[64Cu]Cu-FAP-NOX, a novel cyclic peptide for FAP PET imaging with a flexible imaging time window

Introduction: Developing tracers with an extended and flexible imaging time window can not only enhance the convenience and flexibility of routine scans but also potentially improve image contrast and detectability by enabling radioactivity clearance from normal tissue. The aim of the present study was to develop a novel ⁶⁴Cu-labeled cyclic peptide ([⁶⁴Cu]Cu-FAP-NOX) that targets fibroblast activation protein (FAP) and may offer advantages in terms of image contrast, imaging time window, and low uptake in normal tissues.

Methods: The novel ⁶⁴Cu-labeled cyclic peptide featuring with a N-oxalyl modified tail was constructed and conjugated to NOTA for ⁶⁴Cu labeling. Biochemical and cellular assays were performed with A549.hFAP cells. The performance of [⁶⁴Cu]Cu-FAP-NOX was compared to that of two established tracers, namely [⁶⁸Ga]Ga-FAP-2286 and [⁶⁴Cu]Cu-FAPI-04, in HEK-293T.hFAP xenograft mice using micro-PET imaging. Ex vivo biodistribution studies were performed to confirm the FAP specificity and to validate the PET data. Furthermore, a first-in-human study of this novel tracer was conducted on one patient with lung cancer.

Results: Compared to [⁶⁴Cu]Cu-FAPI-04, [⁶⁴Cu]Cu-FAP-NOX demonstrated faster and higher rates of cellular uptake and internalization in A549.hFAP cells, but lower rates of cellular efflux. All three radiotracers were rapidly taken up by the tumor within the first 4 h post-injection. However, [⁶⁴Cu]Cu-FAP-NOX had more intense tumor accumulation and slower washout from the target. The ratios of the tumor to normal tissue (including kidneys and muscles) increased significantly over time, with [⁶⁴Cu]Cu-FAP-NOX reaching the highest ratio among all tracers. In the patient, [⁶⁴Cu]Cu-FAP-NOX PET showed a comparable result to FDG PET in the primary malignant lesion while exhibiting higher uptake in pleural metastases, consistent with elevated FAP expression as confirmed by immunohistochemistry.

Conclusions: [⁶⁴Cu]Cu-FAP-NOX is a promising FAP-targeted tracer with a highly flexible imaging time window, as evidenced by preclinical evaluation encompassing biodistribution and micro-PET studies, along with a successful patient application. Furthermore, [⁶⁴Cu]Cu-FAP-NOX showed enhanced image contrast compared with [⁶⁴Cu]Cu-FAPI-04 and [⁶⁸Ga]Ga-FAP-2286 for FAP PET imaging, warranting translation into large cohort studies.

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