PLK1 Inhibition Radiosensitizes Breast Cancer Cells, but Shows Low Efficacy as Monotherapy or in Combination with other Cytotoxic Drugs

Background and Purpose: Over the last decade, the inhibition of PLK1 has proven potent antiproliferative activity in vitro. However, the effectiveness of most synthetic targeted drugs has not yet been translated into clinics. Herein, we investigated the in vitro effects of two second-generation PLK1 inhibitors BI 6727 and GSK461364 in breast cancer cell lines as monotherapy or in combination with other drugs or ionizing radiation.

Material and Methods: Cell survival was analyzed through XTT®, clonogenicity and caspase-3 activation assays were also studied, and drug interactions analyzed through a nonlinear regression of a sigmoid doseresponse model. Sensibilization to radiation was assessed through enhancement ratio calculation.

Results: Mild effects on the viability of both cell lines tested (MCF-7 and Hs578T) were observed irrespective of the used PLK1 inhibitor. Alternatively, abrogation of PLK1 significantly reduced clonogenicity while effectively sensitized cells to ionizing radiation. Drug interactions showed dissimilar results with antagonistic effects with any drug combination in MCF-7 and clear synergic interactions between both PLK1 inhibitors and cisplatin, temozolomide or doxorubicin in Hs578T, which is TP53 mutated.

Conclusion: Targeting kinases involved in mitotic checkpoints are expected to prevent mitotic exit and enhance chemosensitization. Nonetheless, despite overexpressing PLK1, in our model, expressive results after its inhibition were only seen through clonogenic assays or when BI 6727 and GSK461364 were combined with ionizing radiation. Disparate responses of cell lines to drug combinations might denote a partial reflection of the substantial differences in the vast spectrum of genetic, biological and epigenetic burden observed in breast cancer. In the near future, individual genomic/proteomic profiling will allow its further classification and will consent the initiation of novel strategies for therapy. Even though the future impact of PLK1-tailored treatment still needs validation, much more pre-clinical and clinical research for this kinase are warranted.

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