5. Extended Endocrine Therapy (EET)
There is a wide range of variation regarding survival endpoint achievement in RCTs with EET beyond the standard five year duration
[4,56][4][43]. However, the consensus is that EET should be dedicated to patients harboring pathologically high long-term risks for a total duration of no longer than ten years
[46,57][44][45].
The ATLAS study revealed that continuing Tamoxifen to ten years, versus concluding at five years, notably extends OS and DFS, with an absolute risk reduction for BC recurrence and mortality by 3.7% and 2.8%, at five years after the extended therapy was completed
[58][46]. Nevertheless, it elevates the absolute cumulative risk to develop endometrial cancer by 1.7%
[59][47]. Irrespective of whether the initial five years on ET used Tamoxifen, AI, or both (sequential switching therapy), EET with AI for an additional two to five years improves the risk of DFS by approximately 23% in high-risk postmenopausal women. Unfortunately, compilations of RCTs in a high-level evidence systematic review and meta-analysis did not demonstrate OS with an extended duration of AI therapy
[40][29]. On the contrary, its prolonged exposure significantly increases musculoskeletal pain and increases the risk for cardiovascular events, fractures, and osteoporosis
[40,60,61][29][48][49].
6. Genomic Expression Assays (GEAs)
Through evaluation of tumor biology using reference molecular drivers of cancer-related genes, GEAs generate prognostic information to estimate recurrence rates in the ER+ HER2- early BC
[6,7][6][7]. Producing a grading risk score, GEAs identify those for whom adjuvant chemotherapy is not advantageous. Albeit, for those in whom the magnitude of effect of chemotherapy is not substantial, ET has a paramount role
[6,7,8,9][6][7][8][9].
There is paucity of evidence that genomic profiling can determine adjuvant ET duration
[9]. However, emerging studies have started to portray support in this area. Among the GEAs, the Breast Cancer Index (BCI) appraises the ratio of estrogen signaling and tumor proliferation, and thereby predicts ET efficacy
[6,65][6][50]. Regardless of whether the ET class administered in the EET setting is the same as the primary adjuvant therapy, BCI has demonstrated to be a prognostic and predictive tool suitable for identifying patients in whom EET is beneficial
[65,66,67][50][51][52]. The Clinical Treatment Score post-5 years (CTS5) is an online algorithm-based predictor tool that uses clinical and pathological features to calculate the ratio for late distant BC relapse after five years of adjuvant ET completion
[9,68][9][53].
7. Adjuvant CDK4/6 Inhibitors and Endocrine Therapy
Critical measures to advance novel therapies are necessary for improving treatment outcomes in the high-risk groups. The successful results of CDK4/6 inhibitors in advanced BC patients prompted the emergence of new studies extending this drug class to non-metastatic scenarios
[69,70,71,72,73][54][55][56][57][58]. The Pallas and Penelope-B trials combined Palbociclib with ET in adjuvant and neoadjuvant BC populations, respectively. While the first study investigated Palbociclib for two years, the latter planned its administration for one year only. Markedly, both studies failed to demonstrate their survival- and efficacy-related endpoints of adding Palbociclib to ET
[74,75,76][59][60][61].
Meanwhile, the MonarchE trial, combining ET with two years of Abemaciclib in the postoperative setting in patients with adverse pathological LN+ presentation, decreased the risk of local–regional and distant recurrence by at least 25% when compared with ET alone
[77][62]. Regardless of the index Ki-67, the absolute benefit of adding Abemaciclib to improving the risk of BC relapse reached 5.4% at 3 years
[73,78][58][63]. As a response to these outstanding results, the ASCO guideline optimized recommendation of Abemaciclib plus ET to patients categorized within the high-risk group (
Figure 1 and
Figure 2)
[47][64]. Notably, independent of the menopausal status, either Tamoxifen or AI plus or minus OFS (if applicable) were used in the MonarchE study
[77][62].
8. Bone-Modifying Agents (BMAs)
Bisphosphonates act by inhibiting osteoclasts by way of apoptosis, and thereby decrease bone resorption and increase mineralization
[80,81][65][66]. Independent of the ER and HER2 status, the usage of bisphosphonates improves OS and DFS, and lower rates of bone metastasis in adjuvant breast cancer
[82][67]. These effects are restricted to postmenopausal women and a higher magnitude of treatment effects may be encountered in those with an elevated risk for BC recurrence
[83][68]. In this group, the time-to-event outcome showed a reduction in risk of mortality by 23% and disease recurrence by 18% when compared with no BMAs. Nevertheless, employing adjuvant bisphosphonates has a protective factor by reducing the risk of bone fracture events by more than 25%.
Cancer Care Ontario, in conjunction with ASCO, recommend one of following bisphosphonate agents: oral clodronate, oral ibandronate, or intravenous zoledronic acid (
Figure 1 and
Figure 2). An early start, within two-to-three months from the end of adjuvant chemotherapy or curative-intent surgery, leads to better BMAs efficacy
[83][68]. Its usage is not exempt from side effects, such as bone pain, fatigue, potential rare episodes of hypocalcaemia, and osteonecrosis of the jaw, and should be disclosed to patients
[82,83][67][68]. The latter encompasses 0.7% of cases and it has an increased likelihood with invasive dental surgical procedures
[82,83,85][67][68][69].
9. Conclusions
Breast tumors associated with luminal differentiation ER+ HER2−, comprise the largest subgroup of female BC. In the adjuvant setting, its cornerstone treatment relies on ET, and its benefits translate dramatically to lengthen life expectancy with bearable side-effects. Nonetheless, relapse events steadily continue beyond the time of treatment completion, regardless of ET duration
[4]. Tailoring the breadth of endocrine therapies hinges on a wide array of factors to be appraised by the prescribing physician, such as the patient’s menopausal status and the pathological tumor landscape. Classifying the risk category for the BC assists in deciding the treatment route and its optimal duration. In a select group of patients, GEAs predict those for whom chemotherapy is not beneficial and thereby for whom ET is the preferred choice. A meticulous disclosure of each suitable ET helps clinicians and patients to choose the appropriate therapy for each individualized case, outweighing its benefit and conceivable harm. Additionally, emphasizing an adequate treatment adherence is a crucial factor in contributing to satisfactory outcomes.
Furthermore, elderly patients are commonly underrepresented in randomized controlled studies. Hence, a thorough collection of medical history and special attention is required with respect to potential detrimental drug interaction in this population, and any added medicine should be cautiously selected.