Cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors: existing and emerging differences

Absolute lymphocyte count predicts efficacy of palbociclib in patients with metastatic luminal breast cancer

BACKGROUND

Absolute lymphocyte count (ALC) is a predictive and prognostic factor for various tumor types, including breast cancer. Palbociclib is a CDK4/6 inhibitor widely used for the treatment of metastatic estrogen receptor (ER)-positive, HER2-negative breast cancer. However, predictive biomarkers of the efficacy of palbociclib remain unelucidated. We conducted a retrospective study to examine the predictive value of the baseline ALC in patients treated with palbociclib.

METHODS

The medical records of patients with ER-positive, HER2-negative breast cancer treated with palbociclib plus hormonal therapy between December 2017 and December 2021 were analyzed retrospectively. The cutoff value of ALC was set at 1800 cells/μL at the initiation of palbociclib treatment. The clinical benefit rate (CBR) was defined as the rate of complete or partial response or stable disease for at least 6 months. Progression-free survival (PFS) rates were estimated using the Kaplan-Meier method and compared using the log-rank test. Univariate and multivariate analyses were performed using Cox proportional hazards regression.

RESULTS

All of the 202 patients were women, with a median age of 59 years and a performance status (PS) of ≤ 2. The median numbers of lines of chemotherapy and endocrine therapy before palbociclib treatment were 0 (range, 0-9) and 1 (range, 0-7), respectively. Fifty-one patients had liver metastases. Forty-six patients tested negative for progesterone receptor (PgR) expression. The median follow-up time was 9.1 months. The CBR was significantly higher in the ALC-high group than in the ALC-low group (79% vs. 60%; P = 0.018). The median PFS was significantly longer in the ALC-high group than in the ALC-low group (26.8 months vs. 8.4 moths, respectively; P = 0.000013). ALC, age, PS, PgR status, prior chemotherapy, prior endocrine therapy, and liver metastasis were entered into the multivariate analysis. ALC was identified as an independent factor for PFS (P = 0.00085), along with liver metastasis (P = 0.0020), PS (P = 0.026), and prior endocrine therapy (P = 0.019).

CONCLUSION

ALC can serve as a predictor of palbociclib efficacy in patients with metastatic ER-positive, HER2-negative breast cancer.

Practical Guidance on Abemaciclib in Combination with Adjuvant Endocrine Therapy for Treating Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-Negative High-Risk Early Breast Cancer

The most common subtype of breast cancer is hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative breast cancer, accounting for 65-70% of all breast cancer cases diagnosed in the United States. Until 2015, single-agent endocrine therapy (ET) was the recommended first-line treatment for metastatic HR-positive, HER2-negative breast cancer. However, the paradigm has since shifted, as targeted therapy is now recommended in combination with ET. The cyclin-dependent kinase (CDK) 4/6 inhibitors have revolutionized the treatment of this breast cancer subtype, and combining either palbociclib, ribociclib, or abemaciclib with ET is now the standard first-line treatment for metastatic disease. Results of clinical trials in the metastatic setting have demonstrated that treatment with the combination of a CDK4/6 inhibitor and ET rather than ET alone is associated with longer overall survival, longer progression-free survival, and better objective response rates. Each of the CDK4/6 inhibitors has been investigated in combination with ET in patients with early-stage HR-positive, HER2-negative breast cancer who are at high risk of relapse. In October 2021, abemaciclib was the first CDK4/6 inhibitor approved in combination with ET by the US Food and Drug Administration for adjuvant treatment of patients with HR-positive, HER2-negative, high-risk early breast cancer. Herein, we provide practical guidance on the use of abemaciclib in combination with ET for HR-positive, HER2-negative, high-risk early breast cancer to assist clinicians in their day-to-day practice, and we review clinically relevant topics of dosing, side effect management, sequencing and optimal timing for initiation, and patient selection.

Open-label, phase II, multicenter study of lasofoxifene plus abemaciclib for treating women with metastatic ER+/HER2- breast cancer and an ESR1 mutation after disease progression on prior therapies: ELAINE 2

BACKGROUND

Acquired ESR1 mutations in estrogen receptor-positive (ER+) metastatic breast cancer (mBC) drive treatment resistance and tumor progression; new treatment strategies are needed. Lasofoxifene, a next-generation, oral, endocrine therapy and tissue-specific ER antagonist, provided preclinical antitumor activity, alone or combined with a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) in ESR1-mutated mBC.

PATIENTS AND METHODS

In the open-label, phase II, ELAINE 2 trial (NCT04432454), women with ESR1-mutated, ER+/human epidermal growth factor receptor 2-negative (HER2-) mBC who progressed on prior therapies (including CDK4/6i) received lasofoxifene 5 mg/day and abemaciclib 150 mg b.i.d until disease progression/toxicity. The primary endpoint was safety/tolerability. Secondary endpoints included progression-free survival (PFS), clinical benefit rate (CBR), and objective response rate (ORR).

RESULTS

Twenty-nine women (median age 60 years) participated; all but one were previously treated with a CDK4/6i (median duration 2 years). The lasofoxifene-abemaciclib combination was well tolerated with primarily grade 1/2 treatment-emergent adverse events (TEAEs), most commonly diarrhea, nausea, fatigue, and vomiting. One patient (with no prior CDK4/6i) discontinued treatment due to grade 2 diarrhea. No deaths occurred during the study. Median PFS was 56.0 weeks [95% confidence interval (CI) 31.9 weeks-not estimable; ∼13 months]; PFS rates at 6, 12, and 18 months were 76.1%, 56.1%, and 38.8%, respectively. CBR at 24 weeks was 65.5% (95% CI 47.3% to 80.1%). In 18 patients with measurable lesions, ORR was 55.6% (95% CI 33.7% to 75.4%). ESR1-mutant circulating tumor DNA (ctDNA) allele fraction decreased from baseline to week 4 in 21/26 (80.8%) patients.

CONCLUSIONS

Lasofoxifene plus abemaciclib had an acceptable safety profile, was well tolerated, and exhibited meaningful antitumor activity in women with ESR1-mutated, ER+/HER2- mBC after disease progression on prior CDK4/6i. Observed decreases in ESR1-mutant ctDNA with lasofoxifene concordant with clinical response suggest target engagement. If the ELAINE 2 findings are confirmed in the initiated, phase III, ELAINE 3 trial, these data could be practice-changing and help address a critical unmet need.