To prepare a ⁶⁸Ga labeled probe targeting integrin alpha M(CD11b) receptor, namely ⁶⁸Ga-1, 4, 7-triazacyclononane-1, 4, 7-triacetic acid-Glycine-Arginine-Glutamate-Arginine-Glutamate-polyethylene glycol₁₁-1, 2, 4, 5-terazine/CD11b antibody-F(ab′)₂-trans-cyclooctene (⁶⁸Ga-NOTA-Polypeptide-PEG₁₁-Tz/anti-CD11b-F(ab′)₂-TCO), and to explore its feasibility as a molecular probe for CD11b receptor through microPET imaging.

Immunofluorescence was used to detect the expression of CD11b on the surface of RAW264.7 cell. CD11b specific monoclonal antibody (M1/70) was conjugated with TCO, and anti-CD11b-F(ab′)₂-TCO fragment was obtained. The ligand NOTA-Polypeptide-PEG₁₁-Tz was labeled with ⁶⁸Ga, and its specific activity and radiochemical purity were detected. Pre-targeted cell binding experiment was conducted to evaluate the binding ability of molecular probe. CT26 colon cancer bearing mouse models were established, and then pre-targeted biodistribution and imaging experiments were performed. Immunohistochemical experiment was used to verify the expression of CD11b receptor in tumor. The one-way analysis of variance was used to compare the data.

The results of immunofluorescence demonstrated CD11b receptor was highly expressed on the surface of RAW264.7 cell. Anti-CD11b-F(ab′)₂-TCO fragment was verified by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). ⁶⁸Ga-NOTA-Polypeptide-PEG₁₁-Tz was successfully synthesized, with the labeling efficiency of 94.6%. The specific activity was 7.0-7.4 MBq/μg, and the radiochemical purity was higher than 95%. Pre-targeted cell binding experiment confirmed that the molecular probe bound to the CD11b receptor. The biodistribution and imaging experiments showed that the kidney radioactivity uptake was high at pre-targeted 4, 12 and 24 h intervals, which proved that probe was excreted through the urinary system. In addition, molecular probe had higher radioactive uptake at the tumor site, with the tumor/muscle ratios of 9.23±1.45, 12.53±1.36 and 10.74±1.11 (F=848.8, P<0.05). When the radioligand was injected 1 h after the pre-positioned 12 h interval, the images contrast was the best, with the standardized uptake value (SUV) in tumor and muscle of 0.67±0.12, 0.09±0.04, respectively. Immunohistochemistry verified the highly expression of CD11b receptor in tumor.

The pre-targeted molecular probe ⁶⁸Ga-NOTA-Polypeptide-PEG₁₁-Tz/anti-CD11b-F(ab′)₂-TCO is successfully synthesized. The molecular probe has targeting ability for CD11b⁺ colon cancer, and is expected to be used as a tracer targeting CD11b receptor in vivo.

Antigens, CD11b; Isotope labeling; Gallium radioisotopes; Click chemistry; Positron-emission tomography; Colonic neoplasms; Mice