Adjuvant chemotherapy and hormonal treatment have substantially reduced the risks of relapse and death that threaten a woman for many years after the diagnosis of breast cancer. Progress in breast-cancer treatment is due in part to the incorporation of powerful cytotoxic agents into chemotherapy regimens and to the optimization of the schedules of administration of these agents. Anthracycline-based chemotherapy combinations, such as the cyclophosphamide–epirubicin–fluorouracil (CEF) regimen evaluated in the National Cancer Institute of Canada Clinical Trials Group Mammary. (MA.5) study, when compared with the regimen of cyclophosphamide, methotrexate, and fluorouracil (CMF), produce on average a further relative reduction in deaths from breast cancer of about 16 percent, which corresponds to a gain in the absolute survival rate at 10 years of approximately 4 percent.

Such progress, however, has a price. As compared with the CMF regimen, the CEF regimen costs more and is associated with more adverse effects, especially the small but clinically important increases in the risks of cardiotoxicity and secondary leukemia. Moreover, retrospective analyses point to clinically significant variations in the magnitude of benefit from adjuvant chemotherapy in different groups of patients. These disparities probably arise from several attributes of the tumor cells: the presence or absence of estrogen and progesterone receptors, the proliferative index of the tumor cells, the presence of p53 mutations, and the overexpression of the HER2 protein. In this issue of the Journal, Pritchard et al., reporting on a companion study to the MA.5 clinical trial, describe an association between the benefit of adjuvant CEF and the overexpression of HER2, a cell-surface receptor in the ErbB family of the epidermal growth factor receptor, or amplification of the HER2 gene (also referred to as HER2/neu), in the tumor cells. This investigation is one of many attempts to tailor complex and expensive adjuvant treatments to individual patients.

The Canadian study by Pritchard et al. is the seventh attempt to correlate overexpression of the HER2 protein or amplification of the HER2 gene with a benefit from anthracycline-based regimens. It is the only one of the seven that tested HER2 or its gene by three parallel methods: immunohistochemical analysis, fluorescence in situ hybridization (FISH), and polymerase-chain-reaction (PCR) analysis. Analyses with the use of these three methods were successfully performed in a central laboratory on 89 percent of the tumors, obtained from 90 percent of the patients who were enrolled in the MA.5 trial.

This study found that treatment with CEF was associated with longer relapse-free survival and overall survival than treatment with CMF only in women whose tumors showed HER2 amplification or HER2 overexpression. In contrast, previous studies of the influence of HER2 on the outcome of adjuvant chemotherapy were based largely on a variety of immunohistochemical assays; only two previous studies used FISH as an alternative11 or a complement12 to immunohistochemical analysis. These other studies had variable success in tumor collection, and the corresponding clinical trials administered outdated or underdosed anthracycline-based regimens or suboptimal CMF treatment. Moreover, they did not uniformly find unequivocal superiority of the anthracycline-containing regimen. In these studies the interaction test for an effect of HER2 overexpression on the outcome of chemotherapy, when performed, was negative, suggestive, of borderline significance,or positive (but only for disease-free survival6). The most convincing interaction between the outcome of treatment and HER2 overexpression was found with the CEF combination regimen used in the Canadian trial, which is the most dose-intensive of all the anthracycline-based regimens studied; an interaction between HER2 overexpression and anthracycline dose intensity or dose density has been suggested in three other clinical trials.

Whether the results of the Canadian MA.5 study are generally applicable to women with breast cancer must be considered in light of four factors. First, the study involved only premenopausal women with node-positive breast cancer; second, the reliability of HER2 assays performed in local laboratories, as compared with a high-volume central laboratory, is questionable; third, there are possible confounding effects of chemotherapy-induced amenorrhea, which can contribute to the efficacy of chemotherapy in young women, and these effects occur somewhat more often in women receiving CEF than in those receiving CMF; and fourth, there is the possibility that topoisomerase II{alpha} will ultimately prove to be a better molecular predictor of the efficacy of anthracyclines in breast cancer than HER2. Topoisomerase II{alpha} is a key enzyme in cell division and the target of anthracyclines. There is as yet no convincing evidence of the value of topoisomerase II{alpha} in predicting a differential benefit of anthracycline-based treatment over CMF regimens. Complicating matters is the lack of correlation between the status of the topoisomerase II{alpha} gene (TOP2A) and levels of the topoisomerase II{alpha} protein in breast cancer; there is, however, a correlation between the levels of topoisomerase II{alpha} protein and the histologic grade and the markers of cell proliferation, such as Ki-67.16 Histologic grade and proliferation were not evaluated in the Canadian study.

In my opinion, the results of Pritchard et al. represent a step forward in anthracycline tailoring, but are not likely to influence current clinical practice. One reason for this is that the anti-HER2 monoclonal antibody trastuzumab has recently been found to be highly effective when it is combined with or follows adjuvant chemotherapy: the risk of relapse, and particularly of lethal distant relapse, is half that found with chemotherapy alone. This benefit seems to be independent of the type of adjuvant chemotherapy regimen used and is of such magnitude that it raises the question of whether some women with HER2-positive disease could be spared chemotherapy. Trastuzumab's disadvantage is the associated risk of congestive heart failure, which increases with prior receipt of anthracycline. The recent suggestion that anthracycline-based regimens are better than other regimens in women treated with trastuzumab only if there is amplification of TOP2A is provocative. If confirmed, this finding will negate the value of anthracycline-based chemotherapy in all cases of HER2-positive breast cancer. For HER2-negative breast cancer, a modest but nevertheless clinically relevant benefit of anthracycline-based chemotherapy over CMF cannot be ruled out: hazard ratios associated with treatment with anthracycline-based chemotherapy, as compared with CMF, range from 0.79 to 1.22 for disease-free survival or relapse-free survival and from 0.82 to 1.64 for overall survival.

High-throughput gene-expression profiling of breast cancers has led to new insights into the molecular heterogeneity of this disease. In particular, HER2-negative status includes at least three different forms of breast cancer: "basal-like," which is hormone receptor–negative and very aggressive; "luminal B," which is hormone receptor–positive and has a poor prognosis; and "luminal A," which is also hormone receptor–positive, but indolent. In the MA.5 trial, the subgroup with HER2-negative breast cancer was probably contaminated by the presence of patients with luminal A breast cancer, for whom any chemotherapy is probably of limited benefit. A danger therefore exists that a benefit from an anthracycline-based-regimen might be overlooked in subgroups with luminal B and basal-like breast cancer.

The time has come to divide breast cancer into clinically relevant molecular subgroups, to prioritize the clinical questions applicable to each subgroup, and to strengthen collaboration between clinicians and laboratory scientists in identifying molecular signatures that can predict the success or failure of treatment. The use of genomics and proteomics could prove to be more fruitful than a focus on a single gene. This new avenue of research in breast cancer promises considerable benefit to women and society.