Evaluation of Adherence to Cell Cycle Inhibitors Used as Adjuvant Therapy in Patients With Localized Breast Cancer at High Risk of Recurrence.

Hormone receptor-positive (HR+) breast cancers represent the most common histological subtype of breast cancer, accounting for approximately 75% of cases, regardless of HER2 (human epidermal growth factor receptor 2) status (1). Adjuvant endocrine therapy (ET), including tamoxifen and aromatase inhibitors (AIs), is an effective pharmacological treatment for improving the prognosis of HR+ breast cancer, reducing the risk of recurrence by up to 50% (2-3-4-6). Despite its proven prognostic benefit, the full potential of endocrine therapy is not realized due to patient non-adherence (i.e., failure to comply with prescribed treatment). Adjuvant endocrine therapy is generally prescribed for a duration of 5 to 10 years. However, up to 40% of patients discontinue treatment prematurely, and 30% take the medication less frequently than prescribed. Poor adherence and low treatment persistence carry a substantial mortality burden: non-adherence is associated with a 49% increase in all-cause mortality. A retrospective analysis of a large database including more than 8,700 patients showed a 10-year survival rate of 80.7% among women who continued treatment, compared with 73.6% among those who discontinued adjuvant therapy prematurely (p < 0.001). Among patients who continued treatment, the survival rate was 82% in those who were fully adherent, versus 78% in those who were only partially adherent (7-16).

The literature has documented a wide range of risk factors associated with non-adherence to or discontinuation of long-term adjuvant endocrine therapy. Treatment-related adverse effects, including hot flashes, joint stiffness, and sexual dysfunction, are common and may lead to treatment discontinuation. Fear of side effects may also prevent some patients from initiating or maintaining endocrine therapy. Others may not be fully convinced of the necessity of adjuvant endocrine therapy, particularly in the absence of overt signs of cancer. In addition, supportive care required to manage side effects is often inadequately reimbursed, making low income-combined with broader socioeconomic factors-a potential barrier to optimal adherence. Some patients may also experience difficulties remembering to take their medication regularly. The relative importance and contribution of these factors to non-adherence may evolve over time. Other factors may also play a role, including sociodemographic characteristics (low income, living alone, or unemployment).

Nevertheless, a residual risk of recurrence persists after five years of well-conducted standard endocrine therapy, extending up to two decades after diagnosis, particularly in patients with early-stage breast cancer stages II and III. In this higher-risk population, two phase III trials, monarchE and NATALEE, have recently evaluated the addition of a cell cycle inhibitor (CDK4/6 inhibitor) to standard adjuvant endocrine therapy and reported positive results with a reduction in the risk of relapse.

In the NATALEE trial, quality of life was assessed in all patients in the ribociclib plus aromatase inhibitor group (n = 2,549) versus the aromatase inhibitor alone group (n = 2,552). Mean scores did not differ significantly from baseline for any of the analyzed domains. Similarly, no significant change from baseline was observed in either treatment group.

However, it is important to note that 33.8% of patients discontinued ribociclib and 20% discontinued both endocrine therapy and ribociclib in the NATALEE trial, which is consistent with data from the literature.

In the monarchE trial, 16.6% of patients discontinued abemaciclib, and 6% discontinued both abemaciclib and endocrine therapy, while only 0.8% discontinued endocrine therapy in the control group. These findings are not consistent with previously published data.

To our knowledge, no real-world study has evaluated CDK4/6 inhibitors in combination with endocrine therapy in the adjuvant treatment of HR+/HER2-negative breast cancer.

AdheRA is a prospective multicenter cohort study of patients with early-stage HR+/HER2-negative breast cancer at high risk of recurrence, eligible for a combination of endocrine therapy and a CDK4/6 inhibitor such as abemaciclib or ribociclib in the adjuvant setting, aiming to assess treatment adherence and the reasons for non-adherence.