Ofatumumab demonstrates activity against rituximab-sensitive and -resistant cell lines, lymphoma xenografts and primary tumour cells from patients with B-cell lymphoma.

Ofatumumab is a new monoclonal antibody (mAb) targeting a novel membrane-proximal epitope on CD20. To better define ofatumumab's activity, we conducted pre-clinical studies in rituximab-sensitive cell lines (RSCL), rituximab-resistant cell lines (RRCL), ofatumumab-exposed cell lines (OECLs), primary lymphoma cells, and a lymphoma xenograft model. RRCL and OECL were generated by repeated exposure of sensitive cells to escalating doses of rituximab or ofatumumab ± human serum. Antibody-dependent cellular cytotoxicity (ADCC) and complement-mediated cytotoxicity (CMC) assays were performed to assess cellular sensitivity to rituximab or ofatumumab. Ofatumumab elicited a higher rate of CMC in RSCL, RRCL and primary tumour cells. The chronic exposure of lymphoma cells to ofatumumab resulted in rituximab resistance but less ofatumumab resistance. In an in vivo severe combined immunodeficiency mouse model of human lymphoma, ofatumumab prolonged median survival compared to rituximab. While rituximab CMC diminished with CD20 down-regulation in RRCL passages, ofatumumab activity in vitro diminished to a lesser degree. Our data suggest that ofatumumab is more potent than rituximab in rituximab-sensitive or rituximab-resistant models and has the potential to decrease the development of biological resistance in patients with repeated exposure to anti-CD20 mAbs.