Rituximab pretreatment before ibritumomab tiuxetan (Zevalin)

Ibritumomab tiuxetan (Zevalin), a yttrium-90-labelled anti-CD20 radioimmunotherapy, is administered only as part of a fixed two-step regimen defined in its FDA label and EMA product information. On day 1, patients receive rituximab 250 mg/m2 by intravenous infusion; seven to nine days later, a second rituximab infusion (250 mg/m2) is given, followed within four hours by the therapeutic yttrium-90 ibritumomab tiuxetan infusion, dosed at 0.4 mCi/kg (14.8 MBq/kg) for patients with platelet counts of 150,000/mm3 or more, or a reduced 0.3 mCi/kg (11.1 MBq/kg) for platelet counts of 100,000-149,000/mm3, with a maximum activity cap of 32 mCi regardless of body weight.

The pharmacological rationale, stated explicitly in the EMA summary of product characteristics, is that rituximab pretreatment clears circulating and marginal-zone CD20-positive B-cells, which would otherwise act as a radioactivity 'sink' and reduce the fraction of the yttrium-90 dose reaching lymphoma cells; the rituximab dose used in the Zevalin regimen is deliberately lower than the dose used when rituximab is given as monotherapy. Zevalin is approved for relapsed or refractory low-grade or follicular B-cell non-Hodgkin lymphoma, and as consolidation after first-line induction chemotherapy in previously untreated follicular lymphoma (the latter still requiring the same rituximab-then-Zevalin sequence).

Because rituximab must be ordered and administered separately from the Zevalin kit, this is one of the clearest examples of a mandated, label-level drug sequence in radiopharmaceutical practice: Zevalin cannot be lawfully administered without preceding rituximab, and the labels carry boxed warnings for infusion reactions (during the rituximab step) and for prolonged, severe cytopenias following the radiolabelled step.