Alpha particles combined with ATR inhibition to activate the immune system: mechanisms and pre-clinical translation

增强肿瘤免疫反应的一种策略是放疗（RT）。最近的数据支持RT-

One strategy to enhance the immune response to tumors is radiotherapy (RT). Recent data support that RT- induced micronuclei (MN) are intrinsically immunostimulatory, as ruptured MN releases double stranded DNA (dsDNA) eliciting the cycling GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING) pathway. Although progress has been made in combining immune checkpoint blockade (ICB) with RT, relatively little is known about the physical mechanisms of RT that elicit immunostimulatory signals and how they can be har- nessed clinically in the context of DNA damage and DNA repair inhibition. Radiation with high ionization density (or linear energy transfer, LET) induces more clustered DNA lesions, more MN and higher cell kill compared to low-LET radiation. α-particles are characterized by their high-LET in contrast to photons and protons and may be ideal for creating high levels of MN and downstream enhanced activation of immunostimulatory signals through the cGAS-STING pathway. A novel modality to deliver α-particles has recently been successfully demon- strated in a phase I clinical trial using a method called diffusing alpha-emitters radiation therapy (DaRT). DaRT consists of interstitial radioactive seeds coated with radium-224, an α-particle emitter. The radium-224 decay chain is unique in that the decay products also emit α-particles and diffuse, allowing the α-particles’ dose to be deposited over 2-3 mm from the seed. Thus, multiple seeds implanted into a tumor allow the high-LET α-particle dose to be deposited within the entire tumor volume. In addition to radiation, pharmacologic inhibition of DNA repair affects the presence of MN. This inhibition of DNA repair can be created with drugs such as Ataxia telan- giectasia and Rad3 related (ATR) inhibitors (ATRi). The combination of an ATRi with α-particle-induced clustered DSB lesions may synergistically enhance the accumulation of MN, ultimately enhancing immunostimulatory sig- nals. These will be investigated with ICB to determine how to synergistically augment RT-induced antitumor immunity. We hypothesize that α-particles combined with an ATRi produces more MN, results in more cGAS binding to dsDNA and consequently potentiate robust antitumor immunity. We propose to: 1) Elucidate the mech- anisms by which cGAS binds to dsDNA in -particles+ATRi treated cells; 2) Elucidate the mechanisms by which -particles+ATRi induces immune signaling; and 3) Evaluate antitumor immunity from -particles+ATRi in vivo. Our research has the potential to define -particles as a tool to augment antitumor immune response, especially for tumors that are known to be unresponsive to ICB. Our proposed research is of critical relevance to address the poor prognosis associated with multiple advanced solid cancers that are immunologically cold. Our proposed work is innovative, in that it aims to define the effects of -particle-induced clustered DNA damage on tumors in the context of antitumor immunity. Our findings will elucidate the mechanisms behind immune modulation by high-LET radiation, which may ultimately guide the combined rational use of modalities that use high-LET radi- ation (including α-emitting radiopharmaceuticals), DNA repair inhibitors and ICB for aggressive cancers.