Late development of resistance to bromocriptine in a patient with macroprolactinoma.

We report the case of a man with an invasive macroprolactinoma who developed resistance to bromocriptine to which he had previously responded satisfactorily for 5 years. Subsequently, hyperprolactinemia was controlled equally well with 600 microg quinagolide daily and later with 4.5 mg cabergoline weekly. This observation suggests that a loss of dopamine receptors at the tumoral cell surface might be the mechanism underlying acquired resistance to bromocriptine. In addition, no tumor growth was observed over a 10-year follow-up, which virtually excludes a malignant transformation of the prolactinoma. This case emphasizes the need for close supervision of patients with macroprolactinoma, even after the serum prolactin concentration has been normalized by bromocriptine. It furthermore illustrates the usefulness of quinagolide and cabergoline when resistance to bromocriptine develops after a prolonged period of adequate response to this drug.