Elzbieta H. Sybirska

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.

Pharmacokinetics of the SPECT benzodiazepine receptor radioligand [123I]iomazenil in human and non-human primates

The pharmacokinetics of [123I]iomazenil (Ro 16-0154) in 5 healthy human volunteers were compared to those in 2 hypothermic and 3 normothermic anesthetized monkeys. Following intravenous injection in humans and monkeys, [123I]iomazenil rapidly diffused outside the vascular bed and was cleared from the arterial plasma triexponentially. The clearance half-times in hypothermic animals were protracted to values closer to those of the human. [123I]Iomazenil was metabolized mainly to a polar radiometabolite (not extracted by ethyl acetate) in the human whereas an additional lipophilic radiometabolite was detected in the monkey. In vitro and in vivo studies showed that [123I]iomazenil established equal concentrations in association with the cellular and plasma component of the blood, indicating that the plasma clearance of [123I]iomazenil mirrors that of the blood. Analysis of organs from a monkey given [123I]iomazenil showed that the parent compound was actively taken up by peripheral organs; the polar radiometabolite accumulated mainly in the bile and the kidneys whereas the non-polar radiometabolite accumulated in the urine and kidneys. Greater than 90% of the radioactivity in the different regions of the brain was unchanged parent [123I]iomazenil.

[123I]Iomazenil spect imaging demonstrates significant benzodiazepine receptor reserve in human and nonhuman primate brain

SPECT imaging with [123I]iomazenil was used to measure benzodiazepine (BZ) neuroreceptor occupancy of the agonist lorazepam administered at therapeutically relevant doses in humans and supratherapeutic doses in monkeys. Lorazepam at therapeutic doses (0.03 mg/kg, i.v.) administered 90 min after the bolus injection of [123I]iomazenil had no statistically significant effect (P > 0.12) on the washout rates of regional brain activities compared to that in control subjects, although human subjects demonstrated marked sedation from the lorazepam. In baboons, the effects of higher doses of lorazepam (cumulative 0.5 mg/kg) were examined in a stepwise displacement paradigm. The in vivo potency was expressed as the ED50 (or dose required to displace 50% of receptor bound activity) and was equal to 0.34 +/- 0.01 mg/kg (mean +/- SD, n = 12). Log-logit analyses of displacement data corrected for endogenous washout showed that therapeutic doses of lorazepam were associated with < 3% BZ receptor occupancy. To examine if endogenous GABA modulates potency of the BZ agonist, the ED50 of lorazepam was compared with and without concurrent administration of tiagabine, a GABA reuptake inhibitor. These experiments were designed to measure an in vivo GABA shift of agonist potency. In vivo microdialysis demonstrated that tiagabine (up to 1 mg/kg, i.v.) increased extracellular GABA levels up to 200% of baseline, but these doses had only a minimal enhancement of lorazepams potency to displace [123I]iomazenil. This study strongly suggests that single therapeutically relevant doses of lorazepam occupy a relatively small percentage (i.e. < 3%) of BZ receptors and that BZ binding sites have a significant (i.e. > 97%) receptor reserve.

Calibration of [123I]iodine-labeled tissue standards for autoradiographic studies

Autoradiographic images can be analyzed with computer-assisted microdensitometry relative to radioactive reference standards to provide quantitative measurements of regional radioactivity concentrations. [123I]Iodine containing sections of brain paste have been calibrated relative to plastic-embedded tritium(3H) and 125Iodine standards. For exposure times of 8, 12 and 24 h, plastic-embedded standards covered a range concentrations of [123I]iodine from 1000 to 100,000 dpm/mg wet weight. Iodine-123 radioactivity was linear with section thickness from 5 to 30 microns. These studies confirmed the feasibility of using commercially available longer-lived reference standards to provide [123I]iodine equivalent values in absolute units of dpm (or microCi) per mg tissue.