Anti-cancer implications of small molecule compounds targeting proliferating cell nuclear antigen

Cancer is a persistent global health concern despite advancements in prevention, detection, and treatment. Acquired chemotherapy resistance and dose-limiting toxicities continue to plague current cancer treatment modalities, indicating a desperate need for new and improved molecularly targeted anti-cancer therapies. Proliferating cell nuclear antigen (PCNA) is a non-oncogenic mediator of DNA replication. For this reason, PCNA function is essential to the uncontrolled proliferation of cancer cells. PCNA is highly expressed in tumors, determining it as a reliable marker of proliferation and a potentially desirable target for cancer therapy.;Functional PCNA is a ring-shaped homotrimer which is loaded onto chromatin by replication factor C (RFC). PCNA plays a crucial role in DNA replication by providing replicative DNA polymerases the high processivity required to duplicate the entire genome. Furthermore, PCNA functions as a scaffold protein, binding a multitude of protein partners involved in many vital cellular processes such as DNA replication, DNA repair, and cell cycle control. Collectively, these many functions of PCNA and its localization at the replisome put PCNA in a central position for determining the fate of the replication fork.;The homotrimeric structure of PCNA is crucial for its function. Therefore, we performed an in silico screen of a drug-like chemical library for small molecule compounds that would bind the monomer-monomer interface of PCNA in attempts to identify compounds that would alter homotrimer stability. The PCNA-inhibitors (PCNA-Is) identified were found to bind PCNA homotrimers with a high affinity and promote homotrimer stabilization by serving as a "linker" at the monomer-monomer interface. This increased stability resulted in a decrease in PCNA chromatin-association in various tumor cell lines. Treatment with PCNA-Is inhibited DNA replication and growth of tumor cells. Additional studies using PCNA-I1, the most potent of the PCNA-Is identified, were carried out to determine the downstream effects of targeting PCNA in two prostate cancer cell lines which differed in mutational status. Treatment with PCNA-I1 resulted in an accumulation of DNA double-strand breaks and induced programmed cell death of PC-3 and LNCaP cells. Moreover, PCNA-I1 sensitized LNCaP prostate cancer cells to cisplatin treatment. However, regardless the molecular drivers, mutational status, or mechanism of programmed cell death, short treatment with PCNA-I1 was cytotoxic to prostate cancer cells. These findings were mirrored in vivo, as intravenous administration of PCNA-I1 inhibited tumor growth and induced apoptosis in a xenograft model of prostate cancer. Furthermore, PCNA-I1 treatment showed no evidence of systemic toxicity at the administered dose.;Structure-activity relationship (SAR) analysis of 46 compounds led to the identification of two compounds with superior potency and improved solubility to PCNA-I1. These compounds stabilized PCNA homotrimers and reduced PCNA chromatin-association, confirming the mechanism of action for this class of compounds. Future studies will focus on further characterizing the downstream effects of PCNA-Is and their potential usefulness as both a monotherapy and combination therapy for the treatment of cancer.