Identification of potent and selective MTH1 inhibitors

Identification and structure-activity relationship of purine derivatives as novel MTH1 inhibitors

The human mutT homolog-1 (MTH1) protein prevents the incorporation of oxidized nucleotides such as 2-OH-dATP and 8-oxo-dGTP during DNA replication by hydrolyzing them into their corresponding monophosphates. It was found previously that cancer cells could tolerate oxidative stress due to this enzymatic activity of MTH1 and its inhibition could be a promising approach to treat several types of cancer. This finding has been challenged recently with increasing line of evidence suggesting that the cancer cell-killing effects of MTH1 inhibitors may be related to their engagement of off-targets. We have previously reported a few purine-based MTH1 inhibitors that enabled us to elucidate the dispensability of MTH1 in cancer cell survival. Here, we provide a detailed process of the identification of purine-based MTH1 inhibitors. Several new compounds with potency in the submicromolar range are disclosed. Furthermore, the structure-activity relationship and associated binding mode prediction using molecular docking have provided insights for the development of highly potent MTH1 inhibitors.

Validation and development of MTH1 inhibitors for treatment of cancer

BACKGROUND

Previously, we showed cancer cells rely on the MTH1 protein to prevent incorporation of otherwise deadly oxidised nucleotides into DNA and we developed MTH1 inhibitors which selectively kill cancer cells. Recently, several new and potent inhibitors of MTH1 were demonstrated to be non-toxic to cancer cells, challenging the utility of MTH1 inhibition as a target for cancer treatment.

MATERIAL AND METHODS

Human cancer cell lines were exposed in vitro to MTH1 inhibitors or depleted of MTH1 by siRNA or shRNA. 8-oxodG was measured by immunostaining and modified comet assay. Thermal Proteome profiling, proteomics, cellular thermal shift assays, kinase and CEREP panel were used for target engagement, mode of action and selectivity investigations of MTH1 inhibitors. Effect of MTH1 inhibition on tumour growth was explored in BRAF V600E-mutated malignant melanoma patient derived xenograft and human colon cancer SW480 and HCT116 xenograft models.

RESULTS

Here, we demonstrate that recently described MTH1 inhibitors, which fail to kill cancer cells, also fail to introduce the toxic oxidized nucleotides into DNA. We also describe a new MTH1 inhibitor TH1579, (Karonudib), an analogue of TH588, which is a potent, selective MTH1 inhibitor with good oral availability and demonstrates excellent pharmacokinetic and anti-cancer properties in vivo.

CONCLUSION

We demonstrate that in order to kill cancer cells MTH1 inhibitors must also introduce oxidized nucleotides into DNA. Furthermore, we describe TH1579 as a best-in-class MTH1 inhibitor, which we expect to be useful in order to further validate the MTH1 inhibitor concept.

Excessive Reactive Oxygen Species and Exotic DNA Lesions as an Exploitable Liability

Although the terms "excessive reactive oxygen species (ROS)" and "oxidative stress" are widely used, the implications of oxidative stress are often misunderstood. ROS are not a single species but a variety of compounds, each with unique biochemical properties and abilities to react with biomolecules. ROS cause activation of growth signals through thiol oxidation and may lead to DNA damage at elevated levels. In this review, we first discuss a conceptual framework for the interplay of ROS and antioxidants. This review then describes ROS signaling using FLT3-mediated growth signaling as an example. We then focus on ROS-mediated DNA damage. High concentrations of ROS result in various DNA lesions, including 8-oxo-7,8-dihydro-guanine, oxazolone, DNA-protein cross-links, and hydantoins, that have unique biological impacts. Here we delve into the biochemistry of nine well-characterized DNA lesions. Within each lesion, the types of repair mechanisms, the mutations induced, and their effects on transcription and replication are discussed. Finally, this review will discuss biochemically inspired implications for cancer therapy. Several teams have put forward designs to harness the excessive ROS and the burdened DNA repair systems of tumor cells for treating cancer. We discuss inhibition of the antioxidant system, the targeting of DNA repair, and ROS-activated prodrugs.

A Chimeric ATP-Linked Nucleotide Enables Luminescence Signaling of Damage Surveillance by MTH1, a Cancer Target

The enzyme MTH1 cleanses the cellular nucleotide pool of oxidatively damaged 8-oxo-dGTP, preventing mutagenesis by this nucleotide. The enzyme is considered a promising therapeutic target; however, methods to measure its activity are indirect and laborious and have low sensitivity. Here we describe a novel ATP-linked chimeric nucleotide (ARGO) that enables luminescence signaling of the enzymatic reaction, greatly simplifying the measurement of MTH1 activity. We show that the reporting system can be used to identify inhibitors of MTH1, and we use it to quantify enzyme activity in eight cell lines and in colorectal tumor tissue. The ARGO reporter is likely to have considerable utility in the study of the biology of MTH1 and potentially in analyzing patient samples during clinical testing.

Proteomic profiling of small-molecule inhibitors reveals dispensability of MTH1 for cancer cell survival

Since recent publications suggested that the survival of cancer cells depends on MTH1 to avoid incorporation of oxidized nucleotides into the cellular DNA, MTH1 has attracted attention as a potential cancer therapeutic target. In this study, we identified new purine-based MTH1 inhibitors by chemical array screening. However, although the MTH1 inhibitors identified in this study targeted cellular MTH1, they exhibited only weak cytotoxicity against cancer cells compared to recently reported first-in-class inhibitors. We performed proteomic profiling to investigate the modes of action by which chemically distinct MTH1 inhibitors induce cancer cell death, and found mechanistic differences among the first-in-class MTH1 inhibitors. In particular, we identified tubulin as the primary target of TH287 and TH588 responsible for the antitumor effects despite the nanomolar MTH1-inhibitory activity in vitro. Furthermore, overexpression of MTH1 did not rescue cells from MTH1 inhibitor–induced cell death, and siRNA-mediated knockdown of MTH1 did not suppress cancer cell growth. Taken together, we conclude that the cytotoxicity of MTH1 inhibitors is attributable to off-target effects and that MTH1 is not essential for cancer cell survival.

MutT Homolog 1 (MTH1): The Silencing of a Target

MTH1 is a member of the nudix phosphohydrolase superfamily of enzymes, and it is involved in nucleotide pool homeostasis. The protein exerts its scavenging action by hydrolyzing oxidized nucleotides, thus avoiding their misincorporation into replicating DNA. Recent reports have validated its inhibition as a potential broad-spectrum target in oncology and have described small molecules able to engage this target with antiproliferative effect. In the accompanying paper (DOI: 10.1021/acs.jmedchem.5b01760 ), a team at AstraZeneca led by Jason G. Kettle elegantly demonstrated the complete lack of antiproliferative activity of cancer cells to highly potent MTH1 inhibitors.