Tumor hypoxia imaging in orthotopic liver tumors and peritoneal metastasis: a comparative study featuring dynamic 18F-MISO and 124I-IAZG PET in the same study cohort

Abstract

Purpose The purpose of this paper is to compare the uptake of two clinically promising positron emission tomography (PET) hypoxia targeting agents, ¹²⁴I-iodoazomycin galactopyranoside (¹²⁴I-IAZG) and ¹⁸F-fluoromisonidazole (¹⁸F-FMISO), by dynamic microPET imaging, in the same rats bearing liver tumors and peritoneal metastasis. Methods Morris hepatoma (RH7777) fragments were surgically implanted into the livers of four nude rats. Tumors formed in the liver and disseminated into the peritoneal cavity. Each rat had a total of two to three liver tumors and peritoneal metastasis measuring 10–15 mm in size. Animals were injected with ¹⁸F-FMISO, followed on the next day (upon complete ¹⁸F decay) by ¹²⁴I-IAZG. The animals were imaged in list mode on the microPET system from the time of injection of each tracer for 3 h and then again at 6 h and 24 h for the long-lived ¹²⁴I-IAZG tracer (4.2-day half-life). Micro computed tomography (CT) scans of each rat were performed for co-registration with the microPET scans acquired with a liver contrast agent, allowing tumor identification. Regions of interest (ROIs) were drawn over the heart, liver, muscle, and the hottest areas of the tumors. Time–activity curves (TACs) were drawn for each tissue ROI. Results The ¹⁸F-FMISO signal increased in tumors over the 3-h time course of observation. In contrast, after the initial injection, the ¹²⁴I-IAZG signal slowly and continuously declined in the tumors. Nevertheless, the tumor-to-normal-tissue ratios of ¹²⁴I-IAZG increased, but more slowly than those of ¹⁸F-FMISO and as a result of the differentially faster clearance from the surrounding normal tissues. These pharmacokinetic patterns were seen in all 11 tumors of the four animals. Conclusions ¹⁸F-FMISO localizes in the same intra-tumor regions as ¹²⁴I-IAZG. The contrast ratios (tumor/background) reach similar values for the two hypoxia tracers, but at later times for ¹²⁴I-IAZG than for ¹⁸F-FMISO and, therefore, with poorer count statistics. As a consequence, the ¹⁸F-FMISO images are of superior diagnostic image quality to the ¹²⁴I-IAZG images in the Morris hepatoma McA-R-7777 tumor model.