Development of novel PET probes, [18F]BCPP-EF, [18F]BCPP-BF, and [11C]BCPP-EM for mitochondrial complex 1 imaging in the living brain

Animal PET scanner with a large field of view is suitable for high-throughput scanning of rodents

OBJECTIVE

In preclinical studies, high-throughput positron emission tomography (PET) imaging, known as simultaneous multiple animal scanning, can reduce the time spent on animal experiments, the cost of PET tracers, and the risk of synthesis of PET tracers. It is well known that the image quality acquired by high-throughput imaging depends on the PET system. Herein, we investigated the influence of large field of view (FOV) PET scanner on high-throughput imaging.

METHODS

We investigated the influence of scanning four objects using a small animal PET scanner with a large FOV. We compared the image quality acquired by four objects scanned with the one acquired by one object scanned using phantoms and animals. We assessed the image quality with uniformity, recovery coefficient (RC), and spillover ratio (SOR), which are indicators of image noise, spatial resolution, and quantitative precision, respectively. For the phantom study, we used the NEMA NU 4-2008 image quality phantom and evaluated uniformity, RC, and SOR, and for the animal study, we used Wistar rats and evaluated the spillover in the heart and kidney.

RESULTS

In the phantom study, four phantoms had little effect on imaging quality, especially SOR compared with that for one phantom. In the animal study as well, four rats had little effect on spillover from the heart muscle and kidney cortex compared with that for one rat.

CONCLUSIONS

This study demonstrated that an animal PET scanner with a large FOV was suitable for high-throughput imaging. Thus, the large FOV PET scanner can support drug discovery and bridging research through rapid pharmacological and pathological evaluation.

Mitochondrial complex I density is associated with IQ and cognition in cognitively healthy adults: an in vivo [18F]BCPP-EF PET study

BACKGROUND

Mitochondrial function plays a key role in regulating neurotransmission and may contribute to general intelligence. Mitochondrial complex I (MC-I) is the largest enzyme of the respiratory chain. Recently, it has become possible to measure MC-I distribution in vivo, using a novel positron emission tomography tracer [18F]BCPP-EF, thus, we set out to investigate the association between MC-I distribution and measures of cognitive function in the living healthy brain.

RESULTS

Analyses were performed in a voxel-wise manner and identified significant associations between [18F]BCPP-EF DVRCS-1 in the precentral gyrus and parietal lobes and WAIS-IV predicted IQ, WAIS-IV arithmetic and WAIS-IV symbol-digit substitution scores (voxel-wise Pearson's correlation coefficients transformed to Z-scores, thresholded at Z = 2.3 family-wise cluster correction at p < 0.05, n = 16). Arithmetic scores were associated with middle frontal and post-central gyri tracer uptake, symbol-digit substitution scores were associated with precentral gyrus tracer uptake. RAVLT recognition scores were associated with [18F]BCPP-EF DVRCS-1 in the middle frontal gyrus, post-central gyrus, occipital and parietal regions (n = 20).

CONCLUSIONS

Taken together, our findings support the theory that mitochondrial function may contribute to general intelligence and indicate that interindividual differences in MC-I should be a key consideration for research into mitochondrial dysfunction in conditions with cognitive impairment.

Neuroimaging in mitochondrial disease

The anatomic complexity of the brain in combination with its high energy demands makes this organ specifically vulnerable to defects of mitochondrial oxidative phosphorylation. Therefore, neurodegeneration is a hallmark of mitochondrial diseases. The nervous system of affected individuals typically shows selective regional vulnerability leading to distinct patterns of tissue damage. A classic example is Leigh syndrome, which causes symmetric alterations of basal ganglia and brain stem. Leigh syndrome can be caused by different genetic defects (>75 known disease genes) with variable disease onset ranging from infancy to adulthood. Other mitochondrial diseases are characterized by focal brain lesions, which is a core feature of MELAS syndrome (mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes). Apart from gray matter, also white matter can be affected by mitochondrial dysfunction. White matter lesions vary depending on the underlying genetic defect and may progress into cystic cavities. In view of the recognizable patterns of brain damage in mitochondrial diseases, neuroimaging techniques play a key role in diagnostic work-up. In the clinical setting, magnetic resonance imaging (MRI) and MR spectroscopy (MRS) are the mainstay of diagnostic work-up. Apart from visualization of brain anatomy, MRS allows the detection of metabolites such as lactate, which is of specific interest in the context of mitochondrial dysfunction. However, it is important to note that findings like symmetric basal ganglia lesions on MRI or a lactate peak on MRS are not specific, and that there is a broad range of disorders that can mimic mitochondrial diseases on neuroimaging. In this chapter, we will review the spectrum of neuroimaging findings in mitochondrial diseases and discuss important differential diagnoses. Moreover, we will give an outlook on novel biomedical imaging tools that may provide interesting insights into mitochondrial disease pathophysiology.

[18F]Tosyl fluoride as a versatile [18F]fluoride source for the preparation of 18F-labeled radiopharmaceuticals

Positron emission tomography (PET) is an in vivo imaging technology that utilizes positron-emitting radioisotope-labeled compounds as PET radiotracers that are commonly used in clinic and in various research areas, including oncology, cardiology, and neurology. Fluorine-18 is the most widely used PET-radionuclide and commonly produced by proton bombardment of ¹⁸O-enriched water in a cyclotron. The [¹⁸F]fluoride thus obtained generally requires processing by azeotropic drying in order to completely remove H₂O before it can be used for nucleophilic radiofluorination. In general, the drying step is important in facilitating the radiofluorination reactions and the preparation of ¹⁸F-labeled PET radiotracers. In this communication, we have demonstrated the feasibility of using [¹⁸F]tosyl fluoride ([¹⁸F]TsF) as a versatile [¹⁸F]fluoride source for radiofluorination to bypass the azeotropic drying step, and we have developed a continuous flow solid-phase radiosynthesis strategy to generate [¹⁸F]TsF in a form that is excellent for radiofluorination. [¹⁸F]TsF shows high reactivity in radiofluorination and provides the features suitable for preparing PET radiotracers on a small scale and exploring novel radiolabeling technologies. Thus, using [¹⁸F]TsF as a [¹⁸F]fluoride source is a promising strategy that facilitates radiofluorination and provides a convenient and efficient solution for the preparation of ¹⁸F-labeled radiopharmaceuticals that is well matched to the emerging trends in PET imaging technologies.

Evaluation of advanced, pathophysiologic new targets for imaging of CNS

The inadequate information about the in vivo pathological, physiological, and neurological impairments, as well as the absence of in vivo tools for assessing brain penetrance and the efficiency of newly designed drugs, has hampered the development of new techniques for the treatment for variety of new central nervous system (CNS) diseases. The searching sites such as Science Direct and PubMed were used to find out the numerous distinct tracers across 16 CNS targets including tau, synaptic vesicle glycoprotein, the adenosine 2A receptor, the phosphodiesterase enzyme PDE10A, and the purinoceptor, among others. Among the most encouraging are [¹⁸ F]FIMX for mGluR imaging, [¹¹ C]Martinostat for Histone deacetylase, [¹⁸ F]MNI-444 for adenosine 2A imaging, [¹¹ C]ER176 for translocator protein, and [¹⁸ F]MK-6240 for tau imaging. We also reviewed the findings for each tracer's features and potential for application in CNS pathophysiology and therapeutic evaluation investigations, including target specificity, binding efficacy, and pharmacokinetic factors. This review aims to present a current evaluation of modern positron emission tomography tracers for CNS targets, with a focus on recent advances for targets that have newly emerged for imaging in humans.

Lower Availability of Mitochondrial Complex I in Anterior Cingulate Cortex in Autism: A Positron Emission Tomography Study

OBJECTIVE

Mitochondrial dysfunction has been implicated in the pathophysiology of autism spectrum disorder (ASD) in previous studies of postmortem brain or peripheral samples. The authors investigated whether and where mitochondrial dysfunction occurs in the living brains of individuals with ASD and to identify the clinical correlates of detected mitochondrial dysfunction.

METHODS

This case-control study used positron emission tomography (PET) with 2-tert-butyl-4-chloro-5-{6-[2-(2-[¹⁸F]fluoroethoxy)-ethoxy]-pyridin-3-ylmethoxy}-2H-pyridazin-3-one ([¹⁸F]BCPP-EF), a radioligand that binds to the mitochondrial electron transport chain complex I, to examine the topographical distribution of mitochondrial dysfunction in living brains of individuals with ASD. Twenty-three adult males with high-functioning ASD, with no psychiatric comorbidities and free of psychotropic medication, and 24 typically developed males with no psychiatric diagnoses, matched with the ASD group on age, parental socioeconomic background, and IQ, underwent [¹⁸F]BCPP-EF PET measurements. Individuals with mitochondrial disease were excluded by clinical evaluation and blood tests for abnormalities in lactate and pyruvate levels.

RESULTS

Among the brain regions in which mitochondrial dysfunction has been reported in postmortem studies of autistic brains, participants with ASD had significantly decreased [¹⁸F]BCPP-EF availability specifically in the anterior cingulate cortex compared with typically developed participants. The regional specificity was revealed by a significant interaction between diagnosis and brain regions. Moreover, the lower [¹⁸F]BCPP-EF availability in the anterior cingulate cortex was significantly correlated with the more severe ASD core symptom of social communication deficits.

CONCLUSIONS

This study provides direct evidence to link in vivo brain mitochondrial dysfunction with ASD pathophysiology and its communicational deficits. The findings support the possibility that mitochondrial electron transport chain complex I is a novel therapeutic target for ASD core symptoms.

Widespread cell stress and mitochondrial dysfunction occur in patients with early Alzheimer's disease

Cell stress and impaired oxidative phosphorylation are central to mechanisms of synaptic loss and neurodegeneration in the cellular pathology of Alzheimer's disease (AD). In this study, we quantified the in vivo expression of the endoplasmic reticulum stress marker, sigma 1 receptor (S1R), using [¹¹C]SA4503 positron emission tomography (PET), the mitochondrial complex I (MC1) with [¹⁸F]BCPP-EF, and the presynaptic vesicular protein SV2A with [¹¹C]UCB-J in 12 patients with early AD and in 16 cognitively normal controls. We integrated these molecular measures with assessments of regional brain volumes and cerebral blood flow (CBF) measured with magnetic resonance imaging arterial spin labeling. Eight patients with AD were followed longitudinally to estimate the rate of change of the physiological and structural pathology markers with disease progression. The patients showed widespread increases in S1R (≤ 27%) and regional reduction in MC1 (≥ -28%) and SV2A (≥ -25%) radioligand binding, brain volume (≥ -23%), and CBF (≥ -26%). [¹⁸F]BCPP-EF PET MC1 binding (≥ -12%) and brain volumes (≥ -5%) showed progressive reductions over 12 to 18 months, suggesting that they both could be used as pharmacodynamic indicators in early-stage therapeutics trials. Associations of reduced MC1 and SV2A and increased S1R radioligand binding with reduced cognitive performance in AD, although exploratory, suggested a loss of metabolic functional reserve with disease. Our study thus provides in vivo evidence for widespread, clinically relevant cellular stress and bioenergetic abnormalities in early AD.

Neuroimaging Methods to Map In Vivo Changes of OXPHOS and Oxidative Stress in Neurodegenerative Disorders

Mitochondrial dysfunction is a pathophysiological hallmark of most neurodegenerative diseases. Several clinical trials targeting mitochondrial dysfunction have been performed with conflicting results. Reliable biomarkers of mitochondrial dysfunction in vivo are thus needed to optimize future clinical trial designs. This narrative review highlights various neuroimaging methods to probe mitochondrial dysfunction. We provide a general overview of the current biological understanding of mitochondrial dysfunction in degenerative brain disorders and how distinct neuroimaging methods can be employed to map disease-related changes. The reviewed methodological spectrum includes positron emission tomography, magnetic resonance, magnetic resonance spectroscopy, and near-infrared spectroscopy imaging, and how these methods can be applied to study alterations in oxidative phosphorylation and oxidative stress. We highlight the advantages and shortcomings of the different neuroimaging methods and discuss the necessary steps to use these for future research. This review stresses the importance of neuroimaging methods to gain deepened insights into mitochondrial dysfunction in vivo, its role as a critical disease mechanism in neurodegenerative diseases, the applicability for patient stratification in interventional trials, and the quantification of individual treatment responses. The in vivo assessment of mitochondrial dysfunction is a crucial prerequisite for providing individualized treatments for neurodegenerative disorders.

Imaging Adipose Tissue Browning using Mitochondrial Complex-I Tracer [18F]BCPP-EF

Browning of white adipose tissue (WAT) into beige adipocytes has been proposed as a strategy to tackle the ongoing obesity epidemic. Thermogenic stimuli have been investigated with the aim of converting existing white adipose tissue, primarily used for energy storage, into beige adipocytes capable of dissipating energy; however, evaluation is complicated by the dearth of noninvasive methodologies to quantify de novo beige adipocytes in WAT. Imaging with [¹⁸F]FDG is commonly used to measure brown adipose tissue (BAT) and beige adipocytes but the relationship between beige adipocytes, thermogenesis and [¹⁸F]FDG uptake is unclear. [¹⁸F]BCPP-EF, a tracer for mitochondrial complex-I (MC-I), acts as a marker of oxidative metabolism and may be useful for the detection of newly formed beige adipocytes. Mice received doses of the β3-adrenergic agonist CL-316,243 subchronically for 7 days to induce formation of beige adipocytes in inguinal white fat. PET imaging was performed longitudinally with both [¹⁸F]FDG (a marker of glycolysis) and [¹⁸F]BCPP-EF (an MC-I marker) to assess the effect of thermogenic stimulation on uptake in browning inguinal WAT and interscapular BAT. Treatment with CL-316,243 led to significant increases in both [¹⁸F]FDG and [¹⁸F]BCPP-EF in inguinal WAT. The uptake of [¹⁸F]BCPP-EF in inguinal WAT was significantly increased above control levels after 3 days of stimulation, whereas [¹⁸F]FDG only showed a significant increase after 7 days. The uptake of [¹⁸F]BCPP-EF in newly formed beige adipocytes was blocked by pretreatment with an adrenoceptor antagonist suggesting that beige adipocyte formation may be associated with the activation of MC-I. However, in BAT, uptake of [¹⁸F]BCPP-EF was unaffected by β3-adrenergic stimulation, potentially due to the high expression of MC-I. [¹⁸F]BCPP-EF can detect newly formed beige adipocytes in WAT generated after subchronic treatment with the β3-adrenergic agonist CL-316,243 and displays both higher inguinal WAT uptake and earlier detection than [¹⁸F]FDG. The MC-I tracer may be a useful tool in the evaluation of new therapeutic strategies targeting metabolic adipose tissues to tackle obesity and metabolic diseases.

Radiopharmaceuticals for PET and SPECT Imaging: A Literature Review over the Last Decade

Positron emission tomography (PET) uses radioactive tracers and enables the functional imaging of several metabolic processes, blood flow measurements, regional chemical composition, and/or chemical absorption. Depending on the targeted processes within the living organism, different tracers are used for various medical conditions, such as cancer, particular brain pathologies, cardiac events, and bone lesions, where the most commonly used tracers are radiolabeled with 18F (e.g., [¹⁸F]-FDG and NA [¹⁸F]). Oxygen-15 isotope is mostly involved in blood flow measurements, whereas a wide array of ¹¹C-based compounds have also been developed for neuronal disorders according to the affected neuroreceptors, prostate cancer, and lung carcinomas. In contrast, the single-photon emission computed tomography (SPECT) technique uses gamma-emitting radioisotopes and can be used to diagnose strokes, seizures, bone illnesses, and infections by gauging the blood flow and radio distribution within tissues and organs. The radioisotopes typically used in SPECT imaging are iodine-123, technetium-99m, xenon-133, thallium-201, and indium-111. This systematic review article aims to clarify and disseminate the available scientific literature focused on PET/SPECT radiotracers and to provide an overview of the conducted research within the past decade, with an additional focus on the novel radiopharmaceuticals developed for medical imaging.

Current Use and Complementary Value of Combining in Vivo Imaging Modalities to Understand the Renoprotective Effects of Sodium-Glucose Cotransporter-2 Inhibitors at a Tissue Level

Sodium-glucose cotransporter-2 inhibitors (SGLT2i) were initially developed to treat diabetes and have been shown to improve renal and cardiovascular outcomes in patients with- but also without diabetes. The mechanisms underlying these beneficial effects are incompletely understood, as is the response variability between- and within patients. Imaging modalities allow in vivo quantitative assessment of physiological, pathophysiological, and pharmacological processes at kidney tissue level and are therefore increasingly being used in nephrology. They provide unique insights into the renoprotective effects of SGLT2i and the variability in response and may thus contribute to improved treatment of the individual patient. In this mini-review, we highlight current work and opportunities of renal imaging modalities to assess renal oxygenation and hypoxia, fibrosis as well as interaction between SGLT2i and their transporters. Although every modality allows quantitative assessment of particular parameters of interest, we conclude that especially the complementary value of combining imaging modalities in a single clinical trial aids in an integrated understanding of the pharmacology of SGLT2i and their response variability.

Positron Emission Tomography in Animal Models of Tauopathies

The microtubule-associated protein tau (MAPT) plays an important role in Alzheimer's disease and primary tauopathy diseases. The abnormal accumulation of tau contributes to the development of neurotoxicity, inflammation, neurodegeneration, and cognitive deficits in tauopathy diseases. Tau synergically interacts with amyloid-beta in Alzheimer's disease leading to detrimental consequence. Thus, tau has been an important target for therapeutics development for Alzheimer's disease and primary tauopathy diseases. Tauopathy animal models recapitulating the tauopathy such as transgenic, knock-in mouse and rat models have been developed and greatly facilitated the understanding of disease mechanisms. The advance in PET and imaging tracers have enabled non-invasive detection of the accumulation and spread of tau, the associated microglia activation, metabolic, and neurotransmitter receptor alterations in disease animal models. In vivo microPET studies on mouse or rat models of tauopathy have provided significant insights into the phenotypes and time course of pathophysiology of these models and allowed the monitoring of treatment targeting at tau. In this study, we discuss the utilities of PET and recently developed tracers for evaluating the pathophysiology in tauopathy animal models. We point out the outstanding challenges and propose future outlook in visualizing tau-related pathophysiological changes in brain of tauopathy disease animal models.

Mitochondrial complex-I abnormalities underlie neurodegeneration and cognitive decline in Alzheimer's disease

BACKGROUND

Abnormal mitochondrial metabolism has been described in Alzheimer's disease (AD) brain. However, the relationship between AD pathophysiology and key mitochondrial processes remains elusive. The purpose of this study is to investigate whether mitochondrial complex I dysfunction is associated with amyloid aggregation, or glucose metabolism and brain atrophy in patients with mild AD using positron emission tomography (PET).

METHODS

Amyloid and tau positive symptomatic AD patients with clinical dementia rating 0.5 or 1 (N=30; mean age ± standard deviation: 71.8 ± 7.6 years) underwent magnetic resonance imaging and PET scans with [¹⁸ F]BCPP-EF, [¹¹ C]PiB and [¹⁸ F]FDG for assessing brain atrophy, mitochondrial complex I dysfunction, amyloid deposition, and glucose metabolism, respectively. Local cortical associations among these biomarkers and gray matter volume were evaluated with voxel-based regressions models.

RESULTS

[¹⁸ F]BCPP-EF standardized uptake value ratio (SUVR) was positively correlated with [¹⁸ F]FDG SUVR in the widespread brain area, while its associations with gray matter volume were restricted to the parahippocampal gyrus. Reductions in [¹⁸ F]BCPP-EF SUVR were associated with domain-specific cognitive performance. We did not observe regional associations between mitochondrial dysfunction and amyloid burden.

CONCLUSIONS

In symptomatic cases, although mitochondrial complex I reduction is linked to a wide range of downstream neurodegenerative processes such as hypometabolism, atrophy, and cognitive decline, the link to amyloid was not observable. The data presented here support [¹⁸ F]BCPP-EF as an excellent imaging tool to investigate mitochondrial dysfunction in AD.

Novel Positron Emission Tomography Radiotracers for Imaging Mitochondrial Complex I

Mitochondrial dysfunction has been indicated in neurodegenerative and other disorders. The mitochondrial complex I (MC-I) of the electron transport chain (ETC) on the inner membrane is the electron entry point of the ETC and is essential for the production of reactive oxygen species. Based on a recently identified β-keto-amide type MC-I modulator from our laboratory, an ¹⁸F-labeled positron emission tomography (PET) tracer, ¹⁸F-2, was prepared. PET/CT imaging studies demonstrated that ¹⁸F-2 exhibited rapid brain uptake without significant wash out during the 60 min scanning time. In addition, the binding of ¹⁸F-2 was higher in the regions of the brain stem, cerebellum, and midbrain. The uptake of ¹⁸F-2 can be significantly blocked by its parent compound. Collectively, the results strongly suggest successful development of MC-I PET tracers from this chemical scaffold that can be used in future mitochondrial dysfunction studies of the central nervous system.

In vivo alterations of mitochondrial activity and amyloidosis in early-stage senescence-accelerated mice: a positron emission tomography study

Purpose

While marked reductions in neural activity and mitochondrial function have been reported in Alzheimer’s disease (AD), the degree of mitochondrial activity in mild cognitive impairment (MCI) or early-stage AD remains unexplored. Here, we used positron emission tomography (PET) to examine the direct relationship between mitochondrial activity (¹⁸F-BCPP-EF) and β-amyloid (Aβ) deposition (¹¹C-PiB) in the same brains of senescence-accelerated mouse prone 10 (SAMP10) mice, an Aβ-developing neuroinflammatory animal model showing accelerated senescence with deterioration in cognitive functioning similar to that in MCI.

Methods

Five- to 25-week-old SAMP10 and control SAMR1 mice, were used in the experiments. PET was used to measure the binding levels (standard uptake value ratios; SUVRs) of [¹⁸F]2-tert-butyl-4-chloro-5-2H-pyridazin-3-one (¹⁸F-BCPP-EF) for mitochondrial complex 1 availability, and ¹¹C-PiB for Aβ deposition, in the same animals, and immunohistochemistry for ATPB (an ATP synthase on the mitochondrial inner membrane) was also performed, to determine changes in mitochondrial activity in relation to amyloid burden during the early stage of cognitive impairment.

Results

The SUVR of ¹⁸F-BCPP-EF was significantly lower and that of ¹¹C-PiB was higher in the 15-week-old SAMP10 mice than in the control and 5-week-old SAMP10 mice. The two parameters were found to negatively correlate with each other. The immunohistochemical analysis demonstrated temporal upregulation of ATPB levels at 15-week-old, but decreased at 25 week-old SAMP10 mice.

Conclusion

The present results provide in vivo evidence of a decrease in mitochondrial energy production and elevated amyloidosis at an early stage in SAMP10 mice. The inverse correlation between these two phenomena suggests a concurrent change in neuronal energy failure by Aβ-induced elevation of neuroinflammatory responses. Comparison of PET data with histological findings suggests that temporal increase of ATPB level may not be neurofunctionally implicated during neuropathological processes, including Aβ pathology, in an animal model of early-phase AD spectrum disorder.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02343-4.

Differences in in vitro microglial accumulation of the energy metabolism tracers [18F]FDG and [18F]BCPP-EF during LPS- and IL4 stimulation

The positron emission tomography probes 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) and 2-tert-butyl-4-chloro-5-{6-[2-(2-[18F]fluoroethoxy)-ethoxy]-pyridin-3-ylmethoxy}-2H-pyridazin-3-one ([18F]BCPP-EF) are designed to evaluate glycolysis and oxidative phosphorylation, respectively, and are both used to estimate neuronal activity. However, previous studies have shown a discrepancy in these probes' accumulation in the compromised region, possibly due to the presence of activated microglia acting like deleterious or neuroprotective phenotypes. Hence, we evaluated lipopolysaccharide (LPS)- and interleukin 4 (IL4)-stimulated microglial uptake of [14C]2DG and [18F]BCPP-EF to give a new insight into the hypothesis that different uptake of [18F]FDG and [18F]BCPP-EF can be ascribed to the different metabolic pathways activated during microglial activation. LPS or IL4 stimulation increased the proinflammatory or anti-inflammatory marker gene expression in microglial cells. In LPS-stimulated cells, [14C]2DG uptake and glycolysis related gene expression were elevated, and [18F]BCPP-EF uptake was reduced. In IL4-stimulated cells, [18F]BCPP-EF uptake was increased, and [14C]2DG uptake was decreased. The expression of genes involved in glycolysis and mitochondrial complex I subunits was not changed by IL4 stimulation. The uptake of [14C]2DG and [18F]BCPP-EF differs in LPS- and IL4-stimulated polarized microglial cells. The present results suggest that the in vivo accumulation of metabolic tracers [18F]FDG and [18F]BCPP-EF can be influenced by the different aspects of neuroinflammation.

Reassessment of Pioglitazone for Alzheimer's Disease

Alzheimer's disease is a quintessential 'unmet medical need', accounting for ∼65% of progressive cognitive impairment among the elderly, and 700,000 deaths in the United States in 2020. In 2019, the cost of caring for Alzheimer's sufferers was $244B, not including the emotional and physical toll on caregivers. In spite of this dismal reality, no treatments are available that reduce the risk of developing AD or that offer prolonged mitiagation of its most devestating symptoms. This review summarizes key aspects of the biology and genetics of Alzheimer's disease, and we describe how pioglitazone improves many of the patholophysiological determinants of AD. We also summarize the results of pre-clinical experiments, longitudinal observational studies, and clinical trials. The results of animal testing suggest that pioglitazone can be corrective as well as protective, and that its efficacy is enhanced in a time- and dose-dependent manner, but the dose-effect relations are not monotonic or sigmoid. Longitudinal cohort studies suggests that it delays the onset of dementia in individuals with pre-existing type 2 diabetes mellitus, which small scale, unblinded pilot studies seem to confirm. However, the results of placebo-controlled, blinded clinical trials have not borne this out, and we discuss possible explanations for these discrepancies.

Deep learning-based attenuation correction for brain PET with various radiotracers

OBJECTIVES

Attenuation correction (AC) is crucial for ensuring the quantitative accuracy of positron emission tomography (PET) imaging. However, obtaining accurate μ-maps from brain-dedicated PET scanners without AC acquisition mechanism is challenging. Therefore, to overcome these problems, we developed a deep learning-based PET AC (deep AC) framework to synthesize transmission computed tomography (TCT) images from non-AC (NAC) PET images using a convolutional neural network (CNN) with a huge dataset of various radiotracers for brain PET imaging.

METHODS

The proposed framework is comprised of three steps: (1) NAC PET image generation, (2) synthetic TCT generation using CNN, and (3) PET image reconstruction. We trained the CNN by combining the mixed image dataset of six radiotracers to avoid overfitting, including [18F]FDG, [18F]BCPP-EF, [11C]Racropride, [11C]PIB, [11C]DPA-713, and [11C]PBB3. We used 1261 brain NAC PET and TCT images (1091 for training and 70 for testing). We did not include [11C]Methionine subjects in the training dataset, but included them in the testing dataset.

RESULTS

The image quality of the synthetic TCT images obtained using the CNN trained on the mixed dataset of six radiotracers was superior to those obtained using the CNN trained on the split dataset generated from each radiotracer. In the [18F]FDG study, the mean relative PET biases of the emission-segmented AC (ESAC) and deep AC were 8.46 ± 5.24 and - 5.69 ± 4.97, respectively. The deep AC PET and TCT AC PET images exhibited excellent correlation for all seven radiotracers (R2 = 0.912-0.982).

CONCLUSION

These results indicate that our proposed deep AC framework can be leveraged to provide quantitatively superior PET images when using the CNN trained on the mixed dataset of PET tracers than when using the CNN trained on the split dataset which means specific for each tracer.

Mitochondrial complex I abnormalities is associated with tau and clinical symptoms in mild Alzheimer's disease

Background

Mitochondrial electron transport chain abnormalities have been reported in postmortem pathological specimens of Alzheimer’s disease (AD). However, it remains unclear how amyloid and tau are associated with mitochondrial dysfunction in vivo. The purpose of this study is to assess the local relationships between mitochondrial dysfunction and AD pathophysiology in mild AD using the novel mitochondrial complex I PET imaging agent [¹⁸F]BCPP-EF.

Methods

Thirty-two amyloid and tau positive mild stage AD dementia patients (mean age ± SD: 71.1 ± 8.3 years) underwent a series of PET measurements with [¹⁸F]BCPP-EF mitochondrial function, [¹¹C]PBB3 for tau deposition, and [¹¹C] PiB for amyloid deposition. Age-matched normal control subjects were also recruited. Inter and intrasubject comparisons of levels of mitochondrial complex I activity, amyloid and tau deposition were performed.

Results

The [¹⁸F]BCPP-EF uptake was significantly lower in the medial temporal area, highlighting the importance of the mitochondrial involvement in AD pathology. [¹¹C]PBB3 uptake was greater in the temporo-parietal regions in AD. Region of interest analysis in the Braak stage I-II region showed significant negative correlation between [¹⁸F]BCPP-EF SUVR and [¹¹C]PBB3 BP_ND (R = 0.2679, p = 0.04), but not [¹¹C] PiB SUVR.

Conclusions

Our results indicated that mitochondrial complex I is closely associated with tau load evaluated by [¹¹C]PBB3, which might suffer in the presence of its off-target binding. The absence of association between mitochondrial complex I dysfunction with amyloid load suggests that mitochondrial dysfunction in the trans-entorhinal and entorhinal region is a reflection of neuronal injury occurring in the brain of mild AD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13024-021-00448-1.

Test-retest variability and reference region-based quantification of 18F-BCPP-EF for imaging mitochondrial complex I in the human brain

Mitochondrial complex I (MC-I) is an essential regulator of brain bioenergetics and can be quantified in the brain using PET radioligand ¹⁸F-BCPP-EF. Here we evaluate the test-retest reproducibility of ¹⁸F-BCPP-EF in humans, and assess the use of a non-invasive quantification method (standardised uptake value ratio - SUVR). Thirty healthy volunteers had a 90-min dynamic ¹⁸F-BCPP-EF scan with arterial blood sampling, five of which received a second scan to be included in the test-retest analysis. Time-activity curves (TAC) were analysed using multilinear analysis 1 (MA1) and the two-tissue compartment model (2TC) to estimate volumes of distribution (V_T). Regional SUVR-1 values were calculated from the 70 to 90-min TAC data using the centrum semiovale as a pseudo reference region, and compared to kinetic analysis-derived outcome measures. The mean absolute test-retest variability of V_T ranged from 12% to 18% across regions. Both DVR-1and SUVR-1 had improved test-retest variability in the range 2%-7%. SUVR-1 was highly correlated with DVR-1 (r² = 0.97, n = 30). In conclusion, ¹⁸F-BCPP-EF has suitable test-retest reproducibility and can be used to quantify MC-I in clinical studies.

Mitochondrial Dysfunction in Alzheimer's Disease: A Biomarker of the Future?

Alzheimer's disease (AD) is the most common cause of dementia worldwide and is characterised pathologically by the accumulation of amyloid beta and tau protein aggregates. Currently, there are no approved disease modifying therapies for clearance of either of these proteins from the brain of people with AD. As well as abnormalities in protein aggregation, other pathological changes are seen in this condition. The function of mitochondria in both the nervous system and rest of the body is altered early in this disease, and both amyloid and tau have detrimental effects on mitochondrial function. In this review article, we describe how the function and structure of mitochondria change in AD. This review summarises current imaging techniques that use surrogate markers of mitochondrial function in both research and clinical practice, but also how mitochondrial functions such as ATP production, calcium homeostasis, mitophagy and reactive oxygen species production are affected in AD mitochondria. The evidence reviewed suggests that the measurement of mitochondrial function may be developed into a future biomarker for early AD. Further work with larger cohorts of patients is needed before mitochondrial functional biomarkers are ready for clinical use.

Targeting mitophagy in Parkinson's disease

The genetics and pathophysiology of Parkinson's disease (PD) strongly implicate mitochondria in disease aetiology. Elegant studies over the last two decades have elucidated complex molecular signaling governing the identification and removal of dysfunctional mitochondria from the cell, a process of mitochondrial quality control known as mitophagy. Mitochondrial deficits and specifically reduced mitophagy are evident in both sporadic and familial PD. Mendelian genetics attributes loss-of-function mutations in key mitophagy regulators PINK1 and Parkin to early-onset PD. Pharmacologically enhancing mitophagy and accelerating the removal of damaged mitochondria are of interest for developing a disease-modifying PD therapeutic. However, despite significant understanding of both PINK1-Parkin-dependent and -independent mitochondrial quality control pathways, the therapeutic potential of targeting mitophagy remains to be fully explored. Here, we provide a summary of the genetic evidence supporting the role for mitophagy failure as a pathogenic mechanism in PD. We assess the tractability of mitophagy pathways and prospects for drug discovery and consider intervention points for mitophagy enhancement. We explore the numerous hit molecules beginning to emerge from high-content/high-throughput screening as well as the biochemical and phenotypic assays that enabled these screens. The chemical and biological properties of these reference compounds suggest many could be used to interrogate and perturb mitochondrial biology to validate promising drug targets. Finally, we address key considerations and challenges in achieving preclinical proof-of-concept, including in vivo mitophagy reporter methodologies and disease models, as well as patient stratification and biomarker development for mitochondrial forms of the disease.

Future Imaging in Dementia

Imaging has made an immense contribution toward supporting the diagnosis of dementias, detecting preclinical and prodromal pathology, and allowing disease progression to be objectively tracked. This has led to consensus guidelines for the use of imaging in dementias to be published and a future task will be to validate these guidelines. Additionally, there needs to be standardised approaches over the use of binary thresholds when assigning an abnormality status. Other medical unmet needs include the need for specific imaging markers of (1) linear tau tangles, TDP-43 and alpha synuclein aggregates; (2) microglial phenotypes that throw light on the activity of these inflammatory cells; (3) activity of intracellular processes which normally act to clear misfolded proteins; (4) epigenetic activity which regulates gene expression. Future imaging studies are predicted to be active in all these areas. Finally, as safer and more effective immunotherapy and other protective strategies against the pathologies of dementias are developed and trialed, imaging will play a major future role in determining the efficacy of neuroprotective treatments and their mechanism of action to be examined.

Imaging mitochondrial complex I activation during a vibrotactile stimulation: A PET study using [18F]BCPP-EF in the conscious monkey brain

In order to evaluate the capability of 2-tert-butyl-4-chloro-5-{6-[2-(2-[¹⁸F]fluoroethoxy)-ethoxy]-pyridin-3-ylmethoxy}-2H-pyridazin-3-one ([¹⁸F]BCPP-EF), a novel positron emission tomography (PET) probe for mitochondrial complex I (MC-I) activity, to assess neuronal activation, an activation PET study was conducted in the conscious monkey brain with a continuous unilateral vibrotactile stimulation. PET scans with [¹⁵O]H₂O, [¹⁸F]BCPP-EF, or 2-deoxy-2-[¹⁸F]fluoroglucose ([¹⁸F]FDG) were conducted under: (1) resting conditions; (2) a continuous vibration stimulation; (3) a continuous vibration stimulation after 15-min pre-vibration; and (4) a continuous vibration stimulation after 30-min pre-vibration. The contralateral/ipsilateral ratio (CIR) in the somatosensory cortex showed significant increases in the uptake of [¹⁵O]H₂O, [¹⁸F]BCPP-EF, and [¹⁸F]FDG with the vibration stimulation. The longer pre-vibration duration induced significantly lower CIR in regional cerebral blood flow (rCBF) measured using [¹⁵O]H₂O, whereas it did not affect the CIR in [¹⁸F]BCPP-EF or the regional cerebral metabolic rate of glucose (rCMRglc) measured using [¹⁸F]FDG 30-60 min after the injection. These results suggest that the [¹⁸F]BCPP-EF response in the later phase of scans was not influenced by the increase in rCBF, indicating the capability of [¹⁸F]BCPP-EF to detect acute changes in MC-I activity induced by neuronal activation. However, the metabolic shift from glycolysis to oxidation was not observed under the stimulation used here.

In vivo positron emission tomography imaging of mitochondrial abnormalities in a mouse model of tauopathy

Damaged mitochondria may be one of the earliest manifestations of Alzheimer's disease. Because oxidative phosphorylation is a primary source of neuronal energy, unlike glycolysis-dependent energy production in inflamed glia, mitochondrial respiration could provide a selective biomarker of neuronal deterioration in Alzheimer's disease. Here we used a recently developed positron emission tomography (PET) probe targeting mitochondrial complex I (MC-I), ¹⁸F-BCPP-EF, to non-invasively visualize mitochondrial abnormalities in the brains of tau transgenic mice (rTg4510). Tauopathy and neuroinflammation were visualized by PET using a tau probe ¹¹C-PBB3 and a translocator protein probe, ¹⁸F-FEBMP, respectively. A marked reduction in ¹⁸F-BCPP-EF uptake was observed in hippocampal and forebrain regions of tau transgenic mice, colocalizing with regions of tauopathy, neuronal damage, and neuroinflammation. MC-I signals were highly correlated with atrophy assayed by magnetic resonance imaging, but negatively associated with inflammatory signals, indicating that neuronal metabolic signals measured by MC-I PET were robust to inflammatory interference. MC-I may be a useful imaging biomarker to detect neuronal damage and metabolic changes with minimal interference from concomitant glial hypermetabolism.

Coexistence of cerebral hypometabolism and neuroinflammation in the thalamo-limbic-brainstem region in young women with functional somatic syndrome

Background

Functional somatic syndrome (FSS) is a disorder characterized by clusters of medically unexplained symptoms. Some women suffer from persistent FSS after human papillomavirus (HPV) vaccination. However, a causal relationship has not been established, and the pathophysiology of FSS remains elusive. Here, we aimed to identify the brain regions showing altered cerebral metabolism and neuroinflammation in patients with FSS and to correlate the measures of positron emission tomography (PET) with clinical data. Twelve women diagnosed with FSS following HPV vaccination (FSS group) underwent both [¹⁸F]FDG-PET to measure glucose metabolism and [¹¹C]DPA713-PET to measure neuroinflammation. [¹⁸F]FDG standardized uptake value ratio (SUVR) and [¹¹C]DPA713 binding potential (BP_ND) values were compared voxel-wise between the FSS and control groups (n = 12 for [¹⁸F]FDG, n = 16 for [¹¹C]DPA713). A region-of-interest (ROI)-based analysis was performed to correlate PET parameters with clinical scores. Statistical significance was set at p < 0.05 corrected for multiple comparisons.

Results

Statistical parametric mapping revealed a concomitant significant decrease of [¹⁸F]FDG SUVR and increase of [¹¹C]DPA713 BP_ND in the regions covering the thalamus, mesial temporal area, and brainstem in the FSS group. Correlation analysis revealed that intelligence and memory scores were significantly positively correlated with [¹⁸F]FDG SUVR and negatively so with [¹¹C]DPA713 BP_ND in these regions. A direct comparison between [¹⁸F]FDG SUVR and [¹¹C]DPA713 BP_ND revealed a significant positive correlation in the right hippocampus and amygdala.

Conclusions

Cerebral hypometabolism with neuroinflammation occurring in the thalamo-limbic-brainstem region may reflect the pathophysiology of FSS.

In vivo mitochondrial and glycolytic impairments in patients with Alzheimer disease

Objective

In vivo glycolysis-related glucose metabolism and electron transport chain-related mitochondrial activity may be different regionally in the brains of patients with Alzheimer disease (AD). To test this hypothesis regarding AD pathophysiology, we measured the availability of mitochondrial complex-I (MC-I) with the novel PET probe [18F]2-tert- butyl-4-chloro-5–2H- pyridazin-3-one ([18F]BCPP-EF), which binds to MC-I, and compared [18F]BCPP-EF uptake with 18F-fluorodeoxyglucose ([18F]FDG) uptake in the living AD brain.

Methods

First, the total distribution volume (VT) of [18F]BCPP-EF from 10 normal controls (NCs) was quantified using arterial blood samples and then tested to observe whether VT could substitute for the standard uptake value relative to the global count (SUVRg). Eighteen NCs and 14 different NCs underwent PET with [18F]BCPP-EF or [18F]FDG, respectively. Second, 32 patients with AD were scanned semiquantitatively with double PET tracers. Interparticipant and intraparticipant comparisons of the levels of MC-I activity ([18F]BCPP-EF) and glucose metabolism ([18F]FDG) were performed.

Results

The [18F]BCPP-EF VT was positively correlated with the [18F]BCPP-EF SUVRg, indicating that the use of the SUVRg was sufficient for semiquantitative evaluation. The [18F]BCPP-EF SUVRg, but not the [18F]FDG SUVRg, was significantly lower in the parahippocampus in patients with AD, highlighting the prominence of oxidative metabolic failure in the medial temporal cortex. Robust positive correlations between the [18F]BCPP-EF SUVRg and [18F]FDG SUVRg were observed in several brain regions, except the parahippocampus, in early-stage AD.

Conclusions

Mitochondrial dysfunction in the parahippocampus was shown in early-stage AD. Mitochondria-related energy failure may precede glycolysis-related hypometabolism in regions with pathologically confirmed early neurodegeneration in AD.

Advances in CNS PET: the state-of-the-art for new imaging targets for pathophysiology and drug development

PURPOSE

A limit on developing new treatments for a number of central nervous system (CNS) disorders has been the inadequate understanding of the in vivo pathophysiology underlying neurological and psychiatric disorders and the lack of in vivo tools to determine brain penetrance, target engagement, and relevant molecular activity of novel drugs. Molecular neuroimaging provides the tools to address this. This article aims to provide a state-of-the-art review of new PET tracers for CNS targets, focusing on developments in the last 5 years for targets recently available for in-human imaging.

METHODS

We provide an overview of the criteria used to evaluate PET tracers. We then used the National Institute of Mental Health Research Priorities list to identify the key CNS targets. We conducted a PubMed search (search period 1st of January 2013 to 31st of December 2018), which yielded 40 new PET tracers across 16 CNS targets which met our selectivity criteria. For each tracer, we summarised the evidence of its properties and potential for use in studies of CNS pathophysiology and drug evaluation, including its target selectivity and affinity, inter and intra-subject variability, and pharmacokinetic parameters. We also consider its potential limitations and missing characterisation data, but not specific applications in drug development. Where multiple tracers were present for a target, we provide a comparison of their properties.

RESULTS AND CONCLUSIONS

Our review shows that multiple new tracers have been developed for proteinopathy targets, particularly tau, as well as the purinoceptor P2X7, phosphodiesterase enzyme PDE10A, and synaptic vesicle glycoprotein 2A (SV2A), amongst others. Some of the most promising of these include 18F-MK-6240 for tau imaging, 11C-UCB-J for imaging SV2A, 11C-CURB and 11C-MK-3168 for characterisation of fatty acid amide hydrolase, 18F-FIMX for metabotropic glutamate receptor 1, and 18F-MNI-444 for imaging adenosine 2A. Our review also identifies recurrent issues within the field. Many of the tracers discussed lack in vivo blocking data, reducing confidence in selectivity. Additionally, late-stage identification of substantial off-target sites for multiple tracers highlights incomplete pre-clinical characterisation prior to translation, as well as human disease state studies carried out without confirmation of test-retest reproducibility.

Characterization of 3 PET Tracers for Quantification of Mitochondrial and Synaptic Function in Healthy Human Brain: 18F-BCPP-EF, 11C-SA-4503, and 11C-UCB-J

Mitochondrial complex 1 is involved in maintaining brain bioenergetics; σ-1 receptor responds to neuronal stress; and synaptic vesicle protein 2A reflects synaptic integrity. Expression of each of these proteins is altered in neurodegenerative diseases. Here, we characterize the kinetic behavior of 3 PET radioligands-¹⁸F-BCPP-EF, ¹¹C-SA-4503, and ¹¹C-UCB-J-for the measurement of mitochondrial complex 1, σ-1 receptor, and synaptic vesicle protein 2A, respectively, and determine appropriate analysis workflows for their application in future studies of the in vivo molecular pathology of these diseases. Methods: Twelve human subjects underwent dynamic PET scans with each radioligand, including associated arterial blood sampling. A range of kinetic models was investigated to identify an optimal kinetic analysis method for each radioligand and a suitable acquisition duration. Results: All 3 radioligands readily entered the brain and yielded heterogeneous uptake consistent with the known distribution of the targets. The optimal models determined for the regional estimates of volume of distribution were multilinear analysis 1 (MA1) and the 2-tissue-compartment model for ¹⁸F-BCPP-EF, MA1 for ¹¹C-SA-4503, and both MA1 and the 1-tissue-compartment model for ¹¹C-UCB-J. Acquisition times of 70, 80, and 60 min for ¹⁸F-BCPP-EF, ¹¹C-SA-4503, ¹¹C-UCB-J, respectively, provided good estimates of regional volume of distribution values. An effect of age was observed on ¹⁸F-BCPP-EF and ¹¹C-UCB-J signal in the caudate. Conclusion: These ligands can be assessed for their potential to stratify patients or monitor the progression of molecular neuropathology in neurodegenerative diseases.

Sensitive and early detection of mitochondrial dysfunction in the liver of NASH model mice by PET imaging with 18F-BCPP-BF

BACKGROUND

Nonalcoholic fatty liver disease is a common disorder that progresses from simple fatty liver (steatosis) to nonalcoholic steatohepatitis (NASH). It is thought that mitochondrial dysfunction plays a critical role in the progression of NASH. In this study, we developed a non-invasive method for early diagnosis and staging of NASH that directly measures mitochondrial complex-I (MC-I) activity in the liver of NASH model mice by positron emission tomography (PET) imaging using the novel tracer 2-tert-butyl-4-chloro-5-[6-(4-[¹⁸F]fluorobutoxy)-pyridin-3-ylmethoxy]-2H-pyridazin-3-one (¹⁸F-BCPP-BF). Liver uptake of ¹⁸F-BCPP-BF in NASH and age-matched control mice was measured as a standard uptake value over a period of 1 to 12 weeks. Histopathological evaluation of the liver tissue was performed by haematoxylin and eosin staining, and fibrosis was assessed by Masson's trichrome staining.

RESULTS

Significant mitochondrial dysfunction was detected as early as 1 week after commencing the diet, and MC-I activity in the liver measured by PET was reduced by > 50% relative to that in age-matched control mice after 6 weeks. Liver uptake of ¹⁸F-BCPP-BF was low throughout the 12-week experimental period. Histopathological examination revealed that steatosis, inflammation, and ballooning progressed from 1 to 6 weeks, with fibrosis observed from 6 to 12 weeks.

CONCLUSIONS

PET scans and histopathological analysis revealed that mitochondrial dysfunction in the liver contributed to the progression of NASH. PET imaging with ¹⁸F-BCPP-BF is a useful tool for detecting NASH at early stages and for monitoring therapeutic response.

Monitoring Mitochondrial Complex-I Activity Using Novel PET Probe 18F-BCPP-EF Allows Early Detection of Radiotherapy Effect in Murine Squamous Cell Carcinoma

Objectives

Aerobic glycolysis, the main pathway of energy production in tumors (Warburg effect) allows detection of tumors by positron emission tomography (PET) using ¹⁸F-fluoro-2-deoxy-D-glucose (¹⁸F-FDG). Since ionizing radiation (IR) is reported to switch aerobic glycolysis to mitochondrial oxidative phosphorylation, radiotherapeutic efficacy was monitored by the activity of mitochondrial complex I (MC-I), using a new PET probe ¹⁸F-BCPP-EF, ¹⁸F-2-tert-butyl-4-chloro-5-{6-[2-(2-fluoro-ethoxy)-ethoxy] -pyridine-3-ylmethoxy}-2H-pyridazin-3-one, compared with ¹⁸F-FDG uptake and the apoptosis index.

Methods

Tumor uptake of ¹⁸F-BCPP-EF or ¹⁸F-FDG was examined in C3H/HeN mice inoculated with murine squamous cell carcinoma SCCVII at various time points after a single dose of x-ray irradiation at 0, 6, 15, or 30 Gy. Apoptosis incidence was determined by TUNEL staining in excised tumor tissue.

Results

Tumor growth suppression was dose-dependent; tumor grew 10-fold (0 Gy), 5-fold (6 Gy), 2-fold (15 Gy), and reduced to half in its volume (30 Gy) 14 days after treatment. ¹⁸F-BCPP-EF uptake was significantly increased as early as 3 days after 15 Gy or 30 Gy, when tumor size and apoptosis index showed no difference among radiation doses. In contrast, ¹⁸F-FDG uptake was initially increased dose-dependently, remained elevated up to 7 days, and eventually decreased 10 days after 30 Gy and also 14 days after 15 Gy when tumor size was already reduced. Apoptosis index was increased after irradiation but failed to correlate with tumor response.

Conclusion

Tumor uptake of ¹⁸F-BCPP-EF was increased dose-dependently early after effective doses of IR when ¹⁸F-FDG uptake as well as apoptosis incidence were not indicative of tumor response. The results suggest that ¹⁸F-BCPP-EF is a promising “positive” MC-I imaging PET probe for early detection of efficacy of tumor radiotherapy.

Effects of acetaminophen on mitochondrial complex I activity in the rat liver and kidney: a PET study with 18F-BCPP-BF

BACKGROUND

In the present study, 2-tert-butyl-4-chloro-5-[6-(4-¹⁸F-fluorobutoxy)-pyridin-3-ylmethoxy]-2H-pyridazin-3-one (¹⁸F-BCPP-BF), a PET probe for mitochondrial complex I (MC-I), was used to validate whether MC-I is a useful biomarker for detecting acetaminophen-induced dysfunctions in the liver and kidney. The kinetic and distribution of ¹⁸F-BCPP-BF were assessed in rats using high-resolution animal PET in vivo. The binding specificity of ¹⁸F-BCPP-BF to MC-I in the liver and kidney was confirmed by the pre-administration of rotenone, a specific MC-I inhibitor. The effects of acetaminophen on MC-I activity were assessed 2 and 24 h after the administration of vehicle or acetaminophen at a dose of 100 or 300 mg/kg. Biochemical parameters in plasma and urine were assessed 2, 6, and 24 h after the administration of vehicle or acetaminophen.

RESULTS

The uptake of ¹⁸F-BCPP-BF by the liver and kidney was significantly inhibited by the pre-administration of rotenone. Two and more hours after the administration of acetaminophen, the uptake of ¹⁸F-BCPP-BF was dose-dependently reduced in the liver, even at 100 mg/kg, and in the kidney at 300 mg/kg, whereas biological parameters started to be affected 6 h or later at doses of 300 mg/kg.

CONCLUSIONS

The present study demonstrated that ¹⁸F-BCPP-BF has potential as a PET probe for the quantitative imaging of hepatic and renal dysfunction as impaired MC-I activity in the early phase of the treatment for an overdose of acetaminophen in the living body with PET.

Non-invasive evaluation of neuroprotective drug candidates for cerebral infarction by PET imaging of mitochondrial complex-I activity

The development of a diagnostic technology that can accurately determine the pathological progression of ischemic stroke and evaluate the therapeutic effects of cerebroprotective agents has been desired. We previously developed a novel PET probe, 2-tert-butyl-4-chloro-5-{6-[2-(2-¹⁸F-fluoroethoxy)-ethoxy]-pyridin-3-ylmethoxy}-2H-pyridazin-3-one ([¹⁸F]BCPP-EF) for detecting activity of mitochondrial complex I (MC-I). This probe was shown to visualize neuronal damage in the living brain of rodent and primate models of neurodegenerative diseases. In the present study, [¹⁸F]BCPP-EF was applied to evaluate the therapeutic effects of a neuroprotectant, liposomal FK506 (FK506-liposomes), on cerebral ischemia/reperfusion (I/R) injury in transient middle cerebral artery occlusion rats. The PET imaging using [¹⁸F]BCPP-EF showed a prominent reduction in the MC-I activity in the ischemic brain hemisphere. Treatment with FK506-liposomes remarkably increased the uptake of [¹⁸F]BCPP-EF in the ischemic side corresponding to the improvement of blood flow disorders and motor function deficits throughout the 7 days after I/R. Additionally, the PET scan could diagnose the extent of the brain damage accurately and showed the neuroprotective effect of FK506-liposomes at Day 7, at which 2, 3, 5-triphenyltetrazolium chloride staining couldn’t visualize them. Our study demonstrated that the PET technology using [¹⁸F]BCPP-EF has a potent capacity to evaluate the therapeutic effect of drug candidates in living brain.

Acute and subacute toxicity studies of CMICE-013, a novel iodinated rotenone-based myocardial perfusion tracer, in Sprague Dawley rats and Gottingen minipigs

PURPOSE

Extensive acute and subacute toxicities studies are required to evaluate the toxicological profile of the novel cardiac perfusion imaging tracer (123)I-CMICE-013 to support applications for clinical trials.

METHODS

Sprague-Dawley rats and Gottingen minipigs received injections of non-radioactive 127I-CMICE-013 at two dosage levels of 1 and 5 μg/kg, and vehicle buffer as control. In the acute toxicity studies, each animal was injected on two occasions 24 h apart and then underwent a 14-day recovery period; in the subacute study, animals received daily injections for 14 days continuously. The health status and mortality of test animals were monitored daily and body weight, food consumption, physiological and biochemical parameters were measured at various time points during the study. Animals were euthanized at the end of the studies and dissected for pathologic examination of organs and tissues.

RESULTS

The acute and subacute administrations of injections of the non-radioactive CMICE-013 in rats and minipigs were well tolerated. Little to no dosing-related adverse effects were observed in animal body and organ weights, hematology, coagulation, clinical chemistry, urinalysis, ophthalmoscopy, electrocardiograms, heart rates, blood pressure, macroscopic and microscopic examination of the preserved animal tissues including the brain.

CONCLUSION

The lack of adverse effects from acute and subacute dosing suggest that the CMICE-013 injection solution has a reasonable safety margin within the designed concentration range to be utilized in imaging applications. The dosage level of 5 μg/kg was considered the no adverse effect level for both rats and minipigs based on our acute and subacute studies.

Comparing α7 nicotinic acetylcholine receptor binding, amyloid-β deposition, and mitochondria complex-I function in living brain: A PET study in aged monkeys

This study was aimed to assess the correlations among α7 nicotinic acetylcholine receptor (α7-nAChR) binding, amyloid-β (Aβ) deposition, and mitochondrial complex I (MC-I) activity in the brain of aged monkeys (Macaca mulatta). Positron emission tomography (PET) measurements with [(11) C](R)-MeQAA, [(11) C]PIB, and [(18) F]BCPP-EF were conducted in monkeys in a conscious condition. [(11) C](R)-MeQAA binding was analyzed by a simplified reference tissue model to calculate nondisplaceable binding potential (BPND), [(11) C]PIB uptake was calculated by standard uptake value ratio (SUVR), and [(18) F]BCPP-EF binding was determined by Logan graphical analysis to calculate total distribution volume (VT) with arterial blood sampling. Higher brain uptake was determined in the thalamus, hippocampus, striatum, and cortical regions for [(11) C](R)-MeQAA, while being lower in the cerebellum. Significant age-related reduction of [(11) C](R)-MeQAA binding to α7-nAChR was determined only in the occipital cortex. The plot of Vt of [(18) F]BCPP-EF against BPND of [(11) C](R)-MeQAA indicated a significant negative correlation in the hippocampus and cortical regions in aged animals. Plotting of SUVR of [(11) C]PIB against BPND of [(11) C](R)-MeQAA showed a positive correlation. The in vivo binding of [(11) C](R)-MeQAA could reflect the upregulation of α7-nAChR induced by neurodegenerative damage determined by Aβ deposition as well as impaired MC-I activity in living brain.

The use of ¹⁸F-BCPP-EF as a PET probe for complex I activity in the brain

Mitochondria are called "cellular power plants" because they exclusively contain a respiratory electron transfer chain consisting of five components (complexes I-V) to generate most of the ATP required to maintain cellular functions. Mitochondrial complex I (MC-I) is the first and the largest macrocomplex in the pathway for oxidative phosphorylation. We recently synthesized a series of novel PET probes for quantitative imaging of MC-I activity in the living brain. Several in vitro biological evaluations suggested that (18)F-BCPP-EF could be applicable for MC-I assessment in vivo, and the probe has been applied to several animal disease models of stroke, aging, and dementia. The data suggested that (18)F-BCPP-EF could be useful to detect ischemic neuronal damage at the subacute phase, 7 days, after ischemic insult, at which time unexpectedly higher (18)F-FDG uptake was observed in the damaged area than in the contralateral intact area. Our studies with the aged monkeys demonstrated that (18)F-BCPP-EF detected the age-related reduction of MC-I activity in the living monkey brain, and also that the monkeys with higher amyloid-β deposition showed lower MC-I activity. Since PET is a sophisticated medical modality for noninvasive assessment of real-time tissue function by using target-specific radiolabeled probes, the development of novel PET probes for MC-I should be useful for diagnostic, prognostic, and treatment monitoring of diseases related to impaired MC-I function.

Comparing amyloid-β deposition, neuroinflammation, glucose metabolism, and mitochondrial complex I activity in brain: a PET study in aged monkeys

PURPOSE

The aim of the present study was to compare amyloid-β (Aβ) deposition, translocator protein (TSPO) activity, regional cerebral metabolic rate of glucose (rCMRglc), and mitochondrial complex I (MC-I) activity in the brain of aged monkeys.

METHODS

PET scans with (11)C-PIB (Aβ), (18)F-BCPP-EF (MC-I), (11)C-DPA-713 (TSPO), and (18)F-FDG (rCMRglc) were performed in aged monkeys (Macaca mulatta) in the conscious state and under isoflurane anaesthesia. (11)C-PIB binding to Aβ and (11)C-DPA-713 binding to TSPO were evaluated in terms of standard uptake values (SUV). The total volume of distribution (V T) of (18)F-BCPP-EF and rCMRglc with (18)F-FDG were calculated using arterial blood sampling.

RESULTS

Isoflurane did not affect MC-I activity measured in terms of (18)F-BCPP-EF uptake in living brain. There was a significant negative correlation between (18)F-BCPP-EF binding (V T) and (11)C-PIB uptake (SUVR), and there was a significant positive correlation between (11)C-DPA-713 uptake (SUV) and (11)C-PIB uptake. In contrast, there was no significant correlation between rCMRglc ratio and (11)C-PIB uptake.

CONCLUSION

(18)F-BCPP-EF could be a potential PET probe for quantitative imaging of impaired MC-I activity that is correlated with Aβ deposition in the living brain.

PET imaging of ischemia-induced impairment of mitochondrial complex I function in monkey brain

To assess the capability of (18)F-2-tert-butyl-4-chloro-5-{6-[2-(2-fluoroethoxy)-ethoxy]-pyridin-3-ylmethoxy}-2H-pyridazin-3-one ((18)F-BCPP-EF), a novel positron emission tomography (PET) probe for mitochondrial complex I (MC-I) activity, as a specific marker of ischemia-induced neuronal death without being disturbed by inflammation, translational research was conducted using an animal PET in ischemic brains of Cynomolgus monkeys (Macaca fascicularis). Focal ischemia was induced by the right middle cerebral artery occlusion for 3 hours, then PET scans were conducted at Day-7 with (15)O-gases for regional cerebral blood flow (rCBF) and regional cerebral metabolism of oxygen (rCMRO₂), and (18)F-BCPP-EF for MC-I with arterial blood sampling. On Day-8, the additional PET scans conducted with (11)C-flumazenil ((11)C-FMZ) for central-type benzodiazepine receptors, (11)C-PBR28 for translocator protein, and (18)F-fluoro-2-deoxy-D-glucose ((18)F-FDG) for regional cerebral metabolic rate of glucose (rCMRglc). The total distribution volume (VT) values of (18)F-BCPP-EF showed the significant reduction in MC-I activity in the damaged area at Day-7. When correlated with rCBF and rCMRO₂, the VT values of (18)F-BCPP-EF provided better correlation with rCMRO₂ than with rCBF. In the inflammatory regions (region of interest, ROIPBR) of the ischemic hemisphere detected with (11)C-PBR28, higher (18)F-FDG uptake and lower VT of (18)F-BCPP-EF, (11)C-FMZ, and rCMRO2 than those in normal contralateral hemisphere were observed. These results strongly suggested that (18)F-BCPP-EF could discriminate the neuronal damaged areas with neuroinflammation, where (18)F-FDG could not owing to its high uptake into the activated microglia.

Novel PET probes 18F-BCPP-EF and 18F-BCPP-BF for mitochondrial complex I: a PET study in comparison with 18F-BMS-747158-02 in rat brain

We developed novel PET probes, 2-tert-butyl-4-chloro-5-{6-[2-(2-(18)F-fluoroethoxy)-ethoxy]-pyridin-3-ylmethoxy}-2H-pyridazin-3-one ((18)F-BCPP-EF) and 2-tert-butyl-4-chloro-5-[6-(4-(18)F-fluorobutoxy)-pyridin-3-ylmethoxy]-2H-pyridazin-3-one ((18)F-BCPP-BF), for quantitative imaging of mitochondrial complex I (MC-I) activity in the brain and preliminarily evaluated their properties in comparison with (18)F-BMS-747158-02 ((18)F-BMS).

METHODS

The affinity of (18)F-BCPP-EF, (18)F-BCPP-BF, and (18)F-BMS to MC-I was analyzed using in vitro binding assays with (3)H-dihydrorotenone and bovine cardiomyocyte submitochondrial particles. (18)F-BCPP-EF, (18)F-BCPP-BF, or (18)F-BMS was intravenously injected into rats, and the uptake (standardized uptake value) in each organ was determined by dissection method. The effects of rotenone, a specific MC-I inhibitor, on the uptake of each probe were assessed by whole-body PET imaging in rats. Ischemic brain model rats were imaged using (18)F-BCPP-EF.

RESULTS

The rank order of affinity to MC-I was (18)F-BCPP-BF > (18)F-BMS > (18)F-BCPP-EF. The uptake of (18)F-BCPP-EF and (18)F-BMS was high in the heart, intermediate in brain, and low in muscle and bone 60 min after the injection. (18)F-BCPP-BF provided increasing bone uptake with time after the injection. The uptake of (18)F-BCPP-EF and (18)F-BMS into the brain and heart was significantly decreased by preadministration of rotenone; however, the reduction degree of (18)F-BCPP-EF was more pronounced than that of (18)F-BMS. Rotenone did not affect (18)F-BCPP-BF uptake in either the brain or the heart. (18)F-BCPP-EF imaged the cortical ischemic neuronal damage without any disturbance by microglial activation even on day 7 when (18)F-FDG showed high uptake in the damaged area.

CONCLUSION

The present study demonstrated that (18)F-BCPP-EF could be a potential PET probe for quantitative imaging of MC-I activity and its ischemic damage in the living brain with PET.

Evaluation of 18F-BCPP-EF for mitochondrial complex 1 imaging in the brain of conscious monkeys using PET

PURPOSE

We have reported on the development of a novel PET probe, (18)F-2-tert-butyl-4-chloro-5-{6-[2-(2-fluoroethoxy)-ethoxy]-pyridin-3-ylmethoxy}-2H-pyridazin-3-one ((18)F-BCPP-EF), for quantitative imaging of mitochondrial complex 1 (MC-1) activity in the brain of the living rat. For clinical application in humans, translational research in the monkey was conducted.

METHODS

PET measurements with (18)F-BCPP-EF were performed in young and old monkeys (Macaca mulatta) in a conscious state with arterial blood sampling. The binding specificity of (18)F-BCPP-EF was evaluated with rotenone, a specific MC-1 inhibitor, in young animals. The binding (total distribution volume, V T) of (18)F-BCPP-EF was calculated using Logan graphical analysis, and one-tissue compartment model (1-TC) and two-tissue compartment model (2-TC) analyses using a metabolite-corrected plasma input function.

RESULTS

F-BCPP-EF was rapidly taken up into the brain just after intravenous injection, peaked between 10 and 20 min after injection, and was then gradually eliminated. The 2-TC analysis provided a better fit than the 1-TC analysis, and the V T values from the 2-TC analysis correlated well with those from the Logan plot. With predosing with rotenone, (18)F-BCPP-EF showed a higher uptake peak in the brain, followed by more rapid elimination thereafter than in the vehicle condition, resulting in significant reductions in 2-TC V T values in all regions. In old animals, the kinetics of (18)F-BCPP-EF were slightly slower with lower peak levels than in young animals, resulting age-related reductions in (18)F-BCPP-EF binding in all brain regions.

CONCLUSION

The present study demonstrated that (18)F-BCPP-EF may be a potential PET probe for quantitative imaging MC-1 activity in the living brain using PET.