The Metabolism of Tamoxifen (I.C.I. 46,474) Part II: In Female Patients

Abstract

1. The metabolism of orally administered [¹⁴C]tamoxifen was investigated in four women. Most of the ¹⁴C was slowly excreted in the faeces, only small amounts appearing in the urine.

2. The labelled material in the faeces was present mainly as conjugates. Unchanged drug and hydroxylated metabolites accounted for less than 30% of the faecal radioactivity.

3. Blood levels of total radioactivity following oral doses of about 0·3 mg/kg reached peak values of 0·06–0·14 μg/ml 4–7 h after dosing. At this time only 20–30% of the radioactive material was present as tamoxifen.

4. The prolongation of blood levels and faecal excretion is thought to be due to enterohepatic circulation.