Autocyclized and oxidized forms of SCR 7 induce cancer cell death by inhibiting nonhomologous DNA end joining in a Ligase IV dependent manner

NonhomologousDNAend joining (NHEJ) is the majorDNAdouble‐strand break (DSB) repair pathway in mammals. Previously, we have described a small molecule inhibitor,SCR7, which can inhibitNHEJin a LigaseIV‐dependent manner. Administration ofSCR7 within the cells resulted in the accumulation ofDNAbreaks, cell death, and inhibition of tumor growth in mice. In the present study, we report that parentalSCR7, which is unstable, can be autocyclized into a stable form. Both parentalSCR7 and cyclizedSCR7 possess the same molecular weight (334.09) and molecular formula (C18H14N4OS), whereas its oxidized form,SCR7‐pyrazine, possesses a different molecular formula (C18H12N4OS), molecular weight (332.07), and structure. While cyclized form ofSCR7 showed robust inhibition ofNHEJin vitro, both forms exhibited efficient cytotoxicity. Cyclized and oxidized forms ofSCR7 inhibitedDNAend joining catalyzed by LigaseIV, whereas their impact was minimal on LigaseIII, Ligase I, and T4DNALigase‐mediated joining. Importantly, both forms inhibited V(D)J recombination, although the effect was more pronounced forSCR7‐cyclized. Both forms blockedNHEJin a LigaseIV‐dependent manner leading to the accumulation ofDSBs within the cells. Although cytotoxicity due toSCR7‐cyclized was LigaseIVspecific, the pyrazine form exhibited nonspecific cytotoxicity at higher concentrations in LigaseIV‐null cells. Finally, we demonstrate that both forms can potentiate the effect of radiation. Thus, we report that cyclized and oxidized forms ofSCR7 can inhibitNHEJin a LigaseIV‐dependent manner, althoughSCR7‐pyrazine is less specific to LigaseIVinside the cell.