A Novel Small Molecule Inhibitor of Human DNA Polymerase Eta Modulates the Efficacy of Cisplatin in Cancer Cells

Chemotherapeutic strategies involving platinum‐based drugs rely on DNA damage to limit tumor growth. Activation of DNA damage response pathways can modulate patient response to these types of therapies. For example, translesion DNA synthesis (TLS) by human DNA polymerase (pol) eta (η) can render platinum‐based chemotherapy ineffective through the direct bypass of DNA damage. Therefore, targeting hpol η with small‐molecule inhibitors is a promising strategy for combating chemoresistance in certain types of cancer, such as ovarian cancer, that rely on hpol η to survive genotoxic chemotherapy. The main objective of our study was to identify small molecule inhibitors of hpol η that modulate the efficacy of genotoxic anti‐cancer drugs in an effort to improve patient outcomes. Towards that end, the catalytic activity of hpol η was measured in the presence of eighty‐five novel indole thio‐barbituric acid (ITBA) and indole barbituric acid (IBA) derivatives. Several compounds that inhibit hpol η activity with low micromolar IC 50 s were identified from this initial screen. One of these compounds, PNR‐7‐02, an ITBA derivative with both N‐ napthoyl moiety and 5‐chloro substituent on the indole ring inhibited hpol η activity with an IC 50 value of 8 μM. PNR‐7‐02 also inhibited hRev1 and hpol lambda (λ) with low micromolar IC 50 , but inhibition by PNR‐7‐02 was approximately ten‐fold more potent against hpol η than B‐family pols. Michaelis‐Menten kinetic analysis revealed a partial competitive mechanism of inhibition of hpol η activity. Chemical labeling and molecular docking results further supported the kinetic mechanism of inhibition. A target‐dependent effect on cell viability was observed when comparing hpol η‐proficient and hpol η‐deficient HAP‐1 cells co‐treated with cisplatin and PNR‐7‐02. The calculated combination index values indicated a strong synergy between cisplatin and PNR‐7‐02 in hpol η‐proficient cells. This synergy was not evident in the hpol η knockout cells, supporting the idea that PNR‐7‐02 modulates cisplatin toxicity in a hpol η‐dependent manner. Results from cell sorting experiments were consistent with synergy observed between cisplatin and PNR‐7‐02. In summary, our lab has performed the first quantitative structure activity relationship for TLS pols to identify potent inhibitor of hpol η that sensitizes cancer cells to cisplatin treatment. Support or Funding Information This work was supported by NIH grants GM 084460 and CA 183895 (to R.L.E.).