The Evolving Role of Ribociclib and Optimizing First-Line Treatment in HR+/HER2- Metastatic Breast Cancer 

Nerea Lopetegui-Lia, MD The Ohio State University

December 1, 2025

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Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors, in combination with endocrine therapy (ET), are the standard first-line treatment for patients with hormone receptor-positive (HR+), HER2-negative metastatic breast cancer and have markedly improved outcomes. Three CDK4/6 inhibitors are approved in this setting: palbociclib (first-in-class), abemaciclib, and ribociclib, all of which have demonstrated meaningful improvements in progression-free survival (PFS) and overall survival (OS) compared to placebo plus ET1. 

No head-to-head trials have established superiority among the available agents and differences in study populations across the phase III trials complicate cross-trial comparisons. However, ribociclib has distinguished itself as the only CDK4/6 inhibitor with a statistically significant OS benefit in the first-line setting when combined with an aromatase inhibitor. 

Specifically, MONALEESA-22 evaluated ribociclib plus letrozole in postmenopausal women in the first-line setting and demonstrated a median OS (mOS) of 63.9 versus 51.4 months (HR 0.76; P = 0.008). In contrast, PALOMA-23, which evaluated palbociclib plus letrozole in postmenopausal women, reported an mOS of 53.9 versus 51.2 months (HR 0.96; P =  .34), and MONARCH-34, which evaluated abemaciclib with letrozole or anastrozole, reported an mOS of  66.8 versus 53.7 months (HR 0.80; P = 0.07); neither was statistically significant. While most studies focused on postmenopausal women, MONALEESA-75 evaluated ribociclib with tamoxifen or a nonsteroidal aromatase inhibitor with ovarian function suppression in pre- and perimenopausal women, demonstrating an mOS of 58.7 versus 48.0 months (HR 0.763; P = 0.00973). 

For patients who experience disease progression on adjuvant ET or have recurrence within 12 months of completing adjuvant therapy, fulvestrant combined with either ribociclib or abemaciclib is considered a category 1, first- or second-line treatment option per NCCN guidelines, based on OS benefit demonstrated in MONALEESA-36 and MONARCH-27, respectively. MONALEESA-36 assessed ribociclib plus fulvestrant in postmenopausal women, showing a significant mOS of 67.6 months versus 51.8 months (HR, 0.67); with an HR for death of 0.72 (P = 0.00455)6,28. MONARCH-27 assessed abemaciclib plus fulvestrant, demonstrating an mOS of 45.8 months versus 37.2 months (HR, 0.78)7. 

Patient- and Disease-Related Factors Influencing First-Line Decision Making 

Beyond OS, several clinical factors guide the selection of a CDK4/6 inhibitor, including toxicity profile, patient comorbidities, and menopausal status. 

Each inhibitor has a distinct toxicity profile. Palbociclib and ribociclib are more frequently associated with neutropenia and leukopenia due to their bone marrow–suppressive effects. This underscores the need for regular blood count monitoring during treatment, both to prevent complications (eg, febrile neutropenia) and to allow for timely dose adjustments. In contrast, abemaciclib is primarily linked to gastrointestinal (GI) toxicity, particularly diarrhea. With ribociclib, close ECG monitoring is essential to assess for any QTc prolongation at initiation8. As a result, patients with baseline GI issues may be better suited for ribociclib or palbociclib, while those with cardiac comorbidities (eg, arrhythmias) may be better candidates for abemaciclib or palbociclib. 

Menopausal status is also an important consideration. Most pivotal trials predominantly enrolled postmenopausal women. However, MONARCH 2 included pre- and postmenopausal patients treated with abemaciclib7, and MONALEESA-7, as noted, exclusively evaluated ribociclib in pre- and perimenopausal women5. It is important to note that ribociclib is not recommended in combination with tamoxifen due to the increased risk of QTc prolongation5.

How First-Line CDK4/6 Inhibitor Selection Affects Subsequent Sequencing Strategies and Long-Term Management 

The choice of a first-line CDK4/6 inhibitor establishes the foundation for subsequent treatment sequencing. Still, it does not fundamentally alter the overarching principles of post-progression management, which involve assessing for targetable mutations.  

Data suggest that sensitivity to some CDK4/6 inhibitors may be retained despite resistance to another CDK4/6 inhibitor. This highlights the likely biological differences across the CDK4/6 inhibitor class9. However, current data do not support the routine continuation or switching of CDK4/6  inhibitors beyond progression on a prior CDK4/6 inhibitor. The MAINTAIN trial10, which primarily included patients who had progressed on palbociclib and switched to ribociclib, and the postMONARCH trial11,  which evaluated patients who progressed on palbociclib or ribociclib and switched to abemaciclib, demonstrated only a modest clinical benefit. Therefore, most guidelines recommend transitioning to a different therapeutic class after progression on a CDK4/6 inhibitor. 

Upon progression on a CDK4/6 inhibitor, subsequent therapy remains individualized and biomarker-driven, with an emphasis on identifying actionable mutations through next-generation sequencing of blood ctDNA and/or tissue12. 

If no actionable mutations are detected, different options can be considered depending on the treatments previously used. These include everolimus + fulvestrant (PrE010213, MANTA14, after BOLERO-215 led to the initial regulatory approval of everolimus + exemestane), abemaciclib + fulvestrant (postMONARCH11), and, pending FDA approval, fulvestrant + gedatolisib +/- palbociclib (Viktoria-116). 

For patients with an ESR1 mutation, elacestrant (EMERALD17) or imlunestrant (EMBER-318) are appropriate treatment options. Giredestrant + everolimus (evERA BC19), pending regulatory approval, is another emerging strategy. 

Given the central role of the PIK3CA/AKT/PTEN pathway in breast cancer biology, targeted approaches should be considered. If patients harbor a PIK3CA, AKT1, or PTEN alteration, capivasertib + fulvestrant (CAPItello-29120) is approved. In patients with a PIK3CA-activating mutation, alpelisib + fulvestrant (SOLAR 121) is an alternative approved option. Lastly, the inavolisib + palbociclib + fulvestrant (INAVO-12022) triplet can be considered in patients with a PIK3CA mutation whose disease progressed during or within 12 months of completing adjuvant ET.

For patients who harbor a germline BRCA1 or BRCA2 mutation, PARP inhibitors such as olaparib (OlympiAD23) or talazoparib (EMBRACA24) are available treatment options. 

In the third-line setting and beyond, and in select earlier-line situations such as visceral crisis, antibody–drug conjugates may be used. These include trastuzumab deruxtecan (for HER2-low or HER2–ultra-low disease, [Destiny-Breast0625]) and sacituzumab govitecan (TROPiCS-0226) or datopotamab deruxtecan (TROPION-Breast0127) in HER2-null. Conventional chemotherapy is typically reserved for later lines of therapy. 

The expanding treatment landscape, increasing personalization of care, emphasis on minimizing unnecessary toxicity, and ongoing efforts to define optimal sequencing underscore the central and evolving role of therapy in HR+/HER2– metastatic breast cancer.

References

  1. Li J, Huo X, Zhao F, Ren D, Ahmad R, Yuan X, Du F, Zhao J. 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 Network Open. 2020 Oct 1;3(10):e2020312-.
  2. Hortobagyi GN, Stemmer SM, Burris HA, Yap YS, Sonke GS, Hart L, Campone M, Petrakova K, Winer EP, Janni W, Conte P. Overall survival with ribociclib plus letrozole in advanced breast cancer. New England Journal of Medicine. 2022 Mar 10;386(10):942-50.
  3. Slamon DJ, Diéras V, Rugo HS, Harbeck N, Im SA, Gelmon KA, Lipatov ON, Walshe JM, Martin M, Chavez MacGregor M, Bananis E. Overall survival with palbociclib plus letrozole in advanced breast cancer. Journal of Clinical Oncology. 2024 Mar 20;42(9):994-1000.
  4. Goetz MP, Toi M, Huober J, Sohn J, Trédan O, Park IH, Campone M, Chen SC, Manso LM, Paluch-Shimon S, Freedman OC. Abemaciclib plus a nonsteroidal aromatase inhibitor as initial therapy for HR+, HER2− advanced breast cancer: final overall survival results of MONARCH 3. Annals of Oncology. 2024 Aug 1;35(8):718-27.
  5. Im SA, Lu YS, Bardia A, Harbeck N, Colleoni M, Franke F, Chow L, Sohn J, Lee KS, Campos-Gomez S, Villanueva-Vazquez R. Overall survival with ribociclib plus endocrine therapy in breast cancer. New England Journal of Medicine. 2019 Jul 25;381(4):307-16.
  6. Neven P, Fasching PA, Chia S, Jerusalem G, De Laurentiis M, Im SA, Petrakova K, Bianchi GV, Martín M, Nusch A, Sonke GS. Updated overall survival from the MONALEESA-3 trial in postmenopausal women with HR+/HER2− advanced breast cancer receiving first-line ribociclib plus fulvestrant. Breast Cancer Research. 2023 Aug 31;25(1):103.
  7. Llombart-Cussac A, Sledge G, Toi M, Neven P, Sohn JH, Inoue K, Pivot X, Okera M, Masuda N, Kaufman PA, Koh H. Abstract PD13-11: Final overall survival analysis of MONARCH 2: a phase 3 trial of abemaciclib plus fulvestrant in HR+, HER2− advanced breast cancer. Cancer Research. 2023 Mar 1;83(5_Supplement):PD13-11.
  8. Onesti CE, Jerusalem G. CDK4/6 inhibitors in breast cancer: differences in toxicity profiles and impact on agent choice. A systematic review and meta-analysis. Expert Review of Anticancer Therapy. 2021 Mar 4;21(3):283-98.
  9. Braal CL, Jongbloed EM, Wilting SM, Mathijssen RH, Koolen SL, Jager A. Inhibiting CDK4/6 in breast cancer with palbociclib, ribociclib, and abemaciclib: similarities and differences. Drugs. 2021 Feb;81(3):317-31.
  10. Kalinsky K, Accordino MK, Chiuzan C, Mundi PS, Sakach E, Sathe C, Ahn H, Trivedi MS, Novik Y, Tiersten A, Raptis G. Randomized phase II trial of endocrine therapy with or without ribociclib after progression on CDK4/6 inhibition in HR+, HER2− metastatic breast cancer: MAINTAIN trial. Journal of Clinical Oncology. 2023 Aug 20;41(24):4004-13.
  11. Kalinsky K, Bianchini G, Hamilton E, Graff SL, Park KH, Jeselsohn R, Demirci U, Martin M, Layman RM, Hurvitz SA, Sammons S. Abemaciclib plus fulvestrant in advanced breast cancer after progression on CDK4/6 inhibition: results from the phase III postMONARCH trial. Journal of Clinical Oncology. 2025 Mar 20;43(9):1101-12.
  12. NCCN Guidelines – Breast Cancer.
  13. Kornblum N, Zhao F, Manola J, Klein P, Ramaswamy B, Brufsky A, Stella PJ, Burnette B, Telli M, Makower DF, Cheema P. Fulvestrant plus everolimus or placebo in postmenopausal women with HR+, HER2− metastatic breast cancer resistant to aromatase inhibitors: PrE0102. Journal of Clinical Oncology. 2018 Jun 1;36(16):1556-63.
  14. Schmid P, Zaiss M, Harper-Wynne C, Ferreira M, Dubey S, Chan S, Makris A, Nemsadze G, Brunt AM, Kuemmel S, Ruiz I. Fulvestrant + vistusertib vs fulvestrant + everolimus vs fulvestrant alone in HR+ metastatic breast cancer: MANTA. JAMA Oncology. 2019 Nov 1;5(11):1556-64.
  15. Piccart M, Hortobagyi GN, Campone M, Pritchard KI, Lebrun F, Ito Y, Noguchi S, Perez A, Rugo HS, Deleu I, Burris HA III. Everolimus + exemestane in HR+, HER2− advanced breast cancer: overall survival results from BOLERO-2. Annals of Oncology. 2014 Dec 1;25(12):2357-62.
  16. Hurvitz SA, Layman RM, Curigliano G, André F, Cristofanilli M, Kim SB, Rodriguez JM, Nadal JC, Kim GM, Emile G, Wesolowski R. LBA17 Gedatolisib + fulvestrant ± palbociclib vs fulvestrant in HR+/HER2−/PIK3CA WT advanced breast cancer: VIKTORIA-1. Annals of Oncology. 2025 Sep 1;36:S1677-8.
  17. Bardia A, Cortés J, Bidard FC, Neven P, Garcia-Sáenz J, Aftimos P, O’Shaughnessy J, Lu J, Tonini G, Scartoni S, Paoli A. Elacestrant in ER+, HER2− metastatic breast cancer with ESR1-mutated tumors: subgroup analyses from the phase III EMERALD trial. Clinical Cancer Research. 2024 Oct 1;30(19):4299-309.
  18. Jhaveri KL, Neven P, Casalnuovo ML, Kim SB, Tokunaga E, Aftimos P, Saura C, O’Shaughnessy J, Harbeck N, Carey LA, Curigliano G. Imlunestrant with or without abemaciclib in advanced breast cancer. New England Journal of Medicine. 2025 Mar 27;392(12):1189-202.
  19. Mayer E, Tolaney S, Martin M, Vidal G, Moscetti L, Jhaveri K, Brufsky A, Gradishar W, Schneeweiss A, Niikura N, Favret A. LBA16 Giredestrant + everolimus in ER+, HER2− advanced breast cancer previously treated with a CDK4/6 inhibitor: evERA BC trial. Annals of Oncology. 2025 Sep 1;36:S1676-7.
  20. Turner NC, Oliveira M, Howell SJ, Dalenc F, Cortes J, Gomez Moreno HL, Hu X, Jhaveri K, Krivorotko P, Loibl S, Morales Murillo S, Okera M, Park YH, Sohn J, Toi M, Tokunaga E, Yousef S, Zhukova L, de Bruin EC, Grinsted L, Schiavon G, Foxley A, Rugo HS; CAPItello-291 Study Group. Capivasertib in hormone receptor–positive advanced breast cancer. New England Journal of Medicine. 2023 Jun 1;388(22):2058-2070.
  21. André F, Ciruelos E, Rubovszky G, Campone M, Loibl S, Rugo HS, Iwata H, Conte P, Mayer IA, Kaufman B, Yamashita T, Lu YS, Inoue K, Takahashi M, Pápai Z, Longin AS, Mills D, Wilke C, Hirawat S, Juric D; SOLAR-1 Study Group. Alpelisib for PIK3CA-mutated, hormone receptor–positive advanced breast cancer. New England Journal of Medicine. 2019 May 16;380(20):1929-1940.
  22. Jhaveri KL, Im SA, Saura C, Loibl S, Kalinsky K, Schmid P, Loi S, Thanopoulou E, Shankar N, Jin Y, Stout TJ, Clark TD, Song C, Juric D, Turner NC. Overall survival with inavolisib in PIK3CA-mutated advanced breast cancer. New England Journal of Medicine. 2025 Jul 10;393(2):151-161.
  23. Senkus E, Delaloge S, Domchek SM, Conte P, Im SA, Xu B, Armstrong A, Masuda N, Fielding A, Robson M, Tung N. Olaparib efficacy in germline BRCA-mutated, HER2-negative metastatic breast cancer: OlympiAD trial subgroup analyses. International Journal of Cancer. 2023 Aug 15;153(4):803-14.
  24. Litton JK, Hurvitz SA, Mina LA, Rugo HS, Lee KH, Gonçalves A, Diab S, Woodward N, Goodwin A, Yerushalmi R, Roché H. Talazoparib vs chemotherapy in germline BRCA1/2-mutated HER2-negative advanced breast cancer: EMBRACA OS results. Annals of Oncology. 2020 Nov 1;31(11):1526-35.
  25. Bardia A, Barrios C, Dent R, Hu X, O’Shaughnessy J, Yonemori K, Darilay A, Boston S, Liu Y, Patel G, Curigliano G. Abstract OT-03-09: Trastuzumab deruxtecan vs investigator’s choice chemo in HR+, HER2-low metastatic breast cancer: DESTINY-Breast06. Cancer Research. 2021 Feb 15;81(4_Supplement):OT-03.
  26. Rugo HS, Bardia A, Marmé F, Cortés J, Schmid P, Loirat D, Trédan O, Ciruelos E, Dalenc F, Pardo PG, Jhaveri KL. Overall survival with sacituzumab govitecan in HR+/HER2− metastatic breast cancer (TROPiCS-02): phase 3 trial. The Lancet. 2023 Oct 21;402(10411):1423-33.
  27. Bardia A, Jhaveri K, Im SA, Pernas S, De Laurentiis M, Wang S, Martínez Jañez N, Borges G, Cescon DW, Hattori M, Lu YS. Datopotamab deruxtecan vs chemotherapy in previously treated HR+/HER2− metastatic breast cancer: TROPION-Breast01. Journal of Clinical Oncology. 2025 Jan 20;43(3):285-96.
  28. Slamon DJ, Neven P, Chia S, Fasching PA, De Laurentiis M, Im SA, Petrakova K, Bianchi GV, Esteva FJ, Martin M, Nusch A. Overall survival with ribociclib plus fulvestrant in advanced breast cancer. New england Journal of medicine. 2020 Feb 6;382(6):514-24.