Abstract 788: QT prolongation induced by CDK-4/6 inhibitors in breast cancer: A prospective cohort study utilizing 12-hour continuous ECG monitoring

Abstract CDK 4/6 inhibitors improve survival in HR+ and HER2 breast cancer patients but carry risks of QT prolongation. Single ECG readings may be unreliable. This study assesses QT prolongation via 12-hour ECG monitoring for three CDK 4/6 inhibitors. Methods: A prospective cohort study (Apr 2023-Aug 2024) included HR+, HER2- breast cancer patients receiving CDK 4/6 inhibitors. Exclusions were QTc >500 msec and persistent Af. Ribociclib/Palbociclib followed a 21/7 regimen; Abemaciclib was continuous. ECG monitoring occurred pre-treatment and on day 14 (Ribociclib/Palbociclib) or day 14 only (Abemaciclib). ECGs were recorded from 8 PM-8 AM and stored on the PCA 500 cloud. Drug blood concentrations were measured on day 8, with dose adjustments as needed. Results: From July 16, 2023, to July 31, 2024, 33 patients were included: Ribociclib (N=19), Abemaciclib (N=9), and Palbociclib (N=5). Case 36 underwent testing at 600 and 400 mg Ribociclib due to QT prolongation. • Ribociclib: Median baseline QTcF was 422 msec, rising to 431 msec (400 mg, N=7) and 440 msec (600 mg, N=13) post-treatment. QTcF Max >500 msec increased from 23% to 54% in the 600 mg group. • Palbociclib: Median QTcF rose from 431 msec (washout) to 439 msec (100 mg, N=4) and decreased to 421 msec (125 mg, N=4). QTcF Max >500 msec rose from 20% to 40% post-treatment. • Abemaciclib: Post-treatment QTcF medians were 425 msec (100 mg, N=1), 433 msec (150 mg, N=5), and 434 msec (alternative 150 mg, N=3). QTcF Max >500 msec increased to 56% post-treatment. Conclusions: Ribociclib and Abemaciclib prolonged QTcF Max >500 msec, especially at higher doses. Palbociclib had less QT prolongation and reduced QTcF at 125 mg. 12-hour ECG monitoring underscores the need for closer cardiac safety measures. Citation Format: Chia-Lun Chang, Wan-Chen Hsieh, Wei-Wen Chang, Ping-Kun Hsiao, Mao-Chih Hsieh, Tzeon-Jye Chiou, Tzu-Yao Liao, H-Eugene Liu, Hsin-Hsien Yu, Chih-Hsin Lee. QT prolongation induced by CDK-4/6 inhibitors in breast cancer: A prospective cohort study utilizing 12-hour continuous ECG monitoring [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 788.