BL22

An anti-CD22 immunotoxin is a monoclonal antibody (targeting CD22) linked to a cytotoxic agent. They are being studied in the treatment of some types of B-cell cancer.

They bind to CD22, a receptor protein on the surface of normal B cells and B-cell tumors, and, upon internalization, kill the cells.

As of August 2009, several anti-CD22 immunotoxins are undergoing clinical trials.

CAT-3888 (or BL22) is an anti-CD22 immunotoxin and completed a Phase I clinical (human) trial for the treatment of hairy cell leukemia at the NIH in the U.S.

Technically, CAT-3888 is an anti-CD22 immunotoxin fusion protein between a murine anti-CD22 disulfide-linked Fv (dsFv) antibody fragment and an edited copy of bacterial Pseudomonas exotoxin PE38. The toxin is activated intracellularly, by the low pH of the lysosome into which the entire protein was internalized via the CD22 receptor. The toxin kills the targeted cell through ribosome inactivation.

CAT-3888 was initially designed and produced at the U.S. National Cancer Institute, one of the agencies which make up the NIH. Early development of BL22 was funded by California biotech Genencor.

In 2001 results were reported of remissions in a phase I trial for leukemia.

CAT-3888 was succeeded by moxetumomab pasudotox (CAT-8015, HA22), an anti-CD22 immunotoxin comprising a modified Pseudomonas exotoxin and an anti-CD22 antibody fragment.

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