Introduction: Src tyrosine kinase overactivation has been correlated with a poor response to human epidermal growth factor receptor 2 (HER2) inhibitors in breast cancer. To identify the mechanism by which Src overexpression sustains this resistance, we tested a panel of breast cancer cell lines either sensitive or resistant to lapatinib. Methods: To determine the role of Src in lapatinib resistance, we evaluated the effects of Src inhibition/silencing in vitro on survival, migration, and invasion of lapatinib-resistant cells. In vivo experiments were performed in JIMT-1 lapatinib-resistant cells orthotopically implanted in nude mice. We used artificial metastasis assays to evaluate the effect of Src inhibition on the invasiveness of lapatinib-resistant cells. Src-dependent signal transduction was investigated with Western blot and ELISA analyses. Results: Src activation was higher in lapatinib-resistant than in lapatinib-sensitive cells. The selective small-molecule Src inhibitor saracatinib combined with lapatinib synergistically inhibited the proliferation, migration, and invasion of lapatinib-resistant cells. Saracatinib combined with lapatinib significantly prolonged survival of JIMT-1-xenografted mice compared with saracatinib alone, and impaired the formation of lung metastases. Unexpectedly, in lapatinib-resistant cells, Src preferentially interacted with epidermal growth factor receptor (EGFR) rather than with HER2. Moreover, EGFR targeting and lapatinib synergistically inhibited survival, migration, and invasion of resistant cells, thereby counteracting Src-mediated resistance. These findings demonstrate that Src activation in lapatinib-resistant cells depends on EGFR-dependent rather than on HER2-dependent signaling. Conclusions: Complete pharmacologic EGFR/HER2 inhibition is required to reverse Src-dependent resistance to lapatinib in breast cancer.

Objective: Targeting the tumor vasculature may offer an alternative or complementary therapeutic approach to targeting growth factor signaling in lung cancer. The aim of these studies was to evaluate the antitumor effects in vivo of the combination of ZD6126, a tumor-selective vascular-targeting agent; ZD1839 (gefitinib, Iressa), an epidermal growth factor receptor tyrosine kinase inhibitor; and ionizing radiation in the treatment of non-small cell lung cancer xenograft model. Methods: Athymic nude mice with established flank A549 human non-small cell lung cancer xenograft model xenografts were treated with fractionated radiation therapy, ZD6126, ZD1839, or combinations of each treatment. ZD6126 (150 mg/kg) was given i.p. the day after each course of radiation. Animals treated with ZD1839 received 100 mg/kg per dose per animal, 5 or 7 days/wk for 2 weeks. Immunohistochemistry was done to evaluate the effects on tumor growth using an anti-Ki67 monoclonal antibody. Effects on tumor-induced vascularization were quantified using an anti–factor VIII–related antigen monoclonal antibody. Results: ZD6126 attenuated the growth of human A549 flank xenografts compared with untreated animals. Marked antitumor effects were observed when animals were treated with a combination of ZD6126 and fractionated radiation therapy with protracted tumor regression. ZD6126 + ZD1839 resulted in a greater tumor growth delay than either agent alone. Similar additive effects were seen with ZD1839 + fractionated radiation. Finally, the addition of ZD6126 to ZD1839 and radiation therapy seemed to further improve tumor growth control, with a significant tumor growth delay compared with animals treated with single agent or with double combinations. Immunohistochemistry showed that ZD1839 induced a marked reduction in A549 tumor cell proliferation. Both ZD1839 and ZD6126 treatment substantially reduced tumor-induced angiogenesis. ZD6126 caused marked vessel destruction through loss of endothelial cells and thrombosis, substantially increasing the level of necrosis seen when combined with radiation therapy. The combination of radiation therapy, ZD6126, and ZD1839 induced the greatest effects on tumor growth and angiogenesis. Conclusion: This first report shows that a selective vascular-targeting agent (ZD6126) + an anti–epidermal growth factor receptor agent (ZD1839) and radiation have additive in vivo effects in a human cancer model. Targeting the tumor vasculature offers an excellent strategy to enhance radiation cytotoxicity. Polytargeted therapy with agents that interfere with both growth factor and angiogenic signaling warrants further investigation.

Adrenocortical carcinomas (ACCs) are rare and aggressive malignancies of adrenal cortex, associated with largely unknown mechanisms of biological development and poor prognosis. Currently, mitotane is the sole approved drug for treating advanced adrenocortical carcinomas (ACCs) and is being utilized more frequently as postoperative adjuvant therapy. Although it is understood that mitotane targets the adrenal cortex and disrupts steroid production, its precise mechanism of action requires further exploration. Additionally, mitotane affects cytochrome P450 enzymes, causes the depolarization of mitochondrial membranes, and leads to an accumulation of free cholesterol, ultimately resulting in cell death. Many patients treated with mitotane develop disease progression over time, underlying the need to understand the mechanisms of primary and acquired resistance. In this manuscript, we provide an overview on the intracellular mechanisms of action of mitotane, exploring data regarding predictive factors of response and evidence associated with the development of primary and acquired resistance mechanisms. In this discussion, mitotane is considered a real target therapy.