PURPOSE: The epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor ZD1839 (Iressa; AstraZeneca Pharmaceuticals, Alderley Park, United Kingdom) is under development as an anticancer agent. We studied the pharmacodynamic effects of ZD1839 on EGFR in the skin, an EGFR-dependent tissue, in cancer patients participating in ZD1839 phase I clinical trials.

PATIENTS AND METHODS: We studied 104 pre– and/or on–ZD1839 therapy (≈ at day 28 of therapy) skin biopsies from 65 patients receiving escalating doses of daily oral ZD1839. We measured ZD1839 effects on EGFR activation by immunohistochemistry using an antibody specific for the activated (phosphorylated) EGFR. Effects on receptor signaling (activated mitogen-activated protein kinase [MAPK]), proliferation, p27^KIP1, and maturation were also assessed.

RESULTS: Histopathologically, the stratum corneum of the epidermis was thinner during therapy (P < .001). In hair follicles, prominent keratin plugs and microorganisms were found in dilated infundibula. ZD1839 suppressed EGFR phosphorylation in all EGFR-expressing cells (P < .001). In addition, ZD1839 inhibited MAPK activation (P < .001) and reduced keratinocyte proliferation index (P < .001). Concomitantly, ZD1839 increased the expression of p27^KIP1 (P < .001) and maturation markers (P < .001) and increased apoptosis (P < .001). These effects were observed at all dose levels, before reaching dose-limiting toxicities.

CONCLUSION: ZD1839 inhibits EGFR activation and affects downstream receptor-dependent processes in vivo. These effects were profound at doses well below the one producing unacceptable toxicity, a finding that strongly supports pharmacodynamic assessments to select optimal doses instead of a maximum-tolerated dose for definitive efficacy and safety trials.