In vivo visualization of pituitary dopaminergic receptors by iodine-123 methoxybenzamide (IBZM) correlates with sensitivity to dopamine agonists in two patients with macroprolactinomas.

We performed in two patients with macroprolactinoma, pituitary scintigraphy with 123 iodine-methoxybenzamide (IBZM), a dopaminergic antagonist that specifically binds to the D2 dopaminergic receptors. In a 34-yr-old woman with basal PRL levels of about 2000 ng/mL, 7.5 mg/day of Bromocriptine (Br) for a month neither reduced PRL levels nor affected tumor size; in this patient single photon emission tomography SPECT failed to show any pituitary accumulation of the tracer. In the other patient, 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 dramatically 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 underwent surgery to correct cerebrospinal fluid leakage. We conclude that, in these two patients, the pituitary scintigraphy with IBZM has given information on the density of dopamine receptors on the adenoma and has correlated with the inhibitory effect of Br on PRL secretion. Whether this tool might be of value in identifying patients with pituitary tumors potentially responsive to Br treatment is still to be investigated.