Preparation of 18F-human serum albumin: a simple and efficient protein labeling method with 18F using a hydrazone-formation method

18F-labeling of proteins and peptides is important for positron emission tomography (PET). Although there are many methods for the labeling of proteins with (18)F, most of these are characterized by complicated procedures or low yields. Here, we report a novel and simple method which includes the preparation of [18F]fluorobenzaldehyde ([18F]FBA) and successive conjugation with hydrazinonicotinic acid-human serum albumin conjugate (HYNIC-HSA) via hydrazone formation. HYNIC-HSA, which can also be used for labeling with (99m)Tc, was prepared via reaction with N-hydroxysuccinimide (NHS) or tetrafluorophenyl (TFP) esters of HYNIC with HSA. No-carrier-added [18F]FBA was prepared by the nucleophilic substitution of [18F]fluoride to 4-trimethylammonium benzaldehyde triflate in the presence of tetrabutylammonium bicarbonate. [18F]FBA was purified by passing ion exchange cartridges (IC-H and QMA) and was adsorbed to a C18 Sep-Pak cartridge. The adsorbed [18F]FBA was then eluted with 50% ethanol. HYNIC-HSA was added to the solution and conjugated with [18F]FBA via hydrazone formation. 18F-HSA was purified with a PD10 column. Biodistribution of 18F-HSA, (99m)Tc-HSA, and [18F]FBA in mice were investigated at 10, 20, and 60 min after intravenous injection. The number of conjugated HYNIC molecules per HSA ranged from 5.2 to 23.2 depending on the reaction conditions. The labeling efficiency of 18F-FBA was 67 +/- 15.7%. The radiochemical purity after purification was over 99%. The conjugation efficiency of HYNIC-HSA with [18F]FBA was between 25% and 90%. The conjugation efficiency was observed to increase with increases in the number of conjugated HYNIC, the HYNIC-HSA concentration, or temperature. 18F-HSA exhibited a biodistribution pattern similar to that of (99m)Tc-HSA while [18F]FBA showed much lower blood activity than that of 18F-HSA and (99m)Tc-HSA. We concluded that 18F-HSA was successfully labeled using a novel method which involves hydrazone formation between [18F]FBA and HYNIC-HSA. This method can be applied to the 18F-labeling of other proteins or peptides.     

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).     

Synthesis and radiopharmacological evaluation of 2'-(4-fluorophenyl)-21-[18F]fluoro-20-oxo-11beta,17alpha-dihydroxy-pregn-4-eno[3,2-c]pyrazole as potential glucocorticoid receptor ligand for positron emission tomography (PET)

The radiosynthesis and the radiopharmacological evaluation of pyrazolo steroid 2'-(4-fluorophenyl)-21-[18F]fluoro-20-oxo-11beta,17alpha-dihydroxy-pregn-4-eno[3,2-c]pyrazole [18F]-2 is described. The radiolabeling was accomplished in 3-4% decay-corrected radiochemical yield within 80 min at an specific radioactivity of 0.8-1.2 Ci/micromol. Biodistribution studies in male Wistar rats showed an initial brain uptake of 0.25+/-0.03% ID/g after 5 min, which remained constant over 60 min. The radiopharmacological evaluation of compound [18F]-2 was completed with autoradiography using rat brain sections and micro-PET imaging.     

1-[3-(2-[18F]fluoropyridin-3-yloxy)propyl]pyrrole-2,5-dione: design, synthesis, and radiosynthesis of a new [18F]fluoropyridine-based maleimide reagent for the labeling of peptides and proteins

FPyME (1-[3-(2-fluoropyridin-3-yloxy)propyl]pyrrole-2,5-dione) was designed as a [(18)F]fluoropyridine-based maleimide reagent for the prosthetic labeling of peptides and proteins via selective conjugation with a thiol (sulfhydryl) function. Its pyridinyl moiety carries the radioactive halogen (fluorine-18) which can be efficiently incorporated via a nucleophilic heteroaromatic substitution, and its maleimido function ensures the efficient alkylation of a free thiol function as borne by cysteine residues. [(18)F]FPyME (HPLC-purified) was prepared in 17-20% non-decay-corrected yield, based on starting [(18)F]fluoride, in 110 min using a three-step radiochemical pathway. The developed procedure involves (1) a high-yield nucleophilic heteroaromatic ortho-radiofluorination on [3-(3-tert-butoxycarbonylaminopropoxy)pyridin-2-yl]trimethylammonium trifluoromethanesulfonate as the fluorine-18 incorporation step, followed by (2) rapid and quantitative TFA-induced removal of the N-Boc-protective group and (3) optimized maleimide formation using N-methoxycarbonylmaleimide. Typically, 4.8-6.7 GBq (130-180 mCi) of radiochemically pure [(18)F]FPyME ([(18)F]-1) could be obtained after semipreparative HPLC in 110 min starting from a cyclotron production batch of 33.3 GBq (900 mCi) of [(18)F]fluoride (overall radiochemical yields, based on starting [(18)F]fluoride: 28-37% decay-corrected). [(18)F]FPyME ([(18)F]-1) was first conjugated with a small model hexapeptide ((N-Ac)KAAAAC), confirming the excellent chemoselectivity of the coupling reaction (CH(2)SH versus CH(2)NH(2)) and then conjugated with two 8-kDa proteins of interest, currently being developed as tumor imaging agents (c-AFIM-0 and c-STxB). Conjugation was achieved in high yields (60-70%, isolated and non-decay-corrected) and used optimized, short-time reaction conditions (a 1/9 (v/v) mixture of DMSO and 0.05 M aq Tris NaCl buffer (pH 7.4) or 0.1 M aq PBS (pH 8), at room temperature for 10 min) and purification conditions (a gel filtration using a Sephadex NAP-10 cartridge or a SuperDex Peptide HR 10/30 column), both compatible with the chemical stability of the proteins and the relatively short half-life of the radioisotope concerned. The whole radiosynthetic procedure, including the preparation of the fluorine-18-labeled reagent, the conjugation with the protein and the final purification took 130-140 min. [(18)F]FPyME ([(18)F]-1) represents a new, valuable, thiol-selective, fluorine-18-labeled reagent for the prosthetic labeling with fluorine-18 of peptides and proteins. Because of its excellent chemoselectivity, [(18)F]FPyME offers an interesting alternative to the use of the nonselective carboxylate and amine-reactive [(18)F]reagents and can therefore advantageously be used for the design and development of new peptide- and protein-based radiopharmaceuticals for PET.     

In vivo evaluation of type 2 transglutaminase contribution to the metastasis formation in melanoma

Three strategies for chemoselective labeling of RGD peptides with ¹⁸F have been compared. Aminooxy [¹⁸F]fluorobenzaldehyde conjugation provided 40 ± 12% decay-corrected radiochemical yield using a fully automated method. An one-pot protocol for ‘click labeling’ of the RGD scaffold with 2-[¹⁸F]fluoroethylazide afforded 47 ± 8% decay-corrected radiochemical yield. Attempted conjugation with 3-[¹⁸F]fluoropropanethiol led to extensive decomposition and was therefore found unsuitable for labeling of the RGD peptide investigated. The results suggest that ‘click labeling’ of RGD peptides provides an attractive alternative to aminooxy aldehyde condensation, however, 2-[¹⁸F]-fluoroethylazide may be too small to allow separation of large ¹⁸F-labeled RGD peptides from their precursors.     

Synthesis and evaluation of [18F]Fluorobutyl ethacrynic amide: a potential PET tracer for studying glutathione transferase

[(18)F]Flurobutyl ethacrynic amide ([(18)F]FBuEA) was prepared from the precursor tosylate N-Boc-N-[4-(toluenesulfonyloxy)butyl]ethacrynic amide with a radiochemical yield of 3%, a specific activity of 48 GBq/μmol and radiochemical purity of 98%. Chemical conjugation of [(18)F]FBuEA with glutathione (GSH) via a self-coupling reaction and enzymatic conjugation under catalysis of glutathiontransferase alpha (GST-α) and π provided about 41% yields of radiochemical conjugated product [(18)F]FBuEA-GSH, 85% and 5-16%, respectively. The catalytic selectivity of this tracer toward GST-alpha was addressed. Positron emission tomography (PET) imaging of [(18)F]FBuEA in normal rats showed that a homogeneous pattern of radioactivity was distributed in the liver, suggesting a catalytic role of GST. By contrast, PET images of [(18)F]FBuEA in rats with thioacetamide-induced cholangiocarcinoma displayed a heterogeneous pattern of radioactive accumulation with cold spots in tumor lesions. PET imaging with [(18)F]FBuEA could be used for early diagnosis of hepatic tumor with a low GST activity as well as liver function.     

Synthesis of 4-(5-[18F]fluoromethyl-3-phenylisoxazol-4-yl)benzenesulfonamide, a new [18F]fluorinated analogue of valdecoxib, as a potential radiotracer for imaging cyclooxygenase-2 with positron emission tomography

Fluoroalkyl and fluoroaryl analogues of valdecoxib were found to possess potent inhibitory activities against cyclooxygenase-2 comparable to that of the parent valdecoxib. Among them, the fluoromethyl analogue was chosen for 18F-labeling. Thus, 4-(5-[18F]fluoromethyl-3-phenylisoxazol-4-yl)benzenesulfonamide (approximately 2000 Ci/mmol at end of synthesis) was synthesized by [18F]fluoride-ion displacement of the corresponding tosylate in approximately 40% decay-corrected radiochemical yield within approximately 120 min from end of bombardment.     

Synthesis of structurally identical fluorine-18 and iodine isotope labeling compounds for comparative imaging

The synthesis of a benzophenone-based labeling compound designed for comparative imaging studies with both in vivo positron emission tomograph (PET) and single-photon computed tomography (SPECT) and ex vivo autoradiography is described. The new compound can be labeled with either F-18 or iodine radioisotopes to give two different radioisotopmers: N-[2-fluoro-5-(3-[I-131]iodobenzoyl)benzyl]-2-bromoacetamide (1) and N-[2-[F-18]fluoro-5-(3-iodobenzoyl)benzyl]-2-bromoacetamide (2). Compound 1 and 2 have a 2-bromoacetyl group, which can be used to conjugate with biomolecules through a nucleophilic substitution reaction. Compound 1 was synthesized from the corresponding tributyltin derivatives via an oxidative destannylation reaction, and compound 2 was prepared via a four-step radiosynthesis (nucleophilic aromatic substitution, reduction, oxidation, and alkylation) starting from 4-(N,N,N-trimethylammonio)-3-cyano-3'-iodobenzophenone triflate. A remarkably high radiochemical yield (>90%) was achieved for the F-18 nucleophilic aromatic substitution under mild conditions (room temperature in less than 10 min), indicating the structural advantage of the designed molecule to facilitate the F-18 for trimethylammonium substitution in the presence of two electron-withdrawing groups (nitrile and carbonyl). The overall radiosynthesis time for compound 2 is less than 3 h after end of bombardment (EOB) with an unoptimized radiochemical yield of about 2% (not decay corrected) and specific activity of 0.8 Ci/micromol at EOB. The radiolabeling precursors for compound 1 and 2 were synthesized via a carbon-carbon bond-forming reaction between 2-substituted-5-lithiobenzonitrile and 3-substituted benzaldehyde derivatives. Compounds 1 and 2 should allow us to label biomolecules with F-18 or iodine isotopes and gives structurally identical products, which are expected to have identical biological properties and should be useful for comparative imaging studies.     

Site-specific labeling of annexin V with F-18 for apoptosis imaging

Annexin V is useful in detecting apoptotic cells by binding to phosphatidylserine (PS) that is exposed on the outer surface of the cell membrane during apoptosis. In this study, we examined the labeling of annexin V-128, a mutated form of annexin V that has a single cysteine residue at the NH 2 terminus, with the thiol-selective reagent (18)F-labeling agent N-[4-[(4-[(18)F]fluorobenzylidene)aminooxy]butyl]maleimide ([(18)F]FBABM). We also examined the cell binding affinity of the (18)F-labeled annexin V-128 ([(18)F]FAN-128). [(18)F]FBABM was synthesized in two-step, one-pot method modified from literature procedure. (Toyokuni et al., Bioconjugate Chem. 2003, 14, 1253-1259). The average yield of [(18)F]FBABM was 23 +/- 4% (n = 4, decay-corrected) and the specific activity was approximately 6000 Ci/mmol. The total synthesis time was approximately 92 min. The critical improvement of this study was identifying and then developing a purification method to remove an impurity N-[4-[(4-dimethylaminobenzylidene)aminooxy]butyl]maleimide 4, whose presence dramatically decreased the yield of protein labeling. Conjugation of [(18)F]FBABM with the thiol-containing annexin V-128 gave [(18)F]FAN-128 in 37 +/- 9% yield (n = 4, decay corrected). Erythrocyte binding assay of [(18)F]FAN-128 showed that this modification of annexin V-128 did not compromise its membrane binding affinity. Thus, an in vivo investigation of [ (18)F]FAN-128 as an apoptosis imaging agent is warranted.     

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.     

Radiolabeling of RGD peptide and preliminary biological evaluation in mice bearing U87MG tumors

2-[(18)F]Fluoroethyl azide ([(18)F]FEA) and terminal alkynyl modified propioloyl RGDfK were selected in this study. [(18)F]FEA was prepared by nucleophilic radiofluorination of 2-azidoethyl 4-toluenesulfonate with radiochemical yield of 71 ± 4% (n = 5, decay-corrected). We assessed the various conditions of the CuAAC reaction between [(18)F]FEA and propioloyl RGDfK, which included peptide concentration, reaction time, temperature and catalyst dosage. The (18)F-labeled-RGD peptide ([(18)F]F-RGDfK) could be obtained in 60 min by a two-step radiochemical synthesis route, with total radiochemical yield of 60 ± 2% (n = 3, decay-corrected) through click chemistry. [(18)F]F-RGDfK showed high stability in phosphate buffered saline and new-born calf serum. Micro-PET imaging at 1 h post injection of [(18)F]F-RGDfK showed medium concentration of radioactivity in tumors while much decreased concentration in tumors in the blocking group. These results showed that [(18)F]F-RGDfK obtained by click chemistry maintained the affinity and specificity of the RGDfK peptide to integrin α(v)β(3). This study provided useful information for peptide radiofluorination by using click chemistry.     

"Click labeling" with 2-[18f]fluoroethylazide for positron emission tomography

As an effort in the development of more flexible (18)F-labeling chemistry, we report herein on the use of the Cu(I)-catalyzed Huisgen cycloaddition, also known as the "click reaction", to form (18)F-labeled 1,2,3-triazoles. Nucleophilic fluorination of 2-azidoethyl-4-toluenesulfonate followed by distillation provided 2-[(18)F]fluoroethylazide in 55% radiochemical yield (decay-corrected). 2-[(18)F]fluoroethylazide was reacted with a small library of terminal alkynes in the presence of excess Cu(2+)/ascorbate or copper powder. The most reactive alkyne, N-benzylpropynamide provided nearly quantitative incorporation of 2-[(18)F]fluoroethylazide after 15 min at ambient temperature, whereas the majority of the alkyne substrates provided excellent yields of the corresponding (18)F-labeled 1,2,3-triazoles following heating to 80 degrees C. Using the method described, a model peptide was obtained in 92.3 +/- 0.3% (n = 3) radiochemical yield (decay-corrected) after purification by semipreparative HPLC.     

18F]Galacto-RGD: synthesis, radiolabeling, metabolic stability, and radiation dose estimates

It has been demonstrated in various murine tumor models that radiolabeled RGD-peptides can be used for noninvasive determination of alphavbeta3 integrin expression. Introduction of sugar moieties improved the pharmacokinetic properties of these peptides and led to tracer with good tumor-to-background ratios. Here we describe the synthesis, radiolabeling, and the metabolic stability of a glycosylated RGD-peptide ([18F]Galacto-RGD) and give first radiation dose estimates for this tracer. The peptide was assembled on a solid support using Fmoc-protocols and cyclized under high dilution conditions. It was conjugated with a sugar amino acid, which can be synthesized via a four-step synthesis starting from pentaacetyl-protected galactose. For radiolabeling of the glycopeptide, 4-nitrophenyl-2-[18F]fluoropropionate was used. This prosthetic group allowed synthesis of [18F]Galacto-RGD with a maximum decay-corrected radiochemical yield of up to 85% and radiochemical purity >98%. The overall radiochemical yield was 29 +/- 5% with a total reaction time including final HPLC preparation of 200 +/- 18 min. The metabolic stability of [18F]Galacto-RGD was determined in mouse blood and liver, kidney, and tumor homogenates 2 h after tracer injection. The average fraction of intact tracer in these organs was approximately 87%, 76%, 69%, and 87%, respectively, indicating high in vivo stability of the radiolabeled glycopeptide. The expected radiation dose to humans after injection of [18F]Galacto-RGD has been estimated on the basis of dynamic PET studies with New Zealand white rabbits. According to the residence times in these animals the effective dose was calculated using the MIRDOSE 3.0 program as 2.2 x 10(-2) mGy/MBq. In conclusion, [18F]Galacto-RGD can be synthesized in high radiochemical yields and radiochemical purity. Despite the time-consuming synthesis of the prosthetic group 185 MBq of [18F]Galacto-RGD, a sufficient dose for patient studies, can be produced starting with approximately 2.2 GBq of [18F]flouride. Moreover, the fast excretion, the suitable metabolic stability and the low estimated radiation dose allow to evaluate this tracer in human studies.     

Synthesis and biological evaluation of anti-1-amino-2-[18F]fluoro-cyclobutyl-1-carboxylic acid (anti-2-[18F]FACBC) in rat 9L gliosarcoma

A new [¹⁸F] labeled amino acid anti-1-amino-2-[¹⁸F]fluoro-cyclobutyl-1-carboxylic acid 9 (anti-2-[¹⁸F]FACBC) was synthesized in 30% decay-corrected yield with high radiochemical purity over 99%. The cyclic sulfamidate precursor was very stable and highly reactive towards nucleophilic radiofluorination. Cell uptake assays with rat 9L gliosarcoma cells showed that [¹⁸F]9 was transported into tumor cells via multiple amino acid transport systems, including L and A systems. Biodistribution study in rats with intracranial 9L gliosarcoma tumors demonstrated that [¹⁸F]9 had a rapid and prolonged accumulation in tumors with 26:1 tumor to brain ratio at 120 min post-injection. In this model, [¹⁸F]9 is a potential PET tracer for brain tumor imaging.     

Streptavidin in antibody pretargeting. 5. chemical modification of recombinant streptavidin for labeling with the alpha-particle-emitting radionuclides 213Bi and 211At

We are investigating the use of recombinant streptavidin (rSAv) as a carrier molecule for the short-lived alpha-particle-emitting radionuclides 213Bi ( t 1/2 = 45.6 min) and 211At ( t 1/2 = 7.21 h) in cancer therapy. To utilize rSAv as a carrier, it must be modified in a manner that permits rapid chelation or bonding with these short-lived radionuclides and also modified in a manner that diminishes its natural propensity for localization in the kidney. Modification for labeling with (213)Bi was accomplished by conjugation of rSAv with the DTPA derivative p-isothiocyanato-benzyl-CHX-A' (CHX-A'), 3a. Modification for direct labeling with 211At was accomplished by conjugation of rSAv with an isothiocyanatophenyl derivative of a nido-carborane (nCB), 3b, or an isothiocyanatophenyl-dPEG/decaborate(2-) derivative, 3c. After conjugation of the chelating or bonding moiety, rSAv was further modified by reaction with an excess (50-100 equivalents) of succinic anhydride. Succinylation of the lysine amines has previously been shown to greatly diminish kidney localization. rSAv modified by conjugation with 3a and succinylated rapidly radiolabeled with 213Bi (<5 min), providing a 72% isolated yield. 211At labeling of modified rSAv was accomplished in aqueous solution using chloramine-T as the oxidant. Astatination of rSAv conjugated with 3b and succinylated occurred very rapidly (<1 min), providing a 50% isolated radiochemical yield. Astatination of rSAv conjugated with 3c and succinylated was also very rapid (<1 min) providing 66-71% isolated radiochemical yields. Astatination of succinylated rSAv, 2a, which did not have conjugated borane cage moieties, resulted in a much lower radiolabeling yield (18%). The 213Bi or 211At-labeled modified rSAv preparations were mixed with the corresponding 125 I-labeled rSAv, and dual-label in vivo distributions were obtained in athymic mice. The in vivo data show that 213Bi-labeled succinylated rSAv [ 213Bi] 6a has tissue concentrations similar to those of 125 I-labeled modified rSAv [ 125 I] 6b, suggesting that (213)Bi is quite stable toward release from the chelate in vivo. In vivo data also indicate that the (211)At-labeled rSAv conjugated with 3b or 3c and succinylated are stable to in vivo deastatination, whereas succinylated rSAv lacking a boron cage moiety is subject to some deastatination. The modified rSAv conjugated with nido-carborane derivative 3b has a higher retention in many tissues than rSAv without the carborane conjugated. Interestingly, the rSAv conjugated with 3c, which also contains an m-dPEG 12 moiety, has significantly decreased concentrations in blood and other tissues when compared with those of direct-labeled rSAv, suggesting that it may be a good candidate for further study. In conclusion, rSAv that has been modified with CHX-A' and succinylated (i.e., 5a) may be useful as a carrier of 213Bi. The encouraging results obtained with the PEGylated decaborate(2-) derivative 3c and succinylated (i.e., 5c) suggests that its further study as a carrier of 211At in pretargeting protocols is warranted.     

Investigations into the synthesis,radiofluorination and conjugation of a new [18F]fluorocyclobutyl prosthetic group and its in vitro stability using a tyrosine model system

The [¹⁸F]fluorocyclobutyl group has the potential to be a metabolically stable prosthetic group for PET tracers. The synthesis of the radiolabeling precursor cis-cyclobutane-1,3-diyl bis(toluene-4-sulfonate) 8 was obtained from epibromohydrin in 7 steps (2% overall yield). The radiolabeling of this precursor 8 and its conjugation to l-tyrosine as a model system was successfully achieved to give the new non-natural amino acid 3-[¹⁸F]fluorocyclobutyl-l-tyrosine (L-3-[¹⁸F]FCBT) [¹⁸F]17 in 8% decay-corrected yield from the non-carrier-added [¹⁸F]fluoride. L-3-[¹⁸F]FCBT was investigated in vitro in different cancer cell lines to determine the uptake and stability. The tracer [¹⁸F]17 showed a time dependent uptake into different tumor cell lines (A549, NCI-H460, DU145) with the best uptake of 5.8% injected dose per 5 × 10⁵ cells after 30 min in human lung carcinoma cells A549. The stability of L-3-[¹⁸F]FCBT in human and rat plasma and the stability of the non-radioactive L-3-FCBT in rat hepatocytes were both found to be excellent. These results show that the non-natural amino acid L-3-[¹⁸F]FCBT is a promising metabolically stable radiotracer for positron emission tomography.     

Fluorine-18 labeled galactosylated chitosan for asialoglycoprotein-receptor-mediated hepatocyte imaging

Galactosylated chitosan (GC) was prepared by reacting lactobionic acid with water-soluble chitosan. GC was labeled with fluorine-18 by conjugation with N-succinimidyl-4-¹⁸F-fluorobenzoate ([¹⁸F]SFB) under a slightly basic condition. After rapid purification with HiTrap desalting column, [¹⁸F]FB-GC was obtained with high radiochemical purity (>97%) determined by radio-HPLC. The total reaction time for [¹⁸F]FB-GC was about 150 min. Typical decay-corrected radiochemical yield was about 4–8%. Ex vivo biodistribution in normal mice showed that [¹⁸F]FB-GC had moderate activity accumulation in liver with very good retention (11.13 ± 1.63, 10.97 ± 1.90 and 10.77 ± 0.95% ID/g at 10, 60, 120 min after injection, respectively). The other tissues except kidney showed relative low radioactivity accumulation. The high liver/background ratio affords promising biological properties to get clear images. The specific binding of this radiotracer to the ASGP receptor was confirmed by blocking experiment in mice. Compared with the non-blocking group the hepatic uptake of [¹⁸F]FB-GC significantly declined in all selected time points. The better liver retention properties of [¹⁸F]FB-GC than that of albumin based imaging agents may improve imaging quality and simplify pharmacokinetic model of liver function in the future application with PET imaging.     

Efficient Method for Site-Specific (18)F-Labeling of Biomolecules Using the Rapid Condensation Reaction between 2-Cyanobenzothiazole and Cysteine

An efficient method based on a rapid condensation reaction between 2-cyanobenzothiazole (CBT) and cysteine has been developed for (18)F-labeling of N-terminal cysteine-bearing peptides and proteins. An (18)F-labeled dimeric cRGD ([(18)F]CBTRGD(2)) has been synthesized with an excellent radiochemical yield (92% based on radio-HPLC conversion, 80% decay-corrected, and isolated yield) and radiochemical purity (>99%) under mild conditions using (18)F-CBT, and shown good in vivo tumor targeting efficiency for PET imaging. The labeling strategy was also applied to the site-specific (18)F-labeling of a protein, Renilla lucifierase (RLuc8) with a cysteine residue at its N-terminus. The protein labeling was achieved with 12% of decay-corrected radiochemical yield and more than 99% radiochemical purity. This strategy should provide a general approach for efficient and site-specific (18)F-labeling of various peptides and proteins for in vivo molecular imaging applications.     

A novel prosthetic group for site-selective labeling of peptides for positron emission tomography

Efficient methodologies for the radiolabeling of peptides with [(18)F]fluoride are a prerequisite to enabling commercialization of peptide-containing radiotracers for positron emission tomography (PET) imaging. It was the purpose of this study to investigate a novel chemoselective ligation reaction comprising conjugation of an [(18)F]-N-methylaminooxy-containing prosthetic group to a functionalized peptide. Twelve derivatives of general formula R1-CO-NH-Lys-Gly-Phe-Gly-Lys-OH were synthesized where R1 was selected from a short list of moieties anticipated to be reactive toward the N-methylaminooxy group. Conjugation reactions were initially carried out with nonradioactive precursors to assess, in a qualitative manner, their general suitability for PET chemistry with only the most promising pairings progressing to full radiochemical assessment. Best results were obtained for the ligation of O-[2-(2-[(18)F]fluoroethoxy)ethyl]-N-methyl-N-hydroxylamine 18 to the maleimidopropionyl-Lys-Gly-Phe-Gly-Lys-OH precursor 10 in acetate buffer (pH 5) after 1 h at 70 degrees C. The non-decay-corrected isolated yield was calculated to be 8.5%. The most encouraging result was observed with the combination 18 and 4-(2-nitrovinyl)benzoyl-Lys-Gly-Phe-Gly-Lys-OH, 9, where the conjugation reaction proceeded rapidly to completion at 30 degrees C after only 5 min. The corresponding non-decay-corrected radiochemical yield for the isolated (18)F-labeled product 27 was 12%. The preliminary results from this study demonstrate the considerable potential of this novel strategy for the radiolabeling of peptides.     

Design and synthesis of a new [18F]fluoropyridine-based haloacetamide reagent for the labeling of oligonucleotides: 2-bromo-N-[3-(2-[18F]fluoropyridin-3-yloxy)propyl]acetamide

Based on the recently highlighted potential of nucleophilic heteroaromatic ortho-radiofluorinations in the preparation of fluorine-18-labeled radiotracers and radiopharmaceuticals for PET, a [(18)F]fluoropyridine-based bromoacetamide reagent has been prepared and used in prosthetic group introduction for the labeling of oligonucleotides. [(18)F]FPyBrA (2-bromo-N-[3-(2-[(18)F]fluoropyridin-3-yloxy)propyl]acetamide) was designed as a radiochemically feasible reagent, its pyridinyl moiety both carrying the radioactive halogen (fluorine-18) and allowing its efficient incorporation via a nucleophilic heteroaromatic substitution, and its 2-bromoacetamide function, ensuring the efficient alkylation of a phosphorothioate monoester group born at the 3'- or 5'-end of single-stranded oligonucleotides. [(18)F]FPyBrA (HPLC-purified) was efficiently prepared in 18-20% non-decay-corrected yield (based on starting [(18)F]fluoride) using a three-step radiochemical pathway in 80-85 min. The developed procedure involves (1) a high-yield nucleophilic heteroaromatic ortho-radiofluorination as the fluorine-18 incorporation-step (70-85% radiochemical yield) and uses [3-(3-tert-butoxycarbonylaminopropoxy)pyridin-2-yl]trimethylammonium trifluoromethanesulfonate as precursor for labeling, followed by (2) rapid and quantitative TFA-removal of the N-Boc-protective group and (3) condensation with 2-bromoacetyl bromide (45-65% radiochemical yield). Typically, 3.3-3.7 GBq (90-100 mCi) of HPLC-purified [(18)F]FPyBrA could be obtained in 80-85 min, starting from 18.5 GBq (500 mCi) of a cyclotron production batch of [(18)F]fluoride. [(18)F]FPyBrA was regioselectively conjugated with 9-mer and 18-mer single-stranded oligonucleotides, provided with a phosphorothioate monoester group at their 3'-end. Both natural phosphodiester DNAs and in vivo-stable 2'-methoxy and -fluoro-modified RNAs were used. Conjugation uses optimized, short-time reaction conditions (MeOH/0.1 M PBS pH 7.4, 15 min, 120 degrees C), both compatible with the chemical stability of the oligonucleotides (ONs) and the half-life of fluorine-18. Conjugated [(18)F]ONs were finally purified by RP-HPLC and desalted using a Sephadex NAP-10 column. The whole radiosynthetic procedure, including the preparation of the fluorine-18-labeled reagent, the conjugation with the oligonucleotide, and the HPLC purification and formulation lasted 140-160 min. [(18)F]FPyBrA represents a valuable alternative to the already reported N-(4-[(18)F]fluorobenzyl)-2-bromoacetamide for the design and development of oligonucleotide-based radiopharmaceuticals for PET.