Tamoxifen as an antifertility agent in the long-tailed macaque (Macaca fascicularis)

The potential use of tamoxifen, a nonsteroidal antiestrogen, as an antifertility agent was studied in the long-tailed macaque (Macaca fascicularis). Twenty-six cycling females were bred, then treated with a single oral dose of tamoxifen (5 mg/kg) (N = 13) or vehicle (N = 13) on day 4 post-ovulation. Serum progesterone (P4) and tamoxifen concentrations were evaluated on post-ovulation days 4, 8, 12, 16, and 18. No effects of treatment were observed on P4 concentrations or on the fertility rate--pregnancy was achieved in 4/13 controls (31%) and 6/13 treated females (46%). Analysis for serum tamoxifen concentrations in samples collected during the fertility and a supplemental pharmacokinetic study (N = 3; single oral dose of 10 mg/kg; urine and serum evaluated) failed to reveal any detectable tamoxifen levels. It was concluded that (1) absorption of tamoxifen may be negligible under the described treatment regimens or (2) tamoxifen metabolism/clearance occurs at a rapid rate.