A dihydropyridine <SIGN> pyridinium salt type of redox, or chemical, delivery system for the site-specific and/or site-enhanced delivery of a radio-nuclide to the brain. A chelating agent capable of chelating with a radionuclide and having a primary, secondary or tertiary amino function can be converted to the corresponding analogue in which said function is replaced with a dihydropyridine <SIGN> pyridinium salt redox system and then complexed with a radionuclide to provide a new radiopharmaceutical that, in its lipoidal dihydropyridine form, penetrates the blood-brain barrier ('BBB') and allows increased levels of radionuclide concentration in the brain, particularly since oxidation of the dihydropyridine moiety in vivo to the ionic pyridinium salt retards elimination from the brain while elimination from the general circulation is accelerated. This radionuclide delivery system is well suited for use in scintigraphy and similar radiographic techniques.
NOVEL RADIOPHARMACEUTICALS AND CHELATING AGENTS USEFUL IN THEIR PREPARATION