Richard A. Milius

Evaluation of the monoamine uptake site ligand [131I]methyl 3β-(4-Iodophenyl)-tropane-2β-carboxylate ([123I]β-CIT) in non-human primates: Pharmacokinetics, biodistribution and SPECT brain imaging coregistered with MRI

Abstract The in vivo properties of a new radioiodinated probe of the dopamine and serotonin transporter, [123I]methyl 3β-(4-iodophenyl)tropane-2β-carboxylate ([123I]β-CIT) were evaluated in baboons and vervet monkeys. The labeled product was prepared in 65.2 ± 2.8% yield (mean ± SEM; n = 18) by reaction of the tributylstannyl precursor with [123I]NaI in the presence of peracetic acid followed by high pressure liquid chromatography (HPLC) purification to give a product with radiochemical purity of 97.5 ± 0.5% and specific activity of 500–1200 Ci/mmol. After intravenous administration, whole brain activity peaked at 6–10% injected dose within 1 h post injection (p.i.) and washed out in a biphasic manner with clearance half-lives of 1–2 and 7–35 h for the rapid and slow components, respectively. Excretion occurred primarily through the hepatobiliary route, with about 30% of the injected dose appearing in the GI tract after 5 h. Estimates of radiation absorbed dose gave 0.01, 0.1, 0.2 and 0.03 mGy/MBq to the brain, gall bladder wall, lower large intestine wall and urinary bladder wall, respectively. High resolution SPECT imaging in a baboon demonstrated high uptake of tracer in the region of the striatum (striatum: cerebellum ratio 4.0), in the hypothalamus (ratio 2.6) and in a midbrain region comprising raphe, substantia nigra and superior colliculus (ratio 2.0), with regional brain uptakes measured at 210 min p.i. of [123I]β-CIT. The anatomical locations of the regions on the SPECT image were confirmed by coregistration with MRI. Plasma metabolites and pharmacokinetics were analyzed in baboons and vervets by ethyl acetate extraction and HPLC. The major metabolite was a polar, non-extractable fraction, which increased to > 50% of the plasma activity by 30–45 min p.i. A minor lipophilic (extractable) metabolite was also observed, increasing to about 4% at 2–3 h p.i. The plasma protein bound fraction, determined by ultrafiltration, was 74.8 ± 1.4% (n = 6). The arterial input function was characterized by the sum of three exponential terms with half-lives of 0.3–1.7, 9.7–24.9 and 77–166 min, respectively, for the concentration of free parent compound. [123I]β-CIT promises to be a useful marker for SPECT study of the monoamine uptake system in primate brain.

[O-methyl-11C]β-CIT-FP, a potential radioligand for quantitation of the dopamine transporter: Preparation, autoradiography, metabolite studies, and positron emission tomography examinations

beta-CIT-FP [N-(3-fluoropropyl)-2 beta-carbomethoxy-3 beta-(4-iodophenyl)nortropane] is a cocaine analogue with a high affinity for the dopamine transporter. [O-methyl-11C]beta-CIT-FP ([11C]beta-CIT-FP) was prepared by O-alkylation of the free acid with [11C]methyl iodide. The total radiochemical yield of [11C]beta-CIT-FP was 50 to 60% with an overall synthesis time of 30 min. The radiochemical purity was > 99%, and the specific radioactivity at time of injection was about 37 GBq/mumol (1000 Ci/mmol). Autoradiographic examination of [11C]beta-CIT-FP binding in human brain postmortem demonstrated specific binding in the caudate nucleus and putamen. Positron emission tomography (PET) examination of [11C]beta-CIT-FP in a Cynomolgus monkey demonstrated accumulation in the striatum with a striatum-to-cerebellum ratio of about 8 after 60 min. Equilibrium in the striatum was attained within 70 to 90 min. The radioactivity ratios of thalamus/cerebellum and neocortex/cerebellum were about 2 and 1.5, respectively. In a displacement experiment, radioactivity in the striatum but not in the cerebellum was reduced after injection of beta-CIT, indicating that striatal radioactivity following injection of [11C]beta-CIT-FP is associated with dopamine transporter sites and that the binding is reversible. The fraction of the total radioactivity in plasma representing [11C]beta-CIT-FP determined by high-performance liquid chromatography (HPLC) was 84% at 15 min and 50% at 95 min. [11C]beta-CIT-FP should be a useful PET radioligand for the quantitation of dopamine transporters in the human brain in vivo.

Regional brain uptake and pharmacokinetics of [123I]N-ω -fluoroalkyl-2β-carboxy-3β-(4-iodophenyl) nortropane esters in baboons

Four N-omega-fluoroalkyl-2 beta-carboxy-3 beta-(4-iodophenyl)nortropane ester (beta-CIT-FE), N-fluoropropyl, methyl ester (beta-CIT-FP), N-fluoroethyl, isopropyl ester (IP-beta-CIT-FE), and N-fluoropropyl, isopropyl ester (IP-beta-CIT-FP)] were labeled with 125I and evaluated in baboons by dynamic SPECT regional brain imaging, measurement of pharmacokinetics in arterial plasma, and whole body imaging. The labeled tracers were prepared by iododestannylation of the corresponding 4-(trimethylstannyl)phenyl compounds in radiochemical yield 63-96% and radiochemical purity > 96%. Regional SPECT brain imaging was carried out over a period of 5 h with a Strichman 810X Brain Imager to assess regional uptake in the striatum and midbrain compared to reference regions in the occipital cortex and cerebellum; arterial blood samples were taken for analysis of metabolites by solvent extraction and HPLC. The methyl esters showed higher total and specific peak uptake in the striatum than the isopropyl esters. Midbrain uptake was uniformly lower than striatal uptake and washed out more rapidly. beta-CIT-FE had more rapid striatal kinetics than beta-CIT-FP, with specific striatal washout rates of 10-14 vs 4-6% peak/h. Biodistribution of [123I] beta-CIT-FE and [123I] beta-CIT-FP measured by whole body conjugate imaging demonstrated major uptake in the brain, liver, and GI tract, with excretion occurring through both the renal and hepatobiliary routes. Absorbed radiation does estimates based on the MIRD schema indicated highest dose rates to the urinary bladder wall and lower large intestine wall (0.7 and 0.6 rad/mCi for [123I] beta-CIT-FE and 0.7 and 0.9 rad/mCi for [123I]beta-CIT-FP, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

N-modified fluorophenyltropane analogs of cocaine with high affinity for cocaine receptors.

The binding properties of three N-modified fluorophenyltropane analogs of cocaine were compared in competition experiments with [3H]cocaine. All three analogs displaced specifically bound [3H]cocaine from caudate-putamen membranes of cynomolgus monkeys with affinities exceeding that of cocaine. The compound with the highest affinity, 2 beta-carbomethoxy-3 beta-(4-fluorophenyl)-N-allyl-nortropane, (N-allyl-CFNT) was about three times more potent than cocaine. N-Allyl-CFNT also had cocaine-like interoceptive effects and was about three times more potent than cocaine in squirrel monkeys trained to discriminate cocaine from vehicle in an operant drug discrimination procedure. The results suggest that N-modified fluorophenyltropane derivatives may be useful precursors for development of pharmacological probes for cocaine receptors.