PARP inhibition as a potential target for canine lymphoma and leukemia

Introduction: Development of novel targeted anticancer therapies is a major goal of contemporary oncology. One of the promising directions, already proven in human medicine, is the use of poly(ADP- ribose) polymerase (PARP) inhibitors (PARPi) in patients with DNA repair pathway disorders. Despite the enormous success of PARPi in human oncology, these drugs are not intensively researched in veterinary medicine. Little interest in this topic may result from the lack of well-defined type of cancer in which these drugs can be effective as it is seen in humans - tumors with inherited mutations in BRCA1 and BRCA2. However, there are multiple studies showing that mutations within these genes occur also in canine cancers. Taking into account the current wide indications for the use of PARPi and the increasing availability of DNA rapair disorders data in various types of cancer in dogs, we decided to check the activity of the first and best-studied PARPi - olaparib in canine lymphoma and leukemia cells. The aim of our study was to demonstrate the way of cytotoxic activity of the studied drug and to indicate the ways of its potential use in hematopoietic cancers in dogs. Materials and Methods: To check how olaparib affects the cell grow of established canine lymphoma/leukemia cell lines (CLBL-1 and GL-1), MTT test and analysis of cell cycle progression by flow cytometry were performed. DNA damage in the cells were detected as phosphorylation of histone H2AX. The level of apoptosis in olaparib- treated cells was assessed using annexin V staining and flow cytometry. Results: Olaparib alone is able to inhibit proliferation of all lymphoma/leukemia cell lines used in the study in both concentration- and time-dependent manner by provoking the accumulation of cells in the G2/M phase of the cell cycle. There were some differences in the sensitivity of individual cell lines with the GL-1 cell line being less susceptible to olaparib than other tested cell lines. Olaparib also induced cell apoptosis, observed as externalization of phosphatidylserine due to DNA damage, verified by increase in γH2AX expression in treated cells. Conclusions: Olaparib causes synthetic lethality in canine lymphoma/leukemia cells in vitro, possibly because of alterations in DNA repair pathways characterizing tested cells. This means that the use of PARPi may be a valuable direction in the treatment of hematopoietic cancers in dogs.