The effect of tamoxifen on tumours induced by cells from a canine mammary carcinoma cell line in athymic nude mice.

Tumours were induced in CBA athymic nude mice by subcutaneous injection of REM 134 cells. These cells were from a continuous line derived from a canine mammary carcinoma and have no detectable oestrogen receptors. In vitro, the anti-oestrogen tamoxifen was growth inhibitory at a concentration of 10(-6) M and adding 10(-8) M oestradiol-17 beta did not reverse this effect. The relative rate of growth of the tumours induced by the cultured cells was the same in male and female mice. Oral tamoxifen at a dose of 1 mg mouse-1 week-1 suppressed the early growth rate significantly and to the same extent in male and female mice; conversely subcutaneous tamoxifen at 2 mg mouse-1 week-1 had only a transient early effect. These results argue against tamoxifen acting solely as an antagonist for oestrogen.