Background. Despite the efficacy of adjuvant endocrine therapy (AET) for improving survival for women with hormone receptor-positive early-stage breast cancer, adherence rates remain low, particularly among younger women who face more barriers. Our THRIVE study (NCT03592771) tested the effectiveness of a remote monitoring intervention on AET adherence and found no significant effects on 1-year AET adherence. We conducted a post hoc trial analysis to examine if age moderated the intervention's effectiveness.

Methods: This non-blinded randomized controlled trial included women with early-stage breast cancer prescribed AET at a large cancer center with 14 clinics. Participants used a pillbox to electronically monitor AET adherence for 1 year and completed surveys at enrollment. Consented participants were randomized into (1) “App”, receiving access to the study adherence and symptom monitoring app for 6 months, with increasing/severe symptoms and missed doses reported in the app triggering follow-ups from the oncology team; (2) “App+Feedback”, receiving additional weekly text messages about managing symptoms, adherence, and communication for 6 months; or (3) “Enhanced Usual Care (EUC).” The primary outcome was 1-year AET adherence captured with the pillbox (≥80% Proportion of Days Covered vs. <80%). We used multiple imputations with chained equations for missing outcomes due to loss of follow-up or missing responses. A linear probability model was then used to assess the interaction between the study arm and age on AET adherence. Marginal effects then calculated to estimate adherence by study arm and age.

Results. Among 304 women randomized (104 EUC, 98 App, and 102 App+Feedback), the 12-month retention rate was 87.5% (n=266); the median age was 60 (range 31 to 83). In adjusted analyses, the App arm was associated with a higher likelihood of adherence, which decreased with increasing age (P<0.01). For example, at age 35, 5.4% of EUC participants were adherent, vs. 48.2% of App, a 42.9 percentage point (ppt) difference (P=0.01). While at age 55, 48.3% of EUC vs. 52.0% of App participants were AET adherent; a 3.7 ppt difference (P=0.62). App+Feedback arm had a similar trend, but the interaction did not reach statistical significance (P=0.09).

Discussion. A remote symptom monitoring app significantly improved AET adherence among younger participants but not older participants. Targeting younger women, who face more adherence barriers and are more likely to benefit from remote symptom monitoring, could enhance adherence, mitigate disparities, and improve longer-term survival outcomes.