Antibody alternative formats: antibody fragments and new frameworks

Antibodies are now recognized as routine molecules in many therapeutic fields, no longer restricted to oncology and inflammation. This explosion of the field leads to new needs that can be better fulfilled by molecules inspired but different from conventional antibodies. In particular, the antibody molecule has multiple functions that are not always necessary, such as its ability to recruit immune system cells, its bivalency, or its high plasma half-life. However, in most applications, its remarkable ability to recognize almost any molecular partner with high affinity and specificity must be preserved. In addition, antibodies are very large molecules, expensive to produce and having limited physicochemical properties that limit their use in aggressive media. Finally, in certain therapeutic applications, the large size of the antibody molecule may also limit its diffusion in tissues and prevent the recognition of some poorly accessible molecular structures. To address these limitations, many alternative formats to whole antibodies have been developed over the last twenty years. These new formats have found applications in many fields like biotechnology, in vitro and in vivo diagnosis, and therapy. Two large families of molecules cover this field and will be presented in this mini-review. The first family is based on antibody by reducing its size, such as classical antibody fragments (Fab, scFv) or those derived from camels or sharks (VHH, V-NAR). The second family was developed by first identifying frameworks fulfilling the desired properties, in particular the stability in extreme medium and the productivity in simple and economic systems like bacteria, then by grafting binding properties comparable to antibodies using methods based on in vitro directed molecular evolution techniques. This mini-review will focus on the most advanced molecules but the field is quickly evolving. It should be noted that many of these molecules, or even these approaches, are covered by patents and are often developed by young innovative companies, some of which have been already bought by large pharmaceutical groups.