Purpose: It is known that major histocompatibility complex class II protein HLA-DR is highly expressed in B-cell lymphomas and in a variety of autoimmune and inflammatory diseases. Therefore, a radiolabelled fully humanized IgG4 monoclonal antibody (mAb) can provide useful prognostic and diagnostic information. Aims of the present study were to radiolabel an anti-HLA-DR mAb with technetium-99m and to evaluate its binding specificity, tissue distribution and targeting potential.
Procedures: For labelling, we compared a direct method, after 2-mercaptoethanol (2-ME) reduction of disulphide bonds, with a two-step labelling method, using a heterobifunctional succinimidyl-6-hydrazinonicotinate hydrochloride chelator. Several in vitro quality controls and in vivo experiments in mice were performed.
Results: We obtained highest labelling efficiency (LE, >98%) and specific activity (SA; 5,550 MBq/mg) via the direct method. In vitro quality control showed good stability, structural integrity and retention of the binding properties of the labelled mAb. The biodistribution in mice showed high and persistent uptake in spleen and suggests kidney and liver-mediated clearance pathways. In tumour targeting experiments, we observed high uptake in HLA-DR-positive xenografts compared to controls. In vivo binding was proportional to the number of injected cells. In the in vivo blocking assay, uptake of radiolabelled mAb was significantly decreased in mice pre-injected with 100-fold molar excess of unlabelled mAb.
Conclusion: We efficiently labelled a humanized anti-HLA-DR mAb with (99m)Tc using a direct labelling method. Radiolabelled mAb binds to human HLA-DR antigens and therefore warrants further evaluation as a prognostic and diagnostic tool for patients with lymphoma or autoimmune diseases.