Target validation and structure–activity analysis of a series of novel PCNA inhibitors

Proliferating cell nuclear antigen ( PCNA ) plays an essential role in DNA replication and repair. Tumor cells express high levels of PCNA , identifying it as a potentially ideal target for cancer therapy. Previously, we identified nine compounds termed PCNA inhibitors ( PCNA ‐Is) that bind directly to PCNA , stabilize PCNA trimer structure, reduce chromatin‐associated PCNA , and selectively inhibit tumor cell growth. Of these compounds, PCNA ‐I1 was most potent. The purpose of this study is to further establish targeting of PCNA by PCNA ‐I1 and to identify PCNA ‐I1 analogs with superior potencies. We found that PCNA ‐I1 does not affect the level of chromatin‐associated PCNA harboring point mutations at the predicted binding site of PCNA ‐I1. Forty‐six PCNA ‐I1 analogs with structures of 1‐hydrazonomethyl‐2‐hydroxy (scaffold A), 2‐hydrazonomethyl‐1‐hydroxy (scaffold B), 2‐hydrazonomethyl‐3‐hydroxy (scaffold C), and 4‐pyridyl hydrazine (scaffold D) were analyzed for their effects on cell growth in four tumor cell lines and PCNA trimer stabilization. Compounds in scaffold group A and group B showed the highest trimer stabilization and the most potent cell growth inhibitory activities with a significant potency advantage observed in the Z isomers of scaffold A. The absence of trimer stabilization and growth inhibitory effects in compounds of scaffold group D confirms the essentiality of the hydroxynaphthyl substructure. Compounds structure–activity relationship ( SAR )‐6 and SAR ‐24 were analyzed for their effects on and found to reduce chromatin‐associated PCNA in tumor cells. This study led to the identification of SAR ‐24, a compound with superior potencies and potentially improved solubility, which will be used for future development of PCNA ‐targeting cancer therapies.