The 2014 Ebola epidemic drew the world’s attention to the virus, which multiplies rapidly inside human tissues, destroying entire organs within days. Although antibodies have proved helpful in treating Ebola, no drugs block the virus. In many cases, all doctors can do is treat the symptoms and wait for the immune system to work. Now, researchers have engineered a group of small molecules that inhibit key cellular proteins, blocking Ebola’s ability to infect mouse cells and offering a new approach for battling the deadly virus (ACS Infect. Dis. 2016, DOI: 10.1021/acsinfecdis.5b00130).
To invade a cell, Ebola takes advantage of enzymes known as cathepsins that are part of the cell’s machinery for breaking down and recycling proteins. When a cathepsin cleaves an Ebola surface protein, it allows the virus to enter and infect the cell. Previous studies in mice had reported that even if the two main cathepsins in cells were blocked, Ebola could find alternate routes of infection using other cathepsins. So Matthew Bogyo of Stanford University and Kartik Chandran of Albert Einstein College of Medicine engineered a group of molecules that block a broad range of these enzymes.
The researchers began their work with a known cathepsin inhibitor named E64. Related compounds appear to be nontoxic and have been tested in humans for other diseases, but they proved ineffective in late-stage clinical trials—probably because they break down easily in the bloodstream and are relatively insoluble. The team wanted to see if they could solve these problems and find a drug effective against Ebola.
The researchers designed a library of 24 compounds by swapping out parts of the E64 structure with more stable and soluble components. For example, they replaced an ethyl ester group that’s unstable in blood with a more stable amide linkage. All 24 compounds retained E64’s central chemical groups, which bind cathepsins and block their activity.
The team engineered harmless viruses to carry coat proteins from Ebola and its relative, Marburg virus, and observed which compounds blocked these engineered strains from entering mouse cells in a culture dish. Three molecules were the most potent inhibitors, and all three effectively blocked infections caused by real Ebola and Marburg viruses in mouse cell lines.
Bogyo says that their collaborators, who conducted the experiments with the real virus, were “very excited” with the results. “They said these are some of the most potent things they’ve seen in terms of being able to shut down infection in a culture dish for authentic virus.”
The three top compounds all carry chemical groups that can acquire a positive charge easily. Once the molecules get into the acidic, intracellular compartments known as lysosomes, where cathepsins work and the virus acts, they acquire a positive charge that prevents them from diffusing out. This activity might explain their potency, according to Bogyo.
Their work is “very promising,” says Jonathan R. Lai, also of Albert Einstein College of Medicine, who was not involved in the study.
Lai added that compared with antibodies—currently the only available treatment option—small-molecule cathepsin inhibitors are likely to offer many advantages for large-scale use: They should be easy to stockpile without refrigeration, would work as oral pills, and would be much less expensive. In upcoming studies, the researchers plan to test whether these small molecules can inhibit Ebola infections in mice and other model animals.