The mechanism of action of a novel benzo[c]phenanthridine alkaloid, NK314 and the cellular responses

NK314 is a novel synthetic benzo[c]phenanthridine alkaloid that is currently in clinical trials as an antitumor compound, based on impressive activities in preclinical models. However, its mechanism of action is unknown. The present investigations were directed at determining the mechanism of action of this agent and cellular responses to NK314. My studies demonstrated that NK314 intercalated into DNA, trapped topoisomerase IIα in its cleavage complex intermediate, and inhibited the ability of topoisomerase IIα to relax super-coiled DNA. CEM/VM1 cells, which are resistant to etoposide due to mutations in topoisomerase IIα, were cross-resistant to NK314. However, CEM/C2 cells, which are resistant to camptothecin due to mutations in topoisomerase I, retained sensitivity. This indicates topoisomerase IIα is the target of NK314 in the cells. NK314 caused phosphorylation of the histone variant, H2AX, which is considered a marker of DNA double-strand breaks. DNA double-strand breaks were also evidenced by pulsed-field gel electrophoresis and visualized as chromosomal aberrations after cells were treated with NK314 and arrested in mitosis. Cell cycle checkpoints are activated following DNA damage. NK314 induced significant G2 cell cycle arrest in several cell lines, independent of p53 status, suggesting the existence of a common mechanism of checkpoint activation. The Chk1-Cdc25C-Cdk1 G2 checkpoint pathway was activated in response to NK314, which can be abrogated by the Chk1 inhibitor UCN-01. Cell cycle checkpoint activation may be a defensive mechanism that provides time for DNA repair. DNA double-strand breaks are repaired either through ATM-mediated homologous recombination or DNA-PK-mediated non-homologous end-joining repair pathways. Clonogenic assays demonstrated a significant decrease of colony formation in both ATM deficient and DNA-PK deficient cells compared to ATM repleted and DNA-PK wild type cells respectively, indicating that both ATM and DNA-PK play important roles in the survival of the cells in response to NK314. The DNA-PK specific inhibitor NU7441 also significantly sensitized cells to NK314. In conclusion, the major mechanism of NK314 is to intercalate into DNA, trap and inhibit topoisomerase IIα, an action that leads to the generation of double-strand DNA breaks, which activate ATM and DNA-PK mediated DNA repair pathways and Chk1 mediated G2 checkpoint pathway. ^