Purpose: To compare serum and urine levels of tamoxifen and metabolites after a loading dose and at the steady state.
Methods: A loading dose of 160 mg of tamoxifen was given to 14 patients with advanced breast cancer. Thereafter a regular daily dose of 30 mg of tamoxifen was given. Serum and urine levels of tamoxifen and metabolites were measured by high-performance liquid chromatography and compared with levels determined in 31 patients with advanced breast cancer at the steady state at a daily dose of 30 mg of tamoxifen.
Results: Serum and urine levels (24-h values) of tamoxifen and metabolites were lower (P < 0.05) after a loading dose than at the steady state. The difference was most pronounced for the metabolites, whereas the tamoxifen loading-dose level was near the steady state.
Conclusion: Tamoxifen steady state can be reached in 1-2 days by the administration of a loading dose of 160 mg of tamoxifen for 2 days. Tamoxifen metabolite steady-state levels are reached regularly after 4 or more weeks during application of a loading dose. Very little tamoxifen or metabolites are excreted into the urine.