Synthesis and biological activity of selenopsammaplin A and its analogues as antitumor agents with DOT1L inhibitory activity

Abstract

Disruptor of telomeric silencing-1 like (DOT1L) is a histone H3 methyltransferase which specifically catalyzes the methylation of histone H3 lysine-79 residue. Recent findings demonstrate that DOT1L is abnormally overexpressed and the upregulated DOT1L evokes the proliferation and metastasis in human breast cancer cells. Therefore, the DOT1L inhibitor is considered a promising strategy to treat breast cancers. Non-nucleoside DOT1L inhibitors, selenopsammaplin A and its analogues, were firstly reported in the present study. Selenopsammaplin A was newly designed and synthesized with 25% overall yield in 8 steps from 3-bromo-4-hydroxybenzaldahyde, and thirteen analogues of selenopsammaplin A were prepared for structure–activity relationship studies of their cytotoxicity against cancer cells and inhibitory activity toward DOT1L for antitumor potential. All synthetic selenopsammaplin A analogues exhibited the higher cytotoxicity compared to psammaplin A with up to 6 – 60 times depending on cancer cells, and most analogues showed significant inhibitory activities against DOT1L. Among the prepared analogues, the phenyl analogue (10) possessed the most potent activity with both cytotoxicity and inhibition of DOT1L. Compound 10 also exhibited the antitumor and antimetastatic activity in an orthotopic mouse metastasis model implanted with MDA-MB-231 human breast cancer cells. These biological findings suggest that analogue 10 is a promising candidate for development as a cancer chemotherapeutic agent in breast cancers.

Introduction

The overall survival rate of cancer patients is affected not only by the use of appropriate anticancer drugs but also by the inhibition of cancer cell metastasis. Disruptor of telomeric silencing-1 like (DOT1L), a histone H3 methyltransferase, specifically catalyzes the methylation of histone H3 on the lysine-79 residue (H3K79). DOT1L-mediated H3K79 methylation is considered to be strongly associated with various biological processes, such as embryonic cell development, cell division checkpoint, and the DNA stability.1, 2 Recent findings also suggested that the overexpression of DOT1L and hyperactivation of DOT1L-mediated H3K79 methylation may play a crucial role in the initiation and maintenance of mixed lineage leukemia (MLL)-rearranged leukemia. Therefore, DOT1L is considered a promising therapeutic target in the development of anticancer agents for MLL-rearranged leukemia.3, 4, 5 In addition, recent evidence has also suggested that DOT1L may solely function as an oncogene in solid tumors including breast cancer without MLL fusion.6, 7, 8 In particular, DOT1L transactivates epithelial-mesenchymal transition (EMT)-promoting genes, which is an essential process for cancer cell invasion and metastasis⁹, suggesting that DOT1L is a suitable molecular target for the development of antitumor and antimetastasis therapeutics for aggressive breast cancer. Since the DOT1L histone H3 methyltransferase has been recognized as a promising target for the development of novel anticancer agents, efforts to identify effective DOT1L inhibitors have also become common.10, 11 To date, however, only a few nucleoside-type inhibitors have been reported and no other classes of small molecules have been developed.¹² (see Fig 1.).

As part of our ongoing search for novel antimetastatic cancer chemotherapeutic agents targeting histone-modifying enzymes, we recently developed very efficient synthetic methods for preparing psammaplin A and its analogues, and evaluated their cytotoxicity.¹⁴ However, only a few compounds have exhibited antitumor activity comparable to psammaplin A itself in both in vitro cell culture and in vivo xenograft mouse models and poor antimetastatic activity (see Fig 2).

Selenium, the element with atomic number 34, belongs to the same family as oxygen and sulfur in the periodic table. It is a trace element present in the body as part of 21st amino acid, selenocysteine (SeC), but it is essential for metabolism, and has been shown to have anticancer effects.²⁵ As oxidative stress is known to be associated with carcinogenesis, the anticancer activity of selenium might be due to the antioxidant properties of selenium-containing proteins (selenoproteins), including proteins in the glutathione peroxidase family and selenoprotein P, which has been tested against bladder cancer, breast cancer, prostate cancer, lung cancer and colon cancer.26, 27, 28, 29 Selenium involved organic compounds have also been reported to induce apoptosis of cancer cells and to exhibit anticancer effects in prostate cancer, colorectal cancer, liver cancer, and blood cancer.³⁰ In addition, as selenium has more lipophilic physicochemical properties than oxygen and sulfur, its presence in organic molecules can result in better oral bioavailability by better penetration across the cell membrane, including the blood–brain barrier (BBB). Based on these findings, seleno-acyclovir (1)³¹, seleno-adefovir³², and seleno-tenofovir (2)33, 34 have been successfully reported as efficient anti-viral agents.

As part of our focus on the anticancer effects of organo-selenium, we designed the novel selenopsammaplin A (9) with improved anticancer activity by replacing the disulfide bond in psammaplin A with a diselenide bond. Herein, we report a new synthetic method for accessing selenopsammaplin A (9) and its analogues with a structure–activity relationship by evaluating cytotoxicity and DOT1L inhibitory activity. In addition, the in vivo antitumor activity and antimetastatic activity of a selected compound was determined by employing an orthotopic mouse metastasis models implanted with human breast cancer cells.