Breast cancer in women using digoxin: tumor characteristics and relapse risk

Incident breast cancer cases in Danish women (N= 49,312; 1995-2008) were identified. Analyses were conducted in women 20-74 years old. Relapse hazard ratios (HR) were compared in women using and not-using digoxin, adjusting for age, calendar period, protocol, tumor size, nodal involvement, histology grade, estrogen-receptor (ER) status, and anti-estrogen therapy in Cox regression models.45 relapses occurred in women already using digoxin at breast cancer diagnosis (1,487 person-years). 24 relapses occurred in women later starting digoxin (384 person-years). At diagnosis, tumors in digoxin users were more likely ER+ (85.4% vs. 78.6%; p=0.002) and have Grade 1 ductal histology, compared to non-users (37.2% vs. 25.7%; p=0.004). Overall relapse HR in digoxin users was 1.13 (95% confidence interval: 0.88-1.46) compared to non-users. Relapse risk in digoxin users was significantly increased in the first year (2.19; 1.26-3.78) but not thereafter (0.99; 0.74-1.32) (p=0.02 for difference in HRs). First-year relapse hazard was high in digoxin-using women with ER+ tumors (2.51; 1.39-4.55) but not ER- tumors (0.72; 0.10-5.27). Recurrence hazard was not significantly changed among digoxin-using women also using tamoxifen.Breast cancers arising in digoxin-using women had better prognostic features. After adjustment for markers, overall breast cancer relapse risk in digoxin users was not increased significantly, although recurrence hazards for ER+ tumors were higher in the first year following diagnosis.