PET imaging of tumor hypoxia using 18F-labeled pimonidazole

Abstract

Background. Tumor hypoxia contributes to loco-regional failure, and for optimal treatment planning, knowledge about tumor hypoxia in individual patients is required. Nitroimidazole-based tracers, which are retained in hypoxic cells, allow PET-based assessment of tumor hypoxia, but current tracers are characterized by slow tracer retention and clearance, resulting in low inter-tissue contrast. Pimonidazole is an immune detectable hypoxia marker widely used for detection of hypoxia in tumor samples. Pimonidazole has excellent chemical properties for hypoxia imaging, but labeling for non- invasive assay has not been attempted. Here we labeled pimonidazole with ¹⁸F ([¹⁸F]FPIMO). Material and methods. [¹⁸F]FPIMO was produced by fluorination of 1-[2-O-tosyl-3-(2-nitroimidazole-1-yl)-propyl]-piperidine, which resulted in two isomeric interchangeable forms (named “5” and “6”) with a radiochemical purity of 91–100%. [¹⁸F]FPIMO was tested by incubation of two different tumor cell lines at high and low oxygen levels. [¹⁸F]FPIMO was also administered to tumor-bearing mice and tracer retention in tumors, non-hypoxic reference tissues and tissues involved in drug metabolism/clearance was evaluated by various techniques. Results and conclusions. Retention of [¹⁸F]FPIMO was strongly hypoxia-driven in vitro, but isomeric form “5” was particularly promising and reached impressive anoxic-to-oxic retention ratios of 36 and 102, in FaDu_DD and SiHa cells, respectively, following three hours of tracer incubation. This was equal to or higher than ratios measured using the established hypoxia tracer [¹⁸F]FAZA. [¹⁸F]FPIMO also accumulated in tumors grown in mice, and reached tumor levels that were two to six-fold higher than in muscle three hours post-administration. Furthermore, the intra-tumoral distribution of [¹⁸F]FPIMO (autoradiography) and unlabeled pimonidazole (immunohistochemistry) was largely identical. Nonetheless, [¹⁸F]FPIMO proved inferior to [¹⁸F]FAZA, since absolute tumor signal and intra-tumoral contrast was low, thus compromising PET imaging. Low tumor signal was coupled to extensive tracer accumulation in liver and kidneys, and analysis of blood metabolites revealed that [¹⁸F]FPIMO was metabolized rapidly, with little parent compound remaining 15 minutes post-administration. Ongoing work focuses on the possibility of labeling pimonidazole in different positions with ¹⁸F to improve tracer stability in vivo.