Previous work has shown that fibres and synapses can be coaxed to regrow by natural signalling molecules called neurotrophins. In the ear, focus has turned to one called neurotrophin-3, which normally promotes synapse formation in the ear in developing embryos, says Gabriel Corfas at the University of Michigan.
One hurdle is that neurotrophins are large protein molecules, which makes them hard to deliver to the inner ear, but there are possible workarounds. Corfas and his colleagues have used gene therapies to boost levels of neurotrophin-3 in the inner ears of mice, which leads to synapse growth. In one experiment, they were even able to give the animals more synapses than normal, which gave them superior hearing.
Niliksha Gunewardene at the Bionics Institute in Melbourne, Australia, and her colleagues have recently shown that in animals, an injection of neurotrophin-3-loaded nanoparticles into the ear releases the compound for several weeks.
Further along is a French firm called Cilcare, which has several drugs in the works that trigger production of multiple neurotrophins. Its lead compound — which Cilcare isn't yet identifying publicly — has already been shown to be safe in people when given orally for a different medical condition. The firm is due to start a trial next year in people with hidden hearing loss, with or without tinnitus.