"This is the first time this type of drug has been able to reactivate silenced genes through oral administration," notes Peter A. Jones, Ph.D., director of the USC/Norris Comprehensive Cancer Center and one of the study's principal investigators.
The research was published in the March 5 issue of the Journal of the National Cancer Institute (JNCI).
Only recently have scientists begun to recognize the important role that DNA methylation-the addition of a methyl group to a stretch of DNA, which can lock, or silence, that gene-can play in the development of cancer. If methylation silences a gene that normally would control cell growth or prompt the cell to commit suicide, then the cell will grow unchecked-the hallmark of cancer.
"The concept that the silencing of genes is a critical part of the cancer process is a major conceptual advance in this field," Jones says. "Realizing that, it becomes very important to find keys to unlock those silenced genes."
Zebularine's inhibition of DNA methylation was first discovered in experiments on a filamentous fungus done by Eric Selker, Ph.D., of the University of Oregon in Eugene, notes Jonathan C. Cheng, an M.D./Ph.D. student at USC who is the first author on the JNCI paper. "It was completely unexpected," Cheng says. "And we have since shown that it can work the same way in mammalian cells."
In the JNCI study, Cheng, Jones and their colleagues gave relatively high doses of zebularine to mice engineered to be susceptible to bladder carcinoma. They administered the drug both intraperitoneally (injecting it into the space between the abdominal muscles and the abdominal organs) and orally.
What they found was that that both routes of administration resulted in the reactivation of a p16 gene that had been previously silenced by the methylation of a second, regulatory gene. In addition, they found that volumes of the bladder tumors in these mice were significantly reduced by the drug.
The fact that zebularine is still effective when taken orally is no small deal, says Cheng. Ease of administration can be a major issue in the success of cancer drugs. Unlike other methylation inhibitors, which break down rapidly in the body, zebularine appears to be both stable in solution and when exposed to an acid environment like the stomach.
"We tested it intraperitoneally and orally," says Cheng, "and found that both routes of administration are equally effective in reducing tumor size in mice."
Preliminary studies in cultured human cells also seem to indicate that zebularine's effects are focused on tumor cells, rather than normal dividing cells. This may be good news in terms of the drugs side effects. "Even at a high dosage, it was very nontoxic in the mice," notes Jones, who added that the main problem experienced by the mice taking zebularine seemed to be weight loss.
Still, says Jones, it may be some time before zebularine is tested in humans. Right now, he said, "it's not very efficiently incorporated into DNA." Future studies will focus on trying to find ways to increase that efficiency.
The work was done in collaboration with Selker, Sheldon Greer of the University of Miami School of Medicine and Victor E. Marquez of the National Cancer Institute (NCI); it was supported by grants from the NCI.
Jonathen C. Cheng, Cindy B. Matsen, Felicidad A. Gonzales, Wei Ye, Sheldon Greer, Victor E. Marquez, Peter A. Jones, Eric U. Selker, "Inhibition of DNA Methylation and Reactivation of Silenced Genes by Zebularine." Journal of the National Cancer Institute, Vol. 95, No. 5, March 5, 2003.