Researchers have created a molecular “cage” that keeps protein-degrading drug molecules inactive until they are exposed to low-energy red light. The technology, called RuPHOTACs, controls when and where these molecules become active, which could help scientists develop treatments that destroy specific proteins while limiting unwanted effects elsewhere.
Protein-degrading technology represents a potentially transformative approach to drug therapies. PROTACs, or PROteolysis-TArgeting Chimeras, work by simultaneously binding to a protein of interest and an enzyme that “tags” that protein for degradation via the natural waste removal processes within cells. These molecules are dubbed “chimeras” because they bind to and bring together two components: the disease-associated protein, and the tag-applying enzyme.
“A PROTAC is a catalyst. As soon as it is finished marking one protein for destruction, it finds another partner and binds, resulting in a much stronger sustained effect. With a PROTAC, you’re not just inhibiting the cancer-driving protein, you’re removing it,” said Edith C. (Phoebe) Glazer, professor of chemistry at North Carolina State University and corresponding author of the study.
However, the main limitation of PROTACs is their effect on surrounding tissue. Many proteins that contribute to cancer and other diseases perform important functions in healthy cells, too. A drug that destroys these proteins throughout the body could cause unwanted side effects.
“Let’s say you’re focused on cancer treatment,” Glazer said. “The problem is there are very few targets – like aberrant proteins – that are only found in cancer. Most of the time, the proteins are important for our healthy tissues too. So we need a ‘cage;’ that is, a way to control when and where these PROTACs are activated.”
Glazer and her colleagues called the cage they developed a RuPHOTAC, or ruthenium-based PHOToActivated Chimera.
Ruthenium, a metal in the platinum group of the periodic table, has properties that make it useful in light-driven processes and chemical reactions. By carefully designing the ruthenium complex, the researchers created a molecular cage that responds to relatively low-energy red light, which penetrates biological tissue effectively.
“Think of the ruthenium as a beach ball that blocks the two proteins from being able to approach each other,” Glazer said. “When we remove the ruthenium by exposing it to a low-energy red light, the PROTAC can get to work.”
The researchers tested the RuPHOTAC both in vitro and in vivo in a zebrafish model. They also demonstrated a new approach to monitoring successful protein destruction by putting fluorescent tags on the target proteins of interest and measuring their numbers after uncaging the RuPHOTAC.
They found that overall, the RuPHOTAC was both non-toxic in the fish model and was effective in decreasing numbers of targeted proteins in specific areas when exposed to low-energy red light. The researchers note, however, that the penetration of the light works best on lighter pigmented tissues.
“This is a proof-of-concept study that we hope will lead to finer control over PROTAC technology,” Glazer said. “The ability to turn on protein degradation precisely where and when it is needed could ultimately help address one of the major challenges in developing targeted medicines for various diseases.”
The research appears in Nature Communications and received the majority of its financial support from the Kentucky Cabinet for Health and Family Services under Grant 1000200168. The work also received support from the National Institutes of Health under grant R01GM162746, which represents approximately 10% of the overall funding for this project. Other NC State contributors include first author and Research Scholar Dmytro Havrylyuk, graduate student Ainsley P. LaMore, and Research Professor David K. Heidary. Graduate student Majd A. Al-Hamaly and Associate Professor of Molecular and Cellular Biochemistry Jessica S. Blackburn, both from the University of Kentucky, also contributed to the work.
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