Sphingosine Kinase Inhibitors May Hold Key To Halting Growth Of Some Tumors

"These compounds our team found are the first 'drug-like' agents that have been shown to inhibit an enzyme called sphingosine kinase," said Charles D. Smith, Ph.D., professor of pharmacology, and director of the Drug Discovery Core, Penn State College of Medicine. "Since sphingosine kinase is involved in growth regulation and certain other biological processes that are important in tumor growth, these compounds have potential use for the treatment of many types of cancer."

This study, titled, "Discovery and evaluation of inhibitors of human sphingosine kinase," appeared in the Sept. 15 issue of Cancer Research and was recently presented at two international scientific meetings. Smith's research team in the Department of Pharmacology included: Kevin J. French, Ph.D., Randy Schrecengost, Brian D. Lee, Yan Zhuang, Ph.D., Staci N. Smith, Justin L. Eberly, and Jong K. Yun, Ph.D., of the Jake Gittlen Cancer Research Institute.

Previous studies have shown that sphingosine kinase (SK) plays a pivotal role in regulating cell growth. Cell membranes contain sphingomyelin, a precursor of two lipids: ceramide, which causes programmed cell death (apoptosis), and sphingosine 1-phosphate (S1P), which causes cell proliferation. The balance of ceramide and S1P determine whether cells multiply or die.

A chain reaction with other enzymes can turn ceramide into sphingosine, which then reacts with SK to form S1P. This promotes cell proliferation, and stops the programmed cell death that would otherwise rid the body of the cancer cells. This study aimed to find a way to stop that chain reaction and create an effective option to treat cancer.

In this project, the Smith team first determined that the amount of mRNA for SK is significantly higher in tumor cells than it is in healthy cells. mRNA is genetic material that holds the information necessary to make SK protein and many other substances. For example, levels of mRNA that produce SK protein were two-fold higher in breast tumors than in healthy tissue. These results suggest that SK protein is important in promoting tumor growth and/or survival, and so may be an excellent target for new anticancer drugs.

Then the group screened about 16,000 compounds searching for inhibitors of SK. Four types of compounds were found to be very effective and were more potent than any other previously described SK inhibitors.

"These compounds are anti-proliferative in small concentrations and are effective against tumor cells that have shown resistance to other cancer therapies," Smith said. "Additionally, all of the compounds cause the tumor cells to undergo apoptosis. These effects are consistent with the hypothesized consequence of reducing S1P levels."

It was then necessary to determine if the new SK inhibitors do in fact promote anti-tumor activity in an intact animal. The group synthesized a variant of one of the inhibitors, and tested its effects on tumors growing in mice (following the procedures and protocols approved by the Animal Care and Use Committee of Penn State College of Medicine.) These studies showed that tumors in mice that were treated with the SK inhibitor had between 50 percent and 85 percent less growth than tumors in untreated animals. Importantly, there were no significant differences in the body weights of those animals treated with the SK inhibitor, suggesting that it was not toxic to the animals. Body weight decrease is one of the first ways an investigator might recognize toxicity of a new compound.

"Overall, these studies demonstrate that halting S1P production by targeting sphingosine kinase will stop tumor growth and cause the tumor cells to die," Smith said. "The compounds we have identified show promise for further development, and may lead to new targeted drugs for our arsenal to fight cancer."

This research was supported by a grant from the National Institutes of Health. Penn State University is pursuing patents for this technology.