Functional maturation and longitudinal imaging of intraportal neonatal porcine islet grafts in genetically diabetic pigs.

Allogeneic intraportal islet transplantation (ITx) has become an established treatment for patients with poorly controlled type 1 diabetes. However, the loss of viable beta-cell mass after transplantation remains a major challenge. Therefore, noninvasive imaging methods for long-term monitoring of the transplant fate are required. In this study, [68Ga]Ga-DOTA-exendin-4 positron emission tomography/computed tomography (PET/CT) was used for repeated monitoring of allogeneic neonatal porcine islets (NPI) after intraportal transplantation into immunosuppressed genetically diabetic pigs. NPI transplantation (3320-15,000 islet equivalents per kg body weight) led to a reduced need for exogenous insulin therapy and finally normalization of blood glucose levels in 3 out of 4 animals after 5 to 10 weeks. Longitudinal PET/CT measurements revealed a significant increase in standard uptake values in graft-bearing livers. Histologic analysis confirmed the presence of well-engrafted, mature islet clusters in the transplanted livers. Our study presents a novel large animal model for allogeneic intraportal ITx. A relatively small dose of NPIs was sufficient to normalize blood glucose levels in a clinically relevant diabetic pig model. [68Ga]Ga-DOTA-exendin-4 PET/CT proved to be efficacious for longitudinal monitoring of islet transplants. Thus, it could play a crucial role in optimizing ITx as a curative therapy for type 1 diabetes.

Image-based dosimetry for [225Ac]Ac-PSMA-I&T therapy and the effect of daughter-specific pharmacokinetics.

PURPOSE: Although 221Fr and 213Bi have sufficient gamma emission probabilities, quantitative SPECT after [225Ac]Ac-PSMA-I&T therapy remains challenging due to low therapeutic activities. Furthermore, 221Fr and 213Bi may underlie a different pharmacokinetics due to alpha recoil. We conducted a quantitative SPECT study and a urine analysis to investigate the pharmacokinetics of 221Fr and 213Bi and the impact on image-based lesion and kidney dosimetry. METHODS: Five patients (7.7 ± 0.2 MBq [225Ac]Ac-PSMA-I&T) underwent an abdominal SPECT/CT (1 h) at 24 and 48 h (Siemens Symbia T2, high-energy collimator, 440 keV/218 keV (width 20%), 78 keV (width 50%)). Quantitative SPECT was reconstructed using MAP-EM with attenuation and transmission-dependent scatter corrections and resolution modelling. Time-activity curves for kidneys (CT-based) and lesions (80% isocontour 24 h) were fitted mono-exponentially. Urine samples collected along with each SPECT/CT were measured in a gamma counter until secular equilibrium was reached. RESULTS: Mean kidney and lesion effective half-lives were as follows: 213Bi, 27 ± 6/38 ± 10 h; 221Fr, 24 ± 6/38 ± 11 h; 78 keV, 23 ± 7/39 ± 13 h. The 213Bi-to-221Fr kidney SUV ratio increased by an average of 9% from 24 to 48 h. Urine analysis revealed an increasing 213Bi-to-225Ac ratio (24 h, 0.98 ± 0.15; 48 h, 1.08 ± 0.09). Mean kidney and lesion absorbed doses were 0.17 ± 0.06 and 0.36 ± 0.1 Sv RBE = 5 /MBq using 221Fr and 213Bi SPECT images, compared to 0.16 ± 0.05/0.18 ± 0.06 and 0.36 ± 0.1/0.38 ± 0.1 Sv RBE = 5 /MBq considering either the 221Fr or 213Bi SPECT. CONCLUSION: SPECT/CT imaging and urine analysis showed minor differences of up to 10% in the daughter-specific pharmacokinetics. These variances had a minimal impact on the lesion and kidney dosimetry which remained within 8%.

Proliferation and apoptosis after whole-body irradiation: longitudinal PET study in a mouse model.

PURPOSE: A reliable method for regional in vivo imaging of radiation-induced cellular damage would be of great importance for the detection of therapy-induced injury to healthy tissue and the choice of adequate treatment of radiation emergency patients in both civilian and military events. This study aimed to investigate in a mouse model if positron emission tomography (PET) imaging with proliferation and apoptosis markers is potentially suitable for this purpose. METHODS: Four groups, including twenty mice (wild-type C57BL/6) each, were whole-body irradiated with 0 Gy, 0.5 Gy, 1 Gy, and 3 Gy and examined by PET over a six-month period at defined time points. 3'-[18F]fluoro-3'-deoxythymidine ([18F]FLT) and 2-(5-[18F]fluoropentyl)-2-methyl malonic acid ([18F]ML-10) were used to visualise proliferation and apoptosis. Regional standard uptake values were compared with respect to irradiation dose over time. Histologic data and peripheral blood cell values were correlated with the PET results. RESULTS: The hematopoietic bone marrow showed a significantly increased [18F]FLT signal at early time points after radiation exposure (day 3 and day 7). This correlated with blood parameters, especially leukocytes, and histological data. A significantly increased [18F]FLT signal also occurred in the gastrointestinal tract and thymus at early time points. An increased [18F]ML-10 signal related to irradiation doses was observed in the bone marrow on day 8, but there was a high variability of standard uptake values and no correlation with histological data. CONCLUSION: [18F]FLT showed potential to visualise the extent, regional distribution and recovery from radiation-induced cellular damage in the bone marrow, gastrointestinal tract and thymus. The potential of [18F]FLT imaging to assess the extent of bone marrow affected by irradiation might be especially useful to predict the subsequent severity of hematopoietic impairment and to adapt the therapy of the bone marrow reserve. [18F]ML-10 PET proved to be not sensitive enough for the reliable detection of radiation induced apoptosis.

Assessment of [18F]PI-2620 Tau-PET Quantification via Non-Invasive Automatized Image Derived Input Function.

PURPOSE: [18F]PI-2620 positron emission tomography (PET) detects misfolded tau in progressive supranuclear palsy (PSP) and Alzheimer's disease (AD). We questioned the feasibility and value of absolute [18F]PI-2620 PET quantification for assessing tau by regional distribution volumes (VT). Here, arterial input functions (AIF) represent the gold standard, but cannot be applied in routine clinical practice, whereas image-derived input functions (IDIF) represent a non-invasive alternative. We aimed to validate IDIF against AIF and we evaluated the potential to discriminate patients with PSP and AD from healthy controls by non-invasive quantification of [18F] PET. METHODS: In the first part of the study, we validated AIF derived from radial artery whole blood against IDIF by investigating 20 subjects (ten controls and ten patients). IDIF were generated by manual extraction of the carotid artery using the average and the five highest (max5) voxel intensity values and by automated extraction of the carotid artery using the average and the maximum voxel intensity value. In the second part of the study, IDIF quantification using the IDIF with the closest match to the AIF was transferred to group comparison of a large independent cohort of 40 subjects (15 healthy controls, 15 PSP patients and 10 AD patients). We compared VT and VT ratios, both calculated by Logan plots, with distribution volume (DV) ratios using simplified reference tissue modelling and standardized uptake value (SUV) ratios. RESULTS: AIF and IDIF showed highly correlated input curves for all applied IDIF extraction methods (0.78 < r < 0.83, all p < 0.0001; area under the curves (AUC): 0.73 < r ≤ 0.82, all p ≤ 0.0003). Regarding the VT values, correlations were mainly found between those generated by the AIF and by the IDIF methods using the maximum voxel intensity values. Lowest relative differences (RD) were observed by applying the manual method using the five highest voxel intensity values (max5) (AIF vs. IDIF manual, avg: RD = -82%; AIF vs. IDIF automated, avg: RD = -86%; AIF vs. IDIF manual, max5: RD = -6%; AIF vs. IDIF automated, max: RD = -26%). Regional VT values revealed considerable variance at group level, which was strongly reduced upon scaling by the inferior cerebellum. The resulting VT ratio values were adequate to detect group differences between patients with PSP or AD and healthy controls (HC) (PSP target region (globus pallidus): HC vs. PSP vs. AD: 1.18 vs. 1.32 vs. 1.16; AD target region (Braak region I): HC vs. PSP vs. AD: 1.00 vs. 1.00 vs. 1.22). VT ratios and DV ratios outperformed SUV ratios and VT in detecting differences between PSP and healthy controls, whereas all quantification approaches performed similarly in comparing AD and healthy controls. CONCLUSION: Blood-free IDIF is a promising approach for quantification of [18F]PI-2620 PET, serving as correlating surrogate for invasive continuous arterial blood sampling. Regional [18F]PI-2620 VT show large variance, in contrast to regional [18F]PI-2620 VT ratios scaled with the inferior cerebellum, which are appropriate for discriminating PSP, AD and healthy controls. DV ratios obtained by simplified reference tissue modeling are similarly suitable for this purpose.

Negative symptoms, striatal dopamine and model-free reward decision-making in schizophrenia.

Negative symptoms, such as lack of motivation or social withdrawal, are highly prevalent and debilitating in patients with schizophrenia. Underlying mechanisms of negative symptoms are incompletely understood, thereby preventing the development of targeted treatments. We hypothesized that in patients with schizophrenia during psychotic remission, impaired influences of both model-based and model-free reward predictions on decision-making ('reward prediction influence', RPI) underlie negative symptoms. We focused on psychotic remission, because psychotic symptoms might confound reward-based decision-making. Moreover, we hypothesized that impaired model-based/model-free RPIs depend on alterations of both associative striatum dopamine synthesis and storage (DSS) and executive functioning. Both factors influence RPI in healthy subjects and are typically impaired in schizophrenia. Twenty-five patients with schizophrenia with pronounced negative symptoms during psychotic remission and 24 healthy controls were included in the study. Negative symptom severity was measured by the Positive and Negative Syndrome Scale negative subscale, model-based/model-free RPI by the two-stage decision task, associative striatum DSS by 18F-DOPA positron emission tomography and executive functioning by the symbol coding task. Model-free RPI was selectively reduced in patients and associated with negative symptom severity as well as with reduced associative striatum DSS (in patients only) and executive functions (both in patients and controls). In contrast, model-based RPI was not altered in patients. Results provide evidence for impaired model-free reward prediction influence as a mechanism for negative symptoms in schizophrenia as well as for reduced associative striatum dopamine and executive dysfunction as relevant factors. Data suggest potential treatment targets for patients with schizophrenia and pronounced negative symptoms.

Depletion and activation of microglia impact metabolic connectivity of the mouse brain.

AIM: We aimed to investigate the impact of microglial activity and microglial FDG uptake on metabolic connectivity, since microglial activation states determine FDG-PET alterations. Metabolic connectivity refers to a concept of interacting metabolic brain regions and receives growing interest in approaching complex cerebral metabolic networks in neurodegenerative diseases. However, underlying sources of metabolic connectivity remain to be elucidated. MATERIALS AND METHODS: We analyzed metabolic networks measured by interregional correlation coefficients (ICCs) of FDG-PET scans in WT mice and in mice with mutations in progranulin (Grn) or triggering receptor expressed on myeloid cells 2 (Trem2) knockouts (-/-) as well as in double mutant Grn-/-/Trem2-/- mice. We selected those rodent models as they represent opposite microglial signatures with disease associated microglia in Grn-/- mice and microglia locked in a homeostatic state in Trem2-/- mice; however, both resulting in lower glucose uptake of the brain. The direct influence of microglia on metabolic networks was further determined by microglia depletion using a CSF1R inhibitor in WT mice at two different ages. Within maps of global mean scaled regional FDG uptake, 24 pre-established volumes of interest were applied and assigned to either cortical or subcortical networks. ICCs of all region pairs were calculated and z-transformed prior to group comparisons. FDG uptake of neurons, microglia, and astrocytes was determined in Grn-/- and WT mice via assessment of single cell tracer uptake (scRadiotracing). RESULTS: Microglia depletion by CSF1R inhibition resulted in a strong decrease of metabolic connectivity defined by decrease of mean cortical ICCs in WT mice at both ages studied (6-7 m; p = 0.0148, 9-10 m; p = 0.0191), when compared to vehicle-treated age-matched WT mice. Grn-/-, Trem2-/- and Grn-/-/Trem2-/- mice all displayed reduced FDG-PET signals when compared to WT mice. However, when analyzing metabolic networks, a distinct increase of ICCs was observed in Grn-/- mice when compared to WT mice in cortical (p < 0.0001) and hippocampal (p < 0.0001) networks. In contrast, Trem2-/- mice did not show significant alterations in metabolic connectivity when compared to WT. Furthermore, the increased metabolic connectivity in Grn-/- mice was completely suppressed in Grn-/-/Trem2-/- mice. Grn-/- mice exhibited a severe loss of neuronal FDG uptake (- 61%, p < 0.0001) which shifted allocation of cellular brain FDG uptake to microglia (42% in Grn-/- vs. 22% in WT). CONCLUSIONS: Presence, absence, and activation of microglia have a strong impact on metabolic connectivity of the mouse brain. Enhanced metabolic connectivity is associated with increased microglial FDG allocation.

[18F]F-DED PET imaging of reactive astrogliosis in neurodegenerative diseases: preclinical proof of concept and first-in-human data.

OBJECTIVES: Reactive gliosis is a common pathological hallmark of CNS pathology resulting from neurodegeneration and neuroinflammation. In this study we investigate the capability of a novel monoamine oxidase B (MAO-B) PET ligand to monitor reactive astrogliosis in a transgenic mouse model of Alzheimer`s disease (AD). Furthermore, we performed a pilot study in patients with a range of neurodegenerative and neuroinflammatory conditions. METHODS: A cross-sectional cohort of 24 transgenic (PS2APP) and 25 wild-type mice (age range: 4.3-21.0 months) underwent 60 min dynamic [18F]fluorodeprenyl-D2 ([18F]F-DED), static 18 kDa translocator protein (TSPO, [18F]GE-180) and ß-amyloid ([18F]florbetaben) PET imaging. Quantification was performed via image derived input function (IDIF, cardiac input), simplified non-invasive reference tissue modelling (SRTM2, DVR) and late-phase standardized uptake value ratios (SUVr). Immunohistochemical (IHC) analyses of glial fibrillary acidic protein (GFAP) and MAO-B were performed to validate PET imaging by gold standard assessments. Patients belonging to the Alzheimer's disease continuum (AD, n = 2), Parkinson's disease (PD, n = 2), multiple system atrophy (MSA, n = 2), autoimmune encephalitis (n = 1), oligodendroglioma (n = 1) and one healthy control underwent 60 min dynamic [18F]F-DED PET and the data were analyzed using equivalent quantification strategies. RESULTS: We selected the cerebellum as a pseudo-reference region based on the immunohistochemical comparison of age-matched PS2APP and WT mice. Subsequent PET imaging revealed that PS2APP mice showed elevated hippocampal and thalamic [18F]F-DED DVR when compared to age-matched WT mice at 5 months (thalamus: + 4.3%; p = 0.048), 13 months (hippocampus: + 7.6%, p = 0.022) and 19 months (hippocampus: + 12.3%, p < 0.0001; thalamus: + 15.2%, p < 0.0001). Specific [18F]F-DED DVR increases of PS2APP mice occurred earlier when compared to signal alterations in TSPO and ß-amyloid PET and [18F]F-DED DVR correlated with quantitative immunohistochemistry (hippocampus: R = 0.720, p < 0.001; thalamus: R = 0.727, p = 0.002). Preliminary experience in patients showed [18F]F-DED VT and SUVr patterns, matching the expected topology of reactive astrogliosis in neurodegenerative (MSA) and neuroinflammatory conditions, whereas the patient with oligodendroglioma and the healthy control indicated [18F]F-DED binding following the known physiological MAO-B expression in brain. CONCLUSIONS: [18F]F-DED PET imaging is a promising approach to assess reactive astrogliosis in AD mouse models and patients with neurological diseases.

Biodistribution and dosimetry for combined [177Lu]Lu-PSMA-I&T/[225Ac]Ac-PSMA-I&T therapy using multi-isotope quantitative SPECT imaging.

PURPOSE: Quantitative SPECT for patient-specific dosimetry is a valuable tool in the scope of radionuclide therapy, although its clinical application for 225Ac-based treatments may be limited due to low therapeutic activities. Therefore, the aim of this study was to demonstrate the feasibility of clinical quantitative low-count SPECT imaging during [177Lu]Lu-PSMA-I&T/[225Ac]Ac-PSMA-I&T treatment. METHODS: Eight prostate cancer patients (1000 MBq/8 MBq [177Lu]Lu-PSMA-I&T/[225Ac]Ac-PSMA-I&T) received a single-bed quantitative 177Lu/225Ac SPECT/CT acquisition (1 h) at 24 h post treatment (high-energy collimator, 16 projections p. head à 3.5 min, 128 × 128 pixel). The gamma peak at 440 keV (width: 10%) of the progeny 213Bi was imaged along with the peak at 208 keV (width: 15%) of 177Lu. Quantification included CT-based attenuation and window-based scatter correction plus resolution modelling. Gaussian post-filtering with a full-width-half-maximum of 30 mm and 40-45 mm was employed to match the signal-to-noise ratio of 225Ac and 177Lu, respectively. RESULTS: Kidney (r = 0.96, p < 0.01) and lesion (r = 0.94, p < 0.01) SUV for [177Lu]Lu-PSMA-I&T and [225Ac]Ac-PSMA-I&T showed a strong and significant correlation. Kidney SUV were significantly higher (p < 0.01) for [225Ac]Ac-PSMA-I&T (2.5 ± 0.8 vs. 2.1 ± 0.9), while for [177Lu]Lu-PSMA-I&T lesion SUV were significantly higher (p = 0.03; 1.8 ± 1.1 vs. 2.1 ± 1.5). For absorbed dose estimates, significant differences regarding the kidneys remained, while no significant differences for lesion dosimetry were found. CONCLUSION: Quantitative low-count SPECT imaging of the peak at 440 keV during [225Ac]Ac-PSMA-I&T therapy is feasible. Multi-isotope imaging for [177Lu]Lu-PSMA-I&T/[225Ac]Ac-PSMA-I&T therapy indicates accumulation of free 213Bi in the kidneys.

Long-Term Pioglitazone Treatment Has No Significant Impact on Microglial Activation and Tau Pathology in P301S Mice.

Neuroinflammation is one disease hallmark on the road to neurodegeneration in primary tauopathies. Thus, immunomodulation might be a suitable treatment strategy to delay or even prevent the occurrence of symptoms and thus relieve the burden for patients and caregivers. In recent years, the peroxisome proliferator-activated receptor γ (PPARγ) has received increasing attention as it is immediately involved in the regulation of the immune system and can be targeted by the anti-diabetic drug pioglitazone. Previous studies have shown significant immunomodulation in amyloid-ß (Aß) mouse models by pioglitazone. In this study, we performed long-term treatment over six months in P301S mice as a tauopathy model with either pioglitazone or placebo. We performed serial 18 kDa translocator protein positron-emission-tomography (TSPO-PET) imaging and terminal immunohistochemistry to assess microglial activation during treatment. Tau pathology was quantified via immunohistochemistry at the end of the study. Long-term pioglitazone treatment had no significant effect on TSPO-PET, immunohistochemistry read-outs of microglial activation, or tau pathology levels in P301S mice. Thus, we conclude that pioglitazone modifies the time course of Aß-dependent microglial activation, but does not significantly modulate microglial activation in response to tau pathology.

Application of machine learning to pretherapeutically estimate dosimetry in men with advanced prostate cancer treated with 177Lu-PSMA I&T therapy.

PURPOSE: Although treatment planning and individualized dose application for emerging prostate-specific membrane antigen (PSMA)-targeted radioligand therapy (RLT) are generally recommended, it is still difficult to implement in practice at the moment. In this study, we aimed to prove the concept of pretherapeutic prediction of dosimetry based on imaging and laboratory measurements before the RLT treatment. METHODS: Twenty-three patients with metastatic castration-resistant prostate cancer (mCRPC) treated with 177Lu-PSMA I&T RLT were included retrospectively. They had available pre-therapy 68 Ga-PSMA-HEBD-CC PET/CT and at least 3 planar and 1 SPECT/CT imaging for dosimetry. Overall, 43 cycles of 177Lu-PSMA I&T RLT were applied. Organ-based standard uptake values (SUVs) were obtained from pre-therapy PET/CT scans. Patient dosimetry was calculated for the kidney, liver, spleen, and salivary glands using Hermes Hybrid Dosimetry 4.0 from the planar and SPECT/CT images. Machine learning methods were explored for dose prediction from organ SUVs and laboratory measurements. The uncertainty of these dose predictions was compared with the population-based dosimetry estimates. Mean absolute percentage error (MAPE) was used to assess the prediction uncertainty of estimated dosimetry. RESULTS: An optimal machine learning method achieved a dosimetry prediction MAPE of 15.8 ± 13.2% for the kidney, 29.6% ± 13.7% for the liver, 23.8% ± 13.1% for the salivary glands, and 32.1 ± 31.4% for the spleen. In contrast, the prediction based on literature population mean has significantly larger MAPE (p < 0.01), 25.5 ± 17.3% for the kidney, 139.1% ± 111.5% for the liver, 67.0 ± 58.3% for the salivary glands, and 54.1 ± 215.3% for the spleen. CONCLUSION: The preliminary results confirmed the feasibility of pretherapeutic estimation of treatment dosimetry and its added value to empirical population-based estimation. The exploration of dose prediction may support the implementation of treatment planning for RLT.

Proof of concept of a multimodal intravital molecular imaging system for tumour transpathology investigation.

BACKGROUND: Transpathology highlights the interpretation of the underlying physiology behind molecular imaging. However, it remains challenging due to the discrepancies between in vivo and in vitro measurements and difficulties of precise co-registration between trans-scaled images. This study aims to develop a multimodal intravital molecular imaging (MIMI) system as a tool for in vivo tumour transpathology investigation. METHODS: The proposed MIMI system integrates high-resolution positron imaging, magnetic resonance imaging (MRI) and microscopic imaging on a dorsal skin window chamber on an athymic nude rat. The window chamber frame was designed to be compatible with multimodal imaging and its fiducial markers were customized for precise physical alignment among modalities. The co-registration accuracy was evaluated based on phantoms with thin catheters. For proof of concept, tumour models of the human colorectal adenocarcinoma cell line HT-29 were imaged. The tissue within the window chamber was sectioned, fixed and haematoxylin-eosin (HE) stained for comparison with multimodal in vivo imaging. RESULTS: The final MIMI system had a maximum field of view (FOV) of 18 mm × 18 mm. Using the fiducial markers and the tubing phantom, the co-registration errors are 0.18 ± 0.27 mm between MRI and positron imaging, 0.19 ± 0.22 mm between positron imaging and microscopic imaging and 0.15 ± 0.27 mm between MRI and microscopic imaging. A pilot test demonstrated that the MIMI system provides an integrative visualization of the tumour anatomy, vasculatures and metabolism of the in vivo tumour microenvironment, which was consistent with ex vivo pathology. CONCLUSIONS: The established multimodal intravital imaging system provided a co-registered in vivo platform for trans-scale and transparent investigation of the underlying pathology behind imaging, which has the potential to enhance the translation of molecular imaging.

Regional Hyperthermia Enhances Mesenchymal Stem Cell Recruitment to Tumor Stroma: Implications for Mesenchymal Stem Cell-Based Tumor Therapy.

The tropism of mesenchymal stem cells (MSCs) for tumors forms the basis for their use as delivery vehicles for the tumor-specific transport of therapeutic genes, such as the theranostic sodium iodide symporter (NIS). Hyperthermia is used as an adjuvant for various tumor therapies and has been proposed to enhance leukocyte recruitment. Here, we describe the enhanced recruitment of adoptively applied NIS-expressing MSCs to tumors in response to regional hyperthermia. Hyperthermia (41°C, 1 h) of human hepatocellular carcinoma cells (HuH7) led to transiently increased production of immunomodulatory factors. MSCs showed enhanced chemotaxis to supernatants derived from heat-treated cells in a 3D live-cell tracking assay and was validated in vivo in subcutaneous HuH7 mouse xenografts. Cytomegalovirus (CMV)-NIS-MSCs were applied 6-48 h after or 24-48 h before hyperthermia treatment. Using 123I-scintigraphy, thermo-stimulation (41°C, 1 h) 24 h after CMV-NIS-MSC injection resulted in a significantly increased uptake of 123I in heat-treated tumors compared with controls. Immunohistochemical staining and real-time PCR confirmed tumor-selective, temperature-dependent MSC migration. Therapeutic efficacy was significantly enhanced by combining CMV-NIS-MSC-mediated 131I therapy with regional hyperthermia. We demonstrate here for the first time that hyperthermia can significantly boost tumoral MSC recruitment, thereby significantly enhancing therapeutic efficacy of MSC-mediated NIS gene therapy.

Dynamic whole-brain metabolic connectivity during vestibular compensation in the rat.

Unilateral damage to the inner ear results in an acute vestibular syndrome, which is compensated within days to weeks due to adaptive cerebral plasticity. This process, called central vestibular compensation (VC), involves a wide range of functional and structural mechanisms at the cellular and network level. The short-term dynamics of whole-brain functional network recruitment and recalibration during VC has not been depicted in vivo. The purpose of this study was to investigate the interplay of separate and distinct brain regions and in vivo networks in the course of VC by sequential [18F]-FDG-PET-based statistical and graph theoretical analysis with the aim of revealing the metabolic connectome before and 1, 3, 7, and 15 days post unilateral labyrinthectomy (UL) in the rat. Temporal changes in metabolic brain connectivity were determined by Pearson's correlation (|r| > 0.5, p < 0.001) of regional cerebral glucose metabolism (rCGM) in 57 segmented brain regions. Metabolic connectivity analysis was compared to univariate voxel-wise statistical analysis of rCGM over time and to behavioral scores of static and dynamic sensorimotor recovery. Univariate statistical analysis revealed an ipsilesional relative rCGM decrease (compared to baseline) and a contralesional rCGM increase in vestibular and limbic networks and an increase in bilateral cerebellar and sensorimotor networks. Quantitative analysis of the metabolic connections showed a maximal increase from baseline to day 3 post UL (interhemispheric: 2-fold, ipsilesional: 3-fold, contralesional: 12-fold) and a gradual decline until day 15 post UL, which paralleled the dynamics of vestibular symptoms. In graph theoretical analysis, an increase in connectivity occurred especially within brain regions associated with brainstem-cerebellar and thalamocortical vestibular networks and cortical sensorimotor networks. At the symptom peak (day 3 post UL), brain networks were found to be organized in large ensembles of distinct and highly connected hubs of brain regions, which separated again with progressing VC. Thus, we found rapid changes in network organization at the subcortical and cortical level and in both hemispheres, which may indicate an initial functional substitution of vestibular loss and subsequent recalibration and reorganization of sensorimotor networks during VC.

Metabolic connectivity-based single subject classification by multi-regional linear approximation in the rat.

Metabolic connectivity patterns on the basis of [18F]-FDG positron emission tomography (PET) are used to depict complex cerebral network alterations in different neurological disorders and therefore may have the potential to support diagnostic decisions. In this study, we established a novel statistical classification method taking advantage of differential time-dependent states of whole-brain metabolic connectivity following unilateral labyrinthectomy (UL) in the rat and explored its classification accuracy. The dataset consisted of repeated [18F]-FDG PET measurements at baseline and 1, 3, 7, and 15 days (= maximum of 5 classes) after UL with 17 rats per measurement day. Classification in different stages after UL was performed by determining connectivity patterns for the different classes by Pearson's correlation between uptake values in atlas-based segmented brain regions. Connections were fitted with a linear function, with which different thresholds on the correlation coefficient (r = [0.5, 0.85]) were investigated. Rats were classified by determining the congruence of their PET uptake pattern with the fitted connectivity patterns in the classes. Overall, the classification accuracy with this method was 84.3% for 3 classes, 75.0% for 4 classes, and 54.1% for 5 classes and outperformed random classification as well as machine learning classification on the same dataset. The optimal classification thresholds of the correlation coefficient and distance-to-fit were found to be |r| > 0.65 and d = 4 when using Siegel's slope estimator for fitting. This connectivity-based classification method can compete with machine learning classification and may have methodological advantages when applied to support PET-based diagnostic decisions in neurological network disorders (such as neurodegenerative syndromes).

Feasibility of short imaging protocols for [18F]PI-2620 tau-PET in progressive supranuclear palsy.

PURPOSE: Dynamic 60-min positron emission tomography (PET) imaging with the novel tau radiotracer [18F]PI-2620 facilitated accurate discrimination between patients with progressive supranuclear palsy (PSP) and healthy controls (HCs). This study investigated if truncated acquisition and static time windows can be used for [18F]PI-2620 tau-PET imaging of PSP. METHODS: Thirty-seven patients with PSP Richardson syndrome (PSP-RS) were evaluated together with ten HCs. [18F]PI-2620 PET was performed by a dynamic 60-min scan. Distribution volume ratios (DVRs) were calculated using full and truncated scan durations (0-60, 0-50, 0-40, 0-30, and 0-20 min p.i.). Standardized uptake value ratios (SUVrs) were obtained 20-40, 30-50, and 40-60 min p.i.. All DVR and SUVr data were compared with regard to their potential to discriminate patients with PSP-RS from HCs in predefined subcortical and cortical target regions (effect size, area under the curve (AUC), multi-region classifier). RESULTS: 0-50 and 0-40 DVR showed equivalent effect sizes as 0-60 DVR (averaged Cohen's d: 1.22 and 1.16 vs. 1.26), whereas the performance dropped for 0-30 or 0-20 DVR. The 20-40 SUVr indicated the best performance of all static acquisition windows (averaged Cohen's d: 0.99). The globus pallidus internus discriminated patients with PSP-RS and HCs at a similarly high level for 0-60 DVR (AUC: 0.96), 0-40 DVR (AUC: 0.96), and 20-40 SUVr (AUC: 0.94). The multi-region classifier sensitivity of these time windows was consistently 86%. CONCLUSION: Truncated and static imaging windows can be used for [18F]PI-2620 PET imaging of PSP. 0-40 min dynamic scanning offers the best balance between accuracy and economic scanning.

Aberrant striatal dopamine links topographically with cortico-thalamic dysconnectivity in schizophrenia.

Aberrant dopamine function in the dorsal striatum and aberrant intrinsic functional connectivity (iFC) between distinct cortical networks and thalamic nuclei are among the most consistent large-scale brain imaging findings in schizophrenia. A pathophysiological link between these two alterations is suggested by theoretical models based on striatal dopamine's topographic modulation of cortico-thalamic connectivity within cortico-basal-ganglia-thalamic circuits. We hypothesized that aberrant striatal dopamine links topographically with aberrant cortico-thalamic iFC, i.e. aberrant associative striatum dopamine is associated with aberrant iFC between the salience network and thalamus, and aberrant sensorimotor striatum dopamine with aberrant iFC between the auditory-sensorimotor network and thalamus. Nineteen patients with schizophrenia during remission of psychotic symptoms and 19 age- and sex-comparable control subjects underwent simultaneous fluorodihydroxyphenyl-l-alanine PET (18F-DOPA-PET) and resting state functional MRI (rs-fMRI). The influx constant kicer based on 18F-DOPA-PET was used to measure striatal dopamine synthesis capacity; correlation coefficients between rs-fMRI time series of cortical networks and thalamic regions of interest were used to measure iFC. In the salience network-centred system, patients had reduced associative striatum dopamine synthesis capacity, which correlated positively with decreased salience network-mediodorsal-thalamus iFC. This correlation was present in both patients and healthy controls. In the auditory-sensorimotor network-centred system, patients had reduced sensorimotor striatum dopamine synthesis capacity, which correlated positively with increased auditory-sensorimotor network-ventrolateral-thalamus iFC. This correlation was present in patients only. Results demonstrate that reduced striatal dopamine synthesis capacity links topographically with cortico-thalamic intrinsic dysconnectivity in schizophrenia. Data suggest that aberrant striatal dopamine and cortico-thalamic dysconnectivity are pathophysiologically related within dopamine-modulated cortico-basal ganglia-thalamic circuits in schizophrenia.

Reduced striatal dopamine synthesis capacity in patients with schizophrenia during remission of positive symptoms.

While there is consistent evidence for increased presynaptic dopamine synthesis capacity in the striatum of patients with schizophrenia during psychosis, it is unclear whether this also holds for patients during psychotic remission. This study investigates whether striatal dopamine synthesis capacity is altered in patients with schizophrenia during symptomatic remission of positive symptoms, and whether potential alterations relate to symptoms other than positive, such as cognitive difficulties. Twenty-three patients with schizophrenia in symptomatic remission of positive symptoms according to Andreasen, and 24 healthy controls underwent 18F-DOPA-PET and behavioural-cognitive assessment. Imaging data were analysed with voxel-wise Patlak modelling with cerebellum as reference region, resulting in the influx constant kicer reflecting dopamine synthesis capacity. For the whole striatum and its subdivisions (i.e. limbic, associative, and sensorimotor), averaged regional kicer values were calculated, compared across groups, and correlated with behavioural-cognitive scores, including a mediation analysis. Patients had negative symptoms (Positive and Negative Syndrome Scale-negative 14.13 ± 5.91) and cognitive difficulties, i.e. they performed worse than controls in Trail-Making-Test-B (TMT-B; P = 0.01). Furthermore, kicer was reduced in patients for whole striatum (P = 0.004) and associative (P = 0.002) and sensorimotor subdivisions (P = 0.007). In patients, whole striatum kicer was negatively correlated with TMT-B (rho = -0.42, P = 0.04; i.e. the lower striatal kicer, the worse the cognitive performance). Mediation analysis showed that striatal kicer mediated the group difference in TMT-B. Results demonstrate that patients with schizophrenia in symptomatic remission of positive symptoms have decreased striatal dopamine synthesis capacity, which mediates the disorder's impact on cognitive difficulties. Data suggest that striatal dopamine dysfunction contributes to cognitive difficulties in schizophrenia.

Dual-targeted NIS polyplexes-a theranostic strategy toward tumors with heterogeneous receptor expression.

Tumor heterogeneity, within and between tumors, may have severe implications for tumor therapy, especially for targeted gene therapy, where single-targeted approaches often result in limited efficacy and therapy resistance. Polymer-formulated nonviral vectors provide a potent delivery platform for cancer therapy. To improve applicability for future clinical use in a broad range of patients and cancer types, a dual-targeting approach was performed. Synthetic LPEI-PEG2kDa-based polymer backbones were coupled to two tumor-specific peptide ligands GE11 (EGFR-targeting) and cMBP (cMET-targeting). The dual-targeting approach was used to deliver the theranostic sodium iodide symporter (NIS) gene to hepatocellular cancer. NIS as auspicious theranostic gene allows noninvasive imaging of functional NIS gene expression and effective anticancer radioiodide therapy. Enhanced tumor-specific transduction efficiency of dual-targeted polyplexes compared to single-targeted polyplexes was demonstrated in vitro using tumor cell lines with different EGFR and cMET expression and in vivo by 124I-PET-imaging. Therapeutic efficacy of the bispecific concept was mirrored by significantly reduced tumor growth and perfusion, which was associated with prolonged animal survival. In conclusion, the dual-targeting approach highlights the benefits of a bifunctional strategy for a future clinical translation of the bioimaging-based NIS-mediated radiotherapy allowing efficient targeting of heterogeneic tumors with variable receptor expression levels.

Imaging correlates of behavioral impairments: An experimental PET study in the rat pilocarpine epilepsy model.

Psychiatric comorbidities are prevalent in patients with epilepsy and greatly contribute to the overall burden of disease. The availability of reliable biomarkers to diagnose epilepsy-associated comorbidities would allow for effective treatment and improved disease management. Due to their non-invasive nature, molecular imaging techniques such as positron emission tomography (PET) are ideal tools to measure pathologic changes. In the current study we investigated the potential of [18F]fluoro-2-deoxy-d-glucose ([18F]FDG) and 2'-methoxyphenyl-(N-2'-pyridinyl)-p-18F-fluoro-benzamidoethylpiperazine ([18F]MPPF) as imaging correlates of neurobehavioral comorbidities in the pilocarpine rat model of epilepsy. Findings from rats with epilepsy revealed a regional reduction in [18F]FDG uptake indicating thalamic hypometabolism. In addition, an increase in septal [18F]MPPF binding was observed in rats with spontaneous recurrent seizures. Both thalamic [18F]FDG and septal [18F]MPPF data proved to correlate with behavioral alterations including decreases in luxury behavior such as burrowing and social interaction, and changes in behavioral patterns in anxiety tests. A correlation with seizure frequency was confirmed for thalamic [18F]FDG data. Moreover, thalamic [18F]FDG and septal [18F]MPPF data exhibited a correlation with brain-derived neurotrophic factor (BDNF) serum concentrations, which were lowered in rats with epilepsy. In conclusion, µPET data from rats with pilocarpine-induced epileptogenesis indicate altered septal 5-HT1A receptor binding. Further research is necessary assessing whether septal 5-HT1A receptor binding may serve as an imaging correlate of neuropsychiatric comorbidities in epilepsy patients and for severity assessment in rodent epilepsy models. In contrast, we obtained evidence that [18F]FDG uptake also reflects the severity of epilepsy and, thus, might not constitute a biomarker with sufficient specificity for psychiatric comorbidities. Evidence has been obtained that BDNF might serve as a peripheral circulatory biomarker. Further experimental and clinical assessment is necessary for validation of the marker candidates.

Imaging biomarkers of behavioral impairments: A pilot micro-positron emission tomographic study in a rat electrical post-status epilepticus model.

OBJECTIVE: In patients with epilepsy, psychiatric comorbidities can significantly affect the disease course and quality of life. Detecting and recognizing these comorbidities is central in determining an optimal treatment plan. One promising tool in detecting biomarkers for psychiatric comorbidities in epilepsy is positron emission tomography (PET). METHODS: Behavioral and biochemical variables were cross-correlated with the results from two µPET scans using the tracers [18 F]fluoro-2-deoxy-D-glucose ([18 F]FDG) and 2'-methoxyphenyl-(N-2'-pyridinyl)-p-18 F-fluoro-benzamidoethylpiperazine ([18 F]MPPF) to explore potential biomarkers for neurobehavioral comorbidities in an electrically induced post-status epilepticus rat model of epilepsy. RESULTS: In rats with epilepsy, µPET analysis revealed a local reduction in hippocampal [18 F]FDG uptake, and a local increase in [18 F]MPPF binding. These changes exhibited a correlation with burrowing as a "luxury" behavior, social interaction, and anxiety-associated behavioral patterns. Interestingly, hippocampal [18 F]FDG uptake did not correlate with spontaneous recurrent seizure activity. SIGNIFICANCE: In the electrically induced post-status epilepticus rat model, we demonstrated hippocampal hypometabolism and its correlation with a range of neurobehavioral alterations. These findings require further confirmation in other preclinical models and patients with epilepsy and psychiatric disorders to address the value of [18 F]FDG uptake as an imaging biomarker candidate for psychiatric comorbidities in patients as well as for severity assessment in rodent epilepsy models.