Labelling of a monoclonal antibody with 68Ga using three DTPA-based bifunctional ligands and their in vitro evaluation for application in radioimmunotherapy

The most commonly used radiometal for dosimetry in radioimmunotherapy is 111In. This radionuclide has suitable physical properties and its chelation chemistry is similar to that of 90Y, which is frequently used in radiotherapy. Since imaging with a γ-ray emitting radionuclide is less accurate than PET-imaging, we evaluated the labelling of a monoclonal antibody with the β +-emitter 68Ga and the in vitro stability of the labelled antibody in human serum. We focused our studies on the bifunctional chelators Bn-DTPA, CHX-A′′-DTPA, and mx-DTPA conjugated to the anti-CD45 monoclonal antibody YAML568. The incorporation of 68Ga into the antibody is rapid for all three ligands. After 5 minutes the radiochemical yield is > 95%. The serum stability differs strongly depending on the chelator. The least stable chelate is [68Ga]Bn-DTPA. After 3 h at 37°C in human serum 66% of 68Ga is transchelated from the antibody to transferrin. The [68Ga]CHX-A′′-DTPA chelate is kinetically more stable. 83% of 68Ga were still chelated to the antibody after 4h in human serum. The best results were obtained using mx-DTPA. Only 5% were transchelated from the labelled antibody to transferrin after 4h in human serum. The high in vitro stability and the low transchelation tendency of the [68Ga]mx-DTPA-conjugate enable the accurate determination of antibody biodistribution for dosimetry using PET in combination with conventional [111In]anti-CD45 scintigraphy.