Combining radiotherapy (RT) with DNA damage response inhibitors may lead to increased tumor cell death through radiosensitization. DNA-dependent protein kinase (DNA-PK) plays an important role in DNA double-strand break repair via the nonhomologous end joining (NHEJ) pathway. We hypothesized that in addition to a radiosensitizing effect from the combination of RT with AZD7648, a potent and specific inhibitor of DNA-PK, combination therapy may also lead to modulation of an anticancer immune response.

AZD7648 and RT efficacy, as monotherapy and in combination, was investigated in fully immunocompetent mice in MC38, CT26, and B16-F10 models. Immunologic consequences were analyzed by gene expression and flow-cytometric analysis.

AZD7648, when delivered in combination with RT, induced complete tumor regressions in a significant proportion of mice. The antitumor efficacy was dependent on the presence of CD8⁺ T cells but independent of NK cells. Analysis of the tumor microenvironment revealed a reduction in T-cell PD-1 expression, increased NK-cell granzyme B expression, and elevated type I IFN signaling in mice treated with the combination when compared with RT treatment alone. Blocking of the type I IFN receptor in vivo also demonstrated a critical role for type I IFN in tumor growth control following combined therapy. Finally, this combination was able to generate tumor antigen-specific immunologic memory capable of suppressing tumor growth following rechallenge.

Blocking the NHEJ DNA repair pathway with AZD7648 in combination with RT leads to durable immune-mediated tumor control.