Biodistribution and radiation dosimetry of the novel hypoxia PET probe [¹⁸F]DiFA and comparison with [¹⁸F]FMISO

To facilitate hypoxia imaging in a clinical setting, we developed 1-(2,2-dihydroxymethyl-3-[¹⁸F]-fluoropropyl)-2-nitroimidazole ([¹⁸F]DiFA) as a new tracer that targets tumor hypoxia with its lower lipophilicity and efficient radiosynthesis. Here, we evaluated the radiation dosage, biodistribution, human safety, tolerability, and early elimination after the injection of [¹⁸F]DiFA in healthy subjects, and we performed a preliminary clinical study of patients with malignant tumors in a comparison with [¹⁸F]fluoromisonidazole ([¹⁸F]FMISO).

The single administration of [¹⁸F]DiFA in 8 healthy male adults caused neither adverse events nor abnormal clinical findings. Dynamic and sequential whole-body scans showed that [¹⁸F]DiFA was rapidly cleared from all of the organs via the hepatobiliary and urinary systems. The whole-body mean effective dose of [¹⁸F]DiFA estimated by using the medical internal radiation dose (MIRD) schema with organ level internal dose assessment/exponential modeling (OLINDA/EXM) computer software 1.1 was 14.4 ± 0.7 μSv/MBq. Among the organs, the urinary bladder received the largest absorbed dose (94.7 ± 13.6 μSv/MBq). The mean absorbed doses of the other organs were equal to or less than those from other hypoxia tracers. The excretion of radioactivity via the urinary system was very rapid, reaching 86.4 ± 7.1% of the administered dose. For the preliminary clinical study, seven patients were subjected to [¹⁸F]FMISO and [¹⁸F]DiFA positron emission tomography (PET) at 48-h intervals to compare the two tracers’ diagnostic ability for tumor hypoxia. The results of the tumor hypoxia evaluation by [¹⁸F]DiFA PET at 1 h and 2 h were not significantly different from those obtained with [¹⁸F]FMISO PET at 4 h ([¹⁸F]DiFA at 1 h, p = 0.32; [¹⁸F]DiFA at 2 h, p = 0.08). Moreover, [¹⁸F]DiFA PET at both 1 h (k = 0.68) and 2 h (k = 1.00) showed better inter-observer reproducibility than [¹⁸F]FMISO PET at 4 h (k = 0.59).

[¹⁸F]DiFA is well tolerated, and its radiation dose is comparable to those of other hypoxia tracers. [¹⁸F]DiFA is very rapidly cleared via the urinary system. [¹⁸F]DiFA PET generated comparable images to [¹⁸F]FMISO PET in hypoxia imaging with shorter waiting time, demonstrating the promising potential of [¹⁸F]DiFA PET for hypoxia imaging and for a multicenter trial.