Epidermal growth factor receptor family and chemosensitization.

Alteration in the expression and activation of protein tyrosine kinases and cell cycle regulatory genes not only can lead directly to the perturbation of growth regulation but also may affect the sensitivity of cancer cells to conventional chemotherapy and radiation therapy. In recent years, numerous studies have explored the effects on chemosensitivity resulting from altered expression and activation of the epidermal growth factor (EGF) receptor and the HER-2/neu receptor. In this issue of the Journal, Dixit et al. ( 1) report that reduced expression of EGF receptors achieved by introduction of EGF receptor antisense oligonucleotides is associated with increased resistance to cisplatin in MDA-468 human breast cancer cells, which express high levels of EGF receptors (>10 /cell). This finding was attributed to abrogation of cisplatin-induced apoptosis in the transfectants. Dixit et al. suggest that critical levels of EGF receptor signaling are necessary for apoptosis induced by cisplatin. A similar effect was not seen with other chemotherapeutic agents tested. While the data appear convincing, the conclusion seems to conflict with the view that human breast cancer cell lines selected for multidrug resistance frequently display increased EGF receptors or HER-2/neu receptors ( 2-4). In addition, experimental elevation of EGF receptor levels in the human breast cancer cell line ZR75B, which normally expresses low receptor levels, led to increased resistance to several cytotoxic chemotherapeutic drugs, including cisplatin (5). An early study with an EGF receptor-blocking agent, monoclonal antibody (MAb) 108, showed that concurrent treatment with cisplatin at the time of KB carcinoma cell implantation resulted in enhanced antitumor activity (6). We have reported the curative treatment of well-established A431 squamous cell carcinoma xenografts with cisplatin followed by anti-EGF receptor MAb 225, which blocks activation of receptor tyrosine kinase and induces receptor down-regulation (i.e., reduction) (7). A possible mechanism was suggested by the preliminary finding that the repair efficiency of a cisplatin-damaged plasmid vector that encoded a luciferase reporter gene (pCMVLuc) was reduced in A431 cells exposed to MAb 225 ( 8). Antitumor activity was similarly enhanced when xenografts of A431 cells or MDA-468 cells were treated with doxorubicin in combination with MAb 225 (9) and when xenografts of MDA-468 cells were treated with paclitaxel in combination with MAb 225 (Baselga J, Kim YM, Mendelsohn J: unpublished observations). MAb 225 also enhanced the cytotoxicity of radiation therapy in A431 cells (10). Further studies with a number of cell lines have shown that treatment with MAb 225 causes a rise in the level of p27 , accompanied by inhibition of cyclin-dependent kinase-2 (CDK2) and cell cycle arrest in the G 1 phase (11,12). A phase I clinical trial with MAb 225 demonstrated tumor localization and lack of toxicity (13), and a human–mouse chimeric version of MAb 225 is presently in clinical trials in combination with chemotherapy. Comparable studies have explored the results of altered expression and activation of the closely related HER-2/neu receptor. Treatment of human breast and ovarian cancer cell lines with cisplatin plus TAb 250 or 4D5 anti-HER-2/neu receptor MAbs, which can block cell proliferation, resulted in enhanced inhibition of cell growth in culture and in xenografts ( 14-16). In the case of MAb 4D5, this enhanced inhibition was associated with a reduction in the repair of cisplatin–DNA adducts in cultured ovarian cancer cells (16). Experimentally induced enhancement of HER-2/neu receptor expression in the human breast cancer cell line MDA-435, which normally expresses low receptor levels, conferred increased resistance to paclitaxel via mdr-1 geneindependent mechanisms ( 17). Chemosensitivity to cisplatin, doxorubicin, or etoposide also could be enhanced by exposure of cultures of non-small-cell lung carcinoma cell lines to tyrphostin AG825, a selective inhibitor of the HER-2/neu receptor tyrosine kinase. The enhanced drug sensitivity was observed in cell lines with high receptor levels but not in those with low receptor levels (18). Thus, there is ample experimental evidence that the sensitivity of tumor cells to cisplatin and other chemotherapeutic agents may be enhanced by antibodies and inhibitors that act on receptors in the EGF and HER-2/neu receptor family to block cell proliferation. The importance of this observation is emphasized by the results of a recent phase I clinical trial demonstrating that treatment of breast cancer patients with MAb 4D5 against HER-2/neu receptor induced objective partial responses in 10% of patients as well as one complete response, which provides evidence that anti-receptor MAb therapy can produce responses in human cancer ( 19). A phase II clinical trial with MAb 4D5 plus chemotherapy has been completed in patients with advanced breast cancer ( 20), and a phase III trial is under way. These results appear to be different from those of Dixit et al. (1) suggesting that a reduction in EGF receptors causes reduced sensitivity to cisplatin. However, their observations are consistent with several reports showing that treatment of cultures with EGF, thereby activating EGF receptors, could enhance the sensitivity of an ovarian cancer cell line and other cancer cell lines to several chemotherapeutic agents (21-24) or to radiation therapy (25). Sensitization to chemotherapy induced by EGF was found to be independent of the mitogenic or anti-mitogenic