Tamoxifen’s Surprising Second Act

AACP Article

Shiga toxin research at The University of Texas at Austin College of Pharmacy illustrates the benefits of repurposing existing treatments.

By Emily Jacobs

Although many cases of foodborne illness last a few days with no long-term effects, a small percentage can cause serious and even fatal complications. Shiga toxin-producing Escherichia coli (STEC) is responsible for many foodborne illnesses in the United States and other developed countries. Of those people diagnosed with STEC, up to 10 percent can develop life-threatening hemolytic uremic syndrome, a type of kidney failure.

Infections involving Shiga toxin-producing bacteria cause more than 1 million deaths around the world annually, with no current effective treatment. Complicating the issue is the fact that some antibiotics trigger STEC to release more toxin. However, new research suggests that the breast cancer drug tamoxifen may block Shiga toxins from invading human cells and causing disease.

Dr. Somshuvra Mukhopadhyay, assistant professor of pharmacology and toxicology at The University of Texas at Austin College of Pharmacy, has been leading research to better understand how Shiga toxins enter human cells. Research had already indicated that the toxin could be prevented from invading and destroying cells. The surprise for Mukhopadhyay’s team was tamoxifen’s efficacy at doing so.

Tamoxifen works against breast cancer by hindering the effects of estrogen and preventing cancerous cells from growing and replicating abnormally in the breast. Its actions on Shiga toxin, however, are entirely independent of anything related to estrogen. Instead, tamoxifen appears to work against Shiga toxin by changing the acidity of parts of the cell. “It's a totally different chemical activity,” said Mukhopadhyay. “It seems like tamoxifen is acting essentially as a base, [making] certain parts of the cells become more alkaline. So it's a very unexpected mechanism of action.”

Dr. Somshuvra MukhopadhyayIt’s a totally different chemical activity. It seems like tamoxifen is acting essentially as a base, [making] certain parts of the cells become more alkaline. So it’s a very unexpected mechanism of action.

Dr. Somshuvra Mukhopadhyay

Previous studies had suggested that changing the pH of compartments within human cells can affect the Shiga toxin’s ability to enter those cells. As part of their research, Mukhopadhyay’s team screened for drugs with this alkaline effect. Tamoxifen was included in their research due to a few lesser-known studies that had indicated its alkaline effects. Though not widely known for changing cell acidity, tamoxifen had the strongest alkaline effects among the drugs tested.

So far, tamoxifen has been effective against purified Shiga toxin that has been injected directly into mice. Researchers have not yet tested its effectiveness against Shiga toxin that has been produced by STEC in the body. This will require further animal testing, after which Mukhopadhyay hopes to see human trials relatively quickly. “If we can just show that tamoxifen is working against bacteria in the mice, essentially anybody could start prescribing tamoxifen off-label,” he said.

Benefits for Patients, Researchers and Students

These findings build on Mukhopadhyay’s previous research on Shiga toxin and its ability to destroy cells. This latest research also could point the way to new or improved treatments for other infectious diseases that resemble Shiga toxin in the way they interact with human cells. Similar diseases include cholera and pertussis, which cause up to 143,000 and 160,700 deaths per year, respectively.

Tamoxifen’s potential for treating STEC is also part of a broader effort to use existing drugs for different treatments. “It's really difficult to get a new small molecule or a new drug into the market because the FDA approval process is extremely difficult,” Mukhopadhyay pointed out. “So for academic labs, it's impossible. You have to have a collaboration within the industry.”

Additionally, many clinical trials for new treatments fail before completion. As a result, researchers are looking to shorten the process by repurposing known treatments for neurological diseases, infectious diseases or cancer. A previously approved medication that has already been found to be safe in humans removes the need for a phase I safety trial. This reduces costs for researchers and pharmaceutical companies and gets a repurposed, approved treatment to patients sooner.

Even before patients can take advantage of research like Mukhopadhyay’s, there are more immediate benefits. Whether it involves studying existing drugs or seeking novel treatments, laboratory work has an enormous impact on the student pharmacists who participate. They see real research in action, which may help them decide to continue their education to receive a Ph.D.

No matter which career path they choose, student pharmacists can continue to apply the knowledge they learn in the laboratory. Whether they practice as a community-based pharmacist, work for a pharmaceutical company or participate in university or government research, early lab experience helps them look at data, sources and conclusions with a careful, critical eye. “They learn how to think,” Mukhopadhyay noted. “That's probably the biggest thing that they get out of lab experience.”

Emily Jacobs is a freelance writer based in Toledo, Ohio.