Abstract 2586: ASP8273 selectively inhibits mutant EGFR signal pathway and induces tumor shrinkage in EGFR mutated tumor models

Activating mutations of epidermal growth factor receptor (EGFR) are associated with the high sensitivity of non-small cell lung cancer (NSCLC) patients to EGFR tyrosine kinase inhibitors (TKIs) like erlotinib. However, acquired resistance limits the clinical efficacy of EGFR-TKIs, which is the most commonly caused by T790M mutation in EGFR. Second generation EGFR-TKIs such as afatinib are able to inhibit T790M mutation but the clinical efficacy in T790M positive patients is limited due to severe side effects associated with wild type (WT) EGFR inhibition. ASP8273 is a mutant-selective irreversible EGFR inhibitor currently in clinical trials (ClinicalTrials.gov Identifier: NCT02113813, NCT02192697). We have previously reported that ASP8273 covalently binds to mutant EGFR via C797 and inhibits kinase activity of mutant EGFR, which results in anti-tumor activity in xenograft models. To further explore the selectivity and the activity of ASP8273 on mutant EGFR, we evaluated effects of ASP8273 and other EGFR-TKIs on EGFR signal pathway, cell growth and anti-tumor activity. Phosphorylation of EGFR, ERK and Akt was determined by Western blot after treatment with EGFR-TKIs at 10, 100 and 1000nM. Apoptosis induction was analyzed by detecting caspase activity after 24h treatment with EGFR-TKIs. Anti-tumor activity of ASP8273 was evaluated in mice xenografted with HCC827 (deletion in exon 19 [del ex19]), NCI-H1975 (T790M/L858R), A431 (WT), and a patient derived LU1868 (T790M/L858R). ASP8273 selectively inhibited phosphorylation of EGFR and its down-stream signal pathway, ERK and Akt from 10nM in HCC827 and NCI-H1975 while inhibitory effects were only detected at 1000nM in A431.In NCI-H1650 (del ex19), ASP8273 inhibited cell growth with an IC50 value of 70nM while other EGFR-TKIs were only partially effective. ERK and Akt phosphorylation were diminished after ASP8273 treatment at 1000nM, however, other EGFR-TKIs only partially reduced the phosphorylation levels of these proteins. ASP8273 potently enhanced the caspase activity in NCI-H1650 after 24h treatment, which is concordant with signal and cell growth inhibitory effect. In HCC827 and NCI-H1975 xenograft models, ASP8273 induced tumor regression at 10, 30 and 100mg/kg without affecting body weight. ASP8273 also produced tumor growth inhibition from 10mg/kg in the NSCLC patient derived tumor xenograft, LU1868 which express T790M/L858R. On the other hand, ASP8273 did not produce significant tumor growth inhibition at