Multivalent recombinant
antibody fragments provide high binding avidity and unique specificity to a wide range of target
antigens and
haptens. This review describes how careful choice of linker length between V-domains creates new types of Fv modules with size, flexibility and valency suited to in vivo imaging and
therapy. Further, we review the design of multi-specific Fv modules suited to cross-linking target
antigens for cell-recruitment, viral delivery and immunodiagnostics.
Single chain Fv antibody fragments (scFvs) are predominantly monomeric when the V(H) and V(L) domains are joined by
polypeptide linkers of at least 12 residues. An scFv molecule with a linker of 3 to 12 residues cannot fold into a functional Fv domain and instead associates with a second scFv molecule to form a bivalent dimer (diabody, approximately 60 kDa). Reducing the linker length below three residues can force scFv association into trimers (triabodies, approximately 90 kDa) or tetramers ( approximately 120 KDa) depending on linker length, composition and V-domain orientation. The increased binding valency in these scFv multimers results in high avidity (long off-rates). A particular advantage for
tumor targeting is that molecules of approximately 60-100 kDa have increased
tumor penetration and fast clearance rates compared to the parent Ig. A number of
cancer-targeting scFv multimers have recently undergone pre-clinical evaluation for in vivo stability and efficacy. Bi- and tri-specific multimers can be formed by association of different scFv molecules and, in the first examples, have been designed as
cross-linking reagents for T-cell recruitment into
tumors (
immunotherapy) and as red blood cell agglutination
reagents (immunodiagnostics).