Biodistribution and Dosimetry of Indigenously Produced ¹³¹I-Rituximab in B-Cell Lymphoma: Pilot Study Estimating Patient-Specific Dose Comparing 2 Different Dosimetric Methods

Cost containment through indigenous production of radioimmunotherapy agents for non-Hodgkin lymphoma (NHL) would be a pivotal step toward wider clinical availability, especially in developing countries. We examined the biodistribution and dosimetry of indigenously developed and radiolabeled ¹³¹I-rituximab, using the monoclonal antibody of chimeric origin, in patients with B-cell lymphoma for potential use in radioimmunotherapy. Methods: This prospective study included 13 patients with B-cell NHL who underwent low-dose diagnostic scanning for dosimetric and biodistribution studies. Soon after rituximab infusion, a diagnostic dose of radioiodinated rituximab was administered. Serial planar whole-body γ-camera images were taken soon afterward and on days 1, 2, 4, and 6. A source of ¹³¹I with known activity was used as a reference standard for dosimetry calculations. Results: The patient-specific administered dose that would give a whole-body absorbed radiation dose of 75 cGy, calculated by the MIRD schema, ranged from 3,095.42 to 6,330.33 MBq (83.66-171.09 mCi), with a mean of 3,986.01 ± 863.95 MBq (107.73 ± 23.35 mCi) and a median of 3,697.41 MBq (99.93 mCi). The mean residence time was 69.54 h. Within the first 48 h at least 50% of the injected activity was cleared, and by 144 h at least 80% was cleared. The patient-specific administered dose that would give a whole-body absorbed radiation dose of 75 cGy, calculated by mean residence time and activity-hours, ranged from 2,654.75 to 6,210.45 MBq (71.75-167.85 mCi), with a mean of 3,576.42 ± 927.59 MBq (96.66 ± 25.07 mCi) and a median of 3,421.02 MBq (92.46 mCi). With respect to organ-specific dosimetry, the mean absorbed doses to organs (apart from blood pool [3.77 Gy] and spleen [4.02 Gy]) were 0.97 Gy to the lungs, 0.69 Gy to the liver, and 0.7 Gy to the kidneys. Conclusion: The indigenous product had kinetics similar to commercial radiopharmaceuticals, with the advantage of a lower human antimouse antibody response because of the pharmaceutical's being a chimeric antibody rather than a murine antibody. Hence, clinical administration was safe. In none of the organs did dose-limiting radiation exposure occur at the proposed therapeutic dose.