On receptor inhibitors and chemotherapy.

In the search for rational targets for drug development in cancer, receptor tyrosine kinases have become a favorite. The rationale for directing efforts to developing inhibitors of these receptors is compelling. Mutated or overexpressed growth factor receptors play a pivotal role in malignant transformation and in maintaining the growth of cancers in model systems. Strategies that inactivate growth factor receptors or their downstream pathways halt tumor growth. Of particular recent interest are members of the EGFR family including the EGFR (c-erbB1/ HER1) and Her2 (c-erbB2/HER2) molecules. Both receptors are overexpressed in a broad range of human cancers of epidermal origin, and their presence has been associated with a poor prognosis and resistance to cytotoxic chemotherapy (1–4). These receptors are composed of an external domain responsible for ligand binding. Binding in turn stimulates dimerization and activates an internal domain that has tyrosine kinase activity. Both the receptor ligand binding function and the kinase domain present potential targets for inhibition of receptor function. In recent years, molecules of both types (external domain antagonists and kinase inhibitors) have reached the clinic and shown evidence of modest activity in breast cancer [Herceptin (5)] and non-small cell lung cancer [EGFR antibody C225 and Iressa (6, 7)]. Although these effects are important in demonstrating the potential of such agents, the ultimate value of the antireceptor approach may lie elsewhere, namely, in the combined use of these inhibitors with cytotoxic drugs or irradiation, a subject addressed in several recent articles in this journal (8–11) and in related publications (12–14). The mechanism whereby tyrosine kinase inhibitors enhance the activity of cytotoxic drugs is complex and incompletely understood. Growth factor signaling pathways provide a stimulus for proliferation through a complex receptor-to-nucleus cascade. In addition, they up-regulate the expression of genes that antagonize apoptosis, or programmed cell death, such as Bcl-2 and Bcl-XL and thereby raise the threshold for cell death in response to DNA damage (15). Finally, these pathways stimulate the secretion of angiogenic factors such as VEGF (16). All of these actions confer a growth or survival advantage, but the apoptotic effects seem most important in combination with drugs. When combined with drugs, inhibitors of growth factor receptors enhance the efficacy of conventional cytotoxic agents. This has been demonstrated convincingly with Herceptin and paclitaxel or Herceptin and doxorubicin in xenograft experiments (17) and in patients with metastatic breast cancer (12), for whom the combination has resulted in a significant increase in response rates and improved survival. Promising preclinical experiments with C225 (18, 19), an anti-EGFR monoclonal antibody, have led to clinical trials of the antibody combined with radiotherapy in head and neck cancer (13, 14). Of nine patients with head and neck cancers treated with C225 and cisplatin, six had major responses, and three of these six had previously failed cisplatin-based chemotherapy (13). Fourteen of 15 patients achieved complete responses after treatment with C225 and onceor twice-daily irradiation for locally advanced head and neck malignancies. In these patients, a complete response rate of 50% or less would have been expected with radiation alone (14). The explanation for these effects may lie in the effects of receptor inhibition on the threshold for apoptosis. In the case of paclitaxel interaction with Her2, paclitaxel disrupts microtubule integrity, leading to cell cycle arrest and apoptosis (20). Overexpression of Her2 confers resistance to paclitaxel by increasing levels of p21, a cyclin-dependent kinase inhibitor, which delays entrance into mitosis and prevents mitotic arrest (20). Resistance to paclitaxel can be overcome by blocking the Her2 receptormediated signaling, and the consequent increase in p21. A second antiapoptotic signal emanates from the EGFR, which, on ligand binding, activates a number of downstream signals (21– 24) including protein kinase A, which, in turn, phosphorylates bcl-2 (22). This interaction provided the rationale for combined therapy with C225, antisense oligonucleotides targeting the RIa regulatory subunit of protein kinase A, and docetaxel in the human breast cancer cell line ZR-75-1 (24). These three agents, used in combination at suboptimal doses, led to a greater degree of Bcl-2 phosphorylation and apoptosis than seen with any single agent alone in this cell line (24). Receptor blockade with ZD1839, a low molecular weight inhibitor of EGFR, exerts synergistic effects with paclitaxel or other chemotherapy in xenograft experiments (25). ZD1839, an anilinoquinazoline, is a potent and selective inhibitor of EGFR receptor kinase in vitro and in vivo. It has strong antiproliferative effects against the growth of the A431 tumor, which has high levels of expression of EGFR (26). Sirotnak et al. (8) report in this issue of Clinical Cancer Research that the combination of ZD1839 with taxanes, platinums, or the folate antagonist edatrexate markedly enhances antitumor activity. The combination of ZD1839 with all cytotoxic agents required a 2-fold or greater attenuation of the ZD1839 dose below its single-agent maximum dose of 150 mg/kg. Interestingly, the degree of enReceived 10/18/00; accepted 10/30/00. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 To whom requests for reprints should be addressed, at Hematology/ Medical Oncology & Massachusetts General Hospital Cancer Center, Massachusetts General Hospital, Cox Building, Room 640, 100 Blossom Street, Boston, MA 02114-2167. 2 The abbreviations used are: EGFR, epidermal growth factor receptor; VEGF, vascular endothelial growth factor; bFGF, basic fibroblast growth factor; IL-8, interleukin 8; TCC, transitional cell carcinoma; PCNA, proliferating cell nuclear antigen. 4607 Vol. 6, 4607–4609, December 2000 Clinical Cancer Research