Antihormones
Tamoxifen is widely used in the treatment of estrogen receptor-positive breast cancer and is known for its inter-individual variability in pharmacokinetics. Previously, a minimal concentration threshold of 5.9 mg/ml of the active metabolite (Z)-endoxifen for the recurrence of breast cancer has been identified 54. Therefore, Therapeutic Drug Monitoring (TDM) has been recommended for tamoxifen to prevent under-treatment based on the (Z)-endoxifen concentration55,56.
In patients after RYGB surgery, reduced absorption of tamoxifen has been described in three women after RYGB 57 with tamoxifen blood concentrations below the therapeutic level of 5.9 mg/ml. For one patient, the time between RYGB and measured tamoxifen concentration was described and was four years. Because of the established relation between (Z)-endoxifen and the recurrence of breast cancer, particularly for patients after bariatric surgery, it seems advisable to apply TDM of (Z)-endoxifen over time after bariatric surgery. The monitoring of side effects can be included in determining the effectiveness/absorption of the therapy, however, because hot flashes are not predictive for serum concentrations of tamoxifen and its metabolites 58, it can not replace TDM. To our best knowledge, there are no known data from other antihormones in patients with bariatric surgery. Tamoxifen seems to be the preferred antihormone therapy because of routine TDM.