A comparison of the in vitro and in vivo activities of IgG and F(ab′)2 fragments of a mixture of three monoclonal anti-Her-2 antibodies

Purpose: We have demonstrated previously that a mixture of three anti-Her-2 monoclonal antibodies (MAbs) that bind to different epitopes on the extracellular domain of Her-2 expressed on a human breast cancer cell line has more potent antitumor activity than the individual MAbs both in vitro and in xenografted severe combined immunodeficient mice. Because the activity of Herceptin is Fc dependent, we determined whether this would also be the, case when a mixture of these three anti-Her-2 MAbs was used. Experimental Design: IgG and highly purified F(ab')2 fragments of the anti-Her-2 MAbs and Herceptin were prepared and evaluated for their ability to induce cell death, inhibit vascular endothelial growth factor secretion, and mediate antibody-dependent cellular cytotoxicity and complement-mediated cytotoxicity in vitro. They were also compared for their abilities to induce regression of large BT474 tumors in severe combined immunodeficient mice. Results: All of the F(ab')2 fragments were >95% pure and, as expected, did not mediate antibody-dependent cellular cytotoxicity or complement-dependent cytotoxicity in vitro. The in vitro antiproliferative and proapoptotic effects of the IgGs and F(ab')2 fragments were similar. In contrast, the IgGs had significant antitumor activity in! vivo, whereas their F(ab')2 fragments were only marginally effective even at 5-fold higher doses to offset their shorter half-lives. Conclusions: These results confirm the importance of the Fc portion of Herceptin for optimal in vivo activity and demonstrate that even a mixture of three anti-Her-2 MAbs that are highly effective at inducing cell death in vitro requires Fc-mediated effector function for optimal in vivo activity.