Mechanism of Action Studies of Lomaiviticin A and the Monomeric Lomaiviticin Aglycon. Selective and Potent Activity Toward DNA Double-Strand Break Repair-Deficient Cell Lines

(−)-Lomaiviticin A (<b>1</b>) and the monomeric lomaiviticin aglycon [aka: (−)-MK7-206, (<b>3</b>)] are cytotoxic agents that induce double-strand breaks (DSBs) in DNA. Here we elucidate the cellular responses to these agents and identify synthetic lethal interactions with specific DNA repair factors. Toward this end, we first characterized the kinetics of DNA damage by <b>1</b> and <b>3</b> in human chronic myelogenous leukemia (K562) cells. DSBs are rapidly induced by <b>3</b>, reaching a maximum at 15 min post addition and are resolved within 4 h. By comparison, DSB production by <b>1</b> requires 2–4 h to achieve maximal values and >8 h to achieve resolution. As evidenced by an alkaline comet unwinding assay, <b>3</b> induces extensive DNA damage, suggesting that the observed DSBs arise from closely spaced single-strand breaks (SSBs). Both <b>1</b> and <b>3</b> induce ataxia telangiectasia mutated- (ATM-) and DNA-dependent protein kinase- (DNA-PK-) dependent production of phospho-SER139-histone H2AX (γH2AX) and generation of p53 binding protein 1 (53BP1) foci in K562 cells within 1 h of exposure, which is indicative of activation of nonhomologous end joining (NHEJ) and homologous recombination (HR) repair. Both compounds also lead to ataxia telangiectasia and Rad3-related- (ATR-) dependent production of γH2AX at later time points (6 h post addition), which is indicative of replication stress. <b>3</b> is also shown to induce apoptosis. In accord with these data, <b>1</b> and <b>3</b> were found to be synthetic lethal with certain mutations in DNA DSB repair. <b>1</b> potently inhibits the growth of breast cancer type 2, early onset- (BRCA2-) deficient V79 Chinese hamster lung fibroblast cell line derivative (VC8), and phosphatase and tensin homologue deleted on chromosome ten- (PTEN-) deficient human glioblastoma (U251) cell lines, with LC₅₀ values of 1.5 ± 0.5 and 2.0 ± 0.6 pM, respectively, and selectivities of >11.6 versus the isogenic cell lines transfected with and expressing functional BRCA2 and PTEN genes. <b>3</b> inhibits the growth of the same cell lines with LC₅₀ values of 6.0 ± 0.5 and 11 ± 4 nM and selectivities of 84 and 5.1, for the BRCA2 and PTEN mutants, respectively. These data argue for the evaluation of these agents as treatments for tumors that are deficient in BRCA2 and PTEN, among other DSB repair factors