Ataluren’s proposed mechanism of action

PTC Therapeutics

A drug in clinical trials for muscular dystrophy and cystic fibrosis might not work through the molecular mechanism that scientists think it does. Although the new findings do not cast doubt on the clinical efficacy of the medication, the new experiments suggest that researchers might need to double-check that therapeutics believed to rescue normal protein production by helping parts of the cell ‘read through’ the genetic sequences actually do just that.

The drug in question, ataluren, was developed by New Jersey’s PTC Therapeutics to treat illnesses caused by nonsense mutations such as Duchenne muscular dystrophy, a disease in which disrupted production of dystrophin, a protein critical to muscle structure and stability, leads to progressive muscle deterioration. The drug has also shown clinical benefit in the 10% of cystic fibrosis patients with a mutation in the CFTR gene that causes truncated proteins and has potential for treating other ailments, including hemophilia.

Overall, about 11% of genetic mutations found in inherited diseases involve changes in the code of messenger RNA that prematurely stop protein translation to produce a dysfunctional or nonexistent protein product. But ataluren, formerly known as PTC124, was thought to work by cozying up to parts of the cell known as ribosomes and allowing them to ‘read through’ these errors and continue to make a normal protein.

The developers of ataluren had conducted experiments in human embryonic kidney cells in which the drug succeeded in reading over a mutation and restoring normal production of the enzyme firefly luciferase (FLuc), which causes cells to glow. However, two subsequent reports found evidence that ataluren boosted production of FLuc without assisting the read-through of the code for the enzyme.

In a new study published today in PLoS Biology, researchers ran additional experiments and came across more data placing the read-through mechanism in doubt. Stuart McElroy, a molecular biologist at the Drug Discovery Unit at the University of Dundee, UK, and his team tested ataluren against G418 activity, a well-documented antibiotic read-through agent, in four non-luciferase assays and found in each case that G418 rescued production of proteins but ataluren did not. Researchers also manipulated the sequence of nonsense mutations in another 12 assays and, again, found no measureable effect of read-through activity with ataluren.

McElroy says that the finding underscores the need to use different tests when trying to affirm the mechanism by which a drug works. “I think it is extremely important to have a robust and diverse array of quantitative assays when developing compounds against complex targets such as translational read-through,” he says.

Now you ‘c’ it

Although ataluren did not appear to assist in the read-through of mutations in the new study, McElroy notes that protein translation was tested with cDNA and it may be that the drug’s mechanism works better in vivo where cells use messenger RNA. Also, other scientists have confirmed efficacy of the drug in read-through experiments that produced CFTR, a protein needed for normal chloride ion transport by cystic fibrosis patients, and dystrophin. Notably, Ataluren has shown clinical benefit in phase 2 trials for both cystic fibrosis and muscular dystrophy.

PTC Therapeutics still hopes to collect more phase 3 data of ataluren in people with cystic fibrosis, and is now actively recruiting for phase 3 clinical trials for patients with Duchenne muscular dystrophy. If the drug is ultimately approved for one of these indications, it wouldn’t be the first therapeutic on the shelves with an unknown molecular mechanism. “Prednisone is the current standard of care drug for Duchenne muscular dystrophy and we don’t actually know why that works,” says Jane Larkindale, vice president of research at the Muscular Dystrophy Association, an Arizona-based organization that funds some of PTC Therapeutics work.

Before moving forward with clinical trials, it would be ideal to know the exact molecular action that drives the clinical benefits credited to ataluren. “But at the end of the day, the human in vivo data trumps everything,” says Richard Boucher director of the Cystic Fibrosis Center at the University of North Carolina at Chapel Hill.