Synthesis and in Vitro Evaluation of 18F- and 19F-Labeled Insulin: A New Radiotracer for PET-based Molecular Imaging Studies

Fully automated 18F-fluorination of N-succinimidyl-4-[18F]fluorobenzoate ([18F]SFB) for indirect labelling of nanobodies

N-succinimidyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB), a widely used labeling agent to introduce the 4-[¹⁸F]fluorobenzoyl-prosthetic group, is normally obtained in three consecutive steps from [¹⁸F]fluoride ion. Here, we describe an efficient one-step labeling procedure of [¹⁸F]SFB starting from a tin precursor. This method circumvents volatile radioactive side-products and simplifies automatization. [¹⁸F]SFB was obtained after HPLC purification in a yield of 42 + 4% and a radiochemical purity (RCP) > 99% (n = 6). In addition, we investigate the automation of the coupling of [¹⁸F]SFB to a nanobody (cAbBcII10, targeting β-lactamase enzyme) and purification by size exclusion chromatography (PD-10 desalting column) to remove unconjugated reagent. Production and use of [¹⁸F]SFB were implemented on a radiosynthesis unit (Neptis^®). The fully automated radiosynthesis process including purification and formulation required 160 min of synthesis time. [¹⁸F]SFB-labeled nanobody was obtained in a yield of 21 + 2% (activity yield 12 + 1% non-decay corrected) and a radiochemical purity (RCP) of > 95% (n = 3). This approach simplifies [¹⁸F]SFB synthesis to one-step, enhances the yield in comparison to the previous report and enables the production of radiolabeled nanobody on the same synthesis module.

Radiolabeling Methods

In vivo measurements of the deposition of an inhaled radiolabeled pharmaceutic have provided useful information related to the inhaler efficiency for depositing drug in the lung. A number of labeling techniques have been developed and applied to pharmaceutical aerosols delivered by pressurized metered-dose inhalers (pMDIs), dry powder inhalers (DPIs) and nebulizers; the choice of radiotracer depends on the type of imaging study being performed and the equipment used to image the lung. Preparation, validation and calibration of the radiolabeled pharmaceutical product is key to successful interpretation of the imaging study. When imaging a subject after inhalation of a radiolabeled formulation, it is the radioactivity that is detected and measured by the scanner; absolute amounts of deposited drug are inferred from the counts of radioactivity in the lung and other regions, based on the assumption that there is a 1:1 relationship between the two components-drug and radioactivity. This relationship holds true for direct-labeled PET products or for those formulations where a firm bond can be demonstrated between the drug and radiotracer for the time taken to acquire all the images. This chapter will discuss radiolabeling methods applied to therapeutic aerosols for the purpose of determining the deposition efficiency of these aerosols in the lung. The techniques apply to both in vivo studies in man and in animal models, and to some extent to in vitro models.

18F-Trifluoromethanesulfinate Enables Direct C-H 18F-Trifluoromethylation of Native Aromatic Residues in Peptides

18F labeling strategies for unmodified peptides with [18F]fluoride require 18F-labeled prosthetics for bioconjugation more often with cysteine thiols or lysine amines. Here we explore selective radical chemistry to target aromatic residues applying C-H 18F-trifluoromethylation. We report a one-step route to [18F]CF3SO2NH4 from [18F]fluoride and its application to direct [18F]CF3 incorporation at tryptophan or tyrosine residues using unmodified peptides as complex as recombinant human insulin. The fully automated radiosynthesis of octreotide[Trp(2-CF218F)] enables in vivo positron emission tomography imaging.

Continuous-Flow Synthesis of N-Succinimidyl 4-[18F]fluorobenzoate Using a Single Microfluidic Chip

In the field of positron emission tomography (PET) radiochemistry, compact microreactors provide reliable and reproducible synthesis methods that reduce the use of expensive precursors for radiolabeling and make effective use of the limited space in a hot cell. To develop more compact microreactors for radiosynthesis of ¹⁸F-labeled compounds required for the multistep procedure, we attempted radiosynthesis of N-succinimidyl 4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB) via a three-step procedure using a microreactor. We examined individual steps for [¹⁸F]SFB using a batch reactor and microreactor and developed a new continuous-flow synthetic method with a single microfluidic chip to achieve rapid and efficient radiosynthesis of [¹⁸F]SFB. In the synthesis of [¹⁸F]SFB using this continuous-flow method, the three-step reaction was successfully completed within 6.5 min and the radiochemical yield was 64 ± 2% (n = 5). In addition, it was shown that the quality of [¹⁸F]SFB synthesized on this method was equal to that synthesized by conventional methods using a batch reactor in the radiolabeling of bovine serum albumin with [¹⁸F]SFB.

(125)I-Tetrazines and Inverse-Electron-Demand Diels-Alder Chemistry: A Convenient Radioiodination Strategy for Biomolecule Labeling, Screening, and Biodistribution Studies

A convenient method to prepare radioiodinated tetrazines was developed, such that a bioorthogonal inverse electron demand Diels-Alder reaction can be used to label biomolecules with iodine-125 for in vitro screening and in vivo biodistribution studies. The tetrazine was prepared by employing a high-yielding oxidative halo destannylation reaction that concomitantly oxidized the dihydrotetrazine precursor. The product reacts quickly and efficiently with trans-cyclooctene derivatives. Utility was demonstrated through antibody and hormone labeling experiments and by evaluating products using standard analytical methods, in vitro assays, and quantitative biodistribution studies where the latter was performed in direct comparison to Bolton-Hunter and direct iodination methods. The approach described provides a convenient and advantageous alternative to conventional protein iodination methods that can expedite preclinical development and evaluation of biotherapeutics.

Positron emission tomography (PET) for assessing aerosol deposition of orally inhaled drug products

The topical distribution of inhaled therapies in the lung can be viewed using radionuclides and imaging. Positron emission tomography (PET) is a three-dimensional functional imaging technique providing quantitatively accurate localization of the quantity and distribution of an inhaled or injected PET radiotracer in the lung. A series of transaxial slices through the lungs are obtained, comparable to an X-ray computed tomography (CT) scan. Subsequent reformatting allows coronal and sagittal images of the distribution of radioactivity to be viewed. This article describes procedures for administering [(18)F]-fluorodeoxyglucose aerosol to human subjects for the purpose of determining dose and distribution following inhalation from an aerosol drug delivery device (ADDD). The advantages of using direct-labeled PET drugs in the ADDD are discussed with reference to the literature. The methods for designing the inhalation system, determining proper radiation shielding, calibration, and validation of administered radioactivity, scanner setup, and data handling procedures are described. Obtaining an X-ray CT or radionuclide transmission scan to provide accurate geometry of the lung and also correct for tissue attenuation of the PET radiotracer is discussed. Protocols for producing accurate images, including factors that need to be incorporated into the data calibration, are described, as well as a proposed standard method for partitioning the lung into regions of interest. Alternate methods are described for more detailed assessments. Radiation dosimetry/risk calculations for the procedures are appended, as well as a sample data collection form and spreadsheet for calculations. This article should provide guidance for those interested in using PET to determine quantity and distribution of inhaled therapeutics.

Lung imaging - two dimensional gamma scintigraphy, SPECT, CT and PET

This review will cover the principles of imaging the deposition of inhaled drugs and some of the state-of-the art imaging techniques being used today. Aerosol deposition can be imaged and quantified by the addition of a radiolabel to the aerosol formulation. The subsequent imaging of the inhaled deposition pattern can be acquired by different imaging techniques. Specifically, this review will focus on the use of two-dimensional planar, gamma scintigraphy, SPECT, CT and PET. This review will look at how these imaging techniques are used to investigate the mechanisms of drug delivery in the lung and how the lung anatomy and physiology have the potential to alter therapeutic outcomes.

Microwave-assisted one-pot synthesis of N-succinimidyl-4[ ¹⁸F]fluorobenzoate ([¹⁸F]SFB)

Biomolecules, including peptides¹⁻⁹, proteins¹⁰⁻¹¹, and antibodies and their engineered fragments¹²⁻¹⁴, are gaining importance as both potential therapeutics and molecular imaging agents. Notably, when labeled with positron-emitting radioisotopes (e.g., Cu-64, Ga-68, or F-18), they can be used as probes for targeted imaging of many physiological and pathological processes.¹⁵⁻¹⁸ Therefore, significant effort has devoted to the synthesis and exploration of ¹⁸F-labeled biomolecules. Although there are elegant examples of the direct ¹⁸F-labeling of peptides,¹⁹⁻²² the harsh reaction conditions (i.e., organic solvent, extreme pH, high temperature) associated with direct radiofluorination are usually incompatible with fragile protein samples. To date, therefore, the incorporation of radiolabeled prosthetic groups into biomolecules remains the method of choice.²³(,)²⁴ N-Succinimidyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB),²⁵⁻³⁷ a Bolton-Hunter type reagent that reacts with the primary amino groups of biomolecules, is a very versatile prosthetic group for the ¹⁸F-labeling of a wide spectrum of biological entities, in terms of its evident in vivo stability and high radiolabeling yield. After labeling with [¹⁸F]SFB, the resulting [F]fluorobenzoylated biomolecules could be explored as potential PET tracers for in vivo imaging studies.¹ Most [¹⁸F]SFB radiosyntheses described in the current literatures require two or even three reactors and multiple purifications by using either solid phase extraction (SPE) or high-performance liquid chromatography (HPLC). Such lengthy processes hamper its routine production and widespread applications in the radiolabeling of biomolecules. Although several module-assisted [¹⁸F]SFB syntheses have been reported²⁹⁻³²,⁴¹⁻⁴² they are mainly based on complicated and lengthy procedures using costly commercially-available radiochemistry boxes (Table 1). Therefore, further simplification of the radiosynthesis of [¹⁸F]SFB using a low-cost setup would be very beneficial for its adaption to an automated process. Herein, we report a concise preparation of [¹⁸F]SFB, based on a simplified one-pot microwave-assisted synthesis (Figure 1). Our approach does not require purification between steps or any aqueous reagents. In addition, microwave irradiation, which has been used in the syntheses of several PET tracers,³⁸⁻⁴¹ can gives higher RCYs and better selectivity than the corresponding thermal reactions or they provide similar yields in shorter reaction times.³⁸Most importantly, when labeling biomolecules, the time saved could be diverted to subsequent bioconjugation or PET imaging step. ²⁸(,)⁴³The novelty of our improved [¹⁸F]SFB synthesis is two-fold: (1) the anhydrous deprotection strategy requires no purification of intermediate(s) between each step and (2) the microwave-assisted radiochemical transformations enable the rapid, reliable production of [¹⁸F]SFB.

Deposition, imaging, and clearance: what remains to be done?

Deposition and clearance studies are used during product development and in fundamental research. These studies mostly involve radionuclide imaging, but pharmacokinetic methods are also used to assess the amount of drug absorbed through the lungs, which is closely related to lung deposition. Radionuclide imaging may be two-dimensional (gamma scintigraphy or planar imaging), or three-dimensional (single photon emission computed tomography and positron emission tomography). In October 2009, a group of scientists met at the "Thousand Years of Pharmaceutical Aerosols" conference in Reykjavik, Iceland, to discuss future research in key areas of pulmonary drug delivery. This article reports the session on "Deposition, imaging and clearance." The objective was partly to review our current understanding, but more importantly to assess "what remains to be done?" A need to standardize methodology and provide a regulatory framework by which data from radionuclide imaging methods could be compared between centers and used in the drug approval process was recognized. There is also a requirement for novel radiolabeling methods that are more representative of production processes for dry powder inhalers and pressurized metered dose inhalers. A need was identified for studies to aid our understanding of the relationship between clinical effects and regional deposition patterns of inhaled drugs. A robust methodology to assess clearance from small conducting airways should be developed, as a potential biomarker for therapies in cystic fibrosis and other diseases. The mechanisms by which inhaled nanoparticles are removed from the lungs, and the factors on which their removal depends, require further investigation. Last, and by no means least, we need a better understanding of patient-related factors, including how to reduce the variability in pulmonary drug delivery, in order to improve the precision of deposition and clearance measurements.

Radiosynthesis of [(18)F]fluoromethyldeoxyspergualin for molecular imaging of heat shock proteins

To probe the in vivo role of stress response factors in normal physiology and in solid tumors we have designed a stable (18)F-labeled molecular imaging agent based on a ligand for heat shock protein 70 (HSP70). We describe the synthesis of [(18)F] fluorodeoxymethylspergualin ([(18)F]MeDSG) as a new radiopharmaceutical probe using a prosthetic group, [(18)F]SFB, for efficient and rapid radiolabeling. Ongoing molecular imaging studies are under way to detect HSP70 expression in tumors by positron emission tomography.

Toward [18F]-labeled aryltrifluoroborate radiotracers: in vivo positron emission tomography imaging of stable aryltrifluoroborate clearance in mice

The use of a boronic ester as a captor of aqueous [(18)F]-fluoride has been previously suggested as a means of labeling biomolecules in one step for positron emission tomography (PET) imaging. For this approach to be seriously considered, the [(18)F]-labeled trifluoroborate should be humorally stable such that it neither leaches free [(18)F]-fluoride to the bone nor accumulates therein. Herein, we have synthesized a biotinylated boronic ester that is converted to the corresponding trifluoroborate salt in the presence of aqueous [(18)F]-fluoride. In keeping with its in vitro aqueous kinetic stability at pH 7.5, the trifluoroborate appears to clear in vivo quite rapidly to the bladder as the stable trifluoroborate salt with no detectable leaching of free [(18)F]-fluoride to the bone. When this labeled biotin is preincubated with avidin, the pharmacokinetic clearance of the resulting complex is visibly altered. This work validates initial claims that boronic esters are potentially useful as readily labeled precursors to [(18)F]-PET reagents.

Microwave-induced nucleophilic [18F]fluorination on aromatic rings: synthesis and effect of halogen on [18F]fluoride substitution of meta-halo (F, Cl, Br, I)-benzonitrile derivatives

The meta-halo-3-methylbenzonitrile derivatives (-F, -Cl, -Br, -I) were synthesized as model compounds to study reactivity towards aromatic nucleophilic substitution. A single-mode microwave system was incorporated into a commercial radiochemical synthetic module for (18)F labeling. Labeling yields of 64% for fluoro-, 13% for bromo- and 9% for chloro-precursors were achieved in DMSO in <3 min. The observed order of reactivity of the leaving groups toward aromatic nucleophilic substitution was F>>Br>Cl>>>I.

Synthesis of N-succinimidyl 4-[18F]fluorobenzoate, an agent for labeling proteins and peptides with 18F

This protocol describes the step-by-step procedure for the synthesis of N-succinimidyl 4-[18F]fluorobenzoate ([18F]SFB), an agent widely used for labeling proteins and peptides with the positron-emitting radionuclide 18F. The protocols for the synthesis of unlabeled SFB and the quaternary salt precursor 4-formyl-N,N,N-trimethyl benzenaminium trifluoromethane sulfonate also are described. For the [18F]SFB synthesis, the quaternary salt is first converted to 4-[18F]fluorobenzaldehyde. Oxidation of the latter provides 4-[18F]fluorobenzoic acid, which is converted to [18F]SFB by treatment with N,N-disuccinimidyl carbonate. Using this method, [18F]SFB can be synthesized in decay-corrected radiochemical yields of 30%-35% and a specific radioactivity of 11-12 GBq micromol(-1). The total synthesis and purification time required is about 80 min, starting from delivery of the [18F]fluoride. [18F]SFB remains an optimal reagent for labeling proteins and peptides with 18F because of good conjugation yields and metabolic stability.

3,4,6-Tri-O-acetyl-2-deoxy-2-[18F]fluoroglucopyranosyl phenylthiosulfonate: a thiol-reactive agent for the chemoselective 18F-glycosylation of peptides

3,4,5-Tri-O-acetyl-2-[18F]fluoro-2-deoxy-d-glucopyranosyl 1-phenylthiosulfonate (Ac3-[18F]FGlc-PTS) was developed as a thiol-reactive labeling reagent for the site-specific 18F-glycosylation of peptides. Taking advantage of highly accessible 1,3,4,6-tetra-O-acetyl-2-deoxy-2-[18F]fluoroglucopyranose, a three-step radiochemical pathway was investigated and optimized, providing Ac3-[18F]FGlc-PTS in a radiochemical yield of about 33% in 90 min (decay-corrected and based on starting [18F]fluoride). Ac3-[18F]FGlc-PTS was reacted with the model pentapeptide CAKAY, confirming chemoselectivity and excellent conjugation yields of >90% under mild reaction conditions. The optimized method was adopted to the 18F-glycosylation of the alphavbeta3-affine peptide c(RGDfC), achieving high conjugation yields (95%, decay-corrected). The alphavbeta3 binding affinity of the glycosylated c(RGDfC) remained uninfluenced as determined by competition binding studies versus 125I-echistatin using both isolated alphavbeta3 and human umbilical vein endothelial cells (Ki = 68 +/- 10 nM (alphavbeta3) versus Ki = 77 +/- 4 nM (HUVEC)). The whole radiosynthetic procedure, including the preparation of the 18F-glycosylating reagent Ac3-[18F]FGlc-PTS, peptide ligation, and final HPLC purification, provided a decay-uncorrected radiochemical yield of 13% after a total synthesis time of 130 min. Ac3-[18F]FGlc-PTS represents a novel 18F-labeling reagent for the mild chemoselective 18F-glycosylation of peptides indicating its potential for the design and development of 18F-labeled bioactive S-glycopeptides suitable to study their pharmacokinetics in vivo by positron emission tomography (PET).