Iobenguane

Iobenguane, or MIBG, is an aralkylguanidine analog of the adrenergic neurotransmitter norepinephrine (noradrenaline), typically used as a radiopharmaceutical. It acts as a blocking agent for adrenergic neurons. When radiolabeled, it can be used in nuclear medicinal diagnostic and therapy techniques as well as in neuroendocrine chemotherapy treatments. It localizes to adrenergic tissue and thus can be used to identify the location of tumors such as pheochromocytomas and neuroblastomas. With iodine-131 it can also be used to treat tumor cells that take up and metabolize norepinephrine. MIBG is absorbed by and accumulated in granules of adrenal medullary chromaffin cells, as well as in pre-synaptic adrenergic neuron granules. The process in which this occurs is closely related to the mechanism employed by norepinephrine and its transporter in vivo. The norepinephrine transporter (NET) functions to provide norepinephrine uptake at the synaptic terminals and adrenal chromaffin cells. MIBG, by bonding to NET, finds its roles in imaging and therapy. Less than 10% of the administered MIBG gets metabolized into m-iodohippuric acid (MIHA), and the mechanism for how this metabolite is produced is unknown. MIBG concentrates in endocrine tumors, most commonly neuroblastoma, paraganglioma, and pheochromocytoma. It also accumulates in norepinephrine transporters in adrenergic nerves in the heart, lungs, adrenal medulla, salivary glands, liver, and spleen, as well as in tumors that originate in the neural crest. When labelled with iodine-123 it serves as a whole-body, non-invasive scintigraphic screening for germ-line, somatic, benign, and malignant neoplasms originating in the adrenal glands. It can detect both intra and extra-adrenal disease. The imaging is highly sensitive and specific. Iobenguane concentrates in presynaptic terminals of the heart and other autonomically innervated organs. This enables the possible non-invasive use as an in vivo probe to study these systems.