Biology drives the discovery of bispecific antibodies as innovative therapeutics

Abstract

A bispecific antibody (bsAb) is able to bind two different targets or two distinct epitopes on the same target. Broadly speaking, bsAbs can include any single molecule entity containing dual specificities with at least one being antigen-binding antibody domain. Besides additive effect or synergistic effect, the most fascinating applications of bsAbs are to enable novel and often therapeutically important concepts otherwise impossible by using monoclonal antibodies alone or their combination. This so-called obligate bsAbs could open up completely new avenue for developing novel therapeutics. With evolving understanding of structural architecture of various natural or engineered antigen-binding immunoglobulin domains and the connection of different domains of an immunoglobulin molecule, and with greatly improved understanding of molecular mechanisms of many biological processes, the landscape of therapeutic bsAbs has significantly changed in recent years. As of September 2019, over 110 bsAbs are under active clinical development, and near 180 in preclinical development. In this review article, we introduce a system that classifies bsAb formats into 30 categories based on their antigen-binding domains and the presence or absence of Fc domain. We further review the biology applications of approximately 290 bsAbs currently in preclinical and clinical development, with the attempt to illustrate the principle of selecting a bispecific format to meet biology needs and selecting a bispecific molecule as a clinical development candidate by 6 critical criteria. Given the novel mechanisms of many bsAbs, the potential unknown safety risk and risk/benefit should be evaluated carefully during preclinical and clinical development stages. Nevertheless we are optimistic that next decade will witness clinical success of bsAbs or multispecific antibodies employing some novel mechanisms of action and deliver the promise as next wave of antibody-based therapeutics.

Keywords: bispecific antibody; bsAb; msAb; multispecific antibody.

Figure 1

The principles, criteria and screening funnel in discovering a good therapeutic bsAb. (A) Three principles of governing the discovery of a good bsAb, (B) Six criteria of defining a bsAb as a clinical development candidate. (C) Detailed function and developability screenings to identify a good therapeutic bsAb molecule.

Figure 2

The classification of bsAb formats based on assembly of antibody fragments as building blocks. The first row and column list the five basic building blocks (SDA, Fv, scFv, Fab, scFab). The different color and shape of VH and VL represent their origins from different parental antibodies. The assembly of different building blocks creates various bsAb formats classified into 30 groups. An exemplary format and its molecular weight of each group are listed. The diagonal line divides the formats into bispecific formats without Fc (top right with number 1-15) and bispecific formats with Fc (bottom left with number 16-30): 1, tandemly linked SDAs; 2, a SDA tandemly linked on the VH of a Fv; 3, a SDA tandemly linked on the VH of a scFv; 4, two SDAs are separately linked on the carboxyl-terminus of constant domain of a Fab; 5, a SDA tandemly linked on the VL of a scFab; 6, the VHs and VLs of two Fvs cross pair to each other to form diabody; 7, a scFv tandemly linked on the VH of a Fv; 8, the VH and VL of a Fv each linked on the carboxyl-terminus of CH1 and CL of a Fab; 9, the VH of a Fv linked to the CH1 of a scFab; 10, two tandemly linked scFvs; 11, a scFv linked on the VH of a Fab; 12, a scFab tandemly linked with a scFv; 13, two tandemly linked Fabs: the light chain of one Fab linked with the heavy chain of another Fab and vice versa; 14, a scFab linked on the VH of a Fab; 15, tandemly linked two scFab; 16, two tandemly linked SDA on Fc to form homodimer; 17, a SDA and the VH of a Fv linked to the FcA to pair with the VL of a Fv linked to another FcB to form heterodimer; 18, a diabody on FcA to pair with FcB to form heterodimer; 19, a SDA on FcA to pair with a scFv on FcB to form heterodimer; 20, a scFv and the VH of a Fv linked to FcA to pair with the VL of the Fv linked with FcB to form heterodimer; 21, two scFv tandemly linked to the amino- and carboxyl-terminus of a Fc to form homodimer; 22, a SDA tandemly link to the light chain of a IgG to form homodimer; 23, a TCR constant domain anchored Fv linked to FcA to pair with a half IgG with FcB to form heterodimer (WuXiBody^TM); 24, a scFv linked FcA to pair with a half IgG with FcB to form heterodimer; 25, two tandemly linked Fabs (the light chain of a Fab linked on the heavy chain of another Fab) on Fc to form homodimer (FIT-Ig); 26, a SDA on FcA to pair with a scFab on FcB to form heterodimer; 27, a scFab and the VH of a Fv linked on FcA to pair with the VL linked on FcB to form heterodimer; 28, a scFv linked on FcA to pair with a scFab linked on FcB to form heterodimer; 29, a half IgG with FcA paired with scFv linked on FcB to form heterodimer; 30, two scFab each linked on FcA and FcB to form heterodimer. Above mentioned FcA and FcB are engineered Fc pair to facilitate Fc heterodimerization.

Figure 3

Fusion sites for antigen-binding building blocks. A) A heterodimeric Fc fragment has at least six fusion sites: amino-terminus (1 and 4), carboxyl-terminus (3 and 6) of Fc and between CH2 and CH3 (2 and 5). B) The fragment made of heterodimeric Fc and two differently heterodimerized CL-CH1 domains provides at least twelve fusion sites: amino-terminus of CL (1 and 7) and CH1 (3 and 9), carboxyl-terminus of CL (2 and 8) and CH3 (6 and 12), hinge region (4 and 10), between CH2 and CH3 (5 and 11).

Figure 4

Statistics showing the booming of bsAb programs. A). The number of clinical studies associated with bsAb in the past fourteen years (up to September 2019). The bsAb programs classified based on B) different clinical stages and C) different disease areas. Data source: Cortellis^TM Competitive Intelligence (CCI) and CortellisTM Drug Discovery Intelligence (CDDI, formerly Integrity) as of Sept 23, 2019.

Figure 5

Licenses and market analysis for bsAb programs. A). Licenses for clinical stage bsAbs. Line represents number of license signed each year. Blue and yellow bars represent the largest deal and total deal values for each year, respectively. B). The largest deals signed from 2014 to 2018. C). Deal values in disease area. D). BsAb market size in 2018 and forecast in 2024. Data source: Cortellis^TM CCI as of Sept 23, 2019.

Figure 6

A network graph characterizing the target pairs of most bispecific programs in both preclinical and clinical investigations. Each node in the network is one target, and each edge connecting two nodes represents one bispecific program. The circular edges are biparatopic programs. The node size shows the degree of a particular target being paired with other different targets. The colors of the edges are marked in black if only one program is available for that particular pair, otherwise in red if more than one are being explored. The popularity of that bispecific program is reflected from the thickness of the red edges. Source data are from Cortellis^TM (Table 1-4). Tri-specific and albumin-relevant bispecific programs are not included. The albumin-relevant tri-specific are analyzed as bispecific projects.