Objective: This study was performed to explore the possibility of using 99mTc-rituximab as an imaging agent to assess expression of CD20 antigen in patients with B-cell non-Hodgkin's lymphoma (NHL) before (radio) immunotherapy, for staging and subsequent evaluation of remission of NHL.
Methods: Rituximab was purified from Mabthera and photoactivated by ultraviolet light. The irradiated solution was aliquoted and labelled with pertechnetate. The effectiveness of the labelling method was evaluated by determination of the number of free thiol groups per photoreduced antibody, radiochemical purity determination and in-vitro stability. Immunoreactivity of 99mTc-rituximab was assessed on Ramos cells using a direct binding assay. Ten patients (age 31-70 years, mean 50 years) were included, nine with CD20+ B-cell NHL and one with CD20-NHL. Whole-body and single photon emission computed tomography images were taken 1, 3, 6 and 20 h postinjection of 99mTc-rituximab. Scintigraphic results were compared with computerized tomography (CT) findings.
Results: In all cases radiochemical purity over 95% was observed with preserved affinity for CD20 antigen. In all patients expected activity was seen in the blood pool, liver, kidneys and spleen. Pathological, moderately to markedly increased 99mTc-rituximab activity was seen in all but one CT-confirmed NHL involved sites 6 and 20 h postinjection. In one patient, increased activity of 99mTc-rituximab was additionally seen in one region not seen on CT. In three patients increased accumulation was seen in bone marrow.
Conclusion: 99mTc-rituximab is a promising imaging agent suitable for assessing expression of CD20 in patients with NHL before (radio) immunotherapy.