Clinical Resistance to CDK4/6 Inhibitors: Why Cyclin E-CDK2 Is the Next Therapeutic Frontier

Resistance to CDK4/6 inhibitors in HR+ breast cancer is oftentimes a consequence of cyclin E-CDK2 reactivating the G1-S checkpoint originally blockaded by the drugs. With the first CDK2-targeting agent now approved for clinical application, the relevance of building a preclinical program around CDK2 inhibition is clear and defined. What remains critically important is whether a candidate CDK2-focused compound can demonstrate activity in a model that reflects clinical resistance to CDK4/6 inhibition.

It is increasingly recognized that clinical resistance to CDK4/6 inhibitors seldom reflects a failure to engage the target kinases. In the majority of cases, palbociclib, ribociclib, and abemaciclib remain bound to CDK4 and CDK6. However, the dependence of tumors on CDK4/6 for autonomous proliferation has diminished or been subverted by the upregulation of alternative pathways amongst the diverse array of other cell cycle proteins. A series of elegant studies demonstrated that increased expression of cyclin E activates CDK2, which then sustains phosphorylation of the retinoblastoma protein (Rb) in the presence of CDK4/6 inhibition and presumptive G1-S blockade, allowing cell cycle progression to resume. In this scenario, CDK4/6 inhibitors continue to directly bind and reduce target activity, but tumor cells no longer require such activity to proliferate, becoming resistant to these therapeutics.

Resistance in this setting is seldom a matter of the inhibitor ceasing to bind. It is a matter of cyclin E-CDK2 performing, independently, the function that CDK4/6 once regulated.

From these studies and other work, it is evident that cyclin E-CDK2 activity is one of the primary drivers of clinical resistance to CDK4/6 inhibitors. This became even more apparent with the subsequent approval of the first CDK2-targeting therapy in December 2025. As such, the focal point is no longer whether CDK2 inhibitors are relevant, but if the candidates being developed can suppress CDK2 activity within a tumor model that has circumvented CDK4/6 inhibition in a manner reflecting how resistance arises in patients.

Resistance is biologically heterogeneous but converges upon the cell cycle

Although numerous mechanism of CDK4/6 inhibitor resistance have been described, including the upregulation of mitogenic and hormonal signaling networks, alterations within the core cell cycle machinery predominate. For example, despite an overall low prevalence in HR+ breast cancer (<5% of all tumors), loss of RB1, typically via inactivating mutation or loss of heterozygosity, disrupts the critical regulatory intersection through which the G1-S checkpoint is orchestrated. More commonly, amplification of CDK6 or loss of the tumor suppressor FAT1 with consequent CDK6 upregulation through the Hippo pathway, generates additional cyclin-CDK complexes that overwhelm the suppressive barrier set by CDK4/6 inhibitors. Elevated p16 (CDKN2A) affects CDK6 in such a way that favors ATP binding over CDK4/6 inhibitors, amplifying drug resistance. And finally, hyperactivation of cyclin E-CDK2 in the presence of CDK4/6 inhibitors restores Rb phosphorylation downstream of the blockade.

Of these mechanisms, upregulation of cyclin E-CDK2 activity is one of the most frequently implicated from clinical evidence, offering distinct pharmacological tractability, with inhibition providing for a complementary approach to more durable suppression of Rb activity in combination with CDK4/6 inhibitors. Data from the PALOMA-3 clinical trial provided early support for these ideas. It was discovered that low cyclin E1 expression was associated with greater benefit from palbociclib treatment, whereas high cyclin E1 correlated with diminished antiproliferative responses. Preclinical studies have further demonstrated that ablating CDK2 or cyclin E1 re-sensitizes CDK4/6 inhibitor-resistant tumors to cell cycle arrest. Together, these findings directly implicate cyclin E-CDK2 activity as not simply a downstream regulatory node, but as a functional driver fueling resistance upon persistent blockade of CDK4/6.

From mechanistic hypothesis to clinical validation

The preclinical evidence supporting CDK2 as a target strengthened considerably in recent years with numerous inhibitors being evaluated. The evidence pool was further bolstered in 2025 when culmerciclib, an oral CDK2/4/6 inhibitor, became the first CDK2-targeting agent to receive regulatory approval for clinical use. In China, culmerciclib was approved as a combination therapy (with fulvestrant) for HR+/HER2-negative advanced breast cancer patients that had progressed following endocrine therapy. In its registrational phase 3 trial, the combination treatment extended median progression-free survival to 16.6 months, against 7.5 months for the fulvestrant alone arm.

Several CDK2-specific inhibitors are presently in clinical development for the post-CDK4/6 setting, including INX-315 (Incyclix), AZD8421 (AstraZeneca), and additional programs from Pfizer (PF-07104091), Incyte (INCB123667), and Novartis (a candidate disclosed at AACR 2026). Two earlier programs, since discontinued, remain instructive notwithstanding the decisions of their sponsors. The Pfizer CDK2/4/6 inhibitor PF-06873600 had demonstrated a risk-benefit profile consistent with the CDK4/6 inhibitor class in its first-in-human study, with an objective response rate of 6.7% in patients previously exposed to CDK4/6 inhibition and 22.7% in a CDK4/6 inhibitor-naive cohort. Pfizer discontinued this program in favor of target-specific CDK4 and CDK2 agents. The selective CDK2 inhibitor BLU-222 (Blueprint Medicines) had a well-characterized preclinical dataset supporting its development before the company elected to complete its phase 1 dose escalation and deprioritize further investment. Nevertheless, the biology established by these programs remains valid today and is concordant with the clinical outcomes now observed for culmerciclib.

The questions that determine clinical translatability

The preclinical work around a CDK2 inhibitor program should address a defined set of questions, the outcomes of which will be dictated in part by the model system(s) deployed, with downstream consequences for clinical translatability.

Does the compound suppress CDK2 activity in vivo?

Rb phosphorylation is the functional readout. Should Rb remain phosphorylated under drug exposure, target coverage within the tumor is insufficient, irrespective of the biochemical IC₅₀. Clinical development requires pharmacodynamic confirmation in situ in addition to in vitro enzymatic data.

Does activity persist in a CDK4/6-resistant context?

Cell cycle re-entry in a resistant tumor is mechanistically distinct from G1 arrest in a treatment-naive state. A compound must be shown to reduce Rb hyperphosphorylation and arrest cell proliferation within a tumor that has already circumvented CDK4/6 blockade.

Does a combination with a CDK4/6 inhibitor confer additional benefit?

The rationale for dual CDK2 and CDK4/6 inhibition is a more enduring and complete repression of Rb phosphorylation in tumors where CDK2 activity is elevated. If preclinical data supports a combinatorial approach, questions of tolerability and scheduling should be resolved through in vivo studies before a clinical design is finalized.

Is there a biomarker that can predict response?

Although CCNE1 amplification is a leading candidate, a recent breast cancer PDX screen indicated that a multigene signature encompassing CCND1, CCNE1, RB1, and CDKN2A may be a more reliable predictor than CCNE1 alone. The construction and testing of a biomarker hypothesis across a panel of patient-derived models of sufficient size is likely of greater value than retrospective annotations in a smaller-scale study.

Each of these questions is answerable in a preclinical setting, but whether the answers have sufficient clinical translatability depends in large part upon the model system(s) employed.

Models that recapitulate the underlying biology, not merely the phenotype

Two approaches to the generation of a CDK4/6-resistant model may be distinguished. The more common is selection of a cell line under prolonged drug pressure in culture, which yields a clonally uniform population characterized by a single dominant resistance mechanism. Such models are informative but not representative of the clinical milieu. The alternative is to derive a model from a patient that progressed on a CDK4/6 inhibitor (with or without associated endocrine therapy) and whose tumor retains the heterogeneity and treatment history that gave rise to drug resistance.

This distinction matters for a CDK2 program because clinical resistance to CDK4/6 inhibition is not monolithic. It emerges through a variety of mechanisms, including cyclin E1 or E2 amplification, RB1 loss, or CDK6 copy number gain, and in patients with a tapestry of prior treatment histories, differing in both agents received and sequence. Cell line models carrying an engineered mechanism of resistance can establish whether a compound acts specifically against that mechanism. However, the very clonality of cell lines that makes them so easy to manipulate and deploy for resistance modeling fails to mimic the rich patchwork of cell heterogeneity and polyclonality found within patient tumors, both of which can modulate how resistance emerges. Evaluating a candidate CDK2 inhibitor across a panel of patient-derived models spanning many potential failure points and molecular features addresses that question directly. The inherent value of such a panel lies not in the number of models screened, but in the breadth of clinical resistance it potentially represents.

Champions Oncology maintains a panel of ER+ breast cancer PDX models developed from patients in an adjuvant setting who demonstrated resistance (primary or acquired) to a CDK4/6 inhibitor-endocrine therapy regimen (see table below), each carrying a documented treatment history and molecular annotation, including CCNE1 and CCNE2 status. Many of these PDX models have already been used extensively in the preclinical development of a host of novel CDK2 inhibitors (highlighted in green in the table below). These models provide an opportunity to test CDK2 inhibitors in an arena where biological resistance to CDK4/6 inhibitors evolved under clinical drug pressure in the therapeutic context the model is meant to represent. For example, CTG-3298 was derived from a patient whose disease was de novo resistant to a combination treatment of ribociclib, everolimus, and exemestane, whilst CTG-3283 was generated from a patient whose disease failed to respond to a combination of fulvestrant and palbociclib after more than four years of prior endocrine therapy. In each case, resistance arose in the patient under the regimen the model represents rather than engineered in culture.

Both CTG-3298 and CTG-3283 have also been characterized in an AstraZeneca CDK2 inhibitor patent application in which each was tested with a selective CDK2 inhibitor alone and in combination with palbociclib. Readouts included tumor volume changes, Rb phosphorylation, and phosphorylation of histone H3 (pHH3), a marker of mitosis and cell cycle progression. The behavior of these models under CDK2 inhibition is therefore documented rather than assumed, which removes one source of uncertainty at the point of model selection.

The models available for CDK2 inhibitor evaluation, with their clinical CDK4/6 failure history and molecular annotation, are summarized below. All models highlighted in green are those with documented characterization in published CDK2 inhibitor studies.

*Phosphoproteomic data on CDK2 targets used as a surrogate for activity. The table indicates the most heavily phosphorylated residue in each protein. Values of 15 to 19 units are considered moderate to high, and values above 20 units are very high. CN, copy number.

For a compound intended for patients who have progressed on CDK4/6 inhibition, activity in a model whose resistance arose in that same setting supports a clearer translational rationale than activity in a line rendered resistant in culture.

Frequently asked questions