The pharmacokinetics of [18F]UCB-H revisited in the healthy non-human primate brain.

BACKGROUND: Positron Emission Tomography (PET) imaging of the Synaptic Vesicle glycoprotein (SV) 2A is a new tool to quantify synaptic density. [18F]UCB-H was one of the first promising SV2A-ligands to be labelled and used in vivo in rodent and human, while limited information on its pharmacokinetic properties is available in the non-human primate. Here, we evaluate the reliability of the three most commonly used modelling approaches for [18F]UCB-H in the non-human cynomolgus primate, adding the coupled fit of the non-displaceable distribution volume (VND) as an alternative approach to improve unstable fit. The results are discussed in the light of the current state of SV2A PET ligands. RESULTS: [18F]UCB-H pharmacokinetic data was optimally fitted with a two-compartment model (2TCM), although the model did not always converge (large total volume of distribution (VT) or large uncertainty of the estimate). 2TCM with coupled fit K1/k2 across brain regions stabilized the quantification, and confirmed a lower specific signal of [18F]UCB-H compared to the newest SV2A-ligands. However, the measures of VND and the influx parameter (K1) are similar to what has been reported for other SV2A ligands. These data were reinforced by displacement studies using [19F]UCB-H, demonstrating only 50% displacement of the total [18F]UCB-H signal at maximal occupancy of SV2A. As previously demonstrated in clinical studies, the graphical method of Logan provided a more robust estimate of VT with only a small bias compared to 2TCM. CONCLUSIONS: Modeling issues with a 2TCM due to a slow component have previously been reported for other SV2A ligands with low specific binding, or after blocking of specific binding. As all SV2A ligands share chemical structural similarities, we hypothesize that this slow binding component is common for all SV2A ligands, but only hampers quantification when specific binding is low.

[(11)C]SL25.1188, a new reversible radioligand to study the monoamine oxidase type B with PET: preclinical characterisation in nonhuman primate.

[(11)C]SL-25.1188 [(S)-5-methoxymethyl-3-[6-(4,4,4-trifluorobutoxy)-benzo[d]isoxazol-3-yl]-oxazolidin-2-one], an oxazolidinone derivative, was characterized in baboons as a radioligand for the in vivo visualization of MAO-B using positron emission tomography (PET). After i.v. injection, [(11)C]SL25.1188 presented a rapid phase of distribution in blood (about 5 min), followed by a T(1/2) elimination of 85 +/- 14 min. Plasma metabolism analysis showed that [(11)C]SL25.1188 is stable in vivo at least for 30 min. Brain uptake was rapid with the highest one observed in the striatum and thalamus, and the lowest in the pons. Calculated distribution volumes (V(T)) were as follows: striatum = 10.3, thalamus = 10.9, hippocampus = 8.9, temporal cortex = 7.7, occipital cortex = 7.2, parietal cortex = 7.4, frontal cortex = 7.4, white matter = 7.4, and pons = 6.1. Pretreatment with deprenyl (2 mg/kg, i.v.) or lazabemide (0.5 mg/kg, i.v.) reduced V(T) values in all brain areas up to 50%. In displacement experiments, injection of SL25.1188 or deprenyl (1 and 2 mg/kg, i.v., respectively) strongly reduced the specific uptake of [(11)C]SL25.1188 in all brain areas (85-100%), while a lesser displacement was observed with lazabemide (0.5 mg/kg, i.v.) (55-70% of specific binding depending on the brain area). Therefore, [(11)C]SL25.1188 is characterized in vivo by reversible binding, high brain uptake and very slow plasma metabolism, strongly suggesting that this radioligand is a potent tool for the in vivo study of brain MAO-B.

Radiosynthesis of (E)-N-(2-[11C]methoxybenzyl)-3-phenyl-acrylamidine, a novel subnanomolar NR2B subtype-selective NMDA receptor antagonist.

Recently, a novel series of amidines has been described, exhibiting high NR2B-subtype selective N-methyl-D-aspartate (NMDA) antagonist activity with nanomolar or subnanomolar affinity. Within the styrylamidine subclass, (E)-N-(2-methoxybenzyl)-3-phenyl-acrylamidine (1), displayed the highest affinity (Ki=0.7 nM versus [(3)H]ifenprodil) and was considered an appropriate candidate for isotopic labelling with carbon-11 (T(1/2): 20.38 min) at its methoxy group for imaging of NMDA receptors with PET. Derivative 1 has been labelled from the corresponding nor-analogue using [(11)C]methyl triflate and the following experimental conditions : (1) trapping at -10 degrees C of [(11)C]methyl triflate in 300 microL of acetone containing 0.6-0.8 mg of precursor 5 (2.4-3.2 micromol) and 5 microL of a 3M solution of NaOH in water (about 5 eq.); (2) concentration to dryness of the reaction mixture (at 110 degrees C, using a helium stream for 1-2 min); (3) taking up the residue with 0.5 mL of the HPLC mobile phase and (4) purification using semi-preparative HPLC (SymmetryPrep) C-18, Waters, 300 x 7.8 mm). Typically, starting from a 1.5 Ci (55.5 GBq) [(11)C]CO(2) production batch, 120-240 m Ci (4.44-8.88 GBq) of [(11)C]-1 (20-40% decay-corrected radiochemical yield, n=5) was obtained within a total synthesis time of 25-30 min. Specific radioactivities ranged from 0.8 to 1.2 Ci/micromol (29.6-44.4 GBq/micromol) at the end of radiosynthesis. No attempts were made to further optimise these reactions, as sufficient material was obtained to allow for preliminary pharmacological characterisation.

Multiinjection approach for D2 receptor binding quantification in living rats using [11C]raclopride and the beta-microprobe: crossvalidation with in vitro binding data.

The purpose of this study was to quantify D2 receptors density and affinity in living rats using [11C]raclopride and to validate the multiinjection modelling approach. To this aim, we used an intracerebral beta+-sensitive probe as a highly sensitive system to quantify the radioligand activity using a single three-injection experimental paradigm. The study was divided into three main parts: (i) [11C]raclopride catabolism evaluation without and with cimetidine pretreatment (cytochrome P450 inhibitor); (ii) quantification of kinetics parameters in the striatum, enthorinal cortex, and cerebellum of living rats using a three-compartment model with an arterial input function; (iii) correlation study of in vivo and in vitro binding density and affinity values in the same striatal tissues. (i) raclopride catabolism was very reproducible between individuals; cimetidine pre-treatment resulted in a 30% reduction of raclopride metabolites. (ii) D2 striatal B'max and KdVr estimates obtained by compartmental modelling were 19.87+/-6.45 and 6.2+/-3.3 nmol/L, respectively. Cerebellum is the best candidate as a reference region with no specific binding detectable in vivo. (iii) When comparing density (Bmax/B'max) and affinity (Kd/KdVr) values in vivo and in vitro for each striatum, a high strict correlation was found (r2=0.90 and 0.72, for density and affinity, respectively). These results validate the multi-injection modelling approach coupled to beta-microprobe acquisitions as a mean to provide accurate and separate estimates of dopamine D2-receptor density and affinity, in the living rodent striatum.

The neuroprotective agent CNTF decreases neuronal metabolites in the rat striatum: an in vivo multimodal magnetic resonance imaging study.

Ciliary neurotrophic factor (CNTF) is neuroprotective against multiple pathologic conditions including metabolic impairment, but the mechanisms are still unclear. To delineate CNTF effects on brain energy homeostasis, we performed a multimodal imaging study, combining in vivo proton magnetic resonance spectroscopy, high-performance liquid chromatography analysis, and in situ glutamate imaging by chemical exchange saturation transfer. Unexpectedly, we found that CNTF expression through lentiviral gene transfer in the rat striatum significantly decreased the levels of neuronal metabolites (N-acetyl-aspartate, N-acetyl-aspartyl-glutamate, and glutamate). This preclinical study shows that CNTF remodels brain metabolism, and suggests that decreased levels of neuronal metabolites may occur in the absence of neuronal dysfunction.

Human ESC-derived dopamine neurons show similar preclinical efficacy and potency to fetal neurons when grafted in a rat model of Parkinson's disease.

Considerable progress has been made in generating fully functional and transplantable dopamine neurons from human embryonic stem cells (hESCs). Before these cells can be used for cell replacement therapy in Parkinson's disease (PD), it is important to verify their functional properties and efficacy in animal models. Here we provide a comprehensive preclinical assessment of hESC-derived midbrain dopamine neurons in a rat model of PD. We show long-term survival and functionality using clinically relevant MRI and PET imaging techniques and demonstrate efficacy in restoration of motor function with a potency comparable to that seen with human fetal dopamine neurons. Furthermore, we show that hESC-derived dopamine neurons can project sufficiently long distances for use in humans, fully regenerate midbrain-to-forebrain projections, and innervate correct target structures. This provides strong preclinical support for clinical translation of hESC-derived dopamine neurons using approaches similar to those established with fetal cells for the treatment of Parkinson's disease.

Mapping the cerebral monoamine oxidase type A: positron emission tomography characterization of the reversible selective inhibitor [11C]befloxatone.

Befloxatone is a competitive and reversible inhibitor of monoamine oxidase-A (MAOI-A). The aim of the study was to characterize the in vivo properties of [(11)C]befloxatone and to validate its use as a ligand for the study of MAO-A by positron emission tomography (PET). PET studies were performed in baboons after i.v. injection of [(11)C]befloxatone (551 +/- 70 MBq, i.e.14.9 +/- 1.9 mCi). [(11)C]Befloxatone enters rapidly in the brain with a maximum uptake at 30 min. Brain concentration of the tracer is high in thalamus, striatum, pons and cortical structures (1.5-1.8% of injected dose per 100 ml of tissue), and lower in cerebellum (1.07% injected dose/100 ml). Nonsaturable uptake, obtained after a pretreatment with a high dose of nonlabeled befloxatone (0.4 mg/kg), is very low and represents only 3% of the total uptake. Brain uptake of [(11)C]befloxatone is not altered by a pretreatment of a high dose with lazabemide (0.5 mg/kg i.v.), a selective MAOI-B but is completely blocked by a pretreatment with moclobemide (MAOI-A; 10 mg/kg). This confirms, in vivo, the selectivity of befloxatone for type A MAO. [(11)C]Befloxatone brain radioactivity was displaced by administration of unlabeled befloxatone (30 min after the tracer injection). The displacement of the tracer from its binding sites is dose-dependent, with an ID(50) of 0.02 mg/kg for all studied structures. These results indicate that [(11)C]befloxatone will be an excellent probe for the study of MAO-A in humans using PET.

Synthesis and in vivo imaging properties of [11C]befloxatone: a novel highly potent positron emission tomography ligand for mono-amine oxidase-A.

Befloxatone (1, (5R)-5-(methoxymethyl)-3-[4-[(3R)-4,4,4-trifluoro-3-hydroxybutoxy]phenyl]-2-oxazolidinone) is an oxazolidinone derivative belonging to a new generation of reversible and selective mono-amine oxidase-A (MAO-A) inhibitors. In vitro and ex vivo studies have demonstrated that befloxatone is a potent, reversible and competitive MAO-A inhibitor with potential antidepressant properties. Befloxatone (1) was labelled with carbon-11 (t(12): 20.4 min) using [(11)C]phosgene as reagent. Typically, starting from a 1.2 Ci (44.4 GBq) cyclotron-produced [(11)C]CH(4) batch, 150-300 mCi (5.55-11.10 GBq) of [(11)C]befloxatone ([(11)C]-1) with a radiochemical- and chemical purity of more than 99% were routinely obtained within 20 min of radiosynthesis (including HPLC purification) with specific radioactivities of 500-2000 mCi/micromol (18.5-74.0 GBq/micromol). The results obtained in vivo with carbon-11-labelled befloxatone not only confirm the biochemical and pharmacological profile of befloxatone found in rodent and in human tissues but also point out [(11)C]befloxatone as an excellent tool for the assessment of MAO-A binding sites using positron emission tomography, a high-resolution, sensitive, non-invasive and quantitative imaging technique.

Synthesis, radiosynthesis and in vivo evaluation of 5-[3-(4-benzylpiperidin-1-yl)prop-1-ynyl]-1,3-dihydrobenzoimidazol-2-[(11)C]one, as a potent NR(1A)/2B subtype selective NMDA PET radiotracer.

Recently, a new series of potent and highly subtype-selective 1-(heteroarylalkynyl)-4-benzylpiperidine antagonists of the NMDA receptors has been described by Pfizer Laboratories. In this series, 5-[3-(4-benzylpiperidin-1-yl)prop-1-ynyl]-1,3-dihydrobenzoimidazol-2-one (1) was identified as a selective antagonist for the NR1(A)/2B subtype, displaying IC(50) values for inhibition of the NMDA responses of 5.3 nM for this subtype (compared to NR1(A)/2A: 35 microM and NR1(A)/2C>100 microM) and was active in rat at a relatively low dosage (10mg/kg po). Derivative 1 has been synthesized in four chemical steps in good overall yield and labelled with carbon-11 at its benzoimidazolone ring using [(11)C]phosgene. The pharmacological profile of [(11)C]-1 was evaluated in vivo in rats with biodistribution studies and brain radioactivity monitored with intracerebral radiosensitive beta-microprobes. The brain uptake of [(11)C]-1 was extremely low (0.07% I.D./mL on average at 30 min) and rather uniform across the different brain structures. This in vivo brain regional distribution of [(11)C]-1 did not match with autoradiographic or binding data obtained with other NR2B subtype-selective NMDA ligands. Competition studies with ifenprodil (20 mg/kg, ip, 30 min before the radiotracer injection) failed to demonstrate specific binding of the radiotracer in the brain. In view of these results, and especially considering the low brain penetration of the radiotracer, [(11)C]-1 does not have the required properties for imaging NMDA receptors using positron emission tomography.

Synthesis of a fluorine-18-labelled derivative of 6-nitroquipazine, as a radioligand for the in vivo serotonin transporter imaging with PET.

Considerable efforts have been engaged in the design, synthesis and pharmacological characterization of radioligands for imaging the serotonin transporter, based on its implication in several neuropsychiatric diseases, such as depression, anxiety and schizophrenia. In the 5-halo-6-nitroquipazine series, the fluoro derivative has been designed for positron emission tomography (PET). The corresponding 5-iodo-, 5-bromo- and 5-chloro N-Boc-protected quipazines as labelling precursors, as well as 5-fluoro-6-nitroquipazine as a reference compound have been synthesized. 5-[(18)F]Fluoro-6-nitroquipazine has been radiolabelled with fluorine-18 (positron-emitting isotope, 109.8 min half-life) by nucleophilic aromatic substitution from the corresponding N-Boc protected 5-bromo- and 5-chloro-precursors using K[(18)F]F-K(222) complex in DMSO by conventional heating (145 degrees C, 2 min) or microwave activation (50 W, 30-45 s), followed by removal of the protective group with TFA. Typically, 15-25 mCi (5.5-9.2 GBq) of 5-[(18)F]fluoro-6-nitroquipazine (1-2 Ci/micromol or 37-72 GBq/micromol) could be obtained in 70-80 min starting from a 550-650 mCi (20.3-24.0 GBq) aliquot of a cyclotron [(18)F]F(-) production batch (2.7-3.8% non decay-corrected yield based on the starting [(18)F]fluoride). Ex vivo studies (biodistribution in rat), as well as PET imaging (in monkey) demonstrated that 5-[(18)F]fluoro-6-nitroquipazine ([(18)F]-1d) readily crossed the blood brain barrier and accumulated in the regions rich in 5-HT transporter (frontal- and posterial cortex, striata). However, the low accumulation of the tracer in the thalamus (rat and monkey) as well as the comparable displacement of the tracer observed with both citalopram, a -HT re-uptake inhibitor and maprotiline, a norepinephrine re-uptake inhibitor (rat), indicate that 5-[(18)F]fluoro-6-nitroquipazine ([(18)F]-1d) does not have the suggested potential for PET imaging of the serotin transporter (SERT).