Drug switch because of treatment-related adverse side effects in endocrine adjuvant breast cancer therapy: how often and how often does it work?

Therapy-related adverse side effects are a main reason for non-persistence to adjuvant endocrine breast cancer therapy. This study reports frequency of drug-related adverse side effects that were so severe that a modification of the therapy was necessary. We evaluated how many patients discontinued adjuvant endocrine therapy because of these side effects (non-persistence). Last, we analyzed how often a drug switch was undertaken for this reason and how often this measure led to the patient successfully continuing their endocrine therapy. Data concerning all postmenopausal breast cancer patients (≤80 years), who initiated endocrine adjuvant therapy between 1998 and 2008 in a Swiss breast center (n = 400), were analyzed. Out of these 400 women, 37 (9.3%) were defined as being non-persistent to the therapy; out of these, 24 (64.9%) because of therapy-related side effects. About 78 patients (19.5%) suffered from severe therapy-related side effects that made a modification of therapy necessary. Out of these 78 cases, 14 patients (17.9%) stopped the therapy without attempting a drug switch (non-persistence). In 64 patients (82.1%; 16% of all women who started endocrine therapy), a drug switch was undertaken. Out of these 64 cases, in 52 cases (81.3%) endocrine therapy was completed after therapy modification. Patients who reported one major adverse effect were more likely to continue the endocrine therapy after a drug switch (P = 0.048) compared with those who suffered from at least two different side effects. In 10 of the 64 cases (15.6%), modification of the therapy was not successful and the patients stopped the treatment prematurely (non-persistence) because of ongoing side effects. In cases when therapy-related side effects occur, a drug switch is a promising step to further improve persistence and, by doing so, the outcome of breast cancer patients.