After elimination of weak inhibitors, other undesirable chemical features, as well as compounds for which there were no convenient commercial sources, further validation of the remaining three compounds, candesartan cilexetil, manoalide, and MK-886 were conducted by radioactive gel-based primer extension assays. The results revealed that these compounds were capable of inhibiting the ability of pol k to catalyze synthesis on either a control nondamaged DNA template or a template adducted with the acroleinderived ring-opened reduced form of c-HOPdG in a dosedependent manner with similar potency. Since the predominant role of pol k is in DNA lesion bypass, these results demonstrated that the primer extension assays using damage-containing DNAs can effectively measure the ability of the compounds to inhibit a biologically relevant activity of pol k. In order to assess the ability of these compounds to target intracellular pol k, cell survival assays were carried out by exposing cells to the combination of pol k Tofacitinib inhibitors and UV. The results showed that candesartan cilexetil could potentiate cellular toxicity induced by UV in XP-V cells. It cannot be ruled out that the cellular effect of candesartan cilexetil may be partly due to its effect on other proteins in addition to pol k, including pol g and pol i, since the compound also inhibited the activities of these polymerases in vitro; however, our in vitro results clearly show that pol k is inhibited by this compound. Additionally, it has been shown that the depletion of either pol g or pol i in XP-V cells did not enhance UV cytotoxicity. Collectively, these observations suggest that pol k is inhibited by this compound in the cells, and thus validate the usefulness of this cell-based assay in identifying compounds with potential to inhibit intracellular pol k. Although manoalide and MK-886 could inhibit pol k activity in vitro, these compounds were unable to enhance UV-induced toxicity in XP-V cells under the conditions tested. Both manoalide and MK-886 have anti-inflammatory activity; manoalide is wellknown as a non-specific phospholipase A2 antagonist, and MK-886 inhibits leukotriene synthesis by blocking 5-lipoxygenaseactivating protein. The reason for the inability of these compounds to potentiate UV cytotoxicity could be due to their significantly lower binding affinity to intracellular pol k relative to other cellular targets. Alternatively, these compounds may take a long time to enter the cells and bind to pol k. Moreover, it is possible that only a small fraction of intracellular pol k is inhibited by these compounds and the remaining pol k may be sufficient to process UV-induced DNA lesions, resulting in unaltered cellular sensitivity to UV. Given the presence of multiple back-up TLS polymerases, nearly-complete inhibition of the activity of all intracellular pol k may be essential for cells to present an apparent phenotype. Further understanding of the inability of these compounds to target intracellular pol k could involve structureactivity relationship analyses. In fact, several structural analogues of these compounds exist such as secomanoalide and luffariellolide for manoalide and L538,916 for MK-886, thus enabling the initiation of such studies. In summary, we presented herein the development of new strategies for the discovery of small molecules that could inhibit pol k activity both in vitro and in vivo. The identification of chemotypes with established drug properties targeting pol k validates this qHTS platform, as well as the secondary assays and sets the stage for exploration of significantly Niraparib larger diverse collections to discover compounds with high potency and specificity towards pol k and thus could potentially be used as pharmaceuticals.