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Development of I-123-Labeled Amines for Brain Studies: Localization of I-123 Iodophenylalkyl Amines in Rat Brain

Medi-Physics, Inc., Emeryville, California

Correspondence: For reprints contact: H. S. Winchell, MD, 1 Via Oneg, Lafayette, CA 94549.

ABSTRACT

Localization in rat brain of forty iodophenylalkyl amines labeled with I-123 was evaluated in an attempt to develop I-123-labeled amines useful for brain studies. For the amines studied, the highest activity in brain and the brain-to-blood activity ratios ranked p > m > o as related to iodine position on the benzene ring: for alkyl groups the rank order was -methylethyl > ethyl > methyl > none; for N additions it was single lipophilic group > H > two lipophilic groups. It is suggested that introduction of a halogen into the ring structure of many amines results in greater concentration of the agent in brain than is seen with the nonhalogenated parent compound.

Thirty-four of the forty compounds showed higher rat brain activity, and higher brain-to-blood activity ratios at 5 min than did [¹²³I]-4-iodoantipyrine. Ten of the compounds showed a tenfold or greater brain-to-blood ratio at 5 min than did [¹²³I]-4-iodoantipyrine. We propose that while initial uptake of these agents in the brain is a consequence of their lipophilicity and that such initial uptake may be a measure of perfusion, progressive brain accumulation of these agents is probably a combined consequence of intravascular/extravascular intracerebral pH gradients, favorable brain lipid/aqueous partition coefficients, and the affinity of the agents for high-capacity, relatively nonspecific binding sites for amines located in the brain and/or brain capillary endothelium.

The agent N-isopropyl-p-iodoamphetamine was chosen for further study because, in the rat, it showed high brain activity (1.57%/g) and brain-blood ratio (12.6) at 5 min; these increased to 2.14%/g and 20.7 at 60 min (R isomer) following its intravenous administration.

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