Small molecule inhibitors of
epidermal growth factor receptor (EGFR)
tyrosine kinase activity, such as
erlotinib and
gefitinib, revolutionized
therapy for
non-small cell lung cancer (NSCLC) patients whose
tumors harbor activating EGFR mutations. However, mechanisms to overcome the invariable development of acquired resistance to such agents, as well as realizing their full clinical potential within the context of wild-type EGFR (WT-EGFR) disease, remain to be established. Here, the antitumor efficacy of targeted EGFR
tyrosine kinase inhibitors (TKIs) and the HSP90 inhibitor
ganetespib, alone and in combination, were evaluated in NSCLC.
Ganetespib potentiated the efficacy of
erlotinib in TKI-sensitive, mutant EGFR-driven NCI-HCC827 xenograft
tumors, with combination treatment causing significant
tumor regressions. In
erlotinib-resistant NCI-H1975 xenografts, concurrent administration of
ganetespib overcame
erlotinib resistance to significantly improve
tumor growth inhibition.
Ganetespib co-treatment also significantly enhanced antitumor responses to
afatinib in the same model. In WT-EGFR cell lines,
ganetespib potently reduced cell viability. In NCI-H1666 cells,
ganetespib-induced loss of client
protein expression, perturbation of oncogenic signaling pathways, and induction of apoptosis translated to robust single-agent activity in vivo. Dual
ganetespib/
erlotinib therapy induced regressions in NCI-H322 xenograft
tumors, indicating that the sensitizing properties of
ganetespib for
erlotinib were conserved within the WT-EGFR setting. Mechanistically, combined
ganetespib/
erlotinib exposure stabilized EGFR
protein levels in an inactive state and completely abrogated
extracellular-signal-regulated kinase (ERK) and AKT signaling activity. Thus, selective HSP90 blockade by
ganetespib represents a potentially important complementary strategy to targeted TKI inhibition alone for inducing substantial antitumor responses and overcoming resistance, in both the mutant and WT-EGFR settings.