A meta-analysis including over 5000 patients with metastatic hormone receptor–positive (HR+) and HER2- breast cancer showed a significant overall survival (OS) benefit with the addition of cyclin-dependent kinase (CDK) 4/6 inhibitors to endocrine therapy (hazard ratio 1.33; P < .001), albeit with higher rates of toxicities, including neutropenia, leukopenia, and diarrhea.3The MONALEESA-2 study randomly assigned 668 postmenopausal women with metastatic HR+/HER2- breast cancer, treatment-naive in the advanced setting, to either ribociclib or placebo plus letrozole. Updated results with a median follow-up of 6.6 years demonstrated a significant OS benefit with ribociclib + letrozole compared with placebo + letrozole (median OS 63.9 months vs 51.4 months; hazard ratio 0.76; P = .008) (Hortobagyi and colleagues). An OS > 5 years with ribociclib plus endocrine therapy is certainly impressive, and efficacy as well as respective toxicities of the various CDK 4/6 inhibitors are factors taken into consideration when choosing the appropriate therapy for an individual patient.
The optimization of adjuvant endocrine therapy (ET) for HR+ early breast cancer, including use of ovarian suppression and extended adjuvant therapy, has improved outcomes for these women. However, there is a high-risk subset for whom the risk for distant recurrence persists. The phase 3 monarchE trial, which included 5637 patients with high-risk early breast cancer (≥ 4 positive nodes, or 1-3 nodes and either tumor size ≥ 5 cm, histologic grade 3, or central Ki-67 ≥ 20%), demonstrated benefits in invasive disease-free and distant-relapse-free survival with the addition of abemaciclib for 2 years to ET. A safety analysis of the monarchE study among patients who had received at least one dose of the study drug (n = 5591) demonstrated an overall manageable side-effect profile, with the majority of these toxicities addressed via dose holds/reductions or supportive medications (Rugo and colleagues). Abemaciclib + ET led to higher incidence of grade ≥ 3 adverse events vs ET alone (49.7% vs 16.3%), with neutropenia being the most frequent (grade 3 = 19.6%) although without significant clinical implications. Diarrhea was common (83.5%), although the majority was low grade (grade 1/2 = 75.7%), with grade 2/3 events characterized by early onset and short duration. Discontinuation of abemaciclib occurred in 18.5%, with two thirds due to grade 1/2 events and in over half without dose reduction.4 These findings show an acceptable safety profile with abemaciclib in the curative setting and highlight the importance of education, recognition, and early management of side effects to maintain patients on treatment.
The heterogeneity of tumor biology within the HR+ breast cancer subtype indicates the need to refine treatment regimens for an individual patient. Genomic assays (70-gene signature and 21-gene recurrence score) have helped tailor adjuvant systemic therapy and in many cases have identified women for whom chemotherapy can be omitted. CDK 4/6 inhibitors have shown impressive activity in the metastatic/advanced setting, although results from trials in the adjuvant setting have produced mixed results. The phase 2 NEOPAL trial evaluated the combination of letrozole + palbociclib vs chemotherapy (sequential anthracycline-taxane) among 106 postmenopausal women with high-risk, HR+/HER2- early breast cancer (luminal B or luminal A with nodal involvement). At a median follow-up of 40.4 months, 3-year PFS (hazard ratio 1.01; P = .98) and invasive disease-free survival (hazard ratio 0.83; P = .71) were similar in the letrozole + palbociclib and chemotherapy arms (Delaloge and colleagues). The phase 2 CORALLEEN trial,5 which investigated neoadjuvant letrozole + ribociclib vs chemotherapy in HR+/HER2- luminal B early breast cancer, demonstrated similar percentages of patients achieving downstaging via molecular assessment at the time of surgery. The neoadjuvant space represents a valuable setting to further study CDK 4/6 inhibitors as well as other novel therapies; endpoints including pathologic complete response and residual cancer burden correlating with long-term outcomes can provide a more rapid means to identify effective therapies. Translational biomarkers can be gathered and adjuvant strategies can be tailored based on response.
- Modi S, Saura C, Yamashita T, et al; DESTINY-Breast01 Investigators. Trastuzumab deruxtecan in previously treated HER2-positive breast cancer. N Engl J Med. 2020;382:610-621. Doi: 10.1056/NEJMoa1914510 Source
- Hurvitz S, Kim S-B, Chung W-P, et al. Trastuzumab deruxtecan (T-DXd; DS-8201a) versus trastuzumab emtansine (T-DM1) in patients (pts) with HER2+ metastatic breast cancer (mBC): Subgroup analyses from the randomized phase 3 study DESTINY-Breast03. Presented at 2021 San Antonio Breast Cancer Symposium; December 7-10, 2021;General Session, GS3-01. Source
- Li J, Huo X, Zhao F, et al. Association of cyclin-dependent kinases 4 and 6 inhibitors with survival in patients with hormone receptor-positive metastatic breast cancer: A systematic review and meta-analysis. JAMA Netw Open. 2020;3:e2020312. Doi: 10.1001/jamanetworkopen.2020.20312 Source
- Harbeck N, Rastogi P, Martin M, et al. Adjuvant abemaciclib combined with endocrine therapy for high-risk early breast cancer: Updated efficacy and Ki-67 analysis from the monarchE study. Ann Oncol. 2021;32:1571-1581. Doi: 10.1016/j.annonc.2021.09.015 Source
- Prat A, Saura C, Pascual T, et al. Ribociclib plus letrozole versus chemotherapy for postmenopausal women with hormone receptor-positive, HER2- negative, luminal B breast cancer (CORALLEEN): An open-label, multicentre, randomised, phase 2 trial. Lancet Oncol. 2020;21:33-43. Doi: 10.1016/S1470-2045(19)30786-7 Source