Objective: Poly (ADP-ribose) polymerase 1 inhibitors (PARPi) are used in the treatment of cancer patients as
monotherapy agents or in combination with approved anticancer drugs to optimize their clinical use. However,
the development of novel anticancer chemotherapy is tempered by the potential detrimental side effects displayed
by cytotoxic drugs in healthy cells. To delineate the rationality of PARPi olaparib (OLA) in combination with the
approved DNA-damaging anticancer drug bleomycin (BLM), we investigated the influence of the drugs on poly
(ADP-ribose) polymerase 1 (PARP 1) activity, chromatin, and DNA structure in healthy rat thymocyte nuclei.
Materials and Methods: Wistar albino male rats (Rattus norvegicus, 6 weeks old) were used throughout the
experiments. Animals were obtained from the stock of the animal house of the faculty of biology at YSU. The
enzymatic assay for PARP 1 activity was performed according to the original method based on the estimation
of residual NAD+ concentration in the PARP assay mix adapted by us to quantify NAD+ consumed by isolated
nuclei. DNA concentrations were measured by the spectrophotometric method using an extinction coefficient
ε260=6600 M-1 cm-1. Statistical differences in results between groups were evaluated by the Student’s t-test. P < 0.05
was considered significant. Key Results: It was observed that in healthy rat thymocyte nuclei, BLM induced
PARP-1 inhibition and DNA and chromatin loosening. In contrast, treatment with OLA maintained chromatin and
DNA condensation, which could partly underlie the cytotoxic effect of OLA as a result of drug-induced downregulation
of chromatin-associated nuclear functions. Our findings provide evidence for the revalidation of the
rationality of OLA application in combination chemotherapy regimens involving BLM.