Revisiting the Logan plot to account for non-negligible blood volume in brain tissue

PET Molecular Imaging in Drug Development: The Imaging and Chemistry Perspective

Positron emission tomography with selective radioligands advances the drug discovery and development process by revealing information about target engagement, proof of mechanism, pharmacokinetic and pharmacodynamic profiles. Positron emission tomography (PET) is an essential and highly significant tool to study therapeutic drug development, dose regimen, and the drug plasma concentrations of new drug candidates. Selective radioligands bring up target-specific information in several disease states including cancer, cardiovascular, and neurological conditions by quantifying various rates of biological processes with PET, which are associated with its physiological changes in living subjects, thus it reveals disease progression and also advances the clinical investigation. This study explores the major roles, applications, and advances of PET molecular imaging in drug discovery and development process with a wide range of radiochemistry as well as clinical outcomes of positron-emitting carbon-11 and fluorine-18 radiotracers.

Mathematical Models for FDG Kinetics in Cancer: A Review

Compartmental analysis is the mathematical framework for the modelling of tracer kinetics in dynamical Positron Emission Tomography. This paper provides a review of how compartmental models are constructed and numerically optimized. Specific focus is given on the identifiability and sensitivity issues and on the impact of complex physiological conditions on the mathematical properties of the models.

Clinical evaluation of [18F] JNJ-64326067, a novel candidate PET tracer for the detection of tau pathology in Alzheimer's disease

PURPOSE

The accumulation of misfolded tau is a common feature of several neurodegenerative disorders, with Alzheimer's disease (AD) being the most common. Earlier we identified JNJ-64326067, a novel isoquinoline derivative with high affinity and selectivity for tau aggregates from human AD brain. We report the dosimetry of [¹⁸F] JNJ-64326067 and results of a proof-of-concept study comparing subjects with probable Alzheimer's disease to age-matched healthy controls.

METHODS

[¹⁸F] JNJ-64326067 PET scans were acquired for 90 min and then from 120 to 180 min in 5 participants with [¹⁸F]-florbetapir PET amyloid positive probable AD (73 ± 9 years) and 5 [¹⁸F]-florbetapir PET amyloid negative healthy controls (71 ± 7 years). Whole-body [¹⁸F] JNJ-64326067 PET CT scans were acquired in six healthy subjects for 5.5 h in 3 scanning sessions. Brain PET scans were visually reviewed. Regional quantification included kinetic analysis of distribution volume ration (DVR) estimated by Logan graphical analysis over the entire scan and static analysis of SUVr in late frames. Both methods used ventral cerebellar cortex as a reference region.

RESULTS

One of the healthy controls had focal areas of PET signal in occipital and parietal cortex underlying the site of a gunshot injury as an adolescent; the other four healthy subjects had no tau brain signal. Four of the 5 AD participants had visually apparent retention of [¹⁸F] JNJ-64326067 in relevant cortical regions. One of the AD subjects was visually negative. Cortical signal in visually positive subjects approached steady state by 120 min. Temporal and frontal cortical SUVr/DVR values in visually positive AD subjects ranged from 1.21 to 3.09/1.2 to 2.18 and from 0.92 to 1.28/0.91 to 1.16 in healthy controls. Whole-body effective dose was estimated to be 0.0257 mSv/MBq for females and 0.0254 mSv/MBq for males.

CONCLUSIONS

[¹⁸F] JNJ-64326067 could be useful for detection and quantitation of tau aggregates.

A PET study in healthy subjects of brain exposure of 11C-labelled osimertinib - A drug intended for treatment of brain metastases in non-small cell lung cancer

Osimertinib is a tyrosine kinase inhibitor (TKI) of the mutated epidermal growth factor receptor (EGFRm) with observed efficacy in patients with brain metastases. Brain exposure and drug distribution in tumor regions are important criteria for evaluation and confirmation of CNS efficacy. The aim of this PET study was therefore to determine brain distribution and exposure of 11C-labelled osimertinib administered intravenously in subjects with an intact blood-brain barrier. Eight male healthy subjects (age 52 ± 8 years) underwent one PET measurement with 11C-osimertinib. The pharmacokinetic parameters Cmax(brain) (standardized uptake value), Tmax(brain) and AUC0-90 minbrain/blood ratio were calculated. The outcome measure for 11C-osimertinib brain exposure was the total distribution volume (VT). 11C-osimertinib distributed rapidly to the brain, with higher uptake in grey than in white matter. Mean Cmax, Tmax and AUC0-90 minbrain/blood ratio were 1.5 (range 1-1.8), 13 min (range 5-30 min), and 3.8 (range 3.3-4.1). Whole brain and white matter VT were 14 mL×cm-3 (range 11-18) and 7 mL×cm-3 (range 5-12). This study in healthy volunteers shows that 11C-osimertinib penetrates the intact blood-brain barrier. The approach used further illustrates the role of molecular imaging in facilitating the development of novel drugs for the treatment of malignancies affecting the brain.

PET Molecular Imaging of Phosphodiesterase 10A: An Early Biomarker of Huntington's Disease Progression

BACKGROUND

Changes in phosphodiesterase 10A enzyme levels may be a suitable biomarker of disease progression in Huntington's disease.

OBJECTIVES

To evaluate phosphodiesterase 10A PET imaging as a biomarker of HD progression using the radioligand, [¹⁸ F]MNI-659.

METHODS

The cross-sectional study (NCT02061722) included 45 Huntington's disease gene-expansion carriers stratified into four disease stages (early and late premanifest and Huntington's disease stages 1 and 2) and 45 age- and sex-matched healthy controls. The primary analysis compared striatal and pallidal phosphodiesterase 10A availability between Huntington's disease gene-expansion carriers and healthy controls as assessed by [¹⁸ F]MNI-659 binding. We assessed changes in phosphodiesterase 10A expression using several PET methodologies and compared with previously proposed measures of Huntington's disease progression (PET imaging of D_2/3 receptors and anatomical volume loss on MRI). The longitudinal follow-up study (NCT02956148) continued evaluation of phosphodiesterase 10A availability in 35 Huntington's disease gene-expansion carriers at a mean of 18 months from baseline of the cross-sectional study.

RESULTS

Primary analyses revealed that phosphodiesterase 10A availability in caudate, putamen, and globus pallidus was significantly lower in Huntington's disease gene-expansion carriers versus healthy controls across all stages. Striatal and pallidal phosphodiesterase 10A availability progressively declined in the premanifest stages and appeared to plateau between stages 1 and 2. The percentage decline of phosphodiesterase 10A availability measured cross-sectionally between Huntington's disease gene-expansion carriers and healthy controls was greater than that demonstrated by D_2/3 receptor availability or volumetric changes. Annualized rates of phosphodiesterase 10A change showed a statistically significant decline between the cross-sectional study and follow-up.

CONCLUSIONS

[¹⁸ F]MNI-659 PET imaging is a biologically plausible biomarker of Huntington's disease progression that is more sensitive than the dopamine-receptor and volumetric methods currently used. © 2020 International Parkinson and Movement Disorder Society.

QModeling: a Multiplatform, Easy-to-Use and Open-Source Toolbox for PET Kinetic Analysis

Kinetic modeling is at the basis of most quantification methods for dynamic PET data. Specific software is required for it, and a free and easy-to-use kinetic analysis toolbox can facilitate routine work for clinical research. The relevance of kinetic modeling for neuroimaging encourages its incorporation into image processing pipelines like those of SPM, also providing preprocessing flexibility to match the needs of users. The aim of this work was to develop such a toolbox: QModeling. It implements four widely-used reference-region models: Simplified Reference Tissue Model (SRTM), Simplified Reference Tissue Model 2 (SRTM2), Patlak Reference and Logan Reference. A preliminary validation was also performed: The obtained parameters were compared with the gold standard provided by PMOD, the most commonly-used software in this field. Execution speed was also compared, for time-activity curve (TAC) estimation, model fitting and image generation. QModeling has a simple interface, which guides the user through the analysis: Loading data, obtaining TACs, preprocessing the model for pre-evaluation, generating parametric images and visualizing them. Relative differences between QModeling and PMOD in the parameter values are almost always below 10^-8. The SRTM2 algorithm yields relative differences from 10^-3 to 10^-5 when [Formula: see text] is not fixed, since different, validated methods are used to fit this parameter. The new toolbox works efficiently, with execution times of the same order as those of PMOD. Therefore, QModeling allows applying reference-region models with reliable results in efficient computation times. It is free, flexible, multiplatform, easy-to-use and open-source, and it can be easily expanded with new models.

Development of 2-(2-(3-(4-([18F]Fluoromethoxy- d2)phenyl)-7-methyl-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione for Positron-Emission-Tomography Imaging of Phosphodiesterase 10A in the Brain

Phosphodiesterase 10A (PDE10A) is a newly identified therapeutic target for central-nervous-system disorders. 2-(2-(3-(4-([¹⁸F]Fluoroethoxy)phenyl)-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ([¹⁸F]MNI-659, [¹⁸F]5) is a useful positron-emission-tomography (PET) ligand for imaging of PDE10A in the human brain. However, the radiolabeled metabolite of [¹⁸F]5 can accumulate in the brain. In this study, using [¹⁸F]5 as a lead compound, we designed four new ¹⁸F-labeled ligands ([¹⁸F]6-9) to find one more suitable than [¹⁸F]5. Of these, 2-(2-(3-(4-([¹⁸F]fluoromethoxy- d₂)phenyl)-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ([¹⁸F]9) exhibited high in vitro binding affinity ( K_i = 2.9 nM) to PDE10A and suitable lipophilicity (log D = 2.2). In PET studies, the binding potential (BP_ND) of [¹⁸F]9 (5.8) to PDE10A in the striatum of rat brains was significantly higher than that of [¹⁸F]5 (4.6). Furthermore, metabolite analysis showed much lower levels of contamination with radiolabeled metabolites in the brains of rats given [¹⁸F]9 than in those given [¹⁸F]5. In conclusion, [¹⁸F]9 is a useful PET ligand for PDE10A imaging in brain.