Breast cancer is the fifth leading cause of cancer death in the United States, with more than 40,000 estimated deaths in 2017.

Many therapies for patients with breast cancer focus on targeting cancer cell proliferation, with a goal to divert cells to a more senescent phenotype.

Deregulation of the cyclin-dependent kinase (CDK) 4&6retinoblastoma (Rb) axis has been demonstrated to contribute to unrestricted growth that is characteristic of many cancers, including breast cancer. &ldquo;CDKs interact with cyclin D proteins to play an integral role in cell cycle progression, and represent attractive therapeutic targets in many tumors, including breast cancer,&rdquo; noted Conleth G. Murphy, MD, of Bon Secours Hospital, Cork, Ireland, and Maura N. Dickler, MD, of Memorial Sloan Kettering Cancer Center, New York, in The Oncologist.

A hallmark of cancer cells, unrestricted growth is often associated with disordered cell cycle regulation.

The cell cycle includes steps that are highly regulated and coordinated by transient activations of cyclins and CDKs.

Together, these elements form bipartite complexes that phosphorylate proteins and transcription factors, which enable the cell to move through the cell cycle.

CDKs are serine/threonine kinases that affect the G1-S phase transition of the cell cycle, which is mediated by an action on the Rb protein (FIGURE).

Rb and the p107 and p130 proteins have been shown to play a key role in cell cycle regulation through exerting a braking influence on cell proliferation in the early G1 phase.

While hypophosphorylated, the Rb protein sequesters proliferative cellular proteins that are released via CDK4&6&minus;cyclin D1 mediated hyperphosphorylation of Rb. These sequestered proteins include the E2F transcription factors that control gene expression that is essential for moving through the cell cycle from G1 to S phase.

When Rb is hyperphosphorylated, transcription factors that initiate DNA replication are released, leading to activation of dihydrofolate reductase, histone H2A, thymidine kinase, and DNA polymerase alpha.

The CIP/KIP and INK4 protein families mediate negative regulation of CDK4&6&minus;cyclin D1.

The INK4 proteins inhibit D-type cyclin activity through selective interaction with CDK4 and CDK6. This inhibition mechanism involves interaction between the p16- and CDK6-charged binding domains that results in reduced kinase activity and a decreased cyclin-binding surface.

Cyclin D1 is susceptible to extrinsic mitogenic stimulation, which provides a link between the external environment and cell cycle machinery.19 Because of the modulatory control over Rb and the resulting function in promoting cellular proliferation, it has been hypothesized that deregulation of cyclin D1 and CDK4&6 may promote tumorigenesis and tumor maintenance.

Many tumors have been shown to increase cyclin D&minus;dependent activity and to avoid senescence via numerous mechanisms, including p16 inactivation, CDK4 mutation with loss of INK4 binding, CDK4 amplification, overexpression of cyclin D1, or translocation or amplification of CCND1, its encoding gene.

The development and maintenance of certain breast cancers may require cyclin D1, as mice lacking cyclin D1 or CDK4 have demonstrated resistance to implanted breast tumor growth driven by human epidermal growth factor receptor 2 (HER2)/neu.

In breast cancer, multiple mechanisms are involved in frequently reported deregulation of CDK4&6&minus;cyclin D1 activity.26,27 For example, in a cancer genome study of 482 patients with invasive breast cancer, 27.4% exhibited genetic deregulation of the cyclin D1/CDK4&6/p16INK4a pathway.

Approximately half of investigated breast cancer cell lines exhibit cyclin D1 overexpression.

In about 16% of breast cancers, CDK4 may be overexpressed via amplification of the CDK4 gene at 12q13-q14.29 CCND1 amplification has been reported in 38% of HER2-positive breast cancers, 29% of luminal A cancers, and in 58% of luminal B cancers.

Altered function of inhibitors of CDK have also been reported in breast cancer, including p16INK4a, p21WAF1/CIP1, and p15INK4b.

Murine breast cancer models have demonstrated that cyclin D1&minus;CDK4&6 axis activation contributes to a tumorigenic phenotype and is crucial for tumorigenesis in HER2-positive breast cancer.

However, viability and normal mammary development are not adversely affected when CDK2 or CDK4&6 are knocked out.

Cyclin D1 has been shown to induce chromosomal instability via transcriptional regulation of CIN-related genes.

A role for cyclin D1 has also been implicated in cellular migration and invasion, DNA damage-sensing and repair, inhibition of mitochondrial metabolism, and angiogenesis enhancement.

These functions are mostly independent of any CDK4&6 interactions and are not thought to be affected by CDK inhibition. Recently, however, CDK6 was reported to have a kinase-independent angiogenesis function, which suggests an alternative role for CDK6 inhibition in tumor therapy.

Estrogen receptor (ER)positive breast cancers have been shown to be dependent on estrogen signaling for both proliferation and survival.

ER inhibition has also been reported to reduce tumor cell viability and cell cycle arrest in the G1 phase.

Furthermore, ER signaling upregulates cyclin D1 levels, stimulating various signaling pathways that subsequently upregulate CDK4&6 activity.

Acquired resistance and lack of response to hormone-based therapies for ER-positive breast cancers may be mediated by deregulation of alternative pathways, such as HER2 and PI3K/AKT, that may induce cyclin D1&minus;CDK4&6 signaling. Cyclin D1 has also been shown to independently activate ER, with a majority of breast cancers that overexpress cyclin D1 being ER-positive.

Most breast cancers maintain intact or functioning Rb, with loss of Rb expression occurring in just 20% to 35% of breast cancers.

Loss of the Rb protein is more frequently reported in triple-negative breast cancer compared with other subtypes.

The inactivation of Rb contributes to tumor progression via proliferation control loss and conversion to a phenotype that is invasive. Generally, this is associated with poor differentiation and an increased metastatic potential. Without functional Rb, cell-to-cell adhesion is disrupted, facilitating the epithelial&minus;mesenchymal transition, which may contribute to the increased frequency of metastases.

In ER-positive tumors, Rb dysregulation is associated with poor prognosis, whereas it is associated with improved outcomes in ER-negative tumors.

These reported differences in outcomes may be a result of different responses to therapy, as loss of Rb has been demonstrated to increase sensitivity to DNA-damaging agents but leads to continued proliferation with tamoxifen treatment.

CDK4&6 inhibitors contribute to G1 phase cell cycle arrest by preventing the phosphorylation of Rb, which may lead to the inhibition of tumor progression in breast cancer and other cancer types. Despite oncogenic signals that are insensitive to endocrine therapy, CDK4&6 inhibitors effectively prevent cell cycle progression and mitosis.

Patients who have loss of Rb expression do not benefit from CDK4&6 inhibitor treatment due to the downstream position of Rb compared with CDK4&6 in the cell cycle.

The selectivity of palbociclib (Ibrance), ribociclib (Kisqali), and abemaciclib (Verzenio)3 FDA-approved CDK4&6 inhibitors&mdash;is thought to be related to binding to the specialized adenosine triphosphate (ATP)&minus;binding pocket of CDK4&6 with specific residue reactions in the ATP-binding cleft.

CDK4 and CDK6 are structurally related proteins that have biological and biochemical similarities and are considered functionally equivalent in their ability to phosphorylate Rb.

CDK4 and CDK6 have overlapping functions and share nearly 75% of amino acid identity.

CDK4&6 are expressed in most cell types and can partner with cyclin D1, D2, and D3.

Palbociclib, ribociclib, and abemaciclib bind to the ATP cleft of CDK4 and CDK6. Abemaciclib has been shown to bind more readily to the ATP cleft, forming a hydrogen bond with a conserved catalytic residue, suggesting that it binds less selectively than ribociclib and palbociclib.55 Palbociclib and ribociclib seem to have greater lipophilicity and larger side chains at the binding site compared with abemaciclib, which may reduce interactions with off-target kinase ATP-binding pockets. A study demonstrated that ribociclib exhibited significantly higher selectivity toward CDK4 and CDK6 compared with palbociclib, which interacted with more than twice as many kinases as did ribociclib.

Additionally, a study of palbociclib in molecularly characterized human breast cancer cell lines showed that ER-positive cell lines with luminal features were the most sensitive to palbociclib, whereas basal cell lines were the most resistant to palbociclib.

Promising preclinical evidence demonstrating the potential therapeutic capability of targeting the cyclin D1&minus;CDK4&6&minus;Rb axis resulted in the development of first-generation CDK inhibitors. Flavopiridol (alvocidib), an intravenous pan-CDK inhibitor, was the first to enter clinical trials based on inhibitory activity in vitro against all human tumor cell lines contained in the tumor cell line panel from the National Cancer Institute.

Results of clinical trials investigating flavopiridol monotherapy have been mixed.

Increased antitumor efficacy was reported for the combination of flavopiridol and chemotherapy agents that promote S-phase accumulation.

In a phase I trial of patients with breast cancer and other solid tumors, encouraging results were reported following weekly administration of sequential docetaxel (Taxotere) and flavopiridol.

Complex pharmacokinetics and dose-limiting toxicities have complicated the use of flavopiridol, however. Distal cellular effects associated with flavopiridol treatment include transcriptional suppression, apoptosis, autophagy, and endoplasmic reticulum stress, which have been associated with high rates of dose-limiting toxicities, including hyperglycemia, neutropenia, and cardiac and pulmonary dysfunction.

Based on the low CDK specificity and the narrow therapeutic window, the development of flavopiridol was discontinued.

Seliciclib (roscovitine) as monotherapy demonstrated minimal disease stabilization in phase I studies of patients with solid tumors.

Seliciclib inhibits CDK4&6 poorly, but also inhibits CDK1, CDK2, CDK7, and CDK9.

Second- and third-generation CDK inhibitors have been developed, including compounds with specificity for CDK4 and CDK6. Second-generation inhibitors started to show preferential inhibition of specific CDK subtypes, with initial efforts primarily focused on inhibition of CDK2 due to crystallographic structure availability.

Later, highly selective inhibitors, including palbociclib, ribociclib, and abemaciclib, were evaluated through chemical screening and optimization by adding pyrido[2,3-d] pyrimidin-7-one compounds with a side chain of 2-amino pyridine at the C2 position.

The major CDK inhibitors (palbociclib, ribociclib, and abemaciclib) currently are orally administered and have limited suppression of other CDK activities. These inhibitors have structures analogous to those of flavopiridol, but have different chemical functions.

Cyclin D1 overexpression has been reported in the majority of breast cancers.

Mice lacking cyclin D1 have been shown to be resistant to mammary carcinomas triggered by the oncogene ErbB-2. The crucial role for cyclin D1 in breast cancer formation is the activation of CDK4.

The continued presence of CDK4-associated kinase activity has also been shown to be required for the maintenance of breast tumorigenesis. A study investigating knock-in mice expressing kinase-deficient cyclin D1&minus;CDK4 complexes demonstrated that these mice develop normally; however, these mice demonstrated resistance to mammary carcinomas triggered by ErbB-2.22 &ldquo;These results point to a differential requirement for cyclin D1&minus;CDK4 kinase activity in mammary neoplasia versus in development,&rdquo; noted a team led by Qunyan Yu, MD, of Dana-Farber Cancer Institute, Boston, Massachusetts, in Cancer Cell.

&ldquo;These findings suggest that the pharmacological inhibition of CDK4 kinase might preferentially target breast cancer cells. Our findings will hopefully stimulate therapeutic strategies aiming at blocking CDK4 kinase activity in patients.&rdquo;

GLOBOCAN 2012: estimated cancer incidence, mortality and prevalence worldwide in 2012. World Health Organization/International Agency for Research on Cancer website. globocan.iarc.fr/Pages/fact_sheets_cancer.aspx. Accessed November 24, 2017.

Cancer stat facts: female breast cancer. National Cancer Institute/Surveillance, Epidemiology, and End Results Program website. seer.cancer.gov/statfacts/html/breast.html. Accessed November 24, 2017.

Dukelow T, Kishan D, Khasraw M, Murphy CG. CDK4/6 inhibitors in breast cancer.

2015;26(8):797-806. doi: 10.1097/CAD.0000000000000249.

Murphy CG, Dickler MN. The role of CDK4/6 inhibition in breast cancer.

2015;20(5):483-490. doi: 10.1634/theoncologist.2014-0443.

Udvardy A. The role of controlled proteolysis in cell-cycle regulation.

1996;240(2):307-313. doi: 10.1111/j.1432-1033.1996.0307h.x.

Coudreuse D, Nurse P. Driving the cell cycle with a minimal CDK control network.

2010;468(7327):1074-1079. doi: 10.1038/nature09543.

Hunt T, Nasmyth K, Novak B. The cell cycle.

2011;366(1584):3494-3497. doi: 10.1098/rstb.2011.0274.

Harper JW, Adams PD. Cyclin-dependent kinases.

Sherr CJ, Roberts JM. Living with or without cyclins and cyclin-dependent kinases.

2004;18(22):2699-2711. doi: 10.1101/gad.1256504.

Ekholm SV, Reed SI. Regulation of G(1) cyclin-dependent kinases in the mammalian cell cycle.

Hanahan D, Weinberg RA. The hallmarks of cancer.

2000;100(1):57-70. doi: 10.1016/S0092-8674(00)81683-9.

Weinberg RA. The retinoblastoma protein and cell cycle control.

1995;81(3): 323-330. doi: 10.1016/0092-8674(95)90385-2.

Rivadeneira DB, Mayhew CN, Thangavel C, et al. Proliferative suppression by CDK4/6 inhibition: complex function of the retinoblastoma pathway in liver tissue and hepatoma cells.

2010;138(5):1920-1930. doi: 10.1053/j.gastro.2010.01.007.

Bosco EE, Knudsen ES. RB in breast cancer: at the crossroads of tumorigenesis and treatment.

Shackelford RE, Kaufmann WK, Paules RS. Cell cycle control, checkpoint mechanisms, and genotoxic stress.

Blain SW. Switching cyclin D-Cdk4 kinase activity on and off.

2008;7(7):892-898. doi: 10.4161/cc.7.7.5637.

Russo AA, Tong L, Lee JO, et al. Structural basis for inhibition of the cyclin-dependent kinase Cdk6 by the tumour suppressor p16INK4a.

1998;395(6699):237-243. doi: 10.1038/26155.

Li J, Poi MJ, Tsai MD. Regulatory mechanisms of tumor suppressor P16(INK4A) and their relevance to cancer.

2011;50(25):5566-5582. doi: 10.1021/bi200642e.

Sherr CJ, Roberts JM. CDK inhibitors: positive and negative regulators of G1-phase progression.

Shapiro GI. Cyclin-dependent kinase pathways as targets for cancer treatment.

. 2006;24(11):1770-1783. doi: 10.1200/JCO.2005.03.7689.

Bowe DB, Kenney NJ, Adereth Y, Maroulakou IG. Suppression of Neu-induced mammary tumor growth in cyclin D1 deficient mice is compensated for by cyclin E.

2002;21(2):291-298. doi: 10.1038/sj.onc.1205025.

Landis MW, Pawlyk BS, Li T, et al. Cyclin D1-dependent kinase activity in murine development and mammary tumorigenesis.

2006;9(1):13-22. doi: 10.1016/j.ccr.2005.12.019.

Yu Q, Geng Y, Sicinski P. Specific protection against breast cancers by cyclin D1 ablation.

2001;411(6841):1017-1021. doi: 10.1038/35082500.

Yu Q, Sicinska E, Geng Y, et al. Requirement for CDK4 kinase function in breast cancer.

2006;9(1):23-32. doi: 10.1016/j.ccr.2005.12.012.

Reddy HK, Mettus RV, Rane SG, et al. Cyclin-dependent kinase 4 expression is essential for neu-induced breast tumorigenesis.

2005;65(22):10174-10178. doi: 10.1158/0008-5472.CAN-05-2639.

Casimiro MC, Velasco-Vel&aacute;zquez M, Aguirre-Alvarado C, Pestell RG. Overview of cyclins D1 function in cancer and the CDK inhibitor landscape: past and present.

2014;23(3):295-304. doi: 10.1517/13543784.2014.867017.

Zhang L, Yang C. Promise of cyclin-dependent kinases 4/6 as therapeutic targets in breast cancer.

2014;5(5):191. doi: 10.4172/2157-2518.1000191.

Bartkova J, Lukas J, Müller H, et al. Cyclin D1 protein expression and function in human breast cancer.

An HX, Beckmann MW, Reifenberger G, et al. Gene amplification and overexpression of CDK4 in sporadic breast carcinomas is associated with high tumor cell proliferation.

1999;154(1):113-118. doi: 10.1016/S0002-9440(10)65257-1.

Cancer Genome Atlas Network. Comprehensive molecular portraits of human breast tumours.

2012;490(7418):61-70. doi: 10.1038/nature11412.

Musgrove EA, Lilischkis R, Cornish AL, et al. Expression of the cyclin-dependent kinase inhibitors p16INK4, p15INK4B and p21WAF1/CIP1 in human breast cancer.

Malumbres M, Barbacid M. Is cyclin D1-CDK4 kinase a bona fide cancer target?

2006;9(1):2-4. doi: 10.1016/j.ccr.2005.12.026.

Rane SG, Dubus P, Mettus RV, et al. Loss of Cdk4 expression causes insulin-deficient diabetes and Cdk4 activation results in beta-islet cell hyperplasia.

Ortega S, Prieto I, Odajima J, et al. Cyclin-dependent kinase 2 is essential for meiosis but not for mitotic cell division in mice.

Malumbres M, Sotillo R, Santamar&iacute;a D, et al. Mammalian cells cycle without the D-type cyclin-dependent kinases Cdk4 and Cdk6.

2004;118(4):493-504. doi: 10.1016/j.cell.2004.08.002.

Casimiro MC, Crosariol M, Loro E, et al. ChIP sequencing of cyclin D1 reveals a transcriptional role in chromosomal instability in mice [published correction appears in

. 2012;122(3):833-843. doi: 10.1172/JCI60256.

2013;183(1):3-9. doi: 10.1016/j.ajpath.2013.03.001.

Kollmann K, Heller G, Schneckenleithner C, et al. A kinase-independent function of CDK6 links the cell cycle to tumor angiogenesis [published correction appears in

2013;24(2):167-181. doi: 10.1016/j.ccr.2013.07.012.

Varma H, Skildum AJ, Conrad SE. Functional ablation of pRb activates Cdk2 and causes antiestrogen resistance in human breast cancer cells.

2007;2(12):e1256. doi: 10.1371/journal.pone.0001256.

Sutherland RL, Green MD, Hall RE, et al. Tamoxifen induces accumulation of MCF 7 human mammary carcinoma cells in the G0/G1 phase of the cell cycle.

Carroll RS, Brown M, Zhang J, et al. Expression of a subset of steroid receptor cofactors is associated with progesterone receptor expression in meningiomas.

Watts CK, King RJ. Overexpression of estrogen receptor in HTB 96 human osteosarcoma cells results in estrogen-induced growth inhibition and receptor cross talk.

1994;9(8):1251-1258. doi: 10.1002/jbmr.5650090815.

Foster JS, Henley DC, Bukovsky A, et al. Multifaceted regulation of cell cycle progression by estrogen: regulation of Cdk inhibitors and Cdc25A independent of cyclin D1-Cdk4 function.

2001;21(3):794-810. doi: 10.1128/MCB.21.3.794-810.2001.

Zwijsen RM, Wientjens E, Klompmaker R, et al. CDK-independent activation of estrogen receptor by cyclin D1.

Trer&eacute; D, Brighenti E, Donati G, et al. High prevalence of retinoblastoma protein loss in triple-negative breast cancers and its association with a good prognosis in patients treated with adjuvant chemotherapy.

2009;20(11):1818-1823. doi: 10.1093/annonc/mdp209.

Arima Y, Inoue Y, Shibata T, et al. Rb depletion results in deregulation of E-cadherin and induction of cellular phenotypic changes that are characteristic of the epithelial-to-mesenchymal transition.

. 2008;68(13):5104-5112. doi: 10.1158/0008-5472.CAN-07-5680.

Musgrove EA, Sutherland RL. RB in breast cancer: differential effects in estrogen receptor-positive and estrogen receptor-negative disease.

. 2010;9(23):4607. doi: 10.4161/cc.9.23.13889.

Bosco EE, Wang Y, Xu H, et al. The retinoblastoma tumor suppressor modifies the therapeutic response of breast cancer.

2007;117(1):218-228. doi: 10.1172/JCI28803.

Finn RS, Dering J, Conklin D, et al. PD 0332991, a selective cyclin D kinase 4/6 inhibitor, preferentially inhibits proliferation of luminal estrogen receptor-positive human breast cancer cell lines in vitro.

Dean JL, Thangavel C, McClendon AK, et al. Therapeutic CDK4/6 inhibition in breast cancer: key mechanisms of response and failure.

2010;29(28):4018-4032. doi: 10.1038/onc.2010.154.

Asghar U, Witkiewicz AK, Turner NC, Knudsen ES. The history and future of targeting cyclin-dependent kinases in cancer therapy.

2015;14(2):130-146. doi: 10.1038/nrd4504.

Lim S, Kaldis P. Cdks, cyclins and CKIs: roles beyond cell cycle regulation.

2013;140(15):3079-3093. doi: 10.1242/dev.091744.

Fry DW, Harvey PJ, Keller PR, et al. Specific inhibition of cyclin-dependent kinase 4/6 by PD 0332991 and associated antitumor activity in human tumor xenografts.

Grossel MJ, Hinds PW. From cell cycle to differentiation: an expanding role for cdk6.

Chen P, Lee N, Hu W, et al. Spectrum and degree of CDK drug interactions predicts clinical performance.

2016;15(10):2273-2281. doi: 10.1158/1535-7163.MCT-16-0300.

Sumi NJ, Kuenzi BM, Knezevic CE, et al. Chemoproteomics reveals novel protein and lipid kinase targets of clinical CDK4/6 inhibitors in lung cancer.

2015;10(12):2680-2686. doi: 10.1021/acschembio.5b00368.

Kaur G, Stetler-Stevenson M, Sebers S, et al. Growth inhibition with reversible cell cycle arrest of carcinoma cells by flavone L86-8275.

Byrd JC, Lin TS, Dalton JT, et al. Flavopiridol administered using a pharmacologically derived schedule is associated with marked clinical efficacy in refractory, genetically high-risk chronic lymphocytic leukemia.

. 2007;109(2):399-404. doi: 10.1182/blood-2006-05-020735.

Articles

Clinical Articles

The second most common cancer worldwide, breast cancer remains a significant global burden despite decreased incidence in Western countries.&nbsp;Breast cancer is the fifth leading cause of cancer death in the United States, with more than 40,000 estimated deaths in 2017.

Rationale For CDK4&6 Inhibition in Breast Cancer

&ldquo;This encouraging efficacy signal is combined with a lack of emerging worrying toxicities at this point in clinical development.&rdquo;

Palbociclib is a reversible, highly selective inhibitor of CDK4&6. In March 2017, palbociclib was approved by the FDA for the treatment of hormone receptor (HR)positive, human epidermal growth factor receptor 2 (HER2)&ndash;negative, advanced or metastatic breast cancer in combination with an aromatase inhibitor as initial endocrine-based therapy in postmenopausal women.

This approval was based on the PALOMA-2 trial that included 666 postmenopausal women who were randomized 2:1 to receive palbociclib plus letrozole or placebo plus letrozole until disease progression or unacceptable toxicity.3 Patients who received palbociclib plus letrozole had a significantly longer median progression-free survival (PFS; 24.8 months) compared with patients who received placebo plus letrozole (14.5 months; HR, 0.58; 95% CI, 0.46-0.72; P <.001). Evaluation of safety data in 444 patients receiving palbociclib plus letrozole revealed neutropenia in 80% of patients. Other commonly reported adverse events (AEs) occurring in &ge;10% of patients include infections, leukopenia, fatigue, nausea, stomatitis, alopecia, diarrhea, rash, anemia, asthenia, thrombocytopenia, vomiting, decreased appetite, dry skin, dysgeusia, and pyrexia. Commonly reported grade &ge;3 AEs include neutropenia, leukopenia, infections, and anemia.

Abemaciclib was approved by the FDA in September 2017 for the treatment of adult patients with HR-positive, HER2-negative advanced or metastatic breast cancer that has progressed following endocrine therapy.

Abemaciclib is approved in combination with fulvestrant following progression on endocrine therapy or as monotherapy in patients previously treated with endocrine therapy and chemotherapy after metastasis. This approval was based on a trial of 669 patients with HR-positive, HER2-negative breast cancer, demonstrating an improved median PFS in patients receiving abemaciclib plus fulvestrant compared with patients receiving placebo plus fulvestrant (16.4 vs 9.3 months). A single-arm trial of 132 patients with HR-positive, HER2-negative breast cancer was designed to evaluate abemaciclib as a monotherapy. Of the patients treated with abemaciclib alone, 19.7% experienced complete or partial tumor shrinkage for a median of 8.6 months.

Frequently reported AEs include diarrhea, nausea, abdominal pain, vomiting, headache, infections, fatigue, decreased appetite, neutropenia, leukopenia, and anemia. Serious AEs include diarrhea, neutropenia, elevated liver function tests, and deep venous thrombosis/pulmonary embolism.

In March 2017, ribociclib, a reversible, highly selective inhibitor of CDK4 and CDK6, was approved by the FDA in combination with an aromatase inhibitor as initial endocrine-based therapy for the treatment of postmenopausal women with HR-positive, HER2-negative advanced or metastatic breast cancer.

This approval was based on results from the MONALEESA-2 trial that included 668 postmenopausal women who received no prior therapy for advanced disease. These women were randomized to receive ribociclib plus letrozole or placebo plus letrozole until disease progression or unacceptable toxicity. A pre-planned interim efficacy analysis revealed a significantly improved median PFS in patients receiving ribociclib plus letrozole compared with patients receiving placebo plus letrozole (not reached vs 14.7 months). An improved objective response rate (ORR) was also reported in patients receiving ribociclib plus letrozole compared with patients receiving placebo plus letrozole (52.7% vs 37.1%). Commonly reported AEs in &ge;20% of patients receiving ribociclib include neutropenia, nausea, diarrhea, fatigue, leukopenia, vomiting, alopecia, headache, constipation, and back pain. Frequent grade 3/4 AEs include vomiting, lymphopenia, neutropenia, leukopenia, and abnormal liver function tests.

In the preclinical setting, estrogen receptor (ER)-positive and HER2-amplified breast cancer cell lines were reported to be the most sensitive to treatment with palbociclib, whereas ER-negative and HER2-negative cell lines were the most resistant to treatment with palbociclib. A synergistic inhibitory effect was also observed when tamoxifen was added to palbociclib.

The PALOMA-1/TRIO-18 trial was designed to evaluate the safety and efficacy of the combination of palbociclib and letrozole as first-line treatment for patients with ER-positive, HER2-negative breast cancer.

The phase Ib study determined the recommended phase II dose of palbociclib of 125 mg once daily for 3 weeks followed by 1 week without treatment, combined with letrozole 2.5 mg once daily in a 28-day cycle. Patients were randomized to receive the combination or letrozole only. The median PFS was significantly longer in patients receiving the combination (20.2 vs 10.2 months). Both the ORR and clinical benefit rate (CBR; complete response, partial response [PR], and stable disease) were higher in the combination arm compared with the letrozole arm (43% vs 33% and 81% vs 58%, respectively). No difference in overall survival was reported.

In the double-blind PALOMA-3 trial, 521 pre- and post-menopausal women with HR-positive, HER2-negative breast cancer who relapsed or progressed during prior endocrine therapy were enrolled to assess the efficacy of palbociclib plus fulvestrant compared with placebo plus fulvestrant. The median PFS was significantly longer in patients receiving palbociclib plus fulvestrant compared with placebo plus fulvestrant (9.5 vs 4.6 months; hazard ratio, 0.46; 95% CI, 0.36-0.59;

A dose-escalation study reported that a dosage break is not necessary with abemaciclib, which is administered at 200 mg every 12 hours.10 Results from a phase I trial that included 47 patients with advanced breast cancer with a median of 7 prior systemic therapies reported a 70% disease control rate for all patients with breast cancer, and 81% for the 36 patients with HR-positive breast cancer. The median PFS was 5.8 months and 8.8 months for all patients with breast cancer and for HR-positive patients, respectively.

This study was expanded to determine the efficacy of abemaciclib plus fulvestrant in patients with HR-positive metastatic breast cancer. A total of 13 patients with a median of 4 prior systemic therapies were enrolled, resulting in 62% confirmed PRs and 23% unconfirmed PRs. No patients discontinued treatment due to toxicity.

The phase II MONARCH-1 trial included 132 patients with previously treated HR-positive, HER2-negative breast cancer, reporting an ORR of 19.7% and a CBR of 42.4% with a median PFS of 6 months from treatment with abemaciclib monotherapy. Due to toxicity, 7.6% of patients discontinued treatment (TABLE 1).

The phase II monarcHER trial (NCT02675231) was designed to evaluate the effectiveness of abemaciclib plus trastuzumab (Herceptin) with or without fulvestrant or chemotherapy. A total of 225 women with HR-positive, HER2-positive locally advanced or metastatic breast cancer who had received at least 2 HER2-directed therapies were enrolled to assess PFS, and the trial is still ongoing.

The phase III MONARCH-2 trial was designed to evaluate PFS in postmenopausal patients with HR-positive, HER2-negative breast cancer treated with abemaciclib plus fulvestrant or fulvestrant alone. The median PFS was significantly longer in patients receiving abemaciclib plus fulvestrant compared with fulvestrant alone (16.4 vs 9.3 months; hazard ratio, 0.553; 95% CI, 0.449-0.681; P <.001). The ORR was also higher in patients with measurable disease who received the combination compared with fulvestrant alone (48.1% vs 21.3%).

An interim analysis of the phase III MONARCH-3 trial, designed to evaluate the effectiveness of non-steroidal aromatase inhibitors (NSAIs) plus abemaciclib in 450 postmenopausal patients with HR-positive, HER2-negative advanced breast cancer patients without prior therapy, demonstrated significantly improved PFS in patients receiving abemaciclib plus anastrozole or letrozole compared with patients receiving placebo plus anastrozole or letrozole (not reached vs 14.7 months). The ORR was also significantly improved in patients receiving abemaciclib vs placebo (59% vs 44%;

The phase III MONARCH plus trial was designed to evaluate abemaciclib combined with NSAI or letrozole in 450 postmenopausal patients with HR-positive, HER2-negative locoregionally recurrent or advanced breast cancer who received no more than 1 line of endocrine therapy or any prior chemotherapy (NCT02763566).

In a dose-escalation study of 132 patients with retinoblastoma (Rb) proteinpositive solid tumors and 18 breast cancer patients, the maximum-tolerated dose (MTD) of ribociclib was reported to be 900 mg, with the recommended phase II dose of 600 mg/day.

The SIGNATURE trial was designed to evaluate the efficacy of ribociclib in patients with solid tumors with CDK4&6 pathway activation who have received at least 1 prior treatment and do not have any standard therapy options remaining (NCT02187783). An ongoing randomized double-blind trial includes 132 patients with HR-positive, HER2-negative breast cancer who progressed on an aromatase inhibitor and a CDK4&6 inhibitor. Primary results are expected in May 2018.

In the advanced setting, phase I/II studies are ongoing, exploring combinations with ribociclib. The TRINITI-1 trial was designed to determine the MTD and CBR of ribociclib with everolimus (Afinitor) and exemestane (NCT02732119). Another trial was designed to determine the MTD and CBR of ribociclib with trastuzumab or trastuzumab emtansine (T-DM1; NCT02657343). A trial was designed to compare dose-limiting toxicities and PFS of ribociclib plus fulvestrant, ribociclib plus BKM120 plus fulvestrant, and ribociclib plus BYL719 plus fulvestrant (NCT02088684).

In the MONALEESA-2 trial, 668 postmenopausal, treatment-na&iuml;ve women with HR-positive, HER2-negative breast cancer were enrolled and randomized to receive ribociclib plus letrozole or placebo plus letrozole. An interim analysis demonstrated a 44% improvement in PFS in patients receiving the combination compared with placebo plus letrozole.16 The CBR and ORR were also significantly improved in patients receiving the combination compared with placebo plus letrozole (53% vs 37% and 80% vs 72%, respectively). The PFS rate at 18 months was 63% with the combination versus 42% with letrozole and placebo.

The MONALEESA-3 trial was designed to evaluate the PFS of ribociclib combined with fulvestrant in 725 men and postmenopausal women with HR-positive, HER2-negative breast cancer who received 1 or no previous lines of endocrine treatment (NCT02422615). COMPLEEMENT-1 was designed to collect additional safety and efficacy data for the combination of ribociclib plus letrozole (NCT02941926). A total of 3000 men and pre-/postmenopausal women with HR-positive, HER2-negative advanced breast cancer without prior hormonal therapy will be enrolled to assess the number of grade 3/4 AEs to determine safety and tolerability.

Palbociclib induces a G1-S cell cycle block by blocking Rb and related protein phosphorylation, followed by the downregulation of S-phase cyclins and mitotic regulatory genes and the suppression of DNA replication and nucleotide biosynthesis.

The inhibition of the phosphorylation of Rb protein and subsequent downregulation of E2F-targeted genes account for cell cycle arrest.

The antitumor effects of palbociclib are limited based on the dependence on active Rb protein presence.

Palbociclib has also been implicated to have a role in the inactivation of FOXM1, a transcription factor, and its transcription targets that induce cellular proliferation.

Palbociclib has also demonstrated sensitization effects on cancer cells to other treatments used in combination, including ionizing radiation and chemotherapy.

Abemaciclib, conversely, has been reported to have increased selectivity for CDK4 versus CDK6.

The most frequently reported toxicity associated with palbociclib is neutropenia, with other reported AEs including leukopenia, anemia, fatigue, back pain, diarrhea, and pulmonary embolism.

Neutropenia has been shown to be the only dose-limiting effect, with common non-hematological AEs including nausea, diarrhea, and fatigue.

The AE profile of ribocicilb is similar to that of palbociclib.

Of patients treated with ribociclib, 10% exhibited asymptomatic QTcF prolongation (>450 ms) at 600 mg/day and 27% exhibited this AE at doses >600 mg/day.

The observed AEs with abemaciclib treatment are different from those reported with palbociclib and ribociclib, which may potentially be a result of a greater selectivity of abemaciclib for CDK4 compared with CDK6.

The dose-limiting toxicity related to abemaciclib is fatigue, with less neutropenia reported compared with palbociclib and ribociclib.

The most commonly reported AEs include fatigue, diarrhea, nausea, vomiting, leukopenia, thrombocytopenia, neutropenia, anorexia, increased creatinine, anemia, and weight loss (TABLE 2).11 Abemaciclib monotherapy treatment has resulted in increased ORRs compared with palbociclib and ribociclib.

Patients receiving palbociclib should avoid simultaneous use of strong CYP3A inhibitors such as ketoconazole, ritonavir (Norvir), clarithromycin, and grapefruit juice. A dose reduction of palbociclib is required if a strong CYP3A inhibitor must be used. Palbociclib use combined with a sensitive CYP3A substrate with a narrow therapeutic index, including fentanyl, everolimus, and midazolam, may require a dose reduction of the CYP3A substrate.

Similar recommendations exist for patients receiving ribociclib. Agents known to prolong the QT interval, including amiodarone, sotalol, and haloperidol (Haldol Decanoate), should be avoided in patients receiving ribociclib.38 Patients receiving abemaciclib should also avoid simultaneous use of strong CYP3A inducers and inhibitors, or a dose reduction of abemaciclib may be warranted.

Following oral administration, palbociclib is slowly absorbed and eliminated in patients.

The median time from dosing to maximum plasma concentration was 4.2 to 5.5 hours. Its half-life was reported to be 23 to 26 hours, with no drug-drug interactions with letrozole.

The MTD recommended for monotherapy and combination strategies was reported as 125 mg once daily over 3 weeks on followed by 1 week off schedule. In a subset of patients, low absorption and drug exposure were observed in the fasted state, which was increased when palbociclib was administered with food. Reduced drug levels and effectiveness may be experienced if palbociclib is taken on an empty stomach.

Following oral administration, ribociclib was absorbed with a median time from dosing to maximum plasma concentration of 1 to 5 hours.

The MTD of ribociclib was reported as 900 mg daily of 3 weeks on and 1 week off over a 28-day schedule.

The development of CDK4&amp;6 inhibitors has provided promising data regarding this new therapeutic option for patients with cancer, particularly for patients with breast cancer. Currently, there are 3 FDA-approved CDK4&amp;6 inhibitors for the treatment of patients with breast cancer.

Breast Cancer: Focus on CDK4&6 Inhibitors

Rationale For CDK4&6 Inhibition in Breast Cancer

Approved CDK4&6 Inhibitors in Breast Cancer

Ongoing Trials in Other Settings for CDK4&6 Inhibitors in Breast Cancer

Expert Shares How CDK4&6 Inhibitors Break the Mold on Breast Cancer Paradigm

CDK4&6 Inhibitors Revolutionize Hormone-Positive Breast Cancer Care