Which Patients Need Chemotherapy? From Pathological Risk Factors to Gene Signatures and Evaluation of Endocrine Response

Background: Chemotherapy, used either before or after surgery, has significantly improved survival in early breast cancer. Accurate risk assessment is essential to avoid both overtreatment and undertreatment. This review provides an overview of the evolution of chemotherapy as well as risk factors for tailored systemic therapies in early breast cancer – from pathologic risk factors to gene expression signatures to endocrine response assessment. Summary: Chemotherapy has improved dramatically in recent decades from its beginnings with conventionally dosed cyclophosphamide plus methotexate plus 5-fluorouracil to dose-dense anthracycline- and taxane-containing regimens. Similarly, risk assessment has evolved starting from traditional pathologic risk factors such as tumor size, axillary nodal status, and grading. In recent decades, gene expression signatures have improved prognostic accuracy with a high level of evidence. In turn, these signatures can be further improved by incorporating the aforementioned pathologic factors. As an important step away from this static assessment, dynamic assessment of proliferation factor Ki-67 after short-term preoperative endocrine treatment has gained interest to improve risk assessment in early hormone receptor-positive breast cancer. Key Message: This review highlights advances in chemotherapy and risk assessment in early breast cancer, from pathologic risk factors for recurrence to gene expression signatures and endocrine response assessment. These developments are leading to better risk stratification and thus better adaptation of therapies.


Introduction
Over the last decades, breast cancer mortality is declining in Western countries.Much of this decline has been attributed to early detection and to the use of adjuvant systemic therapy.Despite the increasing importance of targeted therapies, chemotherapy still plays an important role in the treatment of early breast cancer with increased risk of recurrence (ROR) [1][2][3].Since the groundbreaking work of Bonadonna and colleagues [4] almost five decades ago, combination chemotherapy for early breast cancer has increasingly reduced the ROR and steadily improved survival.In numerous other studies, the addition of newer drug classes such as anthracyclines and taxanes has significantly increased efficacy.It is generally accepted at meta-analysis level that adjuvant chemotherapy containing anthracyclines and taxanes reduces the risk of death from breast cancer by about one third [5].Increasing the dose intensity of adjuvant chemotherapy by shortening the interval between treatment cycles or by sequential administration additionally reduced the 10-year ROR (28.0% vs. 31.4%;risk ratio 0.86, 95% confidence interval [CI]: 0.82-0.89;p < 0.0001) and death (18.9% vs. 21.3%;risk ratio 0.87, 95% CI: 0.83-0.92;p < 0.0001) from breast cancer [6].
For example, Möbus and coworkers [7] conducted a clinical trial in high-risk breast cancer patients with four or more involved axillary lymph nodes using intense dosedense, sequential adjuvant chemotherapy.They showed a 10% absolute survival difference at 10 years of follow-up compared with conventionally dosed anthracycline-and taxane-based chemotherapy (hazard ratio [HR] = 0.72, 95% CI: 0.60-0.87;log-rank p = 0.0007) [7].
When chemotherapy is indicated for early breast cancer, it can be used not only as adjuvant therapy after removal of the breast cancer but also neoadjuvant before surgery, with similar overall survival [8,9].This approach offers the advantage that additional postneoadjuvant therapies can be given depending on the degree of pathologic response.To avoid both overtreatment and undertreatment, it is imperative to select the appropriate treatment strategy based on a careful risk assessment of each patient, carefully balancing the risk and benefit of potentially toxic systemic therapies.

Pathological Risk Factors
According to consensus recommendations and guidelines, conventional clinicopathological factors (Table 1) arguing for the indication of chemotherapy are histological grade 3 carcinomas, high Ki-67 levels, low hormone receptor (HR) status, human epidermal growth factor receptor 2 (HER2) positivity or triple-negative status, and the involvement of more than 3 lymph nodes [1,3,10].
It is well known that the long-term prognosis of early breast cancer depends on the number of axillary lymph nodes involved, histological grading, and tumor size [11,12].In addition to these clinicopathological factors, a plethora of novel prognostic and predictive factors has emerged over the last decades.

Gene Signatures
Especially in HR-positive, HER2-negative tumors, the additional use of multigene assays should help to reliably assess the individual risk profile to avoid both under-and over-therapy of the patient.The history of gene expression signatures in early breast cancer now spans more than 2 decades.Beginning with the seminal work of Perou and coworkers [13], numerous studies have demonstrated the independent prognostic significance of the socalled intrinsic subtypes (e.g., luminal A, luminal B, HER2 enriched, basal like), which were later determined in the form of the commercial Prosigna ® gene expression assay in formalin-fixed, paraffin-embedded tissue which predicts with 50 genes (PAM50) the ROR [13][14][15].This biomarker allows the distinction between two groups of HR-positive, HER2-negative tumors with different prognosis, which is important for the decision to use chemotherapy in addition to endocrine therapy or to use endocrine treatment only.
A second gene expression signature used a supervised clustering approach and found 70 genes that were differentially expressed in breast cancer with high or low ROR [16].The prospective MINDACT study compared the prognostic significance of the 70-gene signature (MammaPrint ® ) with clinicopathologic criteria in patients with early breast cancer (N0-1) [17].At nearly 9 years of follow-up, the 70-gene signature identified a subgroup among women at high clinical risk who had excellent distant metastasis-free survival with endocrine therapy alone.Among these women, the benefit of adding chemotherapy to endocrine therapy remained small (2.6%) and was not enhanced by nodal positivity [18].Further analysis of the MINDACT trial, with a median follow-up of 8.7 years, found that the ultralow-risk subgroup (15%) had an excellent breast cancer-specific longterm survival rate of 99% [19].
By far the most commonly used gene expression signature is the recurrence score (RS).The Oncotype DX ® multigene assay was developed for patients with HR-positive, node-negative breast cancer treated with tamoxifen [20].Further prospective-retrospective analyses showed that this biomarker was predictive for benefit of adjuvant chemotherapy in node-negative and nodepositive patients with HR-positive, HER2-negative early breast cancer [21,22].Prospective studies showed excellent survival of breast cancer patients with HR-positive tumors and a low RS (<11) in the prospective randomized TAILORx trial.1,626 (15.9%) patients classified in the low-risk group based on an RS < 11 had a 5-year diseasefree survival of 93.8% and an overall survival of 98.0% with endocrine therapy alone.The authors therefore considered that patients with low RS can be spared adjuvant chemotherapy [23].Furthermore, the results of the TAILORx study allowed a more reliable statement for patients with an intermediate risk profile.The results demonstrated that the intermediate risk group (RS 11-25) did not benefit from additional chemotherapy Schmidt (HR = 1.08; 95% CI: 0.94-1.24).In younger women (<50 years) with intermediate RS, a slight prognostic advantage of chemotherapy was seen in terms of disease recurrence but not for overall survival [24].Further analysis of the TAILORx trial showed that joint assessment of genomic and clinical risk (tumor size and grading) improved assessment of prognosis [25].In the prospectively randomized PlanB trial, Gluz and coworkers [26] demonstrated that patients with a low RS ≤ 11 have an excellent 3-year disease-free survival of 98% with endocrine therapy alone.In another evaluation of the PlanB trial, Nitz et al. [27] examined traditional clinicopathologic prognostic factors, including nodal status, grading, tumor size, and Ki-67 in addition to RS.The prognostic significance of RS was particularly pronounced in tumors with intermediate Ki-67 (>10%, <40%).The authors concluded that the proliferation factor Ki-67 has the potential to help select patients for genomic testing.
The RNA-based multigene score EndoPredict ® (EP) is based on gene expression analysis of 11 genes, eight cancer-related genes, and three reference genes [29].EPclin ® additionally considers tumor size and the number of affected lymph nodes.The test result distinguishes between patients at low risk (EPclin low risk) and high ROR (EPclin high risk).In postmenopausal, estrogen receptor-positive, and node-negative or -positive patients from the Austrian Breast Cancer Study Group studies 6 and 8 (ABCSG 6 and 8 study), the EPclin test allowed identification of a low-risk group with a 10-year distant metastasis risk of 4% (ABCSG 6 study: 4% vs. 28%; ABCSG 8 study: 4% vs. 22%).Other studies demonstrated prognostic relevance also in premenopausal patients and in patients treated with chemotherapy [30,31].In a consecutive cohort of HR-positive, HER2-negative breast carcinomas with a maximum of 3 positive axillary lymph nodes, Almstedt and co-workers [32] showed that a treatment decision based solely on traditional clinicopathologic parameters was judged inadequate for determining further systemic therapy in 18% of cases.The additional use of EPclin ® led to a change in the therapy recommendation in one-third of the cases, with therapy escalation in 19.2% and de-escalation in 14.1%.
Since multiple gene expression signatures with different genes are used in clinical practice, the concordance and the respective prognostic significance is of interest.In the prospective OPTIMA study, both risk classification and molecular subtype classification were compared between different commercially available gene expression tests [33].For a significant percentage, both risk classification (60.6%, kappa 0.33-0.60)and molecular subtype classification (40.7%, kappa 0.39-0.55)were inconsistent between the tests studied.In this context, Buus et al. [34] identified the molecular features (i.e., proliferation or estrogen related) that significantly influence the multigene tests and lead to these differences.
A direct comparison of the prognostic significance of gene expression signatures was performed in estrogen receptor-positive, HER2-negative, endocrine-treated patients in the TransATAC study [35,36].All multigene assays showed their prognostic value.However, it was shown that the assays that considered clinicopathological parameters in addition to molecular ones had higher prognostic value.

Evaluation of Endocrine Response
The above biomarkers provide an accurate prediction of the individual ROR for a given patient with early HRpositive, Her2-negative breast cancer.However, this risk estimate is static and does not reflect the dynamic interaction between a therapy and a biomarker.An elegant way to circumvent these limitations is to determine the proliferation marker Ki-67 before and after a short (2-4 weeks) preoperative endocrine therapy.The IMPACT trial demonstrated that measuring Ki67 levels in tumors after short-term endocrine treatment can improve prediction of relapse-free survival by relating the prognostic value of Ki67 levels at baseline to changes in Ki67 levels associated with treatment success [37,38].Similarly, the risk of relapse was increased for those with 2-to 4-week Ki67 >10% in the ACOSOG Z1031B study (p = 0.0044) [39].These promising early results were later confirmed in the multicenter, randomized phase 3 POETIC trial [40].In this large-scale confirmatory trial, Smith and coworkers [40] recruited 4,480 postmenopausal women with HR-positive, operable breast cancer.Patients were randomly assigned to preoperative treatment with an aromatase inhibitor for 14 days before and following surgery or no preoperative endocrine therapy.The primary endpoint was time to disease recurrence.An important secondary outcome was the association between Ki67 (dichotomized at 10%) and disease outcomes.Disease recurrence was similar in both groups.Most patients with low baseline Ki67 (Ki67 B ) (5-year recurrence risk 4.3%) or low preoperative treatment-induced Ki67 at 2 weeks (Ki67 2W ) (5-year recurrence risk 8.4%) did Which Patients Need Chemotherapy?Breast Care 2023;18:423-428 DOI: 10.1159/000530818 well with standard adjuvant endocrine therapy.In contrast, patients whose preoperative treatment-induced Ki67 2W remained high had a higher ROR (5-year recurrence risk 21.5%).These results document the relationship of 2-week Ki67 with ROR for estimating whether the prognosis of individual patients is sufficiently good on endocrine therapy alone or whether additional chemotherapy should be considered.
As outlined above, validated gene expression tests can also predict the ROR for individual patients.Therefore, it seems logical to combine both biomarkers for risk assessment in HR-positive, HER2-negative breast cancer.In this regard, the WSG-ADAPT trial is the first study to combine the 21-gene expression assay and response to 3 weeks of preoperative endocrine therapy to guide systemic therapy in early breast cancer [41].In the intention-to-treat population of this study, 2,290 HR-positive, HER2-negative early breast cancer patients with 0-3 axillary lymph nodes involved were assessed with the RS.In addition, Ki67 was analyzed both before and after preoperative endocrine therapy.Patients entered the control arm if RS was ≤11 and the experimental arm if RS was 12-25 and they responded to preoperative endocrine therapy (Ki67 after preoperative endocrine therapy ≤10%).All other patients received dose-dense chemotherapy followed by endocrine therapy.The 5y-iDFS was 92.6% in the experimental arm versus 93.9% in the control arm, demonstrating noninferiority of the control arm.Differences were similar in different subgroups.Thus, the WSG-ADAPT trial demonstrated that guiding systemic treatment by RS and preoperative endocrine response is feasible in routine clinical practice and may spare adjuvant chemotherapy in premenopausal and postmenopausal patients with ≤3 affected lymph nodes.

Recommendations for Biomarkers to Direct Treatment Decisions in Early Breast Cancer
Both the American Society of Clinical Oncology (ASCO) and the Working Group Gynecologic Oncology (AGO) of the German Society of Obstetrics and Gynecology (DGGG) publish regularly updated guidelines for the use of biomarkers for adjuvant therapy decisions in patients with early breast cancer [1,42].The AGO recommends Oncotype DX ® (level of evidence 1a, recommendation grade A, AGO+), MammaPrint ® (1a, A, +), Prosigna ® (2b, B, +), and EndoPredict ® (2b, B, +) in the context of clinicopathological criteria (tumor size, nodal involvement, grade, Ki-67, HR, HER2) for HR-positive, HER2-negative breast cancer with up to 3 affected axillary lymph nodes.Reassessment of Ki-67 after a short preoperative endocrine therapy (2-4 weeks) is also recommended by the AGO (1b, B, +) [1] (Table 2).
According to the latest update of the ASCO recommendations, Oncotype DX ® , MammaPrint ® , Prosigna ® , and EndoPredict ® can be used to guide adjuvant therapy decisions in postmenopausal patients with early HR-positive, HER2-negative breast cancer and up to three affected axillary lymph nodes [42].In premenopausal patients, Oncotype DX ® can be used for node-negative HRpositive, HER2-negative breast cancer.However, premenopausal patients with 1-3 positive axillary lymph nodes benefit from chemotherapy when Oncotype DX ® indicates intermediate risk.Ki-67 can be used to guide decisions about adjuvant therapy in postmenopausal patients who do not have access to genomic testing.However, there is insufficient evidence that baseline Ki-67 expression or Ki-67 levels after 2 weeks of neoadjuvant aromatase inhibitor therapy can be used to guide decisions about adjuvant endocrine therapy and chemotherapy.

Conclusion
The efficacy of chemotherapy for early breast cancer has improved in recent decades.In particular, dose-dense anthracycline and taxane-based chemotherapy has improved the survival of breast cancer patients.To avoid over-or undertreatment, careful risk assessment is critical.In this regard, not only traditional prognostic factors such as nodal status or tumor size play an important role in HRpositive, HER2-negative early breast cancer.Especially in AGO, Working Group Gynecologic Oncology; ER, estrogen receptor; GR, grade of recommendation; HER2, human epidermal growth factor receptor 2; HR, hormone receptor; LoE, level of evidence; PR, progesterone receptor.*Should only be used in the context of clinical-pathological criteria (tumor size, nodal involvement, grade, Ki-67, ER, PR, HER2).
node-negative or limited nodal involvement, validated gene expression signatures or short-term preoperative evaluation of Ki-67 are becoming increasingly important.

Table 2 .
Tumor biological risk factors