Staley J K, Boja J W, Carroll F I, Seltzman H H, Wyrick C D, Lewin A H, Abraham P, Mash D C
Department of Neurology, University of Miami School of Medicine, Florida 33101, USA.
Synapse. 1995 Dec;21(4):364-72. doi: 10.1002/syn.890210412.
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 vitro labeling and localization of the dopamine transporter in the human 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 (KD) of 0.25 +/- 0.04 and 4.9 +/- 1.6 nM (mean +/- SE) and densities (Bmax) of 56.8 +/- 13.8 and 147.7 +/- 23.4 pmol/g tissue, respectively. In contrast, when saturation binding experiments were performed in phosphate-buffered 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-affinity component was noted, suggesting that the low-affinity RTI-121 binding 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 [125I]RTI-121 binding to human caudate membranes demonstrates that the radioligand selectively labels the dopamine transporter (GBR 12909 > RTI-121 > mazindol > nomifensine > (-) cocaine > desipramine > citalopram). Autoradiographic mapping of [125I]RTI-121 revealed very high densities of cocaine recognition sites over areas known to be rich in dopaminergic innervation, including the caudate, putamen, and nucleus accumbens. Moderate densities were also observed over the substantia nigra and the ventral tegmental area. Low-to-background labeling of [125]RTI-121 was seen throughout the cerebral cortex, amygdaloid nuclei, globus pallidus, and thalamus. In comparison with the autoradiographic distribution of the cocaine analogs [3H]WIN 35,428 (or CFT) and [125I]RTI-55 (or beta-CIT), the labeling pattern for [125I]RTI-121 was more restricted. These studies demonstrate that [125I]RTI-121 labels dopamine-rich brain regions with greater selectivity than other currently available cocaine analogs, which makes it a potentially superior imaging probe for mapping the dopamine transporter in the human brain.
新型可卡因类似物RTI-121 [3β-(4-碘苯基)托烷-2β-羧酸异丙酯]被评估作为人脑多巴胺转运体体外标记和定位的探针。在蔗糖-磷酸盐缓冲液（10 mM磷酸钠，pH 7.4，0.32 M蔗糖）中进行的饱和结合实验显示出高亲和力和低亲和力结合成分，其亲和力值（KD）分别为0.25±0.04和4.9±1.6 nM（平均值±标准误），密度（Bmax）分别为56.8±13.8和147.7±23.4 pmol/g组织。相比之下，当在磷酸盐缓冲盐水（10 mM Na2HPO4，1.8 mM KH2PO4，136 mM NaCl，2.8 mM KCl，10 mM NaI，pH 7.4）中进行饱和结合实验时，低亲和力成分的密度下降了9倍，这表明低亲和力RTI-121结合位点与转运体中涉及底物识别和/或摄取的离子依赖性区域相关。抑制[125I]RTI-121与人尾状核膜结合的效力等级顺序表明，该放射性配体选择性地标记多巴胺转运体（GBR 12909 > RTI-121 > 吗茚酮 > 诺米芬辛 > (-)可卡因 > 地昔帕明 > 西酞普兰）。[125I]RTI-121的放射自显影图谱显示，在已知富含多巴胺能神经支配的区域，包括尾状核、壳核和伏隔核，可卡因识别位点密度非常高。在黑质和腹侧被盖区也观察到中等密度。在整个大脑皮层、杏仁核、苍白球和丘脑中可见[125I]RTI-121的低至背景标记。与可卡因类似物[3H]WIN 35,428（或CFT）和[125I]RTI-55（或β-CIT）的放射自显影分布相比，[125I]RTI-121的标记模式更具局限性。这些研究表明，[125I]RTI-121比其他目前可用的可卡因类似物更具选择性地标记富含多巴胺的脑区，这使其成为用于绘制人脑多巴胺转运体图谱的潜在优越成像探针。
