Bystander effect in antibody-drug conjugates: Navigating the fine line in tumor heterogeneity

Antibody-drug conjugates (ADCs) represent a transformative advancement in targeted cancer therapy by combining monoclonal antibodies with cytotoxic payloads. A critical yet underexplored feature of ADCs is the bystander effect, wherein released payloads diffuse into neighboring cells regardless of target antigen expression. This review synthesizes current understanding of the mechanisms, clinical implications, and optimization strategies related to this phenomenon. Mechanistically, cleavable linkers, hydrophobic payloads, and internalization are critical for bystander activity. However, the characteristics of the tumor microenvironment—elevated interstitial fluid pressure, binding site barrier (BSB), and hypoxia—restrict ADC penetration. Clinically, ADCs with bystander effects (e.g., trastuzumab deruxtecan), demonstrate superior efficacy compared to non-bystander ADCs (e.g., trastuzumab emtansine). Despite these advantages, bystander effect raises concerns regarding off-target toxicity and variable efficacy depending on antigen expression. For instance, while the bystander effect allows payloads to penetrate BSB and increase the killing range, non-bystander ADCs like ARX788 may offer comparable efficacy with reduced toxicity in homogeneous settings. Current insights highlight the need to balance bystander potency with target specificity, particularly in tumors with low antigen density or heterogeneous spatial distribution. Future research should focus on three key areas: (1) quantifying bystander contributions in vivo; (2) clarifying spatiotemporal regulation of payload diffusion by TME factors such as hypoxia and binding-site barriers; and (3) validating combinatorial strategies, including Fc engineering, internalization induction, and TME remodeling, to maximize therapeutic indices. Bridging these gaps will refine ADC design paradigms, aligning with precision oncology's goal of optimizing efficacy while minimizing systemic toxicity.