Beginning with puromycin, known for its extreme toxicity, researchers created a prodrug, a masked cytotoxic agent. This prodrug has a sly design feature—it liberates its puromycin only after it has been cleaved by a pair of enzymes. These enzymes, which essentially unmask the puromycin, occur at higher levels with tumor cells rather than with normal cells, and so tumor cells are more likely to run afoul of the prodrug, encountering the hidden toxin face to face.

To create the prodrug, the researchers coupled puromycin with an acetylated lysine group. The prodrug unleashes its toxic effects only after the acetylated lysine group has been removed by the enzymes histone deacetylase (HDAC) and an endogenous protease cathepsin L (CTSL). These enzymes act on the prodrug serially, first removing an acetyl group and then an unacetylated lysine group.

The prodrug has shown promise in early tests, selectively killing human cancer cells with high HDAC and CTSL activities. According to the researchers, in vivo studies confirm tumor growth inhibition in prodrug-treated mice bearing human cancer xenografts.

These results were described in Nature Communications on November 5, in a paper entitled “Selective cancer targeting with prodrugs activated by histone deacetylases and a tumour-associated protease.” In this paper, the authors write, “This cancer-selective cleavage of the masking group is a promising strategy for the next generation of anticancer drug development that could be applied to many other cytotoxic agents.”

The first author of the paper is Nobuhide Ueki, Ph.D., an assistant research professor in the department of molecular genetics and microbiology at Stony Brook University. Dr. Ueki and his team hope their effort will attract the attention of pharmaceutical companies and other researchers, motivating them to make their own compounds using this technique.

“Sometimes drug companies give up on a drug because it is so toxic,” says Dr. Ueki. “If they can use this technique, they might be able to use their drug again. The more new drugs are developed, the more chances of successful treatment are increased. That’s our purpose.”

Speculating about the prodrug’s unmasking mechanism, the authors described how serial activation could be achieved via the spatially separated enyzmes HDAC and CTSL in intact cells: “Puromycin and the prodrug are water-soluble small molecules that are also cell permeable and can diffuse rapidly through the aqueous cytosol in the cell. Thus, it is possible that the serial activation process by the two enzymes takes place in different subcellular locations through simple passive diffusion, regardless of the nucleus or the cytoplasm.”

Dr. Ueki’s team is modifying its puromycin-masking compound to achieve greater efficacy. In addition, they believe that it could be used with other drugs. Finally, it has potential as a tool for marking cancer cells in imaging applications.

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