An Engineered Cysteine-Modified Diabody for Imaging Activated Leukocyte Cell Adhesion Molecule (ALCAM)-Positive Tumors

The purpose of this study was to generate and evaluate a positron emission tomography (PET) radiotracer targeting activated leukocyte cell adhesion molecule (ALCAM). A human anti-ALCAM single chain variable fragment was reformatted to produce a covalent dimer, termed a cys-diabody (CysDb). Purified CysDb was characterized by gel electrophoresis and size exclusion chromatography, and immunoreactivity was assessed by flow cytometry and immunofluorescence. Targeting and imaging of ALCAM-positive tumors using Cu-64-DOTA-CysDb were evaluated in mice bearing human pancreatic adenocarcinoma xenografts (HPAF-II or BxPC-3). CysDb binds specifically to ALCAM-positive cells with an apparent affinity in the range of 1-3 nM. MicroPET images at 4 h showed specific targeting of positive tumors , a finding confirmed by biodistribution analysis, with positive/negative tumor ratios of 1.9 +/- 0.6 and 2.4 +/- 0.6, and positive tumor/blood ratios of 2.5 +/- 0.9 and 2.9 +/- 0.6 (HPAF-II and BxPC-3, respectively). Successful imaging with Cu-64-DOTA-CysDb in animal models suggests further investigation of ALCAM as an imaging biomarker is warranted.