PET Molecular Imaging in Drug Development: The Imaging and Chemistry Perspective

Synthesis of 18F-labeled Aryl Trifluoromethyl Sulfones, -Sulfoxides, and -Sulfides for Positron Emission Tomography

Positron emission tomography (PET) is becoming increasingly important in nuclear medicine and drug discovery. To date, the development of many potential PET tracers is hampered by the lack of suitable synthetic pathways for their preparation. This is particularly true for the highly desired radiolabeling of compounds bearing [18F]CF3-groups. For instance, S(O)nCF3-groups (n=0, 1, 2) serve as structural motif in a range of biologically active compounds, but their radiosynthesis remains largely unprecedented (for n=1, 2). Herein, we describe general methods for the radiosynthesis of 18F-labeled aryl trifluoromethyl sulfones, -sulfoxides, and -sulfides. All three methods are operationally straightforward, start from widely available precursors, i.e., sulfonyl fluorides and thiophenols, and make use of the recently established [18F]Ruppert-Prakash reagent. Further, the syntheses display good functional group tolerance as demonstrated by the 18F-labeling of more than 40 compounds. The applicability of the new method is demonstrated by the radiolabeling of three bioactive molecules, optionally to be used as PET tracers. In a broader context, this work presents a substantial expansion of the chemical space of radiofluorinated structural motifs to be used for the development of new PET tracers.

Structure-guided discovery of orexin receptor-binding PET ligands

Molecular imaging using positron emission tomography (PET) can serve as a promising tool for visualizing biological targets in the brain. Insights into the expression pattern and the in vivo imaging of the G protein-coupled orexin receptors OX1R and OX2R will further our understanding of the orexin system and its role in various physiological and pathophysiological processes. Guided by crystal structures of our lead compound JH112 and the approved hypnotic drug suvorexant bound to OX1R and OX2R, respectively, we herein describe the design and synthesis of two novel radioligands, [18F]KD23 and [18F]KD10. Key to the success of our structural modifications was a bioisosteric replacement of the triazole moiety with a fluorophenyl group. The 19F-substituted analog KD23 showed high affinity for the OX1R and selectivity over OX2R, while the high affinity ligand KD10 displayed similar Ki values for both subtypes. Radiolabeling starting from the respective pinacol ester precursors resulted in excellent radiochemical yields of 93% and 88% for [18F]KD23 and [18F]KD10, respectively, within 20 min. The new compounds will be useful in PET studies aimed at subtype-selective imaging of orexin receptors in brain tissue.

The 18F-Difluoromethyl Group: Challenges, Impact and Outlook

The difluoromethyl functionality has proven useful in drug discovery, as it can modulate the properties of bioactive molecules. For PET imaging, this structural motif has been largely underexploited in (pre)clinical radiotracers due to a lack of user-friendly radiosynthetic routes. This Minireview provides an overview of the challenges facing radiochemists and summarises the efforts made to date to access 18F-difluoromethyl-containing radiotracers. Two distinct approaches have prevailed, the first of which relies on 18F-fluorination. A second approach consists of a 18F-difluoromethylation process, which uses 18F-labelled reagents capable of releasing key reactive intermediates such as the [18F]CF2H radical or [18F]difluorocarbene. Finally, we provide an outlook for future directions in the radiosynthesis of [18F]CF2H compounds and their application in tracer radiosynthesis.

Expedient Access to 18F-Fluoroheteroarenes via Deaminative Radiofluorination of Aniline-Derived Pyridinium Salts

Herein, we disclose that pyridinium salts derived from abundant (hetero)anilines represent a novel precursor class for nucleophilic aromatic substitution reactions with [18F]fluoride. The value of this new 18F-fluorodeamination is demonstrated with the synthesis of over 30 structurally diverse and complex heteroaryl 18F-fluorides, several derived from scaffolds that were yet to be labelled with fluorine-18. The protocol tolerates heteroarenes and functionalities commonly found in drug discovery libraries, and is amenable to scale-up and automation on a commercial radiosynthesiser.

Photoredox Nucleophilic (Radio)fluorination of Alkoxyamines

Herein, we report a photoredox nucleophilic (radio)fluorination using TEMPO-derived alkoxyamines, a class of substrates accessible in a single step from a diversity of readily available carboxylic acids, halides, alkenes, alcohols, aldehydes, boron reagents, and C-H bonds. This mild and versatile one-electron pathway affords radiolabeled aliphatic fluorides that are typically inaccessible applying conventional nucleophilic substitution technologies due to insufficient reactivity and competitive elimination. Automation of this photoredox process is also demonstrated with a user-friendly and commercially available photoredox flow reactor and radiosynthetic platform, therefore expediting access to labeled aliphatic fluorides in high molar activity (Am) for (pre)clinical evaluation.

Developing Low Molecular Weight PET and SPECT Imaging Agents

Imaging agents for positron emission tomography (PET) and single-photon emission computerized tomography (SPECT) have shown their utility in many situations, answering clinical questions related to drug development and medical considerations. The discovery and development of imaging agents follow a well-understood process, with variations related to available starting points and to the envisaged imaging application. This article describes the general development path leading from the expression of an imaging need and project initiation to a clinically usable imaging agent. The definition of the project rationale, the design and optimization of early leads, and the assessment of the imaging potential of an imaging agent candidate are followed by preclinical and clinical development activities that differ from those required for therapeutic agents. These include radiolabeling with a positron emitter and first-in-human clinical studies, to rapidly evaluate the ability of a new imaging agent to address the questions it was designed to answer.

Design and synthesis of novel coumarin-benzimidazole hybrids as human galectin-1 inhibitors

Aim: To develop novel non-carbohydrate inhibitors of human galectin-1 (GAL-1), we have designed a series of coumarin-benzimidazole hybrids. Methods: We synthesized and characterized the coumarin-benzimidazole hybrids and further evaluated them using an in vitro GAL-1 enzyme-linked immunosorbent assay and in silico methods. Results: Among all, the compounds 6p and 6q were found to be potent, with GAL-1 inhibition of 37.61 and 36.92%, respectively, at 10 μM in GAL-1-expressed cell culture supernatant of MCF-7 cells. These two compounds are feasible for fluorine-18 radiolabeling to develop GAL-1 selective PET radiotracers. Computational studies revealed strong binding interactions of GAL-1 with these novel coumarin-benzimidazole hybrids. Conclusion: Coumarin-benzimidazole hybrids can serve as potential leads to develop selective non-carbohydrate GAL-1 inhibitors for cancer therapy.

Preclinical evaluation of an 18F-labeled Tenascin-C aptamer for PET imaging of atherosclerotic plaque in mouse models of atherosclerosis

Tenascin-C is an extracellular matrix glycoprotein strongly expressed in coronary atherosclerotic plaque. Aptamers are single-stranded oligonucleotides that bind to specific target molecules with high affinity. This study hypothesized that tenascin-C expression at atherosclerotic plaque in vivo could be detected by tenascin-C specific aptamers using positron emission tomography (PET). This paper reports the radiosynthesis of a fluorine-18 (18F)-labeled tenascin-C aptamer for the biodistribution and PET imaging of the tenascin-C expression in apolipoprotein E-deficient (ApoE-/-) mice. The aortas ApoE-/- mice showed significantly increased positive areas of Oil red O staining than control C57BL/6 mice, and tenascin-C expression was detected in foam cells accumulated in the subendothelial lesions of ApoE-/- mice. The ex vivo biodistribution of the 18F-labeled tenascin-C aptamer showed significantly increased uptake at the aorta of ApoE-/- mice, and ex vivo autoradiography of aorta revealed the high accumulation of the 18F-labeled tenascin-C aptamer in the atherosclerotic lesions of ApoE-/- mice, which was consistent with the location of the atherosclerotic plaques detected by Oil red O staining. PET imaging of the 18F-labeled tenascin-C aptamer revealed a significantly higher mean standardized uptake in the aorta of the ApoE-/- mice than the control C57BL/6 mice. These data highlight the potential use of tenascin-C aptamer to diagnose atherosclerotic lesions in vivo.

Aza-Heterocyclic Building Blocks with In-Ring CF2 -Fragment

Modern organic chemistry is a titan supporting and reinforcing pharmaceutical, agricultural, food and material science products. Over the past decades, the organic compounds market has been evolving to meet all the research demands. In this regard, medicinal chemistry is especially dependent on available chemical space as subtle tuning of the molecule structure is required to create a drug with relevant physicochemical properties and a remarkable activity profile. The recent rapid evolution of synthetic methodology to deploy fluorine has brought fluorinated compounds to the spotlight of MedChem community. And now unique properties of fluorine still keep fascinating more and more as its justified installation into a molecular framework has a beneficial impact on membrane permeability, lipophilicity, metabolic stability, pharmacokinetic properties, conformation, pKa , etc. The backward influence of medicinal chemistry on organic synthesis has also changed the landscape of the latter towards new fluorinated topologies as well. Such complex relationships create a flexible and ever-changing ecosystem. Given that MedChem investigations strongly lean on the ability to reach suitable building blocks and the existence of reliable synthetic methods in this review we collected advances in the chemistry of respectful, but still enigmatic gem-difluorinated aza-heterocyclic building blocks.

Synthesis and Characterization of a New Carbon-11 Labeled Positron Emission Tomography Radiotracer for Orexin 2 Receptors Neuroimaging

Purpose

Orexin receptors (OXRs) play a crucial role in modulating various physiological and neuropsychiatric functions within the central nervous system (CNS). Despite their significance, the precise role of OXRs in the brain remains elusive. Positron emission tomography (PET) imaging is instrumental in unraveling CNS functions, and the development of specific PET tracers for OXRs is a current research focus.

Methods

The study investigated MDK-5220, an OX2R-selective agonist with promising binding properties (EC50 on OX2R: 0.023 μM, Ki on hOX2R: 0.14 μM). Synthesized and characterized as an OX2R PET probe, [11C]MDK-5220 was evaluated for its potential as a tracer. Biodistribution studies in mice were conducted to assess OX2R binding selectivity, with particular attention to its interaction with P-glycoprotein (P-gp) on the blood-brain barrier.

Results

[11C]MDK-5220 exhibited promising attributes as an OX2R PET probe, demonstrating robust OX2R binding selectivity in biodistribution studies. However, an observed interaction with P-gp impacted its brain uptake. Despite this limitation, [11C]MDK-5220 presents itself as a potential candidate for further development.

Discussion

The study provides insights into the functionality of the OX system and the potential of [11C]MDK-5220 as an OX2R PET probe. The observed interaction with P-gp highlights a consideration for future modifications to enhance brain uptake. The findings pave the way for innovative tracer development and propel ongoing research on OX systems, contributing to a deeper understanding of their role in the CNS.

Conclusion

[11C]MDK-5220 emerges as a promising OX2R PET probe, despite challenges related to P-gp interaction. This study lays the foundation for further exploration and development of PET probes targeting OXRs, opening avenues for advancing our understanding of OX system functionality within the brain.

Preclinical evaluation of an 18F-labeled Nε-acryloyllysine piperazide for covalent targeting of transglutaminase 2

BACKGROUND

Transglutaminase 2 (TGase 2) is a multifunctional protein and has a prominent role in various (patho)physiological processes. In particular, its transamidase activity, which is rather latent under physiological conditions, gains importance in malignant cells. Thus, there is a great need of theranostic probes for targeting tumor-associated TGase 2, and targeted covalent inhibitors appear to be particularly attractive as vector molecules. Such an inhibitor, equipped with a radionuclide suitable for noninvasive imaging, would be supportive for answering the general question on the possibility for functional characterization of tumor-associated TGase 2. For this purpose, the recently developed 18F-labeled Nε-acryloyllysine piperazide [18F]7b, which is a potent and selective irreversible inhibitor of TGase 2, was subject to a detailed radiopharmacological characterization herein.

RESULTS

An alternative radiosynthesis of [18F]7b is presented, which demands less than 300 µg of the respective trimethylammonio precursor per synthesis and provides [18F]7b in good radiochemical yields (17 ± 7%) and high (radio)chemical purities (≥ 99%). Ex vivo biodistribution studies in healthy mice at 5 and 60 min p.i. revealed no permanent enrichment of 18F-activity in tissues with the exception of the bone tissue. In vivo pretreatment with ketoconazole and in vitro murine liver microsome studies complemented by mass spectrometric analysis demonstrated that bone uptake originates from metabolically released [18F]fluoride. Further metabolic transformations of [18F]7b include mono-hydroxylation and glucuronidation. Based on blood sampling data and liver microsome experiments, pharmacokinetic parameters such as plasma and intrinsic clearance were derived, which substantiated the apparently rapid distribution of [18F]7b in and elimination from the organisms. A TGase 2-mediated uptake of [18F]7b in different tumor cell lines could not be proven. Moreover, evaluation of [18F]7b in melanoma tumor xenograft models based on A375-hS100A4 (TGase 2 +) and MeWo (TGase 2 -) cells by ex vivo biodistribution and PET imaging studies were not indicative for a specific targeting.

CONCLUSION

[18F]7b is a valuable radiometric tool to study TGase 2 in vitro under various conditions. However, its suitability for targeting tumor-associated TGase 2 is strongly limited due its unfavorable pharmacokinetic properties as demonstrated in rodents. Consequently, from a radiochemical perspective [18F]7b requires appropriate structural modifications to overcome these limitations.

Quantitative Pharmacokinetics Reveal Impact of Lipid Composition on Microbubble and Nanoprogeny Shell Fate

Microbubble-enabled focused ultrasound (MB-FUS) has revolutionized nano and molecular drug delivery capabilities. Yet, the absence of longitudinal, systematic, quantitative studies of microbubble shell pharmacokinetics hinders progress within the MB-FUS field. Microbubble radiolabeling challenges contribute to this void. This barrier is overcome by developing a one-pot, purification-free copper chelation protocol able to stably radiolabel diverse porphyrin-lipid-containing Definity® analogues (pDefs) with >95% efficiency while maintaining microbubble physicochemical properties. Five tri-modal (ultrasound-, positron emission tomography (PET)-, and fluorescent-active) [64 Cu]Cu-pDefs are created with varying lipid acyl chain length and charge, representing the most prevalently studied microbubble compositions. In vitro, C16 chain length microbubbles yield 2-3x smaller nanoprogeny than C18 microbubbles post FUS. In vivo, [64 Cu]Cu-pDefs are tracked in healthy and 4T1 tumor-bearing mice ± FUS over 48 h qualitatively through fluorescence imaging (to characterize particle disruption) and quantitatively through PET and γ-counting. These studies reveal the impact of microbubble composition and FUS on microbubble dissolution rates, shell circulation, off-target tissue retention (predominantly the liver and spleen), and FUS enhancement of tumor delivery. These findings yield pharmacokinetic microbubble structure-activity relationships that disrupt conventional knowledge, the implications of which on MB-FUS platform design, safety, and nanomedicine delivery are discussed.

PET reporter systems for the brain

Positron emission tomography (PET) can be used as a noninvasive method to longitudinally monitor and quantify the expression of proteins in the brain in vivo. It can be used to monitor changes in biomarkers of mental health disorders, and to assess therapeutic interventions such as stem cell and molecular genetic therapies. The utility of PET monitoring depends on the availability of a radiotracer with good central nervous system (CNS) penetration and high selectivity for the target protein. This review evaluates existing methods for the visualization of reporter proteins and/or protein function using PET imaging, focusing on engineered systems, and discusses possible approaches for future success in the development of high-sensitivity and high-specificity PET reporter systems for the brain.

Generation of Ru(III)-hypochlorite with resemblance to the heme-dependent haloperoxidase enzyme

The reaction of [(Me/BnTPEN)RuII(NCCH3)]2+ (BnTPEN = N1-benzyl-N1,N2,N2-tris(pyridine-2-ylmethyl)ethane-1,2-diamine and MeTPEN = N1-methyl-N1,N2,N2-tris(pyridine-2-ylmethyl)ethane-1,2-diamine) with mCPBA in the presence of chloride ions in CH3CN : H2O generated a novel (Me/BnTPEN)RuIII-OCl species at room temperature. This hypochlorite adduct could also be obtained by the direct reaction of NaOCl and HClO4 with (L)RuII complexes. The current study mimics the synthesis of a metal hypochlorite adduct in a similar fashion as in the heme-dependent haloperoxidase enzyme. As an electrophilic oxidant, the ruthenium hypochlorite adduct catalyzes hydrogen atom abstraction reactions of phenols and their derivatives.

Small-molecule probes from bench to bedside: advancing molecular analysis of drug-target interactions toward precision medicine

Over the past decade, remarkable advances have been witnessed in the development of small-molecule probes. These molecular tools have been widely applied for interrogating proteins, pathways and drug-target interactions in preclinical research. While novel structures and designs are commonly explored in probe development, the clinical translation of small-molecule probes remains limited, primarily due to safety and regulatory considerations. Recent synergistic developments - interfacing novel chemical probes with complementary analytical technologies - have introduced and expedited diverse biomedical opportunities to molecularly characterize targeted drug interactions directly in the human body or through accessible clinical specimens (e.g., blood and ascites fluid). These integrated developments thus offer unprecedented opportunities for drug development, disease diagnostics and treatment monitoring. In this review, we discuss recent advances in the structure and design of small-molecule probes with novel functionalities and the integrated development with imaging, proteomics and other emerging technologies. We further highlight recent applications of integrated small-molecule technologies for the molecular analysis of drug-target interactions, including translational applications and emerging opportunities for whole-body imaging, tissue-based measurement and blood-based analysis.

Biodistribution and radiation dosimetry of multiple tracers on total-body positron emission tomography/computed tomography

Background

[18F]F-FDG, [68Ga]Ga-PSMA-11, and [68Ga]Ga-FAPI-04 have achieved good results in multiple clinical trials and clinical practice, but the imaging of these tracers is limited to traditional short-axis positron emission tomography/computed tomography (PET/CT). Therefore, we aimed to use total-body PET/CT dynamic scanning to describe whole-body biodistribution of these three tracers and to calculate more precise radiation doses.

Methods

Total-body PET/CT (uExplorer, United Imaging Healthcare) dynamic scanning was performed on 54 patients, including 30 patients with [18F]F-FDG, 10 patients with [68Ga]Ga-PSMA-11, and 14 patients with [68Ga]Ga-FAPI-04. A 60-minute dynamic scanning of whole body was performed simultaneously after bedside bolus injection of the corresponding tracers. The dynamic sequence of 92 frames was quantitatively analyzed by the Pmod4.0 software. Whole body biodistribution was calculated as time-activity curves (TACs) describing dynamic uptake patterns in the subject's major organs, followed by calculation of tracer kinetics and cumulative organ activity. Finally, combined with the OLINDA/EXM software, effective doses of the different tracers and individual organ doses were calculated.

Results

In a systematic TAC analysis of three tracers, we identified distinct biodistribution patterns in major organs. [68Ga]Ga-PSMA-11 showed a trend of rapid increasing and slow decreasing in liver, spleen, muscle, and bone. In the heart, stomach, brain, and lung, tracer decreased rapidly after rapid increasing. Similarly, tracer uptake in the kidney and urinary bladder increased gradually. [68Ga]Ga-FAPI-04 showed a rapid increasing and rapid decreasing trend in brain, lung, liver, spleen, bone, heart, kidney, and stomach. The mean effective dose of [68Ga]Ga-PSMA-11 was 1.47E-02 mSv/MBq, and the mean effective doses of [18F]F-FDG and [68Ga]Ga-FAPI-04 were comparable (2.52E-02 mSv/MBq and 2.23E-02 mSv/MBq). The mean effective dose of [18F]F-FDG was lower than that reported in the literature measured by previous short-axis PET, while both [68Ga]Ga-PSMA-11 and [68Ga]Ga-FAPI-04 had higher value than previously reported value.

Conclusions

[18F]F-FDG, [68Ga]Ga-PSMA-11 and [68Ga]Ga-FAPI-04 have good biodistribution in human organs. Real-time high-sensitivity dynamic scanning with total-body PET/CT is a very effective way to accurately calculate biodistribution and effective dose of positron-labeled radiopharmaceuticals.

Reactive Palladium-Ligand Complexes for 11C-Carbonylation at Ambient Pressure: A Breakthrough in Carbon-11 Chemistry

The Pd-Xantphos-mediated ¹¹C-carbonylation protocol (also known as the "Xantphos- method"), due to its simplistic and convenient nature, has facilitated researchers in meeting a longstanding need for preparing ¹¹C-carbonyl-labeled radiopharmaceuticals at ambient pressure for positron emission tomography (PET) imaging and drug discovery. This development could be viewed as a breakthrough in carbon-11 chemistry, as evidenced by the rapid global adoption of the method by the pharmaceutical industry and academic laboratories worldwide. The method has been fully automated for the good manufacturing practice (GMP)-compliant production of novel radiopharmaceuticals for human use, and it has been adapted for "in-loop" reactions and microwave technology; an impressive number of ¹¹C-labeled compounds (>100) have been synthesized. Given the simplicity and efficiency of the method, as well as the abundance of carbonyl groups in bioactive drug molecules, we expect that this methodology will be even more widely adopted in future PET radiopharmaceutical research and drug development.

Improvement of Crystal Identification Accuracy for Depth-of-Interaction Detector System with Peak-to-Charge Discrimination Method

In positron emission tomography (PET), parallax errors degrade spatial resolution. The depth of interaction (DOI) information provides the position in the depth of the scintillator interacting with the γ-rays, thus reducing parallax errors. A previous study developed a Peak-to-Charge discrimination (PQD), which can separate spontaneous alpha decay in LaBr₃:Ce. Since decay constant of GSO:Ce depends on Ce concentration, the PQD is expected to discriminate GSO:Ce scintillators with different Ce concentration. In this study, the PQD-based DOI detector system was developed, which can be processed online and implemented in PET. A detector was composed of four layers of GSO:Ce crystals and a PS-PMT. The four crystals were obtained from both the top and bottom of ingots with a nominal Ce concentration of 0.5 mol% and 1.5 mol%. The PQD was implemented on the Xilinx Zynq-7000 SoC board with 8ch Flash ADC to gain real-time processing, flexibility, and expandability. The results showed that the mean Figure of Merits in 1D between four scintillators are 1.5, 0.99, 0.91 for layers between 1st-2nd, 2nd-3rd, and 3rd-4th respectively, and the mean Error Rate in 1D between four scintillators are 3.50%, 2.96%, 13.3%, and 1.88% for layers 1, 2, 3, and 4, respectively. In addition, the introduction of the 2D PQDs resulted in the mean Figure of Merits in 2D greater than 0.9 and the mean Error Rate in 2D less than 3% in all layers.

Nanoparticles Targeted to Fibroblast Activation Protein Outperform PSMA for MRI Delineation of Primary Prostate Tumors

Accurate delineation of gross tumor volumes remains a barrier to radiotherapy dose escalation and boost dosing in the treatment of solid tumors, such as prostate cancer. Magnetic resonance imaging (MRI) of tumor targets has the power to enable focal dose boosting, particularly when combined with technological advances such as MRI-linear accelerator. Fibroblast activation protein (FAP) is overexpressed in stromal components of >90% of epithelial carcinomas. Herein, the authors compare targeted MRI of prostate specific membrane antigen (PSMA) with FAP in the delineation of orthotopic prostate tumors. Control, FAP, and PSMA-targeting iron oxide nanoparticles were prepared with modification of a lymphotropic MRI agent (FerroTrace, Ferronova). Mice with orthotopic LNCaP tumors underwent MRI 24 h after intravenous injection of nanoparticles. FAP and PSMA nanoparticles produced contrast enhancement on MRI when compared to control nanoparticles. FAP-targeted MRI increased the proportion of tumor contrast-enhancing black pixels by 13%, compared to PSMA. Analysis of changes in R2 values between healthy prostates and LNCaP tumors indicated an increase in contrast-enhancing pixels in the tumor border of 15% when targeting FAP, compared to PSMA. This study demonstrates the preclinical feasibility of PSMA and FAP-targeted MRI which can enable targeted image-guided focal therapy of localized prostate cancer.

Evolution of Detecting Early Onset of Alzheimer's Disease: From Neuroimaging to Optical Immunoassays

Alzheimer's disease (AD) is a pathological disorder defined by the symptoms of memory loss and deterioration of cognitive abilities over time. Although the etiology is complex, it is mainly associated with the accumulation of toxic amyloid-β peptide (Aβ) aggregates and tau protein-induced neurofibrillary tangles (NFTs). Even now, creating non-invasive, sensitive, specific, and cost-effective diagnostic methods for AD remains challenging. Over the past few decades, polymers, and nanomaterials (e.g., nanodiamonds, nanogold, quantum dots) have become attractive and practical tools in nanomedicine for diagnosis and treatment. This review focuses on current developments in sensing methods such as enzyme-linked immunosorbent assay (ELISA) and surface-enhanced Raman scattering (SERS) to boost the sensitivity in detecting related biomarkers for AD. In addition, optical analysis platforms such as ELISA and SERS have found increasing popularity among researchers due to their excellent sensitivity and specificity, which may go as low as the femtomolar range. While ELISA offers easy technological usage and high throughput, SERS has the advantages of improved mobility, simple electrical equipment integration, and lower cost. Both portable optical sensing techniques are highly superior in terms of sensitivity, specificity, human application, and practicality, enabling the early identification of AD biomarkers.

PET radiotracers and fluorescent probes for imaging human carbonic anhydrase IX and XII in hypoxic tumors

Positron emission tomography (PET) and fluorescent imaging play a pivotal role in medical diagnosis, biomedical oncologic research, and drug development process, which include identification of target location, target engagement, but also prove on mechanism of action or pharmacokinetics of new drug candidates. PET estimates physiological changes at the molecular level using specific radiotracers containing a short-lived positron emitting radionuclide such as fluorine-18 or carbon-11, whereas fluorescent imaging techniques use fluorescent probes labeled with suitable drug candidates for detection at the molecular level. The human carbonic anhydrase (hCA) isoforms IX and XII are overexpressed in hypoxic cancer cells, promoting tumor growth by regulating extra/intracellular pH, ferroptosis, and metabolism, being recognized as promising targets for anticancer theranostic agents. In this review, we have focused on PET radiotracers as well as fluorescent probes for diagnosis and treatment of tumors expressing hCA IX and hCA XII.

Translational molecular imaging and drug development in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder that primarily affects elderly people and constitutes a major source of disability worldwide. Notably, the neuropathological hallmarks of PD include nigrostriatal loss and the formation of intracellular inclusion bodies containing misfolded α-synuclein protein aggregates. Cardinal motor symptoms, which include tremor, rigidity and bradykinesia, can effectively be managed with dopaminergic therapy for years following symptom onset. Nonetheless, patients ultimately develop symptoms that no longer fully respond to dopaminergic treatment. Attempts to discover disease-modifying agents have increasingly been supported by translational molecular imaging concepts, targeting the most prominent pathological hallmark of PD, α-synuclein accumulation, as well as other molecular pathways that contribute to the pathophysiology of PD. Indeed, molecular imaging modalities such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) can be leveraged to study parkinsonism not only in animal models but also in living patients. For instance, mitochondrial dysfunction can be assessed with probes that target the mitochondrial complex I (MC-I), while nigrostriatal degeneration is typically evaluated with probes designed to non-invasively quantify dopaminergic nerve loss. In addition to dopaminergic imaging, serotonin transporter and N-methyl-D-aspartate (NMDA) receptor probes are increasingly used as research tools to better understand the complexity of neurotransmitter dysregulation in PD. Non-invasive quantification of neuroinflammatory processes is mainly conducted by targeting the translocator protein 18 kDa (TSPO) on activated microglia using established imaging agents. Despite the overwhelming involvement of the brain and brainstem, the pathophysiology of PD is not restricted to the central nervous system (CNS). In fact, PD also affects various peripheral organs such as the heart and gastrointestinal tract - primarily via autonomic dysfunction. As such, research into peripheral biomarkers has taken advantage of cardiac autonomic denervation in PD, allowing the differential diagnosis between PD and multiple system atrophy with probes that visualize sympathetic nerve terminals in the myocardium. Further, α-synuclein has recently gained attention as a potential peripheral biomarker in PD. This review discusses breakthrough discoveries that have led to the contemporary molecular concepts of PD pathophysiology and how they can be harnessed to develop effective imaging probes and therapeutic agents. Further, we will shed light on potential future trends, thereby focusing on potential novel diagnostic tracers and disease-modifying therapeutic interventions.

Smart Biomimetic Nanozymes for Precise Molecular Imaging: Application and Challenges

New nanotechnologies for imaging molecules are widely being applied to visualize the expression of specific molecules (e.g., ions, biomarkers) for disease diagnosis. Among various nanoplatforms, nanozymes, which exhibit enzyme-like catalytic activities in vivo, have gained tremendously increasing attention in molecular imaging due to their unique properties such as diverse enzyme-mimicking activities, excellent biocompatibility, ease of surface tenability, and low cost. In addition, by integrating different nanoparticles with superparamagnetic, photoacoustic, fluorescence, and photothermal properties, the nanoenzymes are able to increase the imaging sensitivity and accuracy for better understanding the complexity and the biological process of disease. Moreover, these functions encourage the utilization of nanozymes as therapeutic agents to assist in treatment. In this review, we focus on the applications of nanozymes in molecular imaging and discuss the use of peroxidase (POD), oxidase (OXD), catalase (CAT), and superoxide dismutase (SOD) with different imaging modalities. Further, the applications of nanozymes for cancer treatment, bacterial infection, and inflammation image-guided therapy are discussed. Overall, this review aims to provide a complete reference for research in the interdisciplinary fields of nanotechnology and molecular imaging to promote the advancement and clinical translation of novel biomimetic nanozymes.

Pharmacokinetic Evaluation of New Drugs Using a Multi-Labelling Approach and PET Imaging: Application to a Drug Candidate with Potential Application in Neuromuscular Disorders

BACKGROUND AND OBJECTIVE

The determination of pharmacokinetic properties of new chemical entities is a key step in the process of drug development. Positron emission tomography (PET) is an ideal technique to obtain both biodistribution and pharmacokinetic parameters of new compounds over a wide range of chemical modalities. Here, we use a multi-radionuclide/multi-position labelling approach to investigate distribution, elimination, and metabolism of a triazole-based FKBP12 ligand (AHK2) with potential application in neuromuscular disorders.

METHODS

Target engagement and stabilizing capacity of the drug candidate (AHK2) towards FKBP12-RyR was evaluated using competitive ligand binding and proximity ligation assays, respectively. Subsequently, AHK2 was labelled either with the positron emitter carbon-11 (¹¹C) via ¹¹C-methylation to yield both [¹¹C]AHK2.1 and [¹¹C]AHK2.2, or by palladium-catalysed reduction of the corresponding 5-iodotriazole derivative using ³H gas to yield [³H]AHK2. Metabolism was first investigated in vitro using liver microsomes. PET imaging studies in rats after intravenous (IV) administration at different doses (1 µg/Kg and 5 mg/Kg) were combined with determination of arterial blood time-activity curves (TACs) and analysis of plasma samples by high performance liquid chromatography (HPLC) to quantify radioactive metabolites. Arterial TACs were obtained in continuous mode by using an in-house developed system that enables extracorporeal blood circulation and continuous measurement of radioactivity in the blood. Pharmacokinetic parameters were determined by non-compartmental modelling of the TACs.

RESULTS

In vitro studies indicate that AHK2 binds to FKBP12 at the rapamycin-binding pocket, presenting activity as a FKBP12/RyR stabilizer. [¹¹C]AHK2.1, [¹¹C]AHK2.2 and [³H]AHK2 could be obtained in overall non-decay corrected radiochemical yields of 14 ± 2%, 15 ± 2% and 0.05%, respectively. Molar activities were 60-110 GBq/µmol, 68-122 GBq/µmol and 0.4-0.5 GBq/μmol, respectively. In vitro results showed that oxidation of the thioether group into sulfoxide, demethylation of the CH₃O-Ar residue and demethylation of -N(CH₃)₂ were the main metabolic pathways. Fast metabolism was observed in vivo. Pharmacokinetic parameters obtained from metabolite-corrected arterial blood TACs showed a short half-life (12.6 ± 3.3 min). Dynamic PET imaging showed elimination via urine when [¹¹C]AHK2.2 was administered, probably reflecting the biodistribution of [¹¹C]methanol as the major metabolite. Contrarily, accumulation in the gastrointestinal track was observed after administration of [¹¹C]AKH2.1.

CONCLUSIONS

AHK2 binds to FKBP12 at the rapamycin-binding pocket, presenting activity as a FKBP12/RyR stabilizer. Studies performed with the ³H- and ¹¹C-labelled FKBP12/RyR stabilizer AHK2 confirm fast blood clearance, linear pharmacokinetics and rapid metabolism involving oxidation of the sulfide and amine moieties and oxidative demethylation of the CH₃-O-Ar and tertiary amine groups as the main pathways. PET studies suggest that knowledge about metabolic pathways is paramount to interpret images.

Regulated Induced Proximity Targeting Chimeras (RIPTACs): a Novel Heterobifunctional Small Molecule Therapeutic Strategy for Killing Cancer Cells Selectively

While specific cell signaling pathway inhibitors have yielded great success in oncology, directly triggering cancer cell death is one of the great drug discovery challenges facing biomedical research in the era of precision oncology. Attempts to eradicate cancer cells expressing unique target proteins, such as antibody-drug conjugates (ADCs), T-cell engaging therapies, and radiopharmaceuticals have been successful in the clinic, but they are limited by the number of targets given the inability to target intracellular proteins. More recently, heterobifunctional small molecules such as Proteolysis Targeting Chimera (PROTACs) have paved the way for protein proximity inducing therapeutic modalities. Here, we describe a proof-of-concept study using novel heterobifunctional small molecules called Regulated Induced Proximity Targeting Chimeras or RIPTACs, which elicit a stable ternary complex between a target protein selectively expressed in cancer tissue and a pan-expressed protein essential for cell survival. The resulting cooperative protein:protein interaction (PPI) abrogates the function of the essential protein, thus leading to cell death selectively in cells expressing the target protein. This approach not only opens new target space by leveraging differentially expressed intracellular proteins but also has the advantage of not requiring the target to be a driver of disease. Thus, RIPTACs can address non-target mechanisms of resistance given that cell killing is driven by inactivation of the essential protein. Using the HaloTag7-FKBP model system as a target protein, we describe RIPTACs that incorporate a covalent or non-covalent target ligand connected via a linker to effector ligands such as JQ1 (BRD4), BI2536 (PLK1), or multi-CDK inhibitors such as TMX3013 or dinaciclib. We show that these RIPTACs exhibit positive co-operativity, accumulate selectively in cells expressing HaloTag7-FKBP, form stable target:RIPTAC:effector trimers in cells, and induce an anti-proliferative response in target-expressing cells. We propose that RIPTACs are a novel heterobifunctional therapeutic modality to treat cancers that are known to selectively express a specific intracellular protein.

Oxidation-Cyclisation of Biphenyl Thioethers to Dibenzothiophenium Salts for Ultrarapid 18F-Labelling of PET Tracers

¹⁸F-labelled radiotracers are in high demand and play an important role for diagnostic imaging with positron emission tomography (PET). Challenges associated with the synthesis of the labelling precursors and the incorporation of [¹⁸F]fluoride with practical activity yields at batch scale are the main limitations for the development of new ¹⁸F-PET tracers. Herein, we report a high-yielding and robust synthetic method to access naked dibenzothiophenium salt precursors of complex PET tracers and their labelling with [¹⁸F]fluoride. C-S cross-coupling of biphenyl-2-thioacetate with aryl halides followed by sequential oxidation-cyclisation of the corresponding thioethers gives dibenzothiophenium salts in good to excellent yields. Labelling of neutral and electron-deficient substrates with [¹⁸F]fluoride is ultrarapid and occurs under mild conditions (1 min at 90 °C) with high activity yields. The method enables facile synthesis of complex and sensitive radiotracers, as exemplified by radiofluorination of three clinically relevant PET tracers [¹⁸F]UCB-J, [¹⁸F]AldoView and [¹⁸F]FNDP, and can accelerate the development and clinical translation of new ¹⁸F-radiopharmaceuticals.