Radiosynthesis of [(131)I]IAZGP via nucleophilic substitution and its biological evaluation as a hypoxia marker - is specific activity a factor influencing hypoxia-mapping ability of a hypoxia marker?

INTRODUCTION: The hypoxia marker IAZGP, 1-(6-deoxy-6-iodo-beta-d-galactopyranosyl)-2-nitroimidazole, has been labeled with (123)I/(124)I/(125)I/(131)I via iodine-radioiodine exchange, which gives the radiotracer in a specific activity of 10-90 MBq/micromol. We synthesized the same radiotracer possessing several hundred to thousand times higher specific activity (high-SA IAZGP) via nucleophilic substitution and compared its biological behavior with that of conventionally produced IAZGP (low-SA IAZGP) to determine if specific activity is a factor influencing cell uptake kinetics, biodistribution and intratumor microregional localization of the radiotracer.
METHODS: High-SA [(131)I]IAZGP was prepared by substitution of the tosyl functionality with [(131)I]iodide. In vitro uptake of high- and low-SA [(131)I]IAZGP by HCT8 and HT29 cells was assessed in normoxic and hypoxic conditions. Biodistribution and intratumor localization of high- and low-SA [(131)I]IAZGP were determined by injection into HT29 tumor-bearing mice.
RESULTS: The nucleophilic substitution reaction proceeded efficiently in acetonitrile at 150 degrees C, giving the final product in an average yield of 42% and an average specific activity of 30 GBq/micromol. In vitro, high-SA [(131)I]IAZGP was incorporated into the tumor cells with similar kinetics and oxygen dependence to low-SA [(131)I]IAZGP. In HT29 tumor-bearing mice, biodistributions of high- and low-SA [(131)I]IAZGP were equivalent. Ex vivo autoradiography revealed heterogeneous intratumor localization of high-SA [(131)I]IAZGP corresponding closely to distributions of other exogenous and endogenous hypoxia markers. Comparable microregional distribution patterns were observed with low-SA [(131)I]IAZGP.
CONCLUSIONS: Radiolabeled IAZGP produced via nucleophilic substitution is validated as an exogenous hypoxia marker. Specific activity does not appear to influence the in vivo hypoxia-mapping ability of the radiotracer.