Randomized phase 1-2 trial of addition of low-dose endoxifen to tamoxifen for breast cancer in patients with impaired tamoxifen metabolism

Tamoxifen undergoes bioactivation to its active metabolite (Z)-endoxifen, which blocks estrogen-dependent breast tumor growth. The multicenter randomized phase 1-2 TAMENDOX trial in Germany evaluated addition of low-dose (Z)-endoxifen to tamoxifen among patients with impaired tamoxifen metabolism. The study found that addition of low-dose (Z)-endoxifen to standard tamoxifen was safe and provided a new approach to personalized anti-estrogen treatment among patients with impaired tamoxifen metabolism.