Human antibody derivatives against the fibroblast activation protein for tumor stroma targeting of carcinomas

The fibroblast activation protein (FAP) is selectively expressed on activat ed fibroblasts of the tumor stroma on more than 90% of lung, breast and col on carcinomas. The high prevalence and abundance of FAP(+) stroma make it a promising target for in vivo diagnosis and therapy of a variety of carcino mas. We describe the humanization of the murine FAP-specific MAb, F19, whic h has already been clinically used for in vivo diagnostic purposes. Using p hage display technology and human V-repertoires, VL and VH regions of F 19 were replaced by analogous human V-regions while retaining the original HCD R3 sequence in order to maintain F19 epitope specificity. The resulting hum an single-chain fragments of immunoglobulin variable regions (scFv 34, scFv 18) showed affinities of 6 nM on cell membrane-bound FAP, scFv 34 was expr essed as a bivalent minibody (Mb 34). The antigen-binding characteristics o f Mb 34 were comparable to the parental and a complementarity-determining r egion (CDR)-grafted version of F19. This was revealed by binding competitio n studies, FAGS analyses and immunohistochemistry on various tumor samples including breast, colon and lung carcinomas. Importantly, compared with the CDR-grafted humanized scFv version of F19, the V-regions of the selected h uman scFv 34 showed sequence identity with the parental antibody (Ab) only over the short, 15-amino acid long HCDR3. Thus, a largely reduced xenoantig enic potential is expected. These human Ab derivatives are suitable to deve lop novel therapeutic concepts with broad applicability for a wide variety of histological carcinomas based on tumor stroma targeting. (C) 2001 Wiley- Liss, Inc.