Study Overview
Objective. To evaluate the efficacy and safety of the CDK4/6 inhibitor ribociclib in combination with letrozole as initial therapy in patients with hormone-receptor (HR)–positive, human epidermal growth factor receptor 2 (HER-2)–negative advanced breast cancer.
Design. Pre-planned interim analysis of a randomized, double-blind, phase 3 clinical trial.
Setting and participants . This study enrolled patients in 29 countries at 223 centers. A total of 668 postmenopausal women underwent randomization, with 334 assigned to receive ribociclib plus letrozole and 334 assigned to receive placebo plus letrozole. All women had HR-positive, HER-2 negative recurrent or metastatic breast cancer and had not received prior systemic therapy. Enrolled patients had either measurable disease on imaging or at least 1 lytic bone lesion. All patients were required to have an Eastern Cooperative Oncology Group performance status of 0 or 1. Patients were excluded if they had received prior therapy with a CDK4/6 inhibitor, previous systemic chemotherapy or endocrine therapy. If a patient received an aromatase inhibitor for neoadjuvant or adjuvant therapy, the disease-free interval needed to be more than 12 months to be included in the study. Patients with inflammatory breast cancer or central nervous system involvement were also excluded. Normal cardiac function (normal QT interval) was required for enrollment. The randomization was stratified by presence of liver or lung metastases.
Intervention. The patients were randomized to oral ribociclib 600 mg per day 3 weeks on, 1 week off in a 28-day treatment cycle plus letrozole 2.5 mg daily or placebo plus letrozole. The dosing of ribociclib was based on a prior phase 1 study [1]. Treatment was continued until disease progression, unacceptable toxicity, discontinuation, or death. Dose reductions of ribociclib were allowed; however, dose reductions of letrozole were not permitted. Crossover between treatment arms was not allowed. Patients were assessed with computed tomo-graphy at the time of randomization, every 8 weeks for the first 18 months and every 12 weeks there-after. Patients were monitored for hematological toxicity each cycle. Electrocardiographic assessment was done at screening, on day 15 of cycle 1 and on day 1 of all subsequent cycles to monitor for QT prolongation.
Main outcome measures . The primary outcome was progression-free survival. The secondary outcomes were overall survival, overall response rate (complete or partial response), clinical benefit rate, and safety. Clinical benefit rate was defined as overall response plus stable disease lasting 24 weeks or more. A prespecified interim analysis was planned after disease progression or death was reported in 211 of 302 patients (70%).
Results. The baseline characteristics were balanced between the 2 groups. Visceral disease was present in 58.8% and bone-only disease in 22% of the patients. The median duration of therapy exposure was 13 months in the ribociclib group and 12.4 months in the placebo group. The median duration of follow-up was 15.3 months. After 18 months, progression-free survival was 63% (95% confidence interval [CI], 54.6 to 70.3) in the ribociclib/letrozole group versus 42.2% (95% CI, 34.8 to 49.5) in the placebo group ( P < 0.001). The median progression-free survival was not met in the combination group (95% CI, 19.3 to not reached) versus 14.7 months (95% CI, 13.0 to 16.5) in the placebo group. The improved progression-free survival was seen across all subgroups. The overall response rate was higher in the combination arm (52.7% vs. 37.1%) as was the clinical benefit rate (80.1% vs. 71.8%). Serious adverse events occurred in 21.3% of patients in the ribociclib group and 11.8% in the placebo group. Serious adverse events were attributed to the study drug in 7.5% of the ribociclib group and 1.5% of the placebo group. The most common adverse events were myelosuppression, nausea, fatigue and diarrhea. Grade 3 and 4 neutropenia was noted in 59.3% in the ribociclib group versus < 1% in the placebo arm. The discontinuation rate due to adverse events in the ribociclib and placebo groups was 7.5% versus 2.1%, respectively. The most common reason for discontinuation was disease progression in 26% in the ribociclib group and 43.7% in the placebo group. Three deaths occurred in the ribociclib group and one in the placebo group. Interruptions in ribociclib occurred in 76.9% of patients. Dose reductions occurred in 53.9% of patients in the ribociclib group versus 7% in the placebo group. The most common reason a dose reduction occurred was neutropenia.
Conclusion. First-line treatment with ribociclib plus letrozole in postmenopausal women with HR-positive, HER-2 negative advanced breast cancer was associated with significantly longer progression-free survival compared with letrozole plus placebo. The improved progression-free survival was seen across all subgroups.
Commentary
Nearly 80% of all breast cancers express hormone receptor positivity. Hormonal therapy has been an important component of treatment for women with hormone-positive breast cancer in both the local and metastatic setting. Many tumors will eventually develop resistance to such therapy with the median progression-free survival with first-line endocrine therapy alone being around 9 months [2]. Cyclin dependent kinases 4 and 6 (CDK4/6) play an important role in estrogen-receptor signaling and cell cycle progression. CDK 4/6 mediates progression through the cell cycle from G1 to S phase via phosphorylation and inactivation of the retinoblastoma tumor suppressor protein [3]. Overexpression of CDK 4/6 in hormone receptor positive breast cancer is thought to play an important role in the development of endocrine therapy resistance [4].
The previously published PALOMA-2 trial, which compared treatment with the CDK 4/6 inhibitor palbociclib plus letrozole with letrozole alone, reported a significant improvement in progression-free survival with the addition of palbociclib (24.8 months vs. 14.5 months) in the front-line setting for women with advanced, hormone-positive breast cancer [5]. The improved progression-free survival with palbociclib was seen across all subgroups with a favorable toxicity profile. The current study represents the second randomized trial to show that the addition of CDK4/6 inhibitor to endocrine-based therapy significantly improves progression-free survival. This benefit was also seen across all patient subgroups including those with liver and lung metastases. In addition, the combination of ribociclib and letrozole also show significantly higher rates of overall response compared with placebo. In general, the addition of ribociclib to letrozole was well tolerated with a very low rate (7.5%) of discontinuation of therapy. Although neutropenia was a frequent complication in the ribociclib group febrile neutropenia occurred in only 1.5% of patients.
The incorporation of CDK4/6 inhibitors to endocrine-based therapy in the front-line setting has proven effective with an impressive early separation of the progression-free survival curves. Both the PALOMA-2 trial and the current MONALEESA-2 trial have shown similar results with approximately 40% improvement in progression-free survival. Whether the results seen in these trials will translate into an improvement in overall survival is yet to be determined. The results of these 2 trial suggest that CDK4/6 inhibitors have activity in both patients who have not received previous treatment with endocrine therapy and in those who received adjuvant endocrine therapy with late (> 12 months) relapse. Further determination of the subset of women who would benefit from the addition of CDK4/6 inhibitors remains an important clinical question. There are currently no clinical biomarkers that can be used to predict whether a patient would benefit from the addition of these medications.
Applications for Clinical Practice
The results of the current trial represent an exciting step forward in the treatment of advanced breast cancer. Palbociclib in combination with endocrine therapy is currently incorporated into clinical practice. The cost of these agents remains a concern; however, most insurance policies will cover them. Clinical trials are ongoing in the neoadjuvant and adjuvant setting for early breast cancer.
—Daniel Isaac, DO, MS