Late Breaking Abstract – ASCO 2022: KISQALI® with Switch Endocrine Therapy Following Progression on a Prior CDK4/6 Inhibitor in HR+/HER2-negative Metastatic Breast Cancer

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. Approximately 290,560 new cases of breast cancer will be diagnosed in 2022 and about 43,780 individuals will die of the disease, largely due to metastatic recurrence. Approximately 70% of breast tumors in patients with metastatic disease are Estrogen Receptor (ER) and/or Progesterone Receptor (PR) positive and HER2-negative. These patients are often treated with single agent endocrine therapy, endocrine therapy in combination with CDK4/6 inhibitor, or single agent chemotherapy.

Cyclin Dependent Kinases (CDK) 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. Cyclin Dependent Kinases 4 and 6 (CDK4 and CDK6) 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 and phosphorylation of RB protein nullifies its beneficial activities. CDK4 and CDK6 are activated in hormone receptor positive breast cancer, promoting breast cancer cell proliferation. Further, there is evidence to suggest that endocrine resistant breast cancer cell lines depend on CDK4 for cell proliferation and associated with increased expression of CDK4. The understanding of the role of Cyclin Dependent Kinases in the cell cycle, has paved the way for the development of CDK inhibitors.

KISQALI® (Ribociclib) is an orally bioavailable, selective, small-molecule inhibitor of CDK4/6, preferentially inhibiting CDK4 that blocks the phosphorylation of RetinoBlastoma protein, thereby preventing cell-cycle progression and inducing G1 phase arrest.

MAINTAIN trial is an investigator-initiated, multicenter, Phase II, double-blind, placebo-controlled, prospective randomized study, conducted to evaluate the efficacy of Fulvestrant or Exemestane with or without KISQALI®, in patients with HR+/HER2-negative metastatic breast cancer, who had previously progressed on any CDK 4/6 inhibitor plus any endocrine therapy. In this study, 119 evaluable patients were randomized 1:1 to receive either KISQALI® 600mg orally once daily given 3 weeks on and 1 week off plus switch endocrine therapy (N= 60) or placebo plus switch endocrine therapy (N=59). Patients treated with prior Fulvestrant received Exemestane as endocrine therapy in the randomization, whereas if prior Exemestane was endocrine therapy, patients received Fulvestrant. If patients received neither as prior endocrine therapy, Fulvestrant or Exemestane was given per investigator discretion, although Fulvestrant was encouraged. Ultimately, 83% of patients received Fulvestrant and 17% received Exemestane. Eligible patients were postmenopausal, had HR+/HER2- negative metastatic breast cancer and had progressed on prior endocrine therapy and any CDK4/6 inhibitor. With regards to prior CDK 4/6 inhibitor treatment, 84% received IBRANCE® (Palbociclib), 11% received KISQALI®, 2% received VERZENIO® (Abemaciclib) and 3% received IBRANCE® and another CDK 4/6 inhibitor. The median duration of treatment with the prior CDK4/6 inhibitor was 15.5 months in the KISQALI® group and 17 months in the placebo group. Approximately 60% of patients had visceral metastasis, 45% had de novo metastasis at diagnosis, 18% had bone-only disease, 18% had received 2 or more prior endocrine therapies for metastatic disease, and 9% had received chemotherapy. The Primary end point was Progression Free Survival (PFS) and Secondary end points included Overall Response Rate (ORR), Clinical Benefit Rate, safety, and tumor and blood biomarkers. The median follow up was 18.2 months.

There was a statistically significant PFS improvement for patients randomized to KISQALI® plus endocrine therapy and the median PFS for patients in the KISQALI® plus endocrine therapy was 5.33 months, compared with 2.76 months for patients receiving placebo and endocrine therapy (HR=0.56;; P=0.004). At 12 months, the PFS rates were 24.6% in the KISQALI® group versus 7.4% in the placebo group. Similar results were noted in the subset of patients treated with Fulvestrant and KISQALI®, and the median PFS for those randomized to KISQALI® was 5.29 months versus 2.76 months in the placebo group (HR=0.59; P=0.02). The PFS benefit was more evident in KISQALI® group compared to the placebo group, especially among those who received a shorter duration of therapy with a prior CDK4/6 inhibitor (HR=0.36) and among those over age 65 years (HR=0.31). The Overall Response Rate in the KISQALI® group was 20%, compared to 11% in the placebo group, and the median Duration of Response was 18.8 months in those treated with KISQALI® and endocrine therapy, compared with 14.8 months for those treated with placebo plus endocrine therapy. There was also a significant improvement in the Clinical Benefit Rate (CBR), defined as patients who achieved Complete Response, Partial Response, or stable disease lasting at least 24 weeks. The CBR was significantly improved in the KISQALI® group, compared with the placebo group, and was 43% versus 25%, respectively (P=0.06). The most common adverse event in the KISQALI® group was neutropenia at 72%, compared to 15% in the placebo group.

It was concluded from this randomized, placebo-controlled trial that, treatment with KISQALI® and an alternate endocrine therapy, after progression on a prior CDK4/6 inhibitor, showed a 43% reduction in the risk of progression or death, compared with placebo and endocrine therapy, in patients with HR+/HER2-negative metastatic breast cancer.

A randomized, phase II trial of fulvestrant or exemestane with or without ribociclib after progression on anti-estrogen therapy plus cyclin-dependent kinase 4/6 inhibition (CDK 4/6i) in patients (pts) with unresectable or hormone receptor–positive (HR+), HER2-negative metastatic breast cancer (MBC): MAINTAIN trial. Kalinsky K, Accordino MK, Chiuzan C, et al. J Clin Oncol 40, 2022 (suppl 17; abstr LBA1004). DOI: 10.1200/JCO.2022.40.17_suppl.LBA1004.