MAPPING DOPAMINE TRANSPORTERS IN THE HUMAN BRAIN WITH NOVEL SELECTIVECOCAINE ANALOG [I-125] RTI-121

The novel cocaine analog RTI-121 [3 beta-(4-iodophenyl)tropane-2 beta- carboxylic acid isopropyl ester] was evaluated as a probe for the in v itro labeling and localization of the dopamine transporter in the huma n brain. Saturation binding experiments conducted in sucrose-phosphate buffer (10 mM sodium phosphate, pH 7.4, 0.32 M sucrose) revealed high - and low-affinity binding components with affinity values (K-D) of 0. 25 +/- 0.04 and 4.9 +/- 1.6 nM (mean +/- SE) and densities (B-max) of 56.8 +/- 13.8 and 147.7 +/- 23.4 pmol/g tissue, respectively. In contr ast, when saturation binding experiments were performed in phosphate-b uffered saline (10 mM Na2HPO4, 1.8 mM KH2PO4, 136 mM NaCl, 2.8 mM KCl, 10 mM NaI, pH 7.4), a 9-fold decrease in the density of the low-affin ity component was noted, suggesting that the low-affinity RTI-121 bind ing site is associated with the region of the transporter involved in the ionic dependence of substrate recognition and/or uptake. The rank order of potency for inhibition of [I-125]RTI-121 binding to human cau date membranes demonstrates that the radioligand selectively labels th e dopamine transporter (GBR 12909 > RTI-121 > mazindol > nomifensine > (-)cocaine > desipramine > citalopram). Autoradiographic mapping of [I -125]RTI-121 revealed very high densities of cocaine recognition sites over areas known to be rich in dopaminergic innervation, including th e caudate, putamen, and nucleus accumbens. Moderate densities were als o observed over the substantia nigra and the ventral tegmental area. L ow-to-background labeling of [(12)5]RTI-121 was seen throughout the ce rebral cortex, amygdaloid nuclei, globus pallidus, and thalamus. In co mparison with the autoradiographic distribution of the cocaine analogs [H-3]WIN 35, 428 (or CFT) and [I-125]RTI-55 (or beta-CIT), the labeli ng pattern for [I-125]RTI-121 was more restricted. These studies demon strate that [I-125]RTI-121 labels dopamine-rich brain regions with gre ater selectivity than other currently available cocaine analogs, which makes it a potentially superior imaging probe for mapping the dopamin e transporter in the human brain. (C) 1995 Wiley-Liss, Inc.