High avidity scFv multimers; diabodies and triabodies

Multivalent recombinant antibody fragments provide high binding avidity and unique specificity to a wide range of target antigens and haptens. This re view describes how careful choice of linker length between V-domains create s new types of Fv modules with size, flexibility and valency suited to in v ivo imaging and therapy. Further, we review the design of multi-specific Fv modules suited to cross-linking target antigens for cell-recruitment, vira l delivery and immunodiagnostics. Single chain Fv antibody fragments (scFvs ) are predominantly monomeric when the V-H and V-L domains are joined by po lypeptide linkers of at least 12 residues. An scFv molecule with a linker o f 3 to 12 residues cannot fold into a functional Fv domain and instead asso ciates with a second scFv molecule to form a bivalent dimer (diabody, simil ar to 60 kDa). Reducing the linker length below three residues can force sc Fv association into trimers (triabodies, similar to 90 kDa) or tetramers (s imilar to 120 KDa) depending on linker length, composition and V-domain ori entation. The increased binding valency in these scFv multimers results in high avidity (long off-rates). A particular advantage for tumor targeting i s that molecules of similar to 60-100 kDa have increased tumor penetration and fast clearance rates compared to the parent Ig. A number of cancer-targ eting scFv multimers have recently undergone pre-clinical evaluation for in vivo stability and efficacy. Bi- and tri-specific multimers can be formed by association df 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 (immunodiagno stics). (C) 1999 Elsevier Science B.V. All rights reserved.