CD19 (B4, Leu-12) is expressed on pre-B- and B-lymphocytes. In preclinical studies, the anti-CD19 monoclonal antibody HD37 was conjugated to the ribosome-inactivating protein SAP. HD37-SAP inhibited DNA synthesis in fresh CLL cells and was able to exert greater than a 2-log kill in B-NHL cells (150). HD37 has been conjugated to a deglycosylated ricin A chain and tested in patients with NHL, although no CLL patients were enrolled in the phase I trial (151). Anti-B4 was conjugated to blocked ricin to generate an immunotoxin (anti-B4-bR), which was administered by 7-d continuous iv infusion to 34 patients with relapsed or refractory B-cell neoplasms in a phase I clinical trial, including 4 patients with CLL. Five clinical responses (two CR, three PR) were observed, in addition to 11 transient responses (152). The same authors also administered anti-B4-bR by 5 consecutive daily bolus infusions to 25 patients with refractory B-cell neoplasms; three responses (one CR, two PR) were observed (153).

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