Synthesis and preclinical evaluation of [11C]EAI045 as a PET tracer for imaging tumors expressing mutated epidermal growth factor receptor

BACKGROUND

Mutations in the epidermal growth factor receptor (EGFR) kinase domain are common in non-small cell lung cancer. Conventional tyrosine kinase inhibitors target the mutation site in the ATP binding pocket, thereby inhibiting the receptor's function. However, subsequent treatment resistance mutations in the ATP binding site are common. The EGFR allosteric inhibitor, EAI045, is proposed to have an alternative mechanism of action, disrupting receptor signaling independent of the ATP-binding site. The antibody cetuximab is hypothesized to increase the number of accessible allosteric pockets for EAI045, thus increasing the potency of the inhibitor. This work aimed to gain further knowledge on pharmacokinetics, the EGFR mutation-targeting potential, and the influence of cetuximab on the uptake by radiolabeling EAI045 with carbon-11 and tritium.

RESULTS

2-(5-fluoro-2-hydroxyphenyl)-2-((2-iodobenzyl)amino)-N-(thiazol-2-yl)acetamide and 2-(5-fluoro-2-hydroxyphenyl)-N-(5-iodothiazol-2-yl)-2-(1-oxoisoindolin-2-yl)acetamide were synthesized as precursors for the carbon-11 and tritium labeling of EAI045, respectively. [11C]EAI045 was synthesized using [11C]CO in a palladium-catalyzed ring closure in a 10 ± 1% radiochemical yield (decay corrected to end of [11C]CO2 production), > 97% radiochemical purity and 26 ± 1 GBq/µmol molar activity (determined at end of synthesis) in 51 min. [3H]EAI045 was synthesized by a tritium-halogen exchange in a 0.2% radiochemical yield, 98% radiochemical purity, and 763 kBq/nmol molar activity. The ability of [11C]EAI045 to differentiate between L858R/T790M mutated EGFR expressing H1975 xenografts and wild-type EGFR expressing A549 xenografts was evaluated in female nu/nu mice. The uptake was statistically significantly higher in H1975 xenografts compared to A549 xenografts (0.45 ± 0.07%ID/g vs. 0.31 ± 0.10%ID/g, P = 0.0166). The synergy in inhibition between EAI045 and cetuximab was evaluated in vivo and in vitro. While there was some indication that cetuximab influenced the uptake of [3H]EAI045 in vitro, this could not be confirmed in vivo when tumor-bearing mice were administered cetuximab (0.5 mg), 24 h prior to injection of [11C]EAI045.

CONCLUSIONS

EAI045 was successfully labeled with tritium and carbon-11, and the in vivo results indicated [11C]EAI045 may be able to distinguish between mutated and non-mutated EGFR in non-small cell lung cancer mouse models. Cetuximab was hypothesized to increase EAI045 uptake; however, no significant effect was observed on the uptake of [11C]EAI045 in vivo or [3H]EAI045 in vitro in H1975 xenografts and cells.

Development of 18F-Labeled PET Tracer Candidates for Imaging of the Abelson Non-receptor Tyrosine Kinase in Parkinson's Disease

Activated Abelson non-receptor tyrosine kinase (c-Abl) plays a harmful role in neurodegenerative conditions such as Parkinson's disease (PD). Inhibition of c-Abl is reported to have a neuroprotective effect and be a promising therapeutic strategy for PD. We have previously identified a series of benzo[d]thiazole derivatives as selective c-Abl inhibitors from which one compound showed high therapeutic potential. Herein, we report the development of a complementary positron emission tomography (PET) tracer. In total, three PET tracer candidates were developed and eventually radiolabeled with fluorine-18 for in vivo evaluation studies in mice. Candidate [18F]3 was identified as the most promising compound, since it showed sufficient brain uptake, good washout kinetics, and satisfactory metabolic stability. In conclusion, we believe this tracer provides a good starting point to further validate and explore c-Abl as a target for therapeutic strategies against PD supported by PET.

Synthesis and Preclinical Evaluation of [Methylpiperazine-11C]brigatinib as a PET Tracer Targeting Both Mutated Epidermal Growth Factor Receptor and Anaplastic Lymphoma Kinase

Brigatinib, a tyrosine kinase inhibitor (TKI) with specificity for gene rearranged anaplastic lymphoma kinase (ALK), such as the EML4-ALK, has shown a potential to inhibit mutated epidermal growth factor receptor (EGFR). In this study, N-desmethyl brigatinib was successfully synthesized as a precursor in five steps. Radiolabeling with [11C]methyl iodide produced [methylpiperazine-11C]brigatinib in a 10 ± 2% radiochemical yield, 91 ± 17 GBq/μmol molar activity, and ≥95% radiochemical purity in 49 ± 4 min. [Methylpiperazine-11C]brigatinib was evaluated in non-small cell lung cancer xenografted female nu/nu mice. An hour post-injection (p.i.), 87% of the total radioactivity in plasma originated from intact [methylpiperazine-11C]brigatinib. Significant differences in tumor uptake were observed between the endogenously EML4-ALK mutated H2228 and the control xenograft A549. The tumor-to-blood ratio in H2228 xenografts could be reduced by pretreatment with ALK inhibitor crizotinib. Tracer uptake in EGFR Del19 mutated HCC827 and EML4-ALK fusion A549 was not significantly different from uptake in A549 xenografts.

Validation of simplified uptake measures against dynamic Patlak Ki for quantification of lesional 89Zr-Immuno-PET antibody uptake

PURPOSE

Positron emission tomography imaging of zirconium-89-labelled monoclonal antibodies (89Zr-Immuno-PET) allows for visualisation and quantification of antibody uptake in tumours in vivo. Patlak linearization provides distribution volume (VT) and nett influx rate (Ki) values, representing reversible and irreversible uptake, respectively. Standardised uptake value (SUV) and tumour-to-plasma/tumour-to-blood ratio (TPR/TBR) are often used, but their validity depends on the comparability of plasma kinetics and clearances. This study assesses the validity of SUV, TPR and TBR against Patlak Ki for quantifying irreversible 89Zr-Immuno-PET uptake in tumours.

METHODS

Ten patients received 37 MBq 10 mg 89Zr-anti-EGFR with 500 mg/m2 unlabelled mAbs. Five patients received two doses of 37 MBq 89Zr-anti-HER3: 8-24 mg for the first administration and 24 mg-30 mg/kg for the second. Seven tumours from four patients showed 89Zr-anti-EGFR uptake, and 18 tumours from five patients showed 89Zr-anti-HER3 uptake. SUVpeak, TPRpeak and TBRpeak values were obtained from one to six days p.i. Patlak linearization was applied to tumour time activity curves and plasma samples to obtain Ki.

RESULTS

For 89Zr-anti-EGFR, there was a small variability along the linear regression line between SUV (- 0.51-0.57), TPR (- 0.06‒0.11) and TBR (- 0.13‒0.16) on day 6 versus Ki. Similar doses of 89Zr-anti-HER3 showed similar variability for SUV (- 1.3‒1.0), TPR (- 1.1‒0.53) and TBR (- 1.5‒0.72) on day 5 versus Ki. However, for the second administration of 89Zr-anti-HER3 with a large variability in administered mass doses, SUV showed a larger variability (- 1.4‒2.3) along the regression line with Ki, which improved when using TPR (- 0.38-0.32) or TBR (- 0.56‒0.46).

CONCLUSION

SUV, TPR and TBR at late time points were valid for quantifying irreversible lesional 89Zr-Immuno-PET uptake when constant mass doses were administered. However, for variable mass doses, only TPR and TBR provided reliable values for irreversible uptake, but not SUV, because SUV does not take patient and mass dose-specific plasma clearance into account.

Synthesis and preclinical evaluation of two osimertinib isotopologues labeled with carbon-11 as PET tracers targeting the tyrosine kinase domain of the epidermal growth factor receptor

INTRODUCTION

Osimertinib is a third-generation tyrosine kinase inhibitor (TKI) that is able to inhibit the EGFR treatment resistance mutation T790M and primary EGFR mutations Del19 and L858R. The aim of the study was to evaluate the potential of carbon-11 labeled osimertinib to be used as a tracer for the PET imaging of tumors bearing the T790M mutation.

METHODS

Osimertinib was labeled with carbon-11 at two positions, and the effect of the labeling position on the metabolism and biodistribution was studied in female nu/nu mice. The mutation status specificity of osimertinib was confirmed in vitro in a cell growth inhibition experiment, and the tumor-targeting potential of the carbon-11 isotopologues was evaluated using female nu/nu mice xenografted with NSCLC cell lines; the wild-type EGFR expressing A549, the primary Del19 EGFR mutated HCC827 and the resistance T790M/L858R mutated H1975. One of the osimertinib tracers was selected based on the results acquired and evaluated for tracer specificity and selectivity by assessment of tumor uptake in a PET study where HCC827 tumor-bearing mice were pretreated with osimertinib or afatinib.

RESULTS

[Methylindole-¹¹C]- and [dimethylamine-¹¹C]osimertinib were synthesized by ¹¹C-methylation of precursors AZ5104 and AZ7550, respectively. Rapid metabolism of both analogs of [¹¹C]osimertinib was observed. Although the tumor uptake and retention of [methylindole-¹¹C]- and [dimethylamine-¹¹C]osimertinib in tumors were similar, the tumor-to-muscle ratios appeared to be higher for [methylindole-¹¹C]osimertinib. The highest uptake, tumor-to-blood, and tumor-to-muscle ratio were observed in the Del19 EGFR mutated HCC827 tumors. However, the specificity and selectivity of [methylindole-¹¹C]osimertinib PET could not be demonstrated in HCC827 tumors. The uptake of [methylindole-¹¹C]osimertinib was not significantly higher in T790M resistance mutated H1975 xenografts compared to the negative control cell line A549.

CONCLUSIONS

Osimertinib was successfully labeled at two positions with carbon-11, yielding two EGFR PET tracers, [methylindole-¹¹C]osimertinib and [dimethylamine-¹¹C]osimertinib. The preclinical evaluation demonstrated uptake and retention in three NSCLC xenografts; A549, HCC827, and H1975. The highest uptake was observed in the primary Del19 EGFR mutated HCC827. The ability of [methylindole-¹¹C]osimertinib to distinguish between the T790M resistance mutated H1975 xenografts and the wild-type EGFR expressing A549 could not be confirmed in the ex vivo study.

Imaging the TGFβ type I receptor in pulmonary arterial hypertension

Transforming growth factor β (TGFβ) activity is perturbed in remodelled pulmonary vasculature of patients with pulmonary arterial hypertension (PAH), cancer, vascular diseases and developmental disorders. Inhibition of TGFβ, which signals via activin receptor-like kinase 5 (ALK5), prevents progression and development of experimental PAH. The purpose of this study was to assess two ALK5 targeting positron emission tomography (PET) tracers ([¹¹C]LR111 and [¹⁸F]EW-7197) for imaging ALK5 in monocrotaline (MCT)- and Sugen/hypoxia (SuHx)-induced PAH. Both tracers were subjected to extensive in vitro and in vivo studies. [¹¹C]LR111 showed the highest metabolic stability, as 46 ± 2% of intact tracer was still present in rat blood plasma after 60 min. In autoradiography experiments, [¹¹C]LR111 showed high ALK5 binding in vitro compared with controls, 3.2 and 1.5 times higher in SuHx and MCT, respectively. In addition, its binding could be blocked by SB431542, an adenosine triphosphate competitive ALK5 kinase inhibitor. However, [¹⁸F]EW-7197 showed the best in vivo results. 15 min after injection, uptake was 2.5 and 1.4 times higher in the SuHx and MCT lungs, compared with controls. Therefore, [¹⁸F]EW-7197 is a promising PET tracer for ALK5 imaging in PAH.

[18F]FDG and [18F]FES PET/CT Imaging as a Biomarker for Therapy Effect in Patients with Metastatic ER+ Breast Cancer Undergoing Treatment with Rintodestrant

PURPOSE

Positron emission tomography (PET) with 16α-[18F]-fluoro-17β-estradiol ([18F]FES) allows assessment of whole body estrogen receptor (ER) expression. The aim of this study was to investigate [18F]fluorodeoxyglucose ([18F]FDG) and [18F]FES PET/CT imaging for response prediction and monitoring of drug activity in patients with metastatic ER+ breast cancer undergoing treatment with the selective estrogen receptor downregulator (SERD) rintodestrant.

PATIENTS AND METHODS

In this trial (NCT03455270), PET/CT imaging was performed at baseline ([18F]FDG and [18F]FES), during treatment and at time of progression (only [18F]FES). Visual, quantitative and mutational analysis was performed to derive a heterogeneity score (HS) and assess tracer uptake in lesions, in relation to the mutation profile. The primary outcome was progression-free survival (PFS).

RESULTS

The HS and PFS in the entire group did not correlate (n=16, Spearman's rho, P=0.06), but patients with a low HS (<25.0%, n=4) had a PFS of >5 months whereas patients with no [18F]FES uptake (HS 100.0%, n =3) had a PFS of <2 months. [18F]FES uptake was not affected by ESR1 mutations. On-treatment [18F]FES PET/CT scans showed no [18F]FES uptake in any of the baseline [18F]FES positive lesions. At progression, [18F]FES uptake remained blocked in patients scanned ≤1-2 half-lives of rintodestrant whereas it restored in patients scanned ≥5 days after end of treatment.

CONCLUSION

Absence of ER expression on [18F]FES PET is a predictor for no response to rintodestrant. [18F]FES uptake during treatment and at time of progression is useful to monitor the (reversible) effect of therapy and continued mode of action of SERDs.

Synthesis and preclinical evaluation of [11C]LR111 and [18F]EW-7197 as PET tracers of the activin-receptor like kinase-5

The transforming growth factor β (TGFβ) pathway plays a complex role in cancer biology, being involved in both tumour suppression as well as promotion. Overactive TGFβ signalling has been linked to multiple diseases, including cancer, pulmonary arterial hypertension, and fibrosis. One of the key meditators within this pathway is the TGFβ type I receptor, also termed activin receptor-like kinase 5 (ALK5). ALK5 expression level is a key determinant of TGFβ signalling intensity and duration, and perturbation has been linked to diseases. A validated ALK5 positron emission tomography (PET) tracer creates an opportunity, therefore, to study its role in human diseases. To develop ALK5 PET tracers, two small molecule ALK5 kinase inhibitors were selected as lead compounds, which were labelled with carbon-11 and fluorine-18, respectively. [¹¹C]LR111 was synthesized with a yield of 17 ± 6%, a molar activity of 126 ± 79 GBq·μmol^-1 and a purity of >95% (n = 44). [¹⁸F]EW-7197 was synthesized with a yield of 10 ± 5%, a molar activity of 183 ± 126 GBq·μmol^-1 and a purity of >95% (n = 11). Metabolic stability was evaluated in vivo in mice, showing 39 ± 2% of intact [¹¹C]LR111 and 21 ± 2% of intact [¹⁸F]EW-7197 in blood plasma at 45 min p.i. In vitro binding experiments were conducted in breast cancer MDA-MB-231 and lung cancer A431 cell lines. In addition, both tracers were used for PET imaging in MDA-MB-231 xenograft models. Selective uptake of [¹⁸F]EW-7197 and [¹¹C]LR111 was observed in MDA-MB-231 cells, in the MDA-MB-231 tumour xenografts in vivo and in the autoradiograms. As [¹¹C]LR111 and [¹⁸F]EW-7197 showed selectivity of binding to ALK5 in vivo and in vitro. Both tracers are thereby valuable tools for the detection of ALK5 activity.

Relationship between Biodistribution and Tracer Kinetics of 11C-Erlotinib, 18F-Afatinib and 11C-Osimertinib and Image Quality Evaluation Using Pharmacokinetic/Pharmacodynamic Analysis in Advanced Stage Non-Small Cell Lung Cancer Patients

Background: Patients with non-small cell lung cancer (NSCLC) driven by activating epidermal growth factor receptor (EGFR) mutations are best treated with therapies targeting EGFR, i.e., tyrosine kinase inhibitors (TKI). Radiolabeled EGFR-TKI and PET have been investigated to study EGFR-TKI kinetics and its potential role as biomarker of response in NSCLC patients with EGFR mutations (EGFRm). In this study we aimed to compare the biodistribution and kinetics of three different EGFR-TKI, i.e., 11C-erlotinib, 18F-afatinib and 11C-osimertinib. Methods: Data of three prospective studies and 1 ongoing study were re-analysed; data from thirteen patients (EGFRm) were included for 11C-erlotinib, seven patients for 18F-afatinib (EGFRm and EGFR wild type) and four patients for 11C-osimertinib (EGFRm). From dynamic and static scans, SUV and tumor-to-blood (TBR) values were derived for tumor, lung, spleen, liver, vertebra and, if possible, brain tissue. AUC values were calculated using dynamic time-activity-curves. Parent fraction, plasma-to-blood ratio and SUV values were derived from arterial blood data. Tumor-to-lung contrast was calculated, as well as (background) noise to assess image quality. Results: 11C-osimertinib showed the highest SUV and TBR (AUC) values in nearly all tissues. Spleen uptake was notably high for 11C-osimertinib and to a lesser extent for 18F-afatinib. For EGFRm, 11C-erlotinib and 18F-afatinib demonstrated the highest tumor-to-lung contrast, compared to an inverse contrast observed for 11C-osimertinib. Tumor-to-lung contrast and spleen uptake of the three TKI ranked accordingly to the expected lysosomal sequestration. Conclusion: Comparison of biodistribution and tracer kinetics showed that 11C-erlotinib and 18F-afatinib demonstrated the highest tumor-to-background contrast in EGFRm positive tumors. Image quality, based on contrast and noise analysis, was superior for 11C-erlotinib and 18F-afatinib (EGFRm) scans compared to 11C-osimertinib and 18F-afatinib (EGFR wild type) scans.

Validation and test–retest repeatability performance of parametric methods for [11C]UCB-J PET

[¹¹C]UCB-J is a PET radioligand that binds to the presynaptic vesicle glycoprotein 2A. Therefore, [¹¹C]UCB-J PET may serve as an in vivo marker of synaptic integrity. The main objective of this study was to evaluate the quantitative accuracy and the 28-day test–retest repeatability (TRT) of various parametric quantitative methods for dynamic [¹¹C]UCB-J studies in Alzheimer’s disease (AD) patients and healthy controls (HC). Eight HCs and seven AD patients underwent two 60-min dynamic [¹¹C]UCB-J PET scans with arterial sampling over a 28-day interval. Several plasma-input based and reference-region based parametric methods were used to generate parametric images using metabolite corrected plasma activity as input function or white matter semi-ovale as reference region. Different parametric outcomes were compared regionally with corresponding non-linear regression (NLR) estimates. Furthermore, the 28-day TRT was assessed for all parametric methods. Spectral analysis (SA) and Logan graphical analysis showed high correlations with NLR estimates. Receptor parametric mapping (RPM) and simplified reference tissue model 2 (SRTM2) BP_ND, and reference Logan (RLogan) distribution volume ratio (DVR) regional estimates correlated well with plasma-input derived DVR and SRTM BP_ND. Among the multilinear reference tissue model (MRTM) methods, MRTM1 had the best correspondence with DVR and SRTM BP_ND. Among the parametric methods evaluated, spectral analysis (SA) and SRTM2 were the best plasma-input and reference tissue methods, respectively, to obtain quantitatively accurate and repeatable parametric images for dynamic [¹¹C]UCB-J PET.

Novel Thienopyrimidine-Based PET Tracers for P2Y12 Receptor Imaging in the Brain

The P2Y₁₂ receptor (P2Y₁₂R) is uniquely expressed on microglia in the brain, and its expression level directly depends on the microglial activation state. Therefore, P2Y₁₂R provides a promising imaging marker for distinguishing the pro- and anti-inflammatory microglial phenotypes, both of which play crucial roles in neuroinflammatory diseases. In this study, three P2Y₁₂R antagonists were selected from the literature, radiolabeled with carbon-11 or fluorine-18, and evaluated in healthy Wistar rats. Brain imaging was performed with and without blocking of efflux transporters P-glycoprotein and breast cancer resistance protein using tariquidar. Low brain uptake in healthy rats was observed for all tracers at baseline conditions, whereas blocking of efflux transporters resulted in a strong (6–7 fold) increase in brain uptake for both of them. Binding of the most promising tracer, [¹⁸F]3, was further evaluated by in vitro autoradiography on rat brain sections, ex vivo metabolite studies, and in vivo P2Y₁₂R blocking studies. In vitro binding of [¹⁸F]3 on rat brain sections indicated high P2Y₁₂R targeting with approximately 70% selective and specific binding. At 60 min post-injection, over 95% of radioactivity in the brain accounted for an intact tracer. In blood plasma, still 40% intact tracer was found, and formed metabolites did not enter the brain. A moderate P2Y₁₂R blocking effect was observed in vivo by positron emission tomography (PET) imaging with [¹⁸F]3 (p = 0.04). To conclude, three potential P2Y₁₂R PET tracers were obtained and analyzed for P2Y₁₂R targeting in the brain. Unfortunately, the brain uptake appeared low. Future work will focus on the design of P2Y₁₂R inhibitors with improved physicochemical characteristics to reduce efflux transport and increase brain penetration.

Synthesis and evaluation of [18F]cinacalcet for the imaging of parathyroid hyperplasia

INTRODUCTION

Parathyroid hyperplasia is a disease characterized by overactive parathyroid glands secreting increased levels of parathyroid hormone. Surgical removal of the parathyroid glands is the standard treatment but requires precise pre-operative localization of the glands. However, currently available imaging modalities show limited sensitivity. Since positron emission tomography (PET) is a molecular imaging technique with high accuracy and sensitivity, our aim was to develop a new PET tracer for overactive parathyroid glands imaging by radiolabelling cinacalcet, a drug binding to the calcium-sensing receptor of the parathyroid glands.

METHODS

[¹⁸F]Cinacalcet was synthesized by copper-catalysed [¹⁸F]trifluoromethylation of a boronic acid precursor using high molar activity [¹⁸F]fluoroform. Ex vivo biodistribution and metabolism were evaluated in 12 healthy male Wistar rats at 5, 15, 45 and 90 min. PET scans were performed at baseline and after blocking with NPS R-568.

RESULTS

[¹⁸F]Cinacalcet was obtained in an overall radiosynthesis time of 1 h with a radiochemical purity of 98 ± 1%, a radiochemical yield of 8 ± 4% (overall, n = 7, corrected for decay) and a molar activity of 40 ± 11 GBq/μmol (n = 7, at EOS). The ex vivo biodistribution showed uptake in the thyroid and parathyroid glands as well as in other glands such as adrenals, salivary glands and pancreas. The tracer was rapidly cleared from the blood via liver and kidneys and showed fast metabolism. PET images confirmed uptake in the target organ. However, in a blocking study with NPS R-568 specific binding of [¹⁸F]cinacalcet to the CaSR could not be confirmed.

CONCLUSIONS

[¹⁸F]Cinacalcet was successfully synthesized. First in vivo experiments in healthy rats showed uptake of the tracer in the target organ and fast metabolism, encouraging further in vivo evaluation of this tracer.

11C-Sorafenib and 15O-H2O PET for Early Evaluation of Sorafenib Therapy

Sorafenib leads to clinical benefit in a subgroup of patients, whereas all are exposed to potential toxicity. Currently, no predictive biomarkers are available. The purpose of this study was to evaluate whether ¹¹C-sorafenib and ¹⁵O-H₂O PET have potential to predict treatment efficacy. Methods: In this prospective exploratory study, 8 patients with advanced solid malignancies and an indication for sorafenib treatment were included. Microdose ¹¹C-sorafenib and perfusion ¹⁵O-H₂O dynamic PET scans were performed before and after 2 wk of sorafenib therapy. The main objective was to assess whether tumor ¹¹C-sorafenib uptake predicts sorafenib concentrations during therapy in corresponding tumor biopsy samples measured with liquid chromatography tandem mass spectrometry. Secondary objectives included determining the association of ¹¹C-sorafenib PET findings, perfusion ¹⁵O-H₂O PET findings, and sorafenib concentrations after therapeutic dosing with response. Results: ¹¹C-sorafenib PET findings did not predict sorafenib concentrations in tumor biopsy samples during therapy. In addition, sorafenib plasma and tumor concentrations were not associated with clinical outcome in this exploratory study. Higher ¹¹C-sorafenib accumulation in tumors at baseline and day 14 of treatment showed an association with poorer prognosis and correlated with tumor perfusion (Spearman correlation coefficient = 0.671, P = 0.020). Interestingly, a decrease in tumor perfusion measured with ¹⁵O-H₂O PET after only 14 d of therapy showed an association with response, with a decrease in tumor perfusion of 56% ± 23% (mean ± SD) versus 18% ± 32% in patients with stable and progressive disease, respectively. Conclusion: Microdose ¹¹C-sorafenib PET did not predict intratumoral sorafenib concentrations after therapeutic dosing, but the association between a decrease in tumor perfusion and clinical benefit warrants further investigation.

Kinetics and 28-day test-retest repeatability and reproducibility of [11C]UCB-J PET brain imaging

[11C]UCB-J is a novel radioligand that binds to synaptic vesicle glycoprotein 2A (SV2A). The main objective of this study was to determine the 28-day test-retest repeatability (TRT) of quantitative [11C]UCB-J brain positron emission tomography (PET) imaging in Alzheimer's disease (AD) patients and healthy controls (HCs). Nine HCs and eight AD patients underwent two 60 min dynamic [11C]UCB-J PET scans with arterial sampling with an interval of 28 days. The optimal tracer kinetic model was assessed using the Akaike criteria (AIC). Micro-/macro-parameters such as tracer delivery (K1) and volume of distribution (VT) were estimated using the optimal model. Data were also analysed for simplified reference tissue model (SRTM) with centrum semi-ovale (white matter) as reference region. Based on AIC, both 1T2k_VB and 2T4k_VB described the [11C]UCB-J kinetics equally well. Analysis showed that whole-brain grey matter TRT for VT, DVR and SRTM BPND were -2.2% ± 8.5, 0.4% ± 12.0 and -8.0% ± 10.2, averaged over all subjects. [11C]UCB-J kinetics can be well described by a 1T2k_VB model, and a 60 min scan duration was sufficient to obtain reliable estimates for both plasma input and reference tissue models. TRT for VT, DVR and BPND was <15% (1SD) averaged over all subjects and indicates adequate quantitative repeatability of [11C]UCB-J PET.

Identifying advanced stage NSCLC patients who benefit from afatinib therapy using 18F-afatinib PET/CT imaging

OBJECTIVES

Non-small cell lung cancer (NSCLC) tumors harboring common (exon19del, L858R) and uncommon (e.g. G719X, L861Q) activating epidermal growth factor receptor (EGFR) mutations are best treated with EGFR tyrosine kinase inhibitors (TKI) such as the first-generation EGFR TKI erlotinib, second-generation afatinib or third-generation osimertinib. However, identifying these patients through biopsy is not always possible. Therefore, our aim was to evaluate whether ¹⁸F-afatinib PET/CT could identify patients with common and uncommon EGFR mutations. Furthermore, we evaluated the relation between tumor ¹⁸F-afatinib uptake and response to afatinib therapy.

MATERIALS AND METHODS

¹⁸F-afatinib PET/CT was performed in 12 patients: 6 EGFR wild type (WT), 3 EGFR common and 3 EGFR uncommon mutations. Tumor uptake of ¹⁸F-afatinib was quantified using TBR_WB_60-90 (tumor-to-whole blood activity ratio 60-90 min post-injection) for each tumor. Response was quantified per lesion using percentage of change (PC): [(response measurement (RM)-baseline measurement (BM))/BM]×100. Statistical analyses were performed using t-tests, correlation plots and sensitivity/specificity analysis.

RESULTS

Twenty-one tumors were identified. Injected dose was 348 ± 31 MBq. Group differences were significant between WT versus EGFR (common and uncommon) activating mutations (p = 0.03). There was no significant difference between EGFR common versus uncommon mutations (p = 0.94). A TBR_WB_60-90 cut-off value of 6 showed the best relationship with response with a sensitivity of 70 %, a specificity of 100 % and a positive predictive value of 100 %.

CONCLUSION

¹⁸F-afatinib uptake was higher in tumors with EGFR mutations (common and uncommon) compared to WT. Furthermore, a TBR_WB_60-90 cut-off of 6 was found to best predict response to therapy. ¹⁸F-afatinib PET/CT could provide a means to identify EGFR mutation positive patients who benefit from afatinib therapy.

Open-label study with the monoamine stabilizer (-)-OSU6162 in myalgic encephalomyelitis/chronic fatigue syndrome

OBJECTIVES

The purpose of the present study was to investigate the safety and tolerability of the monoaminergic stabilizer (-)-OSU6162 in patients with myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). In addition, a potential therapeutic effect of (-)-OSU6162 in ME/CFS was evaluated by means of observer-rated scales and self-assessment rating scales.

MATERIALS AND METHODS

In the current study using an open-label single-arm design ME/CFS patient received treatment with (-)-OSU6162 during 12 weeks. The patients received the following doses of (-)-OSU6162: 15 mg b.i.d. during the first 4-week period, up to 30 mg b.i.d. during the second 4-week period and up to 45 mg b.i.d. during the third 4-week period, with follow-up visits after 16 and 20 weeks.

RESULTS

Out of 33 included patients, 28 completed the 12 weeks treatment period. (-)-OSU6162 was well tolerated; only one patient discontinued due to an adverse event. Vital signs and physical examinations showed no abnormal changes. Blood analyses showed an increase in serum prolactin. Therapeutically, improvements were seen on the Clinical Global Impression of Change scale, the FibroFatigue scale, the Mental Fatigue Scale, the Fatigue Severity Scale, Beck Depression Inventory, and the Short Form 36 Health Survey Questionnaire.

CONCLUSIONS

(-)-OSU6162 is well tolerated in ME/CFS patients and shows promise as a novel treatment to mitigate fatigue and improve mood and health-related quality of life in ME/CFS. Obviously, the present results need to be confirmed in future placebo-controlled double-blind trials.

Identifying advanced stage NSCLC patients who benefit from afatinib therapy using 18F-afatinib PET/CT imaging

OBJECTIVES

Non-small cell lung cancer (NSCLC) tumors harboring common (exon19del, L858R) and uncommon (e.g. G719X, L861Q) activating epidermal growth factor receptor (EGFR) mutations are best treated with EGFR tyrosine kinase inhibitors (TKI) such as the first-generation EGFR TKI erlotinib, second-generation afatinib or third-generation osimertinib. However, identifying these patients through biopsy is not always possible. Therefore, our aim was to evaluate whether ¹⁸F-afatinib PET/CT could identify patients with common and uncommon EGFR mutations. Furthermore, we evaluated the relation between tumor ¹⁸F-afatinib uptake and response to afatinib therapy.

MATERIALS AND METHODS

¹⁸F-afatinib PET/CT was performed in 12 patients: 6 EGFR wild type (WT), 3 EGFR common and 3 EGFR uncommon mutations. Tumor uptake of ¹⁸F-afatinib was quantified using TBR_WB_60-90 (tumor-to-whole blood activity ratio 60-90 min post-injection) for each tumor. Response was quantified per lesion using percentage of change (PC): [(response measurement (RM)-baseline measurement (BM))/BM]×100. Statistical analyses were performed using t-tests, correlation plots and sensitivity/specificity analysis.

RESULTS

Twenty-one tumors were identified. Injected dose was 348 ± 31 MBq. Group differences were significant between WT versus EGFR (common and uncommon) activating mutations (p = 0.03). There was no significant difference between EGFR common versus uncommon mutations (p = 0.94). A TBR_WB_60-90 cut-off value of 6 showed the best relationship with response with a sensitivity of 70 %, a specificity of 100 % and a positive predictive value of 100 %.

CONCLUSION

¹⁸F-afatinib uptake was higher in tumors with EGFR mutations (common and uncommon) compared to WT. Furthermore, a TBR_WB_60-90 cut-off of 6 was found to best predict response to therapy. ¹⁸F-afatinib PET/CT could provide a means to identify EGFR mutation positive patients who benefit from afatinib therapy.

Repeatability of parametric methods for [18F]florbetapir imaging in Alzheimer's disease and healthy controls: A test-retest study

Accumulation of amyloid beta (Aβ) is one of the pathological hallmarks of Alzheimer's disease (AD), which can be visualized using [¹⁸F]florbetapir positron emission tomography (PET). The aim of this study was to evaluate various parametric methods and to assess their test-retest (TRT) reliability. Two 90 min dynamic [¹⁸F]florbetapir PET scans, including arterial sampling, were acquired (n = 8 AD patient, n = 8 controls). The following parametric methods were used; (reference:cerebellum); Logan and spectral analysis (SA), receptor parametric mapping (RPM), simplified reference tissue model2 (SRTM2), reference Logan (rLogan) and standardized uptake value ratios (SUV_r(50-70)). BP_ND+1, DVR, V_T and SUVr were compared with corresponding estimates (V_T or DVR) from the plasma input reversible two tissue compartmental (2T4k_V_B) model with corresponding TRT values for 90-scan duration. RPM (r² = 0.92; slope = 0.91), Logan (r² = 0.95; slope = 0.84) and rLogan (r² = 0.94; slope = 0.88), and SRTM2 (r² = 0.91; slope = 0.83), SA (r² = 0.91; slope = 0.88), SUVr (r² = 0.84; slope = 1.16) correlated well with their 2T4k_V_B counterparts. RPM (controls: 1%, AD: 3%), rLogan (controls: 1%, AD: 3%) and SUV_r(50-70) (controls: 3%, AD: 8%) showed an excellent TRT reliability. In conclusion, most parametric methods showed excellent performance for [¹⁸F]florbetapir, but RPM and rLogan seem the methods of choice, combining the highest accuracy and best TRT reliability.

In vivo tau pathology is associated with synaptic loss and altered synaptic function

BACKGROUND

The mechanism of synaptic loss in Alzheimer's disease is poorly understood and may be associated with tau pathology. In this combined positron emission tomography (PET) and magnetoencephalography (MEG) study, we aimed to investigate spatial associations between regional tau pathology ([18F]flortaucipir PET), synaptic density (synaptic vesicle 2A [11C]UCB-J PET) and synaptic function (MEG) in Alzheimer's disease.

METHODS

Seven amyloid-positive Alzheimer's disease subjects from the Amsterdam Dementia Cohort underwent dynamic 130-min [18F]flortaucipir PET, dynamic 60-min [11C]UCB-J PET with arterial sampling and 2 × 5-min resting-state MEG measurement. [18F]flortaucipir- and [11C]UCB-J-specific binding (binding potential, BPND) and MEG spectral measures (relative delta, theta and alpha power; broadband power; and peak frequency) were assessed in cortical brain regions of interest. Associations between regional [18F]flortaucipir BPND, [11C]UCB-J BPND and MEG spectral measures were assessed using Spearman correlations and generalized estimating equation models.

RESULTS

Across subjects, higher regional [18F]flortaucipir uptake was associated with lower [11C]UCB-J uptake. Within subjects, the association between [11C]UCB-J and [18F]flortaucipir depended on within-subject neocortical tau load; negative associations were observed when neocortical tau load was high, gradually changing into opposite patterns with decreasing neocortical tau burden. Both higher [18F]flortaucipir and lower [11C]UCB-J uptake were associated with altered synaptic function, indicative of slowing of oscillatory activity, most pronounced in the occipital lobe.

CONCLUSIONS

These results indicate that in Alzheimer's disease, tau pathology is closely associated with reduced synaptic density and synaptic dysfunction.

Non-invasive Standardised Uptake Value for Verification of the Use of Previously Validated Reference Region for [18F]Flortaucipir and [18F]Florbetapir Brain PET Studies

Purpose

The simplified reference tissue model (SRTM) is commonly applied for the quantification of brain positron emission tomography (PET) studies, particularly because it avoids arterial cannulation. SRTM requires a validated reference region which is obtained by baseline-blocking or displacement studies. Once a reference region is validated, the use should be verified for each new subject. This verification normally requires volume of distribution (V_T) of a reference region. However, performing dynamic scanning and arterial sampling is not always possible, specifically in elderly subjects and in advanced disease stages. The aim of this study was to investigate the use of non-invasive standardised uptake value (SUV) approaches, in comparison to V_T, as a verification of the previously validated grey matter cerebellum reference region for [¹⁸F]flortaucipir and [¹⁸F]florbetapir PET imaging in Alzheimer’s disease (AD) patients and controls.

Procedures

Dynamic 130-min [¹⁸F]flortaucipir PET scans obtained from nineteen subjects (10 AD patients) and 90-min [¹⁸F]florbetapir dynamic scans obtained from fourteen subjects (8 AD patients) were included. Regional V_T’s were estimated for both tracers and were considered the standard verification of the previously validated reference region. Non-invasive SUVs corrected for body weight (SUV_BW), lean body mass (SUL), and body surface area (SUV_BSA) were obtained by using later time intervals of the dynamic scans. Simulations were also performed to assess the effect of flow and specific binding (BP_ND) on the SUVs.

Results

A low SUV corresponded well with a low V_T for both [¹⁸F]flortaucipir and [¹⁸F]florbetapir. Simulation confirmed that SUVs were only slightly affected by flow changes and that increases in SUV were predominantly determined by the presence of specific binding.

Conclusions

In situations where dynamic scanning and arterial sampling is not possible, a low SUV_{(80–100 min)} for [¹⁸F]flortaucipir and a low SUV_{(50–70 min)} for [¹⁸F]florbetapir may be used as indication for absence of specific binding in the grey matter cerebellum reference region.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11307-020-01572-y.

PET imaging of P2X7R in the experimental autoimmune encephalomyelitis model of multiple sclerosis using [11C]SMW139

BACKGROUND

Non-invasive imaging of the activation status of microglia and the ability to identify a pro- or anti-inflammatory environment can provide valuable insights not only into pathogenesis of neuro-inflammatory and neurodegenerative diseases but also the monitoring of the efficacy of immunomodulatory therapies. P2X₇R is highly expressed on pro-inflammatory microglia and [¹¹C]SMW139, a specific P2X₇R tracer for positron emission tomography imaging, showed good pharmacokinetics, stability, and brain permeability in vivo. Our objective was to evaluate the potential of [¹¹C]SMW139 for PET imaging of neuroinflammation in vivo in the experimental autoimmune encephalomyelitis (EAE) model.

METHODS

We induced EAE in Lewis rats by immunization with MBP 69-88 in complete Freund's adjuvant (CFA). We determined the affinity of [¹¹C]SMW139 to human and rat P2X₇R using saturation binding assay. Using this tracer, PET imaging was performed at the peak of disease and in the recovery phase. In vivo blocking experiments were conducted to validate the specific brain uptake of the tracer. Immunohistochemistry staining and autoradiography were performed to evaluate the level of neuroinflammation and validate the specific binding of [¹¹C]SMW139.

RESULTS

[¹¹C]SMW139 showed good affinity for the rat P2X₇R with a K_d of 20.6 ± 1.7 nM. The uptake of [¹¹C]SMW139 was significantly higher in EAE animals at the peak of disease compared to the recovery phase but not in CFA control animals. The amplitude of increase of [¹¹C]SMW139 uptake showed significant positive correlation with clinical scores mainly in the spinal cord (Pearson = 0.75, Spearman = 0.76; p < 0.0001). Treating EAE animals with P2X₇R antagonist JNJ-47965567 blocked the uptake of [¹¹C]SMW139 in the spinal cord, cerebellum, and brain stem, demonstrating specific accumulation of the tracer. P-glycoprotein blocking with tariquidar (30 mg/kg) did not affect tracer penetration in the brain showing that [¹¹C]SMW139 is not a Pgp substrate.

CONCLUSION

Our data shows that [¹¹C]SMW139 is a promising PET tracer for imaging neuroinflammation and evaluating the dynamics of pro-inflammatory microglia in the brain. This can provide crucial insights into the role of microglia in disease progression and enables the development of novel treatment strategies aimed at modulating the immune response in order to promote neuroprotection.

Quantification of [18F]afatinib using PET/CT in NSCLC patients: a feasibility study

Introduction

Only a subgroup of non-small cell lung cancer (NSCLC) patients benefit from treatment using epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKI) such as afatinib. Tumour uptake of [¹⁸F]afatinib using positron emission tomography (PET) may identify those patients that respond to afatinib therapy. Therefore, the aim of this study was to find the optimal tracer kinetic model for quantification of [¹⁸F]afatinib uptake in NSCLC tumours.

Methods

[¹⁸F]Afatinib PET scans were performed in 10 NSCLC patients. The first patient was scanned for the purpose of dosimetry. Subsequent patients underwent a 20-min dynamic [¹⁵O]H₂O PET scan (370 MBq) followed by a dynamic [¹⁸F]afatinib PET scan (342 ± 24 MBq) of 60 or 90 min. Using the Akaike information criterion (AIC), three pharmacokinetic plasma input models were evaluated with both metabolite-corrected sampler-based input and image-derived (IDIF) input functions in combination with discrete blood samples. Correlation analysis of arterial on-line sampling versus IDIF was performed. In addition, perfusion dependency and simplified measures were assessed.

Results

Ten patients were included. The injected activity of [¹⁸F]afatinib was 341 ± 37 MBq. Fifteen tumours could be identified in the field of view of the scanner. Based on AIC, tumour kinetics were best described using an irreversible two-tissue compartment model and a metabolite-corrected sampler-based input function (Akaike 50%). Correlation of plasma-based input functions with metabolite-corrected IDIF was very strong (r² = 0.93). The preferred simplified uptake parameter was the tumour-to-blood ratio over the 60- to 90-min time interval (TBR_60–90). Tumour uptake of [¹⁸F]afatinib was independent of perfusion.

Conclusion

The preferred pharmacokinetic model for quantifying [¹⁸F]afatinib uptake in NSCLC tumours was the 2T3K_vb model. TBR_60–90 showed excellent correlation with this model and is the best candidate simplified method.

Trial registration

https://eudract.ema.europa.eu/ nr 2012-002849-38

Evaluation of the Novel PET Tracer [11C]HACH242 for Imaging the GluN2B NMDA Receptor in Non-Human Primates

PURPOSE

There are currently no positron emission tomography (PET) radiotracers for the GluN2B (NR2B) binding sites of brain N-methyl-D-aspartate (NMDA) receptors. In rats, the GluN2B antagonist Ro25-6981 reduced the binding of N-((5-(4-fluoro-2-[¹¹C]methoxyphenyl)pyridin-3-yl)methyl)cyclopentanamin ([¹¹C]HACH242). This paper reports the evaluation of [¹¹C]HACH242 PET in non-human primates at baseline and following administration of the GluN2B negative allosteric modulator radiprodil.

PROCEDURES

Eight 90-min dynamic [¹¹C]HACH242 PET scans were acquired in three male anaesthetised rhesus monkeys, including a retest session of subject 1, at baseline and 10 min after intravenous 10 mg/kg radiprodil. Standardised uptake values (SUV) were calculated for 9 brain regions. Arterial blood samples were taken at six timepoints to characterise pharmacokinetics in blood and plasma. Reliable input functions for kinetic modelling could not be generated due to variability in the whole-blood radioactivity measurements.

RESULTS

[¹¹C]HACH242 entered the brain and displayed fairly uniform uptake. The mean (± standard deviation, SD) T_max was 17 ± 7 min in baseline scans and 24 ± 15 min in radiprodil scans. The rate of radioligand metabolism in plasma (primarily to polar metabolites) was high, with mean parent fractions of 26 ± 10 % at 20 min and 8 ± 5 % at 85 min. Radiprodil increased [¹¹C]HACH242 whole-brain SUV in the last PET frame by 25 %, 1 %, 3 and 17 % for subjects 1, 2, 3 and retest of subject 1, respectively. The mean brain to plasma ratio was 5.4 ± 2.6, and increased by 39 to 110 % in the radiprodil condition, partly due to lower parent plasma radioactivity of -11 to -56 %.

CONCLUSIONS

The present results show that [¹¹C]HACH242 has a suitable kinetic profile in the brain and low accumulation of lipophilic radiometabolites. Radiprodil did not consistently change [¹¹C]HACH242 brain uptake. These findings may be explained by variations in cerebral blood flow, a low fraction of specifically bound tracer, or interactions with endogenous NMDA receptor ligands at the binding site. Further experiments of ligand interactions are necessary to facilitate the development of radiotracers for in vivo imaging of the ionotropic NMDA receptor.

Quantification of PD-L1 Expression with 18F-BMS-986192 PET/CT in Patients with Advanced-Stage Non-Small Cell Lung Cancer

The aim of this work was to quantify the uptake of ¹⁸F-BMS-986192, a programmed cell death ligand 1 (PD-L1) adnectin PET tracer, in patients with non-small cell lung cancer. To this end, plasma input kinetic modeling of dynamic tumor uptake data with online arterial blood sampling was performed. In addition, the accuracy of simplified uptake metrics such as SUV was investigated. Methods: Data from a study with ¹⁸F-BMS-986192 in patients with advanced-stage non-small cell lung cancer eligible for nivolumab treatment were used if a dynamic scan was available and lesions were present in the field of view of the dynamic scan. After injection of ¹⁸F-BMS-986192, a 60-min dynamic PET/CT scan was started, followed by a 30-min whole-body PET/CT scan. Continuous arterial and discrete arterial and venous blood sampling were performed to determine a plasma input function. Tumor time-activity curves were fitted by several plasma input kinetic models. Simplified uptake parameters included tumor-to-blood ratio as well as several SUV measures. Results: Twenty-two tumors in 9 patients were analyzed. The arterial plasma input single-tissue reversible compartment model with fitted blood volume fraction seems to be the most preferred model as it best fitted 11 of 18 tumor time-activity curves. The distribution volume (V _T ) ranged from 0.4 to 4.8 mL⋅cm^-3 Similar values were obtained with an image-derived input function. From the simplified measures, SUV normalized for body weight at 50 and 67 min after injection correlated best with V _T , with an R ² of more than 0.9. Conclusion: A single-tissue reversible model can be used to quantify tumor uptake of the PD-L1 PET tracer ¹⁸F-BMS-986192. SUV at 60 min after injection, normalized for body weight, is an accurate simplified parameter for uptake assessment of baseline studies. To assess its predictive value for response evaluation during programmed cell death protein 1 or PD-L1 immune checkpoint inhibition, further validation of SUV against V _T based on an image-derived input function is recommended.

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.

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

The online version of this article (10.1007/s00259-019-04550-x) contains supplementary material, which is available to authorized users.

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.

Sensitivity of 18F-fluorodihydrotestosterone PET-CT to count statistics and reconstruction protocol in metastatic castration-resistant prostate cancer

Objectives

Whole body [¹⁸F]-fluorodihydrotestosterone positron emission tomography ([¹⁸F]FDHT PET) imaging directly targets the androgen receptor and is a promising prognostic and predictive biomarker in metastatic castration-resistant cancer (mCRPC). To optimize [¹⁸F]FDHT PET-CT for diagnostic and response assessment purposes, we assessed how count statistics and reconstruction protocol affect its accuracy, repeatability, and lesion detectability.

Methods

Whole body [¹⁸F]FDHT PET-CT scans were acquired on an analogue PET-CT on two consecutive days in 14 mCRPC patients harbouring a total of 336 FDHT-avid lesions. Images were acquired at 45 min post-injection of 200 MBq [¹⁸F]FDHT at 3 min per bed position. List-mode PET data were split on a count-wise basis, yielding two statistically independent scans with each 50% of counts. Images were reconstructed according to current EANM Research Ltd. (EARL1, 4 mm voxel) and novel EARL2 guidelines (4 mm voxel + PSF). Per lesion, we measured SUVpeak, SUVmax, SUVmean, and contrast-to-noise ratio (CNR). SUV was normalized to dose per bodyweight as well as to the parent plasma input curve integral. Variability was assessed with repeatability coefficients (RCs).

Results

Count reduction increased liver coefficient of variation from 9.0 to 12.5% and from 10.8 to 13.2% for EARL1 and EARL2, respectively. SUVs of EARL2 images were 12.0–21.7% higher than EARL1. SUVs of 100% and 50% count data were highly correlated (R² > 0.98; slope = 0.97–1.01; ICC = 0.99–1.00). Intrascan variability was volume-dependent, and count reduction resulted in higher intrascan variability for EARL2 than EARL1 images. Intrascan RCs were lowest for SUVmean (8.5–10.6%), intermediate for SUVpeak (12.0–16.0%), and highest for SUVmax (17.8–22.2%). Count reduction increased test-retest variance non-significantly (p > 0.05) for all SUV types and normalizations. For SUVpeak at 50% of counts, RCs remained < 30% when small lesions were excluded. Splitting data reduced CNR by median 4.6% (interquartile range 1.2–8.7%) and 4.6% (interquartile range 1.2–8.7%) for EARL1 and EARL2 images, respectively.

Conclusions

Reducing [¹⁸F]FDHT PET acquisition time from 3 min to 1.5 per bed position resulted in a repeatability of SUVpeak (bodyweight) remaining ≤ 30%, which is generally acceptable for response monitoring purposes. However, EARL2 reconstruction was more affected, especially for SUVmax whose repeatability tended to exceed 30%. Lesion detectability was only slightly impaired by reducing acquisition time, which might not be clinically relevant in mCRPC.

Electronic supplementary material

The online version of this article (10.1186/s13550-019-0531-8) contains supplementary material, which is available to authorized users.

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.

Open study with (-)-OSU6162 in multiple sclerosis-related fatigue

OBJECTIVES

The main objective of this study was to investigate the tolerability and safety of the monoaminergic stabilizer (-)-OSU6162 in patients with multiple sclerosis (MS). In addition, a potential therapeutic effect of (-)-OSU6162 with focus on MS-related fatigue was estimated by means of various self-assessment rating scales as well as a clinical investigator-rated scale.

MATERIALS AND METHODS

In this open-label, single-arm study, 30 MS patients received treatment with the monoaminergic stabilizer (-)-OSU6162 during 12 weeks. The dose of (-)-OSU6162 was 15 mg twice daily during the first 4-week period, up to 30 mg twice daily during the second 4-week period and up to 45 mg twice daily during the third 4-week period, with follow-up visits after 16 and 20 weeks. MS-related fatigue was rated by the clinical investigator or by self-assessments, using mainly established rating scales. Twenty-five patients completed the study.

RESULTS

(-)-OSU6162 was well tolerated by all patients, and no serious adverse events were observed. Therapeutically, improvements were observed with respect to fatigue and mood, as judged by ratings on the Mental Fatigue Scale (MFS), Short Form-36 (SF-36) scale and Beck Depression Inventory (BDI). Furthermore, the large majority of patients were rated as globally improved in the medical observers' rating scale Clinical Global Impression of Change (CGI-C).

CONCLUSIONS

In view of its good tolerability, (-)-OSU6162 may offer a new treatment option for alleviating mental fatigue, as well as depression, in MS. Larger, randomized double-blind controlled trials are warranted to confirm the present preliminary observations.

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

The online version of this article (10.1186/s12974-018-1352-9) contains supplementary material, which is available to authorized users.

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.

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.

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.

In vivo evaluation of two tissue transglutaminase PET tracers in an orthotopic tumour xenograft model

Background

The protein cross-linking enzyme tissue transglutaminase (TG2; EC 2.3.2.13) is associated with the pathogenesis of various diseases, including cancer. Recently, the synthesis and initial evaluation of two high-potential radiolabelled irreversible TG2 inhibitors were reported by us. In the present study, these two compounds were evaluated further in a breast cancer (MDA-MB-231) tumour xenograft model for imaging active tissue transglutaminase in vivo.

Results

The metabolic stability of [¹¹C]1 and [¹⁸F]2 in SCID mice was comparable to the previously reported stability in Wistar rats. Quantitative real-time polymerase chain reaction analysis on MDA-MB-231 cells and isolated tumours showed a high level of TG2 expression with very low expression of other transglutaminases. PET imaging showed low tumour uptake of [¹¹C]1 (approx. 0.5 percentage of the injected dose per gram (%ID/g) at 40–60 min p.i.) and with relatively fast washout. Tumour uptake for [¹⁸F]2 was steadily increasing over time (approx. 1.7 %ID/g at 40–60 min p.i.). Pretreatment of the animals with the TG2 inhibitor ERW1041E resulted in lower tumour activity concentrations, and this inhibitory effect was enhanced using unlabelled 2.

Conclusions

Whereas the TG2 targeting potential of [¹¹C]1 in this model seems inadequate, targeting of TG2 using [¹⁸F]2 was achieved. As such, [¹⁸F]2 could be used in future studies to clarify the role of active tissue transglutaminase in disease.

Identification of the allosteric P2X7 receptor antagonist [11C]SMW139 as a PET tracer of microglial activation

The P2X7 receptor plays a significant role in microglial activation, and as a potential drug target, the P2X7 receptor is also an interesting target in positron emission tomography. The current study aimed at the development and evaluation of a potent tracer targeting the P2X7 receptor, to which end four adamantanyl benzamide analogues with high affinity for the human P2X7 receptor were labelled with carbon-11. All four analogues could be obtained in excellent radiochemical yield and high radiochemical purity and molar activity, and all analogues entered the rat brain. [11C]SMW139 showed the highest metabolic stability in rat plasma, and showed high binding to the hP2X7 receptor in vivo in a hP2X7 receptor overexpressing rat model. Although no significant difference in binding of [11C]SMW139 was observed between post mortem brain tissue of Alzheimer's disease patients and that of healthy controls in in vitro autoradiography experiments, [11C]SMW139 could be a promising tracer for P2X7 receptor imaging using positron emission tomography, due to high receptor binding in vivo in the hP2X7 receptor overexpressing rat model. However, further investigation of both P2X7 receptor expression and binding of [11C]SMW139 in other neurological diseases involving microglial activation is warranted.

Development of [11C]vemurafenib employing a carbon-11 carbonylative Stille coupling and preliminary evaluation in mice bearing melanoma tumor xenografts

Over the last decade kinase inhibitors have witnessed tremendous growth as anti-cancer drugs. Unfortunately, despite their promising clinical successes, a large portion of patients does not benefit from these targeted therapeutics. Vemurafenib is a serine/threonine kinase inhibitor approved for the treatment of melanomas specifically expressing the BRAF^V600E mutation. The aim of this study was to develop vemurafenib as PET tracer to determine its potential for identification of tumors sensitive to vemurafenib treatment. Therefore, vemurafenib was labeled with carbon-11 and analyzed for its tumor targeting potential in melanoma xenografts Colo829 (BRAF^V600E) and MeWo (BRAF^wt) using autoradiography on tissue sections, in vitro tumor cell uptake studies and biodistribution studies in xenografted athymic nu/nu mice. [¹¹C]vemurafenib was synthesized in 21 ± 4% yield (decay corrected, calculated from [¹¹C]CO) in > 99% radiochemical purity and a specific activity of 55 ± 18 GBq/μmol. Similar binding of [¹¹C]vemurafenib was shown during autoradiography and cellular uptake studies in both cell lines. Plasma metabolite analysis demonstrated > 95% intact [¹¹C]vemurafenib in vivo at 45 minutes after injection, indicating excellent stability. Biodistribution studies confirmed the in vitro results, showing similar tumor-to-background ratios in both xenografts models. These preliminary results suggest that identification of BRAF^V600E mutations in vivo using PET with [¹¹C]vemurafenib will be challenging.

Synthesis and Evaluation of New Fluorine-18 Labeled Verapamil Analogs To Investigate the Function of P-Glycoprotein in the Blood-Brain Barrier

P-glycoprotein is an efflux transporter located in the blood-brain barrier. (R)-[11C]Verapamil is widely used as a PET tracer to investigate its function in patients with epilepsy, Alzheimer's disease, and other neurodegenerative diseases. Currently it is not possible to use this successful tracer in clinics without a cyclotron, because of the short half-life of carbon-11. We developed two new fluorine-18 labeled (R)-verapamil analogs, with the benefit of a longer half-life. The synthesis of (R)-N-[18F]fluoroethylverapamil ([18F]1) and (R)-O-[18F]fluoroethylnorverapamil ([18F]2) has been described. [18F]1 was obtained in reaction of (R)-norverapamil with the volatile [18F]fluoroethyltriflate acquired from bromoethyltosylate and a silver trilate column with a radiochemical yield of 2.7% ± 1.2%. [18F]2 was radiolabeled by direct fluorination of precursor 13 and required final Boc-deprotection with TFA resulting in a radiochemical yield of 17.2% ± 9.9%. Both tracers, [18F]1 and [18F]2, were administered to Wistar rats, and blood plasma and brain samples were analyzed for metabolic stability. Using [18F]1 and [18F]2, PET scans were performed in Wistar rats at baseline and after blocking with tariquidar, showing a 3.6- and 2.4-fold increase in brain uptake in the blocked rats, respectively. In addition, for both [18F]1 and [18F]2, PET scans in Mdr1a/b(-/-), Bcrp1(-/-), and WT mice were acquired, in which [18F]2 showed a more specific brain uptake in Mdr1a/b(-/-) mice and no increased signal in Bcrp1(-/-) mice. [18F]2 was selected as the best performing tracer and should be evaluated further in clinical studies.

Synthesis, radiolabeling and preclinical evaluation of a [11C]GMOM derivative as PET radiotracer for the ion channel of the N-methyl-D-aspartate receptor

INTRODUCTION

Presently available PET ligands for the NMDAr ion channel generally suffer from fast metabolism. The purpose of this study was to develop a metabolically more stable ligand for the NMDAr ion channel, taking [¹¹C]GMOM ([¹¹C]1) as the lead compound.

METHODS

[¹¹C]1, its fluoralkyl analogue [¹⁸F]PK209 ([¹⁸F]2) and the newly synthesized fluorovinyloxy analogue [¹¹C]7b were evaluated ex vivo in male Wistar rats for metabolic stability. In addition, [¹¹C]7b was subjected to a biodistribution study and its affinity (K_i) and lipophilicity (logD_7.4) values were determined.

RESULTS

The addition of a vinyl chain in the fluoromethoxy moiety did not negatively alter the affinity of [¹¹C]7b for the NMDAr, while lipophilicity was increased. Biodistribution studies showed higher uptake of [¹¹C]7b in forebrain regions compared with cerebellum. Pre-treatment with MK-801 decreased the overall brain uptake significantly, but not in a region-specific manner. 45min after injection 78, 90 and 87% of activity in the brain was due to parent compound for [¹¹C]1, [¹⁸F]2 and [¹¹C]7b, respectively. In plasma, 26-31% of activity was due to parent compound.

CONCLUSION

Complete substitution of the alpha-carbon increased lipophilicity to more favorable values. Substitution of one or more hydrogens of the alpha-carbon atom in the methoxy moiety improved metabolic stability. In plasma, more parent compound was found for [¹⁸F]2 and [¹¹C]7b then for [¹¹C]1, although differences were not significant. At 45min, significantly more parent [¹⁸F]2 and [¹¹C]7b was measured in the brain compared with [¹¹C]1.

Human Dosimetry of the N-Methyl-d-Aspartate Receptor Ligand 11C-GMOM

The methylguanidine derivative ¹¹C-GMOM (¹¹C-labeled N-(2-chloro-3-thiomethylphenyl)-N'-(3-methoxyphenyl)-N'-methylguanidine) has been used successfully to quantify N-methyl-d-aspartate (NMDA) receptor binding in humans. The purpose of the present study was to estimate the ¹¹C-GMOM radiation dose in healthy humans. Methods: After ¹¹C-GMOM injection, 3 female and 2 male subjects underwent 10 consecutive whole-body PET scans in approximately 77 min. Seven source organs were defined manually, scaled to a sex-specific reference, and residence times were calculated for input into OLINDA/EXM software. Accepted tissue-weighting factors were used to calculate the effective dose. Results: The mean absorbed radiation doses in source organs ranged from 7.7 μGy·MBq^-1 in the brain to 12.7 μGy·MBq^-1 in the spleen. The effective dose (±SD) was 4.5 ± 0.5 μSv·MBq^-1Conclusion: The effective dose of ¹¹C-GMOM is at the lower end of the range seen for other ¹¹C-labeled ligands, allowing for serial PET scanning in a single subject.

Altered GABAA receptor density and unaltered blood-brain barrier [11C]flumazenil transport in drug-resistant epilepsy patients with mesial temporal sclerosis

Studies in rodents suggest that flumazenil is a P-glycoprotein substrate at the blood-brain barrier. This study aimed to assess whether [¹¹C]flumazenil is a P-glycoprotein substrate in humans and to what extent increased P-glycoprotein function in epilepsy may confound interpretation of clinical [¹¹C]flumazenil studies used to assess gamma-aminobutyric acid A receptors. Nine drug-resistant patients with epilepsy and mesial temporal sclerosis were scanned twice using [¹¹C]flumazenil before and after partial P-glycoprotein blockade with tariquidar. Volume of distribution, nondisplaceable binding potential, and the ratio of rate constants of [¹¹C]flumazenil transport across the blood-brain barrier (K₁/k₂) were derived for whole brain and several regions. All parameters were compared between pre- and post-tariquidar scans. Regional results were compared between mesial temporal sclerosis and contralateral sides. Tariquidar significantly increased global K₁/k₂ (+23%) and volume of distribution (+10%), but not nondisplaceable binding potential. At the mesial temporal sclerosis side volume of distribution and nondisplaceable binding potential were lower in hippocampus (both ∼-19%) and amygdala (both ∼-16%), but K₁/k₂ did not differ, suggesting that only regional gamma-aminobutyric acid A receptor density is altered in epilepsy. In conclusion, although [¹¹C]flumazenil appears to be a (weak) P-glycoprotein substrate in humans, this does not seem to affect its role as a tracer for assessing gamma-aminobutyric acid A receptor density.