Switching between endocrine therapies for primary breast cancer: Frequency and timing in Australian clinical practice

Aims To determine the frequency, timing and patterns of endocrine therapy switching in Australian practice for postmenopausal women with primary breast cancer. Methods We identified postmenopausal women in a population‐based cohort commencing endocrine therapy for invasive primary breast cancer between December 2005 and December 2008 ( n = 645). Individual‐level administrative health records and self‐report data were used to determine women's demographic and clinical characteristics, including preexisting and newly‐treated comorbidities, and switches in endocrine therapy. Time to therapy switching was calculated. Chi‐square tests compared the characteristics of women who did and did not switch, and those switching within 2 years or after 2 years of commencing therapy. Results Twenty‐eight percent of women switched from their initial endocrine therapy, most commonly from tamoxifen to anastrozole, or the converse. A small number of anastrozole‐to‐exemestane and letrozole‐to‐exemestane switches were observed ( n = 19). Most women (>80%) who switched therapies did not have newly‐treated comorbidities. Few women (<5%) switched before completing 2 years of therapy, but these women were significantly more likely to have preexisting antidepressant use than women switching later (43% vs 23%, P = 0.048) and remained on the subsequent therapy for less time (6 months vs 2.7 years, P < 0.001). Conclusions Approximately one‐quarter of postmenopausal women with primary breast cancer switched endocrine therapies. The findings suggest that the majority of switching in Australian practice was planned; occurring after 2–3 years of, not precipitated by comorbidity, and in a sequence supported by trial evidence. Early switching, however, was associated with preexisting depression and appeared to be a marker of poor persistence.