A fully automated radiosynthesis of [18F]fluoroethyl-diprenorphine on a single module by use of SPE cartridges for preparation of high quality 2-[18F]fluoroethyl tosylate

Optimization of a Nucleophilic Two-Step Radiosynthesis of 6-O-(2-[18F]fluoroethyl)-6-O-desmethyl-diprenorphine ([18F]FE-DPN) for PET Imaging of Brain Opioid Receptors

We have established a method for nucleophilic one-pot, two-step radiosynthesis of the popular opioid receptor radioligand 6-O-(2-[18F]fluoroethyl)-6-O-desmethyl-diprenorphine ([18F]FE-DPN) from the novel precursor 6-O-(2-tosyloxyethyl)-6-O-desmethyl- 3-O-trityl-diprenorphine (TE-TDDPN), which we designate as the Henriksen precursor. We undertook an optimization of the synthesis conditions, aiming to enhance the accessibility of [18F]FE-DPN for positron emission tomography (PET) studies of μ-opioid receptors. Herein, we report an optimized direct nucleophilic 18F-fluorination and the deprotection conditions for a fully automated radiosynthesis of [18F]FE-DPN on a modified GE Tracerlab FX FE synthesis panel. Starting from 1-1.5 GBq of [18F]fluoride and applying an Oasis Max 1cc cartridge for fluorine-18 trapping with a reduced amount of K2CO3 (5.06 μmol K+ ion), [18F]FE-DPN ([18F]11) was produced with 44.5 ± 10.6 RCY (decay-corrected), high radiochemical purity (>99%), and a molar activity of 32.2 ± 11.8 GBq/μmol in 60-65 min.

Pharmacokinetic analysis of 6-O-[18F]FEE for PET imaging of EGFR mutation

6-O-[¹⁸F]Fluoroethylerlotinib (6-O-[¹⁸F]FEE), with a suitable half-life for commercial distribution, may be a good replacement for [¹¹C]erlotinib to identify epidermal growth factor receptor (EGFR) positive tumors with activating mutations to tyrosine kinase inhibitors therapy. In this study, we explored the fully automated synthesis of 6-O-[¹⁸F]FEE and investigated its pharmacokinetics in tumor-bearing mice. 6-O-[¹⁸F]FEE with high specific activity (28-100 GBq/μmol) and radiochemistry purity (over 99 %) was obtained by two-step reaction and Radio-HPLC separation in PET-MF-2 V-IT-1 automated synthesizer. PET imaging of 6-O-[¹⁸F]FEE in HCC827, A431, and U87 tumor-bearing mice with different EGFR expression and mutation was performed. Uptake and blocking of PET imaging indicated that the probe specifically targeted exon 19 deleted EGFR (the quantitative analysis of tumor-to-mouse ratio for HCC827, HCC827 blocking, U87, A431 was 2.58 ± 0.24, 1.20 ± 0.15, 1.18 ± 0.19, and 1.05 ± 0.13 respectively). Dynamic imaging was used to study the pharmacokinetics of the probe in tumor-bearing mice. Logan plot graphical analysis demonstrated late linearity and a high fitting correlation coefficient (0.998), supporting reversible kinetics. According to the Akaike Information Criterion (AIC) rule, the 2-compartment reversible model was more consistent with the metabolic properties of 6-O-[¹⁸F]FEE. The automated radiosynthesis and pharmacokinetic analysis will promote clinically transformation of 6-O-[¹⁸F]FEE.

A closer look at the synthesis of 2-[18F]fluoroethyl tosylate to minimize the formation of volatile side-products

BACKGROUND

2-[¹⁸F]Fluoroethyltosylate ([¹⁸F]FEtOTs) is a well-known ¹⁸F-fluoroalkylating agent widely used to synthesize radiotracers for positron emission tomography. The widespread use of [¹⁸F]FEtOTs is due in part to its low volatility when compared to other halide and sulfonate building blocks. In this work, the radioactive volatile side-products formed during the synthesis of [¹⁸F]FEtOTs were identified and characterized for the first time, and an optimization of the reaction conditions to minimize their formation was proposed.

RESULTS

In order to characterize the volatiles produced during [¹⁸F]FEtOTs synthesis, the reaction mixtures of both cold FEtOTs and [¹⁸F]FEtOTs were co-injected onto the HPLC system. The radioactive peaks corresponding to the volatile compounds were collected, analyzed through headspace gas chromatography mass spectrometry sampler (HS-GC-MS) and identified as vinyl fluoride ([¹⁹F]VF) and 2-fluoroethanol ([¹⁹F]FEOH). By using a rotatable central composite design with a two-level full factorial core of two factors (2²), it was determined that temperature and time are independent variables which affect the generation of [¹⁸F]VF and [¹⁸F]FEOH during the radiosynthesis of [¹⁸F]FEtOTs. In addition, in order to reduce the formation of the volatiles ([¹⁸F]VF and [¹⁸F]FEOH) and increase the yield of [¹⁸F]FEtOTs, it was demonstrated that the molar ratio of base to precursor must also be considered.

CONCLUSION

[¹⁸F]VF and [¹⁸F]FEOH are volatile side-products formed during the radiosynthesis of [¹⁸F]FEtOTs, whose yields depend on the reaction time, temperature, and the molar ratio of base to precursor. Therefore, special care should be taken during the radiosynthesis and subsequent reactions using [¹⁸F]FEOTs in order to avoid environmental contamination and to improve the yield of the desired products.

Diels-Alder Adducts of Morphinan-6,8-Dienes and Their Transformations

6,14-ethenomorphinans are semisynthetic opiate derivatives containing an ethylene bridge between positions 6 and 14 in ring-C of the morphine skeleton that imparts a rigid molecular structure. These compounds represent an important family of opioid receptor ligands in which the 6,14-etheno bridged structural motif originates from a [4 + 2] cycloaddition of morphinan-6,8-dienes with dienophiles. Certain 6,14-ethenomorphinans having extremely high affinity for opioid receptors are often non-selective for opioid receptor subtypes, but this view is now undergoing some revision. The agonist 20R-etorphine and 20R-dihydroetorphine are several thousand times more potent analgesics than morphine, whereas diprenorphine is a high-affinity non-selective antagonist. The partial agonist buprenorphine is used as an analgesic in the management of post-operative pain or in substitution therapy for opiate addiction, sometimes in combination with the non-selective antagonist naloxone. In the context of the current opioid crisis, we communicated a summary of several decades of work toward generating opioid analgesics with lesser side effects or abuse potential. Our summary placed a focus on Diels-Alder reactions of morphinan-6,8-dienes and subsequent transformations of the cycloadducts. We also summarized the pharmacological aspects of radiolabeled 6,14-ethenomorphinans used in molecular imaging of opioid receptors.

A New Precursor for the Radiosynthesis of 6-O-(2-[18F]Fluoroethyl)-6-Odesmethyl- diprenorphine ([18F]FE-DPN) by Nucleophilic Radiofluorination

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We present the preparation of the new precursor 6-O-(2-tosyloxyethoxy)-6-O-desmethyl-3- O-trityl-diprenorphine (TE-TDDPN) for a one-pot, two-step nucleophilic radiosynthesis of 6-O-(2- [18F]fluoroethyl-6-O-desmethyl-diprenorphine ([18F]FE-DPN). The route to the precursor consists of a five-step synthesis starting from diprenorphine. We also provide alternative synthesis routes for the cold reference standard and the complete 1H- and 13C-NMR assignment of the prepared derivatives.

Fluorinated and [18F]fluorinated morphinan based opioid ligands

It is well documented in the literature that opioid receptors modulate a large number of physiological functions (pain perception, breathing, mood, gastrointestinal motility, etc.). Natural opiates and 4,5α-epoxymorphinan derivatives obtained by their chemical modifications, which are frequently referred to as semi-synthetic opioids, are among the most important types of opioid ligands. On the other hand, fluorinated compounds have a remarkable record in medicinal chemistry providing developmental candidates for therapeutic applications. The reasons are very similar steric impacts of hydrogen and fluorine along with the influence of substituting fluorine for hydrogen in the molecules of exogenous compounds on their lipophilicity, metabolism, conformation and other properties. This review focuses on the functionalization of 4,5α-epoxymorphinans and their derivatives via substitutions with fluorine or fluorine-containing groups in the search for improved pharmacological profile opioid ligands and 18F-containing opioid receptor radioligands for PET. These functionalizations are typically associated with substituents either at the C(3)-O, C(6)-O, and N(17) positions of the 4,5α-epoxymorphinan core or at C(7) in the thebaine based Diels-Alder type adducts. The syntheses resulted in the preparation of both single fluorinated derivatives or short sets of fluorinated derivatives and the families of fluorine-containing opioids allowing, in principle, the structure-activity relationship studies.

Synthesis and Evaluation of [18F]FEtLos and [18F]AMBF3Los as Novel 18F-Labelled Losartan Derivatives for Molecular Imaging of Angiotensin II Type 1 Receptors

Losartan is widely used in clinics to treat cardiovascular related diseases by selectively blocking the angiotensin II type 1 receptors (AT₁Rs), which regulate the renin-angiotensin system (RAS). Therefore, monitoring the physiological and pathological biodistribution of AT₁R using positron emission tomography (PET) might be a valuable tool to assess the functionality of RAS. Herein, we describe the synthesis and characterization of two novel losartan derivatives PET tracers, [¹⁸F]fluoroethyl-losartan ([¹⁸F]FEtLos) and [¹⁸F]ammoniomethyltrifluoroborate-losartan ([¹⁸F]AMBF₃Los). [¹⁸F]FEtLos was radiolabeled by ¹⁸F-fluoroalkylation of losartan potassium using the prosthetic group 2-[¹⁸F]fluoroethyl tosylate; whereas [¹⁸F]AMBF₃Los was prepared following an one-step ¹⁸F-¹⁹F isotopic exchange reaction, in an overall yield of 2.7 ± 0.9% and 11 ± 4%, respectively, with high radiochemical purity (>95%). Binding competition assays in AT₁R-expressing membranes showed that AMBF₃Los presented an almost equivalent binding affinity (K_i 7.9 nM) as the cold reference Losartan (K_i 1.5 nM), unlike FEtLos (K_i 2000 nM). In vitro and in vivo assays showed that [¹⁸F]AMBF₃Los displayed a good binding affinity for AT₁R-overexpressing CHO cells and was able to specifically bind to renal AT₁R. Hence, our data demonstrate [¹⁸F]AMBF₃Los as a new tool for PET imaging of AT₁R with possible applications for the diagnosis of cardiovascular, inflammatory and cancer diseases.

A Survey of Molecular Imaging of Opioid Receptors

The discovery of endogenous peptide ligands for morphine binding sites occurred in parallel with the identification of three subclasses of opioid receptor (OR), traditionally designated as μ, δ, and κ, along with the more recently defined opioid-receptor-like (ORL1) receptor. Early efforts in opioid receptor radiochemistry focused on the structure of the prototype agonist ligand, morphine, although N-[methyl-¹¹C]morphine, -codeine and -heroin did not show significant binding in vivo. [¹¹C]Diprenorphine ([¹¹C]DPN), an orvinol type, non-selective OR antagonist ligand, was among the first successful PET tracers for molecular brain imaging, but has been largely supplanted in research studies by the μ-preferring agonist [¹¹C]carfentanil ([¹¹C]Caf). These two tracers have the property of being displaceable by endogenous opioid peptides in living brain, thus potentially serving in a competition-binding model. Indeed, many clinical PET studies with [¹¹C]DPN or [¹¹C]Caf affirm the release of endogenous opioids in response to painful stimuli. Numerous other PET studies implicate μ-OR signaling in aspects of human personality and vulnerability to drug dependence, but there have been very few clinical PET studies of μORs in neurological disorders. Tracers based on naltrindole, a non-peptide antagonist of the δ-preferring endogenous opioid enkephalin, have been used in PET studies of δORs, and [¹¹C]GR103545 is validated for studies of κORs. Structures such as [¹¹C]NOP-1A show selective binding at ORL-1 receptors in living brain. However, there is scant documentation of δ-, κ-, or ORL1 receptors in healthy human brain or in neurological and psychiatric disorders; here, clinical PET research must catch up with recent progress in radiopharmaceutical chemistry.

Automated production of a N-methyl-D-aspartate receptor radioligand [18F]GE179 for clinical use

N-Methyl-d-aspartate (NMDA) receptors are ligand and voltage-gated heteromeric ion channel receptors. Excessive activation of NMDA receptors is implicated in many neurological and psychiatric disorders, including ischemic stroke, neuropathic pain, epilepsy, drug addition, Alzheimer's disease, and schizophrenia. [¹⁸F]GE179 is a promising PET probe for imaging functional NMDA receptor alterations (activated or 'open' channel) with a high binding affinity (K_d = 2.4 nM). Here, we report the production of the NMDA receptor radioligand [¹⁸F]GE179 in a current Good Manufacturing Practice (cGMP) facility through a one-pot two-step strategy. [¹⁸F]GE179 was produced in approximately 110 min with a radiochemical yield of 12 ± 6% (n = 4, decay corrected), radiochemical purity >95%, molar activity of 146 ± 32 GBq/μmol (at the end of synthesis), an average mass of GE179 at 2.2 μg/batch, and total impurities less than 0.5 μg/batch (n = 4). The radiopharmaceutical dose meets all quality control (QC) criteria for human use, and is suitable for clinical PET studies of activated NMDA receptor ion channels.

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.

Fluorine-18 labelled building blocks for PET tracer synthesis

This review presents a comprehensive overview of the synthesis and application of fluorine-18 labelled building blocks since 2010.

Synthesis and opioid receptor binding of indium (III) and [111In]-labeled macrocyclic conjugates of diprenorphine: novel ligands designed for imaging studies of peripheral opioid receptors

Radiolabeled diprenorphine (DPN) and analogs are widely used ligands for non-invasive brain imaging of opioid receptors. To develop complementary radioligands optimized for studies of the peripheral opioid receptors, we prepared a pair of hydrophilic DPN derivatives, conjugated to the macrocyclic chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), for complexation with trivalent metals. The non-radioactive indium (III) complexes, tethered to the C6-oxygen position of the DPN scaffold by 6- to 9-atom spacers, displayed high affinities for binding to μ, δ and κ opioid receptors in vitro. Use of the 9-atom linker conferred picomolar affinities equipotent to those of the parent ligand DPN. The [¹¹¹In]-labeled complexes were prepared in good yield (>70%), with high radiochemical purity (~99%) and high specific radioactivity (>4000 mCi/μmol). Their log D_7.4 values were -2.21 to -1.66. In comparison, DPN is lipophilic, with a log D_7.4 of +2.25. Further study in vivo is warranted to assess the suitability of these [¹¹¹In]-labeled DPN-DOTA conjugates for imaging trials.

2-[18F]Fluoroethyl tosylate – a versatile tool for building18F-based radiotracers for positron emission tomography

The review highlights the role of 2-[¹⁸F]fluoroethyltosylate ([¹⁸F]FETs) in PET radiotracer design since it is a preferred labeling reagent according to its high reactivity to phenolic, amine, thiophenolic and carboxylic functions.

Synthesis and evaluation of three structurally related ¹⁸F-labeled orvinols of different intrinsic activities: 6-O-[¹⁸F]fluoroethyl-diprenorphine ([¹⁸F]FDPN), 6-O-[¹⁸F]fluoroethyl-buprenorphine ([¹⁸F]FBPN), and 6-O-[¹⁸F]fluoroethyl-phenethyl-orvinol ([¹⁸F]FPEO)

We report the synthesis and biological evaluation of a triplet of 6-O-(18)F-fluoroethylated derivatives of structurally related orvinols that span across the full range of intrinsic activities, the antagonist diprenorphine, the partial agonist buprenorphine, and the full agonist phenethyl-orvinol. [(18)F]fluoroethyl-diprenorphine, [(18)F]fluoroethyl-buprenorphine, and [(18)F]fluoroethyl-phenethyl-orvinol were prepared in high yields and quality from their 6-O-desmethyl-precursors. The results indicate suitable properties of the three 6-O-(18)F-fluoroethylated derivatives as functional analogues to the native carbon-11 labeled versions with similar pharmacological properties.