In this study, we demonstrate that autochthonous breast tumors can be either CDK4/6 dependent or independent. Consistent with previous studies (41
), a potent and selective CDK4/6 inhibitor demonstrated considerable activity in a HER2-driven GEMM. In contrast, inhibition of CDK4/6 did not produce a therapeutic benefit in the Rb-deficient C3-TAg breast cancer model nor, as we previously reported, in a p16INK4a
-deficient melanoma GEMM (8
). Correspondingly, Puyol et al. (57
) recently showed that a CDK4/6 inhibitor had variable activity in a RAS-driven GEMM of lung cancer, with some tumors responding but most exhibiting resistance. These murine findings are consistent with the limited reported experience of CDK4/6 inhibitors in humans (eg, modest but important response rates in certain tumor types like myeloma, mantle cell lymphoma, and teratoma) (58
We believe the evidence suggests that a subset of human tumors will show substantial dependence on CDK4/6 activity, and a clinical test to readily identify CDK4/6-dependent tumors would facilitate the clinical use of these agents as antineoplastics. Our data and other work suggest that Rb loss portends resistance to these agents (8
). Increased expression of nonmutant p16INK4a
, commonly observed in many tumor types (61
), also appears to signify resistant disease given that p16INK4a
is a potent inhibitor of CDK4/6 (24
). Importantly, it is unlikely that mere CCND1
overexpression or p16INK4a
loss will broadly indicate CDK4/6 addiction; consistent with our findings that p16INK4a
-deficient melanoma is resistant in vivo (8
). In accord with these views, a phase II clinical trial has been initiated in non–small cell lung cancer that selects patients for treatment with PD0332991 using Rb expression and p16INK4a
deletion as predictors of response (www.clinicaltrials.gov
, NCT01291017). The present data, however, demonstrate how CDK4/6 inhibitors can be used for therapeutic benefit in tumors, whether CDK4/6 dependent or independent, if a reliable means to discern resistant and sensitive tumors can be developed.
In particular, we show that CDK4/6 inhibitors provided meaningful therapeutic benefit even in tumors that were refractory to CDK4/6 inhibition. Consistent with previous work with ionizing radiation, we showed a marked protective effect of pharmacological quiescence on chemotherapy-induced myelosuppression. In vitro, this protection extended to a number of cell cycle–specific DNA-damaging agents as well as taxanes. Furthermore, by identifying CDK4/6-resistant tumors (eg, Rb-deficient), we have shown successful bone marrow protection without compromising dose intensity or reducing tumor kill. Several retrospective and prospective randomized trials have shown that reductions in the chemotherapy dose intensity compromises long-term disease control and survival (27
). Despite compelling data, surveys in the United States and elsewhere have reported that dose reductions and delays frequently occur in clinical practice because of myelosuppression, even in the potentially curative setting.
Current therapeutic approaches to minimize myelosuppressive effects rely on the use of growth factors (ie, granulocyte colony-stimulating factor or Epo) and have substantial limitations. These expensive injectable biologics each target a single hematopoietic cell lineage and have been associated with long-term toxic effects. Granulocyte colony-stimulating factor and its derivatives have been associated with a modest increase in the risk of myelodysplasia and secondary leukemia (63
). Similarly, the association of Epo and derivatives with increased mortality, thrombosis, and tumor progression has recently garnered a Black Box warning from the US Food and Drug Administration, which indicates that medical studies have shown that the drug carries a substantial risk of serious or even life-threatening adverse effects. Additionally, there are no available approaches to limit chemo-induced thrombocytopenia or lymphopenia. Limiting chemo-induced thrombocytopenia, in particular, is a clinically significant unmet need (65
) of particular importance in breast cancer treated with dose-dense regimens (66
) and (67
). The pharmacological quiescence approach therefore appears to offer the advantage of quadrilineage protection as opposed to treating a single lineage after myelosuppression has occurred. Importantly, the fear of tumor protection has been a major limitation to the use of other adjunctive measures, but the well-understood mechanism of action of pharmacological quiescence limits this concern. By treating patients whose cancers are resistant to CDK4/6 inhibition (eg, Rb-deficient), chemotherapy-induced myelosuppression can be ameliorated without compromising cancer cell death.
The finding that cell cycle modulation affects the toxicity of DNA-damaging agents such as carboplatin and doxorubicin has implications for their use in human tumors. For example, we show that doxorubicin, carboplatin, and PD0332991 are effective in the MMTV-c-neu model, but that concurrent treatment of the DNA-damaging agents with a selective CDK4/6 inhibitor results in reduced antitumor efficacy. Therapeutic antagonism between cytostatic biologic agents and cytotoxic chemotherapy has been suggested previously in other settings. For example, concurrent administration of the epidermal growth factor receptor tyrosine kinase inhibitors gefitinib or erlotinib with chemotherapy in several phase III clinical trials has proven no more effective than chemotherapy alone (68
). Upon subset analysis, it has been suggested that patients with wild-type epidermal growth factor receptor (the majority of patients) treated with erlotinib plus chemotherapy had the worst outcomes of any treatment group (72
). Our data similarly suggest that CDK4/6 inhibitors should not be paired with cytotoxic agents in tumors dependent on CDK4/6 activity for proliferation.
A major hurdle for the routine incorporation of CDK4/6 inhibitors into current treatment paradigms will be the ability to prospectively discern CDK4/6-dependent vs CDK4/6-independent cancers. Although there are common genetic alterations that predict resistance to CDK4/6 inhibition (ie, Rb loss), an incomplete understanding of cancer cell cycle regulation prevents full coverage across the spectrum of human cancers. Therefore, this approach would only be of clinical use if paired with a reliable “companion diagnostic” that provides results in clinical real time. Such a test of CDK4/6 dependence would likely combine mutational analysis (eg, Rb and p16INK4a
) with tests of mRNA and protein expression. For example, using a panel of over 30 breast cancer cells, Finn et al. (24
) identified a deferential gene expression pattern between CDK4/6-sensitive and CDK4/6-resistant cells. Given the fear of unintentionally protecting a patient's cancer, however, pharmacological quiescence will only be of clinical value if this concern can be comprehensively addressed.
To the point above, our study is not without limitations. Although we used two established preclinical models of breast cancer with a well-defined understanding of their dependence on CDK4/6 and Rb, the genetic lesions in these tumors do not fully represent the diverse genetic heterogeneity in cell cycle regulation seen across all cancers. Therefore, the present work provides proof of the potential therapeutic uses of CDK4/6 inhibitors in patients with CDK4/6-depedent and CDK4/6-independent tumors; however, considerable work is still needed to define genetic or other biomarkers that can be used to prospectively identify a specific tumor's dependence on CDK4/6. Additionally, in this study, we only evaluated tumors that were either sensitive or intrinsically resistant to CDK4/6 inhibition and did not evaluate tumor models “transiently” sensitive or with acquired resistance to CDK4/6 inhibition. We believe there are therapeutic strategies for employing CDK4/6 inhibitors in these tumors; however, this study did not evaluate these potential clinical scenarios.
In summary, we believe that a compelling argument can be made for the use of CDK4/6 inhibitors in most human cancers, which generally appear to come in four types based on their dependence on CDK4/6 activity and sensitivity to cytotoxic agents (). Group I tumors are “durably” CDK4/6 dependent (eg, Cyclin D–amplified mantle cell lymphoma), and in this group, CDK4/6 inhibitors could be used as antineoplastic agents, as demonstrated in our study and that of Leonard et al. (58
). Group II tumors are fully CDK4/6 independent (eg, Rb-null small cell lung cancer), and we showed that in these patients, CDK4/6 inhibitors may be used to prevent myelosuppression. Group III tumors are transiently CDK4/6 dependent but rapidly develop resistance as is common in other therapeutics (59
). We believe that CDK4/6 inhibitors could be of use in this setting to synchronize tumor cells in the cell cycle, allowing for increased tumor cell death with other cytotoxic agents. Proof of concept for this approach has been suggested by Di Liberto et al. (74
) in multiple myeloma, where cell cycle synchronization appears to boost the efficacy of bortezomib. Lastly, Group IV tumors are fully CDK4/6 resistant but insensitive to cytotoxic agents. Although we believe this class of tumor is rare, it is unlikely that CDK4/6 inhibitors would be of use in these tumors. We believe that the present data support a possible role for CDK4/6 inhibitors in a majority of patients with advanced cancer: to inhibit tumor growth, ameliorate the dose-limiting toxicities of chemotherapy or ionizing radiation, or to synchronize tumors for increased cell death mediated by other therapeutic agents.
Predicted clinical scenarios for the use of cyclin-dependent kinase (CDK)-4/6 inhibitors*