Uncoupled prolactin suppression and tumor shrinkage in bromocriptine-treated prolactinomas

Prolactin (PRL)-secreting pituitary adenomas (prolactinomas) are the most c ommon type of pituitary tumor. Dopamine agonist pharmacotherapy remains the cornerstone of treatment and usually results in rapid decreases in PRL sec retion. Dopaminergic agents, such as bronocriptine, also decrease tumor mas s due to degradation of the intracellular protein synthetic machinery and u ltimately lactotroph cell size reduction. Generally, bromocriptine-induced tumor regression follows sustained suppression of serum PRL levels. However , approximately one-tenth of patients demonstrate uncoupling of responsiven ess to the PRL-lowering effects of dopamine agonists and prolactinoma shrin kage. Two prolactinoma patients illustrating dissociation of bromocriptine- induced PRL suppression and tumor regression are described. Possible explan ations for this observation include physical constraints limiting tumor shr inkage, inconsistent compliance with medications, misclassification of tumo r type, or misclassification of bromocriptine responsiveness due to delayed tumor regression. The vast majority of cases in which prolactinomas fail t o shrink on dopamine agonist therapy remain unexplained. The molecular mech anisms underlying discordance between successful lowering of serum PRL and tumor shrinkage in bromocriptine-treated prolactinoma patients are unclear and remain to be elucidated.