Towards Optimal Automated 68Ga-Radiolabeling Conditions of the DOTA-Bisphosphonate BPAMD Without Pre-Purification of the Generator Eluate

Abstract

DOTA-functionalized bisphosphonates can be useful tools for PET imaging of bone metastases when radiolabeled with ⁶⁸Ga. Moreover, the versatility of DOTA allows the complexation of radiometals with therapeutic applications (e.g., ¹⁷⁷Lu), positioning these bisphosphonates as attractive theranostic agents. Among these molecules, BPAMD is a compound whose radiolabeling with ⁶⁸Ga has already been described, but only through manual methods. Thus, a fully automated protocol for ⁶⁸Ga radiolabeling of BPAMD on the GAIA® ± LUNA® synthesis module was designed, and a thorough study of the radiolabeling conditions was undertaken. [⁶⁸Ga]Ga-BPAMD was produced in good radiochemical purity (> 93%) and high radiochemical yield (> 91%) using 0.3 M HEPES buffer. The nature of the reaction vessel showed no significant effect on the radiolabeling outcome. Similarly, addition of an antiradiolysis compound to the reaction medium did not significantly improve the already excellent stability of [⁶⁸Ga]Ga-BPAMD over time. The radiolabeled product obtained by automated synthesis was evaluated in vivo in healthy mice and confirmed high accumulation in the joints and along the backbone.

FIGURE 1

Main structure‐activity relationships in bisphosphonate series and chemical structure of BPAMD.

FIGURE 2

Synthesis scheme (A, C) and cassette mounted in the module within the shielded cell (B, D) for the automated production of [⁶⁸Ga]Ga‐BPAMD in configuration A (A, B) using a GAIA® module and configuration B (C, D) using GAIA® and LUNA® modules.

FIGURE 3

Flow chart for [⁶⁸Ga]Ga‐BPAMD synthesis process.

FIGURE 4

Activity distribution in the cassette (up) and RCP and RCY values measured by TLC and HPLC (down) depending on the reaction buffer.

FIGURE 5

Radio‐TLC (left) and radio‐HPLC (right) spectra of [⁶⁸Ga]Ga‐BPAMD after radiolabeling with HEPES 0.3 M.

FIGURE 6

Activity distribution in the cassette (left) and RCP and RCY values measured by TLC and HPLC (right) depending on the amount of BPAMD used for the radiolabeling reaction.

FIGURE 7

Activity distribution in the cassette (left) and RCP and RCY values measured by TLC and HPLC (right) depending on the type of reaction vial.

FIGURE 8

RCP and RCY values measured by TLC and HPLC right after the synthesis (left) and time course of [⁶⁸Ga]Ga‐BPAMD RCP measured by HPLC, with or without antiradiolysis compounds.

FIGURE 9

Example of in vivo accumulation of [⁶⁸Ga]Ga‐BPAMD (18.2 MBq) in bones of healthy mouse, IV tail vein injection, μPET (up) and μPET/CT (down) scans (MIP), 60 min p.i. Blue arrow shows intravesical retention due to urinary excretion.