Histopathological and immunohistochemical features associated with clinical response to neoadjuvant gefitinib therapy in early stage non-small cell lung cancer.

To define the pathological features associated with response to epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) in NSCLC, we have evaluated tumor histopathological features and immunohistochemical markers of proliferation (Ki-67) and epithelial mesenchymal transition (EMT) in 36 resected early stage NSCLC from patients treated preoperatively with gefitinib for 28 days. Tumors studied included 7 squamous cell carcinoma, 27 adenocarcinoma (ADC), one adenosquamous carcinoma, and one large cell carcinoma. Six of the ADC harboured an EGFR tyrosine kinase domain (TKD) mutation; five were the sensitizing type. Five ADC with TKD mutation demonstrated non-mucinous lepidic growth pattern as the dominant histological feature. Post-gefitinib treated EGFR TKD mutant tumors demonstrated lower tumor cellularity and proliferative index compared to wild type ADC and non-ADC cases, features correlating with clinical response. Responding tumors also showed large areas of fibrosis, within which focal residual viable tumor cells were noted. However, there was no significant correlation between the degree of fibrosis and radiological changes in tumor size. Expression of EMT markers was not associated with significant change in tumor size. The results suggest that radiologically assessed response to EGFR TKI in NSCLC is related to loss of tumor cellularity and reduced tumor cell proliferation, but residual viable tumor cells may persist even after prolonged treatment. Neoadjuvant studies in early stage NSCLC offer a unique opportunity to evaluate pathological and biomarker changes induced by targeted drugs.