Biodistribution, pharmacokinetics, dosimetry of [68Ga]Ga-DOTA.SA.FAPi, and the head-to-head comparison with [18F]F-FDG PET/CT in patients with various cancers

Purpose [Ga-68]Ga-labeled fibroblast activation protein inhibitors ([Ga-68]Ga-FAPi) have shown promising preclinical and clinical results in PET imaging. The present study aimed to evaluate the biodistribution, pharmacokinetics, and dosimetry of [Ga-68]Ga-DOTA.SA.FAPi, another modified FAPi tracer, and performed a head-to-head comparison with [F-18]F-FDG PET/CT scans in patients with various cancers. Methods In this prospective study, patients underwent both [F-18]F-FDG and [Ga-68]Ga-DOTA.SA.FAPi PET/CT scans 60 min post-injection (p.i.). Dosimetry studies were conducted in three patients using [Ga-68]Ga-DOTA.SA.FAPi serial time-point imaging. The absorbed dose was calculated using OLINDA/EXM 2.2 software. Quantification of the uptake of the tracers was assessed using standardized uptake values corrected for lean body mass (SUL). Results Fifty-four patients (mean age; 48.4 years) with 14 types of cancers involving 37% breast, 24% lung, 7.4% head and neck (H&N), and remaining 31.6% patients with other histologies were evaluated prospectively. Physiological uptake of [Ga-68]Ga-DOTA.SA.FAPi was observed in the liver, kidneys, pancreas, heart contents, and to a lesser extent in the lacrimals, oral mucosa, salivary glands, and thyroid glands. Uptake in the target lesions on [Ga-68]Ga-DOTA.SA.FAPi scan was initiated at 10 min, and no additional lesions were detected in the delayed acquisition time points. The pancreas was the organ with the highest absorbed dose (5.46E-02 mSv/MBq). While the patient-based comparison between the radiotracers revealed complete concordance in the detection of primary, pleural thickening, bone and liver metastases, and second primary malignancy, discordant findings were observed in the detection of lymph node (7.5%), lung nodules (5.6%), and brain metastases (2%). According to the site of primary disease, patients with H&N cancers demonstrated the highest SULpeak and average (avg) values on [Ga-68]Ga-DOTA.SA-FAPi which was similar to the values of [F-18]F-FDG [(SULpeak: 15.4 vs. 14.2; P-0.680) (SULavg: 8.3 vs. 7.9; P-0.783)]. The lowest uptake was observed in lung cancers with both the radiotracers [(SULpeak: 5.8 vs. 7.4; P-0.238) (SULavg: 4.9 vs. 5.3; P-0.313)]. A significantly higher SULpeak and SULavg for brain metastases to normal brain parenchyma ratios were observed on [Ga-68]Ga-DOTA.SA.FAPi in contrast to the [F-18]F-FDG values {SULpeak: median: 59.3 (IQR: 33.5-130.8) versus 1.5 (1-2.3); P-0.028}. Except for brain metastases, comparable SULpeak and average values were noted between the radiotracers in all other regions of metastases with no significant difference. Conclusion [Ga-68]Ga-DOTA.SA.FAPi is a promising alternative among the FAPI class of molecules and performed well as compared to standard-of-care radiotracer [F-18]F-FDG in the diagnosis of various cancers.