Single-chain variable fragments (scFvs) are small monovalent recombinant antibody fragments that retain the specificity of their parent immunoglobulins. ScFvs are excellent building blocks for new and improved immunodiagnostic and therapeutic proteins. However, the monovalency and the rapid renal elimination of scFvs result in poor tumor accumulation and retention. Engineering divalent antibody fragments is an excellent way to address these shortcomings. In this study, covalent divalent single-chain variable fragments (sc(Fv)₂s), were constructed from the monovalent anti-keratin 8 scFvs, TS1-218 and its mutant, HE1-Q. The scFvs and sc(Fv)₂s were expressed in the methylotrophic yeast Pichia pastoris, utilizing the alpha-factor secretion signal (α-factor) for extracellular secretion. The immunoreactivity and specificity of the antibody fragments were analyzed with enzyme-linked immunosorbent assay (ELISA) and the uptake and retention of the ¹²⁵I labeled antibody fragments were evaluated using HeLa HEp-2 multicellular tumor spheroids (MCTSs). Analysis of the antibody fragments demonstrated that parts of the α-factor remained at the N-terminal of the antibody fragments. Despite incomplete processing of the α-factor, the antibody fragments were functional where the sc(Fv)₂s gave a three-fold stronger signal in ELISA compared to their scFv counterparts and the mutant antibodies demonstrated a stronger signal than their initial wild types. In addition, the sc(Fv)₂s DiTS1-218 and DiHE1-Q displayed an approximately two-fold higher uptake and were retained to a larger extent in the MCTS, demonstrating a 3.9 and 9.4-fold increase in half-life respectively compared to their corresponding scFvs. In conclusion, expression in P. pastoris improved the yield 20-fold and facilitated the purification of the antibody fragments. Furthermore, the sc(Fv)₂s presented a higher functional affinity to K 8 both in ELISA and MCTS compared to the scFvs with DiHE1-Q being the best candidate for further studies.

Antibodies are glycoproteins specifically binding to a variety of antigens and today extensively used in prevention, diagnosis and treatment of diseases. Carcinomas originate from epithelial tissues and are the most common forms of human malignancies. In the necrotic areas of carcinomas significant amounts of keratins (K) are found extracellularly. They remain there due to their low solubility and can be used as targets in immuno-targeting and -therapy. Single-chain fragment variable (scFv) may display several advantages over intact IgGs in various applications but their use in immunotherapy of tumors could be limited. Their small size and monovalency may result in low accumulation of the scFv in the tumors. The targeting efficiency of antibodies needs to be evaluated in vitro prior to in vivo studies in order to exclude poor candidates. In vitro models should preferably resemble the in vivo situation as much as possible.

In the present study a scFv variant (TS1-218) of the anti-keratin 8 (K 8) monoclonal antibody TS1 (mAb TS1) was constructed and characterized using site-directed mutagenesis. In addition, the valency of the TS1-218 and one of its mutants, HE1-Q, were increased by construction of a covalently linked divalent single-chain fragment variable (sc(Fv)₂). To improve the yield of the antibody fragments, the expression host was changed from E. coli to P. pastoris and culture conditions were optimized using Design of Experiments (DoE). Furthermore, a HeLa HEp-2 multicellular tumor spheroid (MCTS) in vitro model was established. The functionality of the radiolabeled TS1-218 alone and in immunocomplex with its anti-idiotype scFv, αTS1 scFv, were evaluated in the MCTS model and compared to a tumor xenograft nude mouse model. The targeting efficiency of the scFv and sc(Fv)₂s were also investigated using MCTS.

TS1-218 in immunocomplex with αTS1 scFv displayed a significantly higher uptake than the TS1-218 alone in both MCTS and tumor xenografts. The sc(Fv)₂s, DiTS1-218 and DiHE1-Q demonstrated a higher functional affinity to K 8 in ELISA and MCTS and were retained to a larger extent in the MCTS than their scFv counterparts, with a 3.9 and 9.4-fold longer half-life, respectively. Furthermore, the yield of the antibody fragments were improved after expression in P. pastoris with an 86-fold improvement for the TS1-218 following optimization using DoE.