Preclinical and pilot clinical evaluation of Al18F-FAP-NUR for PET imaging cancer-associated fibroblasts validated by immunohistochemistry, and clinical comparison with 18F-FDG

Introduction: Fibroblast activation protein (FAP) has gained attention as a promising molecular target with potential utility for cancer diagnosis and therapy. ⁶⁸Ga-labeled FAP-targeting peptides have been successfully applied to positron emission tomography (PET) imaging of various tumor types. To meet the applicable demand for peptide-based FAP tracers with high patient throughput, we herein report the radiosynthesis, preclinical evaluation, and preliminary clinical studies of a novel ¹⁸F-labeled FAP-targeting peptide.

Methods: Al¹⁸F-FAP-NUR was radiolabeled with ¹⁸F using an Al¹⁸F complex on a modified GE TRACERlab FX_FN synthesis platform. For preclinical evaluation, the ¹⁸F-labeled peptide was compared with ⁶⁸Ga-FAP-2286, a ⁶⁸Ga-labeled FAP-targeting peptide, in biochemical and cellular assays conducted in 293T-FAP cells, which had been transfected with human-derived FAP. Additionally, ex vivo biodistribution studies and in vivo micro-PET imaging were performed in 293T-FAP and A549-FAP tumor xenografts.

For the clinical translational study of Al¹⁸F-FAP-NUR, a total of 19 patients underwent Al¹⁸F-FAP-NUR PET/CT examinations. The study population included 10 patients who received dynamic scans at intervals of 5, 15, 30, 45, 60, and 120 min post-injection of Al¹⁸F-FAP-NUR. The remaining 9 patients underwent static scans at 50-70 min post-injection. Radiation dosimetry estimates for Al¹⁸F-FAP-NUR were derived from the dynamic scan data. Excluding 3 patients who did not receive a comparative ¹⁸F-FDG PET/CT scan, the remaining 16 patients (lung cancer, n = 8; appendiceal cancer, n = 1; colon cancer, n = 1; esophagus cancer, n = 1; gastric cancer, n = 1; head and neck cancer, n = 1; hepatic cancer, n = 1; nasopharyngeal carcinoma, n = 1; and thymoma, n = 1) were evaluated with ¹⁸F-FDG PET/CT imaging. In 14/16 patients, the FAP-immunohistochemical results were obtained.

Results: The radiochemical purity (RCP) of Al¹⁸F-FAP-NUR was > 95%. The ¹⁸F-labeled peptide demonstrated more rapid, higher levels of cellular uptake in 293T-FAP cells compared to that of ⁶⁸Ga-FAP-2286 (60 min: 44.78 ± 3.62 %ID/mio cells vs. 19.79 ± 2.55 %ID/mio cells). Based on the biodistribution data of 293T-FAP tumor-bearing mice, at 1 h post-injection, the uptake of Al¹⁸F-FAP-NUR by tumors was two-fold higher compared to that of ⁶⁸Ga-FAP-2286 (26.99 ± 10.10 % ID/g vs. 11.55 ± 3.11 %ID/g, respectively; P = 0.031). In micro-PET imaging, Al¹⁸F-FAP-NUR exhibited a higher level of tumor uptake compared to ⁶⁸Ga-FAP-2286, particularly in 293T-FAP tumors (at 60 min: 17.85 ± 5.12 %ID/g vs. 7.62 ± 1.83 %ID/g, respectively), similar results could be observed in A549-FAP tumors.

The effective dose of Al¹⁸F-FAP-NUR was calculated as 1.90E-02 ± 4.89E-03 mSv/MBq. For the comparison between Al¹⁸F-FAP-NUR and ¹⁸F-FDG in tumor uptake, Al¹⁸F-FAP-NUR PET/CT consistently showed higher SUV_max and TBRs across all primary tumor sites compared to ¹⁸F-FDG PET/CT, particularly notable in lung cancer (SUV_max: 15.19 vs. 12.10, P = 0.012; TBR: 29.74 vs. 21.57, P = 0.036). The FAP-immunohistochemical results of the major lesions found that the immunohistochemical staining for FAP in the tumor was in good agreement with that of the uptake of Al¹⁸F-FAP-NUR PET/CT imaging. Notably, in the detection of metastases, a markedly improved detection rate of bone and visceral metastases for Al¹⁸F-FAP-NUR PET/CT was observed (99% [279/282] vs. 51% [144/282]).

Conclusions: Based on the physical imaging properties and longer half-life of ¹⁸F, Al¹⁸F-FAP-NUR exhibited promising characteristics such as enhanced tumor-specific accumulation and elevated TBRs, which made it a viable candidate for further clinical investigation.

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