Blood pressure medications, antidepressants, antipsychotics: these may seem like unlikely cancer fighters, but in the search for effective new treatments, researchers are casting a wider net in hopes of finding existing compounds that improve patients' chances of survival.
In addition to providing alternative treatments, repurposed—or repositioned—drugs, as they are called, can also help researchers understand how a particular disease works or identify new molecular targets that can lead to even more effective drugs.
Pharmaceutical and biotech companies, academic and research institutions, non-profit organizations, and the government all have a financial (and moral) stake in drug repurposing, especially for rare diseases.
Large-Scale Screening Turns Up New Drugs
Recently, a team led by researchers from the Dana-Farber Cancer Institute and Boston Children’s Hospital identified a 50-year-old antipsychotic as a potential therapy for a rare and aggressive type of cancer called T-cell acute lymphoblastic leukemia (T-ALL).
Researchers screened almost 5,000 compounds, including FDA-approved drugs whose patents had expired and drug-like molecules. They tested these compounds in zebrafish that had been engineered to function as a model for human T-ALL.
The researchers narrowed down the collection to a single compound, the antipsychotic perphenazine. Their study revealed that the drug works by activating a cancer-suppressing enzyme called PP2A that is turned off in cancer cells, a rare find in itself. The researchers also discovered that the drug’s antipsychotic and anti-tumor activities work separately.
This information about how perphenazine works could help the researchers find more effective second-generation drugs—in this case, molecules that are effective against T-ALL, but without the antipsychotic effects.
"T-ALL patients are often on the borderline between a long remission and a cure. If we can push the leukemia cells a little harder, we may get more patients who are actually cured. In this way, PP2A inhibitors may, in combination with other drugs, make a real difference for patients," said study author Thomas Look, M.D., in a press release.
Restoring Blood Flow to Tumors Lets Drugs Reach Their Target
In addition to large-scale screening of compounds, drug repurposing sometimes requires a solid understanding of how a disease affects the body. Researchers at Massachusetts General Hospital used this method, described in a paper published last year in Nature Communications, to find a new role for blood pressure drugs in improving cancer treatment.
When tumors grow in the body, the blood vessels that carry blood and oxygen to the cancer cells compress and collapse. Because many cancer-fighting drugs are delivered to the tumor through the bloodstream, this can limit their effectiveness.
By looking for drugs that could open the collapsed blood vessels and restore the flow of chemotherapy drugs to the tumor, researchers were able to narrow down their search to losartan, a blood pressure medication that has been used for more than a decade.
The key, they found, is to combine losartan with other anti-cancer drugs.
"Increasing tumor blood flow in the absence of anti-cancer drugs might actually accelerate tumor growth, but we believe that combining increased blood flow with chemotherapy, radiation therapy, or immunotherapy will have beneficial results," said Rakesh K. Jain, Ph.D., senior author of the study, in a press release.
A clinical trial of losartan for cancer treatment is currently underway.
Repurposing Drugs for 'Unmet Needs'
For pharmaceutical companies, repurposing their current line of approved drugs for new uses, or resurrecting drugs that never made it through the initial approval process, has a clear benefit for their bottom lines.
However, repurposing drugs also has the potential to fill a treatment void left behind by for-profit companies.
“Repurposing drugs is the most likely way for us to get treatments to patients where there are unmet needs,” says Dr. Bruce Bloom, President and Chief Science Officer of Cures Within Reach, a philanthropic organization focused on repurposing drugs and other treatments.
Unmet needs, explains Bloom, includes rare diseases, which have a very small market and limited earning potential for companies.
Many rare cancers exist. According to a 2010 Public Health Report by the National Institutes of Health, rare cancers—which involve fewer than 15 cases for every 100,000 people—accounted for 25 percent of all adult tumors.
Repurposed drugs can also provide relief where current treatments are effective, but are either too expensive or have many side effects. The latter is often the case with treatments for cancer in children.
“For pediatric cancer, a lot of the treatments that we give kids solves their pediatric cancer,” says Bloom, “but those patients have a much higher incidence of a future cancer than the general population.”
Repurposed Drugs Offer Safety and Speed
One of the driving forces behind drug repurposing is that it can be very cost-effective, especially when working with compounds that are already in use to treat other conditions.
Repurposed drugs that have made it through the Food and Drug Administration’s approval process have already been tested for safety, potential drug interactions, and side effects. This can speed up the drug discovery process and save money in the long run.
For Bloom, the advantage of repurposing drugs, compared with creating new ones, is clear.
“When you have two compounds, both likely to impact a disease in some way,” he says, “and one is a repurposed drug and the other is a new chemical entity, it just seems obvious that it makes sense to look at the repurposed drug first.”