Erlotinib prevents experimental metastases of human small cell lung cancer cells with no epidermal growth factor receptor expression

Epidermal growth factor receptor–tyrosine kinase inhibitors (EGFR–TKIs) show dramatic antitumor activity in a subset of patients with non-small cell lung cancer who have an active mutation in the epidermal growth factor receptor (EGFR) gene. On the other hand, some lung cancer patients with wild type EGFR also respond to EGFR–TKIs, suggesting that EGFR–TKIs have an effect on host cells as well as tumor cells. However, the effect of EGFR–TKIs on host microenvironments is largely unknown. A multiple organ metastasis model was previously established in natural killer cell-depleted severe combined immunodeficient mice using human lung cancer cells. This model was used to investigate the therapeutic efficacy of erlotinib, an EGFR–TKI, on multiple organ metastases induced by human small cell lung cancer cells (SBC-5 cells) that did not express EGFR. Although erlotinib did not have any effect on the proliferation of SBC-5 cells in vitro, it significantly suppressed bone and lung metastases in vivo, but not liver metastases. An immunohistochemical analysis revealed that, erlotinib significantly suppressed the number of osteoclasts in bone metastases, whereas no difference was seen in microvessel density. Moreover, erlotinib inhibited EGF-induced receptor activator of nuclear factor kappa-B expression in an osteoblastic cell line (MC3T3-E1 cells). These results strongly suggested that erlotinib prevented bone metastases by affecting host microenvironments irrespective of its direct effect on tumor cells.