- Class: Monoclonal antibody – anti-CD20 radioimmunotherapy.
- Type: Murine IgG2a mAb conjugated to iodine-131.
Mechanism of Action (MOA)
- Targets CD20 on B-cells → delivers radioactive iodine directly to malignant B-cells.
- Dual action:
- “Cross-fire” effect → kills nearby tumor cells that may not express CD20.
Clinical Uses
- Relapsed or refractory CD20-positive low-grade, follicular, or transformed NHL.
- Often used after failure of standard chemotherapy and rituximab-containing regimens.
- Could be used as consolidation therapy in some patients with partial or complete response.
Dosing
- Two-step process:
- Diagnostic step: small dose of ^131I-tositumomab to calculate biodistribution and dosimetry.
- Therapeutic dose: single IV infusion of calculated ^131I-tositumomab dose.
- Dose depends on patient weight, BSA, and platelet count.
Toxicities
- Hematologic toxicity – primary dose-limiting:
- Neutropenia, thrombocytopenia, anemia (nadir ~4–6 weeks).
- Recovery usually within 8–12 weeks.
- Infusion reactions (mild/moderate).
- Infection risk due to cytopenias.
- Secondary malignancies (rare), including MDS/AML.
- Requires monitoring for hypothyroidism (iodine exposure).
Monitoring
- CBC weekly for at least 8–12 weeks post-infusion.
- Thyroid function (T4/TSH) pre- and post-therapy.
- Platelet count ≥100 × 10⁹/L and neutrophils ≥1.5 × 10⁹/L prior to therapy.
- Radiation safety: thyroid protection and isolation as needed; follow institutional protocols.
Summary
Tositumomab (Bexxar®) is a CD20-targeted radioimmunotherapy for relapsed/refractory follicular NHL. Its key risks are prolonged myelosuppression, infection, and secondary malignancies, and it requires careful coordination with nuclear medicine and radiation safety measures.