Effect of Tamoxifen Administration on Prolactin Release by Invasive Prolactin-Secreting Pituitary Adenomas

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Abstract
Bromocriptine treatment of patients with invasive prolactin (PRL)-secreting pituitary adenomas does not invariably result in normalization of the plasma PRL levels. The antiestrogenic drug tamoxifen was previously shown to inhibit 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 [SE of the mean]; P < 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 < 0.01). Tamoxifen administration suppressed PRL release in patients with giant invasive PRL-secreting pituitary adenomas; it had a slight but significant additive or potentiating effect on the bromocriptine-mediated inhibition of PRL secretion. Despite the simultaneous administration of bromocriptine and tamoxifen, normalization of the circulating PRL levels was not reached in this type of patient.