Life-threatening symptoms occur in patients with asthma, chronic obstructive pulmonary disease (COPD), cystic fibrosis (CF), and cancer-related lung diseases when mucin—a glycoprotein found in mucus—is rapidly and excessively secreted into airways, obstructing breathing. In the United States, about 25 million people have asthma, 16 million adults have COPD, and CF is the most common fatal genetic disease.

“Mucus is a significant problem in pulmonary medicine because, in people with these common lung diseases, thick mucus can block the airways and cause symptoms ranging from a mild cough to very serious decreases in lung function,” said Burton Dickey, MD, professor of pulmonary medicine at the University of Texas MD Anderson Cancer Center. “Most drugs for these conditions reduce inflammation or expand the airways to help people breathe better, but mucus is the most serious issue.”

An article published in Nature, titled, “Inhibition of calcium-triggered secretion by hydrocarbon-stapled peptides,” reports the development of a first-of-a-kind inhaled stapled peptide drug that blocks mucin secretion from epithelial cells that line air passages. The study shows the engineered drug, SP9, successfully enters airway epithelial cells and blocks ATP-induced mucin secretion in airway epithelial cells that are primed by the cytokine IL-13, in vitro and in mice. The study is a multicenter collaborative initiative among MD Anderson, Stanford Medicine, and Ulm University.

“Our research has created the first drug that would stop the secretion of mucins in its tracks,” said Dickey, who is co-corresponding author of the research article. “An inhaled drug like this could help someone during an acute attack of airway disease by stopping the rapid secretion of mucin and, by extension, avoiding the production of thick mucus. You can’t move air through an airway that’s plugged. In asthma, COPD, and CF, it’s been shown that persistent plugs drive the most serious disease. Now we have a drug that could be very important if it’s shown to work in clinical trials.”

Mucin enters the airways when membrane-bound secretory granules carrying mucin fuse with the membranes of cells lining airways, releasing their cargo into the duct. This fusion occurs when the protein Syt2 (synaptotagmin-2), present on mucin-containing secretory granules, binds to free calcium ions and SNARE proteins on the inner surface of cell membranes to form a SNARE complex.

This mechanism was uncovered in earlier studies by Dickey’s team. “We built up a picture of what the secretory machinery looked like, and we knew all of the major players,” said Dickey.

The biological benefit of mucin release into airducts is protection from pathogens and dust particles. Mucin released into airways absorb water to coat airways with a thin layer of mucus that traps particulate matter in the air that is then cleared away by the sweeping action of hair-like cilia on the cell surface. In muco-obstructive lung diseases, mucin secretion is excessive resulting in thick mucus that plugs airways impairs lung function.

Dickey said, “Once we had an idea of how all the pieces worked together, we determined Syt2 was the best protein to target to block mucin secretion because it only becomes activated with a high level of stimulation. Therefore, blocking the activity of Syt2 should prevent sudden massive mucin release without impairing slow, steady baseline mucin secretion that is required for airway health.” The researchers verified Syt2 as a viable therapeutic target protein in different preclinical models.

Philip Jones, PhD, vice president of the therapeutics discovery division and head of the institute for applied cancer science at MD Anderson Cancer Center, designed a hydrocarbon-stapled peptide, SP9, to block Syt2, based on structures developed by senior co-corresponding author Axel Brunger, PhD, professor of molecular and cellular physiology and other collaborators at Stanford University.

Stapled peptides are a new therapeutic modality where modified amino acids bind with increased stability to a target protein due to hydrocarbon cross bridges that maintain the rigidity of the structure. SP9, once approved, would be the first stapled peptide to be used as an inhaled therapeutic.

To generate SP9, the researchers used structural insights on key amino acids in Syt2 to block calcium-induced granule formation. They also designed a peptide tail in SP9 that penetrates through the cell membrane and facilitates the drug’s entry into cells.

Ying Lai, PhD, a postdoctoral associate in Brunger’s laboratory, successfully disrupted calcium triggered membrane fusion in a reconstituted model using SP9. Manfred Frick, PhD, and his team at Ulm University used SP9 conjugated to a cell-penetrating peptide to block rapid mucin secretion in cultured epithelial cells.

Dickey’s team used aerosolized SP9 and confirmed that it blocked the rapid-release pathway to reduce mucin secretion and airway blockage in a mouse model. They also showed SP9 did not affect the slow-release pathway for normal mucin secretion.

Whether SP9 could be administered in doses adequate to prevent mucus-associated disease without impairing the formation of protective mucus needs further research.