Quantitative PET in the 2020s: a roadmap.

Positron emission tomography (PET) plays an increasingly important role in research and clinical applications, catalysed by remarkable technical advances and a growing appreciation of the need for reliable, sensitive biomarkers of human function in health and disease. Over the last 30 years, a large amount of the physics and engineering effort in PET has been motivated by the dominant clinical application during that period, oncology. This has led to important developments such as PET/CT, whole-body PET, 3D PET, accelerated statistical image reconstruction, and time-of-flight PET. Despite impressive improvements in image quality as a result of these advances, the emphasis on static, semi-quantitative 'hot spot' imaging for oncologic applications has meant that the capability of PET to quantify biologically relevant parameters based on tracer kinetics has not been fully exploited. More recent advances, such as PET/MR and total-body PET, have opened up the ability to address a vast range of new research questions, from which a future expansion of applications and radiotracers appears highly likely. Many of these new applications and tracers will, at least initially, require quantitative analyses that more fully exploit the exquisite sensitivity of PET and the tracer principle on which it is based. It is also expected that they will require more sophisticated quantitative analysis methods than those that are currently available. At the same time, artificial intelligence is revolutionizing data analysis and impacting the relationship between the statistical quality of the acquired data and the information we can extract from the data. In this roadmap, leaders of the key sub-disciplines of the field identify the challenges and opportunities to be addressed over the next ten years that will enable PET to realise its full quantitative potential, initially in research laboratories and, ultimately, in clinical practice.

Development and biological evaluation of[18F]FMN3PA & [18F]FMN3PU for leucine-rich repeat kinase 2 (LRRK2) in vivo PET imaging.

PURPOSE: Among all genetic mutations of LRRK2, the G2019S mutation is the most commonly associated with the late-onset of Parkinson's disease (PD). Hence, one potential therapeutic approach is to block the hyperactivity of mutated LRRK2 induced by kinase inhibition. To date, only a few LRRK2 kinase inhibitors have been tested for in vivo quantification of target engagement by positron emission tomography (PET). In this study, we performed biological evaluations of two radiolabeled kinase inhibitors i.e. [18F]FMN3PA (14) and [18F]FMN3PU for LRRK2 (15). PROCEDURES: Radiosyntheses of [18F]FMN3PA (14) and [18F]FMN3PU (15) were performed using K[18F]-F-K222 complex in a TRACERlab FXN module and purification was carried out via C18 plus (Sep-Pak) cartridges. In vitro specific binding assays were performed in rat brain striatum and kidney tissues using GNE-0877 as a blocking agent (Ki = 0.7 nM). For in vivo blocking, 3 mg/kg of GNE-0877 was injected 30 min before radiotracer injection via tail vein in wild-type (WT) mice (n = 4). Dynamic scans by PET/CT (Siemens Inveon) were performed in WT mice (n = 3). RESULTS: Radiofluorinations resulted in radiochemical yields (RCYs) of 25 ± 1.3% (n = 6) ([18F]FMN3PU, 15) and 37 ± 1.6% (n = 6) ([18F]FMN3PA, 14) with ≥96% radiochemical purity (RCP) and a molar activity (MA) of 3.55 ± 1.6 Ci/µmol (131 ± 56 GBq/µmol) for [18F]FMN3PU (15) and 4.57 ± 1.7 Ci/µmol (169 ± 63 GBq/µmol) for [18F]FMN3PA (14), respectively. Saturation assays showed high specific binding for rat brain striatum with Kd 20 ± 1.3 nM ([18F]FMN3PA, 14) and 23.6 ± 4.0 nM ([18F]FMN3PU, 15). In vivo blocking data for [18F]FMN3PA (14) was significant for brain (p < 0.0001, 77% blocking) and kidney (p = 0.0041, 65% blocking). PET images showed uptake in mouse brain striatum. CONCLUSION: In the presence of GNE-0877 as a blocking agent, the specific binding of [18F]FMN3PA (14) and [18F]FMN3PU (15) was significant in vitro. [18F]FMN3PA (14) showed good brain uptake in vivo, though fast clearance from brain was observed (within 10-15 min).

Synthesis and targeting of gold-coated 177Lu-containing lanthanide phosphate nanoparticles-A potential theranostic agent for pulmonary metastatic disease.

Targeted radiotherapies maximize cytotoxicity to cancer cells. In this work, we describe the synthesis, characterization, and biodistribution of antibody conjugated gold-coated lanthanide phosphate nanoparticles containing 177Lu. [177Lu]Lu0.5Gd0.5(PO4)@Au@PEG800@Ab nanoparticles combine the radiation resistance of crystalline lanthanide phosphate for stability, the magnetic properties of gadolinium for facile separations, and a gold coating that can be readily functionalized for the attachment of targeting moieties. In contrast to current targeted radiotherapeutic pharmaceuticals, the nanoparticle-antibody conjugate can target and deliver multiple beta radiations to a single biologically relevant receptor. Up to 95% of the injected dose was delivered to the lungs using the monoclonal antibody mAb-201b to target the nanoparticles to thrombomodulin receptors. The 208 keV gamma ray from 177Lu decay (11%) can be used for SPECT imaging of the radiotherapeutic agent, while the moderate energy beta emitted in the decay can be highly effective in treating metastatic disease.

Multi-isotope SPECT imaging of the 225Ac decay chain: feasibility studies.

Effective use of the [Formula: see text] decay chain in targeted internal radioimmunotherapy requires the retention of both [Formula: see text] and progeny isotopes at the target site. Imaging-based pharmacokinetic tests of these pharmaceuticals must therefore separately yet simultaneously image multiple isotopes that may not be colocalized despite being part of the same decay chain. This work presents feasibility studies demonstrating the ability of a microSPECT/CT scanner equipped with a high energy collimator to simultaneously image two components of the [Formula: see text] decay chain: [Formula: see text] (218 keV) and [Formula: see text] (440 keV). Image quality phantoms were used to assess the performance of two collimators for simultaneous [Formula: see text] and [Formula: see text] imaging in terms of contrast and noise. A hotrod resolution phantom containing clusters of thin rods with diameters ranging between 0.85 and 1.70 mm was used to assess resolution. To demonstrate ability to simultaneously image dynamic [Formula: see text] and [Formula: see text] activity distributions, a phantom containing a [Formula: see text] generator from [Formula: see text] was imaged. These tests were performed with two collimators, a high-energy ultra-high resolution (HEUHR) collimator and an ultra-high sensitivity (UHS) collimator. Values consistent with activity concentrations determined independently via gamma spectroscopy were observed in high activity regions of the images. In hotrod phantom images, the HEUHR collimator resolved all rods for both [Formula: see text] and [Formula: see text] images. With the UHS collimator, no rods were resolvable in [Formula: see text] images and only rods ⩾1.3 mm were resolved in [Formula: see text] images. After eluting the [Formula: see text] generator, images accurately visualized the reestablishment of transient equilibrium of the [Formula: see text] decay chain. The feasibility of evaluating the pharmacokinetics of the [Formula: see text] decay chain in vivo has been demonstrated. This presented method requires the use of a high-performance high-energy collimator.

18F-5-Fluoroaminosuberic Acid as a Potential Tracer to Gauge Oxidative Stress in Breast Cancer Models.

The cystine transporter (system xC-) is an antiporter of cystine and glutamate. It has relatively low basal expression in most tissues and becomes upregulated in cells under oxidative stress (OS) as one of the genes expressed in response to the antioxidant response element promoter. We have developed 18F-5-fluoroaminosuberic acid (FASu), a PET tracer that targets system xC- The goal of this study was to evaluate 18F-FASu as a specific gauge for system xC- activity in vivo and its potential for breast cancer imaging. Methods:18F-FASu specificity toward system xC- was studied by cell inhibition assay, cellular uptake after OS induction with diethyl maleate, with and without anti-xCT small interfering RNA knockdown, in vitro uptake studies, and in vivo uptake in a system xC--transduced xenograft model. In addition, radiotracer uptake was evaluated in 3 breast cancer models: MDA-MB-231, MCF-7, and ZR-75-1. Results: Reactive oxygen species-inducing diethyl maleate increased glutathione levels and 18F-FASu uptake, whereas gene knockdown with anti-xCT small interfering RNA led to decreased tracer uptake. 18F-FASu uptake was robustly inhibited by system xC- inhibitors or substrates, whereas uptake was significantly higher in transduced cells and tumors expressing xCT than in wild-type HEK293T cells and tumors (P < 0.0001 for cells, P = 0.0086 for tumors). 18F-FASu demonstrated tumor uptake in all 3 breast cancer cell lines studied. Among them, triple-negative breast cancer MDA-MB-231, which has the highest xCT messenger RNA level, had the highest tracer uptake (P = 0.0058 when compared with MCF-7; P < 0.0001 when compared with ZR-75-1). Conclusion:18F-FASu as a system xC- substrate is a specific PET tracer for functional monitoring of system xC- and OS imaging. By enabling noninvasive analysis of xC- responses in vivo, this biomarker may serve as a valuable target for the diagnosis and treatment monitoring of certain breast cancers.

Overexpression of HER-2 in MDA-MB-435/LCC6 Tumours is Associated with Higher Metabolic Activity and Lower Energy Stress.

Overexpresssion of HER-2 in the MDA-MB-435/LCC6 (LCC6(HER-2)) tumour model is associated with significantly increased hypoxia and reduced necrosis compared to isogenic control tumours (LCC6(Vector)); this difference was not related to tumour size or changes in vascular architecture. To further evaluate factors responsible for HER-2-associated changes in the tumour microenvironment, small animal magnetic resonance imaging (MRI) and positron emission tomography (PET) were used to measure tumour tissue perfusion and metabolism, respectively. The imaging data was further corroborated by analysis of molecular markers pertaining to energy homeostasis, and measurements of hypoxia and glucose consumption. The results showed a strong trend towards higher perfusion rates (~58% greater, p = 0.14), and significantly higher glucose uptake in LCC6(HER-2) (~2-fold greater; p = 0.025), relative to control tumours. The expression of proteins related to energy stress (P-AMPK, P-ACC) and glucose transporters (GLUT1) were lower in LCC6(HER-2) tumours (~2- and ~4-fold, respectively). The in vitro analysis showed that LCC6(HER-2) cells become more hypoxic in 1% oxygen and utilise significantly more glucose in normoxia compared to LCC6(Vector)cells (p < 0.005). Amalgamation of all the data points suggests a novel metabolic adaptation driven by HER-2 overexpression where higher oxygen and glucose metabolic rates produce rich energy supply but also a more hypoxic tumour mass.

A simple route to [11C]N-Me labeling of aminosuberic acid for proof of feasibility imaging of the x(C)⁻ transporter.

Oxidative stress has been implicated in a variety of conditions, including cancer, heart failure, diabetes, neurodegeneration and other diseases. A potential biomarker for oxidative stress is the cystine/glutamate transporter, system x(C)(-). L-Aminosuberic acid (L-ASu) has been identified as a system x(C)(-) substrate. Here we report a facile method for [(11)C]N-Me labeling of L-ASu, automation of the radiochemical process, and preliminary PET imaging with EL4 tumor bearing mice. The results demonstrate uptake in the tumor above background, warranting further studies on the use of radiolabeled analogs of L-ASu as a PET imaging agent for system x(C)(-).

Development of a PET scanner for simultaneously imaging small animals with MRI and PET.

Recently, positron emission tomography (PET) is playing an increasingly important role in the diagnosis and staging of cancer. Combined PET and X-ray computed tomography (PET-CT) scanners are now the modality of choice in cancer treatment planning. More recently, the combination of PET and magnetic resonance imaging (MRI) is being explored in many sites. Combining PET and MRI has presented many challenges since the photo-multiplier tubes (PMT) in PET do not function in high magnetic fields, and conventional PET detectors distort MRI images. Solid state light sensors like avalanche photo-diodes (APDs) and more recently silicon photo-multipliers (SiPMs) are much less sensitive to magnetic fields thus easing the compatibility issues. This paper presents the results of a group of Canadian scientists who are developing a PET detector ring which fits inside a high field small animal MRI scanner with the goal of providing simultaneous PET and MRI images of small rodents used in pre-clinical medical research. We discuss the evolution of both the crystal blocks (which detect annihilation photons from positron decay) and the SiPM array performance in the last four years which together combine to deliver significant system performance in terms of speed, energy and timing resolution.

Behavioral deficits and striatal DA signaling in LRRK2 p.G2019S transgenic rats: a multimodal investigation including PET neuroimaging.

BACKGROUND: A major risk-factor for developing Parkinson's disease (PD) is genetic variability in leucine-rich repeat kinase 2 (LRRK2), most notably the p.G2019S mutation. Examination of the effects of this mutation is necessary to determine the etiology of PD and to guide therapeutic development. OBJECTIVE: Assess the behavioral consequences of LRRK2 p.G2019S overexpression in transgenic rats as they age and test the functional integrity of the nigro-striatal dopamine system. Conduct positron emission tomography (PET) neuroimaging to compare transgenic rats with previous data from human LRRK2 mutation carriers. METHODS: Rats overexpressing human LRRK2 p.G2019S were generated by BAC transgenesis and compared to non-transgenic (NT) littermates. Motor skill tests were performed at 3, 6 and 12 months-of-age. PET, performed at 12 months, assessed the density of dopamine and vesicular monoamine transporters (DAT and VMAT2, respectively) and measured dopamine synthesis, storage and availability. Brain tissue was assayed for D2, DAT, dopamine and cAMP-regulated phosphoprotein (DARPP32) and tyrosine hydroxylase (TH) expression by Western blot, and TH by immunohistochemistry. RESULTS: Transgenic rats had no abnormalities in measures of striatal dopamine function at 12 months. A behavioral phenotype was present, with LRRK2 p.G2019S rats performing significantly worse on the rotarod than non-transgenic littermates (26% reduction in average running duration at 6 months), but with normal performance in other motor tests. CONCLUSIONS: Neuroimaging using dopaminergic PET did not recapitulate prior studies in human LRRK2 mutation carriers. Consistently, LRRK2 p.G2019S rats do not develop overt neurodegeneration; however, they do exhibit behavioral abnormalities.

DJ-1 and αSYN in LRRK2 CSF do not correlate with striatal dopaminergic function.

Previous studies demonstrated decreased levels of DJ-1 and α-synuclein (αSYN) in human cerebrospinal fluid (CSF) in patients with Parkinson's disease (PD), but neither marker correlated with PD severity, raising the possibility that they may be excellent progression markers during early or preclinical phases of PD. Individuals carrying the leucine-rich repeat kinase 2 (LRRK2) gene mutation are at increased risk for PD, and the phenotype of LRRK2 patients is almost identical to sporadic PD. To determine whether dopaminergic dysfunction in the basal ganglia, as determined by positron emission tomography (PET) scans, correlates with CSF levels of DJ-1 and αSYN during preclinical stages, Luminex assays were used to analyze CSF samples from asymptomatic LRRK2 mutation carriers, along with carriers who presented with a clinical diagnosis of PD. The data revealed no statistically significant relationship between PET scan evidence of loss of striatal dopaminergic function and the CSF biomarkers DJ-1 and αSYN, except for a weak correlation between DJ-1 and methylphenidate binding, suggesting that the use of these potential biomarkers on their own to screen LRRK2 gene mutation carriers for PD is not appropriate.

In vivo measurement of density and affinity of the monoamine vesicular transporter in a unilateral 6-hydroxydopamine rat model of PD.

This is the first in vivo determination of the vesicular monoamine transporter (VMAT2) density (B(max)) and ligand-transporter affinity (K(d)(app)) in six unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats using micro-positron emission tomography (PET) imaging with [(11)C]-(+)-alpha-dihydrotetrabenazine (DTBZ). A multiple ligand concentration transporter assay (MLCTA) was used to determine a B(max) value of 178+/-32 pmol/mL and a K(d)(app) of 47.7+/-9.3 pmol/mL for the non-lesioned side and 30.52+/-5.84 and 43.4+/-15.52 pmol/mL for the lesioned side, respectively. While B(max) was significantly different between the two sides, no significant difference was observed for the K(d)(app). In addition to demonstrating the feasibility of in vivo Scatchard analysis in rats, these data confirm the expectation that a 6-OHDA lesion does not affect the affinity; a much simpler binding potential (BP) measure can thus be used as a marker of lesion severity (LS) in this rat model of Parkinson's disease. A transporter occupancy curve demonstrated negligible transporter occupancy ( approximately 1%) at a specific activity (SA) of 1100 nCi/pmol (assuming an injected dose of 100 microCi/100 g), while 10% occupancy was estimated at 100 nCi/pmol. An indirect measurement indicated that the degree of occupancy as a function of SA is independent of LS. Finally, BP measurement reproducibility was assessed and found to be 11%+/-7% for the healthy and 8%+/-12% for the lesioned side. Quantitative PET results can thus be obtained even for severely lesioned animals with the striatum on one side not clearly visible provided accurate image analysis methods are used.