ImmunoPET Imaging of αvβ6 Expression Using an Engineered Anti-αvβ6 Cys-diabody Site-Specifically Radiolabeled with Cu-64: Considerations for Optimal Imaging with Antibody Fragments.

PURPOSE: Increased expression of the αvβ6 integrin correlates with advanced tumor grade and poor clinical outcome, identifying αvβ6 as a prognostic indicator and an attractive target for molecular imaging. This work investigated the ability of a disulfide-stabilized [64Cu]NOTA-αvβ6 cys-diabody to image αvβ6 expression in vivo using a nu/nu mouse model bearing human melanoma xenografts and positron-emission tomography. PROCEDURES: Small-animal positron emission tomography (PET) imaging, quantitative ROI analysis, and ex vivo biodistribution were conducted to ascertain tumor uptake and organ distribution of the [64Cu]NOTA-αvβ6 cys-diabody. Immunohistochemical staining of tumors and mouse organs and immunoreactivity assays were utilized to correlate in vivo and ex vivo observations.
RESULTS: PET imaging of the [64Cu]NOTA-αvβ6 cys-diabody revealed low tumor uptake at 24 h p.i. in DX3Puroβ6 tumors (2.69 ± 0.45 %ID/g) with comparable results found in the DX3Puro tumors (2.24 ± 0.15 %ID/g). Quantitative biodistribution confirmed that DX3Puroβ6 tumor uptake was highest at 24 h p.i. (4.63 ± 0.18 %ID/g); however, uptake was also observed in the stomach (4.84 ± 2.99 %ID/g), small intestines (4.50 ± 1.69 %ID/g), large intestines (4.73 ± 0.97 %ID/g), gallbladder (6.04 ± 1.88 %ID/g), and lungs (3.89 ± 0.69 %ID/g).
CONCLUSIONS: Small-animal PET imaging was successful in visualizing αvβ6-positive tumor uptake of the [64Cu]NOTA-αvβ6 cys-diabody. Cys-diabody cross-reactivity was observed between human and murine αvβ6 and immunohistochemical staining confirmed the presence of an endogenous αvβ6 antigen sink, which led to suboptimal tumor contrast in this mouse model. Future investigations will focus on dose escalation studies to overcome the endogenous antigen sink while increasing DX3Puroβ6 tumor uptake.