Abstract
There is an unmet need for new antimitotic drug combinations that target cancer-specific vulnerabilities. Based on our finding of elevated biomolecule oxidation in mitotically arrested cancer cells, we combined Plk1 inhibitors with TH588, an MTH1 inhibitor that prevents detoxification of oxidized nucleotide triphosphates. This combination showed robust synergistic killing of cancer, but not normal, cells that, surprisingly, was MTH1-independent. To dissect the underlying synergistic mechanism, we developed VISAGE, a strategy integrating experimental synergy quantification with computational-pathway-based gene expression analysis. VISAGE predicted, and we experimentally confirmed, that this synergistic combination treatment targeted the mitotic spindle. Specifically, TH588 binding to β-tubulin impaired microtubule assembly, which when combined with Plk1 blockade, synergistically disrupted mitotic chromosome positioning to the spindle midzone. These findings identify a cancer-specific mitotic vulnerability that is targetable using Plk1 inhibitors with microtubule-destabilizing agents and highlight the general utility of the VISAGE approach to elucidate molecular mechanisms of drug synergy.
Highlights
The vast majority of tumors are treated with some type of combination chemotherapy (DeVita and Chu, 2008)
The castrationresistant prostate cancer (CRPC) cell line C4-2 was chosen as a starting point for examination of TH588 synergy because: (1) Plk1 expression is elevated in prostate cancer and shown to correlate with Gleason grade (Weichert et al, 2004); (2) antimitotic drugs are widely used for treatment of advanced prostate cancer (Tannock et al, 2004); and (3) we had previously demonstrated elevated levels of cysteine and guanine oxidation following mitotic arrest in these cells (Patterson et al, 2019)
The Cytotoxicity of TH588 and the Synergy between Plk1 Inhibitors and TH588 Are Independent of MTH1 Activity This combination of a Plk1 inhibitor with the MTH1 inhibitor TH588 was chosen based on a model of enhanced reactive oxygen species (ROS) and oxidative damage induced by mitotic arrest, resulting in a subsequent increased reliance on MTH1. To confirm this causal relationship between ROS and sensitivity to BI2536, TH588, or the combination, we examined the effect of the antioxidant N-acetyl cysteine (NAC) on tumor cell viability after treatment with these agents
Summary
The vast majority of tumors are treated with some type of combination chemotherapy (DeVita and Chu, 2008). Inhibitors of mitotic kinases including Plk, the Aurora kinases, and mitotic cyclindependent kinases, which target molecules required mainly for unrestrained cell proliferation, have fared poorly in clinical trials, despite repeatedly demonstrating impressive efficacy in preclinical studies (Dominguez-Brauer et al, 2015). Given these limitations of current antimitotic drugs, ‘‘the challenge is to identify cell cycle regulators that are essential for mitosis of cancer cells rather than normal cells’’, as stated in a seminal and comprehensive review by Tak Mak and colleagues (Dominguez-Brauer et al, 2015)
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