Use of an antibody to target geldanamycin.

The promise of antibodies as mediators of effective targeted anticancer therapy is beginning to be fulfilled. In the past few years, two monoclonal antibodies (MAbs) have been approved for selective use by the U.S. Food and Drug Administration (FDA) (Rockville, MD) and incorporated into standard therapy, resulting in improved clinical responses and prolongation of life. The most dramatic clinical response has been to rituximab, which is directed against the CD-20 antigen on lymphoma cells (1). Herceptin against the HER2 receptor has proved to be effective in combination with chemotherapy for a subpopulation of breast cancer patients whose malignant cells express high levels of this receptor (2). Since the discovery of methods for producing large quantities of MAbs by Kohler and Milstein in 1975 (3), investigators have explored the following three ways of using MAbs to treat cancer: 1) as mediators of immune cytotoxicity through activation of complement or by action of lymphocytes and macrophages, 2) as inhibitors of specific functions mediated by the targeted antigen, and 3) as carriers of cytotoxic molecules or radionuclides to cells bearing the relevant antigen. The article by Mandler et al. (4) in this issue of the Journal combines the second and third approaches. HER2 belongs to the epidermal growth factor (EGF) family of receptors, which also includes HER3 and HER4. When activated by specific ligands, these molecules form homodimers and heterodimers that transmit biochemical signals through stimulation of intrinsic tyrosine kinase activity. The first MAb capable of blocking EGF receptor activity, MAb 225, was reported in 1983 (5). Inhibitory activity depended on the capacity of the antibody to prevent binding of the natural ligands, EGF or transforming growth factor(5). This property inhibited activation of receptor tyrosine kinase and thereby inhibited proliferation of cells bearing EGF receptors, which include nearly all epithelial and mesenchymal cells. In culture and in xenograft models, antibodies against the EGF receptor inhibited cell proliferation (6,7), especially when combined with chemotherapy (8–10). In phase I/II clinical trials, human : murine chimeric antibody C225 against the EGF receptor showed activity in combination with chemotherapy (11) and radiation therapy (12). The first antibody with comparable growth inhibitory properties against the HER2 receptor targeted the rat neu receptor, which is constitutively activated by a single point mutation in the transmembrane region (13,14). Subsequently, Genentech produced MAb 4D5, which became Herceptin in its humanized form (15,16). Herceptin can actually transiently stimulate activation of the HER2 receptor’s intrinsic tyrosine kinase, presumably by the unnatural mechanism of forming HER2–HER2 homodimers (17). This is followed by the internalization and catabolism of receptors, which is believed to be the most likely mechanism of action for these MAbs in inhibiting tyrosine kinase activity and cell proliferation. There is strong clinical evidence that cancer cells expressing high levels of HER2 are resistant to chemotherapy (18,19). A variety of studies (20–22) have demonstrated that the sensitivity of cancer cells growing in culture and in human tumor xenografts to chemotherapy with cisplatin, paclitaxel, or doxorubicin is increased by interventions that reduce HER2 levels and activity. A phase II clinical trial with Herceptin demonstrated a 12% response rate (four partial responses and one complete response) in 43 previously treated patients with advanced metastatic breast cancer. This study (23) provided “proof of concept” that antireceptor treatment can produce clinical responses. A larger study (24) has confirmed these results. A randomized clinical trial demonstrated that the addition of Herceptin to chemotherapy with the combination of doxorubicin and cyclophosphamide or with paclitaxel resulted in an increased response rate of 62% compared with a response rate of 36% for patients with metastatic breast cancer receiving chemotherapy alone (25). At a median follow-up of 25 months, an overall survival advantage for added Herceptin was seen with the combination of doxorubicin and cyclophosphamide (33.4 versus 24.5 months, respectively) and with paclitaxel (22.1 versus 18.4 months, respectively) (2). Unfortunately, in the group treated with Herceptin, doxorubicin, and cyclophosphamide, class III/IV cardiac dysfunction reached 19% (2). The FDA approved administration of Herceptin with paclitaxel for advanced breast cancer in 1998. Currently, there are many trials exploring the use of Herceptin plus chemotherapy in earlier stages of breast cancer and against other types of cancer. To date, responses appear to be confined to patients whose tumors express high levels of HER2, which include only 25% of patients with breast cancer and some patients with lung or gastric cancer. As pointed out in the article by Mandler et al. (4), these data describing modest increases in survival rates are very encouraging, but, at present, the utility of Herceptin is limited to patients with tumors that highly overexpress HER2 (18). The ansamycin group of compounds, including herbamycin A and geldanamycin, has shown strong antitumor activity against human cancer cells in culture and in xenografts. Until recently, these agents were not being studied in clinical trials because of severe toxicity in vivo. Mechanisms that may explain the activity of geldanamycin have accumulated over the past 5 years. Originally, it was found that incubation of cells with ansamycins resulted in reduced activity of a variety of tyrosine kinases, including transmembrane receptors, such as the EGF receptor, HER2, and the insulin-like growth factor receptor, as well as intracellular tyrosine kinases, such as sarc and fyn [reviewed in (26)].