Synthesis and in Vitro and in Vivo Evaluation of SiFA-Tagged Bombesin and RGD Peptides as Tumor Imaging Probes for Positron Emission Tomography

Gastrin-releasing-peptide (GRP)-receptors and α_vβ₃-integrins are widely discussed as potential target structures for oncological imaging with positron emission tomography (PET). Favored by the overexpression of receptors on the surface of tumor cells good imaging characteristics can be achieved with highly specific radiolabeled receptor ligands. PEGylated bombesin (PESIN) derivatives as specific GRP receptor ligands and RGD (one-letter codes for arginine-glycine-aspartic acid) peptides as specific α_vβ₃ binders were synthesized and tagged with a silicon-fluorine-acceptor (SiFA) moiety. The SiFA synthon allows for a fast and highly efficient isotopic exchange reaction at room temperature giving the [¹⁸F]­fluoride labeled peptides in up to 62% radiochemical yields (d.c.) and ≥99% radiochemical purity in a total synthesis time of less than 20 min. Using nanomolar quantities of precursor high specific activities of up to 60 GBq μmol^–1 were obtained. To compensate the high lipophilicity of the SiFA moiety various hydrophilic structure modifications were introduced leading to significantly reduced logD values. Competitive displacement experiments with the PESIN derivatives showed a 32 to 6 nM affinity to the GRP receptor on PC3 cells, and with the RGD peptides a 7 to 3 μM affinity to the α_vβ₃ integrins on U87MG cells. All derivatives proved to be stable in human plasma over at least 120 min. Small animal PET measurements and biodistribution studies revealed an enhanced and specific accumulation of the RGD peptide ¹⁸F-SiFA-LysMe₃-γ-carboxy-d-Glu-RGD (<b>17</b>) in the tumor tissue of U87MG tumor-bearing mice of 5.3% ID/g whereas the PESIN derivatives showed a high liver uptake and only a low accumulation in the tumor tissue of PC3 xenografts. Stability studies with compound <b>17</b> provided further information on its metabolism in vivo. These results altogether demonstrate that the reduction of the overall lipophilicity of SiFA tagged RGD peptides is a promising approach for the generation of novel potent ¹⁸F-labeled imaging agents

Development and Preclinical Evaluation of a Copper-64-Labeled Antibody Targeting Glycine-Alanine Dipeptides for PET Imaging of C9orf72‑Associated Amyotrophic Lateral Sclerosis/Frontotemporal Dementia

Aggregating poly(glycine-alanine) (poly-GA) is derived from the unconventional translation of the pathogenic intronic hexanucleotide repeat expansion in the C9orf72 gene, which is the most common genetic cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). Poly-GA accumulates predominantly in neuronal cytoplasmic inclusions unique to C9orf72 ALS/FTD patients. Poly-GA is, therefore, a promising target for PET/CT imaging of FTD/ALS to monitor disease progression and therapeutic interventions. A novel 64Cu-labeled anti-GA antibody (mAb1A12) targeting the poly-GA protein was developed and evaluated in a transgenic mouse model. It was obtained with high radiochemical purity (RCP), radiochemical yield (RCY), and specific activity, and showed high stability in vitro and ex vivo and specifically bound to poly-GA. The affinity of NODAGA-mAb1A12 for poly-GA was not affected by this modification. [64Cu]Cu-NODAGA-mAb1A12 was injected into transgenic mice expressing GFP-(GA)175 in excitatory neurons driven by Camk2a-Cre and in control littermates. PET/CT imaging was performed at 2, 20, and 40 h post-injection (p.i.) and revealed a higher accumulation in the cortex in transgenic mice than in wild-type mice, as reflected by higher standardized uptake value ratios (SUVR) using the cerebellum as the reference region. The organs were isolated for biodistribution and ex vivo autoradiography. Autoradiography revealed a higher cortex-to-cerebellum ratio in the transgenic mice than in the controls. Results from autoradiography were validated by immunohistochemistry and poly-GA immunoassays. Moreover, we confirmed antibody uptake in the CNS in a pharmacokinetic study of the perfused tissues. In summary, [64Cu]Cu-NODAGA-mAb1A12 demonstrated favorable in vitro characteristics and an increased relative binding in poly-GA transgenic mice compared to wild-type mice in vivo. Our results with this first-in-class radiotracer suggested that targeting poly-GA is a promising approach for PET/CT imaging in FTD/ALS

A Kinome-Wide Selective Radiolabeled TrkB/C Inhibitor for in Vitro and in Vivo Neuroimaging: Synthesis, Preclinical Evaluation, and First-in-Human

The proto-oncogenes <i>NTRK1/2/3</i> encode the tropomyosin receptor kinases TrkA/B/C which play pivotal roles in neurobiology and cancer. We describe herein the discovery of [¹¹C]-(<i>R</i>)-<b>3</b> ([¹¹C]-(<i>R</i>)-IPMICF16), a first-in-class positron emission tomography (PET) TrkB/C-targeting radiolabeled kinase inhibitor lead. Relying on extensive human kinome vetting, we show that (<i>R</i>)-<b>3</b> is the most potent and most selective TrkB/C inhibitor characterized to date. It is demonstrated that [¹¹C]-(<i>R</i>)-<b>3</b> readily crosses the blood–brain barrier (BBB) in rodents and selectively binds to TrkB/C receptors in vivo, as evidenced by entrectinib blocking studies. Substantial TrkB/C-specific binding in human brain tissue is observed in vitro, with specific reduction in the hippocampus of Alzheimer’s disease (AD) versus healthy brains. We additionally provide preliminary translational data regarding the brain disposition of [¹¹C]-(<i>R</i>)-<b>3</b> in primates including first-in-human assessment. These results illustrate for the first time the use of a kinome-wide selective radioactive chemical probe for endogenous kinase PET neuroimaging in human

Additional file 1 of Associations between sex, body mass index and the individual microglial response in Alzheimer’s disease

Additional file 1: Table S1. Detailed regional z-scores of TSPO-PET and Aβ-PET for female and male AD patients in six Braak-stage regions of interest and four amyloidosis regions of interest. CI =95% confidence interval. P –values show false discovery rate (FDR) corrected significance levels for the comparison of medium and high affinity binders (ANOVA). Figure S1. Validation of late-phase [18F]PI-2620 tau-PET quantification via carotid artery image derived input function (IDIF). Images show IDIF derived volume of distribution (VT) of [18F]PI-2620 tau-PET for female and male AD patients and mixed sex cognitively normal individuals, presented as axial overlays on a standard magnetic resonance imaging template. Plots show correlation of tau-PET z-scores for Braak-stage regions I–VI with tau-PET VT. AD female n = 13, AD male n = 9, cognitively normal mixed sex n = 3