SUMMARY: Breast cancer is the most common cancer among women in the US and about 1 in 8 women (12%) will develop invasive breast cancer during their lifetime. The American Cancer Society estimates that in the US, approximately 310,720 new cases of female breast cancer will be diagnosed in 2024, and about 42,250 individuals will die of the disease, largely due to metastatic recurrence. Breast cancer is the second leading cause of cancer death in women, in the U.S.
About 70% of breast tumors express Estrogen Receptors and/or Progesterone Receptors, and Hormone Receptor (HR)-positive/HER2-negative breast cancer is the most frequently diagnosed molecular subtype. About 90% of all breast cancers are detected at an early stage, and these patients are often cured with a combination of surgery, radiotherapy, chemotherapy, and hormone therapy. However approximately 20% of patients will experience local recurrence or distant relapse during the first 10 years of treatment. This may be more relevant for those with high risk disease, among whom the risk of recurrence is even greater during the first 2 years while on adjuvant Endocrine Therapy, due to primary endocrine resistance. More than 75% of the early recurrences are seen at distant sites.
Cyclin Dependent Kinases (CDKs) play a very important role to facilitate orderly and controlled progression of the cell cycle. Genetic alterations in these kinases and their regulatory proteins have been implicated in various malignancies. CDK4 and 6 phosphorylate RetinoBlastoma protein (RB), and initiate transition from the G1 phase to the S phase of the cell cycle. RetinoBlastoma protein has antiproliferative and tumor-suppressor activity. Phosphorylation of RB protein nullifies its beneficial activities. CDK4 and CDK6 are activated in HR-positive breast cancer, by binding to D-cyclins in the ER-positive breast cancer cell, promoting breast cancer cell proliferation. Further, there is evidence to suggest that endocrine resistant breast cancer cell lines depend on CDK4 for cell proliferation. The understanding of the role of CDKs in the cell cycle, has paved the way for the development of CDK inhibitors.
Ribociclib (KISQALI®) is an orally bioavailable, selective, small-molecule inhibitor of CDK4/6, preferentially inhibiting CDK4 and blocking the phosphorylation of RetinoBlastoma protein, thereby preventing cell-cycle progression and inducing G1 phase arrest. The MONALEESA trials of Ribociclib have shown a consistent Overall Survival benefit, regardless of accompanying Endocrine Therapy, line of therapy, or menopausal status, in advanced breast cancer.
NATALEE is a global, multi-center, randomized, open-label Phase III trial, conducted to evaluate the efficacy and safety of Ribociclib with Endocrine Therapy as adjuvant treatment versus Endocrine Therapy alone, in patients with HR+/HER2-negative early breast cancer, who were at risk for disease recurrence. This study conducted in collaboration with Translational Research In Oncology (TRIO), randomly assigned 5,101 eligible men and pre- or postmenopausal women 1:1 to receive either adjuvant Ribociclib 400 mg orally daily for 3 years along with Endocrine Therapy consisting of Letrozole 2.5 mg/day or Anastrozole 1 mg/day, for 5 yrs or more (N= 2,549) or Endocrine Therapy alone for at least 5 years (N = 2,552). This study explored a lower Ribociclib starting dose of 400 mg daily rather than the dose approved for treatment in metastatic breast cancer (600 mg), with the goal to minimize toxicities and disruptions to patient quality of life, without compromising efficacy. Men and premenopausal women also received Goserelin. Eligible patients had an ECOG PS of 0-1 with Stage IIA (either N0 with additional risk factors or N1 with 1-3 axillary lymph nodes), Stage IIB, or Stage III HR-positive, HER2-negative breast cancer who were at risk for disease recurrence. Prior adjuvant Endocrine Therapy was allowed if initiated no more than 1 year before randomization. Stratification factors were menopausal status, disease stage, prior neoadjuvanr/adjuvant chemotherapy, and geographic region. Approximately 44% were premenopausal and 40% had Stage II breast cancer. Majority of patients (88%) received prior chemotherapy. The Primary endpoint of NATALEE was invasive Disease Free Survival (iDFS) as defined by the Standardized Definitions for Efficacy End Points (STEEP) criteria. Secondary endpoints included Distant Disease-Free Survival (DDFS) and Overall Survival (OS).
The authors had previously reported that at a median follow up of 34 months, the addition of Ribociclib to Endocrine Therapy significantly improved in invasive DFS compared with Endocrine Therapy alone (HR=0.748; P=0.0014), reducing the risk of disease recurrence by 25%.
The researchers in this updated analysis of the NATALEE trial presented the efficacy and safety data at data cutoff (29 Apr 2024), with all patients in the Ribociclib plus Endocrine Therapy group (N=2549) off treatment with Ribociclib. This update provided a robust framework for understanding the long-term implications of this therapeutic approach.
The updated analysis revealed that invasive DFS significantly favored the Ribociclib plus Endocrine Therapy combination over Endocrine Therapy alone. At the three-year mark, iDFS rates were 90.8% for the Ribociclib plus Endocrine Therapy group compared to 88.1% for those on Endocrine Therapy alone, with an absolute improvement of 2.7%. By the four-year follow-up, this gap widened, with iDFS rates of 88.5% versus 83.6%, reflecting a 4.9% absolute benefit. This benefit was consistent across various subgroups. Patients with node-negative disease (N0) experienced a 5.1% absolute increase in iDFS at four years, while those with node-positive disease (N+) saw a 5.0% improvement. Similarly, patients in Stage II had an absolute benefit of 4.3%, and those in Stage III achieved a 5.9% increase in their iDFS rates.
The Distant DFS data was similar to the iDFS findings, with Ribociclib plus Endocrine Therapy showing a Hazard Ratio of 0.715 (95% CI, 0.604–0.847; P<0.0001), indicating a substantial reduction in the risk of distant recurrence. While Overall Survival data remains immature, trends suggest a favorable outcome for the Ribociclib group.
Safety data revealed that Ribociclib was well tolerated, and remained consistent with previous analyses. The adverse events of special interest, particularly those Grade 3 or higher, included neutropenia (44.4%), liver-related issues (8.6%), and QT interval prolongation (1.0%).
The researchers concluded that in this 4-year landmark analysis, Ribociclib plus Endocrine Therapy reduced the risk of Invasive and Distant disease recurrence by 28.5% compared with Endocrine Therapy alone. Further, this benefit was maintained even after the end of planned 3-year Ribociclib treatment, for both node-positive and node-negative patients. This deepening efficacy, particularly evident in node-negative and high-risk early breast cancer patients, underscores the necessity of evolving treatment strategies in the fight against breast cancer.
Adjuvant ribociclib (RIB) plus nonsteroidal aromatase inhibitor (NSAI) in patients (Pts) with HR+/HER2− early breast cancer (EBC): 4-year outcomes from the NATALEE trial. Fasching PA, Stroyakovskiy D, Yardley D, et al. DOI: 10.1016/j.annonc.2024.08.2251
