Fully automated peptide radiolabeling from [¹⁸F]fluoride

Abstract

The biological properties of receptor-targeted peptides have made them popular diagnostic imaging and therapeutic agents. Typically, the synthesis of fluorine-18 radiolabeled receptor-targeted peptides for positron emission tomography (PET) imaging is a time consuming, complex, multi-step synthetic process that is highly variable based on the peptide. The complexity associated with the radiolabeling route and lack of robust automated protocols can hinder translation into the clinic. A fully automated batch production to radiolabel three peptides (YGGFL, cRGDyK, and Pyr-QKLGNQWAVGHLM) from fluorine-18 using the ELIXYS FLEX/CHEM® radiosynthesizer in a two-step process is described. First, the prosthetic group, 6-[ 18 F]fluoronicotinyl-2,3,5,6-tetrafluorophenyl ester ([ 18 F]FPy-TFP) was synthesized and subsequently attached to the peptide. The [ 18 F]FPy-peptides were synthesized in 13-26% decay corrected yields from fluorine-18 with high molar activity 1-5 Ci mmol À1 and radiochemical purity of >99% in an overall synthesis time of 97 AE 3 minutes.

Automated Synthesis of Fluorine-18 Labeled CXCR4 Ligand via the Conjugation with Nicotinic Acid N-Hydroxysuccinimide Ester (6-[18F]SFPy)

“…Although there are a few examples of direct fluorine-18 labeling of peptides, labeling is typically performed using an indirect method [ 39 , 40 , 41 , 42 ]. Therefore, a fully automated synthesis is challenging due to the complexity of the indirect labeling method [ 43 , 44 , 45 , 46 , 47 ]. The steps involved for the preparation of fluorine-18 labeled prosthetic group via currently developed Sep-Pak method (catching of fluorine-18 on an anion exchange cartridge, drying of the cartridge, release of fluorine-18 with precursor), are equivalent to the initial processing of fluorine-18 for conventional nucleophilic fluorination (catching of fluorine-18 on an anion exchange cartridge, the release of fluorine-18 with base, azeotropic drying with acetonitrile).…”

4-Nitrophenyl activated esters are superior synthons for indirect radiofluorination of biomolecules

“…3,4 In particular, the use of fluorine-18 to radiolabel peptides combines favourable pharmacology with the ideal physicochemical and imaging characteristics of fluorine-18. [5][6][7][8] However, the preparation of 18 F-labelled peptides remains challenging and time consuming with multistep and laborious processes often required. Typically, sensitive biomolecules are radiolabelled with fluorine-18 indirectly, necessitating the use of a prelabelled synthon (prosthetic groups) as they cannot tolerate the harsh radiofluorination conditions and are often thought to be inert to direct radiofluorination.…”

“…1). 17,18 Herein, we evaluate the stability of PNP and TFP esters to direct radiofluorination conditions for the preparation of 4-[ 18 F]fluorobenzoate and 6-[ 18 F]fluoronicotinate synthons. Furthermore, we report the relative stability and acylation activities of PNP and TFP pre-activated synthons.…”

“…Indeed, the susceptibility of [ 18 F]2 to hydrolysis and subsequent formation of by-products is well established. 17,18 The relative acylation propensities of (Fig. 5).…”

“…In order to understand the relative acylation potential of these esters, it was essential to correct for their decomposition in neat DMSO (control reaction). Since only TFP-ester [ 18 F]27 was achieved in less than 90 minutes on an iPHASE Flexlab automated radiochemistry module. This is in stark contrast with previously reported conditions that employ multistep procedures (>3 radiochemical steps) for the preparation of synthons, leading to overall synthesis time of >130 minutes and ultimately affording fluorine-18 labelled c(RGDyK) in low overall yields and sub-optimal automation procedures.…”