Bystander Effect

Bystander Effect designates the phenomenon where cells not directly targeted by a therapeutic agent are affected through mechanisms including release of toxic metabolites, immune activation, or gap junction communication from treated cells, with implications for both therapeutic efficacy where bystander killing of antigen-negative tumour cells improves coverage, and safety where healthy tissue damage may extend beyond intended targets.

The biopharmaceutical industry considers bystander effects in developing antibody-drug conjugates, gene therapies, and radioimmunotherapy. ADC bystander killing occurs when membrane-permeable payloads released from antigen-positive cells diffuse into adjacent antigen-negative cells, potentially improving efficacy against antigen-heterogeneous tumours. Payload selection influences bystander effect magnitude, with membrane-permeable payloads producing stronger bystander killing than membrane-impermeable alternatives. Oncolytic viruses exploit bystander immune activation as an important mechanism extending anti-tumour activity beyond directly infected cells. Safety assessment must characterise bystander effects in normal tissues. Mechanistic understanding of bystander dynamics continues informing therapeutic design optimising desired tumour bystander killing while limiting normal tissue effects.