Identification of Boronic Acid Derivatives as an Active Form of N-Alkylaminoferrocene-Based Anticancer Prodrugs and Their Radiolabeling with ¹⁸F

Abstract

N-Alkylaminoferrocene (NAAF)-based prodrugs are activated in the presence of elevated amounts of reactive oxygen species (ROS), which corresponds to cancer specific conditions, with formation of NAAF and p-quinone methide. Both products act synergistically by increasing oxidative stress in cancer cells that causes their death. Though it has already been demonstrated that the best prodrugs of this type retain their antitumor activity in vivo, the effects were found to be substantially weaker than those observed in cell cultures. Moreover, the mechanistic studies of these compounds in vivo are missing. For clarification of these important questions, labeling of the prodrugs with radioactive moieties would be necessary. In this paper, we first observed that the representative NAAF-based prodrugs are hydrolyzed in dilute aqueous solutions to the corresponding arylboronic acids. We confirmed that these products are responsible for ROS amplification and anticancer properties of the parent prodrugs. Next, we developed the efficient synthetic protocol for radiolabeling the hydrolyzed NAAF-based prodrugs by [ 18 F]fluoroglucosylation under the conditions of the copper(I)-catalyzed azide−alkyne 1,3-dipolar cycloaddition and used this protocol to prepare one representative hydrolyzed NAAF-based prodrug radiolabeled with 18 F. Finally, we studied the stability of the 18 F-labeled compound in human serum in vitro and in rat blood in vivo and obtained preliminary data on its biodistribution in vivo in mice carrying pancreatic (AR42J) and prostate (PC3) tumors by applying PET imaging studies. The compound described in this paper will help to understand in vivo effects (e.g., pharmacokinetics, accumulation in organs, the nature of side effects) of these prodrugs that will strongly contribute to their advancement to clinical trials.

Radiosynthesis of an 18F‐fluoroglycosylated aminoferrocene for in‐vivo imaging of reactive oxygen species activity by PET

“…As recently reported, glycoconjugate 3 was synthesised as shown in Scheme to afford the reference for its radioactive analogue. In comparison with free boronic acids, cyclic boronic acid esters, such as pinacol esters, are preferably used for synthesis, because the formation of boroxines is avoided .…”

“…The analytical data of these compounds were in accordance with the respective literature data. Compounds 1 , 3 , and 4 were synthesised as recently reported . According to spectroscopic and chromatographic data, the purity of all these reference compounds was higher than 95%.…”

“…No-carrier-added [ 18 F]fluoride was produced through the 18 O(p,n) 18 F reaction on a PETtrace 800 cyclotron using H 2 [ 18 O]O as a target at the University Hospital Würzburg (Center of Radiopharmacy, Würzburg, Germany). It was subsequently fixed on an anion exchange cartridge (QMA, Waters) and transferred in a reaction vial using previously described elution conditions.…”

“…It was subsequently fixed on an anion exchange cartridge (QMA, Waters) and transferred in a reaction vial using previously described elution conditions. 21 After azeotropic drying of the eluted [ 18 F]F − the nucleophilic 18 F-fluorination of 5 was performed under already established conditions 19 to obtain [ 18 F]6, which was then purified using a semipreparative HPLC column (method 1). The product fraction of [ 18 F]6 was fixed on a C18 cartridge, eluted with ethanol, and dried under helium flow at 60°C as described before.…”

“…Synthesis of aminoferrocene glycoconjugate 3 and hydrolysis of cyclic pinacol boronic acid ester to afford 4 . (a) CuI, TEA, THF, room temperature, 12 h…”