Christophe Plisson

In vivo quantification of regional dopamine-D3 receptor binding potential of (+)-PHNO: Studies in non-human primates and transgenic mice.

Examination of dopamine‐D3 (D3) receptors with positron emission tomography (PET) have been hampered in the past by the lack of a PET ligand with sufficient selectivity for D3 over dopamine‐D2 (D2) receptors. The two types co‐localize in the brain, with D2 density significantly higher than D3, hence nonselective PET ligands inform on D2, rather than D3 status. [11C]‐(+)‐PHNO is a novel PET ligand with a preferential affinity for D3 over D2. We used the selective D3 antagonist, SB‐277011 to dissect regional fractions of the [11C]‐(+)‐PHNO signal attributable to D3 and D2 in primate brain. The results were compared with quantitative autoradiography with 3H‐(+)‐PHNO in wild‐type, D2‐knock‐out, and D3‐knock‐out mice examined at baseline and following administration of SB‐277011. Both sets of results converged to indicate a predominant D3‐related component to (+)‐PHNO binding in extra‐striatal regions, with binding in the midbrain being entirely attributable to D3. The midbrain is thus an excellent target region to examine D3 receptor occupancy with [11C]‐(+)‐PHNO PET in vivo. Synapse 63:782–793, 2009.

Within-Subject Comparison of [11C]-( + )-PHNO and [11C]raclopride Sensitivity to Acute Amphetamine Challenge in Healthy Humans

[11C]PHNO is a D2/D3 agonist positron emission tomography radiotracer, with higher in vivo affinity for D3 than for D2 receptors. As [11C]-( + )-PHNO is an agonist, its in vivo binding is expected to be more affected by acute fluctuations in synaptic dopamine than that of antagonist radiotracers such as [11C]raclopride. In this study, the authors compared the effects of an oral dose of the dopamine releaser amphetamine (0.3 mg/kg) on in vivo binding of [11C]-( + )-PHNO and [11C]raclopride in healthy subjects, using a within-subjects, counterbalanced, open-label design. In the dorsal striatum, where the density of D3 receptors is negligible and both tracers predominantly bind to D2 receptors, the reduction of [11C]-( + )-PHNO binding potential (BPND) was 1.5 times larger than that of [11C]raclopride. The gain in sensitivity associated with the agonist [11C]-( + )-PHNO implies that ~65% of D2 receptors are in the high-affinity state in vivo. In extrastriatal regions, where [11C]-( + )-PHNO predominantly binds to D3 receptors, the amphetamine effect on [11C]-( + )-PHNO BPND was even larger, consistent with the higher affinity of dopamine for D3. This study indicates that [11C]- ( + )-PHNO is superior to [11C]raclopride for studying acute fluctuations in synaptic dopamine in the human striatum. [11C]-( + )-PHNO also enables measurement of synaptic dopamine in D3 regions.

Evaluation of 11C-GSK189254 as a Novel Radioligand for the H3 Receptor in Humans Using PET

The histamine H3 receptor is implicated in the pathophysiology of several central nervous system disorders. N-methyl-6-(3-cyclobutyl-2,3,4,5-tetrahydro-1H-benzo[d]azepin-7-yloxy)-nicotamide (GSK189254) is a highly potent, selective, and brain-penetrant H3 receptor antagonist. Previous studies in the pig using PET have shown that 11C-GSK189254 uptake in H3-rich regions of the brain can be blocked by the selective H3 antagonist ciproxifan. The purpose of the present study was to evaluate 11C-GSK189254 as a PET radioligand for human studies and to determine the dose–receptor occupancy relationship of GSK189254 in the human brain. Methods: Dynamic PET scans were obtained in healthy subjects over 90 min after intravenous administration of approximately 370 MBq of 11C-GSK189254. Blood samples were taken throughout the scans to derive the arterial plasma parent input function. Each subject was scanned twice, either with tracer alone (test–retest) or before and after a single oral dose of GSK189254 (10–100 μg). Data were analyzed by compartmental analysis, and regional receptor-occupancy estimates were obtained by graphical analysis of changes in the total volumes of distribution (VT) of the radioligand. Results: 11C-GSK189254 readily entered the brain; its regional brain distribution reflected the known distribution of H3 receptors, with high binding in the caudate and putamen, intermediate binding in cortical regions, and low binding in the cerebellum. GSK189254 displayed a high receptor affinity, and a marked reduction in VT was apparent at all the doses tested. The oral dose equaling 50% occupancy of the available receptor sites (ED50) was estimated as 4.33 μg. Additional data on plasma pharmacokinetics after oral dosing and the plasma free fraction gave a corresponding estimate of the free concentration of GSK189254 required to occupy 50% of the available receptor sites (EC50) (0.011 nM). The test–retest data showed reductions in regional VT on the second scan in all subjects. A nonlinear compartmental analysis of this effect demonstrated that this reduction was consistent with carryover of a tracer mass dose effect with an estimated in vivo apparent dissociation constant of 0.010 nM, close to the independent estimate of the plasma EC50. Conclusion: 11C-GSK189254 can be used to quantify H3 receptor availability in humans in vivo using PET but requires high specific activity; the possibility of tracer mass dose effects should be carefully analyzed.

Degradative transport of cationic amphiphilic drugs across phospholipid bilayers

Drug molecules must cross multiple cell membrane barriers to reach their site of action. We present evidence that one of the largest classes of pharmaceutical drug molecules, the cationic amphiphilic drugs (CADs), does so via a catalytic reaction that degrades the phospholipid fabric of the membrane. We find that CADs partition rapidly to the polar–apolar region of the membrane. At physiological pH, the protonated groups on the CAD catalyse the acid hydrolysis of the ester linkage present in the phospholipid chains, producing a fatty acid and a single-chain lipid. The single-chain lipids rapidly destabilize the membrane, causing membranous fragments to separate and diffuse away from the host. These membrane fragments carry the drug molecules with them. The entire process, from drug adsorption to drug release within micelles, occurs on a time-scale of seconds, compatible with in vivo drug diffusion rates. Given the rate at which the reaction occurs, it is probable that this process is a significant mechanism for drug transport.

Radiosynthesis and in vivo evaluation of [11C]MP-10 as a positron emission tomography radioligand for phosphodiesterase 10A

INTRODUCTION The aim of this study was to evaluate a newly reported positron emission tomography (PET) radioligand [(11)C]MP-10, a potent and selective inhibitor of the central phosphodiesterase 10A enzyme (PDE10A) in vivo, using PET. METHODS A procedure was developed for labeling MP-10 with carbon-11. [(11)C]MP-10 was evaluated in vivo both in the pig and baboon brain. RESULTS Alkylation of the corresponding desmethyl compound with [(11)C]methyl iodide produced [(11)C]MP-10 with good radiochemical yield and specific activity. PET studies in the pig showed that [(11)C]MP-10 rapidly entered the brain reaching peak tissue concentration at 1-2 min postadministration, followed by washout from the tissue. Administration of a selective PDE10A inhibitor reduced the binding in all brain regions to the levels of the cerebellum, demonstrating the saturability and selectivity of [(11)C]MP-10 binding. In the nonhuman primate, the brain tissue kinetics of [(11)C]MP-10 were slower, reaching peak tissue concentrations at 30-60 min postadministration. In both species, the observed rank order of regional brain signal was striatum>diencephalon>cortical regions=cerebellum, consistent with the known distribution and concentration of PDE10A. [(11)C]MP-10 brain kinetics were well described by a two-tissue compartment model, and estimates of total volume of distribution (V(T)) were obtained. Blocking studies with unlabeled MP-10 revealed the suitability of the cerebellum as a reference tissue and enabled the estimation of regional binding potential (BP(ND)) as the outcome measure of specific binding. Quantification of [(11)C]MP-10 binding using the simplified reference tissue model with cerebellar input function produced BP(ND) estimates consistent with those obtained by the two-tissue compartment model. CONCLUSION We demonstrated that [(11)C]MP-10 possesses good characteristics for the in vivo quantification of the PDE10A in the brain by PET.

Phosphodiesterase 10A PET Radioligand Development Program: From Pig to Human

Four novel phosphodiesterase 10A (PDE10A) PET tracers have been synthesized, characterized in preclinical studies, and compared with the previously reported 11C-MP-10. Methods: On the basis of in vitro data, IMA102, IMA104, IMA107, and IMA106 were identified as potential PDE10A radioligand candidates and labeled with either 11C via N-methylation or with 18F through an SN2 reaction, in the case of IMA102. These candidates were compared with 11C-MP-10 in pilot in vivo studies in the pig brain. On the basis of these data, 11C-IMA106 and 11C-IMA107 were taken into further evaluation and comparison with 11C-MP-10 in the primate brain. Finally, the most promising radioligand candidate was progressed into human evaluation. Results: All 5 tracers were produced with good radiochemical yield and specific activity. All candidates readily entered the brain and demonstrated a heterogeneous distribution consistent with the known expression of PDE10A. Baseline PET studies in the pig and baboon showed that 11C-IMA107 and 11C-MP-10 displayed the most favorable tissue kinetics and imaging properties. The administration of selective PDE10A inhibitors reduced the binding of 11C-IMA107 and 11C-MP-10 in the PDE10A-rich brain regions, in a dose-dependent manner. In the nonhuman primate brain, the tissue kinetics of 11C-IMA107 and 11C-MP-10 were well described by a 2-tissue-compartment model, allowing robust estimates of the regional total volume of distribution. Blockade with unlabeled MP-10 confirmed the suitability of the cerebellum as a reference tissue and enabled the estimation of regional binding potential as the outcome measure of specific binding. Conclusion: 11C-IMA107 was identified as the ligand with the highest binding potential while still possessing reversible kinetics. The first human administration of 11C-IMA107 has demonstrated the expected regional distribution and suitably fast kinetics, indicating that 11C-IMA107 will be a useful tool for the investigation of PDE10A status in the living human brain.

11C-GSK189254: A Selective Radioligand for In Vivo Central Nervous System Imaging of Histamine H3 Receptors by PET

The histamine H3 receptor is a G-protein–coupled presynaptic auto- and heteroreceptor whose activation leads to a decrease in the release of several neurotransmitters including histamine, acetycholine, noradrenaline, and dopamine. H3 receptor antagonists such as 6-[(3-cyclobutyl-2,3,4,5-tetrahydro-1H-3-benzazepin-7-yl)oxy]-N-methyl-3-pyridinecarboxamide hydrochloride (GSK189254) can increase the release of these neurotransmitters and thus may offer potential therapeutic benefits in diseases characterized by disturbances of neurotransmission. The aim of this study was to synthesize and evaluate 11C-labeled GSK189254 (11C-GSK189254) for imaging the histamine H3 receptor in vivo by PET. Methods: GSK189254 exhibits high affinity (0.26 nM) and selectivity for the human histamine H3 receptor. Autoradiography experiments were performed using 3H-GSK189254 to evaluate its in vitro binding in porcine brain tissues. GSK189254 was labeled by N-alkylation using 11C-methyl iodide in good yields, radiochemical purity, and specific activity. A series of PET experiments was conducted to investigate 11C-GSK189254 binding in the porcine brain. Results: In vitro autoradiography demonstrated specific 3H-GSK189254 binding in the porcine brain; therefore, 11C-GSK189254 was evaluated in vivo in pigs and showed good brain penetration and high uptake in regions such as the striatum and cortices, known to contain high densities of the histamine H3 receptors. The radioligand kinetics were reversible, and quantitative analysis was achieved with a 2-tissue-compartmental model yielding the distribution volume as the outcome measure of interest. The distribution volume was reduced to a homogeneous level in all regions after blocking by the coadministration of either unlabeled GSK189254 or ciproxifan, a structurally distinct histamine H3 antagonist. Further coadministration studies allowed for the estimation of the radioligand affinity (0.1 nM) and the density of histamine H3 receptor sites in the cerebellum (0.74 nM), cortex (2.05 nM), and striatum (2.65 nM). Conclusion: These findings suggest that 11C-GSK189254 possesses appropriate characteristics for the in vivo imaging of the histamine H3 receptor by PET.

In vivo imaging of microglial activation by positron emission tomography with [11C]PBR28 in the 5XFAD model of Alzheimer's disease

Microglial activation has been linked with deficits in neuronal function and synaptic plasticity in Alzheimers disease (AD). The mitochondrial translocator protein (TSPO) is known to be upregulated in reactive microglia. Accurate visualization and quantification of microglial density by PET imaging using the TSPO tracer [11C]‐R‐PK11195 has been challenging due to the limitations of the ligand. In this study, it was aimed to evaluate the new TSPO tracer [11C]PBR28 as a marker for microglial activation in the 5XFAD transgenic mouse model of AD. Dynamic PET scans were acquired following intravenous administration of [11C]PBR28 in 6‐month‐old 5XFAD mice and in wild‐type controls. Autoradiography with [3H]PBR28 was carried out in the same brains to further confirm the distribution of the radioligand. In addition, immunohistochemistry was performed on adjacent brain sections of the same mice to evaluate the co‐localization of TSPO with microglia. PET imaging revealed that brain uptake of [11C]PBR28 in 5XFAD mice was increased compared with control mice. Moreover, binding of [3H]PBR28, measured by autoradiography, was enriched in cortical and hippocampal brain regions, coinciding with the positive staining of the microglial marker Iba‐1 and amyloid deposits in the same areas. Furthermore, double‐staining using antibodies against TSPO demonstrated co‐localization of TSPO with microglia and not with astrocytes in 5XFAD mice and human post‐mortem AD brains. The data provided support of the suitability of [11C]PBR28 as a tool for in vivo monitoring of microglial activation and assessment of treatment response in future studies using animal models of AD. GLIA 2016;64:993–1006

Characterization of in vivo pharmacological properties and sensitivity to endogenous serotonin of [11C] P943: A positron emission tomography study in Papio anubis

[11C] P943 is a recently developed PET radiotracer for serotonin 5‐HT1B receptors. We characterized a number of its in vivo pharmacokinetic properties, including the evaluation of its two stereo‐isomers, saturability of specific binding, selectivity for 5‐HT1B and 5‐HT1D receptors, and vulnerability to pharmacologically induced increases in endogenous 5‐HT levels. Six isoflurane‐anesthetized baboons were scanned with [11C] P943 at baseline, and following various pharmacological manipulations. The interventions included the administration of pharmacological doses of P943, SB‐616234‐S (a 5‐HT1B selective antagonist), SB‐714786 (a 5‐HT1D selective antagonist), as well as the administration of 5‐HT releasing agents (fenfluramine, amphetamine) and 5‐HT reuptake inhibitor (citalopram). [11C] P943 was observed to bind saturably and specifically to 5‐HT1B receptors and to be sensitive to all three challenges known to alter 5‐HT levels in the proximity of receptors. [11C] P943 shows promise as a tracer to image serotonin function in healthy subjects as well as subjects with psychiatric or neurologic conditions. Synapse, 2011.

One-pot multi-tracer synthesis of novel (18)F-labeled PET imaging agents.

(18)F labeled phosphonium salts are increasingly important molecular probes for targeting the mitochondrial membrane potential depletion during apoptosis and for detecting myocardial perfusion deficit. Here, we introduce three new tracers, [(18)F]MitoPhos_04, [(18)F]MitoPhos_05, and [(18)F]MitoPhos_07, that have the potential to act as mitochondrial imaging agents. Moreover, they have the added advantage of being synthesized in the same reaction vial from one radiolabeled synthon, demonstrating a new approach to synthesizing multiple tracers in one-pot, which is a highly useful means for increasing the throughput of radiotracer development. The radiosynthesis of the tracers was carried out on a fully automated system via a facile two-step reaction. Utilizing the radiolabeling of an ethyl azide, a copper-mediated 1,3-cycloaddition reaction and isolation via semiprep high-performance liquid chromatography (HPLC) allowed for the simultaneous synthesis of two or three tracers with a total synthesis time of less than 1 h.