Therapeutic bispecific antibodies: The selection of stable single-chain fragments to overcome engineering obstacles

The clinical success of mAbs continues to reinforce antibody engineering as an essential tool for the development of biologics. Research focused on discovering the next generation of therapeutics has prompted a revisiting of the concept of bispecific antibodies (bsAbs). Recently, clinical programs investigating combinations of mAb therapies have renewed interest in the applications of bsAbs. However, because of challenges with production, efforts directed toward the development of bsAbs have yet to yield a product approved by the FDA. The current status of these proteins implies that the strategies for constructing therapeutic bsAbs will likely require a highly refined design plan at the outset of the engineering process. Antibody fragments are attractive building blocks for the assembly of bsAbs. Of the recombinant antibody fragments, single-chain variable fragments (scFvs) offer the advantage of expression as a single polypeptide, thereby greatly simplifying production. However, issues with stability have plagued these proteins and limit the application of scFvs as therapeutics. Recent advances in selection processes using display platforms have been reported that facilitate the 'evolution' of scFvs to obtain stabilities comparable with those of mAbs. The timely advances in scFv engineering parallel the resurgence of bsAbs and enable the construction of dual-targeting proteins that can be manufactured as therapeutics.