib in NSCLC patients withsomatic activating EGFR mutations has been associated with thedevelopment of a secondary T790M mutation and/or MET amplification in about 50% and 20% of the patients, respectively. The mechanisms for acquired resistance in the remaining tumors are still unknown. Heat shock protein 90 (HSP90) is a molecular chaperone thatplays a key role in the conformational maturation of oncogenicsignaling proteins andhas become a potential therapeutic target for thetreatment of some cancers. IPI-504, the highly soluble hydroquinone hydrochloride derivative of 17-AAG, was synthesized as an HSP90 inhibitor with favorable pharmaceutical properties. Herein, we explored IPI-504 as an agent to overcome the T790M and non-T790M gefitinib-resistance mechanisms. Methods and Results: We have developed gefitinib-resistant clones from gefitinib-hypersensitive EGFR L858R mutant (H3255) and exon 19 deletion (del E746_A750) mutant (HCC827) NSCLC cell lines by exposing them to increasing concentrations of gefitinib in vitro. We then investigated the effects of IPI-504 on cells according to the mechanism of resistance to gefitinib; KRAS mutation (A549 and H441), EGFR T790M mutation (H1975, H820 and H3255GR) and non-T790M mutation (MET amplification) (HCC827GR). In time course experiments of Western blot, mutant EGFR proteins in H1975, H820, H3255GR and HCC827GR were depleted after only 6 hrs of exposure to 100 nM IPI-504, while diminution of wild-type EGFR in A549 and H441 was less substantial. Phospho-Akt levels were also reduced in parallel with EGFR depletion and more rapidly than total Akt in EGFR-mutant cell lines, although phospho-Akt levels in wild-type cells were correlated with total Akt levels. In addition, MET protein expression in HCC827GR was also decreased in time and dose dependent manner. Exposure to IPI-504 showed dose and time dependent growth inhibition exhibiting IC₅₀ values for cell viabilityat 72 h that ranged from 41 - 293 nM for all cell lines. All cell lines tested were found to be sensitive to IPI-504 regardless of gefitinib-resistance mechanisms but apoptosis was only evident in the EGFR mutant cell lines. We are now testing IPI-504 alone or in combination with gefitinib in HCC827GR xenograft models. Conclusions: These data support the conclusion that proteins that mediate resistance to Gefitinib (mutant EGFR and amplified MET) are dependent on Hsp90 for activity. Taken together, inhibition of Hsp90 by IPI-504 represents a novel strategy for the treatment of NSCLC patients having either EGFR T790M and/or non-T790M gefitinib resistance mechanisms and suggests that IPI-504 should be evaluated clinically in NSCLC patients that have developed gefitinib or erlotinib resistance.