Inhibition of the epidermal growth factor receptor tyrosine kinase by PD153035 in human A431 tumors in athymic nude mice

PD153035 is a potent (Ki = 6 pm) and specific inhibitor of the epidermal growth factor (EGF) receptor tyrosine kinase that suppresses tyrosine phosphorylation of the EGF receptor in A431 cells at nanomolar concentrations in Cell Culture. We have examined the pharmacokinetics of this compound and its ability to rapidly suppress phosphorylation of the EGF receptor in A431 human epidermoid tumors grown as xenografts in immunodeficient nude mice. Following a single i.p. dose of 80 mg/kg, the drug levels in the plasma and tumor rose to 50 and 22 microM within 15 minutes. While the plasma levels of PD153035 fell below 1 microM by 3 hours, in the tumors it remained at micromolar concentrations for at least 12 hours. The tyrosine phosphorylation of the EGF receptor was rapidly suppressed by 80-90% in the tumors. However receptor phosphorylation returned to control levels after 3 hours despite the continued presence of the drug at concentrations which, based on previous in vitro results, were predicted to maintain inhibition. EGF-stimulated tyrosine kinase activity in tumor extracts was decreased and recovered in parallel with the effects of PD153035 on receptor phosphorylation though the activity had reached only about half of the control activity after three hours. These results demonstrate the potential for using small molecule inhibitors to inhibit the EGF receptor tyrosine kinase in vivo, though a fair evaluation of their potential anti-cancer activity will have to wait for solutions to problems with sustained delivery which may allow us to maintain suppression of EGF receptor phosphorylation.