Iobenguane I 131 is a guanidine analog with specific affinity for tissues of the sympathetic nervous system and related tumors. The radiolabeled forms are used as antineoplastic agents and radioactive imaging agents. MIBG serves as a neuron-blocking agent which has a strong affinity for, and retention in, the adrenal medulla and also inhibits ADP-ribosyltransferase.