title: Therapeutic CDK4/6 inhibition in breast cancer: key mechanisms of response and failure.
abstract: A hallmark of cancer is the deregulation of cell-cycle machinery, ultimately facilitating aberrant proliferation that fuels tumorigenesis and disease progression. Particularly, in breast cancers, cyclin D1 has a crucial role in the development of disease. Recently, a highly specific inhibitor of CDK4/6 activity (PD-0332991) has been developed that may have efficacy in the treatment of breast cancer. To interrogate the utility of PD-0332991 in treating breast cancers, therapeutic response was evaluated on a panel of breast cancer cell lines. These analyses showed that the chronic loss of Rb is specifically associated with evolution to a CDK4/6-independent state and, ultimately, resistance to PD-0332991. However, to interrogate the functional consequence of Rb directly, knockdown experiments were performed in models that represent immortalized mammary epithelia and multiple subtypes of breast cancer. These studies showed a highly specific role for Rb in mediating the response to CDK4/6 inhibition that was dependent on transcriptional repression manifest through E2F, and the ability to attenuate CDK2 activity. Acquired resistance to PD-03322991 was specifically associated with attenuation of CDK2 inhibitors, indicating that redundancy in CDK functions represents a determinant of therapeutic failure. Despite these caveats, in specific models, PD-0332991 was a particularly effective therapy, which induced Rb-dependent cytostasis. Combined, these findings indicate the critical importance of fully understanding cell-cycle regulatory pathways in directing the utilization of CDK inhibitors in the clinic.
authors.affiliation: Kimmel Cancer Center, Philadelphia, PA, USA.
authors.forename: J L
authors.initials: JL
authors.lastname: Dean
authors.forename: C
authors.initials: C
authors.lastname: Thangavel
authors.forename: A K
authors.initials: AK
authors.lastname: McClendon
authors.forename: C A
authors.initials: CA
authors.lastname: Reed
authors.forename: E S
authors.initials: ES
authors.lastname: Knudsen
mesh_terms: D001943:Breast Neoplasms / Q000188:drug therapy* / Q000201:enzymology / Q000473:pathology; D045744:Cell Line, Tumor; D051358:Cyclin-Dependent Kinase 4 / Q000037:antagonists & inhibitors*; D051361:Cyclin-Dependent Kinase 6 / Q000037:antagonists & inhibitors*; D005260:Female; D006801:Humans; D010879:Piperazines / Q000627:therapeutic use*; D047428:Protein Kinase Inhibitors / Q000627:therapeutic use*; D011725:Pyridines / Q000627:therapeutic use*
chemical_list: D010879:Piperazines; D047428:Protein Kinase Inhibitors; D011725:Pyridines; D051358:Cyclin-Dependent Kinase 4; D051361:Cyclin-Dependent Kinase 6; C500026:palbociclib