Imaging Early-Stage Metastases Using an ¹⁸F-Labeled VEGFR-1-Specific Single Chain VEGF Mutant

Metastatic breast cancer is the second leading cause of cancer-related death in women. The 5-year survival rate for metastatic breast cancer has remained near 26.9 % for over a decade. The recruitment of hematopoietic stem cells with high expression of the vascular endothelial growth factor receptor 1 (VEGFR-1) has been implicated in early stages of metastasis formation. We propose the use of an ¹⁸F-labeled single-chain version of VEGF121, re-engineered to be selective for VEGFR-1 (scVR1), as a positron emission tomography (PET) imaging agent to non-invasively image early-stage metastases.

scVR1 was ¹⁸F-labeled via a biorthogonal click reaction between site-specifically trans-cyclooctene functionalized scVR1 and an Al¹⁸F labeled tetrazine-NODA (1,4,7-triazacyclononane-1,4-diiacetic acid). The [¹⁸F]AlF-NODA-scVR1 was purified using a PD10 column and subsequently analyzed on HPLC to determine radiochemical purity. Animal experiments were performed in 6-8-week-old female BALB/c mice bearing orthotopic primary 4T1 breast tumors or 4T1 metastatic lesions. The [¹⁸F]AlF-NODA-scVR1 tracer was administered via tail vein injection; PET imaging and ex vivo analysis was performed 2 h post-injection.

The [¹⁸F]AlF-NODA-scVR1 was prepared with a 98.2 ± 1.5 % radiochemical purity and an apparent molar activity of 7.5 ± 1.2 GBq/μmol. The specific binding of scVR1 to VEGFR-1 was confirmed via bead-based assay. The ex vivo biodistribution showed tumor uptake of 3.5 ± 0.5 % ID/g and was readily observable in PET images. Metastasis formation was detected with [¹⁸F]AlF-NODA-scVR1 tracer showing colocalization with bioluminescent imaging as well as ex vivo autoradiography and immunofluorescent staining of VEGFR-1.

The diagnostic capabilities of the [¹⁸F]AlF-NODA-scVR1 PET tracer was confirmed in both orthotopic and metastatic murine cancer models. These results support the potential use of [¹⁸F]AlF-NODA-scVR1 as a PET tracer that could image metastases, providing clinicians with an additional tool to assess a patient's need for adjuvant therapies.