Adjuvant Endocrine Therapy in Early Stage Breast Cancer: Impact of Drug Adherence/Persistence on Clinical Outcome in Various Breast Cancer Subtypes

Breast cancer is the most common cancer among women in Norway. In 2016, 3402 new cases were diagnosed (3371 in women). Breast cancer is still the second most common cause of death from cancer among women with 585 breast cancer deaths in Norway in 2015. The majority of the patients (70-75 %) belong to the Luminal subtypes, which comprise the hormone receptor (oestrogen receptor (ER) and/or progesterone receptor (PR)) positive tumours.

The most important systemic adjuvant therapy in luminal breast cancers is a long-lasting administration of per-oral anti-oestrogen medication. A systemic hypo estrogenic state in the body may be created by the selective oestrogen receptor modulator tamoxifen or by inhibitors of the peripheral systemic aromatization of adrenal androgens into estrogens (aromatase inhibitors). Initially, tamoxifen was given adjuvant for 2 years, later prolonged to 5 years and recently an extension to 10 years has been recommended for premenopausal women. Aromatase inhibitors were introduced in Norwegian treatment guidelines in 2002. Currently, they are recommended in postmenopausal patients for 5 years, either as monotherapy or in concert with tamoxifen (aromatase inhibitors for 2 years followed by tamoxifen for 3 years).

In premenopausal breast cancer patients, tamoxifen still is the drug of choice. Two of the major underlying reasons for late recurrences in luminal breast cancer subtypes are development of endocrine resistance to tamoxifen and aromatase inhibitors or failure of taking the medication as prescribed. Higher mortality has been shown for breast cancer patients with reduced tamoxifen adherence. The patients' ability to follow instructions and recommendations are probably overestimated in controlled trials due to patient selection and close follow-up in the study setting. Some patients experience distressing side effects like hot flushes, fatigue, joint pain, mood swings and vaginal dryness. To the investigators' knowledge, there are few studies in Norway regarding discontinuation of endocrine treatment in breast cancer. In this study they will investigate the long-term discontinuation pattern to oral adjuvant systemic endocrine therapy in a large cohort of breast cancer patients treated in St. Olav's hospital in Trondheim, Norway, and the association between adherence to endocrine treatment and long-term survival.