The article by Rouzier and colleagues, published in the August 15, 2005, issue of Clinical Cancer Research, demonstrated that different molecular subtypes of breast cancer have different degrees of sensitivity to chemotherapy, but the extent of response to neoadjuvant therapy has a different meaning by subtype. Several molecular subtype–specific clinical trials are under way to maximize pathologic complete response rates in triple-negative breast cancer and HER2-positive cancers, and to provide adjuvant treatment options for patients with residual invasive disease. Clin Cancer Res; 21(16); 3575–7. ©2015 AACR.

See related article by Rouzier et al., Clin Cancer Res 2005;11(16) Aug 15, 2005;5678–85

The most useful disease classification systems are those that not only capture salient biologic features of disease subsets but also guide treatment strategies. Ten years ago, we published an article that demonstrated significantly different chemotherapy sensitivity across different molecular subtypes of breast cancers (1). We found that basal-like and HER2-positive cancers are the most chemotherapy sensitive, followed by the luminal B and luminal A subtypes. Chemotherapy sensitivity was assessed in the context of preoperative therapy of newly diagnosed, stage I–III cancers using a multidrug, third-generation chemotherapy regimen. Pathologic complete response (pCR) rate was used as the study endpoint. Similar observations were subsequently made by many other investigators (2). The realization that luminal B cancers have poorer prognosis compared with luminal A cancers when treated with adjuvant endocrine therapy alone but at the same time have greater chemotherapy sensitivity enabled the development of several molecular diagnostic assays that assist physicians today to select adjuvant chemotherapy for estrogen receptor (ER)–positive breast cancers. The initial success in finding prognostic gene signatures for ER-positive cancers and associating gene expression patterns with chemotherapy sensitivity raised expectations that similar prognostic and predictive gene signatures, and perhaps even individual drug- and regimen-specific predictors, can also be developed for each breast cancer subtype separately.

Not all clinical prediction problems turned out to be equally easy to solve in the gene expression space. The greater chemotherapy sensitivity of luminal B cancers is associated with, and probably caused by, high expression of a large number of proliferation related genes that provided an easy basis to select molecular variables to build multivariable prediction models. However, proliferation-related genes carried only modest chemotherapy predictive or prognostic values in triple-negative breast cancer (TNBC), partly due to the almost uniformly high proliferative activity of these cancers (3). Although TNBC showed greater chemotherapy sensitivity than the other molecular subtypes, it has been difficult to identify molecular predictors that could separate the more chemotherapy-sensitive from the less-sensitive cancers within the TNBC subgroup. The only consistent biologic feature that shows a broad association with chemotherapy response in early-stage TNBC resided not in the cancer cells but in the tumor microenvironment. The level of tumor-infiltrating lymphocytes (TIL), and molecular markers that reflect the extent of immune infiltration, are the only currently available (although not yet standardized or routinely reported) predictors of chemotherapy response in TNBC (4). However, although a statistically significant association exists between greater TIL count (and immune gene signatures) and higher probability of pCR, it is difficult to define a TIL threshold below which benefit from treatment could be excluded with certainty.

Efforts to define clinically useful prognostic gene signatures within TNBC also met with limited success. Although it is possible to define better and worse prognostic groups among these cancers, the practical value of these observations is limited because even patients who are in the “good risk” group have 15% to 20% risk of recurrence if treated with surgery alone. Remarkably, the biologic processes that carry the greatest prognostic information in TNBC are also immune and inflammatory parameters in the tumor stroma (5). Considering the clinical decision-making context of TNBC after surgery, which involves choosing between observation or adjuvant chemotherapy, and the lack of data to support that adjuvant chemotherapy could not further improve the outcome of patients assigned to the “good”-prognosis group, which tends to include the most immune-rich tumors, the clinical utility of molecular prognostic predictors for TNBC is modest at best. Not surprisingly, in contrast with the multitude of prognostic and predictive tests for ER-positive cancers, no clinically useful similar tests exist for TNBC.

However, two important discoveries were made during the search for molecular prognostic and predictive markers for TNBC. First, the substantial within-class heterogeneity of TNBC has become apparent, suggesting that each tumor may be resistant or sensitive to a particular drug, or combination of drugs, on its own way (6). Although this makes the prospect of empirical predictive marker development rather daunting, it also raises the intriguing possibility of using metrics of tumor heterogeneity or “genome disorganization” as a new type of biomarker. Indeed, the most promising candidate for a drug-specific predictive marker is a composite score that captures telomeric allelic imbalance, loss of heterozygosity, and mutation load as a summary readout of genomic scarring due to DNA repair deficiency that might predict sensitivity to platinum therapy (7). The second important discovery was the association between higher immune cell infiltration and better outcome that extends beyond TNBC and also holds true for high-risk luminal B and HER2-positive cancers. High TIL count is associated with better prognosis in TNBC patients treated with surgery alone and also in patients who received adjuvant chemotherapy (5, 8). Even among patients who have residual cancer after neoadjuvant chemotherapy, high TIL count in the residual cancer predicts for better prognosis (9). Although the association between immune cells and prognosis has been noted for several decades, what makes these observations particularly relevant today is the availability of effective immune-modulating drugs that will allow us to test whether these associations represent a cause-and-effect relationship. If the immune cells in the tumor microenvironment exert at least partial control over the cancer and cause the improved survival, activation of the immune surveillance through immune checkpoint inhibitor therapy could further improve survival.

Complete imaging and pathologic resolution of metastatic breast cancer in response to therapy is rare (although it is becoming more common with multimodality HER2-targeted therapies in HER2-positive cancers) and, therefore, in clinical trials, tumor shrinkage ≥ 30% is often considered as a meaningful signal for drug activity. We, and others, have proposed that in the neoadjuvant treatment setting of early-stage breast cancer, pCR defined as complete eradication of invasive cancer from the breast and lymph nodes (ypT0/is, ypN0) represents a more meaningful endpoint than clinical or imaging response because of its strong association with survival. Patients with breast cancer who achieve pCR, or have minimal residual cancer burden, have excellent survival regardless of molecular subtype, whereas those with residual invasive disease (RD) have variable prognosis that is influenced by disease subtype and extent of RD (10). Patients with TNBC and residual cancer after neoadjuvant chemotherapy have a poor prognosis that is proportionate to the extent of residual cancer, whereas many patients with ER-positive cancer have excellent survival in spite of RD. This strong association between pCR and survival at the individual patient level in TNBC, and also in HER2-positive cancers, motivates the efforts to develop regimens that maximize pCR rates. In 2013, the FDA granted its first accelerated approval for a drug, pertuzumab, based on its success at improving the pCR rate in HER2-positive breast cancer. However, the relationship between improvements in pCR rate and clinical trial-arm level survival remains controversial because several, admittedly underpowered, trials failed to demonstrate significant survival improvement in trial arms with a higher pCR rate. There are many reasons (e.g., good baseline prognosis, effective post-neoadjuvant therapies, and pCR preferentially occurring in patients with good prognosis) why a regimen with substantially increased cytotoxic activity demonstrated by increased pCR rate could produce only small improvement in survival (11). Only substantially larger trials, or trials restricted to high-risk patient populations with high event rates, will have the adequate power to prospectively test the relationship between higher pCR rate and improved survival. It is important to remember that absence of evidence (from underpowered trials) is not evidence of absence for an effect.

An important corollary of these observations is that pCR itself can be used as the elusive predictive marker that defines the patient population among TNBC and HER2-positive cancers that benefitted the most from a given neoadjuvant chemotherapy regimen, and RD defines the patients who need further novel therapies to improve their outcome. Several neoadjuvant clinical trials have been designed to increase pCR rates for TNBC that capitalize on the improved molecular understanding of the disease and its microenvironment. A number of trials are being planned to combine neoadjuvant chemotherapy with immune checkpoint inhibitors based on preclinical results that indicate that chemotherapy-induced cytotoxicity is partly mediated by an antitumor immune response in the tumor microenvironment, which is also consistent with the higher pCR rates seen in immune-rich tumors. Until very recently, no adjuvant treatment options existed for TNBC and HER2-positive patients with RD; however, today at least four randomized clinical trials are ongoing or planned to test the efficacy of different novel adjuvant treatment strategies in this clinical setting. TDM1 is being tested as adjuvant treatment in HER2-positive breast cancers that have RD after trastuzumab/pertuzumab-containing neoadjuvant chemotherapy (NCT01196052). Olaparib is being tested in germline BRCA-mutated TNBC patients, including those with residual cancer after neoadjuvant chemotherapy (NCT02032823). Two other adjuvant trials are planned or will soon be activated for TNBC with RD that will test carboplatin (EA 1131A) and the anti–PD-1 antibody pembrolizumab, respectively (SWOG S1418). If these trials show positive results, the treatment paradigm for these molecular subtypes will fundamentally change. Positive results would establish neoadjuvant chemotherapy as the most logical and cost-effective initial therapy using pCR (and minimal residual cancer burden) as an important biologic early surrogate marker to determine who will need subsequent adjuvant therapy.

No potential conflicts of interest were disclosed.

Conception and design: L. Pusztai, R. Rouzier, W.F. Symmans

Writing, review, and/or revision of the manuscript: L. Pusztai, R. Rouzier, W.F. Symmans

This work was supported in part by grants from the Breast Cancer Research Foundation (L. Pusztai and W.F. Symmans) and Susan G. Komen (W.F. Symmans).

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