The Pharmacological Profile of Cyclin-dependent Kinase (CDK) 4/6 Inhibitors: Clinical Management of Toxicity and Drug Interactions Related to CDK 4/6 Inhibitor-based Treatment in Advanced/Metastatic Breast Cancer

One of the most important hallmarks of cancer is the ability of cancer cells to sustain aberrant proliferative signaling, resulting in immortality [1]. In normal tissues, a number of proteins have the significant role of tightly regulating the process of cell proliferation [2]. In this way, under the strict regulation of the cell cycle machinery, cells divide without aberrations [3]. Cyclin D-dependent kinases, in particular CDK 4 and 6, play a critical role as regulators of cell cycle by controlling the transition from the G1 to the S phase [4]. CDK1 regulates the transition from the G2 to the M phase, while CDK2 regulates the transition through the S phase [5]. A number of preclinical trials in breast cancer have shown that abnormal tumor cell proliferation is related to the hyperactivity of the cyclin D-CDK4/6 axis, making the inhibition of this pathway an interesting target for the treatment approach of advanced/metastatic estrogen receptor positive (ER(+)) and human epidermal growth factor receptor 2 negative (HER2(−)) breast cancer in combination with endocrine therapy [6]. Additionally, the role of the tumor suppressor retinoblastoma protein (Rb) for the transition from the G1 to the S phase of cell cycle is well known [7], [8]. Retinoblastoma is a gene product that promotes this transition after being hyperphosphorylated by the complex cyclin D-CDK 4 and 6 [9]. Several alterations of the CDK4/6-Rb pathway can result in overactivation of CDK4/6 that can lead to attenuation of senescence and promotion of uncontrolled cell division [5], [7], [10]. In this short review we will focus on the mechanism of action of CDK4/6 inhibitors and the management of the most common adverse events and drug interactions related to the treatment of breast cancer with these new agents.

To date, the subtypes of breast cancer in which CDK4/6 inhibitors have shown clinical benefit are hormone receptor positive (HR+) and HER2− [3], [11], [12], [13], [14]. In HR+ breast cancer there is high expression of ER and/or progesterone receptor (PR) and dependence from the ER signaling pathway for the growth and survival of cancer cells [15]. While estrogen drives the ER signaling pathway to facilitate a number of cellular functions, such as the cell proliferation and procedures of apoptosis and angiogenesis, HR+ breast cancers use these regulating functions in favor of tumor growth, development, and progression [15]. This observation led to the introduction of the following agents in the therapeutic field of HR+ breast cancer: the aromatase inhibitors such as anastrozole, letrozole, and examestane, selective ER modulators such as tamoxifen, and selective ER down regulators such as fulvestrant. CDK4/6 inhibitors in combination with endocrine therapy have shown improved progression-free survival (PFS) in HR+/HER2− advanced/metastatic breast cancer [13].

Since the promotion of tumor growth is driven by the action of ER pathway in HR+ breast cancer, its inhibition would prevent the cancer’s development [15]. Indeed, the efficacy of endocrine therapy in this subtype of breast cancer is undeniable. In some patients, though, this efficacy can be limited due to acquired or preexisting resistance [15]. The fact that both ER signaling pathway and alternative survival pathways use the cyclin D-CDK4/6-INK4-Rb pathway led to the addition of CDK4/6 inhibitors to endocrine therapy in order to prevent the tumor growth [7], [11], [12], [13], [14], [16], [17].

The development and proliferation of normal cells undergo strict regulation by external growth signals and the cell cycle machinery [2]. All phases of cell cycle, G1, S, G2, and M phase, are controlled by a number of “checkpoints” that prevent any damage or defect before entering the M phase, in order to protect daughter cells from carrying damaged DNA [18]. Breast cancer cells, like all tumor cells, are characterized by dysregulation of these “checkpoints,” resulting in uncontrolled proliferation and survival of cancer cells [6].

The passage from the G1 to the S phase of cell cycle is very crucial, because it promotes the cell to enter the M phase irreversibly [19]. The cyclin D-CDK4/6-INK4-Rb pathway plays a key role in the control of transition from the G1 to the S phase [20]. During phase G, the inactive complex between Rb and E2F transcription factor regulates the prevention of gene’s expressions required for the cell to enter the S phase and proceed to cell division [7]. Concomitantly, the ER pathway, a mitogenic signaling pathway, regulates the expression of cyclin D through CCND1 gene, allowing the formation of cyclin D-CDK 4 or 6 complex [21]. CDK 4 and 6 are serine-threonine kinases with structural, biochemical, and biological similarities [20]. The complex cyclin D-CDK4/6 is stabilized by proteins, such as p21. The active holoenzyme monophosphorylates Rb protein that causes derepression of the transcription factors E2F family [10]. E2F is then active and enables the expression of another, very important factor for the S phase, cyclin E [10]. Binding of cyclin E to CDK2 causes its activation by which Rb protein becomes hyperphosphorylated, leading to further liberation of E2F transcription factors [22], [23]. This sequence of functions results in promotion of transition from the G1 to the S phase through the expression of multiple genes [7], [17], [20].

In ER+ breast cancer there is disruption of the abovementioned axis leading to promotion of tumor growth through hyperactivity of this pathway [6]. The mode of action of the three CDK4/6 inhibitors, palbociclib, ribociclib, and abemaciclib, consists of blockade of Rb phosphorylation, leading to inactive E2F transcription factors and induction of cancer cell arrest [16], [17], [23], [24], [25]. Indeed, the phenotype of cancer cells after the influence of CDK4/6 inhibitors resembles the cellular senescence [16]. In particular, all three CDK4/6 inhibitors are highly selective reversible inhibitors of CDK 4 and 6. Their efficacy has been shown in preclinical and clinical level in ER+/HER2− advanced/metastatic breast cancer, in combination with hormone therapy, or as a single agent in the case of abemaciclib [11], [13], [26].

In particular, the data of the clinical trials concerning all three CDK 4/6 inhibitors are as follows:

Palbociclib: This CDK 4/6 inhibitor was studied in the PALOMA trials. The first-in-human phase I study demonstrated a favorable safety profile of the agent with neutropenia being the main dose-limiting toxicity, but with a very low risk of febrile neutropenia, compared to cytotoxic chemotherapy [27]. The initial dose of 125 mg daily (3 weeks on/1week off) was tested as a single agent in metastatic breast cancer patients with Rb expression [28]. PALOMA 1 was a phase II trial, where palbociclib+letrozole was administered as first-line therapy in advanced ER(+)/HER2(−) breast cancer and showed a 10-month improvement in PFS (20.2 versus 10.2 months) [29]. These impressive results were confirmed by PALOMA 2, a phase III trial, with median PFS being 24.8 and 14.5 months for the palbociclib and the placebo group, respectively. Also its final analysis demonstrated that in a follow-up of almost 38 months there was a significant delay in the use of chemotherapy after palbociclib was stopped (40.4 and 29.9 months for the palbociclib and the placebo arm, respectively), while quality of patients’ life was maintained [30]. Also, in PALOMA 3 palbociclib was studied in endocrine therapy-resistant metastatic breast cancer in combination with fulvestrant as second-line therapy with median PFS 9.5 and 4.6 months for the palbociclib and the placebo group, respectively [31] [32]. The final analysis in PALOMA 3 showed improvement in OS (34.9 and 28 months for the palbociclib and the placebo group, respectively) [28].

Table 1 summarizes all data concerning clinical trials for palbociclib, ribociclib, and abemaciclib.

The first CDK4/6 inhibitor designed was palbociclib. Its enzymatic IC50 for CDK 4 and 6 is approximately 11 and 15 nM, respectively. In the case of ribociclib the IC50s are 10 and 40 nM, respectively, for CDK 4 and 6, while the most potent CDK4/6 inhibitor seems to be abemaciclib with IC50s 2 and 10 nM for CDK 4 and 6, respectively [12]. All CDK4/6 inhibitors are orally bioavailable agents and demonstrate their action by binding to the ATP-binding pocket within kinases. This binding results in blocking of Rb’s phosphorylation mediated by CDK4/6, which in turn causes the cell cycle arrest at the G1–S checkpoint, because E2F remains inactive as a complex with Rb [7], [17]. These observations were demonstrated by inhibition of cancer cell proliferation in Rb(+) cell lines and dose-dependent inhibition of tumor growth in Rb(+) xenograft tumor models [13].

CDK4/6 inhibitors, palbociclib, ribociclib, and abemaciclib, are small molecules that underwent strict evaluation after their design in order to prove their efficacy in breast cancer [11], [13]. The trial programs were PALOMA for palbociclib [31], [32], [42], [43], [44], [45], MONALEESA for ribociclib [33], [34], [35], [36], [46], and MONARCH for abemaciclib [38], [41], [47], while a number of trials are in progress as there is the suggestion that CDK4/6 inhibitors could offer clinical benefit in other breast cancer subtypes as well [12] [11].

Palbociclib was approved by Food and Drug Administration (FDA) in February 2015, ribociclib was approved in March 2017, and abemaciclib in October 2015 received FDA breakthrough therapy designation. The first two agents, palbociclib and ribociclib, are approved by European Medicines Agency (EMA) as well [14]. It should be taken into account that there are differences between the three agents as far as drug administration is concerned. The dosing schedule for the three agents is as follows: The recommended initial dose of palbociclib is 125 mg daily for 3 weeks on, 1 week off, constituting a 28-day cycle [45], of ribociclib is 600 mg daily for 3 weeks on, 1 week off, constituting a 28-day cycle [46], and of abemaciclib is 150 mg twice daily continuously throughout the cycle when given as monotherapy and in combination with endocrine therapy [47]. While ribociclib and abemaciclib can be taken orally with or without food, in the case of palbociclib it was found that the fasted state caused decreased absorption and drug exposure, and it is recommended that the drug should be administered with food [45], [46], [47].

The three drugs differ in Tmax (time to maximum concentration). The agent with the slower absorption is palbociclib, with Tmax between 6 and 12 hours [45], while ribociclib is rapidly absorbed, with Tmax between 1 and 5 hours [46], and abemaciclib’s Tmax is between 4 and 6 hours [47]. The mean half-life (T1/2) for the CDK4/6 inhibitors is approximately 26 hours for palbociclib [45], 33 hours for ribociclib [46], and 17–38 hours for abemaciclib [47]. Steady-state levels of drugs can be reached within 8 days, in the case of palbociclib and ribociclib, while more days are required for abemaciclib to reach steady-state levels due to the shorter half-life of the drug [13].

If there is any need of modification of the dosing schedule, recommendations are as follows: For palbociclib the recommended initial dose is 125 mg daily (3 weeks on/1 week off), with the first dose reduction 100 mg daily and the second 75 mg daily. For ribociclib the initial dose is 600 mg daily (3 weeks on/1 week off) with the first dose reduction 400 mg daily and the final reduction 200 mg daily. Finally, the recommended initial dose for abemaciclib is 150 mg every 12 hours as monotherapy and in combination with endocrine therapy. The first dose reduction, if required, is 100 mg twice daily and the second and final dose reduction is 50 mg twice daily. Following the second reductions mentioned above for the three agents, no further reduction is recommended and the drug should be permanently discontinued if further unacceptable toxicity occurs [3], [14], [45], [46], [47].

In vitro and in vivo studies have shown that all CDK4/6 inhibitors, palbociclib, ribociclib, and abemaciclib, undergo primary hepatic metabolism by CYP3A4 and SULT2A1 (sulfotransferase) in humans. Additionally, it has been shown that CDK4/6 inhibitors are weak, time-dependent inhibitors of CYP3A [3], [14]. The observation above led to specific recommendations regarding the co-administration of CDK4/6 inhibitors with CYP3A4 inhibitors or inducers.

The family of CDKs, by improvement in PFS, has proven its efficacy in HR+ and HER2− advanced/metastatic breast cancer [11]. However, like all drugs, the use of CDK4/6 inhibitors demonstrated some adverse events, with neutropenia being the most common [3], [13], [14]. Yet, the side effects associated with chemotherapy are more severe than those induced by CDK4/6 inhibitors. Other adverse events during CDK4/6 inhibitor-based treatment are fatigue, low severity gastrointestinal toxicity, such as vomiting, nausea, and diarrhea, QTc prolongation, hepatobiliary toxicity, pulmonary embolism, and alopecia [45], [46], [47]. According to the data above, accurate patient monitoring and management of CDK4/6 inhibitors’ side effects are recommended [3], [13], [14], [26].

CDK4/6 inhibitors are innovative drugs that entered the treatment landscape of HR(+)/HER2(−) advanced/metastatic breast cancer, exhibiting PFS for patients (pre- and postmenopausal) of this subtype of breast cancer in combination with endocrine therapy, compared to endocrine therapy alone. In preclinical and clinical level it has been shown that CDK4/6 inhibitors are all well-tolerated oral agents. Yet, like all medicinal products, there are a number of adverse events related to these agents, with neutropenia being the most common. Other side effects of CDK4/6 inhibitor-based treatment are gastrointestinal and hepatobiliary toxicity, QT prolongation, pulmonary embolism, fatigue, and alopecia. However, the toxicity profile of CDK4/6 inhibitors was proven to be less severe, compared to that of chemotherapy. The key to achieve efficacy and avoid toxicity during CDK4/6 inhibitor-based treatment is early and sufficient monitoring, regular clinical assessments, and reporting of signs or symptoms of any possible adverse event. As time goes, our understanding and knowledge of CDK4/6 inhibitors’ mechanism of action and toxicity improves and this will facilitate the use of this highly effective oral treatment in larger populations, reducing the need for chemotherapy in the early treatment lines of metastatic disease, thus allowing for more patients to receive active treatment at home, instead of the hospital. Meanwhile, the ongoing studies will hopefully offer answers to whether these new agents can contribute clinical benefit in other breast cancer subtypes in the near future.

Vasiliki C. Tzelepi, and Eleni T. Timotheadou conceived and designed the study. Vasiliki C. Tzelepi, Aristeidis T. Gogadis, and Christos K. Adamidis provided the study material, collected and assembled the data, analyzed and interpreted the data, and wrote the manuscript. Vasiliki C. Tzelepi, Aristeidis T. Gogadis, Christos K. Adamidis, and Eleni T. Timotheadou approved the manuscript.

The authors reported no financial relationships.