BLOCKADE OF EPIDERMAL GROWTH-FACTOR RECEPTOR FUNCTION BY BIVALENT ANDMONOVALENT FRAGMENTS OF 225 ANTIEPIDERMAL GROWTH-FACTOR RECEPTOR MONOCLONAL-ANTIBODIES

We have previously described anti-epidermal growth factor (EGF) recept or monoclonal antibodies (MAbs) which can block binding of transformin g growth factor alpha (TGF-alpha) and EGF to receptors and inhibit act ivation of receptor protein tyrosine kinase. Studies with these MAbs i nvolving cell cultures and nude mouse xenografts demonstrated their ca pacity to inhibit the growth of a variety of tumor cell lines, which e xpress EGF receptors and TGF-alpha and appear to depend upon receptor activation for cell proliferation. To explore the mechanism(s) by whic h anti-EGF receptor 225 MAb inhibits cell proliferation, we have compa red the activity of native 225 MAb with the response to bivalent 225 F (ab')2 and monovalent 225 Fab' fragments. Both native 225 MAb and its fragments could inhibit the binding of I-125-EGF to EGF receptors. Sca tchard analysis revealed that the K(d) of 225 F(ab')2 is comparable to that of 225 MAb (1 nM), whereas the K(d) of 225 Fab' is 5 nm. Both bi valent 225 MAb and 225 F(ab')2 and monovalent 225 Fab' were able to co mpletely inhibit TGF-alpha-induced EGF receptor tyrosine kinase activa tion, as assayed by autophosphorylation of tyrosine residues of EGF re ceptors on MCF10A nonmalignant human mammary cells, MDA468 human breas t adenocarcinoma cells, and A431 human vulvar squamous carcinoma cells . The bivalent forms of MAb could inhibit proliferation stimulated by endogenous (autocrine) TGF-alpha in cultures of these three cell lines . They also blocked growth stimulation by added exogenous TGF-alpha in cultures of MCF10A cells and the growth-inhibitory effect of exogenou s TGF-alpha upon MDA468 and A431 cell cultures. Monovalent 225 Fab' ha d weaker inhibitory effects upon the proliferation of these cell lines . To determine whether the in vivo antiproliferative activity of anti- EGF receptor MAb can occur without the participation of the Fc portion of MAb, the capacities of 225 F(ab')2 and native 225 MAb to inhibit g rowth of s.c. A431 cell xenografts were compared. Equimolar amounts of either 225 MAb or 225 F(ab')2 were administered at intervals equivale nt to the half-lives of the molecules, to attempt to maintain comparab le plasma levels. Both 225 MAb and 225 F(ab')2 inhibited A431 cell xen ograft growth in a dose-dependent manner, with a more sustained respon se in the case of the intact antibody. These experiments establish the capacity of the bivalent 225 F(ab')2 and monovalent 225 Fab' fragment s of an anti-EGF receptor antibody to mimic the properties of native 2 25 MAb in inhibiting ligand-induced tyrosine kinase activation, althou gh the Fab' fragment is a weaker inhibitor of proliferation compared w ith bivalent forms of antibody. They also provide strong evidence that inhibition of A431 cell growth in vivo can result from pharmacologica l blockade of EGF receptors, without participation of immune responses mediated by the Fc fragment of the antibody.