Question: Which enzyme is inhibited by ribociclib?

Context:
Three CDK 4/6 inhibitors have been investigated for the treatment of HR(+) breast cancer, including palbociclib (PD 0332991), ribociclib (LEE011), and abemaciclib (LY2835219). 
The CDK 4/6 inhibitors are a novel class of therapeutics that target the CDK 4/6 kinases that promote transition through the cell cycle. Currently, palbociclib (PD0332991, Pfizer), abemaciclib (LY2835219, Lilly) and ribociclib (LEE011, Novartis) are being investigated in clinical trials. 
SUMMARY: Palbociclib, abemaciclib and ribociclib have demonstrated very promising clinical activity in breast cancer, liposarcoma, mantel cell lymphoma and melanoma. Moreover, CDK4/6 inhibitors have shown promising preclinical activity in glioblastoma, renal and ovarian cancer models that may provide directions for their future clinical development.
Currently, three selective CDK4/6 inhibitors have been approved or are in late-stage development: palbociclib (PD-0332991), ribociclib (LEE011), and abemaciclib (LY2835219).
The combination of antiestrogen therapy and ribociclib, an investigational CDK4/6 inhibitor, led to improved outcomes in women with metastatic HR-positive, HER2-negative breast cancer, according to findings presented at a meeting of the European Society for Medical Oncology. 
OBJECTIVES: Cyclin D-cyclin-dependent kinase (CDK) 4/6-inhibitor of CDK4/6-retinoblastoma (Rb) pathway hyperactivation is associated with hormone receptor-positive (HR+) breast cancer (BC). This study assessed the biological activity of ribociclib (LEE011; CDK4/6 inhibitor) plus letrozole compared with single-agent letrozole in the presurgical setting
Importantly, 2 other CDK4/6 inhibitors, abemaciclib (LY2835219; Lilly) and ribociclib (LEE011; Novartis), are in the late stage of clinical development. 
After noting an inverse correlation with sensitivity to ribociclib (CDK4/6 inhibitor), we studied the combinatorial effect of these two agents using proliferation assays, cell cycle analysis, Ki67 immunostaining, timelapse microscopy and xenograft studies.
Methods In this randomized, placebo-controlled, phase 3 trial, we evaluated the efficacy and safety of the selective CDK4/6 inhibitor ribociclib combined with letrozole for first-line treatment in 668 postmenopausal women with HR-positive, HER2-negative recurrent or metastatic breast cancer who had not received previous systemic therapy for advanced disease. 
Three selective CDK4/6 inhibitors, palbociclib (Ibrance; Pfizer), ribociclib (Novartis), and abemaciclib (Lilly), are in various stages of development in a variety of pRb-positive tumor types, including breast cancer, melanoma, liposarcoma, and non-small cell lung cancer. 
Clinical Development of the CDK4/6 Inhibitors Ribociclib and Abemaciclib in Breast Cancer.
Targeted inhibition of CDK4/6 pathway by small-molecule inhibitors palbociclib (PD-0332991) and ribociclib (LEE011) resulted in inhibition of cell-cycle progression, amelioration of kidney injury, and improved overall survival. 
Clinical Development of the CDK4/6 Inhibitors Ribociclib and Abemaciclib in Breast Cancer.

Answer: Ribociclib is inhibitor of cyclin D-cyclin-dependent kinase 4/6 (CDK 4/6). It is used for breast cancer treatment.
Exact answer: cyclin D-cyclin-dependent kinase 4/6, CDK4/6