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Molecular Imaging and Contrast Agent Database (MICAD) [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2004-2013.

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Molecular Imaging and Contrast Agent Database (MICAD) [Internet].

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131I-B1 MAb
, PhD
National Center for Biotechnology Information, NLM, NIH, Bethesda, MD, vog.hin.mln.ibcn@dacim

Created: ; Last Update: June 2, 2008.

Chemical name:131I-Tositumomab
Abbreviated name:131I-B1 MAb
Synonym:Bexxar®, 131I-anti-CD20 antibody, 131I-anti-B1 antibody
Agent Category:Antibody
Target:CD20 antigen on B-cell non-Hodgkin’s lymphoma
Target Category:Antibody-antigen binding
Method of detection:Single-photon emission computed tomography (SPECT), gamma imaging
Source of signal/contrast:131I
  • Checkbox In vitro
  • Checkbox Rodents
  • Checkbox Non-human primates
  • Checkbox Humans
Click on protein, nucleotide (RefSeq), and gene for more information about CD20.



131I-Tositumomab (131I-B1 MAb) is a conjugation of 131I with a murine anti-CD20 monoclonal antibody (MAb) that can be used for both imaging and therapy of lymphoid malignancies (1-8). It was approved in the United States in 2003 for the therapeutic treatment of patients with relapsed, rituximab-refractory, CD20+, follicular non-Hodgkin’s lymphoma (NHL), with or without transformation.

The CD20 antigen (B1) is a 35-kDa, cell-surface nonglycosylated, hydrophobic phosphoprotein expressed on normal and malignant B cells, and it does not shed, modulate, or internalize (3, 7, 9). It is a transmembrane protein that acts as a calcium channel and plays an important role in cell cycle progression and differentiation of B cells. B1 is present on approximately 9% of the peripheral blood mononuclear cell fraction and >90% of B cells from blood and lymphoid organs. Lymphoma cells from >90% of patients with B cell NHL expressed this antigen. When antibodies bind to this antigen, they induce apoptosis, antibody-dependent cellular cytotoxicity, and complement-dependent cytotoxicity of lymphoma cells. Despite the presence of CD20 on normal B cells, it is a good tumor target for molecular targeting with antibodies for the management of NHL.

Radiolabeled MAbs have been developed for both diagnosis and therapy of tumors (3, 8, 10). Tositumomab (B1 MAb) was initially developed by the Dana-Farber Cancer Institute (8, 11). It is an intact murine IgG2a lambda MAb directed against the CD20 antigen and is composed of two murine gamma 2a heavy chains of 451 amino acids each and two lambda light chains of 220 amino acids each (5, 12). 131I-B1 MAb has an approximate molecular weight of 150 kDa and is covalently linked to 131I. 131I emits both gamma and beta radiation, and it can be used for imaging at low doses and therapy at high doses (2). It is also relatively inexpensive and easy to conjugate to proteins. Although 131I-B1 MAb is only approved by the U.S. Food and Drug Administration for therapy, a dosimetric low dose of 185 MBq (5 mCi) has been used in the therapy protocol for whole-body imaging/counting on at least three occasions to determine unacceptable abnormal distribution and to establish the individualized therapeutic dose for a specific patient based on his/her tissue distribution and body clearance of 131I-B1 MAb.

The B1 MAb in the commercial preparation is produced in an antibiotic-free culture of mammalian cells and is supplied at a nominal concentration of 14 mg/ml of antibody (12). 131I-B1 MAb is supplied as a sterile, preservative-free liquid at nominal protein and activity concentrations of 0.1 mg/ml and 22.57 MBq (0.6 mCi)/ml (at the date of calibration), respectively. The formulation contains 4.4%-6.6% (w/v) providone, 1-2 mg/ml of maltose, 8.5-9.5 mg/ml of sodium chloride, and 0.9-1.3 mg/ml of ascorbic acid at pH 7.0.



B1 MAb was synthesized based on the hybridoma technique developed by Kohler and Milstein (11, 13-16). The antibody was isolated from serum-free hybridoma supernatants and purified by ion-exchange chromatography to produce a preparation with >98% pure monomeric IgG. It was labeled with 131I using direct iodination methods (10, 16, 17). Several studies used the Iod-o-Gen method. The radiochemical yield was > 90% of radioactivity bound to the antibody, as measured by thin-layer chromatography The mean specific activities were reported to be 30.71-325.6 MBq (0.83-8.8 mCi)/mg. Other methods such as the chloramine-T method had also been used (18, 19). In 2003, Beeson and colleagues (11) demonstrated a new method that involved attaching the radioactive iodide to this antibody through a linker (cephalosporin) that could be cleaved in vivo by an administered enzyme (β-lactamase) for rapid clearance as a means to reduce patient radiation exposure.

In Vitro Studies: Testing in Cells and Tissues


Stashenko and colleagues (20) in 1980 first reported the generation of a specific B1 MAb using the hybridoma technique. This MAb showed a 94.5% of absorption by human B cells from healthy volunteer donors. In 1991, 125I-B1 MAb supplied by the commercial source reported a mean binding affinity (Kd) of 1.37 ± 0.22 nm (n=3) to a human CD20-expressing B-cell line (BALL-1) (1). In comparison, another anti-CD20 MAb (125I-rituximab) had a Kd of 4.36 ± 0.34 nm (n=3).

Animal Studies



Localization and imaging of radiolabeled B1 MAb were studied in nude mice bearing malignant CD20+ B-lymphoblastoid Namalwa tumors (21). Live cell radioimmunoassay indicated an 11.9% binding of 125I-B1 MAb to this human cell line. The mouse biodistribution of 18.5 kBq (0.5 µCi) of 125I-B1 MAb by i.p. injection showed tumor/blood ratios of 1.2 and 0.7 at 4 and 6 days after injection (n = 6-8), respectively. On the 6th day after injection, more than 5-fold greater activity was reported in tumor as compared with other soft tissue organs (liver, kidney, spleen, muscle). Imaging of 1.11 MBq (30 µCi) of 131I-B1 MAb in nude mice at 2 and 4 days after i.p. injection showed tumor/nontumor (T/NT) ratios of 0.33 and 0.39, respectively.

Other Non-Primate Mammals


No publication is currently available.

Non-Human Primates


Radiolabeled B1 MAb was studied in rhesus monkeys at the New England Primate Research Center in 1987 (22). Approximately 18.5 MBq (0.5 mCi) of 131I-B1 MAb (0.3 131I atom/MAb molecule) were administered i.v. to each monkey (n=2). One monkey received 2.0 mg of 131I-B1 MAb protein (high dose) and the other one received only 0.14 mg (low dose). Imaging of both animals 24 h after the infusions showed radioactivity primarily in the blood pool, heart and liver. The spleens (lymphocyte-rich) contained a high concentration of radioactivity, and the ratio of spleen to heart activity increased by 111% over a period of 5 days in the high-dose monkey and increased only 53% in the low-dose monkey. The serum clearance of total radioactivity was slightly more rapid in the high-dose animal (t½ = 24.1 h) than in the low-dose animal (t½ = 28.8 h). The cumulative urine excretions over 7 days were 33.6% and 28.7% for the low-dose monkey and high-dose monkey, respectively.

Human Studies


A human pharmacokinetics study indicated that the half-life of 131I-B1 MAb was affected by the extent of disease of the patient. In 53 patients with different degrees of disease involvement, the mean body clearance (t½) was 68.9 h. Patients with splenomegaly had a t½ of 50 h and patients with greater disease burden had t½ of 68.0 hours. No change was found in patients with bone marrow involvement (1). In another study of 43 patients with B-cell lymphoma in relapse, the mean serum retention half-times of 185-370 MBq (5-10 mCi) of 131I-B1 MAb were 35.5 ± 16.8, 48.2 ± 17, and 48.1 ± 23.3 h after doses of 0.5 ± 2.5 and 10 mg/kg, respectively (23). Tumor uptake averaged 0.009 ± 0.003% injected dose/g of tumor in patients with favorable biodistribution (greater radiation doses to the tumors than other organs) and 0.002 ± 0.002% in patients with unfavorable biodistribution.

In a Phase I trial, increasing the predoses of unlabeled B1 MAb from 0 to 475 mg was associated with prolongation of the body clearance t½ of 131I-anti-B1-MAb from 63 to 85 h. The predosing with 475 mg of unlabeled B1 MAb appeared to provide superior tumor targeting (24). In another study with 685 mg of B1 MAb predosing and 185 MBq (5 mCi) in 15-20 mg of 131I-B1 MAb, definite tumor imaging was observed in all NHL CD20+ patients (n=34) with tumor sites larger than >2 cm. On average, the tumors received approximately 17 times the radiation dose delivered to the whole body (25).

Koral and colleagues (26) in 1994 demonstrated the use of computed tomography-single photon emission computed tomography (CT-SPECT) image fusion plus conjugate views to provide quantitative SPECT imaging of 131I-B1 MAb and to allow the quantification of tumor radioactivity in vivo. This hybrid SPECT-conjugate-view method was further refined, and a review of the results of 33 NHL patients who received 450 mg of B1 MAb predose and 185 MBq(5 mCi) of 131I-B1 MAb indicated that it could be used to provide radiation absorbed dose estimates of 131I-B1 MAb for the tumors in individual patient (27, 28).

On the basis of the sequential whole body imaging and the MIRDOSE 3 program, the median absorbed doses (mGy/MBq) of 131I-B1 MAb to the major organs were 2.71 (thyroid), 1.96 (kidneys), 1.3 (intestine walls), 1.25 (heart wall), 1.14 (spleen), and 0.65 (red marrow) (12). The critical organ appeared to be the bone marrow, because the dose-limiting toxicity was hematologic. The most common adverse reactions recorded during the clinical trials included neutropenia, thrombocytopenia, and anemia.

Various other studies have been published on the dosimetry and efficacy of the therapeutic application of treating NHL using high doses of 131I-B1 MAb [PubMed].

Supplemental Information

NIH Support

NIH DE-04881, AI 12069, CA 19589, CA 06516, RR 05526.


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