In women with estrogen receptor–positive (ER+) breast cancer (BC), endocrine therapy (ET) can significantly decrease mortality. Patients with mental illness (MI) are often nonadherent to prescribed treatment regimens. When such noncompliance occurs in patients with BC, the results can be catastrophic.

Data from the SEER (Surveillance, Epidemiology and End-Results) Medicare database were examined to determine the prevalence and influence of preexisting MI on adherence to ET.

The analysis included a cohort of women from the SEER Medicare database who were aged 68 years or older and who were diagnosed between 2007 and 2013 with stages I-IV ER+ BC. The SEER registries include 28% of the United States population, making it representative of women with BC in the U.S.

In addition to positive hormone-receptor status, to be included in the study women had to have continuous fee-for-service Parts A and B Medicare coverage for at least 36 months before and for at least 18 months after the BC diagnosis. They also had to have Medicare Part D coverage for at least 4 months before and for at least 18 months after BC diagnosis.

Data were gathered on 21,894 ethnically diverse women with BC. The database of women with a BC diagnosis was further screened for the presence of ICD-9 codes for 11 Diagnostic and Statistical Manual of Mental Disorders—Fourth Edition MI diagnoses, which included unipolar depression, anxiety, adjustment disorder, delirium, dementia, personality disorder, bipolar disorder, schizophrenia, nonschizophrenic psychosis, drug use, and alcohol use. The MI diagnosis had to be present for 36 months prior to the BC diagnosis and had to be coded on two outpatient claims for the same illness that were separated by at least 30 days.

ET included selective estrogen receptor modulators (SERMs; i.e., tamoxifen, toremifene) and aromatase inhibitors (AIs; i.e., anastrozole, exemestane, letrozole). Patterns of ET use were analyzed to determine discontinuation rates that occurred within the first 5 years after ET initiation.

To determine adherence, the proportion of days covered (PDC) was calculated. The PDC was defined as the days covered by ET prescriptions divided by the days in follow-up. Although patients were maintained in the study if they had switched between SERMs and AIs, any time spent hospitalized or in a skilled nursing facility was excluded from analysis. Patients receiving ET were followed for 5 years as per recommendations from the 2014 American Society of Clinical Oncology guidelines for the management of hormone receptor–positive BC.

Investigators found that 25% of BC patients had had an MI diagnosis prior to receiving a BC diagnosis. Among the MI illnesses present in this population were unipolar depression (11%), anxiety (9.5%), dementia (4.6%), nonschizophrenic psychotic disorder (4.6%), drug-use disorder (4.4%), delirium (2.4%), bipolar disorder (1.2%), schizophrenia (0.8%), adjustment disorder (0.4%), alcohol use (0.3%), and personality disorder (0.3%).

The study specifically focused on initiation, adherence, and discontinuation of ET.

Eighty percent of BC patients were initiated on ET upon BC diagnosis. This included AIs in 78%, SERMs in 11.1%, and a combination of AIs and SERMs in 10.9%. Among the 20% of BC patients who were not initiated on therapy, the most common MI diagnoses were dementia, bipolar disorder, and psychotic disorder. Patients with dementia were 3.5% and those with bipolar disorder or psychosis were 2.5% less likely to be initiated on ET compared with those without these MI diagnoses. Factors associated with initiation of ET included younger age (68-74 years compared with those aged 75-84 years, 85-94 years, or 95+ years), having stage II or IV BC, less frequent MI diagnoses, Asian race, and more recent BC diagnosis (i.e., 2010-2012 versus 2008-2009).

Most patients with MI adhered to ET as well as the general population did. Factors associated with increased adherence included older age (85-94 years), Asian race, and recent BC diagnosis (i.e., 2009-2012 versus 2008). Substance-use disorder was associated with 2.3% worse adherence. Other factors that were associated with nonadherence included advanced BC stage (III-IV).

Of the patients receiving ET, 25.7% discontinued therapy within 5 years of follow-up. Discontinuation was more common among patients with MI, especially those with anxiety disorder and substance-use disorder. Patients with stage I BC were also more likely to discontinue therapy. Conversely, younger (age 68-74 years compared with those ages 75-84 years, 85-94 years, or 95+ years), black, Asian, or Hispanic patients and those with stage II or III BC were more likely to continue with treatment for 5 years.

This study helps shed light on adherence patterns among women with BC receiving ET, which can assist pharmacists in tailoring their patient-education efforts.

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