Currently submitted to: JMIR Research Protocols
Date Submitted: Sep 22, 2026
Open Peer Review Period: Sep 23, 2026 - Nov 18, 2026
(currently open for review)
Warning: This is an author submission that is not peer-reviewed or edited. Preprints - unless they show as "accepted" - should not be relied on to guide clinical practice or health-related behavior and should not be reported in news media as established information.
Medication adherence in patients with early-stage breast cancer receiving adjuvant aromatase inhibitor therapy with or without abemaciclib: a cohort study protocol
ABSTRACT
Background:
Adjuvant endocrine therapy (ET) with an aromatase inhibitor (AI) combined with the cyclin-dependent kinase 4/6 (CDK4/6) inhibitor abemaciclib has recently become the standard of care for high-risk hormone receptor positive HER2 negative (HR+/HER2-) early breast cancer (BC) patients. However, the treatment is associated with adverse events (AEs) that may jeopardise treatment adherence.
Objective:
We will assess medication adherence in relation to beliefs about medicines and quality of life (QoL) among patients undergoing adjuvant therapy with AIs, either alone or combined with abemaciclib, and explore factors influencing medication adherence.
Methods:
We will conduct a prospective cohort study in at least 319 patients diagnosed with early-stage, HR+/HER2- BC undergoing adjuvant treatment with AI and abemaciclib (approximately 100 patients) and AI alone (approximately 200 patients). Medication adherence will be assessed using the 5-item Medication Adherence Report Scale and pill counts at 12 and 24 weeks. The discontinuation rate for either AI and/or the CDK4/6 inhibitor will be calculated. Factors influencing treatment adherence will be investigated; the following data will be collected: tumour characteristics and anticancer treatment data, AE related to AI and abemaciclib, patients’ sociodemographic characteristics, QoL (European Organisation for Research and Treatment of Cancer QoL Core Questionnaire and BC Module), beliefs about medicines (Beliefs about Medicines Questionnaire), and cognitive functioning (Functional Assessment of Cancer Therapy).
Results:
The study started in January 2025 and is ongoing. As of September 2026, 117 patients have been enrolled: 67 in the abemaciclib group and 50 in the control group. Data collection is expected to be completed in September 2028.
Conclusions:
Medication adherence data in early-stage BC patients treated with CDK 4/6 inhibitors are scarce. This study will provide the first real-world insights into adherence, discontinuation rates, and factors such as demographics and AEs influencing adherence. Identifying adherence barriers will inform targeted interventions, improve patient management, and guide future research and practice to maximise treatment effectiveness and reduce relapse and mortality. Clinical Trial: The study is registered with ClinicalTrials.gov (NCT06650423) and is ongoing.
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