Inhibition of epidermal growth factor receptor-associated tyrosine phosphorylation in human carcinomas with CP-358,774: Dynamics of receptor inhibition in situ and antitumor effects in athymic mice

Phosphorylation of tyrosine residues on the epidermal growth factor (EGF) r eceptor (EGFr) is an important early event in signal transduction, leading to cell replication for major human carcinomas. CP-358,774 is a potent and selective inhibitor of the EGFr tyrosine kinase and produces selective inhi bition of EGF-mediated tumor cell mitogenesis. To assess the pharmacodynami c aspects of EGFr inhibition, we devised an ex vivo enzyme-linked immunosor bent assay for quantification of EGFr-specific tyrosine phosphorylation in human tumor tissue specimens obtained from xenografts growing s.c. in athym ic mice. When coupled with pharmacokinetic analyses, this measurement can b e used to describe the extent and duration of kinase inhibition in vivo. CP -358,774 is an effective, orally active inhibitor of EGFr-specific tyrosine phosphorylation (ED50 = 10 mg/kg, single dose). It has a significant durat ion of action, producing, on average, a 70% reduction in EGFr-associated ph osphotyrosine over a 24-h period after a single 100 mg/kg dose. Inhibition of EGFr phosphotyrosine in an ex vivo assay format effectively estimates th e potency and degree of inhibition of EGFr-dependent human LICR-LON-HN5 hea d and neck carcinoma tumor growth. Substantial growth inhibition of human t umor xenografts was achieved with p.o. doses of the compound (ED50 = 10 mg/ kg q.d. for 20 days). Combination chemotherapy with cisplatin produced a si gnificant response above that of cisplatin alone with no detectable effects on body weight or lethal toxicity. Taken together, these observations sugg est that CP-358,774 may be useful for the treatment of EGFr-driven human ca rcinomas.