Tc-99m-rituximab radiolabelled by photo-activation: a new non-Hodgkin's lymphoma imaging agent

Purpose: Rituximab was the first chimeric monoclonal antibody to be approved for treatment of indolent B-cell non-Hodgkin's lymphoma (NHL). It is directed against the CD20 antigen, which is expressed by 95% of B-cell NHLs. The aim of this study was to explore the possibility of radiolabelling rituximab with Tc-99m for use as an imaging agent in NHL for early detection, staging, remission assessment, monitoring for metastatic spread and tumour recurrence, and assessment of CD20 expression prior to ( radio) immunotherapy. Methods: Rituximab was purified from Mabthera solution ( Roche), photo-activated at 302 nm by UV irradiation and radiolabelled with Tc-99m. The effectiveness of the labelling method was evaluated by determination of the number of free thiol groups per photoreduced antibody, radiochemical purity and in vitro stability of Tc-99m-rituximab. Results: On average, 4.4 free thiol groups per photoreduced antibody were determined. Radiolabelling yields greater than 95% were routinely observed after storage of the photo-activated antibody at - 80 degrees C for 195 days. The direct binding assay showed preserved ability of Tc-99m-rituximab to bind to CD20, with an average immunoreactive fraction of 93.3%. The internalisation rate was proven to be low, with only 5.3% of bound Tc-99m-rituximab being internalised over 4 h at 37 degrees C. Conclusion: Our results demonstrate that Tc-99m-rituximab of high radiochemical purity and with preserved binding affinity for the antigen can be prepared by photoreduction and that the method shows good reproducibility. Tc-99m-rituximab will be further explored as an imaging agent applicable in NHL for the purposes mentioned above.