Blocking CDK2 may rescue CDK4/6 inhibitor activity in resistant breast cancer models
In patient-derived xenografts, adding a CDK2 inhibitor to CDK4/6 inhibitors boosted antitumor activity in HR+ and triple-negative breast cancer, guided by a 4-gene signature.
Summary
CDK4/6 inhibitors like ribociclib and palbociclib are standard care for HR+/HER2- breast cancer, but many tumors resist them, often by activating CDK2 through high cyclin E1. Researchers tested the selective CDK2 inhibitor BLU-222, alone and with CDK4/6 inhibitors, in breast cancer cell lines and patient-derived xenograft (PDX) models. The combination was broadly active in CDK4/6i-resistant and -naïve HR+ and triple-negative models. A four-gene mRNA signature (CCND1, CCNE1, RB1, CDKN2A) predicted response, and adding endocrine therapy improved activity in HR+ models. Early clinical data suggest BLU-222 is active alone (CCNE1-amplified tumors) and with ribociclib plus fulvestrant. The work is preclinical and company-sponsored, so clinical validation is still needed.
Detailed Summary
Why it matters: CDK4/6 inhibitors plus endocrine therapy are first-line standard care for HR+/HER2- breast cancer and have improved survival. About 20% of patients are primarily resistant, and most initial responders eventually relapse. A recurring resistance route is aberrant CDK2 activity, often driven by high cyclin E1 (CCNE1), which lets cells pass the G1/S checkpoint without CDK4/6. CDK2 inhibition is therefore a rational strategy, but earlier studies used a single inhibitor and few models.
What was studied: The team (Blueprint Medicines with MD Anderson collaborators) profiled the selective CDK2 inhibitor BLU-222 against palbociclib in 38 breast cancer cell lines. They also compared five selective CDK2 inhibitors, including three that have been dosed in patients (BLU-222, INX-315, PF-07104091), and built isogenic T-47D cells overexpressing CCNE1, CDKN2A (p16), or both. The main efficacy work used PDX models of intrinsic and acquired CDK4/6i resistance as well as CDK4/6i-naïve HR+ and triple-negative breast cancer (TNBC) models. Combinations tested CDK2i with ribociclib, with and without endocrine therapy.
Key results: In cell lines, strong BLU-222 responders tended to be palbociclib-resistant, and BLU-222 nonresponders tended to be palbociclib-sensitive. BLU-222 responders had lower RB1, CCND1 and ESR1 and higher CCNE1, CDKN2A and CDKN2B expression. No single biomarker predicted CDK2i sensitivity, but a CCNE1-high, Rb-intact, p16-high profile enriched for responders, and all five CDK2 inhibitors behaved similarly. In T-47D cells, only co-overexpression of CCNE1 and p16 produced CDK2i sensitivity and resistance to palbociclib and ribociclib. This also reversed the cell-cycle effect: BLU-222 caused G1 accumulation while palbociclib did not, and the G1 effect depended on Rb. The effect was specific to the CDK2–CDK4/6 axis, since CDK1, pan-CDK, Wee1 and ATR inhibitor sensitivity did not shift. Per the abstract, the CDK2i + CDK4/6i combination was broadly active across the PDX models. A weighted mRNA signature of CCND1, CCNE1, RB1 and CDKN2A predicted response to the combination. Adding endocrine therapy significantly enhanced activity in HR+ models.
Implications: The authors report early clinical activity of BLU-222 as monotherapy in CCNE1-amplified disease and in combination with ribociclib and fulvestrant in HR+/HER2- patients. Together, the data suggest CDK2 inhibition can address several CDK4/6i resistance mechanisms and that a simple expression signature could help select patients, possibly beyond CCNE1-high tumors.
Caveats: The evidence is mainly preclinical, and the clinical data are early. The study was sponsored by the developer of BLU-222, and the signature still needs prospective validation. Note that the text supplied for this summary was truncated after the cell-line results, so PDX and clinical details here rely on the abstract.
Key Findings
- BLU-222 responders among 38 breast cancer cell lines tended to be palbociclib-resistant, and nonresponders palbociclib-sensitive (P<0.01, P<0.05).
- A CCNE1-high/Rb-intact/p16-high profile enriched for CDK2i responders across five selective CDK2 inhibitors; no single biomarker sufficed.
- In T-47D cells, co-overexpressing CCNE1 and p16 was required for CDK2i sensitivity and CDK4/6i resistance, via an Rb-dependent G1 arrest.
- CDK2i plus CDK4/6i was broadly active in PDX models; a CCND1/CCNE1/RB1/CDKN2A signature predicted response, and endocrine therapy added benefit in HR+ models.
- Early clinical data showed BLU-222 activity alone (CCNE1-amplified) and with ribociclib plus fulvestrant in HR+/HER2- breast cancer.
Methodology
Preclinical study combining in vitro proliferation assays in 38 breast cancer cell lines, CCLE gene-expression analysis, isogenic T-47D overexpression and siRB1 experiments, and a panel of breast cancer PDX models (HR+ and TNBC; CDK4/6i-resistant and -naïve) treated with CDK2i, CDK4/6i and endocrine therapy. Early clinical observations with BLU-222 were also reported.
Study Limitations
Findings are largely from cell lines and mouse PDX models, and the clinical data are early and limited. The study was funded by the company developing BLU-222, and the predictive signature lacks prospective validation. The supplied text was truncated after the cell-line section, so PDX and clinical details come from the abstract and this summary may omit specifics.
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