Effect of tamoxifen administration on prolactin release by invasive prolactin-secreting pituitary adenomas

Bromocriptine treatment of patients with invasive prolactin (PRL)-secreting pituitary adenomas does not invariably result in normalization of the plasma PRL levels. We previously showed that the antiestrogenic drug tamoxifen inhibited hormone release from transplantable PRL-secreting pituitary tumors in rats. In 8 patients with invasive PRL-secreting pituitary adenomas with extrasellar extension, the effect of the administration of tamoxifen was investigated on the plasma PRL concentration and on the bromocriptine-mediated inhibition of PRL release. Treatment for 5 days with tamoxifen (20 mg/day) suppressed plasma PRL levels as measured in 5 samples over the day significantly by 20 +/- 3% (means +/- SEM; p less than 0.01). During tamoxifen administration the inhibition of PRL secretion by 2.5 mg bromocriptine was further suppressed by 36 +/- 7%, in comparison with the plasma PRL levels after bromocriptine alone (p less than 0.01). Tamoxifen administration suppressed PRL release in patients with giant invasive PRL-secreting pituitary adenomas, and it had a slight but significant additive or potentiating effect on the bromocriptine-mediated inhibition of PRL secretion. However, despite the simultaneous administration of bromocriptine and tamoxifen, normalization of the circulating PRL levels was not reached in this type of patient.