HPLC and TLC methods for analysis of [18F]FDG and its metabolites from biological samples

Development and preclinical validation of 2-deoxy 2-[18F]fluorocellobiose as an Aspergillus-specific PET tracer

The global incidence of invasive fungal infections (IFIs) has increased over the past few decades, mainly in immunocompromised patients, and is associated with high mortality and morbidity. Aspergillus fumigatus is one of the most common and deadliest IFI pathogens. Major hurdles to treating fungal infections remain the lack of rapid and definitive diagnosis, including the frequent need for invasive procedures to provide microbiological confirmation, and the lack of specificity of structural imaging methods. To develop an Aspergillus-specific positron emission tomography (PET) imaging agent, we focused on fungal-specific sugar metabolism. We radiolabeled cellobiose, a disaccharide known to be metabolized by Aspergillus species, and synthesized 2-deoxy-2-[18F]fluorocellobiose ([18F]FCB) by enzymatic conversion of 2-deoxy-2-[18F]fluoroglucose ([18F]FDG) with a radiochemical yield of 60 to 70%, a radiochemical purity of >98%, and 1.5 hours of synthesis time. Two hours after [18F]FCB injection in A. fumigatus pneumonia as well as A. fumigatus, bacterial, and sterile inflammation myositis mouse models, retained radioactivity was only seen in foci with live A. fumigatus infection. In vitro testing confirmed production of β-glucosidase enzyme by A. fumigatus and not by bacteria, resulting in hydrolysis of [18F]FCB into glucose and [18F]FDG, the latter being retained by the live fungus. The parent molecule was otherwise promptly excreted through the kidneys, resulting in low background radioactivity and high target-to-nontarget ratios at A. fumigatus infectious sites. We conclude that [18F]FCB is a promising and clinically translatable Aspergillus-specific PET tracer.

Peptidoglycan-Targeted [18F]3,3,3-Trifluoro-d-alanine Tracer for Imaging Bacterial Infection

Imaging is increasingly used to detect and monitor bacterial infection. Both anatomic (X-rays, computed tomography, ultrasound, and MRI) and nuclear medicine ([111In]-WBC SPECT, [18F]FDG PET) techniques are used in clinical practice but lack specificity for the causative microorganisms themselves. To meet this challenge, many groups have developed imaging methods that target pathogen-specific metabolism, including PET tracers integrated into the bacterial cell wall. We have previously reported the d-amino acid derived PET radiotracers d-methyl-[11C]-methionine, d-[3-11C]-alanine, and d-[3-11C]-alanine-d-alanine, which showed robust bacterial accumulation in vitro and in vivo. Given the clinical importance of radionuclide half-life, in the current study, we developed [18F]3,3,3-trifluoro-d-alanine (d-[18F]-CF3-ala), a fluorine-18 labeled tracer. We tested the hypothesis that d-[18F]-CF3-ala would be incorporated into bacterial peptidoglycan given its structural similarity to d-alanine itself. NMR analysis showed that the fluorine-19 parent amino acid d-[19F]-CF3-ala was stable in human and mouse serum. d-[19F]-CF3-ala was also a poor substrate for d-amino acid oxidase, the enzyme largely responsible for mammalian d-amino acid metabolism and a likely contributor to background signals using d-amino acid derived PET tracers. In addition, d-[19F]-CF3-ala showed robust incorporation into Escherichia coli peptidoglycan, as detected by HPLC/mass spectrometry. Based on these promising results, we developed a radiosynthesis of d-[18F]-CF3-ala via displacement of a bromo-precursor with [18F]fluoride followed by chiral stationary phase HPLC. Unexpectedly, the accumulation of d-[18F]-CF3-ala by bacteria in vitro was highest for Gram-negative pathogens in particular E. coli. In a murine model of acute bacterial infection, d-[18F]-CF3-ala could distinguish live from heat-killed E. coli, with low background signals. These results indicate the viability of [18F]-modified d-amino acids for infection imaging and indicate that improved specificity for bacterial metabolism can improve tracer performance.

A fingerprint of 2-[18F]FDG radiometabolites - How tissue-specific metabolism beyond 2-[18F]FDG-6-P could affect tracer accumulation

Studies indicate that the radiotracer 2-[18F]fluoro-2-deoxy-D-glucose (2-[18F]FDG) can be metabolized beyond 2-[18F]FDG-6-phosphate (2-[18F]FDG-6-P), but its metabolism is incompletely understood. Most importantly, it remains unclear whether downstream metabolism affects tracer accumulation in vivo. Here we present a fingerprint of 2-[18F]FDG radiometabolites over time in cancer cells, corresponding tumor xenografts and murine organs. Strikingly, radiometabolites representing glycogen metabolism or the oxPPP correlated inversely with tracer accumulation across all examined tissues. Recent studies suggest that not only hexokinase, but also hexose-6-phosphate dehydrogenase (H6PD), an enzyme of the oxidative pentose phosphate pathway (oxPPP), determines 2-[18F]FDG accumulation. However, little is known about the corresponding enzyme glucose-6-phosphate dehydrogenase (G6PD). Our mechanistic in vitro experiments on the role of the oxPPP propose that 2-[18F]FDG can be metabolized via both G6PD and H6PD, but data from separate enzyme knockdown suggest diverging roles in downstream tracer metabolism. Overall, we propose that tissue-specific metabolism beyond 2-[18F]FDG-6-P could matter for imaging.

Method development, validation, and impurity measurement of β-estradiol from radiolabeled [131I]β-estradiol using radio-high-performance liquid chromatography for radioligand of saturation binding assay

β-estradiol is an estrogen steroid hormone and acts as an estrogen receptor agonist. Radiolabeled β-estradiol is widely used as a radioligand for binding assays. In this present study, the synthesis of [131I]β-estradiol has been successfully carried out. Accordingly, the measurement of the radiochemical purity (RCP) value and the presence of chemical impurities are needed. To validate the method for identifying the RCP and chemical impurities from [131I]β-estradiol using high-performance liquid chromatography (HPLC). The synthesis of [131I]β-estradiol was accomplished by a radioiodination reaction, and the RCP was determined by radio-HPLC. The method for β-estradiol measurement was validated by reversed-phase HPLC radio-analytical employing ultraviolet-visible (UV-Vis) and radioactive detector. The method for radio-HPLC analysis was validated and established using a C-18 column and MeCN: H2O (55:45 v/v) as the mobile phase. The following conditions were applied: a flow rate of 1.2 mL/min, isocratic, and a UV-Vis detector at 280 nm. The RCP of [131I]β-estradiol measured by thin-layer chromatography and radio-HPLC was 99.27% ± 1.25% and 95.75% ± 2.41%, respectively. The validation parameters were appropriate and met the requirements for acceptance. HPLC analysis was able to identify the presence of unlabeled estradiol (24.51%-27.29%) in the mixture of [131I]β-estradiol. As a result, purification using preparative HPLC or other methods will be required in future studies.

Positive effects of dietary fiber from sweet potato [Ipomoea batatas (L.) Lam.] peels by different extraction methods on human fecal microbiota in vitro fermentation

The purpose of this study was to compare the effects of sweet potato peels dietary fiber obtained by different extraction methods on intestinal health. Specifically, four different dietary fibers were extracted by hot water, microwave, ultrasonic and subcritical water methods. And the prebiotics effects of sweet potato peels dietary fibers were explored in an in vitro fermentation model, by determining intestinal gas content, short-chain fatty acid content, pH, ammonia content and the gut microbiota composition. The results showed that dietary fiber obtained by four different extraction methods could be utilized by GM and improve human health by increasing the abundance of beneficial bacteria (e.g., Bifidobacterium, Faecalibacterium, and Prevotella) and reducing the abundance of harmful bacteria (e.g., Proteobacteria, Romboutsia and Dorea), enhancing the relative abundance of SCFA-producing bacteria, promoting the production of short-chain fatty acids, reducing intestinal pH from 6.89 to 4.63 and ammonia. Among them, dietary fiber extracted by ultrasound is better than the other three extraction methods. This study suggests that all the four different extraction methods are available for sweet potato peels dietary fiber, and the extracted dietary fiber could be served as potential functional foods with great development value. In addition, it is beneficial to reduce the environmental pollution of sweet potato peels and improve the high-value processing and utilization of sweet potato by-products.

Hepatic Positron Emission Tomography: Applications in Metabolism, Haemodynamics and Cancer

Evaluating in vivo the metabolic rates of the human liver has been a challenge due to its unique perfusion system. Positron emission tomography (PET) represents the current gold standard for assessing non-invasively tissue metabolic rates in vivo. Here, we review the existing literature on the assessment of hepatic metabolism, haemodynamics and cancer with PET. The tracer mainly used in metabolic studies has been [¹⁸F]2-fluoro-2-deoxy-D-glucose (¹⁸F-FDG). Its application not only enables the evaluation of hepatic glucose uptake in a variety of metabolic conditions and interventions, but based on the kinetics of ¹⁸F-FDG, endogenous glucose production can also be assessed. 14(R,S)-[¹⁸F]fluoro-6-thia-Heptadecanoic acid (¹⁸F-FTHA), ¹¹C-Palmitate and ¹¹C-Acetate have also been applied for the assessment of hepatic fatty acid uptake rates (¹⁸F-FTHA and ¹¹C-Palmitate) and blood flow and oxidation (¹¹C-Acetate). Oxygen-15 labelled water (¹⁵O-H₂O) has been used for the quantification of hepatic perfusion. ¹⁸F-FDG is also the most common tracer used for hepatic cancer diagnostics, whereas ¹¹C-Acetate has also shown some promising applications in imaging liver malignancies. The modelling approaches used to analyse PET data and also the challenges in utilizing PET in the assessment of hepatic metabolism are presented.

Quantitative radio-thin-layer chromatography and positron emission tomography studies for measuring streptavidin transduced chimeric antigen receptor T cells

The proliferation of chimeric antigen receptor (CAR) T cells is closely related to their efficacy, but it is still a great challenge to monitor and quantify CAR T cells in vivo. Based on the high affinity (Kd ≈ 10^-15 M) of streptavidin (SA) and biotin, radiolabeled biotin may be used to quantify SA-transduced CAR T cells (SA-CAR T cells). Radio-thin-layer chromatography (radio-TLC) and positron emission tomography (PET) are highly sensitive for trace analysis. Our aim was to develop radio-TLC and PET methods to quantify SA-CAR T cells in vitro and in vivo. First, we developed [⁶⁸Ga]-DOTA-biotin. Commercially available SA was used as a standard, and quantitative standard curves were established in vitro and in vivo by radio-TLC and PET. Furthermore, the feasibility of the method was verified in Raji model mice. The linear range of radio-TLC was 0.02 ∼ 0.15 pmol/μL with R² = 0.9993 in vitro. The linear range of PET was 0.02 ∼ 0.76 pmol/μL with R² = 0.9986 in vivo. SA in CAR T cells can also be accurately quantified in a Raji leukemia model according to PET imaging. The radio-TLC/PET method established in this study is promising for using in the dynamic monitoring and analysis of SA-CAR T cells during therapy.

If It Works, Don't Touch It? A Cell-Based Approach to Studying 2-[18F]FDG Metabolism

The glucose derivative 2-[¹⁸F]fluoro-2-deoxy-D-glucose (2-[¹⁸F]FDG) is still the most used radiotracer for positron emission tomography, as it visualizes glucose utilization and energy demand. In general, 2-[¹⁸F]FDG is said to be trapped intracellularly as 2-[¹⁸F]FDG-6-phosphate, which cannot be further metabolized. However, increasingly, this dogma is being questioned because of publications showing metabolism beyond 2-[¹⁸F]FDG-6-phosphate and even postulating 2-[¹⁸F]FDG imaging to depend on the enzyme hexose-6-phosphate dehydrogenase in the endoplasmic reticulum. Therefore, we aimed to study 2-[¹⁸F]FDG metabolism in the human cancer cell lines HT1080, HT29 and Huh7 applying HPLC. We then compared 2-[¹⁸F]FDG metabolism with intracellular tracer accumulation, efflux and the cells’ metabolic state and used a graphical Gaussian model to visualize metabolic patterns. The extent of 2-[¹⁸F]FDG metabolism varied considerably, dependent on the cell line, and was significantly enhanced by glucose withdrawal. However, the metabolic pattern was quite conserved. The most important radiometabolites beyond 2-[¹⁸F]FDG-6-phosphate were 2-[¹⁸F]FDMannose-6-phosphate, 2-[¹⁸F]FDG-1,6-bisphosphate and 2-[¹⁸F]FD-phosphogluconolactone. Enhanced radiometabolite formation under glucose reduction was accompanied by reduced efflux and mirrored the cells’ metabolic switch as assessed via extracellular lactate levels. We conclude that there can be considerable metabolism beyond 2-[¹⁸F]FDG-6-phosphate in cancer cell lines and a comprehensive understanding of 2-[¹⁸F]FDG metabolism might help to improve cancer research and tumor diagnosis.

Magnetic Resonance Imaging Is More Sensitive Than PET for Detecting Treatment-Induced Cell Death-Dependent Changes in Glycolysis

Metabolic imaging has been widely used to measure the early responses of tumors to treatment. Here, we assess the abilities of PET measurement of [18F]FDG uptake and MRI measurement of hyperpolarized [1-13C]pyruvate metabolism to detect early changes in glycolysis following treatment-induced cell death in human colorectal (Colo205) and breast adenocarcinoma (MDA-MB-231) xenografts in mice. A TRAIL agonist that binds to human but not mouse cells induced tumor-selective cell death. Tumor glycolysis was assessed by injecting [1,6-13C2]glucose and measuring 13C-labeled metabolites in tumor extracts. Injection of hyperpolarized [1-13C]pyruvate induced rapid reduction in lactate labeling. This decrease, which correlated with an increase in histologic markers of cell death and preceded decrease in tumor volume, reflected reduced flux from glucose to lactate and decreased lactate concentration. However, [18F]FDG uptake and phosphorylation were maintained following treatment, which has been attributed previously to increased [18F]FDG uptake by infiltrating immune cells. Quantification of [18F]FDG uptake in flow-sorted tumor and immune cells from disaggregated tumors identified CD11b+/CD45+ macrophages as the most [18F]FDG-avid cell type present, yet they represented <5% of the cells present in the tumors and could not explain the failure of [18F]FDG-PET to detect treatment response. MRI measurement of hyperpolarized [1-13C]pyruvate metabolism is therefore a more sensitive marker of the early decreases in glycolytic flux that occur following cell death than PET measurements of [18F]FDG uptake. SIGNIFICANCE: These findings demonstrate superior sensitivity of MRI measurement of hyperpolarized [1-13C]pyruvate metabolism versus PET measurement of 18F-FDG uptake for detecting early changes in glycolysis following treatment-induced tumor cell death.

Vascular dysfunction-The disregarded partner of Alzheimer's disease

Increasing evidence recognizes Alzheimer's disease (AD) as a multifactorial and heterogeneous disease with multiple contributors to its pathophysiology, including vascular dysfunction. The recently updated AD Research Framework put forth by the National Institute on Aging-Alzheimer's Association describes a biomarker-based pathologic definition of AD focused on amyloid, tau, and neuronal injury. In response to this article, here we first discussed evidence that vascular dysfunction is an important early event in AD pathophysiology. Next, we examined various imaging sequences that could be easily implemented to evaluate different types of vascular dysfunction associated with, and/or contributing to, AD pathophysiology, including changes in blood-brain barrier integrity and cerebral blood flow. Vascular imaging biomarkers of small vessel disease of the brain, which is responsible for >50% of dementia worldwide, including AD, are already established, well characterized, and easy to recognize. We suggest that these vascular biomarkers should be incorporated into the AD Research Framework to gain a better understanding of AD pathophysiology and aid in treatment efforts.

Blood-Brain Barrier: From Physiology to Disease and Back

The blood-brain barrier (BBB) prevents neurotoxic plasma components, blood cells, and pathogens from entering the brain. At the same time, the BBB regulates transport of molecules into and out of the central nervous system (CNS), which maintains tightly controlled chemical composition of the neuronal milieu that is required for proper neuronal functioning. In this review, we first examine molecular and cellular mechanisms underlying the establishment of the BBB. Then, we focus on BBB transport physiology, endothelial and pericyte transporters, and perivascular and paravascular transport. Next, we discuss rare human monogenic neurological disorders with the primary genetic defect in BBB-associated cells demonstrating the link between BBB breakdown and neurodegeneration. Then, we review the effects of genes underlying inheritance and/or increased susceptibility for Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease, and amyotrophic lateral sclerosis (ALS) on BBB in relation to other pathologies and neurological deficits. We next examine how BBB dysfunction relates to neurological deficits and other pathologies in the majority of sporadic AD, PD, and ALS cases, multiple sclerosis, other neurodegenerative disorders, and acute CNS disorders such as stroke, traumatic brain injury, spinal cord injury, and epilepsy. Lastly, we discuss BBB-based therapeutic opportunities. We conclude with lessons learned and future directions, with emphasis on technological advances to investigate the BBB functions in the living human brain, and at the molecular and cellular level, and address key unanswered questions.

Blood-brain barrier breakdown in Alzheimer disease and other neurodegenerative disorders

The blood-brain barrier (BBB) is a continuous endothelial membrane within brain microvessels that has sealed cell-to-cell contacts and is sheathed by mural vascular cells and perivascular astrocyte end-feet. The BBB protects neurons from factors present in the systemic circulation and maintains the highly regulated CNS internal milieu, which is required for proper synaptic and neuronal functioning. BBB disruption allows influx into the brain of neurotoxic blood-derived debris, cells and microbial pathogens and is associated with inflammatory and immune responses, which can initiate multiple pathways of neurodegeneration. This Review discusses neuroimaging studies in the living human brain and post-mortem tissue as well as biomarker studies demonstrating BBB breakdown in Alzheimer disease, Parkinson disease, Huntington disease, amyotrophic lateral sclerosis, multiple sclerosis, HIV-1-associated dementia and chronic traumatic encephalopathy. The pathogenic mechanisms by which BBB breakdown leads to neuronal injury, synaptic dysfunction, loss of neuronal connectivity and neurodegeneration are described. The importance of a healthy BBB for therapeutic drug delivery and the adverse effects of disease-initiated, pathological BBB breakdown in relation to brain delivery of neuropharmaceuticals are briefly discussed. Finally, future directions, gaps in the field and opportunities to control the course of neurological diseases by targeting the BBB are presented.

Alzheimer's disease: A matter of blood-brain barrier dysfunction?

Montagne et al. examine the role of blood–brain barrier (BBB) dysfunction in Alzheimer’s neurodegeneration and how targeting the BBB can influence the course of neurological disorder in transgenic models with human APP, PSEN1 and TAU mutations, APOE4 (major genetic risk), and pericyte degeneration causing loss of BBB integrity.

The blood–brain barrier (BBB) keeps neurotoxic plasma-derived components, cells, and pathogens out of the brain. An early BBB breakdown and/or dysfunction have been shown in Alzheimer’s disease (AD) before dementia, neurodegeneration and/or brain atrophy occur. However, the role of BBB breakdown in neurodegenerative disorders is still not fully understood. Here, we examine BBB breakdown in animal models frequently used to study the pathophysiology of AD, including transgenic mice expressing human amyloid-β precursor protein, presenilin 1, and tau mutations, and apolipoprotein E, the strongest genetic risk factor for AD. We discuss the role of BBB breakdown and dysfunction in neurodegenerative process, pitfalls in BBB measurements, and how targeting the BBB can influence the course of neurological disorder. Finally, we comment on future approaches and models to better define, at the cellular and molecular level, the underlying mechanisms between BBB breakdown and neurodegeneration as a basis for developing new therapies for BBB repair to control neurodegeneration.