*Article* **PARP Inhibitor Olaparib Causes No Potentiation of the Bleomycin E**ff**ect in VERO Cells, Even in the Presence of Pooled ATM, DNA-PK, and LigIV Inhibitors**

**Valentina Perini <sup>1</sup> , Michelle Schacke <sup>1</sup> , Pablo Liddle <sup>1</sup> , Salomé Vilchez-Larrea <sup>2</sup> , Deborah J. Keszenman 3,\* and Laura Lafon-Hughes 1,\***


Received: 30 September 2020; Accepted: 19 October 2020; Published: 5 November 2020

**Abstract:** Poly(ADP-ribosyl)polymerase (PARP) synthesizes poly(ADP-ribose) (PAR), which is anchored to proteins. PAR facilitates multiprotein complexes' assembly. Nuclear PAR affects chromatin's structure and functions, including transcriptional regulation. In response to stress, particularly genotoxic stress, PARP activation facilitates DNA damage repair. The PARP inhibitor Olaparib (OLA) displays synthetic lethality with mutated homologous recombination proteins (BRCA-1/2), base excision repair proteins (XRCC1, Polβ), and canonical nonhomologous end joining (LigIV). However, the limits of synthetic lethality are not clear. On one hand, it is unknown whether any limiting factor of homologous recombination can be a synthetic PARP lethality partner. On the other hand, some BRCA-mutated patients are not responsive to OLA for still unknown reasons. In an effort to help delineate the boundaries of synthetic lethality, we have induced DNA damage in VERO cells with the radiomimetic chemotherapeutic agent bleomycin (BLEO). A VERO subpopulation was resistant to BLEO, BLEO + OLA, and BLEO + OLA + ATM inhibitor KU55933 + DNA-PK inhibitor KU-0060648 + LigIV inhibitor SCR7 pyrazine. Regarding the mechanism(s) behind the resistance and lack of synthetic lethality, some hypotheses have been discarded and alternative hypotheses are suggested.

**Keywords:** poly(ADP-ribosylation); PARP-1; Olaparib; KU55933; KU-0060648; SCR7 pyrazine; VERO cells; synergism; resistance; CDKN2A
