Katalin Marthi

Kinetics of the metabolism of four PET radioligands in living minipigs

Most radioligands are substantially metabolised in peripheral organs during the course of positron emission tomography (PET) recordings. Accurate determination of plasma concentrations of unmetabolised radioligands is often important for quantification of data from PET studies. The fractions of untransformed radioligand and radioactive metabolites in plasma extracts must then be measured. Temporal changes in these fractions are influenced by the rate constant of appearance of total radioactive metabolites in plasma (apparent rate constant of metabolism in plasma, k(0)) and the net rate constant of elimination of all radioactive metabolites from plasma (k(-1)). In order to clarify the relationship between radioligand fractions and rate constants, plasma samples collected from Göttingen minipigs during PET recordings using four different binding site ligands were analysed by radio high performance liquid chromatography. The calculated plasma concentrations of parent compounds and their radioactive metabolites were used to calculate k(0) and k(-1) for 11C-labelled NNC 112, NS 2214, PK 11195 and raclopride in minipigs using a novel application of the tissue-slope intercept plot. In general, the apparent rate constant of metabolism in plasma was found to be greater in the minipig than in man. The reported kinetic analysis enables the apparent metabolism of PET radioligands in plasma to be quantified.

Normalization of markers for dopamine innervation in striatum of MPTP-lesioned miniature pigs with intrastriatal grafts

As part of the DaNeX study, the uptake and binding of several positron emitting tracers was recorded in brain of healthy Göttingen minipigs, in minipigs with a syndrome of parkinsonism due to MPTP intoxication, and in parkinsonian minipigs which had received intrastriatal grafts of mesencephalic neurons from fetal pigs. The specific binding of [11C]NS 2214 to catecholamine uptake sites was reduced by two thirds in striatum of the intoxicated animals, while the rate constant for the decarboxylation of [18F]fluorodopa was reduced by 50% in the intoxicated animals. Several months after grafting, both pre‐synaptic markers of dopamine fibres were normal in striatum. Dopamine depletion or grafting were without effect on the cerebral perfusion rate, measured with [15O]‐water, did not alter the rate of oxygen metabolism (CMRO2) in brain, and did not alter the binding potential of tracers for dopamine D1 or D2 receptors in pig striatum. However, the grafting was associated with a local increase in the binding of [11C]PK 11195, a tracer for reactive gliosis, suggesting that an immunological reaction occurs at the site of graft, which might potentially have reduced the graft patency. However, this apparent immunological response did not preclude the re‐establishment of normal [18F]fluorodopa and [11C]NS 2214 uptake in the allografted striatum.

Biodistribution and radiation dosimetry of [N-methyl-11C]mirtazapine, an antidepressant affecting adrenoceptors.

Central adrenoceptors cannot currently be studied by PET neuroimaging due to a lack of appropriate radioligands. The fast-acting antidepressant drug mirtazapine, radiolabelled for PET, may be of value for assessing central adrenoceptors, provided that the radiation dosimetry of the radioligand is acceptable. To obtain that information, serial whole-body images were made for up to 70 min following intravenous injection of 326 and 185 MBq [N-methyl-11C]mirtazapine (specific activities E.O.S. of 119 and 39G Bq/micromol, respectively) in a healthy volunteer. Ten source organs plus remaining body were considered in estimating absorbed radiation doses calculated using MIRD 3.1. The highest absorbed organ doses were found to the lungs (3.4 x 10(-2) mGy/MBq), adrenals (1.2 x 10(-2) mGy/MBq), spleen (1.2 x 10(-2) mGy/MBq), and gallbladder wall (1.1 x 10(-2) mGy/MBq). The effective dose was estimated to be 6.8 x 10(-3) mSv/MBq, which is similar to that produced by several radioligands used routinely for neuroimaging.

[11C]Mirtazapine for PET neuroimaging: radiosynthesis and initial evaluation in the living porcine brain

We radiolabelled mirtazapine, a tetracyclic, atypical, antidepressant drug, for positron emission tomography (PET) and evaluated its regional kinetics in the living porcine brain. We produced [N-methyl-11C]mirtazapine with a radiochemical-purity >98% in a 21% decay-corrected radiochemical yield by alkylation of N-desmethyl mirtazapine with [11C]methyl iodide, followed by HPLC purification and formulation. [N-Methyl-11C]mirtazapine entered the brain readily and, under baseline conditions, it had an apparent volume of distribution (V(e)) of 9-13 in the basal ganglia, thalamus, and frontal cortex. Reference region and graphical analyses based on a one-compartment model showed that the binding of [N-methyl-11C]mirtazapine was reversible, with an apparent binding potential of more than two in thalamus and frontal cortex. Infusion of unlabelled mirtazapine markedly displaced [N-methyl-11C]mirtazapine from binding sites in the basal ganglia, thalamus and frontal cortex, but not in reference regions (cerebellum and olfactory tubercle). Thus, [N-methyl-11C]mirtazapine showed rapid passage into the living brain, slow metabolism in blood, and reversible, competitive binding, which may make it useful for PET neuroimaging of neuroreceptors involved in antidepressant actions.

[N-methyl-11C]Mirtazapine for positron emission tomography neuroimaging of antidepressant actions in humans

RationaleMany actions of antidepressant drugs cannot yet be studied using positron emission tomography (PET) neuroimaging due to lack of suitable radioligands. We believe that mirtazapine, radiolabeled with C-11, might be suitable for PET neuroimaging of α2-adrenoceptors in selected regions of the living human brain.ObjectiveTo determine the regional central biodistribution and pharmacokinetics of [N-methyl-11C]mirtazapine in humans.MethodsFive healthy volunteers received an intravenous injection of [N-methyl-11C]mirtazapine for evaluating its metabolism, biodistribution and pharmacokinetics.Results[N-methyl-11C]Mirtazapine entered the brain readily, with initial clearance from blood to tissue (K1) ranging from 0.31xa0ml/ml/min in amygdala to 0.54xa0ml/ml/min in thalamus. The rate of metabolism of [N-methyl-11C]mirtazapine in the bloodstream was relatively slow, with 20–40% of [11C]-derived radioactivity still present as parent compound at 60xa0min post-injection. The clearance of [N-methyl-11C]mirtazapine from the tissue compartment (k2’) ranged from a low of 0.03xa0min−1 in amygdala to a high of 0.06–0.07xa0min−1 in thalamus and cerebellum. The volume of distribution (Ve’) of [N-methyl-11C]mirtazapine was markedly greater in hippocampus and amygdala (11.3–12.0) than in cerebellum (6.7), with intermediate levels in the thalamus (9.4).Conclusions[N-methyl-11C]Mirtazapine has suitable properties for PET neuroimaging. We envision [N-methyl-11C]mirtazapine as a molecular probe for PET imaging of antidepressant actions at sites such as α2-adrenoceptors in the living human brain.

PET neuroimaging of [11C]mirtazapine enantiomers in pigs

Previously, we used positron emission tomography (PET) for studying the pharmacokinetics of rac-[11C]mirtazapine in living brain. Our findings showed that rac-[11C]mirtazapine has suitable properties for PET neuroimaging. However, separate studies of enantiomers are typically required for characterizing the pharmacokinetics of a racemic drug. Therefore, we have determined the whole-body distribution and brain pharmacokinetics of S- and R-[11C]mirtazapine in pigs. The enantiomers of [11C]mirtazapine produced similar effective doses of radioactivity in most body organs, except for the brain, in which the dose was approximately 40% higher after injection of S-[11C]mirtazapine than the antipode. Kinetic analyses of dynamic brain PET recordings showed that values for regional accumulation of compound (k3) were significantly higher for S-[11C]mirtazapine than for the antipode, while the values for clearance of compounds from tissue to circulation (k2) were consistently lower for S-[11C]mirtazapine than for the R-form. No reliable difference occurred in the rate of metabolism of S- and R-[11C]mirtazapine in the bloodstream of the pigs. The present findings indicate that enantioselective processes affect the cerebral pharmacokinetics of rac-mirtazapine.

Structures of racemic monofluoro‐substituted mandelic acids, their relation to the thermochemical properties and an analysis of short intermolecular fluorine–carbon contacts

The structures of the three monofluoro-substituted mandelic acids (C 8 H 7 FO 3 , M r =170.14) have been determined from low-temperature [122 (1) K] X-ray diffraction data [λ(Cu Kα)=1.54184 A]. o-Fluoromandelic acid, monoclinic, P2 1 /c, a=8.4238 (12), b=5.4766 (7), c=15.959 (2) A, β=95.962 (11) o , V=732.3 (3) A 3 , Z=4, D x =1.543 g cm -3 , μ=11.25 cm -1 , F(000)=352, R=0.040 for 1357 contributing reflections, m.p. 388.3 (5) K. m-Fluoromandelic acid (metastable modification), monoclinic, P2 1 /a, a=10.8657 (14), b=9.2663 (10), c=15.722 (2)A, β=107.474 (10)°, V=1509.9 (6)A 3 , Z=8, D x =1.497 g cm -3 , μ=10.92 cm -1 , F(000)=704, R=0.048 for 2977 contributing reflections, m.p. 368.4 (5)K

PET evaluation of a tetracyclic, atypical antidepressant, [N-methyl-11C]mianserin, in the living porcine brain

We synthesized [N-methyl-11C]mianserin by alkylation of N-desmethyl mianserin with [11C]methyl iodide followed by HPLC purification. We used PET for determining the regional cerebral pharmacokinetics of the radiotracer in anesthetized swine. [N-methyl-11C]Mianserin entered most brain regions readily (range of K1 values: 0.66-1.13), reaching highest levels in the basal ganglia and thalamus. The binding potential of [N-methyl-11C]mianserin was relatively low (range: 0.07-0.50), but regional differences were nonetheless observed, with highest values in the temporal cortex and lowest values in the brainstem. These PET findings, which are the first ones for a tetracyclic, antidepressant drug, show that [N-methyl-11C]mianserin has only a limited degree of regional specificity of binding in the living brain.

Optical resolution of 1-(1-naphthyl)ethylamine by its dicarboxylic acid derivatives: Structural features of the oxalic acid derivative diastereomeric salt pair

Optical resolution methods were established for racemic 1-(1-naphthyl) ethylamine. The resolving agents were synthesized by N-derivatizing (R)-1-(1-naphthyl) ethylamine with dicarboxylic acids. Oxalic, malonic, and succinic acid derivatives were found to be suitable resolving agents. These resolutions are parallel to a series of optical resolutions of 1-phenylethylamine which had been previously performed by our research group using similar derivative resolving agents (Balint et al., Tetrahedron: Asymmetry 2001;12:1511-1518.) The comparison of the results of the enantiomer separations is performed. The diastereomeric salts formed with (R)-N-[1-(1-naphthyl)ethyl]oxalamic acid were investigated by single crystal X-ray diffraction. The crystal structures were compared with the previously published structures of the diastereomers of the phenyl-substituted analogue, namely (R)- and (S)-1-phenylethylammonium (R)-N-(1-phenylethyl)oxalamates (Balint et al., Tetrahedron: Asymmetry 2001;12:1511-1518).

Structures of racemic halogen‐substituted 3‐hydroxy‐3‐phenylpropionic acids; relations between spontaneously resolved and racemic compounds

Low-temperature [122.0(5)K] X-ray diffraction data [λ(CuKα)=1.54184A] showed that racemic 3-hydroxy-3-(3-chlorophenyl)propionic acid and its bromo-substituted analogue are isostructural. 3-Hydroxy-3-(3-chlorophenyl)propionic acid : C 9 H 9 ClO 3 , M r = 200.61, monoclinic, P2 1 /c, a = 15.301 (2), b = 5.7117 (8), c = 11.2462 (13) A, β = 110.873 (10)°, V = 918.4 (2) A 3 , Z = 4, D x = 1.451 gcm -3 , μ = 35.15 cm -1 , F(000) = 416, wR 2 = 0.0775 for 1880 contributing reflections [R = 0.0274 for F > 4σ(F)], m.p. 342.7 (5) K. 3-Hydroxy-3-(3-bromophenyl)propionic acid : C 9 H 9 BrO 3 , M r = 245.07, monoclinic, P2 1 /c, a = 15.498 (3), b = 5.7513 (12), c = 11.298 (2) A, β = 111.280 (14)°, V = 938.4 (3)A 3 , Z = 4, D x = l.735 gcm -3 , μ = 58.07cm -1 , F(000) = 488, wR 2 = 0.0769 for 1926 contributing reflections [R = 0.0284 for F > 4σ(F)], m.p. 348.7(5)K. The results from these structure determinations were combined with results from crystal structure determinations of other halogen-substituted 3-hydroxy-3-phenylpropionic acids in an analysis performed to obtain insight into their conglomerate versus racemic compound formation. In all the structures studied, the O-H...O hydrogen bonds are formed between identical donor and acceptor atoms leading to two distinct motifs. The crystallization of the racemates as either racemic compounds or mechanical mixtures of enantiomers can only be rationalized if weaker intermolecular interactions are taken into account.