A generic approach for the generation of stable humanized single-chain Fv fragments from rabbit monoclonal antibodies

Despite their favorable pharmacokinetic properties single-chain Fv antibody fragments (scFvs) are not commonly used as therapeutics, mainly due to generally low stabilities and poor production yields. In this work we describe the identification and optimization of a human scFv scaffold, termed FW1.4 that is suitable for humanization and stabilization of a broad variety of rabbit antibody variable domains. A motif consisting of five structurally relevant framework residues that are highly conserved in rabbit variable domains was introduced into FW1.4 to generate a generically applicable scFv scaffold, termed FW1.4gen. Grafting of complementarity determining regions (CDRs) from 15 different rabbit monoclonal antibodies onto FW1.4 and derivatives thereof, resulted in humanized scFvs with binding affinities in the range from 4.7 x 10 -9 M to 1.5 x 10 -11 M. Interestingly, minimalistic grafting of CDRs onto FW1.4gen, without any substitutions in the framework regions, resulted in affinities ranging from 5.7 x 10 -10 M to <1.8 x 10 -12 M. When compared to progenitor rabbit scFvs, affinities of most humanized scFvs were similar. Moreover, in contrast to progenitor scFvs, which were difficult to produce, biophysical properties of the humanized scFvs were significantly improved, as exemplified by generally good production yields in a generic refolding process, and by apparent melting temperatures between 53 and 86 °C. Thus, minimalistic grafting of rabbit CDRs on the FW1.4gen scaffold presents a simple and reproducible approach to humanize and stabilize rabbit variable