Tamoxifen and CYP 2D6 inhibitors: caution

Tamoxifen, an oestrogen antagonist, is the standard hormone treatment for breast cancer. It is extensively transformed into its active metabolites by the cytochrome P450 enzyme system, especially into endoxifen by isoenzyme CYP 2D6. Co-administration of tamoxifen with isoenzyme CYP 2D6 inhibitors reduces this metabolism. Selective serotonin reuptake inhibitor (SSRI) antidepressants inhibit isoenzyme CYP 2D6. Paroxetine and fluoxetine reduce the plasma concentration of endoxifen by about 50%. Two epidemiological studies involving about 3700 women have shown a link between the use of SSRI antidepressants and an increased frequency of breast cancer recurrence. Other studies, with a lower level of evidence, were less convincing. Studies of other isoenzyme CYP 2D6 inhibitors showed no increase in the risk of breast cancer recurrence, but they lacked statistical power. It is better to avoid prescribing isoenzyme CYP 2D6 inhibitors to women treated with tamoxifen for breast cancer, especially SSRI antidepressants such as paroxetine and fluoxetine. Depression does not always require antidepressant drug therapy, and antidepressants have no proven preventive impact on hot flushes linked to the menopause. If in certain cases, an antidepressant is considered necessary, it may be advisable to replace tamoxifen with anastrozole.