Discovery of a CSF-1R inhibitor and PET tracer for imaging of microglia and macrophages in the brain.

Colony stimulating factor 1 receptor (CSF-1R) is a kinase expressed on macrophages and microglia in the brain. It has been recognized as a potential drug and imaging target in treatment of neuroinflammatory diseases and glioblastoma. Despite several attempts, no validated CSF-1R PET tracer is currently available. The aim of this work was to develop a brain permeable CSF-1R PET tracer for non-invasive imaging of CSF-1R in vivo. Based on fragments of two potent and selective CSF-1R inhibitors, novel hybrid molecules were designed and synthesized. Affinity for human recombinant CSF-1R and selectivity over c-KIT and PDGFR-ß was determined using a FRET based in vitro assay. Radiosynthesis was performed by fully automated [11C]CH3I methylation of the corresponding des-methyl precursor. PET imaging was performed at baseline, efflux transporter blocking and CSF-1R blocking conditions. Moreover, tracer distribution and blood and plasma radiometabolites were determined following injection in healthy mice. The most promising CSF-1R inhibitor, compound 4, showed high selectivity and high affinity for CSF-1R (IC50: 12 ± 3 nM) and no affinity for kinase family members c-KIT and PDGFR-beta. [11C]4 was obtained in good yield (15 ± 0.2 % decay corrected yield, (2.0 ± 0.26 GBq at end of synthesis) and excellent purity. The compound demonstrated high brain penetration and good metabolic stability (>2 %ID/g at 60 min post injection and 79 ± 8 % intact [11C]4 in brain at 60 min post injection) and no strong efflux transporter substrate behavior. Blocking CSF-1R prior to imaging with [11C]4 resulted in significant decrease in brain uptake. In conclusion, [11C]4 shows good potential as a novel PET tracer for imaging of CSF-1R in the CNS and future experiments in relevant animal models are warranted.

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.

Effects of the sigma-1 receptor agonist blarcamesine in a murine model of fragile X syndrome: neurobehavioral phenotypes and receptor occupancy.

Fragile X syndrome (FXS), a disorder of synaptic development and function, is the most prevalent genetic form of intellectual disability and autism spectrum disorder. FXS mouse models display clinically-relevant phenotypes, such as increased anxiety and hyperactivity. Despite their availability, so far advances in drug development have not yielded new treatments. Therefore, testing novel drugs that can ameliorate FXS' cognitive and behavioral impairments is imperative. ANAVEX2-73 (blarcamesine) is a sigma-1 receptor (S1R) agonist with a strong safety record and preliminary efficacy evidence in patients with Alzheimer's disease and Rett syndrome, other synaptic neurodegenerative and neurodevelopmental disorders. S1R's role in calcium homeostasis and mitochondrial function, cellular functions related to synaptic function, makes blarcamesine a potential drug candidate for FXS. Administration of blarcamesine in 2-month-old FXS and wild type mice for 2 weeks led to normalization in two key neurobehavioral phenotypes: open field test (hyperactivity) and contextual fear conditioning (associative learning). Furthermore, there was improvement in marble-burying (anxiety, perseverative behavior). It also restored levels of BDNF, a converging point of many synaptic regulators, in the hippocampus. Positron emission tomography (PET) and ex vivo autoradiographic studies, using the highly selective S1R PET ligand [18F]FTC-146, demonstrated the drug's dose-dependent receptor occupancy. Subsequent analyses also showed a wide but variable brain regional distribution of S1Rs, which was preserved in FXS mice. Altogether, these neurobehavioral, biochemical, and imaging data demonstrates doses that yield measurable receptor occupancy are effective for improving the synaptic and behavioral phenotype in FXS mice. The present findings support the viability of S1R as a therapeutic target in FXS, and the clinical potential of blarcamesine in FXS and other neurodevelopmental disorders.

BLZ945 derivatives for PET imaging of colony stimulating factor-1 receptors in the brain.

BACKGROUND: The kinase colony stimulating factor-1 receptor (CSF-1R) has recently been identified as a novel therapeutic target for decreasing tumor associated macrophages and microglia load in cancer treatment. In glioblastoma multiforme (GBM), a high-grade cancer in the brain with extremely poor prognosis, macrophages and microglia can make up to 50% of the total tumor mass. Currently, no non-invasive methods are available for measuring CSF-1R expression in vivo. The aim of this work is to develop a PET tracer for imaging of CSF-1R receptor expression in the brain for future GBM patient selection and treatment monitoring. METHODS: BLZ945 and a derivative that potentially allows for fluorine-18 labeling were synthesized and evaluated in vitro to determine their affinity towards CSF-1R. BLZ945 was radiolabeled with carbon-11 by N-methylation of des-methyl-BLZ945 using [11C]CH3I. Following administration to healthy mice, metabolic stability of [11C]BLZ945 in blood and brain and activity distribution were determined ex vivo. PET scanning was performed at baseline, efflux transporter blocking, and CSF-1R blocking conditions. Finally, [11C]BLZ945 binding was evaluated in vitro by autoradiography on mouse brain sections. RESULTS: BLZ945 was the most potent compound in our series with an IC50 value of 6.9 ± 1.4 nM. BLZ945 was radiolabeled with carbon-11 in 20.7 ± 1.1% decay corrected radiochemical yield in a 60 min synthesis procedure with a radiochemical purity of >95% and a molar activity of 153 ± 34 GBq·µmol-1. Ex vivo biodistribution showed moderate brain uptake and slow wash-out, in addition to slow blood clearance. The stability of BLZ945 in blood plasma and brain was >99% at 60 min post injection. PET scanning demonstrated BLZ945 to be a substrate for efflux transporters. High brain uptake was observed, which was shown to be mostly non-specific. In accordance, in vitro autoradiography on brain sections revealed high non-specific binding. CONCLUSIONS: [11C]BLZ945, a CSF-1R PET tracer, was synthesized in high yield and purity. The tracer has high potency for the target, however, future studies are warranted to address non-specific binding and tracer efflux before BLZ945 or derivatives could be translated into humans for brain imaging.

Evaluation of carbon-11 labeled 5-(1-methyl-1H-pyrazol-4-yl)-N-(2-methyl-5-(3-(trifluoromethyl)benzamido)phenyl)nicotinamide as PET tracer for imaging of CSF-1R expression in the brain.

Pharmacological targeting of tumor associated macrophages and microglia in the tumor microenvironment is a novel therapeutic strategy in the treatment of glioblastoma multiforme. As such, the colony stimulating factor-1 receptor (CSF-1R) has been identified as a druggable target. However, no validated companion diagnostic marker for these therapies exists to date. Towards development of a CSF-1R PET tracer, a set of six compounds based on recently reported CSF-1R inhibitor 5-(1-methyl-1H-pyrazol-4-yl)-N-(2-methyl-5-(3-(trifluoromethyl)benzamido)phenyl)nicotinamide (Compound 5) was designed, synthesized and evaluated in vitro for potency and selectivity. The highest affinity for CSF-1R was found for compound 5 (IC50: 2.7 nM). Subsequent radiosynthesis of [11C]5 was achieved in 2.0 ± 0.2% yield (decay corrected to start of synthesis) by carbon-11 carbon monoxide aminocarbonylation in 40 min after end of bombardment. In vitro autoradiography with [11C]5 on rat brain sections demonstrated high specific binding, but also strong off-target binding. Ex vivo, only intact tracer was observed in blood plasma at 90 min post injection in healthy rats. PET scanning results demonstrated negligible brain uptake under baseline conditions and this brain uptake did not increase by blocking of efflux transporters using Tariquidar. To conclude, [11C]5 was successfully synthesized and evaluated in healthy rats. However, the inability of [11C]5 to cross the blood-brain-barrier excludes its use for imaging of CSF-1R expression in the brain.

Efficient synthesis of carbon-11 labelled acylsulfonamides using [11C]CO carbonylation chemistry.

Herein, a novel method for carbon-11 labeling of acyl sulfonamides by a one-step insertive [11C]CO carbonylative cross-coupling reaction between aryl halides and sulfonamides is presented. Various model compounds as well as drug molecules LY573636 (tasisulam) and ABT-199 were obtained in excellent yields. This method provides a valuable and widely applicable contribution to the continuously expanding radiochemical toolbox for PET research.

A [11 C] CO dispensing system for rapid screening of carbonylation reactions.

[11 C] CO is a highly versatile synthon that allows for labeling at carbonyl positions of many molecules by means of transition metal-mediated carbonylation reactions. The intrinsic complexity of carbonylation reactions often requires tedious screening of reaction conditions for obtaining satisfying yields. Herein, a [11 C] CO dispending system for performing multiple reactions with a single batch of cyclotron-produced [11 C]CO2 is described. This semiautomated setup allows for more rapid and efficient screening of reactions and reaction conditions compared with the traditional "one beam for one reaction" strategy.

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.

Strategies towards in vivo imaging of active transglutaminase type 2 using positron emission tomography.

Transglutaminase type 2 (TG2) is increasingly linked to the pathogenesis of several diseases, such as celiac disease, cancer, and fibrotic and neurodegenerative diseases. In parallel with becoming an attractive target for therapy, interest in the development of compounds for in vivo imaging of TG2 is rising. Such imaging biomarkers might assist in clarifying the role of TG2 in pathology and in monitoring TG2 inhibition in vivo and thus assist in drug development. In this review, the latest results together with various strategies in TG2 PET tracer development are discussed, including radiolabelling of irreversible and reversible active-site inhibitors, as well as allosteric inhibitors, acyl-donor and acyl-acceptor substrates, and anti-TG2 monoclonal antibodies.

Development of fluorine-18 labeled peptidic PET tracers for imaging active tissue transglutaminase.

INTRODUCTION: The protein-protein crosslinking activity of the enzyme tissue transglutaminase (TG2; EC 2.3.2.13) is associated with the pathogenesis of various diseases, including celiac disease, lung-, liver- and kidney fibrosis, cancer and neurodegenerative diseases. This study aims at developing a TG2 PET tracer based on the peptidic irreversible TG2 inhibitor Z006. METHODS: Initially, the carbon-11 labeling of Z006 at the diazoketone position was explored. Subsequently, a set of analogues that allow for fluorine-18 labeling was synthesized. Two potent analogues, 6f and 6g, were radiolabeled with fluorine-18 and biodistribution and metabolite analysis in Wistar rats was performed. The identity of the main metabolite of [18F]6g was elucidated using LC-MS/MS. In vitro binding to isolated TG2 and in vitro autoradiography on MDA-MB-231 breast cancer tissue using [18F]6g was performed. RESULTS: [18F]6f and [18F]6g were obtained in 20 and 9% yields, respectively. Following administration to healthy Wistar rats, rapid metabolism of both tracers was observed. Remarkably, full conversion to just one single metabolite was observed for one of the tracers, [18F]6g. By LC-MS/MS analysis this metabolite was identified as C-terminally saponified [18F]6g. This metabolite was also found to be a potent TG2 inhibitor in vitro. In vitro binding to isolated TG2 and in vitro autoradiography on MDA-MB-231 tumor sections using [18F]6g demonstrated high specific and selective binding of [18F]6g to active TG2. CONCLUSIONS: Whereas based on the intensive metabolism [18F]6f seems unsuitable as a TG2 PET tracer, the results warrant further evaluation of [18F]6gin vivo.

Development of carbon-11 labeled acryl amides for selective PET imaging of active tissue transglutaminase.

INTRODUCTION: Tissue transglutaminase (TG2) is a ubiquitously expressed enzyme capable of forming metabolically and mechanically stable crosslinks between the γ-carboxamide of a glutamine acyl-acceptor substrate and the ε-amino functionality of a lysine acyl-donor substrate resulting in protein oligomers. High TG2 crosslinking activity has been implicated in the pathogenesis of various diseases including celiac disease, cancer and fibrotic and neurodegenerative diseases. Development of a PET tracer specific for active TG2 provides a novel tool to further investigate TG2 biology in vivo in disease states. Recently, potent irreversible active site TG2 inhibitors carrying an acrylamide warhead were synthesized and pharmacologically characterized. METHODS: Three of these inhibitors, compound 1, 2 and 3, were successfully radiolabeled with carbon-11 on the acrylamide carbonyl position using a palladium mediated [(11)C]CO aminocarbonylation reaction. Ex vivo biodistribution and plasma stability were evaluated in healthy Wistar rats. Autoradiography was performed on MDA-MB-231 tumor sections. RESULTS: [(11)C]1, -2 and -3 were obtained in decay corrected radiochemical yields of 38-55%. Biodistribution showed low uptake in peripheral tissues, with the exception of liver and kidney. Low brain uptake of <0.05% ID/g was observed. Blood plasma analysis demonstrated that [(11)C]1 and [(11)C]2 were rapidly metabolized, whereas [(11)C]3 was metabolized at a more moderate rate (63.2 ± 6.8 and 28.7 ± 10.8% intact tracer after 15 and 45 min, respectively). Autoradiography with [(11)C]3 on MDA-MB-231 tumor sections showed selective and specific binding of the radiotracer to the active state of TG2. CONCLUSIONS: Taken together, these results identify [(11)C]3 as the most promising of the three compounds tested for development as PET radiotracer for the in vivo investigation of TG2 activity.

[¹¹C]Sorafenib: radiosynthesis and preclinical evaluation in tumor-bearing mice of a new TKI-PET tracer.

INTRODUCTION: Tyrosine kinase inhibitors (TKIs) like sorafenib are important anticancer therapeutics with thus far limited treatment response rates in cancer patients. Positron emission tomography (PET) could provide the means for selection of patients who might benefit from TKI treatment, if suitable PET tracers would be available. The aim of this study was to radiolabel sorafenib (1) with carbon-11 and to evaluate its potential as TKI-PET tracer in vivo. METHODS: Synthetic methods were developed in which sorafenib was labeled at two different positions, followed by a metabolite analysis in rats and a PET imaging study in tumor-bearing mice. RESULTS: [methyl-(11)C]-1 and [urea-(11)C]-1 were synthesized in yields of 59% and 53%, respectively, with a purity of >99%. The identity of the products was confirmed by coinjection on HPLC with reference sorafenib. In an in vivo metabolite analysis [(11)C]sorafenib proved to be stable. The percentage of intact product in blood-plasma after 45 min was 90% for [methyl-(11)C]-1 and 96% for [urea-(11)C]-1, respectively. Due to the more reliable synthesis, further research regarding PET imaging was performed with [methyl-(11)C]-1 in nude mice bearing FaDu (head and neck cancer), MDA-MB-231 (breast cancer) or RXF393 (renal cancer) xenografts. Highest tracer accumulation at a level of 2.52 ± 0.33%ID/g was observed in RXF393, a xenograft line extensively expressing the sorafenib target antigen Raf-1 as assessed by immunohistochemistry. CONCLUSION: In conclusion, we have synthesized [(11)C]sorafenib as PET tracer, which is stable in vivo and has the capability to be used as PET tracer for imaging in tumor-bearing mice.

Tissue transglutaminase colocalizes with extracellular matrix proteins in cerebral amyloid angiopathy.

Cerebral amyloid angiopathy (CAA) is a key histopathological hallmark of Alzheimer's disease (AD) and hereditary cerebral hemorrhage with amyloidosis of the Dutch type (HCHWA-D). CAA is characterized by amyloid-beta (Aß) depositions and remodeling of the extracellular matrix (ECM) in brain vessels and plays an important role in the development and progression of both AD and HCHWA-D. Tissue transglutaminase (tTG) modulates the ECM by molecular cross-linking of ECM proteins. Here, we investigated the distribution pattern, cellular source, and activity of tTG in CAA in control, AD, and HCHWA-D cases. We observed increased tTG immunoreactivity and colocalization with Aß in the vessel wall in early stage CAA, whereas in later CAA stages, tTG and its cross-links were present in halos enclosing the Aß deposition. In CAA, tTG and its cross-links at the abluminal side of the vessel were demonstrated to be either of astrocytic origin in parenchymal vessels, of fibroblastic origin in leptomeningeal vessels, and of endothelial origin at the luminal side of the deposited Aß. Furthermore, the ECM proteins fibronectin and laminin colocalized with the tTG-positive halos surrounding the deposited Aß in CAA. However, we observed that in situ tTG activity was present throughout the vessel wall in late stage CAA. Together, our data suggest that tTG and its activity might play a differential role in the development and progression of CAA, possibly evolving from direct modulation of Aß aggregation to cross-linking of ECM proteins resulting in ECM restructuring.

Spacer effects on in vivo properties of DOTA-conjugated dimeric [Tyr3]octreotate peptides synthesized by a "Cu(I)-click" and "sulfo-click" ligation method.

We report on the SSTR2-binding properties of a series of four dimeric [Tyr3]octreotate analogues with different spacer lengths (nine, 19, 41, and 57 atoms) between the peptides. Two analogues (9 and 57 atoms) were selected as precursors for the design, synthesis, and biological evaluation of DOTA-conjugated dimeric [Tyr3]octreotate analogues for tumor targeting. These compounds were synthesized by using a two-stage click ligation procedure: a Cu(I) -catalyzed 1,3-dipolar cycloaddition ("copper-click" reaction) and a thio acid/sulfonyl azide amidation ("sulfo-click" reaction). The IC(50) values of these DOTA-conjugated [Tyr3]octreotate analogues were comparable, and internalization studies showed that the nine-atom (111) In-DOTA-labeled [Tyr3]octreotate dimer had rapid and high receptor binding. Biodistribution studies with BALB/c nude mice bearing subcutaneous AR42J tumors showed that the (111) In-labeled [Tyr3]octreotate dimer (nine atoms) had a high tumor uptake at 1 h p.i. (38.8 ± 8.3 % ID g(-1) ), and excellent tumor retention at 4 h p.i. (40.9 ± 2.5 % ID g(-1) ). However, the introduction of the extended hydrophilic 57 atoms spacer led to rapid clearance from the circulation; this limited tumor accumulation of the radiotracer (21.4 ± 4.9 % ID g(-1) at 1 h p.i.). These findings provide important insight on dimerization and spacer effects on the in vivo properties of DOTA-conjugated [Tyr3]octreotate dimers.