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

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 99mTc 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. Rituximab was purified from Mabthera solution (Roche), photo-activated at 302 nm by UV irradiation and radiolabelled with 99mTc. 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 99mTc-rituximab. 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 C for 195 days. The direct binding assay showed preserved ability of 99mTc-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 99mTc-rituximab being internalised over 4 h at 37 C. Our results demonstrate that 99mTc-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. 99mTc-rituximab will be further explored as an imaging agent applicable in NHL for the purposes mentioned above. (orig.)