Cytotoxic enhancement of a bispecific diabody by format conversion to tandem single-chain variable fragment (taFv): the case of the hEx3 diabody.

Diabodies (Dbs) and tandem single-chain variable fragments (taFv) are the most widely used recombinant formats for constructing small bispecific antibodies. However, only a few studies have compared these formats, and none have discussed their binding kinetics and cross-linking ability. We previously reported the usefulness for cancer immunotherapy of a humanized bispecific Db (hEx3-Db) and its single-chain format (hEx3-scDb) that target epidermal growth factor receptor and CD3. Here, we converted hEx3-Db into a taFv format to investigate how format affects the function of a small bispecific antibody; our investigation included a cytotoxicity assay, surface plasmon resonance spectroscopy, thermodynamic analysis, and flow cytometry. The prepared taFv (hEx3-taFv) showed an enhanced cytotoxicity, which may be attributable to a structural superiority to the diabody format in cross-linking target cells but not to differences in the binding affinities of the formats. Comparable cross-linking ability for soluble antigens was observed among hEx3-Db, hEx3-scDb, and hEx3-taFv with surface plasmon resonance spectroscopy. Furthermore, drastic increases in cytotoxicity were found in the dimeric form of hEx3-taFv, especially when the two hEx3-taFv were covalently linked. Our results show that converting the format of small bispecific antibodies can improve their function. In particular, for small bispecific antibodies that target tumor and immune cells, a functional orientation that avoids steric hindrance in cross-linking two target cells may be important in enhancing the growth inhibition effect.

Build-up functionalization of anti-EGFR × anti-CD3 bispecific diabodies by integrating high-affinity mutants and functional molecular formats.

Designing non-natural antibody formats is a practical method for developing highly functional next-generation antibody drugs, particularly for improving the therapeutic efficacy of cancer treatments. One approach is constructing bispecific antibodies (bsAbs). We previously reported a functional humanized bispecific diabody (bsDb) that targeted epidermal growth factor receptor and CD3 (hEx3-Db). We enhanced its cytotoxicity by constructing an Fc fusion protein and rearranging order of the V domain. In this study, we created an additional functional bsAb, by integrating the molecular formats of bsAb and high-affinity mutants previously isolated by phage display in the form of Fv. Introducing the high-affinity mutations into bsDbs successfully increased their affinities and enhanced their cytotoxicity in vitro and in vivo. However, there were some limitations to affinity maturation of bsDb by integrating high-affinity Fv mutants, particularly in Fc-fused bsDb with intrinsic high affinity, because of their bivalency. The tetramers fractionated from the bsDb mutant exhibited the highest in vitro growth inhibition among the small bsAbs and was comparable to the in vivo anti-tumor effects of Fc-fused bsDbs. This molecule shows cost-efficient bacterial production and high therapeutic potential.

Rearranging the domain order of a diabody-based IgG-like bispecific antibody enhances its antitumor activity and improves its degradation resistance and pharmacokinetics.

One approach to creating more beneficial therapeutic antibodies is to develop bispecific antibodies (bsAbs), particularly IgG-like formats with tetravalency, which may provide several advantages such as multivalent binding to each target antigen. Although the effects of configuration and antibody-fragment type on the function of IgG-like bsAbs have been studied, there have been only a few detailed studies of the influence of the variable fragment domain order. Here, we prepared four types of hEx3-scDb-Fc, IgG-like bsAbs, built from a single-chain hEx3-Db (humanized bispecific diabody [bsDb] that targets epidermal growth factor receptor and CD3), to investigate the influence of domain order and fusion manner on the function of a bsDb with an Fc fusion format. Higher cytotoxicities were observed with hEx3-scDb-Fcs with a variable light domain (VL)-variable heavy domain (VH) order (hEx3-scDb-Fc-LHs) compared with a VH-VL order, indicating that differences in the Fc fusion manner do not affect bsDb activity. In addition, flow cytometry suggested that the higher cytotoxicities of hEx3-scDb-Fc-LH may be attributable to structural superiority in cross-linking. Interestingly, enhanced degradation resistance and prolonged in vivo half-life were also observed with hEx3-scDb-Fc-LH. hEx3-scDb-Fc-LH and its IgG2 variant exhibited intense in vivo antitumor effects, suggesting that Fc-mediated effector functions are dispensable for effective anti-tumor activities, which may cause fewer side effects. Our results show that merely rearranging the domain order of IgG-like bsAbs can enhance not only their antitumor activity, but also their degradation resistance and in vivo half-life, and that hEx3-scDb-Fc-LHs are potent candidates for next-generation therapeutic antibodies.

Domain order of a bispecific diabody dramatically enhances its antitumor activity beyond structural format conversion: the case of the hEx3 diabody.

The domains of bispecific diabodies (BsDbs) can be ordered in four different ways; however, the influence of domain order on the cytotoxicity of BsDbs that retarget immune cells against tumor cells had not been addressed. We previously reported the marked antitumor effects of a humanized BsDb that targets epidermal growth factor receptor and CD3 (hEx3-Db). Here, we rearranged the domains of hEx3-Db to examine the influence of domain order on the function of BsDbs. We successfully prepared homogenous dimers of hEx3-Db in all four domain configurations. Interestingly, all three rearranged hEx3s inhibited cancer growth more effectively than did the original hEx3-Db, in which both components were in variable heavy domain (VH)-variable light domain (VL) order (redesignated as hEx3-HL), and the highest effects were observed with hEx3-LH (hEx3-Db with both components in VL-VH order). In addition, hEx3-LH had comparable in vitro growth inhibitory effects to those of the tandem single-chain variable fragment (scFv) format of hEx3-Db (hEx3-tandem scFv (taFv)), which we previously showed to have greater cytotoxicity than does hEx3-HL. Flow cytometry suggested that the enhanced cytotoxicity of hEx3-LH is attributable to structural superiority for cross-linking, similar to that of hEx3-taFv. Furthermore, hEx3-LH inhibited cancer growth in mice more effectively than did hEx3-taFv; this difference may be due to differences in antibody stability. Our results show that merely rearranging the domain order of BsDbs can enhance their effects beyond those with structural format conversion.

In vitro and in vivo antitumor effects of recombinant bispecific antibodies based on humanized anti-EGFR antibody.

We performed in vitro and in vivo experiments of the anti-epidermal growth factor receptor (EGFR) x anti-CD3 bispecific diabody (hEx3-Db) with the IgG-like bispecific antibodies (BsAbs) (hEx3-scFv-Fc and hEx3-scDb-Fc) and the anti-EGFR therapeutic antibody cetuximab to assess the effect of BsAbs on cancer growth inhibition. In vitro, efficacy of the BsAbs and cetuximab were compared by growth inhibition assays of human cell lines of bile duct (TFK-1, HuCC-T1, OCUCh-LM1), epidermoid (A431), gastric (Kato-III), colon (DLD-1, SW480), and breast (SK-BR-3, MCF-7) cancer. In vivo, in three mouse models, we evaluated the anti-tumor activity of hEx3-Db and cetuximab, assessed the effect of hEx3-Db alone, and compared the antitumor activity of hEx3-Db with the IgG-like BsAbs. In vitro, hEx3-scFv-Fc showed nearly 100% killing activity for all cell lines. Both in vitro and in vivo, hEx3-Db needed CD3-positive phenotypes to induce a growth inhibitory effect. In contrast, IgG-like BsAbs showed monotherapeutic effects in vivo by inducing antibody-dependent cellular cytotoxicity (ADCC) similar to cetuximab. However, enhancement was not observed when lymphokine-activated killer cells with the T-cell phenotype were co-injected. Results suggest that IgG-like BsAbs could not efficiently direct T lymphocytes toward tumor cells to induce ADCC due to steric hindrance on binding to CD3- and Fc-receptor-positive phenotypes. Although hEx3-scFv-Fc showed high cytotoxicity in vitro, its high molecular weight limits its usefulness. With an in vivo effect comparable to hEx3-scFv-Fc and its realistic molecular weight, hEx3-scDb-Fc shows promise as a novel recombinant therapeutic antibody and may be modified to enhance its potency by prevention of steric hindrance.