CD20 is a nonglycosylated 33-37 kDa phosphoprotein member of the MS4A family which is widely expressed on normal B cell surfaces during all stages of development as well as by most B cell malignancies1,2. The biological role of CD20 remains poorly understood; however, it is thought to be involved in calcium ion influx. CD20 has no natural ligand and is not immediately internalized upon antibody binding. Thus, mAbs directed against CD20 depend on the recruitment of a host response. Anti-CD20 mAbs bind to the 44 amino acid extracellular portion.
Obinutuzumab (GA101) is a new generation, type II, anti-CD20 antibody2. Obinutuzumab was humanized by grafting the complementarity-determining sequences of murine IgG1-κ antibody B-Ly1 onto human VH and VL acceptor frameworks3. The Fc segment was glycoengineered to attach bisected, complex, nonfucosylated oligosaccharides to asparagine 297, leading to increased affinity to FcgRIII.
Obinutuzumab causes homotypic adhesion4,5,6, induces direct cell death via largely caspase-independent mechanisms4,6,7,8,9, does not localize into lipid rafts4,10,11, displays half-maximal CD20 binding at saturating conditions7, and displays minimal complement dependent cytotoxicity7.
Compared to rituximab, obinutuzumab recognizes a distinct but overlapping CD20 epitope, in a different orientation that results in increased pro-apoptotic potential12,13,14. A modified elbow-hinge residue, characterized by a leucine to valine mutation at Kabat position 11, is key to superior phosphatidylserine exposure and cell death relative to rituximab3.
