Fate of 177Lu-CHX-A”-DTPA-Rituximab: In vitro Evaluation in Raji Cell Line

Context: Radioimmunotherapy is an emerging treatment modality for various types of cancers. While immunotherapy using monoclonal antibodies has shown promising results, particularly in hematological malignancies, a significant number of patients develop resistance to the treatment, which may be overcome using monoclonal antibodies labeled with suitable therapeutic radioisotopes. Aim: In this study, in vitro evaluation studies of 177Lu-CHX-A''-DTPA-rituximab were performed in Raji cells that overexpress CD20. The extent of internalization of 177Lu-CHX-A”-DTPA rituximab inside the target cell as well as the impact of cellular toxicity in Raji cells was studied. Materials and Methods: The monoclonal antibody rituximab was labeled with 177Lu using CHX-A”-DTPA as the bifunctional chelator. In vitro cell binding and inhibition studies were performed in Raji cells to ascertain the specificity of the radioimmunoconjugate toward the CD20 receptors. The immunoreactive fraction was determined to evaluate the integrity of the radioimmunoconjugate. A cellular internalization assay was performed to evaluate the extent of internalization of the radioimmunoconjugate, and the extent of cytotoxicity was determined using flow cytometry in comparison with unlabeled rituximab. Results: Radiochemical purity of 177Lu-CHX-A''-DTPA-rituximab was determined to be 97.4% ± 1%. In vitro cell-binding studies in Raji cells showed a cell concentration-dependent increase in the percent cell binding, which surged from 11.7% ± 0.7% to 22.7% ± 0.9%, as the cell concentration increased from 0.94 × 10^6 to 7.5 × 10^6 successively. Inhibition in binding was observed in the presence of unlabeled rituximab (11.7% ± 0.7% to 7.8% ± 1.2% and from 22.7% ± 0.9% to 12.1% ± 1.3%). The immunoreactive fraction was found to be 78.5%. A time-dependent increase in the cellular internalization from 25.21 ± 1.7 to 60.47 ± 0.20 was observed. The percent cell viability decreased from 56% to 41% when the cell was treated with rituximab compared with 177Lu-rituximab. Conclusions: Thus, the results show a potential of 177Lu-rituximab as a promising radiopharmaceutical against non-Hodgkin's lymphoma.