18 F-Fluorination of Nitroimidazolyl-Containing Sultone: A Direct Access to a Highly Hydrophilic Radiotracer for High-Performance Positron Emission Tomography Imaging of Hypoxia

Abstract

Hypoxia, characterized by nonphysiological levels of oxygen tension, is a key phenomenon common to the majority of malignant tumors with poor prognosis. Many efforts have been made to develop hypoxia imaging for diagnosis, staging, and monitoring of diseases, as well as for evaluating therapies. PET Imaging using ¹⁸F-fluoronitroimidazoles (i.e., [¹⁸F]FMISO as a lead radiotracer) has demonstrated potential for clinical investigations, but the poor contrast and prolonged acquisition times (>2.5 h) strongly limit its accuracy and routine developments. Here, we report an original [¹⁸F]fluoronitroimidazole bearing a sulfo group ([¹⁸F]FLUSONIM) that displays highly hydrophilic properties and rapid clearance, providing high-performance hypoxia specific PET imaging. We describe the synthesis and radiosynthesis of [¹⁸F]FLUSONIM, its in vivo preclinical evaluation by PET imaging in healthy rats and a rhabdomyosarcoma rat model, as well as its radiometabolization and histological studies. [¹⁸F]FLUSONIM was prepared in a single step by high yielding radiofluorination of a sultone precursor, highlighting the advantages of this new radiolabeling approach not yet explored for radiopharmaceutical development. PET imaging experiments were conducted by systematically comparing [¹⁸F]FLUSONIM to [¹⁸F]FMISO as a reference. The overall results unequivocally demonstrate that the developed radiopharmaceutical meets the criteria of an ideal candidate for hypoxia PET imaging-rapid and efficient radiosynthesis, total stability, exclusive urinary elimination, high specificity for hypoxic regions, unprecedented tumor/background ratios, short acquisition delays (<60 min), and promising potential for further preclinical and clinical applications.

Figure 1

Usual 2-nitroimidazole-based radiopharmaceuticals for clinical PET imaging of hypoxia and the rationale for the development of a new hydrophilic sulfo analogue.

Figure 2

(A) Synthesis and radiosynthesis of the investigated FLUSONIM; (B) preparative HPLC (up, γ trace; down, UV trace) for [¹⁸F]FLUSONIM purification at the end of radiosynthesis; (C) quality control of [¹⁸F]FLUSONIM ready for injection (up, RadioHPLC analysis; down, radio TLC analysis); (D) analytical RadioHPLC for stability studies of [¹⁸F]FLUSONIM in plasma (up, in vitro after incubation for 150 min at 37 °C in plasma; down, ex vivo after extraction of blood 150 min post-injection).

Figure 3

Biodistribution by PET imaging of [¹⁸F]FLUSONIM in the main organs after i.v. injection in healthy rats. (A) Fused PET/CT images. (B) Time–activity curves. (C) Repartition at 15, 30, 60, and 150 min.

Figure 4

PET Imaging in the rhabdomyosarcoma model (n = 8). (A) Fused PET/CT images hidden from elimination organs and time activity curves for [¹⁸F]FMISO. (B) Fused PET/CT images and time activity curves for [¹⁸F]FLUSONIM. (C) Tumor-to-muscle ratio results for [¹⁸F]FMISO and [¹⁸F]FLUSONIM (*P < 0.05).

Figure 5

Perfusion vs hypoxia imaging in the rhabdomyosarcoma model (n = 8). Fused PET/CT images were hidden from elimination organs at early and late times for [¹⁸F]FMISO (A) and for [¹⁸F]FLUSONIM (B). TACs for [¹⁸F]FMISO (C) and [¹⁸F]FLUSONIM (D). TMRs for [¹⁸F]FMISO (E) and [¹⁸F]FLUSONIM (F). Voxel-by-voxel correlation between early (perfusion) and late (hypoxia) uptakes for [¹⁸F]FMISO (G) and [¹⁸F]FLUSONIM (H). Comparison of TMRs for [¹⁸F]FLUSONIM and [¹⁸F]FMISO (I, J).

Figure 6

Histological, immunostaining, and autoradiography results for tumor sections of rhabdomyosarcoma rats.

Figure 7

RadioHPLC of tumor extract at 60 min (top) vs [¹⁸F]FLUSONIM from the quality control (bottom).