Chemistry Considerations for the Clinical Translation of Oncology PET Radiopharmaceuticals

Radiological and Molecular Analysis of Radioiodinated Anastrozole and Epirubicin as Innovative Radiopharmaceuticals Targeting Methylenetetrahydrofolate Dehydrogenase 2 in Solid Tumors

In the dynamic field of radiopharmaceuticals, innovating targeted agents for cancer diagnosis and therapy is crucial. Our study enriches this evolving landscape by evaluating the potential of radioiodinated anastrozole ([125I]anastrozole) and radioiodinated epirubicin ([125I]epirubicin) as targeting agents against MTHFD2-driven tumors. MTHFD2, which is pivotal in one-carbon metabolism, is notably upregulated in various cancers, presenting a novel target for radiopharmaceutical application. Through molecular docking and 200 ns molecular dynamics (MD) simulations, we assess the binding efficiency and stability of [125I]anastrozole and [125I]epirubicin with MTHFD2. Molecular docking illustrates that [125I]epirubicin has a superior binding free energy (∆Gbind) of -41.25 kJ/mol compared to -39.07 kJ/mol for [125I]anastrozole and -38.53 kJ/mol for the control ligand, suggesting that it has a higher affinity for MTHFD2. MD simulations reinforce this, showing stable binding, as evidenced by root mean square deviation (RMSD) values within a narrow range, underscoring the structural integrity of the enzyme-ligand complexes. The root mean square fluctuation (RMSF) analysis indicates consistent dynamic behavior of the MTHFD2 complex upon binding with [125I]anastrozole and [125I]epirubicin akin to the control. The radius of gyration (RG) measurements of 16.90 Å for MTHFD2-[125I]anastrozole and 16.84 Å for MTHFD2-[125I]epirubicin confirm minimal structural disruption upon binding. The hydrogen bond analysis reveals averages of two and three stable hydrogen bonds for [125I]anastrozole and [125I]epirubicin complexes, respectively, highlighting crucial stabilizing interactions. The MM-PBSA calculations further endorse the thermodynamic favorability of these interactions, with binding free energies of -48.49 ± 0.11 kJ/mol for [125I]anastrozole and -43.8 kJ/mol for MTHFD2-. The significant contribution of Van der Waals and electrostatic interactions to the binding affinities of [125I]anastrozole and [125I]epirubicin, respectively, underscores their potential efficacy for targeted tumor imaging and therapy. These computational findings lay the groundwork for the future experimental validation of [125I]anastrozole and [125I]epirubicin as MTHFD2 inhibitors, heralding a notable advancement in precision oncology tools. The data necessitate subsequent in vitro and in vivo assays to corroborate these results.

Design, Synthesis, and Biological Evaluation of a Novel [18F]-Labeled Arginine Derivative for Tumor Imaging

To better diagnose and treat tumors related to arginine metabolism, (2S,4S)-2-amino-4-(4-(2-(fluoro-18F)ethoxy)benzyl)-5-guanidinopentanoic acid ([18F]7) was designed and prepared by introducing [18F]fluoroethoxy benzyl on carbon-4 of arginine. [18F]7 and 7 were successfully prepared using synthesis methods similar to those used for (2S,4S)-4-[18F]FEBGln and (2S,4S)-4-FEBGln, respectively. In vitro experiments on cell transport mechanisms showed that [18F]7 was similar to (2S,4S)4-[18F]FPArg and was transported into tumor cells by cationic amino acid transporters. However, [18F]7 can also enter MCF-7 cells via ASC and ASC2 amino acid transporters. Further microPET-CT imaging showed that the initial uptake and retention properties of [18F]7 in MCF-7 subcutaneous tumors were good (2.29 ± 0.09%ID/g at 2.5 min and 1.71 ± 0.09%ID/g at 60 min after administration), without significant defluorination in vivo. However, compared to (2S,4S)4-[18F]FPArg (3.06 ± 0.59%ID/g at 60 min after administration), [18F]7 exhibited lower tumor uptake and higher nonspecific uptake. When further applied to U87MG imaging, [18F]7 can quickly visualize brain gliomas (tumor-to-brain, 1.85 at 60 min after administration). Therefore, based on the above results, [18F]7 will likely be applied for the diagnosis of arginine nutrition-deficient tumors and efficacy evaluations.

Automated sulfur-[18F]fluoride exchange radiolabelling of a prostate specific membrane antigen (PSMA) targeted ligand using the GE FASTlab™ cassette-based platform

Sulfur-[18F]fluoride exchange radiochemistry is a rapid and convenient method for incorporating fluorine-18 into biologically active molecules. We report a fully automated radiolabelling procedure for the synthesis of a [18F]SO3F-bearing prostate specific membrane antigen (PSMA) targeted ligand ([18F]5) using the GE FASTLab™ cassette-based platform in a 25.0 ± 2.6% radiochemical yield (decay corrected). Uptake in vitro and in vivo correlated with PSMA expression, and the radioligand exhibited favourable biodistribution and pharmacokinetic profiles.

Fully Automated Radiosynthesis of [18F]mG4P027 for mGluR4 Imaging

Fluorine-18 labeled N-(4-chloro-3-(((fluoro-¹⁸F)methyl-d₂)thio)phenyl)picolinamide, [¹⁸F]mG4P027, is a potent positron emission tomography (PET) radiotracer for metabotropic glutamate receptor 4 (mGluR4). Our previous in vitro and in vivo evaluations have demonstrated that this tracer is promising for further translational studies. To automate the radiosynthesis of [¹⁸F]mG4P027, significant modifications were made to the manual process by carefully examining this process and addressing the root causes of the challenges associated with its automation. We successfully implemented its automated radiosynthesis using the TRACERlab FX2N module and consequently, obtained a high-purity radiolabeled [¹⁸F]mG4P027 in high yield, meeting the requirements for future human studies.

Positron Emission Tomography Probes for Imaging Cytotoxic Immune Cells

Non-invasive positron emission tomography (PET) imaging of immune cells is a powerful approach for monitoring the dynamics of immune cells in response to immunotherapy. Despite the clinical success of many immunotherapeutic agents, their clinical efficacy is limited to a subgroup of patients. Conventional imaging, as well as analysis of tissue biopsies and blood samples do not reflect the complex interaction between tumour and immune cells. Consequently, PET probes are being developed to capture the dynamics of such interactions, which may improve patient stratification and treatment evaluation. The clinical efficacy of cancer immunotherapy relies on both the infiltration and function of cytotoxic immune cells at the tumour site. Thus, various immune biomarkers have been investigated as potential targets for PET imaging of immune response. Herein, we provide an overview of the most recent developments in PET imaging of immune response, including the radiosynthesis approaches employed in their development.

Lanthanide porphyrinoids as molecular theranostics

In recent years, lanthanide (Ln) porphyrinoids have received increasing attention as theranostics. Broadly speaking, the term 'theranostics' refers to agents designed to allow both disease diagnosis and therapeutic intervention. This Review summarises the history and the 'state-of-the-art' development of Ln porphyrinoids as theranostic agents. The emphasis is on the progress made within the past decade. Applications of Ln porphyrinoids in near-infrared (NIR, 650-1700 nm) fluorescence imaging (FL), magnetic resonance imaging (MRI), radiotherapy, and chemotherapy will be discussed. The use of Ln porphyrinoids as photo-activated agents ('phototheranostics') will also be highlighted in the context of three promising strategies for regulation of porphyrinic triplet energy dissipation pathways, namely: regioisomeric effects, metal regulation, and the use of expanded porphyrinoids. The goal of this Review is to showcase some of the ongoing efforts being made to optimise Ln porphyrinoids as theranostics and as phototheranostics, in order to provide a platform for understanding likely future developments in the area, including those associated with structure-based innovations, functional improvements, and emerging biological activation strategies.

A practical guide to automating fluorine-18 PET radiochemistry using commercially available cassette-based platforms

This Tutorial Account aims to be a useful educational resource which describes how to automate fluorine-18 positron emission tomography (PET) radiochemistry using cassette-based automated radiosynthesis platforms.

A kit-based aluminium-[18F]fluoride approach to radiolabelled microbubbles

The production of 18F-labelled microbubbles (MBs) via the aluminium-[18F]fluoride ([18F]AlF) radiolabelling method and facile inverse-electron-demand Diels-Alder (IEDDA) 'click' chemistry is reported. An [18F]AlF-NODA-labelled tetrazine was synthesised in excellent radiochemical yield (>95% RCY) and efficiently conjugated to a trans-cyclooctene (TCO) functionalised phospholipid (40-50% RCY), which was incorporated into MBs (40-50% RCY). To demonstrate the potential of producing 18F-labelled MBs for clinical studies, we also describe a kit-based approach which is amenable for use in a hospital radiopharmacy setting.

Development and Preclinical Evaluation of Radiolabeled Covalent G12C-Specific Inhibitors for Direct Imaging of the Oncogenic KRAS Mutant

Although KRAS has been an important target for many cancers, direct inhibition of oncogenic RAS remains challenging. Until recently, covalent KRAS G12C-specific inhibitors have been developed and progressed to the clinics. Nevertheless, not all patients benefit from these covalent inhibitors. At present, identification of candidates for this treatment requires tissue biopsies and gene sequencing, which are invasive, time-consuming, and could be of insufficient quality and limited predictive value owing to tumor heterogeneity. The use of noninvasive molecular imaging techniques such as PET and SPECT for spying KRAS G12C mutation in tumors provide a promising strategy for circumventing these hurdles. In the present study, based on the covalent G12C-specific inhibitor ARS-1620, we sought to develop radiolabeled small molecules for direct imaging of the KRAS mutation status in tumors. [¹³¹I]I-ARS-1620 and [¹⁸F]F-ARS-1620 were successfully prepared with high radiochemical yield, radiochemical purity, and molar activity. In vitro and in vivo studies have demonstrated the affinity, specificity, and capacity of [¹³¹I]I-ARS-1620 for direct imaging of the oncogenic KRAS G12C mutant. This initial attempt allows us to directly screen the KRAS G12C mutant for the first time in vivo.

The aluminium-[18F]fluoride revolution: simple radiochemistry with a big impact for radiolabelled biomolecules

The aluminium-[¹⁸F]fluoride ([¹⁸F]AlF) radiolabelling method combines the favourable decay characteristics of fluorine-18 with the convenience and familiarity of metal-based radiochemistry and has been used to parallel gallium-68 radiopharmaceutical developments. As such, the [¹⁸F]AlF method is popular and widely implemented in the development of radiopharmaceuticals for the clinic. In this review, we capture the current status of [¹⁸F]AlF-based technology and reflect upon its impact on nuclear medicine, as well as offering our perspective on what the future holds for this unique radiolabelling method.

Metabolic activity by FDG-PET/CT after neoadjuvant chemotherapy in borderline resectable and locally advanced pancreatic cancer and association with survival

BACKGROUND

The optimal prognostic markers for neoadjuvant chemotherapy in patients with borderline resectable or locally advanced pancreatic cancer are not yet established.

METHOD

Patients who received neoadjuvant chemotherapy prior to surgery and underwent FDG-PET/CT between July 2012 and December 2017 were included. Metabolic parameters including standardised uptake value (SUV), metabolic tumour volume (MTV), and total lesion glycolysis (TLG) on PET/CT, and response evaluations using PERCIST criteria, were investigated for its impact on survival and recurrence. Cox proportional hazards model was performed. Differences in risk were expressed as hazard ratio [HR] with 95% confidence interval [c.i.].

RESULTS

The patients with borderline resectable (N = 106) or locally advanced pancreatic cancer (N = 82) were identified. The median survival was 33.6 months. Decreased metabolic parameters of PET/CT after neoadjuvant chemotherapy were associated with positive impacts on survival and recurrence such as SUVmax (HR 1.16, 95% c.i. 1.01 to 1.32, P = 0.025), SUVpeak (HR 1.26, 95% c.i. 1.05 to 1.51, P = 0.011), and MTV (HR 1.15, 95% c.i. 1.04 to 1.26, P = 0.005). Large delta values were related to a positive impact on recurrence such as SUVmax (HR 1.21, 95% c.i. 1.06 to 1.38, P = 0.005). Post-neoadjuvant chemotherapy SUVmax ≥3 (HR 3.46, 95% c.i. 1.21 to 9.91; P = 0.036) was an independent prognostic factor for negative impact on survival. Patients with post-neoadjuvant chemotherapy SUVmax <3 showed more chemotherapy cycles (8.7 versus 6.2, P = 0.001), more frequent complete metabolic response (25 vs 2.2%, P = 0.002), smaller tumour size (2.1 vs 3.1 cm, P = 0.002), and less frequent lymphovascular invasion (23.7 vs 51.1%, P = 0.020) than patients with SUVmax ≥3.

CONCLUSION

Reduction in metabolic tumour parameters of FDG- PET/CT after neoadjuvant chemotherapy indicates improved overall survival and recurrence-free survival.

Scalable 18F processing conditions for copper-mediated radiofluorination chemistry facilitate DoE optimization studies and afford an improved synthesis of [18F]olaparib

A convenient and scalable base-free method for processing [¹⁸F]fluoride as [¹⁸F]TBAF is reported and applied to copper-mediated radiofluorination radiosyntheses. A central feature of this method is that a single production of [¹⁸F]TBAF can be divided into small aliquots that can be used to perform multiple small-scale reactions in DoE optimization studies. The results of these studies can then be reliably translated to full batch tracer productions using automated synthesizers. The processing method was applied to the DoE optimization of [¹⁸F]olaparib, affording the tracer in high radiochemical yields via both manual (%RCY = 78 ± 6%, n = 4 (CMRF step only)) and automated (up to 80% (%RCY); 41% activity yield (%AY)) radiosynthesis procedures.

Radiolabelling an 18F biologic via facile IEDDA "click" chemistry on the GE FASTLab™ platform

The use of biologics in positron emission tomography (PET) imaging is an important area of radiopharmaceutical development and new automated methods are required to facilitate their production. We report an automated radiosynthesis method to produce a radiolabelled biologic via facile inverse electron demand Diels-Alder (IEDDA) "click" chemistry on a single GE FASTLab™ cassette. We exemplified the method by producing a fluorine-18 radiolabelled interleukin-2 (IL2) radioconjugate from a trans-cyclooctene (TCO) modified IL2 precursor. The radioconjugate was produced using a fully automated radiosynthesis on a single FASTLab™ cassette in a decay-corrected radiochemical yield (RCY, d.c.) of 19.8 ± 2.6% in 110 min (from start of synthesis); the molar activity was 132.3 ± 14.6 GBq μmol-1. The in vitro uptake of [18F]TTCO-IL2 correlated with the differential receptor expression (CD25, CD122, CD132) in PC3, NK-92 and activated human PBMCs. The automated method may be adapted for the radiosynthesis of any TCO-modified protein via IEDDA chemistry.

Synthesis and evaluation of 3'-[18F]fluorothymidine-5'-squaryl as a bioisostere of 3'-[18F]fluorothymidine-5'-monophosphate

The squaryl moiety has emerged as an important phosphate bioisostere with reportedly greater cell permeability. It has been used in the synthesis of several therapeutic drug molecules including nucleoside and nucleotide analogues but is yet to be evaluated in the context of positron emission tomography (PET) imaging. We have designed, synthesised and evaluated 3'-[18F]fluorothymidine-5'-squaryl ([18F]SqFLT) as a bioisostere to 3'-[18F]fluorothymidine-5'-monophosphate ([18F]FLTMP) for imaging thymidylate kinase (TMPK) activity. The overall radiochemical yield (RCY) was 6.7 ± 2.5% and radiochemical purity (RCP) was >90%. Biological evaluation in vitro showed low tracer uptake (<0.3% ID mg-1) but significantly discriminated between wildtype HCT116 and CRISPR/Cas9 generated TMPK knockdown HCT116shTMPK-. Evaluation of [18F]SqFLT in HCT116 and HCT116shTMPK- xenograft mouse models showed statistically significant differences in tumour uptake, but lacked an effective tissue retention mechanism, making the radiotracer in its current form unsuitable for PET imaging of proliferation.

Non-oncological applications of RGD-based single-photon emission tomography and positron emission tomography agents

INTRODUCTION

Non-invasive imaging techniques (especially single-photon emission tomography and positron emission tomography) apply several RGD-based imaging ligands developed during a vast number of preclinical and clinical investigations. The RGD (Arg-Gly-Asp) sequence is a binding moiety for a large selection of adhesive extracellular matrix and cell surface proteins. Since the first identification of this sequence as the shortest sequence required for recognition in fibronectin during the 1980s, fundamental research regarding the molecular mechanisms of integrin action have paved the way for development of several pharmaceuticals and radiopharmaceuticals with clinical applications. Ligands recognizing RGD may be developed for use in the monitoring of these interactions (benign or pathological). Although RGD-based molecular imaging has been actively investigated for oncological purposes, their utilization towards non-oncology applications remains relatively under-exploited.

METHODS AND SCOPE

This review highlights the new non-oncologic applications of RGD-based tracers (with the focus on single-photon emission tomography and positron emission tomography). The focus is on the last 10 years of scientific literature (2009-2020). It is proposed that these imaging agents will be used for off-label indications that may provide options for disease monitoring where there are no approved tracers available, for instance Crohn's disease or osteoporosis. Fundamental science investigations have made progress in elucidating the involvement of integrin in various diseases not pertaining to oncology. Furthermore, RGD-based radiopharmaceuticals have been evaluated extensively for safety during clinical evaluations of various natures.

CONCLUSION

Clinical translation of non-oncological applications for RGD-based radiopharmaceuticals and other imaging tracers without going through time-consuming extensive development is therefore highly plausible. Graphical abstract.