PURPOSE: Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) combined with endocrine therapy (ET) are the standard first-line treatment for hormone receptor–positive (HR+), human epidermal growth factor receptor 2–negative (HER2–) advanced breast cancer (ABC); however, disease progression occurs in almost all patients and additional treatment options are needed. Herein, we report outcomes of the postMONARCH trial investigating a switch in ET with/without CDK4/6 inhibition with abemaciclib after disease progression on CDK4/6i. METHODS: This double-blind, randomized phase III study enrolled patients with disease progression on previous CDK4/6i plus aromatase inhibitor as initial therapy for advanced disease or recurrence on/after adjuvant CDK4/6i + ET. Patients were randomly assigned (1:1) to abemaciclib + fulvestrant or placebo + fulvestrant. The primary end point was investigator-assessed progression-free survival (PFS). Secondary end points included PFS by blinded independent central review, objective response rate (ORR), and safety. RESULTS: This study randomly assigned 368 patients (abemaciclib + fulvestrant, n = 182 placebo + fulvestrant, n = 186). At the primary analysis (258 events), the hazard ratio (HR) was 0.73 (95% CI, 0.57 to 0.95; nominal P = .017), with median PFS 6.0 (95% CI, 5.6 to 8.6) versus 5.3 (95% CI, 3.7 to 5.6) months and 6-month PFS rates of 50% and 37% in the abemaciclib + fulvestrant and placebo + fulvestrant arms, respectively. These results were supported by BICR-assessed PFS (HR, 0.55 [95% CI, 0.39 to 0.77]; nominal P < .001). A consistent treatment effect was seen across major clinical and genomic subgroups, including with/without ESR1 or PIK3CA mutations. Among patients with measurable disease, investigator-assessed ORR was improved with abemaciclib + fulvestrant versus placebo + fulvestrant (17% v 7%; nominal P = .015). No new safety signals were observed, with findings consistent with the known safety profile of abemaciclib. CONCLUSION: Abemaciclib + fulvestrant significantly improved PFS after disease progression on previous CDK4/6i + ET in patients with HR+, HER2– ABC, offering an additional targeted therapy option for these patients.

Background: Findings from randomized clinical trials may have limited generalizability to patients treated in routine clinical practice. This study examined the effectiveness of first-line palbociclib plus letrozole versus letrozole alone on survival outcomes in patients with hormone receptor–positive (HR+)/human epidermal growth factor receptor–negative (HER2−) metastatic breast cancer (MBC) treated in routine clinical practice in the USA. Patients and methods: This was a retrospective observational analysis of electronic health records within the Flatiron Health Analytic Database. A total of 1430 patients with ≥ 3 months of follow-up received palbociclib plus letrozole or letrozole alone in the first-line setting between February 3, 2015, and February 28, 2019. Stabilized inverse probability treatment weighting (sIPTW) was used to balance baseline demographic and clinical characteristics. Real-world progression-free survival (rwPFS) and overall survival (OS) were analyzed. Results: After sIPTW adjustment, median follow-up was 24.2 months (interquartile range [IQR], 14.2–34.9) in the palbociclib group and 23.3 months (IQR, 12.7–34.3) in those taking letrozole alone. Palbociclib combination treatment was associated with significantly longer median rwPFS compared to letrozole alone (20.0 vs 11.9 months; hazard ratio [HR], 0.58; 95% CI, 0.49–0.69; P < 0.0001). Median OS was not reached in the palbociclib group and was 43.1 months with letrozole alone (HR, 0.66; 95% CI, 0.53–0.82; P = 0.0002). The 2-year OS rate was 78.3% in the palbociclib group and 68.0% with letrozole alone. A propensity score matching analysis showed similar results. Conclusions: In this “real-world” population of patients with HR+/HER2− MBC, palbociclib in combination with endocrine therapy was associated with improved survival outcomes compared with patients treated with letrozole alone in the first-line setting. Trial registration: Clinicaltrials.gov; NCT04176354

Inhibition of histone lysine acetyltransferases (KATs) KAT6A and KAT6B has shown antitumor activity in estrogen receptor-positive (ER⁺) breast cancer preclinical models. PF-07248144 is a selective catalytic inhibitor of KAT6A and KAT6B. In the present study, we report the safety, pharmacokinetics (PK), pharmacodynamics, efficacy and biomarker results from the first-in-human, phase 1 dose escalation and dose expansion study (n = 107) of PF-07248144 monotherapy and fulvestrant combination in heavily pretreated ER⁺ human epidermal growth factor receptor-negative (HER2⁻) metastatic breast cancer (mBC). The primary objectives of assessing the safety and tolerability and determining the recommended dose for expansion of PF-07248144, as monotherapy and in combination with fulvestrant, were met. Secondary endpoints included characterization of PK and evaluation of antitumor activity, including objective response rate (ORR) and progression-free survival (PFS). Common treatment-related adverse events (any grade; grades 3–4) included dysgeusia (83.2%, 0%), neutropenia (59.8%, 35.5%) and anemia (48.6%, 13.1%). Exposure was approximately dose proportional. Antitumor activity was observed as monotherapy. For the PF-07248144–fulvestrant combination (n = 43), the ORR (95% confidence interval (CI)) was 30.2% (95% CI = 17.2–46.1%) and the median PFS was 10.7 (5.3–not evaluable) months. PF-07248144 demonstrated a tolerable safety profile and durable antitumor activity in heavily pretreated ER⁺HER2⁻ mBC. These findings establish KAT6A and KAT6B as druggable cancer targets, provide clinical proof of concept and reveal a potential avenue to treat mBC. clinicaltrial.gov registration: NCT04606446.

Randomized controlled trials have shown inconsistent overall survival (OS) benefit among the three cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) as first-line (1L) treatment of patients with hormone receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2-) metastatic breast cancer (mBC). Several real-world studies compared CDK4/6i effectiveness, with inconsistent findings. This study compared overall survival (OS) of patients with HR+/HER2- mBC receiving 1L palbociclib, ribociclib, or abemaciclib, in combination with an aromatase inhibitor (AI), in US clinical practice. This retrospective study used real-world data from the Flatiron Health electronic health record-derived deidentified longitudinal database. Patients with HR+/HER2- mBC aged ≥18 years at mBC diagnosis started 1L CDK4/6i therapy (index treatment) between February 2015 and November 2023, with a potential ≥6-month follow-up. OS was defined as months from start of index treatment to death. Stabilized inverse probability of treatment weighting (sIPTW; primary analysis) was used to balance baseline patient characteristics. Multivariable Cox proportional hazards model was carried out as a sensitivity analysis. Of 9146 eligible patients, 6831, 1279, and 1036 received palbociclib plus AI, ribociclib plus AI, or abemaciclib plus AI, respectively. After sIPTW, baseline characteristics were balanced between treatment groups. After sIPTW, no significant OS differences were found between treatment groups [ribociclib versus palbociclib: adjusted hazard ratio (aHR) 0.98, 95% confidence interval (CI) 0.87-1.10, P = 0.7531; abemaciclib versus palbociclib: aHR 0.95, 95% CI 0.84-1.08, P = 0.4292; abemaciclib versus ribociclib: aHR 0.97, 95% CI 0.82-1.14, P = 0.6956]. Sensitivity analysis including a subanalysis of patients who started index treatment in 2017 or later also showed no significant OS differences between treatment groups. This large real-world study suggested that there were no significant OS differences between 1L ribociclib, abemaciclib, and palbociclib in combination with an AI for patients with HR+/HER2- mBC. These findings together with other factors such as safety and quality of life are helpful in the selection of CDK4/6i combination therapy for patients with HR+/HER2- mBC.