The next dimension in drug testing
Researchers from The Scripps Research Institute (TSRI) believe they have discovered a new way to screen potential cancer drugs for efficacy. By utilizing 3D spheroids instead of 2D samples tested flat on a plate, researchers are able to more closely mimic how drugs would interact with a tumor in the body. Outlining their findings in an article recently appearing in Oncogene, the authors are hopeful the new methodology will allow for increased high-throughput testing on a broad library of drugs seeking new applications.
“Until now, most of the research to screen for cancer drugs has used cells that are growing flat on a plate,” says Louis Scampavia, director of high-throughput screening (HTS) chemistry and technologies at TSRI and one of the study’s co-authors. “With these 3D spheroids, we emulate much more closely what’s found in living tissues.”
Using a form of 3D bioprinting, researchers can create spheroids ranging from 100 to 600 μm in diameter—equivalent to the thickness of a few sheets of paper. In contrast to 2D layers of cells normally used to screen for drugs, which tend to all grow at the same rate because they get the same exposure to oxygen and nutrients, in the spheroids some cells are on the outside and some are on the inside.
“We are agnostic about which drugs to test,” says Timothy Spicer, director of lead identification discovery biology and HTS at Scripps Research. “Our goal is high throughput at a low cost and the direction it takes us doesn’t matter. We are starting with known standard-of-care treatments, but this methodology allows us to test everything.”
The team began screening an existing library of FDA-approved drugs with a phenotypic screen, meaning they were testing every drug they could without a preconceived idea of how it might affect the culture. In their early rounds of testing, they focused on RAS proteins, which control cellular signaling pathways responsible for growth, migration and adhesion. Mutations including the cancer-driving KRAS are found in 30 percent of all cancers and in up to 90 percent of pancreatic cancers. Scientists took pancreatic cell lines and tested a variety of drugs, only looking for something that impacted cell growth pathways, not something that would impact KRAS specifically.
“In the past, KRAS has been a very tricky protein to target. People have spent several decades trying, but so far there has been little success,” commented Dr. Joseph Kissil, a professor at Scripps Research Medicine and the other co-corresponding author for this paper. “The KRAS protein is relatively small, and that’s made it hard to attack it directly. But the method of screening that we used in this study allowed us to come at the question in a different way.”
Researchers discovered that a compound called Proscillardin A, used in some drugs that target heart disease, also served as a selective inhibitor of KRAS—a finding that would not have been possible using just a 2D culturing method.
“From our perspective, this is a proof-of-principle study,” Kissil adds. “It shows you can look at libraries of drugs that have already been approved for other diseases and find drugs that may also work for cancer. In theory, you could use this screening method for any line of cancer cells and any mutation you want.”
According to Spicer, the promise of this finding is just beginning. Each drug effect discovered raises a series of questions and potential avenues for further research. Why does a drug impact a 3D model but not a 2D model? What is the mode of action of specific inhibitors? Can this methodology be used to test for drug toxicity as well as efficacy?
“We would love to use this research to create a pipeline for new oncology drugs,” he notes. “We can use this miniaturization and automation to develop new drug cocktails to treat cancer. Even taking a translational approach, if we can improve on the small molecule work that is already happening, it will be a success and will contribute to precision oncology applications. We are just getting going.”