IN-VIVO VISUALIZATION OF PITUITARY DOPAMINERGIC RECEPTORS BY I-123 METHOXYBENZAMIDE (IBZM) CORRELATES WITH SENSITIVITY TO DOPAMINE AGONISTSIN 2 PATIENTS WITH MACROPROLACTINOMAS

We performed in two patients with macroprolactinoma, pituitary scintig raphy with 123 iodine-methoxybenzamide (IBZM), a dopaminergic antagoni st that specifically binds to the D2 dopaminergic receptors. In a 34-y r-old woman with basal PRL levels of about 2000 ng/mL, 7.5 mg/day of B romocriptine (Br) for a month neither reduced PRL levels nor affected tumor size; in this patient single photon emission tomography SPECT fa iled to show any pituitary accumulation of the tracer. In the other pa tient, a 27-yr-old man presenting with cerebrospinal fluid rhinorrhea, basal PRL levels were at 5000 ng/mL; magnetic resonance imaging (MRI) demonstrated a huge pituitary tumor, and SPECT showed a very intense concentration of IBZM at the level of the adenoma. PRL levels fell dra matically to 530 ng/mL with only 2.5 mg/day of Br after 4 days; after 6 days with 7.5 mg/day Br, PRL levels were 63 ng/mL, and the patient u nderwent surgery to correct cerebrospinal fluid leakage. We conclude t hat, in these two patients, the pituitary scintigraphy with IBZM has g iven information on the density of dopamine receptors on the adenoma a nd has correlated with the inhibitory effect of Br on PRL secretion. W hether this tool might be of value in identifying patients with pituit ary tumors potentially responsive to Br treatment is still to be inves tigated.