Using positron emission tomography to facilitate CNS drug development

A Blueprint for Translational Precision Medicine in Autism Spectrum Disorder and Related Neurogenetic Syndromes

Objectives: Despite growing knowledge of the underlying neurobiology of autism spectrum disorder (ASD) and related neurogenetic syndromes, treatment discovery has remained elusive. In this review, we provide a blueprint for translational precision medicine in ASD and related neurogenetic syndromes. Methods: The discovery of trofinetide for Rett syndrome (RTT) is described, and the role of nonmammalian, mammalian, and stem cell model systems in the identification of molecular targets and drug screening is discussed. We then provide a framework for translating preclinical findings to human clinical trials, including the role of biomarkers in selecting molecular targets and evaluating target engagement, and discuss how to leverage these findings for future ASD drug development. Results: Multiple preclinical model systems for ASD have been developed, each with tradeoffs with regard to suitability for high-throughput small molecule screening, conservation across species, and behavioral face validity. Future clinical trials should incorporate biomarkers and intermediate phenotypes to demonstrate target engagement. Factors that contributed to the approval of trofinetide for RTT included replicated findings in mouse models, a well-studied natural history of the syndrome, development of RTT-specific outcome measures, and strong engagement of the RTT family community. Conclusions: The translation of our growing understanding of the neurobiology of ASD to human drug discovery will require a precision medicine approach, including the use of multiple model systems for molecular target selection, evaluation of target engagement, and clinical trial design strategies that address heterogeneity, power, and the placebo response.

Radiosynthesis and evaluation of novel 18F labeled PET ligands for imaging monoacylglycerol lipase

Monoacylglycerol lipase (MAGL) is a 33 kDa cytosolic serine hydrolase that is widely distributed in the central nervous system and peripheral tissues. MAGL hydrolyzes monoacylglycerols into fatty acids and glycerol, playing a crucial role in endocannabinoid degradation. Inhibition of MAGL in the brain elevates levels of 2-arachidonoylglycerol and leads to decreased pro-inflammatory prostaglandin and thromboxane production. As such, MAGL is considered a potential target for treating neuropsychiatric disorders, metabolic syndromes, and cancer. Based on a novel spirocyclic system, we synthesized two fluorinated carbamate scaffolds as reversible MAGL inhibitors (epimers: (R)-6, IC50 = 18.6 nM and (S)-6, IC50 = 1.6 nM). In vitro autoradiography studies of [18F](R)-6 (codenamed [18F]MAGL-2304) and [18F](S)-6 (codenamed [18F]MAGL-2305) demonstrated heterogeneous distribution and specific binding affinity to MAGL-rich brain regions. Autoradiography with MAGL knockout mouse brain tissues confirmed the binding specificity of [18F](S)-6. Dynamic PET imaging studies revealed that [18F](S)-6 exhibited limited brain uptake and homogenous distribution in rat brains. In vivo P-gp inhibition enhanced [18F](S)-6 uptake in the brain, suggesting that [18F](S)-6 constitutes a P-gp efflux substrate. This research could provide new directions in the design of MAGL PET ligands that are based on spirocyclic scaffolds.

PET Mapping of Receptor Occupancy Using Joint Direct Parametric Reconstruction

Receptor occupancy (RO) studies using PET neuroimaging play a critical role in the development of drugs targeting the central nervous system (CNS). The conventional approach to estimate drug receptor occupancy consists in estimation of binding potential changes between two PET scans (baseline and post-drug injection). This estimation is typically performed separately for each scan by first reconstructing dynamic PET scan data before fitting a kinetic model to time activity curves. This approach fails to properly model the noise in PET measurements, resulting in poor RO estimates, especially in low receptor density regions.

OBJECTIVE

In this work, we evaluate a novel joint direct parametric reconstruction framework to directly estimate distributions of RO and other kinetic parameters in the brain from a pair of baseline and post-drug injection dynamic PET scans.

METHODS

The proposed method combines the use of regularization on RO maps with alternating optimization to enable estimation of occupancy even in low binding regions.

RESULTS

Simulation results demonstrate the quantitative improvement of this method over conventional approaches in terms of accuracy and precision of occupancy. The proposed method is also evaluated in preclinical in-vivo experiments using 11C-MK-6884 and a muscarinic acetylcholine receptor 4 positive allosteric modulator drug, showing improved estimation of receptor occupancy as compared to traditional estimators.

CONCLUSION

The proposed joint direct estimation framework improves RO estimation compared to conventional methods, especially in intermediate to low-binding regions.

SIGNIFICANCE

This work could potentially facilitate the evaluation of new drug candidates targeting the CNS.

The Bifunctional Dimer Caffeine-Indan Attenuates α-Synuclein Misfolding, Neurodegeneration and Behavioral Deficits after Chronic Stimulation of Adenosine A1 Receptors

We previously found that chronic adenosine A1 receptor stimulation with N6-Cyclopentyladenosine increased α-synuclein misfolding and neurodegeneration in a novel α-synucleinopathy model, a hallmark of Parkinson's disease. Here, we aimed to synthesize a dimer caffeine-indan linked by a 6-carbon chain to cross the blood-brain barrier and tested its ability to bind α-synuclein, reducing misfolding, behavioral abnormalities, and neurodegeneration in our rodent model. Behavioral tests and histological stains assessed neuroprotective effects of the dimer compound. A rapid synthesis of the 18F-labeled analogue enabled Positron Emission Tomography and Computed Tomography imaging for biodistribution measurement. Molecular docking analysis showed that the dimer binds to α-synuclein N- and C-termini and the non-amyloid-β-component (NAC) domain, similar to 1-aminoindan, and this binding promotes a neuroprotective α-synuclein "loop" conformation. The dimer also binds to the orthosteric binding site for adenosine within the adenosine A1 receptor. Immunohistochemistry and confocal imaging showed the dimer abolished α-synuclein upregulation and aggregation in the substantia nigra and hippocampus, and the dimer mitigated cognitive deficits, anxiety, despair, and motor abnormalities. The 18F-labeled dimer remained stable post-injection and distributed in various organs, notably in the brain, suggesting its potential as a Positron Emission Tomography tracer for α-synuclein and adenosine A1 receptor in Parkinson's disease therapy.

Identification of (R)-[18F]YH134 for Monoacylglycerol Lipase Neuroimaging and Exploration of Its Use for Central Nervous System and Peripheral Drug Development

This study aimed to evaluate (R)-[18F]YH134 as a novel PET tracer for imaging monoacylglycerol lipase (MAGL). Considering the ubiquitous expression of MAGL throughout the whole body, the impact of various MAGL inhibitors on (R)-[18F]YH134 brain uptake and its application in brain-periphery crosstalk were explored. Methods: MAGL knockout and wild-type mice were used to evaluate (R)-[18F]YH134 in in vitro autoradiography and PET experiments. To explore the impact of peripheral MAGL occupancy on (R)-[18F]YH134 brain uptake, PET kinetics with an arterial input function were studied in male Wistar rats under baseline and blocking conditions. Results: In in vitro autoradiography, (R)-[18F]YH134 revealed a heterogeneous distribution pattern with high binding to MAGL-rich brain regions in wild-type mouse brain slices, whereas the radioactive signal was negligible in MAGL knockout mouse brain slices. The in vivo brain PET images of (R)-[18F]YH134 in wild-type and MAGL knockout mice demonstrated its high specificity and selectivity in mouse brain. A Logan plot with plasma input function was applied to estimate the distribution volume (V T) of (R)-[18F]YH134. V T was significantly reduced by a brain-penetrant MAGL inhibitor but was unchanged by a peripherally restricted MAGL inhibitor. The MAGL target occupancy in the periphery was estimated using (R)-[18F]YH134 PET imaging data from the brain. Conclusion: (R)-[18F]YH134 is a highly specific and selective PET tracer with favorable kinetic properties for imaging MAGL in rodent brain. Our results showed that blocking of the peripheral target influences brain uptake but not the V T of (R)-[18F]YH134. (R)-[18F]YH134 can be used for estimating the dose of MAGL inhibitor at half-maximal peripheral target occupancy.

Bio-evaluation of poly(lactic-co-glycolic) acid nanoparticles loaded with radiolabelled rifampicin

AIMS

The poly(lactic-co-glycolic) acid (PLGA) nanoparticles of tubercular drugs have been demonstrated to have a sustained release profile over 7 days. There is no information on the location or mode of release of these nanoparticles in living systems. Therefore, we have planned the study to explore the pharmacokinetics and biodistribution of PLGA rifampicin nanoparticles in healthy human volunteers. We aim to study the distribution pattern of PLGA-loaded nano-formulation of radiolabelled rifampicin in humans.

METHODS

Rifampicin was labelled with 99m Tc by indirect method and nanoparticles were prepared by a single emulsion evaporation method. To investigate the pharmacokinetics and biodistribution of nanoparticles, a single dose of 450 mg of rifampicin was administered orally to healthy human volunteers divided into two different groups.

RESULTS

Following a single oral dosage of the rifampicin nanoformulation, the pharmacokinetic (PK) parameters were significantly different between the nanoparticle and conventional groups: area under the concentration-time curve (AUC = 113.8 vs. 58.6; P < .001), mean residence time (MRT = 16.2 vs. 5.8; P < .01) and elimination rate constant (Ke  = 0.04 vs. 0.10; P < .05). Also, Single-photon emission computed tomography/computed tomography (SPECT/CT) images revealed biodistribution of nanoparticles in the distal portions of the intestine, which is consistent with our dosimetry analysis.

CONCLUSIONS

Significant difference in PK parameters and biodistribution of nanoparticles in spleen and lymph nodes with maximum deposition were observed in the large intestine. The nanoparticle distribution pattern may be advantageous for the treatment of intestinal or lymph node tuberculosis (TB) and has the potential to result in a lower dose of rifampicin nanoformulation for the treatment of pulmonary TB.

New Advances in the Exploration of Esterases with PET and Fluorescent Probes

Esterases are hydrolases that catalyze the hydrolysis of esters into the corresponding acids and alcohols. The development of fluorescent probes for detecting esterases is of great importance due to their wide spectrum of biological and industrial applications. These probes can provide a rapid and sensitive method for detecting the presence and activity of esterases in various samples, including biological fluids, food products, and environmental samples. Fluorescent probes can also be used for monitoring the effects of drugs and environmental toxins on esterase activity, as well as to study the functions and mechanisms of these enzymes in several biological systems. Additionally, fluorescent probes can be designed to selectively target specific types of esterases, such as those found in pathogenic bacteria or cancer cells. In this review, we summarize the recent fluorescent probes described for the visualization of cell viability and some applications for in vivo imaging. On the other hand, positron emission tomography (PET) is a nuclear-based molecular imaging modality of great value for studying the activity of enzymes in vivo. We provide some examples of PET probes for imaging acetylcholinesterases and butyrylcholinesterases in the brain, which are valuable tools for diagnosing dementia and monitoring the effects of anticholinergic drugs on the central nervous system.

Brain exposure of osimertinib in patients with epidermal growth factor receptor mutation non-small cell lung cancer and brain metastases: A positron emission tomography and magnetic resonance imaging study

Brain metastases (BMs) are associated with poor prognosis in epidermal growth factor receptor mutation-positive (EGFRm) non-small cell lung cancer (NSCLC). Osimertinib is a third-generation, irreversible, EGFR-tyrosine kinase inhibitor that potently and selectively inhibits EGFR-sensitizing and T790M resistance mutations with efficacy in EGFRm NSCLC including central nervous system (CNS) metastases. The open-label phase I positron emission tomography (PET) and magnetic resonance imaging (MRI) study (ODIN-BM) assessed [¹¹ C]osimertinib brain exposure and distribution in patients with EGFRm NSCLC and BMs. Three dynamic 90-min [¹¹ C]osimertinib PET examinations were acquired together with metabolite-corrected arterial plasma input functions at: baseline, after first oral osimertinib 80 mg dose, and after greater than or equal to 21 days of osimertinib 80 mg q.d. treatment. Contrast-enhanced MRI was performed at screening and after 25-35 days of osimertinib 80 mg q.d.; treatment effect was assessed per CNS Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 and per volumetric changes in total BM using a novel analysis approach. Four patients (aged 51-77 years) completed the study. At baseline, ~1.5% injected radioactivity reached the brain (ID_max[brain] ) 22 min (median, T_max[brain] ) after injection. Total volume of distribution (V_T ) in whole brain was numerically higher compared with the BM regions. After a single oral osimertinib 80 mg dose, there was no consistent decrease in V_T in whole brain or BMs. After greater than or equal to 21 days' daily treatment, V_T in whole brain and BMs were numerically higher versus baseline. MRI revealed 56%-95% reduction in total BMs volume after 25-35 days of osimertinib 80 mg q.d. treatment. The [¹¹ C]osimertinib crossed the blood-brain and brain-tumor barriers and had a high, homogeneous brain distribution in patients with EGFRm NSCLC and BMs.

Proof of lung muscarinic receptor occupancy by tiotropium: Translational Positron Emission Tomography studies in non-human primates and humans

Introduction

Molecular imaging has not been used to support the development of drugs for the treatment of pulmonary disorders. The aim of the present translational study was to advance quantitative pulmonary PET imaging by demonstrating occupancy of the reference asthma drug tiotropium at muscarinic acetylcholine receptors (mAChR).

Methods

PET imaging was performed using the muscarinic radioligand [11C]VC-002. The key methodological step involved estimating muscarinic receptor binding while disentangling it from the background of non-specific binding. The relationship between tiotropium exposure and receptor occupancy (RO) was assessed in non-human primates (NHPs) after intravenous injection of tiotropium doses at a broad dose interval (0.03-1 µg/kg). The feasibility of measuring RO in the human lung was then confirmed in seven healthy human subjects after inhalation of a single therapeutic dose of tiotropium (18 µg).

Results

There was an evident effect of tiotropium on [11C]VC-002 binding to mAChRs in lungs in both NHPs and humans. In NHPs, RO was 11 to 78% and increased in a dose dependent manner. Non-displaceable binding in NHPs was about 10% of total binding. In humans, RO was 6%-65%, and non-displaceable binding was about 20% of total binding at baseline.

Discussion

The results demonstrate that [11C]VC-002 binds specifically to mAChRs in the lungs enabling the assessment of RO following administration of muscarinic antagonist drugs. Furthermore, the methodology has potential not only for dose finding and comparison of drug formulations in future applied studies, but also for evaluating changes in lung receptor distribution during disease or in response to therapy.

Clinical Trial Registration

ClinicalTrials.gov, identifier: NCT03097380.

Strategic, feasibility, economic, and cultural aspects of phase 0 approaches: Is it time to change the drug development process in order to increase productivity?

Research conducted over the past 2 decades has enhanced the validity and expanded the applications of microdosing and other phase 0 approaches in drug development. Phase 0 approaches can accelerate drug development timelines and reduce attrition in clinical development by increasing the quality of candidates entering clinical development and by reducing the time to "go-no-go" decisions. This can be done by adding clinical trial data (both healthy volunteers and patients) to preclinical candidate selection, and by applying methodological and operational advantages that phase 0 have over traditional approaches. The main feature of phase 0 approaches is the limited, subtherapeutic exposure to the test article. This means a reduced risk to research volunteers, and reduced regulatory requirements, timelines, and costs of first-in-human (FIH) testing. Whereas many operational aspects of phase 0 approaches are similar to those of other early phase clinical development programs, they have some unique strategic, regulatory, ethical, feasibility, economic, and cultural aspects. Here, we provide a guidance to these operational aspects and include case studies to highlight their potential impact in a range of clinical development scenarios.

In Vitro to In Vivo Extrapolation Linked to Physiologically Based Pharmacokinetic Models for Assessing the Brain Drug Disposition

Drug development for the central nervous system (CNS) is a complex endeavour with low success rates, as the structural complexity of the brain and specifically the blood-brain barrier (BBB) poses tremendous challenges. Several in vitro brain systems have been evaluated, but the ultimate use of these data in terms of translation to human brain concentration profiles remains to be fully developed. Thus, linking up in vitro-to-in vivo extrapolation (IVIVE) strategies to physiologically based pharmacokinetic (PBPK) models of brain is a useful effort that allows better prediction of drug concentrations in CNS components. Such models may overcome some known aspects of inter-species differences in CNS drug disposition. Required physiological (i.e. systems) parameters in the model are derived from quantitative values in each organ. However, due to the inability to directly measure brain concentrations in humans, compound-specific (drug) parameters are often obtained from in silico or in vitro studies. Such data are translated through IVIVE which could be also applied to preclinical in vivo observations. In such exercises, the limitations of the assays and inter-species differences should be adequately understood in order to verify these predictions with the observed concentration data. This report summarizes the state of IVIVE-PBPK-linked models and discusses shortcomings and areas of further research for better prediction of CNS drug disposition.

Brain exposure of the ATM inhibitor AZD1390 in humans-a positron emission tomography study

BACKGROUND

The protein kinase ataxia telangiectasia mutated (ATM) mediates cellular response to DNA damage induced by radiation. ATM inhibition decreases DNA damage repair in tumor cells and affects tumor growth. AZD1390 is a novel, highly potent, selective ATM inhibitor designed to cross the blood-brain barrier (BBB) and currently evaluated with radiotherapy in a phase I study in patients with brain malignancies. In the present study, PET was used to measure brain exposure of 11C-labeled AZD1390 after intravenous (i.v.) bolus administration in healthy subjects with an intact BBB.

METHODS

AZD1390 was radiolabeled with carbon-11 and a microdose (mean injected mass 1.21 µg) was injected in 8 male subjects (21-65 y). The radioactivity concentration of [11C]AZD1390 in brain was measured using a high-resolution PET system. Radioactivity in arterial blood was measured to obtain a metabolite corrected arterial input function for quantitative image analysis. Participants were monitored by laboratory examinations, vital signs, electrocardiogram, adverse events.

RESULTS

The brain radioactivity concentration of [11C]AZD1390 was 0.64 SUV (standard uptake value) and reached maximum 1.00% of injected dose at Tmax[brain] of 21 min (time of maximum brain radioactivity concentration) after i.v. injection. The whole brain total distribution volume was 5.20 mL*cm-3. No adverse events related to [11C]AZD1390 were reported.

CONCLUSIONS

This study demonstrates that [11C]AZD1390 crosses the intact BBB and supports development of AZD1390 for the treatment of glioblastoma multiforme or other brain malignancies. Moreover, it illustrates the potential of PET microdosing in predicting and guiding dose range and schedule for subsequent clinical studies.

Synthesis and preliminary evaluation of novel 11C-labeled GluN2B-selective NMDA receptor negative allosteric modulators

N-methyl-D-aspartate receptors (NMDARs) play critical roles in the physiological function of the mammalian central nervous system (CNS), including learning, memory, and synaptic plasticity, through modulating excitatory neurotransmission. Attributed to etiopathology of various CNS disorders and neurodegenerative diseases, GluN2B is one of the most well-studied subtypes in preclinical and clinical studies on NMDARs. Herein, we report the synthesis and preclinical evaluation of two 11C-labeled GluN2B-selective negative allosteric modulators (NAMs) containing N,N-dimethyl-2-(1H-pyrrolo[3,2-b]pyridin-1-yl)acetamides for positron emission tomography (PET) imaging. Two PET ligands, namely [11C]31 and [11C]37 (also called N2B-1810 and N2B-1903, respectively) were labeled with [11C]CH3I in good radiochemical yields (decay-corrected 28% and 32% relative to starting [11C]CO2, respectively), high radiochemical purity (>99%) and high molar activity (>74 GBq/μmol). In particular, PET ligand [11C]31 demonstrated moderate specific binding to GluN2B subtype by in vitro autoradiography studies. However, because in vivo PET imaging studies showed limited brain uptake of [11C]31 (up to 0.5 SUV), further medicinal chemistry and ADME optimization are necessary for this chemotype attributed to low binding specificity and rapid metabolism in vivo.

In vitro binding affinity vs. in vivo site occupancy: A PET study of four diastereomers of dihydrotetrabenazine (DTBZ) in monkey brain

INTRODUCTION

In vivo imaging methods such as Positron Emission Tomography (PET) can be used to examine the relationship between in vitro binding affinity and in vivo occupancy of binding sites in the brain for new drug candidates. In this study, PET imaging in monkey brain was used to evaluate that correlation for a set of four diastereomers of the compound dihydrotetrabenazine (DTBZ), the pharmacologically active metabolite of the drug tetrabenazine.

METHODS

PET studies of DTBZ diastereomers were completed in a single monkey brain. In vivo occupancies (ED50) were estimated using multiple drug doses and the vesicular monoamine transporter 2 specific radioligand (+)-α-[11C] DTBZ, employing a test-retest sequence of control PET scan, drug administration and a second PET scan completed on a single day.

RESULTS

DTBZ has three chiral carbon centers and eight possible stereoisomers, and in vivo occupancy of the target site VMAT2 was observed only for the four diastereomers of DTBZ having the 11bR absolute configuration. The estimated in vivo occupancies (ED50 values from 0.023 to >3.15 mg/kg) correlated well (R2 = 0.95) with the in vitro binding affinities (Ki values of 4 to 600 nM for the VMAT2), and an even better correlation (R2 = 0.99) was found for the three isomers with in vitro binding affinities <100 nM.

CONCLUSIONS

If the physiochemical (MW, log P, pKa) or physiological (metabolism, transport, protein binding) properties of a set of drug stereoisomers are considered similar, the binding affinities determined from in vitro assays may predict the in vivo occupancies of the target binding site in the monkey brain.

Phase 0/microdosing approaches: time for mainstream application in drug development?

Phase 0 approaches - which include microdosing - evaluate subtherapeutic exposures of new drugs in first-in-human studies known as exploratory clinical trials. Recent progress extends phase 0 benefits beyond assessment of pharmacokinetics to include understanding of mechanism of action and pharmacodynamics. Phase 0 approaches have the potential to improve preclinical candidate selection and enable safer, cheaper, quicker and more informed developmental decisions. Here, we discuss phase 0 methods and applications, highlight their advantages over traditional strategies and address concerns related to extrapolation and developmental timelines. Although challenges remain, we propose that phase 0 approaches be at least considered for application in most drug development scenarios.

Quantification and reliability of [11C]VC - 002 binding to muscarinic acetylcholine receptors in the human lung - a test-retest PET study in control subjects

BACKGROUND

The radioligand [11C]VC-002 was introduced in a small initial study long ago for imaging of muscarinic acetylcholine receptors (mAChRs) in human lungs using positron emission tomography (PET). The objectives of the present study in control subjects were to advance the methodology for quantification of [11C]VC-002 binding in lung and to examine the reliability using a test-retest paradigm. This work constituted a self-standing preparatory step in a larger clinical trial aiming at estimating mAChR occupancy in the human lungs following inhalation of mAChR antagonists.

METHODS

PET measurements using [11C]VC-002 and the GE Discovery 710 PET/CT system were performed in seven control subjects at two separate occasions, 2-19 days apart. One subject discontinued the study after the first measurement. Radioligand binding to mAChRs in lung was quantified using an image-derived arterial input function. The total distribution volume (VT) values were obtained on a regional and voxel-by-voxel basis. Kinetic one-tissue and two-tissue compartment models (1TCM, 2TCM), analysis based on linearization of the compartment models (multilinear Logan) and image analysis by data-driven estimation of parametric images based on compartmental theory (DEPICT) were applied. The test-retest repeatability of VT estimates was evaluated by absolute variability (VAR) and intraclass correlation coefficients (ICCs).

RESULTS

The 1TCM was the statistically preferred model for description of [11C]VC-002 binding in the lungs. Low VAR (< 10%) across analysis methods indicated good reliability of the PET measurements. The VT estimates were stable after 60 min.

CONCLUSIONS

The kinetic behaviour and good repeatability of [11C]VC-002 as well as the novel lung image analysis methodology support its application in applied studies on drug-induced mAChR receptor occupancy and the pathophysiology of pulmonary disorders.

TRIAL REGISTRATION

ClinicalTrials.gov identifier: NCT03097380, registered: 31 March 2017.

Effect of clinically relevant doses of vortioxetine and citalopram on serotonergic PET markers in the nonhuman primate brain

Vortioxetine is a multimodal antidepressant approved for treatment of major depressive disorder. Preclinical studies have demonstrated that the mechanism of action of vortioxetine might be different from selective serotonin reuptake inhibitors (SSRIs), including larger serotonin (5-HT) release and direct modulation of several 5-HT receptors. In the current positron emission tomography (PET) study, we evaluated the mechanism of action of vortioxetine by comparing its effect to the SSRI citalopram on the binding of [¹¹C]AZ10419369 to the 5-HT_1B receptor in the nonhuman primate brain. Initially, the 5-HT transporter (5-HTT) binding of vortioxetine was determined by [¹¹C]MADAM PET measurements before and after administration of vortioxetine (0.1-3.0 mg/kg) and data were used to confirm clinically relevant dosing in subsequent PET measurements with [¹¹C]AZ10419369. The 5-HT_1B receptor binding was significantly decreased after 0.3 mg/kg of citalopram in the dorsal raphe nucleus (5%), as well as after 0.3 mg/kg of vortioxetine in six brain regions (~25%) or 1.0 mg/kg of vortioxetine in all 12 examined regions (~48%). Moreover, there was no effect of 1.0 mg/kg of vortioxetine on the binding of [¹¹C]Cimbi-36 to the 5-HT_2A receptor, which has comparable sensitivity to 5-HT release as [¹¹C]AZ10419369 binding. In conclusion, at clinically relevant doses, vortioxetine induced larger reductions in [¹¹C]AZ10419369 binding than citalopram. These observations suggest that vortioxetine binds to the 5-HT_1B receptor at clinically relevant doses. Future studies are warranted to evaluate the role of the 5-HT_1B receptor in the therapeutic effects of vortioxetine and as a potential target for the development of novel antidepressant drugs.

Pulmonary PET imaging confirms preferential lung target occupancy of an inhaled bronchodilator

BACKGROUND

Positron emission tomography (PET) is a non-invasive molecular imaging technique that traces the distribution of radiolabeled molecules in experimental animals and human subjects. We hypothesized that PET could be used to visualize the binding of the bronchodilator drug ipratropium to muscarinic receptors (MR) in the lungs of living non-human primates (NHP). The objectives of this study were two-fold: (i) to develop a methodology for quantitative imaging of muscarinic receptors in NHP lung and (ii) to estimate and compare ipratropium-induced MR occupancy following drug administration via intravenous injection and inhalation, respectively.

METHODS

A series of PET measurements (n = 18) was performed after intravenous injection of the selective muscarinic radioligand ¹¹C-VC-002 in NHP (n = 5). The lungs and pituitary gland (both rich in MR) were kept in the field of view. Each PET measurement was followed by a PET measurement preceded by treatment with ipratropium (intravenous or inhaled).

RESULTS

Radioligand binding was quantified using the Logan graphical analysis method providing the total volume of distribution (V_T). Ipratropium reduced the V_T in the lung and pituitary in a dose-dependent fashion. At similar plasma ipratropium concentrations, administration by inhalation produced larger reductions in V_T for the lungs. The plasma-derived apparent affinity for ipratropium binding in the lung was one order of magnitude higher after inhalation (K_iih = 1.01 nM) than after intravenous infusion (K_iiv = 10.84 nM).

CONCLUSION

Quantitative muscarinic receptor occupancy imaging by PET articulates and quantifies the therapeutic advantage of the inhaled route of delivery and provides a tool for future developments of improved inhaled drugs.

Peptide-based PET quantifies target engagement of PD-L1 therapeutics

Immune checkpoint therapies have shown tremendous promise in cancer therapy. However, tools to assess their target engagement, and hence the ability to predict their efficacy, have been lacking. Here, we show that target engagement and tumor-residence kinetics of antibody therapeutics targeting programmed death ligand-1 (PD-L1) can be quantified noninvasively. In computational docking studies, we observed that PD-L1-targeted monoclonal antibodies (atezolizumab, avelumab, and durvalumab) and a high-affinity PD-L1-binding peptide, WL12, have common interaction sites on PD-L1. Using the peptide radiotracer [64Cu]WL12 in vivo, we employed positron emission tomography (PET) imaging and biodistribution studies in multiple xenograft models and demonstrated that variable PD-L1 expression and its saturation by atezolizumab, avelumab, and durvalumab can be quantified independently of biophysical properties and pharmacokinetics of antibodies. Next, we used [64Cu]WL12 to evaluate the impact of time and dose on the unoccupied fraction of tumor PD-L1 during treatment. These quantitative measures enabled, by mathematical modeling, prediction of antibody doses needed to achieve therapeutically effective occupancy (defined as >90%). Thus, we show that peptide-based PET is a promising tool for optimizing dose and therapeutic regimens employing PD-L1 checkpoint antibodies, and can be used for improving therapeutic efficacy.

Synthesis, pharmacology and preclinical evaluation of 11C-labeled 1,3-dihydro-2H-benzo[d]imidazole-2-ones for imaging γ8-dependent transmembrane AMPA receptor regulatory protein

a-Amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are implicated in the pathology of neurological diseases such as epilepsy and schizophrenia. As pan antagonists for this target are often accompanied with undesired effects at high doses, one of the recent drug discovery approaches has shifted to subtype-selective AMPA receptor (AMPAR) antagonists, specifically, via modulating transmembrane AMPAR regulatory proteins (TARPs). The quantification of AMPARs by positron emission tomography (PET) would help obtain insights into disease conditions in the living brain and advance the translational development of AMPAR antagonists. Herein we report the design, synthesis and preclinical evaluation of a series of TARP γ-8 antagonists, amenable for radiolabeling, for the development of subtype-selective AMPAR PET imaging agents. Based on the pharmacology evaluation, molecular docking studies and physiochemical properties, we have identified several promising lead compounds 3, 17-19 and 21 for in vivo PET studies. All candidate compounds were labeled with [¹¹C]COCl₂ in high radiochemical yields (13-31% RCY) and high molar activities (35-196 GBq/μmol). While tracers 30 ([¹¹C]17) &32 ([¹¹C]21) crossed the blood-brain barrier and showed heterogeneous distribution in PET studies, consistent with TARP γ-8 expression, high nonspecific binding prevented further evaluation. To our delight, tracer 31 ([¹¹C]3) showed good in vitro specific binding and characteristic high uptake in the hippocampus in rat brain tissues, which provides the guideline for further development of a new generation subtype selective TARP γ-8 dependent AMPAR tracers.

Radionuclide imaging of VEGFR2 in glioma vasculature using biparatopic affibody conjugate: proof-of-principle in a murine model

Vascular endothelial growth factor receptor-2 (VEGFR2) is a key mediator of angiogenesis and therefore a promising therapeutic target in malignancies including glioblastoma multiforme (GBM). Molecular imaging of VEGFR2 expression may enable patient stratification for antiangiogenic therapy. The goal of the current study was to evaluate the capacity of the novel anti-VEGFR2 biparatopic affibody conjugate (Z_VEGFR2-Bp₂) for in vivo visualization of VEGFR2 expression in GBM. Methods: Z_VEGFR2-Bp₂ coupled to a NODAGA chelator was generated and radiolabeled with indium-111. The VEGFR2-expressing murine endothelial cell line MS1 was used to evaluate in vitro binding specificity and affinity, cellular processing and targeting specificity in mice. Further tumor targeting was studied in vivo in GL261 glioblastoma orthotopic tumors. Experimental imaging was performed. Results: [¹¹¹In]In-NODAGA-Z_VEGFR2-Bp₂ bound specifically to VEGFR2 (K_D=33±18 pM). VEGFR2-mediated accumulation was observed in liver, spleen and lungs. The tumor-to-organ ratios 2 h post injection for mice bearing MS1 tumors were approximately 11 for blood, 15 for muscles and 78 for brain. Intracranial GL261 glioblastoma was visualized using SPECT/CT. The activity uptake in tumors was significantly higher than in normal brain tissue. The tumor-to-cerebellum ratios after injection of 4 µg [¹¹¹In]In-NODAGA-Z_VEGFR2-Bp₂ were significantly higher than the ratios observed for the 40 µg injected dose and for the non-VEGFR2 binding size-matched conjugate, demonstrating target specificity. Microautoradiography of cryosectioned CNS tissue was in good agreement with the SPECT/CT images. Conclusion: The anti-VEGFR2 affibody conjugate [¹¹¹In]In-NODAGA-Z_VEGFR2-Bp₂ specifically targeted VEGFR2 in vivo and visualized its expression in a murine GBM orthotopic model. Tumor-to-blood ratios for [¹¹¹In]In-NODAGA-Z_VEGFR2-Bp₂ were higher compared to other VEGFR2 imaging probes. [¹¹¹In]In-NODAGA-Z_VEGFR2-Bp₂ appears to be a promising probe for in vivo noninvasive visualization of tumor angiogenesis in glioblastoma.

Synthesis, Radiosynthesis, and Preliminary in vitro and in vivo Evaluation of the Fluorinated Ceramide Trafficking Inhibitor (HPA-12) for Brain Applications

Ceramide levels are increased in blood and brain tissue of Alzheimer's disease (AD) patients. Since the ceramide transporter protein (CERT) is the only known protein able to mediate non-vesicular transfer of ceramide between organelle membranes, the modulation of CERT function may impact on ceramide accumulation. The competitive CERT inhibitor N-(3-hydroxy-1-hydroxymethyl-3-phenylpropyl) dodecanamide (HPA-12) interferes with ceramide trafficking. To understand the role of ceramide/CERT in AD, HPA-12 can be a useful tool to modulate ceramide trafficking. Here we first report the synthesis and in vitro properties of HPA-12 radiolabeled with fluorine-18 and present preliminary in vitro and in vivo positron emission tomography (PET) imaging and biodistribution data. In vitro results demonstrated that the fluorination did not alter the biological properties of HPA-12 since the [fluorine-19]HPA-12, interferes with 5-DMB-ceramide trafficking in HeLa cells. Radiolabeled HPA-12, [fluorine-18]HPA-12, was obtained with a radiochemical yield of 90% and a specific activity of 73 MBq/μmol. PET imaging on wild-type mice showed hepatobiliary clearance and a brain uptake on the order of 0.3 standard uptake value (SUV) one hour post injection. Furthermore, the biodistribution data showed that after removal of the blood by intracardial perfusion, radioactivity was still measurable in the brain demonstrating that the [fluorine-18]HPA-12 crosses the blood brain barrier and is retained in the brain.

Small Molecule PET Tracers in Drug Discovery

The process of discovering and developing a new pharmaceutical is a long, difficult, and risky process that requires numerous resources. Molecular imaging techniques such as PET have recently become a useful tool for making decisions along a drug candidate's development timeline. PET is a translational, noninvasive imaging technique that provides quantitative information about a potential drug candidate and its target at the molecular level. Using this technique provides decisional information to ensure that the right drug candidate is being chosen, for the right target, at the right dose within the right patient population. This review will focus on small molecule PET tracers and how they are used within the drug discovery process. PET provides key information about a drug candidate's pharmacokinetic and pharmacodynamic properties in both preclinical and clinical studies. PET is being used in all phases of the drug discovery and development process, and the goal of these studies are to accelerate the process in which drugs are developed.

In vivo imaging biomarkers of neuroinflammation in the development and assessment of stroke therapies - towards clinical translation

Modulation of the inflammatory microenvironment after stroke opens a new avenue for the development of novel neurorestorative therapies in stroke. Understanding the spatio-temporal profile of (neuro-)inflammatory imaging biomarkers in detail thereby represents a crucial factor in the development and application of immunomodulatory therapies. The early integration of quantitative molecular imaging biomarkers in stroke drug development may provide key information about (i) early diagnosis and follow-up, (ii) spatio-temporal drug-target engagement (pharmacodynamic biomarker), (iii) differentiation of responders and non-responders in the patient cohort (inclusion/exclusion criteria; predictive biomarkers), and (iv) the mechanism of action. The use of targeted imaging biomarkers for may thus allow clinicians to decipher the profile of patient-specific inflammatory activity and the development of patient-tailored strategies for immunomodulatory and neuro-restorative therapies in stroke. Here, we highlight the recent developments in preclinical and clinical molecular imaging biomarkers of neuroinflammation (endothelial markers, microglia, MMPs, cell labeling, future developments) in stroke and outline how imaging biomarkers can be used in overcoming current translational roadblocks and attrition in order to advance new immunomodulatory compounds within the clinical pipeline.

In Vitro and in Vivo Evaluation of 11C-Labeled Azetidinecarboxylates for Imaging Monoacylglycerol Lipase by PET Imaging Studies

Monoacylglycerol lipase (MAGL) is the principle enzyme for metabolizing endogenous cannabinoid ligand 2-arachidonoyglycerol (2-AG). Blockade of MAGL increases 2-AG levels, resulting in subsequent activation of the endocannabinoid system, and has emerged as a novel therapeutic strategy to treat drug addiction, inflammation, and neurodegenerative diseases. Herein we report a new series of MAGL inhibitors, which were radiolabeled by site-specific labeling technologies, including 11C-carbonylation and spirocyclic iodonium ylide (SCIDY) radiofluorination. The lead compound [11C]10 (MAGL-0519) demonstrated high specific binding and selectivity in vitro and in vivo. We also observed unexpected washout kinetics with these irreversible radiotracers, in which in vivo evidence for turnover of the covalent residue was unveiled between MAGL and azetidine carboxylates. This work may lead to new directions for drug discovery and PET tracer development based on azetidine carboxylate inhibitor scaffold.

Synthesis and Preliminary Studies of a Novel Negative Allosteric Modulator, 7-((2,5-Dioxopyrrolidin-1-yl)methyl)-4-(2-fluoro-4-[11C]methoxyphenyl) quinoline-2-carboxamide, for Imaging of Metabotropic Glutamate Receptor 2

Metabotropic glutamate 2 receptors (mGlu2) are involved in the pathogenesis of several CNS disorders and neurodegenerative diseases. Pharmacological modulation of this target represents a potential disease-modifying approach for the treatment of substance abuse, depression, schizophrenia, and dementias. While quantification of mGlu2 receptors in the living brain by positron emission tomography (PET) would help us better understand signaling pathways relevant to these conditions, few successful examples have been demonstrated to image mGlu2 in vivo, and a suitable PET tracer is yet to be identified. Herein we report the design and synthesis of a radiolabeled negative allosteric modulator (NAM) for mGlu2 PET tracer development based on a quinoline 2-carboxamide scaffold. The most promising candidate, 7-((2,5-dioxopyrrolidin-1-yl)methyl)-4-(2-fluoro-4-[11C]methoxyphenyl) quinoline-2-carboxamide ([11C]QCA) was prepared in 13% radiochemical yield (non-decay-corrected at the end of synthesis) with >99% radiochemical purity and >74 GBq/μmol (2 Ci/μmol) specific activity. While the tracer showed limited brain uptake (0.3 SUV), probably attributable to effects on PgP/Bcrp efflux pump, in vitro autoradiography studies demonstrated heterogeneous brain distribution and specific binding. Thus, [11C]QCA is a chemical probe that provides the basis for the development of a new generation mGlu2 PET tracers.

Use of PET Imaging to Evaluate Transporter-Mediated Drug-Drug Interactions

Several membrane transporters belonging to the adenosine triphosphate-binding cassette (ABC) and solute carrier (SLC) families can transport drugs and drug metabolites and thereby exert an effect on drug absorption, distribution, and excretion, which may potentially lead to transporter-mediated drug-drug interactions (DDIs). Some transporter-mediated DDIs may lead to changes in organ distribution of drugs (eg, brain, liver, kidneys) without affecting plasma concentrations. Positron emission tomography (PET) is a noninvasive imaging method that allows studying of the distribution of radiolabeled drugs to different organs and tissues and is therefore the method of choice to quantitatively assess transporter-mediated DDIs on a tissue level. There are 2 approaches to how PET can be used in transporter-mediated DDI studies. When the drug of interest is a potential perpetrator of DDIs, it may be administered in unlabeled form to assess its influence on tissue distribution of a generic transporter-specific PET tracer (probe substrate). When the drug of interest is a potential victim of DDIs, it may be radiolabeled with carbon-11 or fluorine-18 and used in combination with a prototypical transporter inhibitor (eg, rifampicin). PET has already been used both in preclinical species and in humans to assess the effects of transporter-mediated DDIs on drug disposition in different organ systems, such as brain, liver, and kidneys, for which examples are given in the present review article. Given the growing importance of membrane transporters with respect to drug safety and efficacy, PET is expected to play an increasingly important role in future drug development.

Palladium-mediated11C-carbonylations using aryl halides and cyanamide

A rapid, efficient and high-yielding synthesis of¹¹C-cyanobenzamides, including novel analogs of various drug molecules, is described.

Guidelines to PET measurements of the target occupancy in the brain for drug development

This guideline summarizes the current view of the European Association of Nuclear Medicine Drug Development Committee. The purpose of this guideline is to guarantee a high standard of PET studies that are aimed at measuring target occupancy in the brain within the framework of development programs of drugs that act within the central nervous system (CNS drugs). This guideline is intended to present information specifically adapted to European practice. The information provided should be applied within the context of local conditions and regulations.

Strategies to facilitate the discovery of novel CNS PET ligands

Positron Emission Tomography (PET), as a non-invasive translatable imaging technology, can be incorporated into various stages of the CNS drug discovery process to provide valuable information for key preclinical and clinical decision-making. Novel CNS PET ligand discovery efforts in the industry setting, however, are facing unique challenges associated with lead design and prioritization, and budget constraints. In this review, three strategies aiming toward improving the central nervous system (CNS) PET ligand discovery process are described: first, early determination of receptor density (B_max) and bio-distribution to inform PET viability and resource allocation; second, rational design and design prioritization guided by CNS PET design parameters; finally, a cost-effective in vivo specific binding assessment using a liquid chromatography-mass spectrometry (LC-MS/MS) “cold tracer” method. Implementation of these strategies allowed a more focused and rational CNS PET ligand discovery effort to identify high quality PET ligands for neuroimaging.

Brain imaging of pain: state of the art

Pain is a complex sensory and emotional experience that is heavily influenced by prior experience and expectations of pain. Before the development of noninvasive human brain imaging, our grasp of the brain's role in pain processing was limited to data from postmortem studies, direct recording of brain activity, patient experience and stimulation during neurosurgical procedures, and animal models of pain. Advances made in neuroimaging have bridged the gap between brain activity and the subjective experience of pain and allowed us to better understand the changes in the brain that are associated with both acute and chronic pain. Additionally, cognitive influences on pain such as attention, anticipation, and fear can now be directly observed, allowing for the interpretation of the neural basis of the psychological modulation of pain. The use of functional brain imaging to measure changes in endogenous neurochemistry has increased our understanding of how states of increased resilience and vulnerability to pain are maintained.

Fluorine-18 Radiolabeled PET Tracers for Imaging Monoamine Transporters: Dopamine, Serotonin, and Norepinephrine

This review focuses on the development of fluorine-18 radiolabeled PET tracers for imaging the dopamine transporter (DAT), serotonin transporter (SERT), and norepinephrine transporter (NET). All successful DAT PET tracers reported to date are members of the 3β-phenyl tropane class and are synthesized from cocaine. Currently available carbon-11 SERT PET tracers come from both the diphenylsulfide and 3β-phenyl nortropane class, but so far only the nortropanes have found success with fluorine-18 derivatives. NET imaging has so far employed carbon-11 and fluorine-18 derivatives of reboxetine but due to defluorination of the fluorine-18 derivatives further research is still necessary.

Applications of in vivo imaging in the evaluation of the pathophysiology of viral and bacterial infections and in development of countermeasures to BSL3/4 pathogens

While preclinical and clinical imaging have been applied to drug discovery/development and characterization of disease pathology, few examples exist where imaging has been used to evaluate infectious agents or countermeasures to biosafety level (BSL)3/4 threat agents. Viruses engineered with reporter constructs, i.e., enzymes and receptors, which are amenable to detection by positron emission tomography (PET), single photon emission tomography (SPECT), or magnetic resonance imaging (MRI) have been used to evaluate the biodistribution of viruses containing specific therapeutic or gene transfer payloads. Bioluminescence and nuclear approaches involving engineered reporters, direct labeling of bacteria with radiotracers, or tracking bacteria through their constitutively expressed thymidine kinase have been utilized to characterize viral and bacterial pathogens post-infection. Most PET, SPECT, CT, or MRI approaches have focused on evaluating host responses to the pathogens such as inflammation, brain neurochemistry, and structural changes and on assessing the biodistribution of radiolabeled drugs. Imaging has the potential when applied preclinically to the development of countermeasures against BSL3/4 threat agents to address the following: (1) presence, biodistribution, and time course of infection in the presence or absence of drug; (2) binding of the therapeutic to the target; and (3) expression of a pharmacologic effect either related to drug mechanism, efficacy, or safety. Preclinical imaging could potentially provide real-time dynamic tools to characterize the pathogen and animal model and for developing countermeasures under the U.S. FDA Animal Rule provision with high confidence of success and clinical benefit.

Profiling neuronal ion channelopathies with non-invasive brain imaging and dynamic causal models: Case studies of single gene mutations

Clinical assessments of brain function rely upon visual inspection of electroencephalographic waveform abnormalities in tandem with functional magnetic resonance imaging. However, no current technology proffers in vivo assessments of activity at synapses, receptors and ion-channels, the basis of neuronal communication. Using dynamic causal modeling we compared electrophysiological responses from two patients with distinct monogenic ion channelopathies and a large cohort of healthy controls to demonstrate the feasibility of assaying synaptic-level channel communication non-invasively. Synaptic channel abnormality was identified in both patients (100% sensitivity) with assay specificity above 89%, furnishing estimates of neurotransmitter and voltage-gated ion throughput of sodium, calcium, chloride and potassium. This performance indicates a potential novel application as an adjunct for clinical assessments in neurological and psychiatric settings. More broadly, these findings indicate that biophysical models of synaptic channels can be estimated non-invasively, having important implications for advancing human neuroimaging to the level of non-invasive ion channel assays.

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Dynamic causal modeling (DCM) for M/EEG includes ion channel parameter estimates.

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Parameter estimates from patients with monogenic ion channelopathies were compared.

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Synaptic channel abnormality was identified in patients, with specificity above 89%.

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DCM could serve as a platform for non-invasively assaying brain molecular dynamics.

Radiosynthesis and in vitro validation of (3)H-NS14492 as a novel high affinity alpha7 nicotinic receptor radioligand

The neuronal α7 nicotinic acetylcholine receptor is a homo-pentameric ligand-gated ion channel that is a promising drug target for cognitive deficits in Alzheimer׳s disease and schizophrenia. We have previously described (11)C-NS14492 as a suitable agonist radioligand for in vivo positron emission tomography (PET) occupancy studies of the α7 nicotinic receptor in the pig brain. In order to investigate the utility of the same compound for in vitro studies, (3)H-NS14492 was synthesized and its binding properties were characterized using in vitro autoradiography and homogenate binding assays in pig frontal cortex. (3)H-NS14492 showed specific binding to α7 nicotinic receptors in autoradiography, revealing a dissociation constant (Kd) of 2.1±0.7nM and a maximum number of binding sites (Bmax) of 15.7±2.0fmol/mg tissue equivalent. Binding distribution was similar to that of another selective ligand (125)I-α-bungarotoxin ((125)I-BTX) in autoradiography, and unlabeled NS14492 displaced (125)I-BTX with an inhibition constant (Ki) of 23nM. (3)H-NS14492 bound to α7 nicotinic receptors in homogenized pig frontal cortex with a Kd of 0.8±0.3nM and a Bmax of 30.2±11.6fmol/mg protein. This binding assay further revealed the Ki rank order for a number of α7 nicotinic receptor agonists, and positive allosteric modulators (PAMs). Further, we saw increased binding of (3)H-NS14492 to pig frontal cortex membranes when co-incubated with PNU-120596, a type II PAM. Taken together, these findings show that (3)H-NS14492 is a useful new in vitro radioligand for the pig α7 nicotinic receptor.

Development of a series of novel carbon-11 labeled PDE10A inhibitors

Phosphodiesterase 10A (PDE10A) is a member of the PDE family of enzymes that degrades cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Our aim was to label a series of structurally related PDE10A inhibitors with carbon-11 and evaluate them as potential positron emission tomography (PET) radioligands for PDE10A using nonhuman primates. The series consisted of seven compounds based on the 3-(1H-pyrazol-5-yl)pyridazin-4(1H)-one backbone. These compounds were selected from the initial larger library based on a number of parameters such as affinity, selectivity for hPDE10A in in vitro tests, lipophilicity, and on the results of multidrug resistance protein 1 (MDR1)-LLCPK1 and the parallel artificial membrane permeability assays. Seven radioligands (KIT-1, 3, 5, 6, 7, 9, and 12) were radiolabeled with carbon-11 employing O-methylation on the hydroxyl moiety using [(11)C]methyl triflate. In vivo examination of each radioligand was performed using PET in rhesus monkeys; analysis of radiometabolites in plasma also was conducted using HPLC. All seven radioligands were labeled with high (>90%) incorporation of [(11)C]methyl triflate into their appropriate precursors and with high specific radioactivity. Carbon-11 labeled KIT-5 and KIT-6 showed high accumulation in the striatum, consistent with the known anatomical distribution of PDE10A in brain, accompanied by fast washout and high specific binding ratio. In particular [(11)C]KIT-6, named [(11)C]T-773, is a promising PET tool for further examination of PDE10A in human brain.

Large Variation in Brain Exposure of Reference CNS Drugs: a PET Study in Nonhuman Primates

BACKGROUND

Positron emission tomography microdosing of radiolabeled drugs allows for noninvasive studies of organ exposure in vivo. The aim of the present study was to examine and compare the brain exposure of 12 commercially available CNS drugs and one non-CNS drug.

METHODS

The drugs were radiolabeled with (11)C (t 1/2 = 20.4 minutes) and examined using a high resolution research tomograph. In cynomolgus monkeys, each drug was examined twice. In rhesus monkeys, a first positron emission tomography microdosing measurement was repeated after preadministration with unlabeled drug to examine potential dose-dependent effects on brain exposure. Partition coefficients between brain and plasma (KP) were calculated by dividing the AUC0-90 min for brain with that for plasma or by a compartmental analysis (VT). Unbound KP (KP u,u) was obtained by correction for the free fraction in brain and plasma.

RESULTS

After intravenous injection, the maximum radioactivity concentration (C max, %ID) in brain ranged from 0.01% to 6.2%. For 10 of the 12 CNS drugs, C max, %ID was >2%, indicating a preferential distribution to brain. A lower C max, %ID was observed for morphine, sulpiride, and verapamil. K P ranged from 0.002 (sulpiride) to 68 (sertraline) and 7 of 13 drugs had KP u,u close to unity. For morphine, sulpiride, and verapamil, K P u,u was <0.3, indicating impaired diffusion and/or active efflux. Brain exposure at microdosing agreed with pharmacological dosing conditions for the investigated drugs.

CONCLUSIONS

This study represents the largest positron emission tomography study on brain exposure of commercially available CNS drugs in nonhuman primates and may guide interpretation of positron emission tomography microdosing data for novel drug candidates.

Development of rapid multistep carbon-11 radiosynthesis of the myeloperoxidase inhibitor AZD3241 to assess brain exposure by PET microdosing

INTRODUCTION

The myeloperoxidase inhibitor AZD3241 has been selected as a candidate drug currently being developed to delay progression in patients with neurodegenerative brain disorders. Part of the decision tree for translation of AZD3241 into clinical studies included the need for assessment of brain exposure in non-human primates by PET microdosing. For that purpose a rapid multistep method for (11)C-labeling of AZD3241 was developed.

METHODS

AZD3241 was labeled in the thio-carbonyl position starting from [(11)C]potassium cyanide in a 4-step procedure using microwave assisted heating. In the first step [(11)C]potassium cyanide was converted to [(11)C]potassium thiocyanate followed by reaction with benzoyl chloride to yield benzoyl [(11)C]isothiocyanate. The benzoyl [(11)C]isothiocyanate was subsequently reacted with the precursor ethyl 3-(2-isopropoxyethylamino)-1H-pyrrole-2-carboxylate and the formed intermediate underwent a base catalyzed cyclization to obtain [(11)C]AZD3241 in the final step. To assess [(11)C]AZD3241 brain exposure PET measurements were performed in three cynomolgus monkeys.

RESULTS

[(11)C]AZD3241 was produced in good and reproducible radiochemical yield 710 ± 294 MBq (mean ± SD, n = 7). Total time of synthesis was 60 min from end of bombardment. The specific radioactivity was 9 ± 4GBq/μmol and the radiochemical purity was >98%. Following iv administration of [(11)C]AZD3241 there was a rapid presence of radioactivity in brain in each of the three monkeys. The distribution of [(11)C]AZD3241 to brain was fast and a Cmax of 1.9 to 2.6% of the injected radioactivity was observed within 1.5 min. [(11)C]AZD3241 was homogeneously distributed in brain.

CONCLUSION

The MPO inhibitor AZD3241 was successfully labeled with carbon-11 in a challenging 4-step procedure in good radiochemical yield allowing PET microdosing studies in cynomolgus monkey. [(11)C]AZD3241 rapidly entered brain and confirmed adequate brain exposure to support translation of AZD3241 to phase 2a studies in patients.

Application of cross-species PET imaging to assess neurotransmitter release in brain

RATIONALE

This review attempts to summarize the current status in relation to the use of positron emission tomography (PET) imaging in the assessment of synaptic concentrations of endogenous mediators in the living brain.

OBJECTIVES

Although PET radioligands are now available for more than 40 CNS targets, at the initiation of the Innovative Medicines Initiative (IMI) "Novel Methods leading to New Medications in Depression and Schizophrenia" (NEWMEDS) in 2009, PET radioligands sensitive to an endogenous neurotransmitter were only validated for dopamine. NEWMEDS work-package 5, "Cross-species and neurochemical imaging (PET) methods for drug discovery", commenced with a focus on developing methods enabling assessment of changes in extracellular concentrations of serotonin and noradrenaline in the brain.

RESULTS

Sharing the workload across institutions, we utilized in vitro techniques with cells and tissues, in vivo receptor binding and microdialysis techniques in rodents, and in vivo PET imaging in non-human primates and humans. Here, we discuss these efforts and review other recently published reports on the use of radioligands to assess changes in endogenous levels of dopamine, serotonin, noradrenaline, γ-aminobutyric acid, glutamate, acetylcholine, and opioid peptides. The emphasis is on assessment of the availability of appropriate translational tools (PET radioligands, pharmacological challenge agents) and on studies in non-human primates and human subjects, as well as current challenges and future directions.

CONCLUSIONS

PET imaging directed at investigating changes in endogenous neurochemicals, including the work done in NEWMEDS, have highlighted an opportunity to further extend the capability and application of this technology in drug development.

Modeling and design of challenge tests: Inflammatory and metabolic biomarker study examples

Given the complexity of pharmacological challenge experiments, it is perhaps not surprising that design and analysis, and in turn interpretation and communication of results from a quantitative point of view, is often suboptimal. Here we report an inventory of common designs sampled from anti-inflammatory, respiratory and metabolic disease drug discovery studies, all of which are based on animal models of disease involving pharmacological and/or patho/physiological interaction challenges. The corresponding data are modeled and analyzed quantitatively, the merits of the respective approach discussed and inferences made with respect to future design improvements. Although our analysis is limited to these disease model examples, the challenge approach is generally applicable to the vast majority of pharmacological intervention studies. In the present five Case Studies results from pharmacodynamic effect models from different therapeutic areas were explored and analyzed according to five typical designs. Plasma exposures of test compounds were assayed by either liquid chromatography/mass spectrometry or ligand binding assays. To describe how drug intervention can regulate diverse processes, turnover models of test compound-challenger interaction, transduction processes, and biophase time courses were applied for biomarker response in eosinophil count, IL6 response, paw-swelling, TNFα response and glucose turnover in vivo. Case Study 1 shows results from intratracheal administration of Sephadex, which is a glucocorticoid-sensitive model of airway inflammation in rats. Eosinophils in bronchoalveolar fluid were obtained at different time points via destructive sampling and then regressed by the mixed-effects modeling. A biophase function of the Sephadex time course was inferred from the modeled eosinophil time courses. In Case Study 2, a mouse model showed that the time course of cytokine-induced IL1β challenge was altered with or without drug intervention. Anakinra reversed the IL1β induced cytokine IL6 response in a dose-dependent manner. This Case Study contained time courses of test compound (drug), challenger (IL1β) and cytokine response (IL6), which resulted in high parameter precision. Case Study 3 illustrates collagen-induced arthritis progression in the rat. Swelling scores (based on severity of hind paw swelling) were used to describe arthritis progression after the challenge and the inhibitory effect of two doses of an orally administered test compound. In Case Study 4, a cynomolgus monkey model for lipopolysaccharide LPS-induced TNFα synthesis and/or release was investigated. This model provides integrated information on pharmacokinetics and in vivo potency of the test compounds. Case Study 5 contains data from an oral glucose tolerance test in rats, where the challenger is the same as the pharmacodynamic response biomarker (glucose). It is therefore convenient to model the extra input of glucose simultaneously with baseline data and during intervention of a glucose-lowering compound at different dose levels. Typically time-series analyses of challenger- and biomarker-time data are necessary if an accurate and precise estimate of the pharmacodynamic properties of a test compound is sought. Erosion of data, resulting in the single-point assessment of drug action after a challenge test, should generally be avoided. This is particularly relevant for situations where one expects time-curve shifts, tolerance/rebound, impact of disease, or hormetic concentration-response relationships to occur.

Characterization of the binding properties of T-773 as a PET radioligand for phosphodiesterase 10A

INTRODUCTION

Phosphodiesterase 10A (PDE10A) is a dual-substrate PDE that hydrolyzes both cAMP and cGMP and is selectively expressed in striatal medium spiny neurons. Recent studies have suggested that PDE10A inhibition is a novel approach for the treatment of disorders such as schizophrenia and Huntington's disease. A positron emission tomography (PET) occupancy study can provide useful information for the development of PDE10A inhibitors. We discovered T-773 as a candidate PET radioligand for PDE10A and investigated its properties by in vitro autoradiography and a PET study in a monkey.

METHODS

Profiling of T-773 as a PET radioligand for PDE10A was conducted by in vitro enzyme inhibitory assay, in vitro autoradiography, and PET study in a monkey.

RESULTS

T-773 showed a high binding affinity and selectivity for human recombinant PDE10A2 in vitro; the IC50 value in an enzyme inhibitory assay was 0.77nmol/L, and selectivity over other PDEs was more than 2500-fold. In autoradiography studies using mouse, rat, monkey, or human brain sections, radiolabeled T-773 selectively accumulated in the striatum. This selective accumulation was not observed in the brain sections of Pde10a-KO mice. The binding of [(3)H]T-773 to PDE10A in rat brain sections was competitively inhibited by MP-10, a selective PDE10A inhibitor. In rat brain sections, [(3)H]T-773 bound to a single high affinity site of PDE10A with Kd values of 12.2±2.2 and 4.7±1.2nmol/L in the caudate-putamen and nucleus accumbens, respectively. In a monkey PET study, [(11)C]T-773 showed good brain penetration and striatum-selective accumulation.

CONCLUSION

These results suggest that [(11)C]T-773 is a potential PET radioligand for PDE10A.

Pathways for small molecule delivery to the central nervous system across the blood-brain barrier

The treatment of central nervous system (CNS) disease has long been difficult due to the ineffectiveness of drug delivery across the blood-brain barrier (BBB). This review summarizes important concepts of the BBB in normal versus pathophysiology and how this physical, enzymatic, and efflux barrier provides necessary protection to the CNS during drug delivery, and consequently treatment challenging. Small molecules account for the vast majority of available CNS drugs primarily due to their ability to penetrate the phospholipid membrane of the BBB by passive or carrier-mediated mechanisms. Physiochemical and biological factors relevant for designing small molecules with optimal capabilities for BBB permeability are discussed, as well as the most promising classes of transporters suitable for small-molecule drug delivery. Clinically translatable imaging methodologies for detecting and quantifying drug uptake and targeting in the brain are discussed as a means of further understanding and refining delivery parameters for both drugs and imaging probes in preclinical and clinical domains. This information can be used as a guide to design drugs with preserved drug action and better delivery profiles for improved treatment outcomes over existing therapeutic approaches.

Synthesis and biological evaluation of carbon-11 and fluorine-18 labeled tracers for in vivo visualization of PDE10A

INTRODUCTION

In vivo visualization of PDE10A using PET provides a tool to evaluate the role of PDE10A in various neuropsychiatric diseases and can also be useful in the clinical evaluation of PDE10A inhibitor drug candidates. We evaluated several carbon-11 and fluorine-18 labeled PDE10A inhibitors as potential PDE10A PET radioligands.

MATERIALS & METHODS

[(11)C]MP10, [(11)C]JNJ42071965 and four other tracers were developed. Their biodistribution was evaluated in rats. Rat plasma and brain radiometabolites were quantified. Baseline microPET imaging was performed in normal rats and PDE10A knockout (KO) and wild-type (WT) mice. Blocking and displacement studies were conducted. The selectivity of the tracer binding was further studied in an ex vivo autoradiography experiment in PDE10A KO and WT mice.

RESULTS

Biodistribution showed brain uptake for all tracers in the striatum and wash-out from the cerebellum. [(11)C]1 ((11)C-MP10) had the highest specific uptake index (striatum (S) vs. cerebellum (C) ratios (S/C)-1) at 60 min (7.4). [(11)C]5 ([(11)C]JNJ42071965) had a high index at the early time points (1.0 and 3.7 at 2 and 30 min p.i., respectively). The affinity of [(11)C]4, [(18)F]3 and [(18)F]6 was too low to visualize PDE10A using microPET. [(11)C] 2 showed a specific binding, while kinetics of [(11)C]1 were too slow. [(11)C]5 reached equilibrium after 10 min (uptake index=1.2). Blocking and displacement experiments in rats and baseline imaging in PDE10A KO mice showed specific and reversible binding of [(11)C]5 to PDE10A.

CONCLUSIONS

We successfully radiolabeled and evaluated six radiotracers for their potential to visualize PDE10A in vivo. While [(11)C]1 had the highest striatal specific uptake index, its slow kinetics likely compromise clinical use of this tracer. [(11)C]5 has a relatively high striatum-to-background ratio and fast kinetic profile, which makes it a valuable carbon-11 alternative.

Syntheses, radiolabelings, and in vitro evaluations of fluorinated PET radioligands of 5-HT6 serotoninergic receptors

The 5-HT6 receptors are potent therapeutic targets for psychiatric and neurological diseases (schizophrenia, Alzheimer's disease, etc.). However, with lack of specific radiopharmaceuticals, their pharmacology is still incomplete and their exploration is limited to animal models. In this context, we have designed a fluorinated PET radiotracer, [(18)F]2FNQ1P, that possesses a high affinity and selectivity for 5-HT6. In vitro PET autoradiographies in rat brain sections with this radiotracer were in accordance with the 5-HT6 distribution pattern.