Effect of high-dose oestrogen administration on the growth and prolactin receptor content of N-nitrosomethylurea-induced mammary tumours in the rat

The effect of chronic administration of a high dose of oestradiol benzoate (OB) on growth and prolactin receptor content of the N-nitrosomethylurea (NMU)-induced rat mammary tumour was investigated. Daily i.m. administration of 50 μg OB for 8 days to tumour-bearing rats induced marked reduction in tumour size which was already significant after only three injections. Simultaneous daily administration of 1 mg perphenazine to stimulate endogenous prolactin release counteracted the inhibitory effect on tumour growth of OB but only partially since the tumours grew significantly more when perphenazine was injected alone. No treatment significantly affected prolactin receptor content in the tumours. We concluded that administration of pharmacological doses of OB has a potent antitumour action on the NMU-induced rat mammary tumour. This effect can only partially be explained by inhibition of the peripheral action of prolactin.