Preferential potentiation of topoisomerase I poison cytotoxicity by PARP inhibition in S phase

Br J Cancer. 2014 Sep 23;111(7):1319-26. doi: 10.1038/bjc.2014.378. Epub 2014 Jul 8.


Background: Topoisomerase I (Topo I) poisons (e.g., camptothecin (CPT)), used to treat cancer, cause DNA breaks that are most cytotoxic during S phase. PARP-1 promotes DNA repair and PARP inhibitors (PARPi) sensitise cells to Topo I poisons. We aimed to determine whether chemosensitisation is also S phase specific using rucaparib, a potent PARPi in advanced clinical evaluation.

Methods: The impact of rucaparib, on CPT-induced cytotoxicity was measured in human colon cancer (LoVo) and leukaemic (K562) cells in asynchronous and cell cycle phase-separated cultures. Topoisomerase I and PARP levels and activity and the effect of rucaparib on DNA single-strand breaks (SSBs), double-strand breaks (DSBs) and collapsed replication fork induction and repair were determined in cell cycle phase-separated cells.

Results: The cytotoxicity of CPT was greatest during S phase, partially attributable to high Topo I activity, and rucaparib preferentially sensitised S-phase cells. Rucaparib increased CPT-induced DNA SSBs in all phases of the cell cycle, and increased DSB and γH2AX foci in S and G2, with γH2AX foci being highest in S-phase cells. Repair of SSBs and DSBs was most rapid during S then G2 phases and was substantially hindered by rucaparib.

Conclusions: Rucaparib preferentially sensitises S-phase cells by increasing the frequency of collapsed replication forks.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Camptothecin / pharmacology*
  • Cell Survival / drug effects
  • DNA Breaks, Double-Stranded
  • DNA Repair / drug effects
  • DNA Topoisomerases, Type I / metabolism
  • Drug Screening Assays, Antitumor
  • Drug Synergism
  • Humans
  • Indoles / pharmacology*
  • K562 Cells
  • Poly (ADP-Ribose) Polymerase-1
  • Poly(ADP-ribose) Polymerase Inhibitors*
  • Poly(ADP-ribose) Polymerases / metabolism
  • S Phase Cell Cycle Checkpoints / drug effects*
  • Topoisomerase I Inhibitors / pharmacology*


  • Indoles
  • Poly(ADP-ribose) Polymerase Inhibitors
  • Topoisomerase I Inhibitors
  • rucaparib
  • PARP1 protein, human
  • Poly (ADP-Ribose) Polymerase-1
  • Poly(ADP-ribose) Polymerases
  • DNA Topoisomerases, Type I
  • TOP1 protein, human
  • Camptothecin