Targeting VPAC1 Receptors for Imaging Glioblastoma

Scintigraphic imaging of malignant glioblastoma (MG) continues to be challenging. We hypothesized that VPAC1 cell surface receptors can be targeted for positron emission tomography (PET) imaging of orthotopically implanted MG in a mouse model, using a VPAC1-specific peptide [⁶⁴Cu]TP3805.

The expression of VPAC1 in mouse GL261 and human U87 glioma cell lines was determined by western blot. The ability of [⁶⁴Cu]TP3805 to bind to GL261 and U87 cells was studied by cell-binding. Receptor-blocking studies were performed to validate receptor specificity. GL261 tumors were implanted orthotopically in syngeneic T-bet knockout C57BL/6 mouse brain (N = 15) and allowed to grow for 2–3 weeks. Mice were injected i.v., first with ~ 150 μCi of 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) then 24 h later with ~ 200 μCi of [⁶⁴Cu]TP3805. In another set of tumor-bearing mice, (N = 5), ionic [⁶⁴Cu]Cl₂ was injected as a control. Mice were imaged at a 2-h post-injection using an Inveon micro-PET/CT, sacrificed and % ID/g of [⁶⁴Cu]TP3805 and [⁶⁴Cu]Cl₂ were calculated in a tumor, normal brain, and other tissues. For histologic tissue examination, 3-μm thick sections of the tumors and normal brain were prepared, digital autoradiography (DAR) was performed, and then the sections were H&E stained for histologic examination.

Western blots showed a strong signal for VPAC1 on both cell lines. [⁶⁴Cu]TP3805 cell-binding was 87 ± 1.5 %. Receptor-blocking reduced cell-binding to 24.3 ± 1.5 % (P < 0.01). PET imaging revealed remarkable accumulation of [⁶⁴Cu]TP3805 in GL261 MG with a negligible background in the normal brain, as compared to [¹⁸F]FDG. Micro-PET/CT image analyses and tissue distribution showed that the brain tumor uptake for [⁶⁴Cu]TP3805 was 8.2 ± 1.7 % ID/g and for [⁶⁴Cu]Cl₂ 2.1 ± 0.5 % ID/g as compared to 1.0 ± 0.3 % ID/g and 1.4 ± 0.3 % ID/g for normal mouse brains, respectively. The high tumor/normal brain ratio for [⁶⁴Cu]TP3805 (8.1 ± 1.1) allowed tumors to be visualized unequivocally. Histology and [⁶⁴Cu]TP3805 DAR differentiated malignant tumors from healthy brain and confirmed PET findings.

Targeting VPAC1 receptors using [⁶⁴Cu]TP3805 for PET imaging of MG is a promising novel approach and calls for further investigation.