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Multiple myeloma is a malignancy of plasma cells for which there is no effective treatment. To develop an immunotherapeutic agent, we have raised a high affinity mAb (AT13/5) against CD38, one of the few well-characterized surface Ags present on myeloma cells. Since murine monoclonals have many disadvantages as human therapeutics, we prepared two engineered forms of the Ab: a CDR-grafted humanized IgG1 and a chimeric FabFc2 (mouse Fab cross-linked to two human gamma 1 Fc). To retain affinity in the humanized Ab, a number of changes were required to the human framework regions of the heavy chain. In particular, through systematic mutagenesis and computer modeling, we identified a critical interaction between the side chains of residues 29 and 78, which may be important for the humanization of other Abs. The properties of the humanized IgG1 and FabFc2 constructs were compared in a series of in vitro tests. Both constructs efficiently directed Ab-dependent cellular cytotoxicity against CD38-positive cell lines, but C was activated only poorly. Neither construct caused down-modulation of CD38, nor did they affect the NADase activity of CD38. Despite their differing structures, both Abs showed similar activity in most assays, although the humanized IgG1 was more potent at inducing monocyte cytotoxicity. These data represent the first direct comparison of CDR-grafted and chimeric FabFc2 forms of the same Ab, and offer no support for the perceived advantages of the FabFc2. These Abs show promise for therapy of multiple myeloma and other diseases involving CD38-positive cells.
Ellis et al. (Sat,) studied this question.