The newly revealed structure of a cystic fibrosis drug, captured in tandem with its target channel protein, could help scientists design better medications for the condition.
Cystic fibrosis results from mutations in the gene for the cystic fibrosis transmembrane conductance regulator (CFTR), an ion channel that allows chloride ions to cross the cell membrane. While the most common mutation causes protein misfolding and degradation, such that CFTR is rarely found at the cell membrane, several other mutations result in a channel that doesn’t allow enough chloride through. The result is poor transit of chloride out of the cell, which leads to the production of a thick, sticky mucus that blocks breathing and promotes lung infections.
Although cystic fibrosis doesn’t have a cure, current medications help sufferers live into their 40s. One popular drug is ivacaftor. First approved in 2012 and extended this year to children as young as six months, the drug is of a class called potentiators: it encourages the faulty CFTR channel to stay open longer. Doctors often prescribe ivacaftor in conjunction with another class of medication called correctors, which help the mutant CFTR protein mature and reach the cell membrane. Used together, correctors and potentiators maximize the CFTR available to cells and promote channel opening, but still can’t fully restore channel activity. Researchers are after improvements.
Ivacaftor wasn’t so much designed as discovered: by screening for molecules that would improve channel function and chloride movement. “The drug just makes [chloride] flow better,” explains structural biophysicist Jue Chen of the Rockefeller University in New York City, NY. But exactly how the medicine achieved better flow wasn’t clear.
Hoping to better understand CFTR and learn how to make better potentiators, Chen and colleagues used cryoelectron microscopy to wrest a structure from the small, floppy protein bound to the drug, as they reported in Science. “CFTR as a molecule has difficulty folding, so biochemically you have to be very careful to make the protein stable,” she says. The researchers used a stable mutant of the protein in their study.
Ivacaftor squeezed into the hinge of the ion channel’s gate, interacting with the protein as well as the surrounding membrane. Chen surmises that this binding stabilizes the open-gate arrangement, like a stick crammed in the hinge of an open door. To check their work, the researchers mutated each amino acid in the drug-binding pocket, which limited ivacaftor binding to the protein. This confirmed their importance in the drug interaction.
“This paper definitely answers the question of where ivacaftor binds,” says Tzyh-Chang Hwang, a biophysicist at the University of Missouri School of Medicine in Columbia. Hwang was not involved in the study, though he did advise the researchers on some experimental techniques.
Chen’s group also examined the binding of a different potential cystic fibrosis drug, GLPG1837, which also promotes channel opening. It stuck to the same cleft. “We basically identify a hotspot on the molecule,” says Chen. Using this information, scientists can design new medications that bind at the same place, but might be more effective than current potentiators.
Ivacaftor already works well, but there is room for improvement, says Richard Moss, a pediatric pulmonologist at the Stanford Medical School in California who wasn’t involved in the study. The medication can interfere with the action of a common corrector medication, and the liver quickly metabolizes ivacaftor, so patients must take the medicine twice a day. If chemists could use Chen’s structure to improve ivacaftor’s half-life or potency, that would be beneficial, he says.
But Moss sees a greater need for improvements in corrector drugs, which would require structures of those bound to CFTR. Chen says she intends to analyze the channel bound to correctors as well. She also plans to check other potentiators, not yet on the market, to see if there are additional hotspots for drug action.
Potentiators that bind to different CFTR sites could work together to improve the channel’s function, notes Hwang. “Ivacaftor is very effective,” he says, “but it cannot fully restore the function of CFTR mutants.”