Imaging DNA damage response by γH2AX in vivo predicts treatment response to Lutetium-177 radioligand therapy and suggests senescence as a therapeutically desirable outcome

Rationale: An effective absorbed dose response relationship is yet to be established for Lutetium-177 based radionuclide therapies such as (177)Lu-DOTATATE and (177)Lu-PSMA. The inherent biological heterogeneity of neuroendocrine and prostate cancers may make the prospect of establishing cohort-based dose-response relationships unobtainable. Instead, an individual-based approach, monitoring the dose-response within each tumor could provide the necessary metric to monitor treatment efficacy. Methods: We developed a dual isotope SPECT imaging strategy to monitor the change over time in the relationship between (177)Lu-DOTATATE and (111)In-anti-γH2AX-TAT, a modified radiolabelled antibody that allows imaging of DNA double strand breaks, in mice bearing rat pancreatic cancer xenografts. The dynamics of γH2AX foci, apoptosis and senescence following exposure to (177)Lu-DOTATATE was further investigated in vitro and in ex vivo tumor sections. Results: The change in slope of the (111)In-anti-γH2AX-TAT to (177)Lu signal between days 5 and 7 was found to be highly predictive of survival (r = 0.955, P < 0.0001). This pivotal timeframe was investigated further in vitro: clonogenic survival correlated with the number of γH2AX foci at day 6 (r = -0.995, P < 0.0005). While there was evidence of continuously low levels of apoptosis, delayed induction of senescence in vitro appeared to better account for the γH2AX response to (177)Lu. The induction of senescence was further investigated by ex vivo analysis and corresponded with sustained retention of (177)Lu within tumor regions. Conclusions: Dual isotope SPECT imaging can provide individualized tumor dose-responses that can be used to predict lutetium-177 treatment efficacy. This bio-dosimeter metric appears to be dependent upon the extent of senescence induction and suggests an integral role that senescence plays in lutetium-177 treatment efficacy.