Oxidation of p-[125I]Iodobenzoic Acid and p-[211At]Astatobenzoic Acid Derivatives and Evaluation In Vivo

Abstract

The alpha particle-emitting radionuclide astatine-211 (²¹¹At) is of interest for targeted radiotherapy; however, low in vivo stability of many ²¹¹At-labeled cancer-targeting molecules has limited its potential. As an alternative labeling method, we evaluated whether a specific type of astatinated aryl compound that has the At atom in a higher oxidation state might be stable to in vivo deastatination. In the research effort, para-iodobenzoic acid methyl ester and dPEG₄-amino acid methyl ester derivatives were prepared as HPLC standards. The corresponding para-stannylbenzoic acid derivatives were also prepared and labeled with ¹²⁵I and ²¹¹At. Oxidization of the [¹²⁵I]iodo- and [²¹¹At]astato-benzamidyl-dPEG₄-acid methyl ester derivatives provided materials for in vivo evaluation. A biodistribution was conducted in mice with coinjected oxidized ¹²⁵I- and ²¹¹At-labeled compounds. The oxidized radioiodinated derivative was stable to in vivo deiodination, but unfortunately the oxidized [²¹¹At]astatinated benzamide derivative was found to be unstable under the conditions of isolation by radio-HPLC (post animal injection). Another biodistribution study in mice evaluated the tissue concentrations of coinjected [²¹¹At]NaAtO₃ and [¹²⁵I]NaIO₃. Comparison of the tissue concentrations of the isolated material from the oxidized [²¹¹At]benzamide derivative with those of [²¹¹At]astatate indicated the species obtained after isolation was likely [²¹¹At]astatate.

Keywords: astatate; astatine-211; biodistribution; iodate; iodine-125; oxidation; radiolabel; stability.

Figure 1

Structures of p-(radio)halobenzoic acid derivatives synthesized.

Figure 2

Reactions to prepare halobenzoic acid methyl esters and oxidized forms, including p-[¹²⁵I]iodoxybenzoic acid methyl ester [¹²⁵I]6 and p-[²¹¹At]astatoxybenzoic acid methyl ester [²¹¹At]8..

Figure 3

Synthetic scheme for the preparation of p-(radio)halobenzamidyl-dPEG₄-carboxylic acid methyl ester derivatives 9–12 and subsequent oxidation to form [¹²⁵I]11 and [²¹¹At]13.

Figure 4

Radiochromatograms of reaction solutions and isolated products in the preparation of [¹²⁵I]11 and [²¹¹At]13 for animal biodistributions. (Panel A) Reaction solution containing [¹²⁵I]10; (Panel B) Reaction solution containing [²¹¹At]12; (Panel C) Reaction mixture from oxidation of [¹²⁵I]10; (Panel D) Reaction mixture from oxidation of [²¹¹At]12; (Panel E) [¹²⁵I]11 in PBS after isolation from HPLC; (Panel F) Isolated products in PBS from oxidation of [²¹¹At]12 (note that the desired product in panel D at 5.8 min decomposed on isolation).

Figure 5

Graph showing %ID/g of isolated products for [¹²⁵I]11 and [²¹¹At]13 at 1, 4 and 24 h p.i. in athymic mice. (n = 5) See Table S1 in Supplementary Materials for a tabulation of %ID/g data.

Figure 6

Radiochromatograms of reaction solutions and isolated products in the preparation of [¹²⁵I]NaIO₃ and [²¹¹At]NaAtO₃ for biodistribution study. (Panel A) Reaction solution containing [¹²⁵I]NaIO₃. (Panel B) Reaction solution containing [²¹¹At]NaAtO₃; (Panel C) Reaction solution containing [¹²⁵I]NaIO₃ after solvent evaporation under heating and dissolution in PBS; (Panel D) Reaction solution containing [²¹¹At]NaAtO₃ after solvent evaporation under heating and dissolution in PBS; (Panel E) Reaction solution containing [¹²⁵I]NaIO₃ after solvent evaporation at room temperature and dissolution in PBS; and (Panel F) Reaction solution containing [²¹¹At]NaAtO₃ after solvent evaporation at room temperature and dissolution in PBS. Note that retention times can slightly change when PBS solutions are injected on radio-HPLC.

Figure 7

Graph showing concentration as %ID/g of [¹²⁵I]NaIO₃^– and [²¹¹At]NaAtO₃^– in tissues of athymic mice at 1, 4 and 24 h p.i. (n = 5). See Table S2 in Supplementary Materials for a tabulation of %ID/g data.