An experimental drug that has been tested as a possible treatment for Alzheimer's disease shows promise for its potential to counteract  severe hearing loss, an impairment that affects more than 250 million people worldwide.

According to a study published in Neuron, the compound, known as LY-411575, significantly improved the hearing of mice deafened by loud noise. The molecule works by allowing new hair cells to grow inside the inner ear. These cells, in turn, play a key role in the brain’s ability to interpret and understand sound.

"It is possible to regenerate hair cells, and that's something that had not been possible before," says Albert Edge, an ear specialist at Harvard Medical School, who led the study.

The ability to hear depends on a tiny snail-shaped organ called the cochlea, which is found in the inner ear. Hair cells within the cochlea turn sound-wave variationsinto neurological signals. These cells, however, can be easily damaged by harmful viruses or loud noises—and once destroyed, they will not naturally regenerate. Neighboring cells called supporting cells have the potential to become hair cells, but their transformation is blocked by certain biochemical signals.

Edge targeted those inhibitory signals with LY-411575, applying it directly to the inner ears of his deafened mice. After three months researchers measured an estimated 20 percent increase in auditory hair cells by using a genetic color-coding system to track new growth. The mice were newly able to detect sounds at low frequencies—the equivalent of a car passing by or a door closing.

"Most of deafness is because of lost hair cells," says Elizabeth Olson, an auditory biophysicist at Columbia University. 

"It's very convincing science," she adds. "They've really discovered something."

Despite such positive results, LY-411575 is not ready for clinical trials in humans, Edge cautions. The compound is not medically or economically superior to current treatments for hearing loss, such as cochlear implants. And although LY-411575’s potential for improving deafness is significant, it may have unknown side effects.

Now that the preliminary work is complete, Edge says he must further refine his research by finding other inhibiting signals and new drugs that might counteract them. Scientists don't understand why mammals are unable to naturally regenerate ear hair cells, but many theorize that supporting cells are essential components of the hearing process—remove supporting cells and risk reducing the quality of sound that humans can perceive.

Still, in a case of quantity over quality, Olson says, something may be better than nothing. She likens the approach to playing a symphony.

"If half your orchestra dies off, you'd be happy to get another instrument," she says. "You need something there."