Future Studies Hope to Move CDK4&6 Inhibition Into New Terrain

Evolving ParadigmsCDK4&6 Inhibition

Significant strides have been made in developing cyclin-dependent kinase 4 (CDK4) and CDK6 inhibitors to treat estrogen receptor (ER)‒positive breast cancer.

Evidence from recent studies suggests that CDK4&6 inhibitors may be effective in other cancer types, as well. Preclinical trials show that CDK4&6 inhibitors demonstrate activity in human epidermal growth factor receptor 2 (HER2)‒positive breast cancer cells, and may prove effective in overcoming resistance to standard therapy. Data from preclinical and early-phase clinical trials in melanoma and mantle cell lymphoma (MCL) have produced encouraging results that point to CDK4&6 inhibitors as potential alternative therapies. Clinical studies of CDK4&6 inhibitors in breast cancer subtypes and other cancers are currently ongoing.

CDK4&6 Inhibitors in ER-Positive Breast Cancer

The development of CDK4&6 inhibitors is most advanced in the area of breast cancer treatment, particularly for the ERpositive, HER2-negative type. Based on the results of PALOMA-3, palbociclib was approved by the FDA in February 2016 for treating advanced or metastatic breast cancer in combination with fulvestrant.1In another phase III trial, PALOMA-2, palbociclib plus letrozole was reported to prolong progression-free survival (PFS) compared with letrozole alone, and results will be presented at the 2016 ASCO Annual Meeting.2

Abemaciclib has demonstrated high disease control rates (DCRs) and prolonged PFS as a single agent and in combination with fulvestrant in phase I/II studies.3The lack of hematologic and other serious dose-limiting adverse events (AEs) fuels the hope that abemaciclib may be an effective monotherapy for ER-positive, HER2-negative breast cancer.4Abemaciclib is being studied as a single-agent treatment in a phase II study, MONARCH 1 (NCT02102490). Phase III studies of abemaciclib in combination with nonsteroidal aromatase inhibitors (AIs; MONARCH 3, NCT02246621) and fulvestrant (MONARCH 2, NCT02107703) are ongoing.

Preclinical and phase I studies have shown promising results for ribociclib as a treatment for ER-positive, HER2-negative breast cancer. Single-agent activity has been demonstrated in retinoblastoma (Rb)‒positive tumors, although myelosuppression remains a dose-limiting toxicity (DLT).3,4Ribociclib has advanced to phase III trials as combination treatment with letrozole (MONALEESA-2, NCT01958021), fulvestrant (MONALEESA- 3, NCT02422615), and goserelin with either tamoxifen or an AI (MONALEESA-7, NCT02278120).

Overcoming Resistance Mechanisms

CDK4&6 inhibitors have shown efficacy for treating ER-positive breast cancer that is refractory to standard therapy, but many patients who initially respond to CDK4&6 inhibitors will develop resistance to this treatment modality, as well.5,6Multiple pathways exist that enable ER-positive tumors to become resistant, and most patients treated with palbociclib were eventually found to progress on therapy. CCNE1 amplification and RB1 gene loss are 2 mechanisms that have been demonstrated to mediate resistance. Notably, cells with CCNE1 amplification were sensitive to CDK2 inhibition, whereas those with RB1 loss were not. Thus, CDK2 may serve as a target for future therapies to overcome resistance to CDK4&6 inhibitors.9

Resistance to CDK4&6 inhibition may also develop via the PI3K pathway. Constitutive activation of the PI3K pathway is associated with relapses in ER-positive breast cancer, and up to 40% of all breast cancer types have a mutation in the PIK3CA gene, which encodes the catalytic site of PI3Kα. In addition, more than 50% of patients with breast cancer have abnormalities in components of the PI3K pathway, such as PTEN and AKT1-3, that lead to increased PI3K signaling. In both preclinical studies and the clinical setting, persistent retinoblastoma product (pRb) phosphorylation, the result of incomplete PI3K pathway inhibition, predicted treatment failure with CDK4&6 inhibitors. Mutations in PI3KCA may serve as a biomarker to identify the patients who are most likely to progress on CDK4&6 inhibitors.8

In a breast cancer cell line study by Sadhna Vora, MD, with Massachusetts General Hospital Cancer Center in Boston, MA, and colleagues, 1 of 4 models responded to CDK4 and 6 plus PI3K inhibitor (PI3Ki) and did not regrow after treatment was discontinued. However, the other 3 models were resistant to treatment, suggesting that more biomarkers are needed to appropriately select patients for targeted therapy.10Interestingly, fulvestrant was found to have synergy with CDK4 and CDK6 and PI3Ki in 1 of the resistant models, which highlights residual ER signaling as an alternate pathway of resistance in tumors with PI3K dysregulation.8

In other preclinical studies, both palbociclib and ribociclib have demonstrated synergy with PI3Ki.4,9Several trials are under way that evaluate ribociclib plus everolimus, a rapamycin analog that inhibits the PI3K pathway, in ER-positive breast cancer that has progressed on CDK4&6 inhibitor treatment (NCT02732119 and NCT01857193). A phase II study is also investigating ribociclib plus fulvestrant versus fulvestrant alone in a similar population (NCT02632045).

In an article that previewed the work of Vora and colleagues, senior author Gordon Mills, MD, PhD, with MD Anderson Cancer Center in Houston, TX, noted that “a comprehensive elucidation of the mechanisms underlying persistent cyclin D1 expression and CDK4 activity in the presence of PI3Ki and the downstream events mediating cell survival and proliferation may provide a ‘suite’ of targets warranting exploration in combination with PI3Ki in the future.”8

CDK4&6 Inhibitors in HER2-Positive Breast Cancer

Even though CDK4&6 inhibitors have been extensively studied in ER-positive breast cancer, preclinical data show that CDK4&6 inhibitors have activity in HER2-positive subtypes, as well.5Although the majority of breast cancers are ER-positive, HER2 is overexpressed in up to 15% of all patients with breast cancer. HER2 exerts its oncogenic effect by dimerizing with other HER molecules, which initiate downstream signaling pathways that ultimately lead to tumorigenesis, cell proliferation, and metastasis.6

“While anti-HER2 therapy provides tremendous benefit in HER2-positive breast cancer, resistance to treatment does develop,” stated Sara Tolaney, MD, with Dana-Farber Cancer Institute (DFCI) in Boston, MA. The mainstay of therapy for HER2-positive breast cancer is to target HER2 using monoclonal antibodies, a HER2/epidermal growth factor receptor (EGFR) kinase inhibitor, or an antibody-drug conjugate (ADC). However, tumors that resist initial treatment relapse as metastases at distant sites, which continue to render advanced disease resistant to targeted treatment.6

“Data in mouse models and human breast cancer cell lines suggest that HER2-positive cells that are resistant to treatment have high levels of cyclin D and CDK4,” Tolaney noted. “Synergy may also exist between CDK4&6 inhibition and anti-HER2 therapy. Preclinical work done by Shom Goel and his group suggests that CDK4&6 inhibition can help overcome this resistance and resensitize cells to anti-HER2 therapy.”

More than 1 mechanism may explain the synergistic effects of CDK4&6 inhibition and HER2 blockade. Shom Goel, MD, PhD, with DFCI, and colleagues, showed that administering CDK4&6 inhibitors reduced TSC2 phosphorylation and attenuated mTORC1 activity in HER2-positive breast cancer models. Decreased mTORC1 activity resulted in decreased feedback inhibition of upstream EGFR kinases, thus restoring tumor sensitivity to EGFR/HER2 blockade. When administered together, CDK4&6 inhibitors and HER2-blocking agents induced greater-than-expected suppression of phosphorylated TSC2 and subsequent mTORC1 activity.6

CDK4&6 inhibitors palbociclib, abemaciclib, and ribociclib have each shown activity in HER2-positive models in preclinical trials.5,7Palbociclib appeared to complement the activity of the ADC trastuzumab emtansine (T-DM1), a HER2-blocker that was recently approved in 2013 for HER2-positive metastatic breast cancer.5In HER2-positive clones, palbociclib suppressed proliferation of cells that were previously treated with T-DM1. Sequencing treatment by first administering T-DM1, followed by palbociclib, may constitute a synergistic combination, and this strategy is being studied in a phase I trial (NCT01976169).

Goel and colleagues found that abemaciclib had activity in HER2-positive cells, both as a single agent and in combination with HER2-blockers. Synergy with lapatinib or trastuzumab was demonstrated, but the authors felt that abemaciclib plus lapatinib was unlikely to be a viable combination in clinical treatment since the 2 agents have significant overlap of gastrointestinal toxicity. Abemaciclib and trastuzumab administered separately did not induce tumor regression, but in combination they decreased tumor growth and delayed growth to a greater extent than either agent alone.6

Abemaciclib also showed promise in the clinical setting. In a phase I trial that enrolled 16 HER2-positive patients, the partial response rate was 36% and the DCR was 100% among those with hormone receptor (HR)‒positive, HER2-positive status. Patients with HR-negative, HER2-positive disease experienced only brief periods of stable disease. Despite these differences in DCR, the entire cohort had a median PFS and median duration of response of 8.8 months and 13.4 months, respectively.4

The preclinical and phase I findings led to a phase II trial, monarcHER (NCT02675231), which will evaluate abemaciclib plus trastuzumab with or without fulvestrant in patients with advanced HR-positive, HER2-positive breast cancer. A phase II study of palbociclib plus trastuzumab with or without letrozole is also under way (NCT02448420). Ribociclib is also being studied in HER2-positive breast cancer in combination with trastuzumab or T-DM1 in a phase Ib/II trial (NCT02657343).

While CDK4&6 inhibitors show promise in ER-positive and HER2-positive breast cancers, their potential benefits may extend to a subtype known as triple-negative breast cancer (TNBC), in which tumors do not overexpress ER, HER2, or progesterone receptor. “Preclinical data suggest that CDK4&6 inhibitors may have activity in the luminal androgen receptor subtype of TNBC. Currently, studies are being planned that will examine the efficacy of CDK4&6 inhibitors in Rb-positive or androgen receptorpositive TNBC,” Tolaney stated.

TABLE. CDK4&6 Inhibitors in Development for Other Cancer Types




Non—Small Cell Lung Cancer


Abemaciclib versus docetaxel

Phase II


Abemaciclib versus erlotinib

Phase III


Ribociclib plus ceritinib in ALK+ NSCLC

Phase I/II

Mantle Cell Lymphoma



Phase II


Palbociclib plus ibrutinib

Phase I

Brain Tumors


Ribociclib in glioblastoma or anaplastic glioma

Phase I


Abemaciclib for brain metastases

Phase II




Phase II



Phase II

CDK4&6 Inhibitors in Other Cancers

CDK4&6 inhibitors are now being explored in cancers types outside of breast cancer. D-type cyclins depend on RAS signaling, and preclinical trials have shown synergy between CDK4&6 inhibitors and MEK inhibitors in NRAS-driven melanoma. Anti- MEK agents cause apoptosis but not cell cycle arrest, but when combined with CDK4&6 inhibitors, they induced senescence in cancer cells as well as tumor regression.4A current phase Ib/ II trial is investigating ribociclib plus a MEK inhibitor in NRASmutant melanoma (NCT01781572).

Pancreatic cancer cell lines that are deficient in p16INK4a do not respond to CDK4&6 inhibitors, but adding a PI3K or IGF1R inhibitor improved sensitivity to CDK4&6 inhibitors. IGF1R plus CDK4&6 inhibition led to decreased mTORC1 activity, which may explain the synergistic effect.4 Clinical trials evaluating CDK4&6 inhibitors in combination with TGF-β receptor inhibitors or IGF1R inhibitors are being planned.11

In a phase I trial in non‒small cell lung cancer (NSCLC), patients with the KRAS mutation had a higher DCR (55%) in response to CDK4&6 inhibition than those without (39%).4An ongoing study is evaluating palbociclib in KRAS mutation-positive NSCLC (NCT02022982).

In liposarcoma cell lines, CDK4&6 inhibitors induced rapid and durable cell cycle arrest in cells with chromosome 12q14 amplification of linked CDK4 and MDM2. Reduced MDM2 expression as the result of proteolytic turnover was associated with PFS of up to 800 days in duration.4A phase II trial examining the efficacy of ribociclib monotherapy in advanced liposarcoma is under way (NCT02571829).

The subtype of MCL associated with the [t(11:14(q13;q32)] translocation has been shown to respond to palbociclib in a pilot study. The [t(11:14(q13;q32)] translocation induces B cells to overexpress cyclin D1. In a pilot study, palbociclib decreased tumor metabolism on fluorothymidine-PET scan, decreased pRb phosphorylation and Ki67 expression, and induced G1 cell cycle arrest.

MCL may also respond to combination therapy with CDK4 and CDK6 and PI3Ki. Preclinical models showed that in MCL cells, treatment with CDK4&6 inhibitors induced higher levels of PIK31P1, a negative PI3K regulator. In turn, MCL sensitivity to PI3Ki increased, resulting in apoptosis. Clinical studies are being planned that will evaluate CDK4&6 inhibitors as a monotherapy or in combination with PI3Ki.11

In glioblastoma multiforme (GBM), codeletions in CDKN2A and CDKN2C, which encode p16INK4a and p18INK4c, respectively, were markers of increased sensitivity to CDK4&6 inhibitors.4A phase I study is being planned to investigate CDK4&6 inhibitors as monotherapy in patients with p16-deficient, RB-positive GBM (NCT02345824).

In trials that recruit patients based on mutation status, genetic features are also being explored as potential biomarkers. These include NRAS in melanoma, CDKN2A in pancreatic cancer and GBM, KRAS in NSCLC, and PIK3CA in ER-positive breast cancer. “[It] remains critical to identify and refine biomarkers that will allow assignment to combination treatments that would lead to the most efficacious response and to continue to identify novel drug combinations that would be well tolerated in patients,” Mills concluded.8


  1. Pfizer Receives Expanded FDA Approval For IBRANCE (palbociclib) In HR+, HER2- Metastatic Breast Cancer [news release]. New York, NY: Pfizer Inc. February 19, 2016. http://www.pfizer.com/news/press-release/press-release-detail/pfizer_receives_ expanded_fda_approval_for_ibrance_palbociclib_in_hr_her2_metastatic_ breast_cancer. Accessed May 10, 2016.
  2. Pfizer Announces Positive Top-Line Results for Phase 3 PALOMA-2 Clinical Trial of IBRANCE® (palbociclib) [news release]. New York, NY: Pfizer Inc. April 19, 2016. http://www.pfizer.com/news/press-release/press-release-detail/pfizer_announces_ positive_top_line_results_for_phase_3_paloma_2_clinical_trial_of_ibrance_palbociclib. Accessed May 10, 2016.
  3. Finn RS, Aleshin A, Slamon DJ. Targeting the cyclin-dependent kinases (CDK) 4/6 in estrogen receptor-positive breast cancers.Breast Cancer Res. 2016;18(1):17.
  4. Sherr CJ, Beach D, Shapiro GI. Targeting CDK4 and CDK6: from discovery to therapy.Cancer Discov. 2016;6(4):353-367.
  5. Murphy CG, Dickler MN. The role of CDK4/6 inhibition in breast cancer.Oncologist. 2015;20(5):483-490.
  6. Goel S, Wang Q, Watt AC, et al. Overcoming therapeutic resistance in HER2-positive breast cancers with CDK4/6 inhibitors.Cancer Cell. 2016;29(3):255-269.
  7. Beckman MG, Hulihan MM, Byams VR, et al. Public health surveillance of nonmalignant blood disorders.Am J Prev Med.2014;47(5):664-668.
  8. Muranen T, Meric-Bernstam F, Mills GB. Promising rationally derived combination therapy with PI3K and CDK4/6 inhibitors.Cancer Cell.2014;26(1):7-9.
  9. Breast Cancer Resistance to CDK4/6 Inhibitors Arises in Many Ways [news release]. Philadelphia, PA: American Association for Cancer Research. March 28, 2016. http://www.aacr.org/Newsroom/Pages/News-Release-Detail.aspx?ItemID=847#. VzIt7PmDFBc. Accessed May 10, 2016.
  10. Vora SR, Juric D, Kim N, et al. CDK 4/6 inhibitors sensitize PIK3CA mutant breast cancer to PI3K inhibitors.Cancer Cell.2014;26(1):136-149.
  11. O’Leary B, Finn RS, Turner NC. Treating cancer with selective CDK4/6 inhibitors [published online March 31, 2016].Nat Rev Clin Oncol. 2016. doi: 10.1038/nrclinonc. 2016.26.

CDK4&6 inhibitors have many potential applications in multiple cancer types and subtypes. Greater understanding of the oncogenic pathways in treatment-resistant breast cancer and in other cancers have led to new strategies that offer hope to patients who do not respond to standard therapy. In addition, a stronger emphasis has been placed on the need for prognostic biomarkers. New anticancer regimens, combined with the ability to provide personalized targeted therapy, will enable patients to receive the most appropriate and effective treatment possible.

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