Assessing the radiation-induced bystander effect in radionuclide therapy using targeted radiopharmaceuticals

Radiation-induced biologic bystander effects (RIBBEs) where non-irradiated cells display radiation like responses are well established in the context of external beam radiotherapy. However, their presence and characteristics in radionuclide therapy have been explored in only a limited number of studies. With the growing application of radionuclide therapy, this study was designed to investigate RIBBEs using a targeting agent labeled with three different types of emitters, aiming to quantify the varying biological effects attributable to each radionuclide and to devise strategies for achieving maximum therapeutic effect.Trastuzumab antibody targeting human epidermal growth factor receptor 2 (HER2) was labeled with radionuclides of varying energy profile; Y-90 (β^-emitter), Lu-177 (β^-/γ-emitter), and I-125 (Auger electron emitter). The radiolabeled conjugates were characterized, and their specificity was evaluated. Studies to evaluate both direct and bystander effects in (HER2) receptor overexpressing cell lines were performed via a media transfer protocol.

The study highlights distinct cellular responses depending on the radionuclide employed, with significant bystander induced reductions in clonogenic survival observed for all radiolabeled formulations. Notably, HER2-overexpressing SK-OV-3 and SK-BR-3 cells displayed dose-dependent variations in survival, with ⁹⁰Y and ¹⁷⁷Lu demonstrating greater bystander toxicity than ¹²⁵I. The survival fraction of recipient cells, which were exposed solely to media containing bystander factors, decreased significantly, indicating the potency of bystander factors in influencing therapeutic outcomes. Additionally, cytotoxicity assays revealed that higher doses of ⁹⁰Y and ¹⁷⁷Lu induced substantial apoptosis and necrosis, whereas ¹²⁵I exhibited lower cytotoxicity, consistent with its lower energy emission profile. Reactive oxygen species (ROS) generation assays corroborated these findings, showing elevated ROS levels in direct and donor cell groups treated with ⁹⁰Y and ¹⁷⁷Lu, while recipient cells exhibited relatively lower ROS induction.

This study elucidates the complex interplay of direct and bystander effects in targeted radionuclide therapy, demonstrating that the therapeutic efficacy and cellular response depend on the specific radionuclide and its energy profile. The findings emphasize the need for further exploration of RIBBEs to optimize radionuclide-based cancer therapies to enhance their clinical efficacy.