Parametric methods for [18F]flortaucipir PET

[18F]Flortaucipir is a PET tau tracer used to visualize tau binding in Alzheimer's disease (AD) in vivo. The present study evaluated the performance of several methods to obtain parametric images of [18F]flortaucipir. One hundred and thirty minutes dynamic PET scans were performed in 10 AD patients and 10 controls. Parametric images were generated using different linearization and basis function approaches. Regional binding potential (BPND) and volume of distribution (VT) values obtained from the parametric images were compared with corresponding values derived using the reversible two-tissue compartment model (2T4k_VB). Performance of SUVr parametric images was assessed by comparing values with distribution volume ratio (DVR) and SRTM-derived BPND estimates obtained using non-linear regression (NLR). Spectral analysis (SA) (r2 = 0.92; slope = 0.99) derived VT correlated well with NLR-derived VT. RPM (r2 = 0.95; slope = 0.98) derived BPND correlated well with NLR-derived DVR. Although SUVr80-100 min correlated well with NLR-derived DVR (r2 = 0.91; slope = 1.09), bias in SUVr appeared to depend on uptake time and underlying level of specific binding. In conclusion, RPM and SA provide parametric images comparable to the NLR estimates. Individual SUVr values are biased compared with DVR and this bias requires further study in a larger dataset in order to understand its consequences.

PET and CSF amyloid-β status are differently predicted by patient features: information from discordant cases

BACKGROUND

Amyloid-β PET and CSF Aβ42 yield discordant results in 10-20% of memory clinic patients, possibly providing unique information. Although the predictive power of demographic, clinical, genetic, and imaging features for amyloid positivity has previously been investigated, it is unknown whether these features differentially predict amyloid-β status based on PET or CSF or whether this differs by disease stage.

METHODS

We included 768 patients (subjective cognitive decline (SCD, n = 194), mild cognitive impairment (MCI, n = 127), dementia (AD and non-AD, n = 447) with amyloid-β PET and CSF Aβ42 measurement within 1 year. Ninety-seven (13%) patients had discordant PET/CSF amyloid-β status. We performed parallel random forest models predicting separately PET and CSF status using 17 patient features (demographics, APOE4 positivity, CSF (p)tau, cognitive performance, and MRI visual ratings) in the total patient group and stratified by syndrome diagnosis. Thereafter, we selected features with the highest variable importance measure (VIM) as input for logistic regression models, where amyloid status on either PET or CSF was predicted by (i) the selected patient feature and (ii) the patient feature adjusted for the status of the other amyloid modality.

RESULTS

APOE4, CSF tau, and p-tau had the highest VIM for PET and CSF in all groups. In the amyloid-adjusted logistic regression models, p-tau was a significant predictor for PET-amyloid in SCD (OR = 1.02 [1.01-1.04], pFDR = 0.03), MCI (OR = 1.05 [1.02-1.07], pFDR < 0.01), and dementia (OR = 1.04 [1.03-1.05], pFDR < 0.001), but not for CSF-amyloid. APOE4 (OR = 3.07 [1.33-7.07], punc < 0.01) was associated with CSF-amyloid in SCD, while it was only predictive for PET-amyloid in MCI (OR = 9.44 [2.93, 30.39], pFDR < 0.01). Worse MMSE scores (OR = 1.21 [1.03-1.41], punc = 0.02) were associated to CSF-amyloid status in SCD, whereas worse memory (OR = 1.17 [1.05-1.31], pFDR = 0.02) only predicted PET positivity in dementia.

CONCLUSION

Amyloid status based on either PET or CSF was predicted by different patient features, and this varied by disease stage, suggesting that PET-CSF discordance yields unique information. The stronger associations of both APOE4 carriership and worse memory z-scores with CSF-amyloid in SCD suggest that CSF-amyloid is more sensitive early in the disease course. The higher predictive value of CSF p-tau for a positive PET scan suggests that PET is more specific to AD pathology.

Hippocampal [18F]flortaucipir BPND corrected for possible spill-in of the choroid plexus retains strong clinico-pathological relationships

BACKGROUND

Off-target [¹⁸F]flortaucipir (tau) PET binding in the choroid plexus causes spill-in into the nearby hippocampus, which may influence the correlation between [¹⁸F]flortaucipir binding and measures of cognition. Previously, we showed that partial volume correction (combination of Van Cittert iterative deconvolution and HYPR denoising; PVC HDH) and manually eroding the hippocampus resulted in a significant decrease of the choroid plexus spill-in. In this study, we compared three different approaches for the quantification of hippocampal [¹⁸F]flortaucipir signal using a semi-automated technique, and assessed correlations with cognitive performance across methods.

METHODS

Dynamic 130 min [¹⁸F]flortaucipir PET scans were performed in 109 subjects (45 cognitively normal subjects (CN) and 64 mild cognitive impairment/Alzheimer's disease (AD) dementia patients. We extracted hippocampal binding potential (BP_ND) using receptor parametric mapping with cerebellar grey matter as reference region. PVC HDH was performed. Based on our previous study in which we manually eroded 40% ± 10% of voxels of the hippocampus, three hippocampal volumes-of-interest (VOIs) were generated: a non-optimized 100% hippocampal VOI [100%], and combining HDH with eroding a percentage of the highest hippocampus BP_ND voxels (i.e. lowering spill-in) resulting in optimized 50%[50%HDH] and 40%[40%HDH] hippocampal VOIs. Cognitive performance was assessed with the Mini-Mental State Examination (MMSE) and Rey auditory verbal learning delayed recall. We performed receiver operating characteristic analyses to investigate which method could best discriminate MCI/AD from controls. Subsequently, we performed linear regressions to investigate associations between the hippocampal [¹⁸F]flortaucipir BP_ND VOIs and MMSE/delayed recall adjusted for age, sex and education.

RESULTS

We found higher hippocampal [¹⁸F]flortaucipir BP_ND in MCI/AD patients (BP_ND100%=0.27±0.15) compared to CN (BP_ND100%= 0.07±0.13) and all methods showed comparable discriminative effects (AUC_100%=0.85[CI=0.78-0.93]; AUC_50%HDH=0.84[CI=0.74-0.92]; AUC_40%HDH=0.83[CI=0.74-0.92]). Across groups, higher [¹⁸F]flortaucipir BP_ND was related to lower scores on MMSE (standardized β_100%=-0.38[CI=-0.57-0.20]; β_50%HDH= -0.37[CI=-0.54-0.19]; β_40%HDH=-0.35[CI=-0.53-0.17], all p<0.001) and delayed recall (standardized β_100%=-0.64[CI=-0.79-0.49]; β_50%HDH= -0.61[CI=-0.76-0.46]; β_40%HDH=-0.59[CI=-0.75-0.44]; all p<0.001), with comparable effect sizes for all hippocampal VOIs.

CONCLUSIONS

Hippocampal tau load measured with [¹⁸F]flortaucipir PET is strongly associated with cognitive function. Both discrimination between diagnostic groups and associations between hippocampal [¹⁸F]flortaucipir BP_ND and memory were comparable for all methods. The non-optimized 100% hippocampal VOI may be sufficient for clinical interpretation. However, proper correction for choroid plexus spillover and may be required in case of smaller effect sizes between subject groups or for longitudinal studies.

The P2X7 receptor tracer [11C]SMW139 as an in vivo marker of neuroinflammation in multiple sclerosis: a first-in man study

Purpose

The novel PET tracer [¹¹C]SMW139 binds with high affinity to the P2X₇ receptor, which is expressed on pro-inflammatory microglia. The purposes of this first in-man study were to characterise pharmacokinetics of [¹¹C]SMW139 in patients with active relapsing remitting multiple sclerosis (RRMS) and healthy controls (HC) and to evaluate its potential to identify in vivo neuroinflammation in RRMS.

Methods

Five RRMS patients and 5 age-matched HC underwent 90-min dynamic [¹¹C]SMW139 PET scans, with online continuous and manual arterial sampling to generate a metabolite-corrected arterial plasma input function. Tissue time activity curves were fitted to single- and two-tissue compartment models, and the model that provided the best fits was determined using the Akaike information criterion.

Results

The optimal model for describing [¹¹C]SMW139 kinetics in both RRMS and HC was a reversible two-tissue compartment model with blood volume parameter and with the dissociation rate k₄ fixed to the whole-brain value. Exploratory group level comparisons demonstrated an increased volume of distribution (V_T) and binding potential (BP_ND) in RRMS compared with HC in normal appearing brain regions. BP_ND in MS lesions was decreased compared with non-lesional white matter, and a further decrease was observed in gadolinium-enhancing lesions. In contrast, increased V_T was observed in enhancing lesions, possibly resulting from disruption of the blood-brain barrier in active MS lesions. In addition, there was a high correlation between parameters obtained from 60- to 90-min datasets, although analyses using 60-min data led to a slight underestimation in regional V_T and BP_ND values.

Conclusions

This first in-man study demonstrated that uptake of [¹¹C]SMW139 can be quantified with PET using BP_ND as a measure for specific binding in healthy controls and RRMS patients. Additional studies are warranted for further clinical evaluation of this novel neuroinflammation tracer.

Electronic supplementary material

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Quantification of [18F]florbetapir: A test-retest tracer kinetic modelling study

Accumulation of amyloid beta can be visualized using [¹⁸F]florbetapir positron emission tomography. The aim of this study was to identify the optimal model for quantifying [¹⁸F]florbetapir uptake and to assess test-retest reliability of corresponding outcome measures. Eight Alzheimer's disease patients (age: 67 ± 6 years, Mini-Mental State Examination (MMSE): 23 ± 3) and eight controls (age: 63 ± 4 years, MMSE: 30 ± 0) were included. Ninety-minute dynamic positron emission tomography scans, together with arterial blood sampling, were acquired immediately following a bolus injection of 294 ± 32 MBq [¹⁸F]florbetapir. Several plasma input models and the simplified reference tissue model (SRTM) were evaluated. The Akaike information criterion was used to identify the preferred kinetic model. Compared to controls, Alzheimer's disease patients had lower MMSE scores and evidence for cortical Aβ pathology. A reversible two-tissue compartment model with fitted blood volume fraction (2T4k_V_B) was the preferred model for describing [¹⁸F]florbetapir kinetics. SRTM-derived non-displaceable binding potential (BP_ND) correlated well (r²= 0.83, slope = 0.86) with plasma input-derived distribution volume ratio. Test-retest reliability for plasma input-derived distribution volume ratio, SRTM-derived BP_ND and SUVr_(50-70) were r = 0.88, r = 0.91 and r = 0.86, respectively. In vivo kinetics of [¹⁸F]florbetapir could best be described by a reversible two-tissue compartmental model and [¹⁸F]florbetapir BP_ND can be reliably estimated using an SRTM.

Discordant amyloid-β PET and CSF biomarkers and its clinical consequences

Background

In vivo, high cerebral amyloid-β load has been associated with (i) reduced concentrations of Aβ₄₂ in cerebrospinal fluid and (ii) increased retention using amyloid-β positron emission tomography. Although these two amyloid-β biomarkers generally show good correspondence, ~ 10–20% of cases have discordant results. To assess the consequences of having discordant amyloid-β PET and CSF biomarkers on clinical features, biomarkers, and longitudinal cognitive trajectories.

Methods

We included 768 patients (194 with subjective cognitive decline (SCD), 127 mild cognitive impairment (MCI), 309 Alzheimer’s dementia (AD), and 138 non-AD) who were categorized as concordant-negative (n = 315, 41%), discordant (n = 97, 13%), or concordant-positive (n = 356, 46%) based on CSF and PET results. We compared discordant with both concordant-negative and concordant-positive groups on demographics, clinical syndrome, apolipoprotein E (APOE) ε4 status, CSF tau, and clinical and neuropsychological progression.

Results

We found an increase from concordant-negative to discordant to concordant-positive in rates of APOE ε4 (28%, 55%, 70%, Z = − 10.6, P < 0.001), CSF total tau (25%, 45%, 78%, Z = − 13.7, P < 0.001), and phosphorylated tau (28%, 43%, 80%, Z = − 13.7, P < 0.001) positivity. In patients without dementia, linear mixed models showed that Mini-Mental State Examination and memory composite scores did not differ between concordant-negative (β [SE] − 0.13[0.08], P = 0.09) and discordant (β 0.08[0.15], P = 0.15) patients (P_interaction = 0.19), while these scores declined in concordant-positive (β − 0.75[0.08] patients (P_interaction < 0.001). In patients with dementia, longitudinal cognitive scores were not affected by amyloid-β biomarker concordance or discordance. Clinical progression rates from SCD to MCI or dementia (P = 0.01) and from MCI to dementia (P = 0.003) increased from concordant-negative to discordant to concordant-positive.

Conclusions

Discordant cases were intermediate to concordant-negative and concordant-positive patients in terms of genetic (APOE ε4) and CSF (tau) markers of AD. While biomarker agreement did not impact cognition in patients with dementia, discordant biomarkers are not benign in patients without dementia given their higher risk of clinical progression.

Electronic supplementary material

The online version of this article (10.1186/s13195-019-0532-x) contains supplementary material, which is available to authorized users.

A new perspective for advanced positron emission tomography-based molecular imaging in neurodegenerative proteinopathies

Recent studies in neurodegenerative conditions have increasingly highlighted that the same neuropathology can trigger different clinical phenotypes or, vice-versa, that similar phenotypes can be triggered by different neuropathologies. This evidence has called for the adoption of a pathology spectrum-based approach to study neurodegenerative proteinopathies. These conditions share brain deposition of abnormal protein aggregates, leading to aberrant biochemical, metabolic, functional, and structural changes. Positron emission tomography (PET) is a well-recognized and unique tool for the in vivo assessment of brain neuropathology, and novel PET techniques are emerging for the study of specific protein species. Today, key applications of PET range from early research and clinical diagnostic tools to their use in clinical trials for both participants screening and outcome evaluation. This position article critically reviews the role of distinct PET molecular tracers for different neurodegenerative proteinopathies, highlighting their strengths, weaknesses, and opportunities, with special emphasis on methodological challenges and future applications.

Simplified Methods for Quantification of 18F-DCFPyL Uptake in Patients with Prostate Cancer

Radiolabeled prostate-specific membrane antigen (PSMA) PET has demonstrated promising results for prostate cancer (PCa) imaging. Quantification of PSMA radiotracer uptake is desired as it enables reliable interpretation of PET images, use of PSMA uptake as an imaging biomarker for tumor characterization, and evaluation of treatment effects. The aim of this study was to perform a full pharmacokinetic analysis of 2-(3-(1-carboxy-5-[(6-¹⁸F-fluoro-pyridine-3-carbonyl)-amino]-pentyl)-ureido)-pentanedioic acid (¹⁸F-DCFPyL), a second-generation ¹⁸F-labeled PSMA ligand. On the basis of the pharmacokinetic analysis (reference method), simplified methods for quantification of ¹⁸F-DCFPyL uptake were validated. Methods: Eight patients with metastasized PCa were included. Dynamic PET acquisitions were performed at 0-60 and 90-120 min after injection of a median dose of 313 MBq of ¹⁸F-DCFPyL (range, 292-314 MBq). Continuous and manual arterial blood sampling provided calibrated plasma tracer input functions. Time-activity curves were derived for each PCa metastasis, and ¹⁸F-DCFPyL kinetics were described using standard plasma input tissue-compartment models. Simplified methods for quantification of ¹⁸F-DCFPyL uptake (SUVs; tumor-to-blood ratios [TBRs]) were correlated with kinetic parameter estimates obtained from full pharmacokinetic analysis. Results: In total, 46 metastases were evaluated. A reversible 2-tissue-compartment model was preferred for ¹⁸F-DCFPyL kinetics in 59% of the metastases. The observed k ₄ was small, however, resulting in nearly irreversible kinetics during the course of the PET study. Hence, k ₄ was fixated (0.015) and net influx rate, K_i, was preferred as the reference kinetic parameter. Whole-blood TBR provided an excellent correlation with K_i from full kinetic analysis (R ² = 0.97). This TBR could be simplified further by replacing the blood samples with an image-based, single measurement of blood activity in the ascending aorta (image-based TBR, R ² = 0.96). SUV correlated poorly with K_i (R ² = 0.47 and R ² = 0.60 for SUV normalized to body weight and lean body mass, respectively), most likely because of deviant blood activity concentrations (i.e., tumor tracer input) in patients with higher tumor volumes. Conclusion: ¹⁸F-DCFPyL kinetics in PCa metastases are best described by a reversible 2-tissue-compartment model. Image-based TBRs were validated as a simplified method to quantify ¹⁸F-DCFPyL uptake and might be applied to clinical, whole-body PET scans. SUV does not provide reliable quantification of ¹⁸F-DCFPyL uptake.

Assessment of Simplified Methods for Quantification of 18F-FDHT Uptake in Patients with Metastatic Castration-Resistant Prostate Cancer

¹⁸F-fluorodihydrotestosterone (¹⁸F-FDHT) PET/CT potentially provides a noninvasive method for assessment of androgen receptor expression in patients with metastatic castration-resistant prostate cancer (mCRPC). The objective of this study was to assess simplified methods for quantifying ¹⁸F-FDHT uptake in mCRPC patients and to assess effects of tumor perfusion on these ¹⁸F-FDHT uptake metrics. Methods: Seventeen mCRPC patients were included in this prospective observational multicenter study. Test and retest 30-min dynamic ¹⁸F-FDHT PET/CT scans with venous blood sampling were performed in 14 patients. In addition, arterial blood sampling and dynamic ¹⁵O-H₂O scans were obtained in a subset of 6 patients. Several simplified methods were assessed: Patlak plots; SUV normalized to body weight (SUV_BW), lean body mass (SUV_LBM), whole blood (SUV_WB), parent plasma activity concentration (SUV_PP), area under the parent plasma curve (SUV_AUC,PP), and area under the whole-blood input curve (SUV_AUC,WB); and SUV_BW corrected for sex hormone-binding globulin levels (SUV_SHBG). Results were correlated with parameters derived from full pharmacokinetic ¹⁸F-FDHT and ¹⁵O-H₂O. Finally, the repeatability of individual quantitative uptake metrics was assessed. Results: Eighty-seven ¹⁸F-FDHT-avid lesions were evaluated. ¹⁸F-FDHT uptake was best described by an irreversible 2-tissue-compartment model. Replacing the continuous metabolite-corrected arterial plasma input function with an image-derived input function in combination with venous sample data provided similar K _i results (R ² = 0.98). Patlak K _i and SUV_AUC,PP showed an excellent correlation (R ² > 0.9). SUV_BW showed a moderate correlation to K _i (R ² = 0.70, presumably due to fast ¹⁸F-FDHT metabolism. When calculating SUV_SHBG, correlation to K _i improved (R ² = 0.88). The repeatability of full kinetic modeling parameters was inferior to that of simplified methods (repeatability coefficients > 36% vs. < 28%, respectively). ¹⁸F-FDHT uptake showed minimal blood flow dependency. Conclusion: ¹⁸F-FDHT kinetics in mCRPC patients are best described by an irreversible 2-tissue-compartment model with blood volume parameter. SUV_AUC,PP showed a near-perfect correlation with the irreversible 2-tissue-compartment model analysis and can be used for accurate quantification of ¹⁸F-FDHT uptake in whole-body PET/CT scans. In addition, SUV_SHBG could potentially be used as an even simpler method to quantify ¹⁸F-FDHT uptake when less complex scanning protocols and accuracy are required.

Binding characterization of N-(2-chloro-5-thiomethylphenyl)-N'-(3-[3 H]3 methoxy phenyl)-N'-methylguanidine ([3 H]GMOM), a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist

Labeled with carbon‐11, N‐(2‐chloro‐5‐thiomethylphenyl)‐N′‐(3‐methoxyphenyl)‐N′‐methylguanidine ([¹¹C]GMOM) is currently the only positron emission tomography (PET) tracer that has shown selectivity for the ion‐channel site of N‐methyl‐D‐aspartate (NMDA) receptors in human imaging studies. The present study reports on the selectivity profile and in vitro binding properties of GMOM. The compound was screened on a panel of 80 targets, and labeled with tritium ([³H]GMOM). The binding properties of [³H]GMOM were compared to those of the reference ion‐channel ligand [³H](+)‐dizocilpine maleate ([³H]MK‐801), in a set of concentration‐response, homologous and heterologous inhibition, and association kinetics assays, performed with repeatedly washed rat forebrain preparations. GMOM was at least 70‐fold more selective for NMDA receptors compared to all other targets examined. In homologous inhibition and concentration‐response assays, the binding of [³H]GMOM was regulated by NMDA receptor agonists, albeit in a less prominent manner compared to [³H]MK‐801. Scatchard transformation of homologous inhibition data produced concave upward curves for [³H]GMOM and [³H]MK‐801. The radioligands showed bi‐exponential association kinetics in the presence of 100 μmol L⁻¹l‐glutamate/30 μmol L⁻¹ glycine. [³H]GMOM (3 nmol L⁻¹ and 10 nmol L⁻¹) was inhibited with dual affinity by (+)‐MK‐801, (R,S)‐ketamine and memantine, in both presence and absence of agonists. [³H]MK‐801 (2 nmol L⁻¹) was inhibited in a monophasic manner by GMOM under baseline and combined agonist conditions, with an IC₅₀ value of ~19 nmol L⁻¹. The non‐linear Scatchard plots, biphasic inhibition by open channel blockers, and bi‐exponential kinetics of [³H]GMOM indicate a complex mechanism of interaction with the NMDA receptor ionophore. The implications for quantifying the PET signal of [¹¹C]GMOM are discussed.

Fully Automated 89Zr Labeling and Purification of Antibodies

Dozens of monoclonal antibodies (mAbs) have been approved for clinical use, and hundreds more are under development. To support these developments and facilitate a personalized medicine approach, PET imaging and quantification of mAbs, after chelation with desferrioxamine B (DFO) and radiolabeling with ⁸⁹Zr, has become attractive. Also, the use of ⁸⁹Zr-mAbs in preclinical and clinical studies is expanding rapidly. Despite these rapid developments, ⁸⁹Zr radiolabeling is still performed manually. Therefore, we aimed to develop a simple, fully automated, good-manufacturing-practice (GMP)-compliant production procedure for the ⁸⁹Zr labeling of mAbs. Such procedures should increase the robustness and capacity of ⁸⁹Zr-mAb production while minimizing the radiation dose to the operator. Here, the procedures for fully automated ⁸⁹Zr-mAb production are described and applied to produce batches of ⁸⁹Zr-DFO-N-suc-cetuximab and ⁸⁹Zr-DFO-N-suc-rituximab suitable for clinical use. Both products had to meet the GMP-compliant quality standards with respect to yield, radiochemical purity, protein integrity, antigen binding, sterility, and endotoxin levels. Methods: Automated ⁸⁹Zr labeling of mAbs was developed on a Scintomics GRP 2V module and comprised the following steps: reagent transfer to the ⁸⁹Zr-containing reaction vial, mixing of the reagents followed by a 60-min reaction at room temperature to obtain optimal radiolabeling yields, and product purification using a PD-10 desalting column. Results: Radiochemical yields of ⁸⁹Zr-DFO-N-suc-cetuximab and ⁸⁹Zr-DFO-N-suc-rituximab were all more than 90% according to instant thin-layer chromatography. Isolated yields were 74.6% ± 2.0% and 62.6% ± 3.0% for ⁸⁹Zr-DFO-N-suc-cetuximab and ⁸⁹Zr-DFO-N-suc-rituximab, respectively, which are similar to isolated yields obtained using GMP protocols for manual ⁸⁹Zr labeling of mAbs. To meet the GMP-compliant quality standards, only the radiochemically pure fractions were collected from PD-10, resulting in a lower isolated yield than the radiochemical yield according to instant thin-layer chromatography. The radiochemical purity and protein integrity were more than 95% for both products, and the antigen binding was 95.6% ± 0.6% and 87.1% ± 2.2% for ⁸⁹Zr-DFO-N-suc-cetuximab and ⁸⁹Zr-DFO-N-suc-rituximab, respectively. The products were sterile, and the endotoxin levels were within acceptable limits, allowing future clinical production using this procedure. Conclusion: Procedures for fully automated GMP-compliant production of ⁸⁹Zr-mAbs were developed on a commercially available synthesis module, which also allows the GMP production of other radiolabeled mAbs.

In vivo assessment of neuroinflammation in progressive multiple sclerosis: a proof of concept study with [18F]DPA714 PET

Background

Over the past decades, positron emission tomography (PET) imaging has become an increasingly useful research modality in the field of multiple sclerosis (MS) research, as PET can visualise molecular processes, such as neuroinflammation, in vivo. The second generation PET radioligand [¹⁸F]DPA714 binds with high affinity to the 18-kDa translocator-protein (TSPO), which is mainly expressed on activated microglia. The aim of this proof of concept study was to evaluate this in vivo marker of neuroinflammation in primary and secondary progressive MS.

Methods

All subjects were genotyped for the rs6971 polymorphism within the TSPO gene, and low-affinity binders were excluded from participation in this study. Eight patients with progressive MS and seven age and genetic binding status matched healthy controls underwent a 60 min dynamic PET scan using [¹⁸F]DPA714, including both continuous on-line and manual arterial blood sampling to obtain metabolite-corrected arterial plasma input functions.

Results

The optimal model for quantification of [¹⁸F]DPA714 kinetics was a reversible two-tissue compartment model with additional blood volume parameter. For genetic high-affinity binders, a clear increase in binding potential was observed in patients with MS compared with age-matched controls. For both high and medium affinity binders, a further increase in binding potential was observed in T2 white matter lesions compared with non-lesional white matter. Volume of distribution, however, did not differentiate patients from healthy controls, as the large non-displaceable compartment of [¹⁸F]DPA714 masks its relatively small specific signal.

Conclusion

The TSPO radioligand [¹⁸F]DPA714 can reliably identify increased focal and diffuse neuroinflammation in progressive MS when using plasma input-derived binding potential, but observed differences were predominantly visible in high-affinity binders.

Electronic supplementary material

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Synthesis and Preclinical Evaluation of the First Carbon-11 Labeled PET Tracers Targeting Substance P1-7

Two potent SP_1–7 peptidomimetics have been successfully radiolabeled via [¹¹C]CO₂-fixation with excellent yields, purity, and molar activity. l-[¹¹C]SP_1–7-peptidomimetic exhibited promising ex vivo biodistribution profile. Metabolite analysis showed that l-[¹¹C]SP_1–7-peptidomimetic is stable in brain and spinal cord, whereas rapid metabolic degradation occurs in rat plasma. Metabolic stability can be significantly improved by substituting l-Phe for d-Phe, preserving 70% more of intact tracer and resulting in better brain and spinal cord tracer retention. Positron emission tomography (PET) scanning confirmed moderate brain (1.5 SUV; peak at 3 min) and spinal cord (1.0 SUV; peak at 10 min) uptake for l- and d-[¹¹C]SP_1–7-peptidomimetic. A slight decrease in SUV value was observed after pretreatment with natural peptide SP_1–7 in spinal cord for l-[¹¹C]SP_1–7-peptidomimetic. On the contrary, blocking using cold analogues of l- and d-[¹¹C]tracers did not reduce the tracers’ brain and spinal cord exposure. In summary, PET scanning of l- and d-[¹¹C]SP_1–7-peptidomimetics confirms rapid blood–brain barrier and blood–spinal-cord barrier penetration. Therefore, further validation of these two tracers targeting SP_1–7 is needed in order to define a new PET imaging target and select its most appropriate radiopharmaceutical.

Identification of new molecular targets for PET imaging of the microglial anti-inflammatory activation state

Microglia are potential targets for therapeutic intervention in neurological and neurodegenerative diseases affecting the central nervous system. In order to assess the efficacy of therapies aimed to reduce the tissue damaging activities of microglia and/or to promote the protective potential of these cells, suitable pre-clinical and clinical tools for the in vivo analysis of microglia activities and dynamics are required. The aim of this work was to identify new translational markers of the anti-inflammatory / protective state of microglia for the development of novel PET tracers.

Methods: New translational markers of the anti-inflammatory/protective activation state of microglia were selected by bioinformatic approaches and were in vitro and ex vivo validated by qPCR and immunohistochemistry in rodent and human samples. Once a viable marker was identified, a novel PET tracer was developed. This tracer was subsequently confirmed by autoradiography experiments in murine and human brain tissues.

Results: Here we provide evidence that P2RY12 expression increases in murine and human microglia following exposure to anti-inflammatory stimuli, and that its expression is modulated in the reparative phase of experimental and clinical stroke. We then synthesized a novel carbon-11 labeled tracer targeting P2RY12, showing increased binding in brain sections of mice treated with IL4, and low binding to brain sections of a murine stroke model and of a stroke patient.

Conclusion: This study provides new translational targets for PET tracers for the anti-inflammatory/protective activation state of microglia and shows the potential of a rationale-based approach. It therefore paves the way for the development of novel non-invasive methodologies aimed to monitor the success of therapeutic approaches in various neurological diseases.

A dual-labeled cRGD-based PET/optical tracer for pre-operative staging and intraoperative treatment of colorectal cancer

cRGD peptides target integrins associated with angiogenesis (e.g., α_vβ₃) and cancer, and have been used as binding ligands for both positron emission tomography (PET) and near-infrared fluorescence (NIRF) optical imaging. This study introduces the hybrid tracer cRGD-ZW800-1-Forte-[⁸⁹Zr]Zr-DFO, which is based on a novel zwitterionic fluorophore structure that reduces non-specific background uptake during molecular imaging of tumors. An in vitro binding assay was used to validate tracer performance. 10 nmol ZW800F-cRGD-Zr-DFO was injected in mice (n=7) bearing orthotopic human colorectal tumors (HT29-luc2) for tumor detection with NIRF imaging. Subsequently, ZW800F-cRGD-Zr-DFO was loaded with ⁸⁹Zr and 10 nmol cRGD-ZW800-1-Forte-[⁸⁹Zr]Zr-DFO (3 MBq) was injected in mice (n=8) for PET/CT imaging. Imaging and biodistribution was performed at 4 and 24 h. NIRF imaging was performed up to 168 h after administration. Sufficient fluorescent signals were measured in the tumors of mice injected with ZW800F-cRGD-Zr-DFO (emission peak ~800 nm) compared to the background. The signal remained stable for up to 7 days. The fluorescence signal of cRGD-ZW800-1-Forte-[⁸⁹Zr]Zr-DFO remained intact after labeling with ⁸⁹Zr. PET/CT permitted clear visualization of the colorectal tumors at 4 and 24 h. Biodistribution at 4 h showed the highest uptake of the tracer in kidneys and sufficient uptake in the tumor, remaining stable for up to 24 h. A single molecular imaging agent, ZW800F-cRGD-[⁸⁹Zr]Zr-DFO, permits serial PET and NIRF imaging of colorectal tumors, with the latter permitting image-guided treatment intraoperatively. Due to its unique zwitterionic structure, the tracer is rapidly renally cleared and fluorescent background signals are low.

Reproducibility and Repeatability of Semiquantitative 18F-Fluorodihydrotestosterone Uptake Metrics in Castration-Resistant Prostate Cancer Metastases: A Prospective Multicenter Study

18F-fluorodihydrotestosterone (18F-FDHT) is a radiolabeled analog of the androgen receptor's primary ligand that is currently being credentialed as a biomarker for prognosis, response, and pharmacodynamic effects of new therapeutics. As part of the biomarker qualification process, we prospectively assessed its reproducibility and repeatability in men with metastatic castration-resistant prostate cancer. Methods: We conducted a prospective multiinstitutional study of metastatic castration-resistant prostate cancer patients undergoing 2 (test/retest) 18F-FDHT PET/CT scans on 2 consecutive days. Two independent readers evaluated all examinations and recorded SUVs, androgen receptor-positive tumor volumes, and total lesion uptake for the most avid lesion detected in each of 32 predefined anatomic regions. The relative absolute difference and reproducibility coefficient (RC) of each metric were calculated between the test and retest scans. Linear regression analyses, intraclass correlation coefficients (ICCs), and Bland-Altman plots were used to evaluate repeatability of 18F-FDHT metrics. The coefficient of variation and ICC were used to assess interobserver reproducibility. Results: Twenty-seven patients with 140 18F-FDHT-avid regions were included. The best repeatability among 18F-FDHT uptake metrics was found for SUV metrics (SUVmax, SUVmean, and SUVpeak), with no significant differences in repeatability among them. Correlations between the test and retest scans were strong for all SUV metrics (R2 ≥ 0.92; ICC ≥ 0.97). The RCs of the SUV metrics ranged from 21.3% (SUVpeak) to 24.6% (SUVmax). The test and retest androgen receptor-positive tumor volumes and TLU, respectively, were highly correlated (R2 and ICC ≥ 0.97), although variability was significantly higher than that for SUV (RCs > 46.4%). The prostate-specific antigen levels, Gleason score, weight, and age did not affect repeatability, nor did total injected activity, uptake measurement time, or differences in uptake time between the 2 scans. Including the most avid lesion per patient, the 5 most avid lesions per patient, only lesions 4.2 mL or more, only lesions with an SUV of 4 g/mL or more, or normalizing of SUV to area under the parent plasma activity concentration-time curve did not significantly affect repeatability. All metrics showed high interobserver reproducibility (ICC > 0.98; coefficient of variation < 0.2%-10.8%). Conclusion: Uptake metrics derived from 18F-FDHT PET/CT show high repeatability and interobserver reproducibility.

Synthesis and preliminary preclinical evaluation of fluorine-18 labelled isatin-4-(4-methoxyphenyl)-3-thiosemicarbazone ([18F]4FIMPTC) as a novel PET tracer of P-glycoprotein expression

Background

Several P-glycoprotein (P-gp) substrate tracers are available to assess P-gp function in vivo, but attempts to develop a tracer for measuring expression levels of P-gp have not been successful. Recently, (Z)-2-(5-fluoro-2-oxoindolin-3-ylidene)-N-(4-methoxyphenyl)hydrazine-carbothioamide was described as a potential selective P-gp inhibitor that is not transported by P-gp. Therefore, the purpose of this study was to radiolabel two of its analogues and to assess their potential for imaging P-gp expression using PET.

Results

[¹⁸F]2-(4-fluoro-2-oxoindolin-3-ylidene)-N-(4-methoxyphenyl)hydrazine-carbothioamide ([¹⁸F]5) and [¹⁸F]2-(6-fluoro-2-oxoindolin-3-ylidene)-N-(4-methoxyphenyl)hydrazine-carbothioamide ([¹⁸F]6) were synthesized and both their biodistribution and metabolism were evaluated in rats. In addition, PET scans were acquired in rats before and after tariquidar (P-gp inhibitor) administration as well as in P-gp knockout (KO) mice.

Both [¹⁸F]5 and [¹⁸F]6 were synthesized in 2–3% overall yield, and showed high brain uptake in ex vivo biodistribution studies. [¹⁸F]6 appeared to be metabolically unstable in vivo, while [¹⁸F]5 showed moderate stability with limited uptake of radiolabelled metabolites in the brain. PET studies showed that transport of [¹⁸F]5 across the blood-brain barrier was not altered by pre-treatment with the P-gp inhibitor tariquidar, and uptake was significantly lower in P-gp KO than in wild-type animals and indeed transported across the BBB or bound to P-gp in endothelial cells.

Conclusion

In conclusion, [¹⁸F]5 and [¹⁸F]6 were successfully and reproducibly synthesized, albeit with low radiochemical yields. [¹⁸F]5 appears to be a radiotracer that binds to P-gp, as showed in P-gp knock-out animals, but is not a substrate for P-gp.

Prophylactic and therapeutic activity of alkaline phosphatase in arthritic rats: single-agent effects of alkaline phosphatase and synergistic effects in combination with methotrexate

Alkaline phosphatase (AP) is a gate-keeper of innate immune system responses by detoxifying inflammation triggering moieties released from endogenous and external sources. We examined whether AP's broad mechanism of action constitutes a safe therapeutic, either as single agent or combined with methotrexate (MTX), for chronic inflammatory disorders, for example, rheumatoid arthritis (RA). A rat model for RA was used with repeated intra-articular methylated bovine serum albumin (mBSA) injections in 1 knee ("arthritic" knee), with the contralateral knee serving as internal control. AP (200 µg, subcut) was administered before mBSA injections (prophylactic setting) or after arthritis induction (therapeutic setting) or combined with MTX (0.3 mg/kg or 1 mg/kg; intraperitoneally). As end point of treatment outcome, macrophage infiltration in knees, liver, and spleen was assessed by immunohistochemistry (ED1 and ED2 expression), immunofluoresence (macrophage marker folate receptor-β [FRβ]), and [¹⁸F]fluoro-polyethylene glycol-folate positron emission tomography (PET) (macrophage imaging) and ex vivo tissue distribution. Single-agent AP treatment and combinations with MTX were well tolerated. Both prophylactic and therapeutic AP markedly reduced synovial macrophage infiltration in arthritic knees (ED1: 3.5- to 4-fold; ED2: 3.5- to 6-fold), comparable with MTX treatment. AP-MTX combinations slightly improved on single agent effects. PET monitoring and ex vivo tissue distribution studies corroborated the impact of AP, MTX, and AP-MTX on reducing synovial macrophage infiltration. Beyond localized articular effects, AP also revealed systemic anti-inflammatory effects by a 2-fold reduction of ED1, ED2, and FRβ⁺ macrophages in liver and spleen of arthritic rats. Collectively, single-agent AP and AP combined with MTX elicited local and systemic anti-arthritic activity in arthritic rats.

Quantification of O-(2-[18F]fluoroethyl)-L-tyrosine kinetics in glioma

BACKGROUND

This study identified the optimal tracer kinetic model for quantification of dynamic O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET) positron emission tomography (PET) studies in seven patients with diffuse glioma (four glioblastoma, three lower grade glioma). The performance of more simplified approaches was evaluated by comparison with the optimal compartment model. Additionally, the relationship with cerebral blood flow-determined by [15O]H2O PET-was investigated.

RESULTS

The optimal tracer kinetic model was the reversible two-tissue compartment model. Agreement analysis of binding potential estimates derived from reference tissue input models with the distribution volume ratio (DVR)-1 derived from the plasma input model showed no significant average difference and limits of agreement of - 0.39 and 0.37. Given the range of DVR-1 (- 0.25 to 1.5), these limits are wide. For the simplified methods, the 60-90 min tumour-to-blood ratio to parent plasma concentration yielded the highest correlation with volume of distribution VT as calculated by the plasma input model (r = 0.97). The 60-90 min standardized uptake value (SUV) showed better correlation with VT (r = 0.77) than SUV based on earlier intervals. The 60-90 min SUV ratio to contralateral healthy brain tissue showed moderate agreement with DVR with no significant average difference and limits of agreement of - 0.24 and 0.30. A significant but low correlation was found between VT and CBF in the tumour regions (r = 0.61, p = 0.007).

CONCLUSION

Uptake of [18F]FET was best modelled by a reversible two-tissue compartment model. Reference tissue input models yielded estimates of binding potential which did not correspond well with plasma input-derived DVR-1. In comparison, SUV ratio to contralateral healthy brain tissue showed slightly better performance, if measured at the 60-90 min interval. SUV showed only moderate correlation with VT. VT shows correlation with CBF in tumour.

First in human evaluation of [18F]PK-209, a PET ligand for the ion channel binding site of NMDA receptors

Background

Efforts to develop suitable positron emission tomography (PET) tracers for the ion channel site of human N-methyl-d-aspartate (NMDA) receptors have had limited success. [¹⁸F]PK-209 is a GMOM derivative that binds to the intrachannel phencyclidine site with high affinity and selectivity. Primate PET studies have shown that the volume of distribution in the brain was reduced by administration of the NMDA receptor antagonist MK-801, consistent with substantial specific binding. The purpose of the present study was to evaluate [¹⁸F]PK-209 in 10 healthy humans by assessing test–retest reproducibility and binding specificity following intravenous S-ketamine administration (0.5 mg ∙ kg⁻¹). Five healthy subjects underwent a test–retest protocol, and five others a baseline-ketamine protocol. In all cases dynamic, 120-min PET scans were acquired together with metabolite-corrected arterial plasma input functions. Additional input functions were tested based on within-subject and population-average parent fractions.

Results

Best fits of the brain time-activity curves were obtained using an irreversible two-tissue compartment model with additional blood volume parameter. Mean test–retest variability of the net rate of influx K_i varied between 7 and 24% depending on the input function. There were no consistent changes in [¹⁸F]PK-209 PET parameters following ketamine administration, which may be a consequence of the complex endogenous ligand processes that affect channel gating.

Conclusions

The molecular interaction between [¹⁸F]PK-209 and the binding site within the NMDA receptor ion channel is insufficiently reproducible and specific to be a reliable imaging agent for its quantification.

Trial registration

EudraCT 2014-001735-36. Registered 28 April 2014

Quantification of Tau Load Using [18F]AV1451 PET

Purpose

The tau tracer [¹⁸F]AV1451, also known as flortaucipir, is a promising ligand for imaging tau accumulation in Alzheimer’s disease (AD). Most of the previous studies have quantified tau load using standardized uptake value ratios (SUVr) derived from a static [¹⁸F]AV1451 scan. SUVr may, however, be flow dependent and, especially for longitudinal studies, should be validated against a fully quantitative approach. The objective of this study was to identify the optimal tracer kinetic model for measuring tau load using [¹⁸F]AV1451.

Procedures

Following intravenous injection of 225 ± 16 MBq [¹⁸F]AV1451, 130 min dynamic PET scans were performed in five biomarker confirmed AD patients and five controls. Arterial blood sampling was performed to obtain a metabolite-corrected plasma input function. Next, regional time–activity curves were generated using PVElab software. These curves were analysed using several pharmacokinetic models.

Results

The reversible single tissue compartment model (1T2k_V_B) was the preferred model for all but one control. For AD patients, however, model preference shifted towards a reversible two tissue compartmental model (2T4k_V_B). The simplified reference tissue model (SRTM) derived binding potential (BP_ND) showed good correlation (AD: r² = 0.87, slope = 1.06; controls: r² = 0.87, slope = 0.86) with indirect plasma input binding (distribution volume ratio-1). Standardized uptake value ratios (80–100 min) correlated well with DVR (r² = 0.93, slope = 1.07) and SRTM-derived BP_ND (r² = 0.84, slope = 0.95). In addition, regional differences in tracer binding between subject groups in different tau-specific regions were observed.

Conclusions

Model preference of [¹⁸F]AV1451 appears to depend on subject status and, in particular, V_T. The relationship between model preference and V_T suggests that (higher) tau load may be reflected by a second tissue compartment. Nevertheless, consistent results can be obtained using a 2T4k_V_B model. In addition, SRTM can be used to derive BP_ND.

Electronic supplementary material

The online version of this article (doi:10.1007/s11307-017-1080-z) contains supplementary material, which is available to authorized users.