Determination of the radioimmunoreactivity for anti-CD66-antibody therapy

Objectives: Radioimmunoreactivity, i.e. the binding fraction of radiolabeled antibodies, is an important parameter which considerably affects the absorbed doses to the target tissue and the organ at risk in radioimmunotherapy. The precise knowledge of radioimmunoreactivity is important 1) for compound and process optimization during therapy development, 2) to improve prediction accuracy, and 3) to distinguish low receptor density from low radioimmunoreactivity. Based on the application of a DTPA-coupled CEACAM8-specific antibody for the intensification of conditioning for high-risk leukemia patients, an optimized determination of radioimmunoreactivity is presented.

Methods: The determination of the radioimmunoreactivity was performed by isolating granulocytes from leukocyte-platelet-concentrate. Cell numbers in the range of 10⁴ to 10⁸ were distributed on Eppendorf tubes and 1 ng of radiolabeled antibody was added per reaction. DTPA-coupled antibody (DTPA-mAb) was labeled with Y-90 or In-111 (in-house coupling), uncoupled antibody with Tc-99m (commercial kit). The batches (triplicates) were incubated in an overhead shaker for 60 min and after removal of the unbound antibodies the accumulated activity was determined using a gamma counter. A general theoretical derivation [1] was adapted to the experiment and the measured data were fitted using GraphPad Prism v9.0.

Results: The chelator-to-mAb-ratio was in the range of 4-6. After radiolabeling a therapeutic specific activity of 5.7 MBq/µg was achieved. For the derived fit function, a very good fit in visual assessment with R² values ranging from 0.88 to 0.99 was obtained. For Y-90 radioimmunoreactivities were determined to be 0.68 ± 0.06 (n=7, average ± standard deviation), for In-111; 0.85 ± 0.07 (n=4) and for Tc-99m 0.90 ± 0.05 (n=5).

Conclusions: The desired radioimmunoreactivity of 1 (100%), cannot be achieved for radiolabeled antibodies. With the method used and the adapted fit function, valid values could be determined reproducibly. A suitable absorbed dose distribution for therapy was achieved for the investigated high specific activities. References: [1] G. Glatting and S.N. Reske. Determination of the immunoreactivity of radiolabeled monoclonal antibodies: A theoretical analysis. Cancer Biother Radiopharm 2006; 21(1):15-21

• Download figure
• Open in new tab
• Download powerpoint