DNA polymerase ␤ (Pol ␤) is an error-prone enzyme whose up-regulation has been shown to be a genetic instability enhancer as well as a contributor to cisplatin resistance in tumor cells. In this work, we describe the isolation of new Pol ␤ inhibitors after high throughput screening of 8448 semipurified natural extracts. In vitro, the selected molecules affect specifically Pol ␤-mediated DNA synthesis compared with replicative extracts from cell nuclei. One of them, masticadienonic acid (MA), is particularly attractive because it perturbs neither the activity of the purified replicative Pol ␦ nor that of nuclear HeLa cell extracts. With an IC 50 value of 8 M, MA is the most potent of the Pol ␤ inhibitors found so far. Docking simulation revealed that this molecule could substitute for single-strand DNA in the binding site of Pol ␤ by binding Lys35, Lys68, and Lys60, which are the main residues involved in the interaction Pol ␤/singlestrand DNA. Selected inhibitors also affect the Pol ␤-mediated translesion synthesis (TLS) across cisplatin adducts; MA was still the most efficient. Therefore, masticadienonic acid sensitized the cisplatin-resistant 2008C13*5.25 human tumor cells. Our data suggest that molecules such as masticadienonic acid could be suitable in conjunction with cisplatin to enhance anticancer treatments.

The constitution of the aglycon of nudicaulin has been revised to be a pentacyclic indole alkaloid. The relative and absolute configurations of the two diastereomers, nudicaulins I (3a) and II (3b), have been assigned by NMR, conformational analyses, and interpretation of the experimental ECD spectra by quantum-chemical calculations.