Moxetumomab is a reformulation of an older study drug, BL22, which was composed the variable region (Fv) of an anti-CD22 monoclonal antibody fused to Pseudomonas aeruginosa exotoxin A.93 BL22 was shown to be highly active against Hairy Cell Leukemia in a phase 2 trial.94 In a phase 1 trial of children with relapsed/refractory ALL, BL22 was well tolerated and exhibited anti-leukemic activity at all doses, but clinical benefits were transient and modest.95 Therefore, BL22 was reformulated as moxetumomab to contain a Fv fragment with greater affinity for CD22. Here, CD22 is linked to hairy cell leukemia.