Cristian Salinas

Imaging dopamine receptors in humans with [11C]-(+)-PHNO: Dissection of D3 signal and anatomy

[(11)C]-(+)-PHNO is a D3 preferring PET radioligand which has recently opened the possibility of imaging D3 receptors in the human brain in vivo. This imaging tool allows characterisation of the distribution of D3 receptors in vivo and further investigation of their functional role. The specific [(11)C]-(+)-PHNO signal is a mixture of D3 and D2 components with the relative magnitude of each component determined by the regional receptor densities. An accurate and reproducible delineation of regions of interest (ROI) is therefore important for optimal analysis of human PET data. We present a set of anatomical guidelines for the delineation of D3 relevant ROIs including substantia nigra, hypothalamus, ventral pallidum/substantia innominata, ventral striatum, globus pallidus and thalamus. Delineation of these structures using this approach allowed for high intra- and inter-operator reproducibility. Subsequently we used a selective D3 antagonist to dissect the total [(11)C]-(+)-PHNO signal in each region into its D3 and D2 components and estimated the regional fraction of the D3 signal (f(PHNO)(D3)). In descending order of magnitude the following results for the f(PHNO)(D3) were obtained: hypothalamus=100%, substantia nigra=100%, ventral pallidum/substantia innominata=75%, globus pallidus=65%, thalamus=43%, ventral striatum=26% and precommissural-ventral putamen=6%. An automated approach for the delineation of these anatomical regions of interest was also developed and investigated in terms of its reproducibility and accuracy.

Prediction of repeat-dose occupancy from single-dose data: characterisation of the relationship between plasma pharmacokinetics and brain target occupancy

Positron emission tomography (PET) is used in drug development to assist dose selection and to establish the relationship between blood and tissue pharmacokinetics (PKs). We present a new biomathematical approach that allows prediction of repeat-dose (RD) brain target occupancy (TO) using occupancy data obtained after administration of a single dose (SD). A PET study incorporating a sequential adaptive design was conducted in 10 healthy male adults who underwent 4 PET scans with [11C]DASB ([11C]N,N-dimethyl-2-(2-amino-4-cyanophenylthio) benzylamine): 1 at baseline, 2 after 20 mg SD of the 5-hydroxytryptamine transporter (5-HTT) inhibitor duloxetine, and 1 after 4 days daily administration of 20 mg duloxetine. An adaptive design was used to select optimal times after SD for measurement of occupancy. Both direct and indirect PK/TO models were fitted to the SD data to characterise the model parameters and then applied to a predicted RD duloxetine plasma time course to predict the 5-HTT occupancy after RD. Repeat-dose prediction from the indirect model (OC50=2.62±0.93 ng/mL) was significantly better (P<0.05) than that from the direct model (OC50=2.29±1.11 ng/mL). This approach increases the value of SD occupancy studies that are performed as part of first time in human drug development programmes by providing an estimate of the dose required to achieve the desired TO at RD.

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.

Translational characterization of [11C]GSK931145, a PET ligand for the glycine transporter type 1

The current interest in developing Glycine transporter Type 1 (GlyT‐1) inhibitors, for diseases such as schizophrenia, has led to the demand for a GlyT‐1 PET molecular imaging tool to aid drug development and dose selection. We report on [11C]GSK931145 as a novel GlyT‐1 imaging probe in primate and man. Primate PET studies were performed to determine the level of specific binding following homologous competition with GSK931145 and the plasma‐occupancy relationship of the GlyT‐1 inhibitor GSK1018921. Human PET studies were performed to determine the test–retest reproducibility of [11C]GSK931145 and the plasma‐occupancy relationship of GSK1018921. [11C]GSK931145 entered primate and human brain and yielded a heterogeneous pattern of uptake which was similar in both species with highest uptake in midbrain, thalamus, and cerebellum. Homologous competition in primates indicated no viable reference region and gave binding potential estimates between 1.5 and 3 for midbrain, thalamus and cerebellum, While the distribution and binding potential values were similar across species, both the plasma free fraction (fP: 0.8 vs. 8%) and delivery (K1: 0.025 vs. 0.126 ml cm−3 min−1) were significantly lower in humans. Test–retest reproducibility in humans calculated using a two tissue compartmental model was poor (VAR(VT): 29–38%), but was improved using a pseudo reference tissue model (VAR(BPND): 16–23%). GSK1018921 EC50 estimates were 22.5 and 45.7 ng/ml in primates and humans, respectively. Synapse, 2011.

Combining PET biodistribution and equilibrium dialysis assays to assess the free brain concentration and BBB transport of CNS drugs.

The passage of drugs in and out of the brain is controlled by the blood—brain barrier (BBB), typically, using either passive diffusion across a concentration gradient or active transport via a protein carrier. In-vitro and preclinical measurements of BBB penetration do not always accurately predict the in-vivo situation in humans. Thus, the ability to assay the concentration of novel drug candidates in the human brain in vivo provides valuable information for derisking of candidate molecules early in drug development. Here, positron emission tomography (PET) measurements are combined with in-vitro equilibrium dialysis assays to enable assessment of transport and estimation of the free brain concentration in vivo. The PET and equilibrium dialysis data were obtained for 36 compounds in the pig. Predicted P-glycoprotein (P-gp) status of the compounds was consistent with the PET/equilibrium dialysis results. In particular, Loperamide, a well-known P-gp substrate, exhibited a significant concentration gradient consistent with active efflux and after inhibition of the P-gp process the gradient was removed. The ability to measure the free brain concentration and assess transport of novel compounds in the human brain with combined PET and equilibrium dialysis assays can be a useful tool in central nervous system (CNS) drug development.

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.

Discovery of 6-(Fluoro-18F)-3-(1H-pyrrolo[2,3-c]pyridin-1-yl)isoquinolin-5-amine ([18F]-MK-6240): A Positron Emission Tomography (PET) Imaging Agent for Quantification of Neurofibrillary Tangles (NFTs)

Neurofibrillary tangles (NFTs) made up of aggregated tau protein have been identified as the pathologic hallmark of several neurodegenerative diseases including Alzheimers disease. In vivo detection of NFTs using PET imaging represents a unique opportunity to develop a pharmacodynamic tool to accelerate the discovery of new disease modifying therapeutics targeting tau pathology. Herein, we present the discovery of 6-(fluoro-(18)F)-3-(1H-pyrrolo[2,3-c]pyridin-1-yl)isoquinolin-5-amine, 6 ([(18)F]-MK-6240), as a novel PET tracer for detecting NFTs. 6 exhibits high specificity and selectivity for binding to NFTs, with suitable physicochemical properties and in vivo pharmacokinetics.

Radiosynthesis and Characterization of 11C-GSK215083 as a PET Radioligand for the 5-HT6 Receptor

The development of a PET radioligand for imaging 5-hydroxytryptamine (5-HT) 6 receptors in the brain would, for the first time, enable in vivo imaging of this target along with assessment of its involvement in disease pathophysiology. In addition, such a tool would assist in the development of novel drugs targeting the 5-HT6 receptor. Methods: On the basis of in vitro data, GSK215083 was identified as a promising 5-HT6 radioligand candidate and was radiolabeled with 11C via methylation. The in vivo properties of 11C-GSK215083 were evaluated first in pigs (to investigate brain penetration and specific binding), second in nonhuman primates (to confirm brain penetration, specific binding, selectivity, and kinetics), and third in human subjects (to confirm brain penetration and biodistribution). Results: 11C-GSK215083 readily entered the brain in all 3 species, leading to a heterogeneous distribution (striatum > cortex > cerebellum) consistent with reported 5-HT6 receptor densities and distribution determined by tissue-section autoradiography in preclinical species and humans. In vivo saturation studies using escalating doses of GSK215083 in primates demonstrated saturable, dose-dependent binding to the 5-HT6 receptor in the striatum. Importantly, 11C-GSK215083 also exhibited affinity for the 5-HT2A receptor; however, given the differential localization of these 2 receptors in the central nervous system, the discrete 5-HT6 binding properties of this radioligand were able to be determined. Conclusion: These data demonstrate the utility of 11C-GSK215083 as a promising PET radioligand for probing the 5-HT6 receptor in vivo in both preclinical and clinical species.

Identification and evaluation of [11C]GSK931145 as a novel ligand for imaging the type 1 glycine transporter with positron emission tomography.

The type‐1 glycine transporter (GlyT1) is an important target for the development of new medications for schizophrenia. A specific and selective positron emission tomography (PET) GlyT1 ligand would facilitate drug development studies to determine whether a drug reaches this target and help establish suitable doses for clinical trials. This article describes the evaluation of three candidate GlyT1 PET radioligands (GSK931145, GSK565710, and GSK991022) selected from a library of compounds based on favorable physicochemical and pharmacological properties. Each candidate was successfully labeled using [11C]methyl iodide or [11C]methyl triflate and administered to a pig pre‐ and postadministration with a pharmacological dose of a GlyT1 inhibitor to determine their suitability as PET ligands in the porcine brain in vivo. All three candidate ligands were analyzed quantitatively with compartment analyses employing a plasma input function. [11C]GSK931145 showed good brain penetration and a heterogeneous distribution in agreement with reported GlyT1 localization. Following pretreatment with GSK565710, uptake of [11C]GSK931145 was reduced to homogeneous levels. Although [11C]GSK565710 also showed good brain penetration and a heterogeneous distribution, the apparent level of specific binding was reduced compared to [11C]GSK931145. In contrast, [11C]GSK991022 showed a much lower brain penetration and resultant signal following pretreatment with GSK565710. Based on these findings [11C]GSK931145 was identified as the most promising ligand for imaging GlyT1 in the porcine brain, possessing good brain penetration, specific signal, and reversible kinetics. [11C]GSK931145 is now being progressed into higher species. Synapse 64:542–549, 2010.

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.