Tositumomab was launched as a new radioimmunotherapeutic antibody for the
treatment of β-cell non-Hodgkin’s lymphoma (NHL). It is specifically indicated for
patients with CD20-positive, follicular NHL, with or without transformation, whose
disease is refractory to rituximab and has relapsed following chemotherapy.
Tositumomab is an anti-CD20 murine IgG2a lambda monoclonal antibody produced in
an antibiotic-free culture of mammalian cells. The therapeutic regimen of the drug is
composed of tositumomab and 131I-tositumomab. This dual-action therapy combines the
tumor-targeting ability of a cytotoxic monoclonal antibody and the therapeutic potential
of radiation with patient-specific dosing. The binding of tositumomab to CD20 antigen on
NHL cells initiates an immune response to the cancer and the radiolabeled antibody, 131Itositumomab,
delivers a dose of radiation directly to tumor cells. Iodine-131 has a
physical half-life of eight days. It emits b particles and γ rays, but the primary antitumor
effect results from the β particles. In contrast to the external beam radiation, 131Itositumomab
delivers radiation to the tumor at a continuous low dose rate. The dose rate
initially increases, as radiolabelled antibodies accumulate at the tumor site, with a
maximal tumor dose rate of ＜0.10 Gy/h. The dose rate then decreases with physical
decay of the radioisotope and clearance from the body. The tositumomab and 131Itositumomab
therapeutic regimen is dosed in two steps: a dosimetric step followed 7–14
days later by a therapeutic step. The dosimetric step consists of successive infusions of
tositumomab (450 mg) and 131I-tositumomab (5.0 mCi, 35 mg). The therapeutic step
consists of 450 mg tositumomab infusion, followed by 131I-tositumomab as calculated to
patient-specific activity. Since non-tumor localization of 131I is primarily in the thyroid,
the patients are also treated with a thyro-protective agent such as potassium iodide
throughout the therapeutic course. In a clinical study in patients who had an average of four prior chemotherapies and who did not respond to or relapsed from rituximab
therapy, overall response to tositumomab and 131I-tositumomab treatment was achieved
in 63% of the patients and complete response was achieved in 29% of the patients. The
duration of response in this category of patients was 25 months. The most common
adverse reactions associated tositumomab regimens are severe or life-threatening
cytopenia, infections, hemorrhage, allergic reactions, secondary leukemia, and
myelodysplasia.
