"Internalization of Anti-CD33 Antibodies "
Human CD33 is expressed by early myeloid and leukemic progenitors, but not by pluripotent hematopoietic stem cells, and has been successfully used for targeted therapy with immunoconjugates (e.g. gemtuzumab ozogamicin, GO; humanized anti-CD33 antibody conjugated with the potent anti-tumor antibiotic calicheamicin) in acute myeloid leukemia (AML). The success of this therapeutic approach requires internalization of the antibody-CD33 complex, but the underlying mechanism has not been studied so far. We have found in preliminary studies that the rate of internalization of the antibody is greatest in AML blast cells from patients who responded favorably to treatment with GO, and that rapid internalization of CD33 may be dependent upon tyrosine kinase activity. We therefore propose to study the factors responsible for receptor-mediated internalization of anti-CD33 antibodies in detail. Specifically, we plan to investigate which tyrosine kinases and which motifs on the cytoplasmic tail of CD33 are of importance for fast CD33 internalization. We will also determine why the rapid CD33 internalization is apparently not functional in some cell lines with slower internalization of CD33 and whether rapid internalization can be restored in such cell lines and primary AML cells. Elucidation of the mechanism(s) of CD33 internalization would substantially expand the current understanding of CD33 biology and could theoretically lead to novel approaches for enhanced clinical efficacy of immunoconjugate therapy of AML.