The Evolving Paradigm of Antibody-Drug Conjugates Targeting the ErbB/HER Family of Receptor Tyrosine Kinases

Abstract

Current therapies targeting the human epidermal growth factor receptor (HER) family, including monoclonal antibodies (mAbs) and tyrosine kinase inhibitors (TKIs), are limited by drug resistance and systemic toxicities. Antibody-drug conjugates (ADCs) are one of the most rapidly expanding classes of anti-cancer therapeutics with 13 presently approved by the FDA. Importantly, ADCs represent a promising therapeutic option with the potential to overcome traditional HER-targeted therapy resistance by delivering highly potent cytotoxins specifically to HER-overexpressing cancer cells and exerting both mAb- and payload-mediated antitumor efficacy. The clinical utility of HER-targeted ADCs is exemplified by the immense success of HER2-targeted ADCs including trastuzumab emtansine and trastuzumab deruxtecan. Still, strategies to improve upon existing HER2-targeted ADCs as well as the development of ADCs against other HER family members, particularly EGFR and HER3, are of great interest. To date, no HER4-targeting ADCs have been reported. In this review, we extensively detail clinical-stage EGFR-, HER2-, and HER3-targeting monospecific ADCs as well as novel clinical and pre-clinical bispecific ADCs (bsADCs) directed against this receptor family. We close by discussing nascent trends in the development of HER-targeting ADCs, including novel ADC payloads and HER ligand-targeted ADCs.

Keywords: EGFR; HER2; HER3; antibody–drug conjugates; bispecific antibodies; combination therapy; drug resistance; receptor tyrosine kinase; therapeutics.

Figure 1

HER-targeted therapy resistance mechanisms. HER-targeted mAbs and TKIs are subject to a wide variety of resistance mechanisms. These may include (A) altered trafficking of the mAb–receptor complex away from degradation and toward recycling; (B) impaired ADCC/ADCP-mediated killing; (C) resistance mutations in downstream signaling mediators; (D) mutations in the receptor itself; and (E) activation of alternative receptors, such as c-MET. Asterisks indicate commonly mutated signaling mediators.

Figure 2

ADC mechanisms of cytotoxicity. ADCs exert both mAb- and payload-mediated effects. Therefore, the mechanisms of ADC cytotoxicity may include (A) neutralization of downstream signaling pathways; (B) ADCC/ADCP; (C) degradation of the target receptor; and (D) payload-mediated cytotoxicity, which may be mediated through anti-microtubule, topoisomerase-inhibiting, and DNA-damaging mechanisms, among others.

Figure 3

Mechanisms of ADC resistance. While ADCs circumvent some of the resistance mechanisms encountered by traditional HER-targeting mAbs and TKIs, alternative resistance mechanisms emerge. Among these are (A) increased ADC–receptor recycling/routing away from degradation pathways; (B) upregulation of drug efflux transporters (e.g., ABCB1 and P-gp); (C) mutations in the machinery involved in payload-mediated cytotoxicity, such as TOP1; and (D) target antigen downregulation following ADC treatment.

Figure 4

BsAb generation strategies. (A) Fab-Fc-scFv involves the fusion of Fab and scFv antibody fragments targeting separate antigens. Notably, the scFv region may be attached to regions other than those depicted above, such as the Fab N-terminus. (B) Knobs-into-holes require the incorporation of complementary “knob” (T366W) and “hole” (T366S; L368A; Y407V) mutations into the antibody CH3 region to facilitate heterodimerization. BsAbs may have common or different light chains. (C) Strand-exchange engineered domain (SEED) relies on the generation of CH3 domains with alternating IgG/IGA domains (“GA”/“AG”) that facilitate heterodimerization. SEED technology is also amenable to the incorporation of alternative antibody formats, such as scFv, as depicted above. (D) Bispecific Antibody by Protein Trans-Splicing (BAPTS) generates two antibody fragments fused to the split intein Npu DnaE. Following protein-splicing, the full-length intein is removed and the full-length bsAb is generated. (E) Common strategies to enhance bsAb efficacy, selectivity, and production include affinity maturation to alter antibody affinity for a target antigen and the incorporation of a common light chain or engineered cysteine/disulfide bond to eliminate heavy–light chain mispairing. Asterisks depict amino acid mutations in the complementarity-determining region.

Figure 5

Emergent HER-targeting antibody-conjugate modalities. (A) BsADCs engage two tumor-associated antigens, such as two RTKs. The development of bsADCs that engage a rapidly internalizing receptor is another common motif for bsADCs. (B) ISACs, loaded with immune agonists, such as TLR7/8 agonists, engage the FcγR on antigen-presenting cells (APCs), mediating internalization, immune activation, and cytokine release, resulting in target cell death. (C) DACs are loaded with protein degraders such as PROTACs or molecular glues that, upon internalization, mediate ubiquitination and subsequent proteasomal degradation of the degrader-targeted protein (i.e., BRD4, GSPT1). (D) ADCs with novel inhibitors, such as BCL-XL and CDK inhibitors function by specifically targeting these cytotoxins to target-expressing cells. (E) Ligand-targeted ADCs may recognize full-length surface-bound ligands, solubilized ligands, or novel ligand epitopes remaining following ligand proteolytic cleavage. Furthermore, ligand-targeted ADCs may have the added benefit of signaling neutralization through the associated ligand’s receptor.