Rituximab is a genetically engineered chimeric anti-CD 20 Mab that has murine light-and heavy-chain variable regions and human γ1 and κ light-chain constant regions (IgG1 κ isotype) (Anderson et al. 1997). The chimeric Mab is composed of two heavy chains of 451 amino acids and two light chains of 213 amino acids and has an approximate molecular weight of 145 kDa (Grillo-López 2003). Rituximab was engineered with human constant regions to decrease immunogenicity and to enable it to bind complement and effector cells (Reff et al. 1994). It is more effective than in its murine parent in fixing complement and mediating antibody-dependent cellular cytotoxicity and it effects complement-dependent cytotoxicity in the presence of human complement (Anderson et al. 1997).

Iodine-131 radioiodination of rituximab is performed in a hospital nuclear medicine radiopharmaceutical laboratory in a simple thirty minute semi-automated procedure (Turner et al. 2003). Briefly, protein-labelling grade ¹³¹I-sodium iodide, which contains no stabilizing agent, is used within 24 h of manufacture, being shipped in a 10 ml sterile vial which constitutes the reaction vessel. Rituximab, 15 mg from a sterile, undiluted patient dose, is added to 6 GBq radioiodine with Chloramine T and incubated at room temperature for 5 min and the reaction is then quenched with sodium thiosulphate. Purification is performed on a shielded proprietary sterile semi-automated commercially available kit (Therap-I Mab kit™ Go Medical Industries Pty Ltd Australia) and quality control performed on ITLC silica gel. Manufacture is performed on protocols approved by the National Association of Testing Authorities (NATA). Radiochemical purity is routinely above 99 % with retention of immunoreactivity >80 % (Turner et al. 2003). It is however important to administer therapeutic activities to patients within one hour of radiolabeling, to avoid radiolysis of the antibody, since no radioprotective agents are used.

Solid state methods, such as iodogen, for radioiodination of proteins (Markwell 1982) have been subsequently modified (Visser et al. 2001) and recently refined (Tran et al. 2009) to achieve labelling efficacy of ¹³¹I-rituximab of 95–98 % and immunoreactivity of around 70 % (Illidge et al. 2009; Tran et al. 2009). The iodogen and Chloramine T radiolabelling methods for ¹³¹I-rituximab are both simple, rapid and efficient and radiation exposure to laboratory personnel is minimal.

The mean circulating half-life in patients with non-Hodgkin lymphoma is similar for ¹³¹I-rituximab labelled by Chloramine T 85.4 h (46–115) (Turner et al. 2003) or Iodogen 87.9 h (40–134) (Scheidhauer et al. 2002) and is longer than that of its murine anti-CD 20 Mab counterpart ¹³¹I-tositumomab at 56 h (Kaminski et al. 2000).

Whole body activity decreases monoexponentially with a wide range of effective half-lives which mandate individual prospective dosimetry for safe, effective ¹³¹I-rituximab RIT. The mean effective ¹³¹I-rituximab half-life of 85–90 h is seen in NHL lymphoma undergoing standard RIT with predose unlabelled rituximab at 375 mg/m² which improves tumour targeting and obviates splenic sequestration of activity (Illidge et al. 2009). Studies of patients receiving ¹³¹I-rituximab in the absence of prior unlabelled Mab show mean effective half-life of 45 h (40–55) compared with 93 h (66–113) in the same patients after rituximab predosing (Illidge et al. 2009). Fractionation of ¹³¹I-rituximab therapy may also significantly increase the effective half-life of the subsequent dose (Illidge et al. 2009).

Prescription of the whole body radiation absorbed dose of 0.75 Gy (equivalent to <2.0 Gy to red marrow) by prospective individual dosimetry gamma imaging of a tracer 200 MBq ¹³¹ I-rituximab determines the safe, therapeutic activity for each patient and minimizes myelotoxicity. Following predosing with 375 mg/m² unlabelled rituximab the tracer 200 MBq ¹³¹I-rituximab is administered intravenously and whole body gamma imaging performed at 1 h, 4 days and 7 days under identical geometry on the same gamma camera. The automated generation of a dose-activity relation curve based upon individual patient whole body retention of the tracer activity of ¹³¹I-rituximab calculates the therapy activity (GBq) which will limit whole body radiation absorbed dose to 0.75 Gy (Turner 2009). Validation of this dosimetric methodology by quantitative SPECT/CT imaging of lumbo sacral spine and blood sampling after tracer and therapy activities of ¹³¹I-rituximab demonstrates a median radiation absorbed dose of 1.56 Gy (1.09–1.9) to red marrow when the administered activity was prescribed according to a calculated whole body radiation absorbed dose of 0.75 Gy (Boucek and Turner 2005).