CTNI-62. LONG-TERM EFFICACY AND SAFETY OF LAROTRECTINIB IN PATIENTS WITH TROPOMYOSIN RECEPTOR KINASE (TRK) FUSION PRIMARY CENTRAL NERVOUS SYSTEM (CNS) TUMORS

BACKGROUND

Larotrectinib is a highly selective TRK inhibitor that demonstrated an objective response rate (ORR) of 30% and a 24-week disease control rate (DCR) of 73% across 33 evaluable adult and pediatric patients with TRK fusion primary CNS tumors, as of July 2020 (Doz et al, Neuro Oncol 2021). We report updated data on an expanded dataset.

METHODS

Patients with TRK fusion primary CNS tumors in two clinical trials (NCT02637687, NCT02576431) were included. Responses were investigator-assessed.

RESULTS

As of July 2021, 38 patients with TRK fusion primary CNS tumors (median age, 10.8 [range 1.3–79.0] years) were identified: high-grade glioma (HGG; n = 23), low-grade glioma (LGG; n = 9), and other (n = 6). Sixteen (42%) patients had ≥ 2 prior systemic therapies. ORR for 37 evaluable patients was 30% (95% confidence interval [CI] 16–47): three complete responses, eight partial responses, 21 stable disease (16 patients ≥ 24 weeks), and five progressive disease. For pediatric patients (n = 28), the ORR was 39% (95% CI 22–59). For pediatric patients with HGG and LGG, ORRs were 43% (95% CI 18–71) and 38% (95% CI 9–76), respectively. ORRs for patients with 0, 1, 2, and ≥ 3 prior therapies were 33%, 20%, 38%, and 38%, respectively. Median time to response was 1.9 months. The 24-week DCR was 73% (95% CI 56–86). Median duration of response (DoR) was not reached; 12-month DoR rate was 64%. Median progression-free survival was 16.5 months (95% CI 6.7–not estimable). Median overall survival (OS) was not reached; 24-month OS rate was 65%. Treatment duration ranged from 0.1+ to 38.7+ months. Treatment-related adverse events (TRAEs) were mostly Grade 1–2. No patients discontinued treatment due to TRAEs.

CONCLUSION

Larotrectinib demonstrated a high DCR, rapid and durable responses, and a manageable safety profile in patients with TRK fusion primary CNS tumors.

Preliminary Evaluations of [11C]Verubulin: Implications for Microtubule Imaging With PET

[¹¹C]Verubulin (a.k.a.[¹¹C]MCP-6827), [¹¹C]HD-800 and [¹¹C]colchicine have been developed for imaging microtubules (MTs) with positron emission tomography (PET). The objective of this work was to conduct an in vivo comparison of [¹¹C]verubulin for MT imaging in mouse and rat brain, as well as an in vitro study with this radiotracer in rodent and human Alzheimer’s Disease tissue. Our preliminary PET imaging studies of [¹¹C]verubulin in rodents revealed contradictory results between mouse and rat brain uptake under pretreatment conditions. In vitro autoradiography with [¹¹C]verubulin showed an unexpected higher uptake in AD patient tissue compared with healthy controls. We also conducted the first comparative in vivo PET imaging study with [¹¹C]verubulin, [¹¹C]HD-800 and [¹¹C]colchicine in a non-human primate. [¹¹C]Verubulin and [¹¹C]HD-800 require pharmacokinetic modeling and quantification studies to understand the role of how these radiotracers bind to MTs before translation to human use.

Radiofluorination of oxazole-carboxamides for preclinical PET neuroimaging of GSK-3

Glycogen synthase kinase 3 (GSK-3) is an enzyme that is dysregulated in oncology neurodegeneration, neuroinflammation and several mental health illnesses. As such, GSK-3 is a long-sought after target for positron emission tomography (PET) imaging and therapeutic intervention. Herein, we report on the development and radiofluorination of two oxazole-4-carboxamides, including one bearing a non-activated aromatic ring. Both compounds demonstrated excellent selectivity in a kinase screen and inhibit GSK-3 with high affinity. [¹⁸F]OCM-49 was synthesized from [¹⁸F]fluoride using a copper-mediated reaction of an aryl boronic acid precursor, while [¹⁸F]OCM-50 used a trimethylammonium triflate precursor, and both radiotracers were translated for preclinical PET imaging in rodents. Due to superior radiochemical yields and brain uptake (peak standardized uptake value of ~2.0), [¹⁸F]OCM-50 was further evaluated in non-human primate and also showed good brain uptake and rapid clearance. Further studies to consider clinical translation of both radiotracers are underway.

Efficient radiosynthesis and preclinical evaluation of [18 F]FOMPyD as a positron emission tomography tracer candidate for TrkB/C receptor imaging

Herein we report an efficient radiolabeling of a ¹⁸ F-fluorinated derivative of dual inhibitor GW2580, with its subsequent evaluation as a positron emission tomography (PET) tracer candidate for imaging of two neuroreceptor targets implicated in the pathophysiology of neurodegeneration: tropomyosin receptor kinases (TrkB/C) and colony stimulating factor receptor (CSF-1R). [¹⁸ F]FOMPyD was synthesized from a boronic acid pinacolate precursor via copper-mediated ¹⁸ F-fluorination concerted with thermal deprotection of the four Boc groups on a diaminopyrimidine moiety in an 8.7±2.8% radiochemical yield, a radiochemical purity >99%, and an effective molar activity of 187±93 GBq/μmol. [¹⁸ F]FOMPyD showed moderate brain permeability in wild-type rats (SUV_max = 0.75) and a slow washout rate. The brain uptake was partially reduced (ΔAUC_40-90 = 11.6%) by administration of the nonradioactive FOMPyD (up to 30 μg/kg). In autoradiography, [¹⁸ F]FOMPyD exhibits ubiquitous distribution in rat and human brain tissues with relatively high nonspecific binding revealed by self-blocking experiment. The binding was blocked by TrkB/C inhibitors, but not with a CSF-1R inhibitor, suggesting selective binding to the former receptor. Although an unfavorable pharmacokinetic profile will likely preclude application of [¹⁸ F]FOMPyD as a PET tracer for brain imaging, the concomitant one-pot copper-mediated ¹⁸ F-fluorination/Boc-deprotection is a practical technique for the automated radiosynthesis of acid-sensitive PET tracers.

Structural Basis for Achieving GSK-3β Inhibition with High Potency, Selectivity, and Brain Exposure for Positron Emission Tomography Imaging and Drug Discovery

Using structure-guided design, several cell based assays, and microdosed positron emission tomography (PET) imaging, we identified a series of highly potent, selective, and brain-penetrant oxazole-4-carboxamide-based inhibitors of glycogen synthase kinase-3 (GSK-3). An isotopologue of our first-generation lead, [3H]PF-367, demonstrates selective and specific target engagement in vitro, irrespective of the activation state. We discovered substantial ubiquitous GSK-3-specific radioligand binding in Tg2576 Alzheimer's disease (AD), suggesting application for these compounds in AD diagnosis and identified [11C]OCM-44 as our lead GSK-3 radiotracer, with optimized brain uptake by PET imaging in nonhuman primates. GSK-3β-isozyme selectivity was assessed to reveal OCM-51, the most potent (IC50 = 0.030 nM) and selective (>10-fold GSK-3β/GSK-3α) GSK-3β inhibitor known to date. Inhibition of CRMP2T514 and tau phosphorylation, as well as favorable therapeutic window against WNT/β-catenin signaling activation, was observed in cells.

First-in-Human Brain Imaging of [18F]TRACK, a PET tracer for Tropomyosin Receptor Kinases

The tropomyosin receptor kinase TrkA/B/C family is responsible for human neuronal growth, survival, and differentiation from early nervous system development stages onward. Downregulation of TrkA/B/C receptors characterizes numerous neurological disorders including Alzheimer's disease (AD). Abnormally expressed Trk receptors or chimeric Trk fusion proteins are also well-characterized oncogenic drivers in a variety of neurogenic and non-neurogenic human neoplasms and are currently the focus of intensive clinical research. Previously, we have described the clinical translation of a highly selective and potent carbon-11-labeled pan-Trk radioligand and the preclinical characterization of the optimized fluorine-18-labeled analogue, [¹⁸F]TRACK, for in vivo Trk positron emission tomography (PET) imaging. We describe herein central nervous system selectivity assessment and first-in-human study of [¹⁸F]TRACK.

Probing Anti-inflammatory Properties Independent of NF-κB Through Conformational Constraint of Peptide-Based Interleukin-1 Receptor Biased Ligands

Interleukin-1β (IL-1β) binds to the IL-1 receptor (IL-1R) and is a key cytokine mediator of inflammasome activation. IL-1β signaling leads to parturition in preterm birth (PTB) and contributes to the retinal vaso-obliteration characteristic of oxygen-induced retinopathy (OIR) of premature infants. Therapeutics targeting IL-1β and IL-1R are approved to treat rheumatoid arthritis; however, all are large proteins with clinical limitations including immunosuppression, due in part to inhibition of NF-κB signaling, which is required for immuno-vigilance and cytoprotection. The all-D-amino acid peptide 1 (101.10, H-d-Arg-d-Tyr-d-Thr-d-Val-d-Glu-d-Leu-d-Ala-NH₂) is an allosteric IL-1R modulator, which exhibits functional selectivity and conserves NF-κB signaling while inhibiting other IL-1-activated pathways. Peptide 1 has proven effective in experimental models of PTB and OIR. Seeking understanding of the structural requirements for the activity and biased signaling of 1, a panel of twelve derivatives was synthesized employing the various stereochemical isomers of α-amino-γ-lactam (Agl) and α-amino-β-hydroxy-γ-lactam (Hgl) residues to constrain the D-Thr-D-Val dipeptide residue. Using circular dichroism spectroscopy, the peptide conformation in solution was observed to be contingent on Agl, Hgl, and Val stereochemistry. Moreover, the lactam mimic structure and configuration influenced biased IL-1 signaling in an in vitro panel of cellular assays as well as in vivo activity in murine models of PTB and OIR. Remarkably, all Agl and Hgl analogs of peptide 1 did not inhibit NF-κB signaling but blocked other pathways, such as JNK and ROCK2 phosphorylation contingent on structure and configuration. Efficacy in preventing preterm labor correlated with a capacity to block IL-1β-induced IL-1β synthesis. Furthermore, the importance of inhibition of JNK and ROCK2 phosphorylation for enhanced activity was highlighted for prevention of vaso-obliteration in the OIR model. Taken together, lactam mimic structure and stereochemistry strongly influenced conformation and biased signaling. Selective modulation of IL-1 signaling was proven to be particularly beneficial for curbing inflammation in models of preterm labor and retinopathy of prematurity (ROP). A class of biased ligands has been created with potential to serve as selective probes for studying IL-1 signaling in disease. Moreover, the small peptide mimic prototypes are promising leads for developing immunomodulatory therapies with easier administration and maintenance of beneficial effects of NF-κB signaling.

Fluorine-18: an untapped resource in inorganic chemistry

Advances in the field of fluorine chemistry have been applied extensively to the syntheses of 18F-labelled organic compounds and radiopharmaceuticals. However, 18F has sparely been used as a tool to explore inorganic chemistry and can be viewed as a research area worthy of further development. This review highlights the application of 18F in development of inorganic fluorinating agents, mechanistic studies and imaging tools.

Automated synthesis of PET radiotracers by copper-mediated 18 F-fluorination of organoborons: Importance of the order of addition and competing protodeborylation

In this paper, we describe the use of Cu-mediated [18 F]fluorodeboronation for the automated production of positron emission tomography radiotracers suitable for clinical use. Two recurrent issues with the method, low radiochemical conversion on automation and protoarene byproduct purification issues, have been successfully addressed. The new method was utilized to produce sterile injectable doses of [18 F]-(±)-IPMICF17, a positron emission tomography radiotracer for tropomyosin receptor kinase B/C, using an automated synthesis module. The product was isolated in 1.9 ± 0.1% isolated radiochemical yield, excellent radiochemical purity (>99%), and high specific activity (5294 ± 1227 Ci/mmol). Quality control testing confirmed that doses were suitable for clinical use.

Emerging PET Radiotracers and Targets for Imaging of Neuroinflammation in Neurodegenerative Diseases: Outlook Beyond TSPO

The dynamic and multicellular processes of neuroinflammation are mediated by the nonneuronal cells of the central nervous system, which include astrocytes and the brain's resident macrophages, microglia. Although initiation of an inflammatory response may be beneficial in response to injury of the nervous system, chronic or maladaptive neuroinflammation can have harmful outcomes in many neurological diseases. An acute neuroinflammatory response is protective when activated neuroglia facilitate tissue repair by releasing anti-inflammatory cytokines and neurotrophic factors. On the other hand, chronic neuroglial activation is a major pathological mechanism in neurodegenerative diseases, likely contributing to neuronal dysfunction, injury, and disease progression. Therefore, the development of specific and sensitive probes for positron emission tomography (PET) studies of neuroinflammation is attracting immense scientific and clinical interest. An early phase of this research emphasized PET studies of the prototypical imaging biomarker of glial activation, translocator protein-18 kDa (TSPO), which presents difficulties for quantitation and lacks absolute cellular specificity. Many alternate molecular targets present themselves for PET imaging of neuroinflammation in vivo, including enzymes, intracellular signaling molecules as well as ionotropic, G-protein coupled, and immunoglobulin receptors. We now review the lead structures in radiotracer development for PET studies of neuroinflammation targets for neurodegenerative diseases extending beyond TSPO, including glycogen synthase kinase 3, monoamine oxidase-B, reactive oxygen species, imidazoline-2 binding sites, cyclooxygenase, the phospholipase A2/arachidonic acid pathway, sphingosine-1-phosphate receptor-1, cannabinoid-2 receptor, the chemokine receptor CX3CR1, purinergic receptors: P2X7 and P2Y12, the receptor for advanced glycation end products, Mer tyrosine kinase, and triggering receptor expressed on myeloid cells-1. We provide a brief overview of the cellular expression and function of these targets, noting their selectivity for astrocytes and/or microglia, and highlight the classes of PET radiotracers that have been investigated in early-stage preclinical or clinical research studies of neuroinflammation.

Recent Advances in 18F Radiochemistry: A Focus on B-18F, Si-18F, Al-18F, and C-18F Radiofluorination via Spirocyclic Iodonium Ylides

Straightforward radiosynthesis protocols for ¹⁸F-labeled radiopharmaceuticals are an indispensable but often overlooked prerequisite to successfully perform molecular imaging studies in vivo by PET. In recent years, thanks to the expansion of the ¹⁸F chemical toolbox, structurally diverse and novel clinically relevant radiopharmaceuticals have been synthesized with both high efficiency and ready implementation. This article provides an overview of recent ¹⁸F-labeling methodologies, specifically for B-¹⁸F, Si-¹⁸F, Al-¹⁸F, and iodine (III)-mediated radiofluorination via the spirocyclic iodonium ylide technology.

Discovery of PET radiopharmaceuticals at the academia-industry interface

Project-specific collaborations between academia and pharmaceutical partners are a growing phenomenon within molecular imaging and in particular in the positron emission tomography (PET) radiopharmaceutical community. This cultural shift can be attributed in part to decreased public funding in academia in conjunction with the increased reliance on outsourcing of chemistry, radiochemistry, pharmacology and molecular imaging studies by the pharmaceutical industry. This account highlights some of our personal experiences working with industrial partners to develop new PET radiochemistry methodologies for drug discovery and neuro-PET research studies. These symbiotic academic-industrial partnerships have not only led to novel radiotracers for new targets but also to the application of new carbon-11 and fluorine-18 labeling methodologies and technologies to label previously unprecedented compounds for in vivo evaluations.

Small Prosthetic Groups in 18F-Radiochemistry: Useful Auxiliaries for the Design of 18F-PET Tracers

Prosthetic group (PG) applications in ¹⁸F-radiochemistry play a pivotal role among current ¹⁸F^-labeling techniques for the development and availability of ¹⁸F-labeled imaging probes for PET (Wahl, 2002) (¹). The introduction and popularization of PGs in the mid-80s by pioneers in ¹⁸F-radiochemistry has profoundly changed the landscape of available tracers for PET and has led to a multitude of new imaging agents based on simple and efficiently synthesized PGs. Because of the chemical nature of anionic ¹⁸F^- (apart from electrophilic low specific activity ¹⁸F-fluorine), radiochemistry before the introduction of PGs was limited to simple nucleophilic substitutions of leaving group containing precursor molecules. These precursors were not always available, and some target compounds were either hard to synthesize or not obtainable at all. Even with the advent of recently introduced "late-stage fluorination" techniques for the ¹⁸F-fluorination of deactivated aromatic systems, PGs will continue to play a central role in ¹⁸F-radiochemistry because of their robust and almost universal usability. The importance of PGs in radiochemistry is shown by its current significance in tracer development and exemplified by an overview of selected methodologies for PG attachment to PET tracer molecules. Especially, click-chemistry approaches to PG conjugation, while furthering the historical evolution of PGs in PET tracer design, play a most influential role in modern PG utilization. All earlier and recent multifaceted approaches in PG development have significantly enriched the contingent of modern ¹⁸F-radiochemistry procedures and will continue to do so.

Tropomyosin receptor kinase inhibitors: an updated patent review for 2010-2016 - Part II

INTRODUCTION

TrkA/B/C receptor activation supports growth, survival, and differentiation of discrete neuronal populations during development, adult life, and ageing but also plays numerous roles in human disease onset and progression. Trk-specific inhibitors have therapeutic applications in cancer and pain and thus constitute a growing area of interest in oncology and neurology. There has been substantial growth in the number of structural classes of Trk inhibitors and the number of industrial entrants to the Trk inhibitor field over the past six years. Areas covered: In Part II of this two-part review, the discussion of recent patent literature covering Trk family inhibitors is continued from Part I and clinical research with Trk inhibitors is considered. Expert opinion: Trk has been molecularly targeted for over a decade resulting in the progressive evolution of structurally diversified Trk inhibitors arising from scaffold hopping and HTS efforts. Correspondingly, there have been a growing number of clinical investigations utilizing Trk inhibitors in recent years, with a particular focus on the treatment of NTRK-fusion positive cancers and chronic pain. The observed potential of Trk inhibitors to cause adverse CNS side effects however suggests the need for a more rigorous consideration of BBB permeation capabilities during drug development.

Tropomyosin receptor kinase inhibitors: an updated patent review for 2010-2016 - Part I

INTRODUCTION

Tropomyosin receptor kinases (TrkA/B/C) play crucial roles in the development and maintenance of the nervous system, and aberrant expression of Trk has been implicated in neurological disorders as well as neural and non-neural neoplasms. Patent activity encompassing Trk inhibitors has grown substantially over the last 6 years, recognized by a rise in the number of pharmaceutical entrants to the field and the escalation of novel inhibitor chemotypes. Area covered: In Part I of this two part review, a biological and structural overview of Trk is provided in the context of Trk as a therapeutic target for cancer and pain, followed by the report of recent patent literature claiming small molecule inhibitors of Trk family kinases or which describe inhibitors developed for other kinase targets but include noteworthy Trk inhibition/application. The discussion of the patent literature continues in Part II of this review, which includes an in-depth view of the current clinical applications of Trk inhibitors. Expert opinion: Substantial synthetic efforts in Trk inhibitor development has propagated numerous and diverse inhibitor chemotypes, including TrkA-specific inhibitors. While many novel Trk inhibitors remain the original progeny of Trk-specific development programs, kinase inhibitors initially developed for other kinases have also been successfully repositioned for Trk.

From Unorthodox to Established: The Current Status of (18)F-Trifluoroborate- and (18)F-SiFA-Based Radiopharmaceuticals in PET Nuclear Imaging

Unorthodox (18)F-labeling strategies not employing the formation of a carbon-(18)F bond are seldom found in radiochemistry. Historically, the formation of a boron- or silicon-(18)F bond has been introduced very early on into the repertoire of labeling chemistries, but is without translation into any clinical radiotracer besides inorganic B[(18)F]F4(-) for brain tumor diagnosis. For many decades these labeling methodologies were forgotten and have just recently been revived by a handful of researchers thinking outside the box. When breaking with established paradigms such as the inability to obtain labeled compounds of high specific activity via isotopic exchange or performing radiofluorination in aqueous media, the research community often reacts skeptically. In 2005 and 2006, two novel labeling methodologies were introduced into radiochemistry for positron emission tomography (PET) tracer development: RBF3(-) labeling reported by Perrin et al. and the SiFA methodology by Schirrmacher, Jurkschat, and Waengler et al. which is based on isotopic exchange (IE). Both labeling methodologies have been complemented by other noncanonical strategies to introduce (18)F into biomolecules of diagnostic importance, thus profoundly enriching the landscape of (18)F radiolabeling. B- and Si-based labeling strategies finally revealed that IE is a viable alternative to established and traditional radiochemistry with the advantage of simplifying both the labeling effort as well as the necessary purification of the radiotracer. Hence IE will be the focus of this contribution over other noncanonical labeling methods. Peptides for tumor imaging especially lend themselves favorably toward one-step labeling via IE, but small molecules have been described as well, taking advantage of these new approaches, and have been used successfully for brain imaging. This Review gives an account of both radiochemistries centered on boron and silicon, describing the very beginnings of their basic research, the path that led to optimization of their chemistries, and the first encouraging preclinical results paving the way to their clinical use. This side by side approach will give the reader the opportunity to follow the development of a new basic discovery into a clinically applicable radiotracer including all the hurdles that have had to be overcome.

In Vivo Evaluation of ¹⁸F-SiFAlin-Modified TATE: A Potential Challenge for ⁶⁸Ga-DOTATATE, the Clinical Gold Standard for Somatostatin Receptor Imaging with PET

Radiolabeled peptides for tumor imaging with PET that can be produced with kits are currently in the spotlight of radiopharmacy and nuclear medicine. The diagnosis of neuroendocrine tumors in particular has been a prime example for the usefulness of peptides labeled with a variety of different radionuclides. Among those, (68)Ga and (18)F stand out because of the ease of radionuclide introduction (e.g., (68)Ga isotope) or optimal nuclide properties for PET imaging (slightly favoring the (18)F isotope). The in vivo properties of good manufacturing practice-compliant, newly developed kitlike-producible (18)F-SiFA- and (18)F-SiFAlin- (SiFA = silicon-fluoride acceptor) modified TATE derivatives were compared with the current clinical gold standard (68)Ga-DOTATATE for high-quality imaging of somatostatin receptor-bearing tumors.

METHODS

SiFA- and SiFAlin-derivatized somatostatin analogs were synthesized and radiolabeled using cartridge-based dried (18)F and purified via a C18 cartridge (radiochemical yield 49.8% ± 5.9% within 20-25 min) without high-performance liquid chromatography purification. Tracer lipophilicity and stability in human serum were tested in vitro. Competitive receptor binding affinity studies were performed using AR42J cells. The most promising tracers were evaluated in vivo in an AR42J xenograft mouse model by ex vivo biodistribution and in vivo PET/CT imaging studies for evaluation of their pharmacokinetic profiles, and the results were compared with those of the current clinical gold standard (68)Ga-DOTATATE.

RESULTS

Synthetically easily accessible (18)F-labeled silicon-fluoride acceptor-modified somatostatin analogs were developed. They exhibited high binding affinities to somatostatin receptor-positive tumor cells (1.88-14.82 nM). The most potent compound demonstrated comparable pharmacokinetics and an even slightly higher absolute tumor accumulation level in ex vivo biodistribution studies as well as higher tumor standardized uptake values in PET/CT imaging than (68)Ga-DOTATATE in vivo. The radioactivity uptake in nontumor tissue was higher than for (68)Ga-DOTATATE.

CONCLUSION

The introduction of the novel SiFA building block SiFAlin and of hydrophilic auxiliaries enables a favorable in vivo biodistribution profile of the modified TATE peptides, resulting in high tumor-to-background ratios although lower than those observed with (68)Ga-DOTATATE. As further advantage, the SiFA methodology enables a kitlike labeling procedure for (18)F-labeled peptides advantageous for routine clinical application.

Syntheses and evaluation of carbon-11- and fluorine-18-radiolabeled pan-tropomyosin receptor kinase (Trk) inhibitors: exploration of the 4-aza-2-oxindole scaffold as Trk PET imaging agents

Tropomyosin receptor kinases (TrkA/B/C) are critically involved in the development of the nervous system, in neurological disorders as well as in multiple neoplasms of both neural and non-neural origins. The development of Trk radiopharmaceuticals would offer unique opportunities toward a more complete understanding of this emerging therapeutic target. To that end, we first developed [(11)C]GW441756 ([(11)C]9), a high affinity photoisomerizable pan-Trk inhibitor, as a lead radiotracer for our positron emission tomography (PET) program. Efficient carbon-11 radiolabeling afforded [(11)C]9 in high radiochemical yields (isolated RCY, 25.9% ± 5.7%). In vitro autoradiographic studies in rat brain and TrkB-expressing human neuroblastoma cryosections confirmed that [(11)C]9 specifically binds to Trk receptors in vitro. MicroPET studies revealed that binding of [(11)C]9 in the rodent brain was mostly nonspecific despite initial high brain uptake (SUVmax = 2.0). Modeling studies of the 4-aza-2-oxindole scaffold led to the successful identification of a small series of high affinity fluorinated and methoxy derivatized pan-Trk inhibitors based on our lead compound 9. Out of this series, the fluorinated compound 10 was selected for initial evaluation and radiolabeled with fluorine-18 (isolated RCY, 2.5% ± 0.6%). Compound [(18)F]10 demonstrated excellent Trk selectivity in a panel of cancer relevant kinase targets and a promising in vitro profile in tumors and brain sections but high oxidative metabolic susceptibility leading to nonspecific brain distribution in vivo. The information gained in this study will guide further exploration of the 4-aza-2-oxindole scaffold as a lead for Trk PET ligand development.

Development of subnanomolar radiofluorinated (2-pyrrolidin-1-yl)imidazo[1,2-b]pyridazine pan-Trk inhibitors as candidate PET imaging probes

Dysregulation of tropomyosin receptor kinases (TrkA/B/C) expression and signalling is recognized as a hallmark of numerous neurodegenerative diseases including Parkinson's, Huntington's and Alzheimer's disease.

Automated radiosynthesis of N-succinimidyl 3-(di-tert-butyl[(18)F]fluorosilyl)benzoate ([(18)F]SiFB) for peptides and proteins radiolabeling for positron emission tomography

Recently, silicon fluoride building blocks (SiFA) have emerged as valuable and promising tools to overcome challenges in the labeling of peptides and proteins for positron emission tomography (PET). Herein, we report a fully automated synthesis of N-succinimidyl 3-(di-tert-butyl[(18)F]fluorosilyl)benzoate ([(18)F]SiFB) by a commercially available Scintomics Hot Box 3 synthesis module, to be used as a prosthetic group for peptide and protein labeling. The drying of K2.2.2./K (18)F complex was performed according to the Munich method modified by our group (avoiding azeotropic drying) using oxalic acid to neutralize the base from the (18)F(-) containing QMA eluent. This K2.2.2./K (18)F complex was then used for SiFA (18)F-(19)F isotopic exchange followed by a fast purification by a solid-phase-extraction (SPE) to afford [(18)F]SiFB with an average preparative radiochemical yield (RCY) of 24±1% (non-decay corrected (NDC)) within a synthesis time of 30 min. The [(18)F]SiFB produced by automated synthesis was then used for the (18)F-labeling of rat serum albumin (RSA) as a proof of applicability.

Towards tropomyosin-related kinase B (TrkB) receptor ligands for brain imaging with PET: radiosynthesis and evaluation of 2-(4-[(18)F]fluorophenyl)-7,8-dihydroxy-4H-chromen-4-one and 2-(4-([N-methyl-(11)C]-dimethylamino)phenyl)-7,8-dihydroxy-4H-chromen-4-one

The interaction of tropomyosin-related kinase B (TrkB) with the cognate ligand brain-derived neurotrophic factor (BDNF) mediates fundamental pathways in the development of the nervous system. TrkB signaling alterations are linked to numerous neurodegenerative diseases and conditions. Herein we report the synthesis, biological evaluation and radiosynthesis of the first TrkB radioligands based on the recently identified 7,8-dihydroxyflavone chemotype. 2-(4-[(18)F]fluorophenyl)-7,8-dihydroxy-4H-chromen-4-one ([(18)F]10b) was synthesized in high radiochemical yields via an efficient SNAr radiofluorination involving a para-Michael acceptor substituted aryl followed by BBr3-promoted double demethylation. Selective N-[(11)C]methylation afforded 2-(4-([N-methyl-(11)C]-dimethylamino)phenyl)-7,8-dihydroxy-4H-chromen-4-one ([(11)C]10c) from the fully deprotected catechol-bearing normethyl precursor 13 with [(11)C]MeOTf. In vitro autoradiography of [(18)F]10b with transverse rat brain sections revealed high specific binding in the cortex, striatum, hippocampus and thalamus in accordance with expected TrkB distribution. Blockade experiments with both 7,8-dihydroxyflavone (1a) and TrkB cognate ligand, BDNF, led to decreases of 80% and 85% of radioligand binding strongly supporting the hypothesis that 7,8-dihydroxyflavones exert their effect on TrkB phosphorylation via direct TrkB extracellular domain (ECD) binding. Positron emission tomography (PET) studies revealed that [(18)F]10b and [(11)C]10c brain uptake is minimal and that they are rapidly eliminated from the plasma (effective plasma half-life 5-10 min) via hepatic secretion. Nevertheless, the high specific binding and TrkB specificity derived from in vitro experiments suggests that the 7,8-disubstituted flavone chemotype represents a promising scaffold for the development of TrkB radiotracers for PET.