MIBG and catecholamine storage in the brain: an in-vitro study

Brain uptake of iodine-131 metaiodobenzylguanidine following therapy of malignant pheochromocytoma

Intracranial metaiodobenzylguanidine (MIBG) uptake is occasionally and only faintly visualized on diagnostic studies. Recently, intense normal cerebellar uptake was described on posttherapy MIBG images. Experience at the University of Michigan with posttherapy MIBG scintigraphy of pheochromocytoma was reviewed. The patterns and correlates of intracranial uptake after therapeutic 1-131 MIBG in 25 patients (61 patient treatment encounters) were evaluated by review of records and blinded consensus interpretation of diagnostic and posttherapeutic MIBG scans. Thirty-nine (64%) patient treatment encounters demonstrated at least faint (grade 1) MIBG uptake in one or more brain sites; the most common site was the cerebellum. There was a statistically significant relation between intracranial uptake and 1) size of therapeutic dose and 2) patient age, but no relation between intracranial uptake and gender, body mass index, plasma epinephrine level, plasma norepinephrine level, urine metanephrine level, or the therapy-to-imaging interval. Although the influence of age on the pattern and intensity of intracranial uptake is unexplained, the relation to therapy dose may be explained by the possible generation of MIBG metabolites that can cross the blood-brain barrier (high activity administered and the delay until imaging). Further studies are needed to define mechanisms of intracranial uptake and relation to responses and toxicity after MIBG therapy of neuroendocrine tumors.

Effect of adrenergic receptor ligands on metaiodobenzylguanidine uptake and storage in neuroblastoma cells

The effects of adrenergic receptor ligands on uptake and storage of the radiopharmaceutical [125I]metaiodobenzylguanidine (MIBG) were studied in the human neuroblastoma cell line SK-N-SH. For uptake studies, cells were incubated for 15 min with varying concentrations of alpha-agonist (clonidine, methoxamine, and xylazine), alpha-antagonist (phentolamine, tolazoline, phenoxybenzamine, yohimbine, and prazosin), beta-antagonist (propranolol, atenolol), beta-agonist (isoprenaline and salbutamol), mixed alpha/beta antagonist (labetalol), or the neuronal blocking agent guanethidine, prior to the addition of [125I]MIBG (0.1 microM). The incubation was continued for 2 h and specific cell-associated radioactivity was measured. For the storage studies, cells were incubated with [125I]MIBG for 2 h, followed by replacement with fresh medium with or without drug (MIBG, clonidine, or yohimbine). Cell-associated radioactivity was measured at various times over the next 20 h. Propanolol reduced [125I]MIBG uptake by approximately 30% (P<0.01) at all concentrations tested, most likely due to nonspecific membrane changes. However, incubation with the other beta-agonists or antagonists failed to elicit significant reductions in uptake. In contrast, all of the alpha-agonists significantly inhibited uptake (P<0.05); guanethidine >xylazine >clonidine=methoxamine. The alpha-antagonists demonstrated a broad range of inhibition (phenoxybenzamine >>phentolamine >prazosin >>yohimbine=tolazoline)(P< 0.05). The mixed ligand, labetolol, inhibited MIBG uptake in a dose-dependent manner with an apparent IC50 of 0.65 microM. The retention studies demonstrated that unlabeled MIBG caused profound self-inhibition (P<0.01). Clonidine produced a modest inhibition of retention and yohimbine had no effect. Labetalol, phenoxybenzamine, guanethidine, and propranolol reduced uptake of [125I]MIBG by neuroblastoma cells in culture. Although only labetalol has been reported to cause false-negative MIBG scans, our results suggest that these other drugs have the potential to interfere with MIBG imaging and therapy, particularly at high doses. Adrenergic drugs did not alter cytoplasmic retention of [125I]MIBG in neuroblastoma cells but may have potential in tumors such as phenochromocytoma, where granular storage of MIBG has been observed. Inhibition of [125I]MIBG retention by unlabeled MIBG supports the use of high specific activity radioiodinated MIBG for both diagnosis and therapy.