It was the late 1980s, and Boston College undergraduate John Brigande was exhausted. It wasn’t because he had been partying too much or because he had pulled too many all-nighters. He was wearing himself out simply trying to hear. “A hearing-impaired person burns an enormous amount of energy working to understand the meaning behind sounds in their environment, especially voices,” says Brigande, who started losing his hearing by age nine. “At the end of the day, I was totally spent doing the work to hear.”
By the time he started graduate research, also at Boston College, the budding developmental biologist had lost much of his hearing in his left ear, but he was able to get by thanks to the amplified sounds emitted by a hearing aid he wore in his right ear. Over the next several years, as the hearing in his right ear waned as well, he elected to focus his research on the auditory system, studying chick and mouse inner ears. In addition to relating to his own experience as a hearing-impaired person, he figured this line of research would engage him with a group of researchers likely to be attuned to his condition. “That would be the best group for me to think about my developmental questions, but also for me to communicate with,” he says.
Now an associate professor at Oregon Health & Science University (OHSU) in Portland, Brigande works to identify genes involved in development of the mammalian inner ear and to prevent congenital genetic hearing loss by reprogramming cells during embryonic development in mouse models of human hearing loss. He feels that his experiences give him a deeper connection to the disorders he hopes to treat. “I live the life of the patient,” he says.
Unfortunately, while the hearing field has made great strides over the last several decades in understanding the biology of the inner ear and the causes of hearing loss, there are still no approved drugs to treat the condition. Rather, the 360 million people worldwide who suffer disabling hearing loss rely on imperfect devices in order to hear. Brigande, for example, uses a hearing aid in his right ear paired with microphones that deliver sound straight to the device. But hearing aids can be tough to tune, sometimes emit painfully loud sounds, and only work for those patients who retain at least some functional hair cells—the sensory cells of the ear that translate sound into nerve impulses. The other main option, the cochlear implant, bypasses the inner-ear hair cells by directly exciting the neurons that carry auditory signals to the brain. Cochlear implants have accomplished the remarkable feat of allowing deaf people to understand human speech and more confidently navigate their environments. But patients with cochlear implants attest that the experience is unlike natural hearing, consisting of buzzing or staticky sounds. And for some, as was the case for Brigande’s left ear, a cochlear implant may not work at all, perhaps because of auditory nerve deterioration that can occur after years without stimulation.
Better solutions may be on the horizon, however. Researchers are now using their knowledge of the ear’s biology to develop drugs and therapies that could rebuild hair cells and even, someday, auditory nerves. “I think that it’s an extremely exciting time to be involved in regenerative medicine,” says Brigande. “There’s much deeper knowledge of the genes that are important or even required to specify sensory hair cell identity and cochlear function.” (See “Inner Ear Cartography.”) Other groups are working to protect the inner ear’s delicate cells from damage before hearing begins to wane.
While many of these therapies are still at the earliest stages of development, clinical testing is underway for at least half a dozen small-molecule and gene therapies that may prevent hearing loss or even reverse it to some degree. And more than a dozen small biotechs, along with pharma giants such as Novartis, Eli Lilly, and Pfizer, are now active in the area.
“I think we will probably get some [hearing loss] drugs that are hitting the market within the next decade,” says Stefan Heller, a developmental neurobiologist at Stanford University. “The question is, ‘Who will benefit from these drugs?’ I think we cannot really tell.”
Staving off hearing loss
Switzerland-based biotech Auris Medical is preparing for two Phase 3 clinical trials that will test a drug called AM-111 for the treatment of sudden hearing loss within three days of its onset. AM-111 is made of a cross-membrane transporter linked to a synthetic peptide that inhibits an enzyme called JNK stress kinase, which contributes to inflammation and apoptosis in hair cells and neurons of the inner ear when they are stressed by loud sounds, loss of blood flow, infection, or toxic chemicals. Researchers inject the drug as a gel into the middle ear, where it diffuses into the cochlea and enters dying cells. “We actually allow the cochlear hair cells and cochlear neurons to recover from [insults] or to remain protected,” says Thomas Meyer, Auris’s CEO.
Meanwhile, a recent Phase 3 trial by North Carolina–based Fennec Pharmaceuticals indicated that sodium thiosulfate can safely and successfully neutralize harmful metabolites of cisplatin, to prevent loss of hair cells and cochlear neurons in young cancer patients being treated with the chemotherapeutic agent. The company is now running a second Phase 3 trial. And at Sound Pharmaceuticals in Seattle, Washington, researchers are eyeing yet another step on the pathway to hair-cell damage: attack by reactive oxygen species during times of stress. Different forms of the small molecule ebselen are in Phase 2 clinical development for protection of hearing in people at risk of exposure to loud noises, as well as for patients receiving cisplatin or related chemotherapeutic agents. The company also hopes to prevent hearing loss in patients exposed to aminoglycosides, a class of antibiotics—commonly taken by cystic fibrosis sufferers, among others—that also damages hair cells.
“We are not targeting . . . chronic hearing loss, but we are coming in here with the potentially first-in-class treatment for acute hearing loss,” Meyer says of the work ongoing at his own company. “If left untreated, that acute hearing loss will become chronic.”
Repairing hair cells
Others are working to rebuild hair cells entirely. In 1988, hearing scientists Ed Rubel of the University of Washington and Brenda Ryals, now at James Madison University in Harrisonburg, Virginia, discovered that adult birds can regrow their inner-ear hair cells following damage.2 This was the first time any adult vertebrate had been shown to regenerate hair cells after damage, but it soon became clear that many nonmammalian species regenerate hair cells, either directly converting supporting cells of the inner ear into sensory hair cells or generating hair cells as supporting cells divide.
The prevalence of regeneration in the animal kingdom “leads me to think that regeneration maybe was the default condition in the inner ear and that somehow that regenerative ability has been lost in mammals,” says Mark Warchol, a neurobiologist at Washington University School of Medicine in St. Louis. “Rather than ask, ‘Why do birds regenerate?’, I think the more productive question would be, ‘Why do mammals not regenerate?’ ”
One mechanism researchers have fingered as preventing the conversion of supporting cells into hair cells in mammals is Notch signaling. In 2013, Albert Edge, who studies cellular repair in the nervous system at Harvard Medical School and Massachusetts Eye and Ear Infirmary, demonstrated that blocking Notch signaling with a γ-secretase inhibitor caused regeneration of hair cells and restored responses to sound in adult mice whose hearing had been damaged by loud noises.3 Based on Edge’s work, Amsterdam–based Audion Therapeutics is developing a drug to inhibit Notch signaling. Eli Lilly, which developed the γ-secretase inhibitor used by Edge, is supporting Audion’s program. “I think a good thing with the γ-secretase Notch approach is that this is a known drug target,” says Rolf Jan Rutten, Audion cofounder and CEO. “There are molecules available that target this mechanism.”
Components of the Notch pathway can also be manipulated by gene therapy. Last year, Novartis began a Phase 1/2 trial to treat hearing loss and balance problems by transfecting cells in the inner ear with the gene for transcription factor Atoh1, which spurs hair-cell regeneration in birds and regulates hair-cell development in mammalian embryos when Notch signaling is inhibited. Novartis’s therapy involves injecting an adenovirus vector into the inner ear via a hole surgically drilled in the footplate of the stapes—one of the three tiny bones that amplify sound vibrations in the middle ear. The viruses carry ATOH1 into cells throughout the inner ear, along with a promoter that confines ATOH1 expression to the supporting cells. At the University of Kansas Medical Center, Hinrich Staecker has performed the surgery on five people with severe-to-profound hearing loss. A sixth patient underwent the procedure at Johns Hopkins School of Medicine. The participants are now being monitored for changes in balance, hearing, and neural activity in a brain region involved in the auditory pathway. But some researchers wonder if turning on just a single gene will be enough to trigger clinically significant hair-cell regeneration. “In mammalian research studies, there’s some evidence that simply overexpressing Atoh1 is not going to have potent and lasting effects on hair cells or restoring function,” says Jennifer Stone, who studies hair cell development and regeneration at the University of Washington.
“The story that is emerging is that there is a complex interplay between multiple signaling pathways that are required to specify cell fate and to differentiate cells into fully functioning inner and outer hair cells or vestibular hair cells,” Brigande says. “To have exquisite control, temporally and spatially, multiple therapeutic genes [will likely be necessary] to establish functional auditory recovery that’s persistent.”
To identify more genes involved in hair-cell regeneration, the Hearing Health Foundation in 2011 launched the Hearing Restoration Project (HRP), whose participants include Stone, Brigande, Rubel, Heller, Edge, and Warchol, among others. In the initial phase of the project, HRP researchers sequenced the RNA of the regenerating hair cells of chickens and zebrafish, as well as the RNA of nonregenerating mouse hair cells following injury, hoping to reveal a clearer picture of which genes are turned on and off during regeneration. In the next phase of the project, the researchers will test the key genes they identify to see which aid regeneration, then screen for small molecules that modulate the expression of these regenerative genes.
Even if human hair cells can be regenerated, however, it remains to be seen how much hearing might return. Regenerated hair cells would need to grow delicate stereocilia to respond to sound and may only be functional if they arrange themselves in a precise configuration within the cochlea and properly connect to the auditory neurons. (See “Hurdles for Hearing Restoration.”) Moreover, the mass of the regenerated cells could affect how the inner ear vibrates in response to sound. “The cochlea may be sufficiently precise that adding in a couple of new hair cells would screw things up,” says HRP head Peter Barr-Gillespie, who studies mechanotransduction by hair cells at OHSU.
“We shouldn’t expect a magic treatment that immediately restores everything to high fidelity,” says Heller. “It will be stepwise. I will be happy if in my lifetime we get something that is as effective as a cochlear implant, and provides [natural] hearing to profoundly deaf patients.”
Beyond hair cells
Housley hopes to make cochlear implant–assisted hearing richer by delivering nerve growth factors to the spiral ganglion neurons. In guinea pigs, for instance, he and his colleagues have pumped copies of the neurotrophin gene BDNF into the cochlea, then surgically installed cochlear implant electrode arrays into the animals. Using the electrodes, the researchers delivered jolts of electricity to the cochlea, coaxing nearby support cells to take up the DNA.1 These cells then produce the protein BDNF, luring spiral ganglion neurons to grow towards them—and the electrodes. In this way, the team aims to create a more intimate relationship between surviving neurons of the inner ear and the electrodes the cochlear implant uses to stimulate neurons and generate the perception of sound. Specifically, by linking each of the implant’s electrodes with a smaller group of neurons, the researchers hope to replicate natural hearing better than current devices that stimulate large numbers of neurons at once. “We are very hopeful that with optimization of the cochlear implant, we can perhaps improve pitch perception, which is a major challenge for cochlear implants,” says Housley, who has partnered with the implant maker Cochlear to test the gene therapy in a small group of patients in Sydney, Australia, next year.
Meanwhile, other groups of researchers aim to more precisely stimulate neurons in the inner ear by targeting them with light rather than electricity. While electricity from a cochlear implant scatters as it travels through the fluid-filled cochlea, light can shoot through fluids with minimal scattering. Tobias Moser at the University of Göttingen in Germany, for example, has engineered neurons of the inner ears of deaf rodents to express the light-sensitive protein channelrhodopsin-2, resulting in animals that show activity in the auditory brain stem in response to light stimulation.2 Alternatively, infrared light can excite cochlear neurons without any genetic engineering, possibly by locally heating the fluid in different parts of the cochlea and depolarizing the membranes of neurons, leading to a change in charge.
Claus-Peter Richter, who develops novel cochlear implants at Northwestern University’s Feinberg School of Medicine, has implanted cochlear devices that use infrared light into the ears of cats. The cats appeared to respond to the stimulus, although it’s unclear whether it produced the sensation of hearing or stimulated some other type of sensory perception.3 Richter, who hopes to produce a prototype for humans by the end of next year, says that while current cochlear implants provide fewer than half a dozen independent frequency channels, such infrared implants could theoretically target many more channels. “That would be very beneficial for understanding speech in noise or having an appreciation of music.”
Scientists once assumed that loud noises primarily damage hair cells. But in 2009, Charles Liberman and Sharon Kujawa at Harvard Medical School and Massachusetts Eye and Ear Infirmary demonstrated that mice exposed to two hours of intense, high-pitched sound can suffer damage to the synapses that link hair cells in the inner ear with the spiral ganglion neurons that relay the signal to the brain.4 Specifically, the tips of the spiral ganglion neurons degenerate following exposure to loud noise, possibly as a result of excitotoxicity, a process by which nerve cells are poisoned by excess exposure to the neurotransmitter glutamate. This degeneration can translate to hearing loss even in the absence of damage to the hair cells themselves.
On the bright side, it appears that the spiral ganglion neurons often retain their cell bodies and their processes projecting to the brain, suggesting that all it would take to restore hearing in that case would be to close the less-than-1-millimeter gap between the hair cells and neurons. Last year, Liberman and his colleagues achieved just that in mice by boosting levels of a protein called neurotrophin-3 (Ntf-3), which stimulates and guides neuronal growth. Mice lacking Ntf-3 from supporting cells had fewer synaptic connections between neurons and hair cells than wild-type mice, while mice engineered to overexpress Ntf-3 showed regrowth of synapses after noise damage.5
Liberman hopes to eventually develop a gene therapy to deliver genes for neurotrophin to the human inner ear, find a small molecule that boosts neurotrophin levels, or apply neurotrophin proteins themselves to the ear. This may help prevent and repair synapse degeneration in people exposed to bomb blasts, for example. More ambitiously, Liberman wonders if neurotrophins could restore synapses to millions of elderly people with age-related hearing loss. “If a lot of people are walking around with reasonable hair cell populations but half their neurons are gone, and if you could partially or totally reverse that, that would be huge,” Liberman says.
Hearing loss may also have roots beyond the ear entirely, in the auditory processing regions of the brain. U.K.-based biotech Autifony Therapeutics aims to treat age-related hearing loss with an oral drug that crosses the blood-brain barrier and modulates the firing of neurons deep in the auditory cortex and other brain regions in the auditory pathway. Dubbed AUT00063, the drug modulates Kv3 potassium channels that decline in number and likely in function with age. Autifony is now recruiting for a Phase 2 clinical trial called CLARITY-1 that will test AUT00063 in elderly people with mild to moderate hearing loss.
“These ion channels basically are responsible for enabling certain kinds of neurons to fire very rapidly and very precisely,” explains Barbara Domayne-Hayman, Autifony’s chief business officer. “Speech is a series of rapid, transient sounds. If your neurons are not firing rapidly and precisely enough, you’re going to be missing key elements.” (Autifony is also testing AUT00063 as a treatment for tinnitus, a persistent ringing in the ears. See “The Sounds of Silence” here.)
Hearing the future
The final frontier of restoring lost hearing is stem cell therapy. Researchers hope to differentiate stem cells into new hair cells and spiral ganglion neurons and implant them in the inner ear, replacing damaged or degenerated cells. While many therapies are geared towards regenerating structures in the recently deafened, stem cell therapy could theoretically help people who never developed proper inner ear structures at all, or whose cells are completely degenerated. “Conceptually, you could rebuild the whole organ, if you have the technology,” says Marcelo Rivolta, who studies stem cell therapies at the University of Sheffield in the U.K.
Rivolta’s lab began by studying stem cells collected from the inner ears of human fetuses, unraveling which pathways direct the differentiation of auditory neurons and hair cells. Using this knowledge, the team has successfully differentiated human embryonic stem cells (hESCs) into hair cells and spiral ganglion neurons. Transplanting hESC-derived inner ear–cell progenitors into gerbils with degenerated auditory nerves, the researchers found that the cells differentiated and took root in the animals, which showed improved sensitivity to sound.6
It will be a few years at least before Rivolta and other groups working on cellular approaches to hearing loss will test such therapies in humans, however. For now, hearing researchers agree that it’s best to hedge their bets and continue studying any possible solution that could restore hearing. “If you haven’t solved the problem, then not being pluralistic is a real mistake,” says Rubel.
“At the end of the day, we are not going to have one treatment . . . to treat all deafness,” Rivolta adds. “We’re going to have different treatments, different alternatives that will be suitable for different people.”
Brigande’s own hearing loss is progressive—and still very slowly worsening. He anticipates that over the next 10 years, his increasing deafness may affect his life more and more, and believes that his experience with hearing loss drives his research. “[It’s] a very personal, very intimate challenge,” he says. “It causes me to work a whole lot harder in the lab.”
Kate Yandell is a freelance science writer living in Philadelphia.
|HEARING-LOSS DRUGS IN DEVELOPMENT|
|Company/ Investigator||Therapy||Target||Function||Stage of Development|
|Auris Medical||AM-111 (cell-permeable peptide)||Hair cells and spiral ganglion neurons||Blocks apoptosis and reduces inflammation directly following damage||Phase 3|
|Fennec Pharmaceuticals||Sodium thiosulfate (small molecule)||Cells of the cochlea||Neutralizes toxic metabolites of the chemotherapy drug cisplatin circulating in plasma before they can make it to the inner ear||Phase 3|
|Sound Pharmaceuticals||Two oral formulations of ebselen (small molecule)||Cells of the cochlea||Protects against damage from reactive oxygen species||Phase 2|
|Autifony||AUT00063 (small molecule)||Hearing regions of the brain||Stems age-related decline of Kv3 potassium channels||Phase 2|
|Novartis||CGF166 (gene therapy)||Hair cells||Introduces the transcription factor Atoh1 to supporting cells of the inner ear, hoping to spur hair-cell regeneration||Phase 1/2|
|Audion Therapeutics||Notch inhibitor||Hair cells||Blocks Notch signaling, which may suppress hair-cell regeneration in mammals||Preclinical|
|Charles Liberman, Harvard University||Neurotrophins||Spiral ganglion neurons||Guides growth of neurons to form synapses with hair cells||Preclinical|
|Marcelo Rivolta, University of Sheffield||Stem cell therapy||Spiral ganglion neurons and potentially other inner-ear cells||Progenitor cells generated from stem cells differentiate into neurons after introduction into the inner ear||Preclinical|
|Oricula Therapeutics||BPN-13661 (small molecule)||Hair cells||Protects hair cells from damage by aminoglycoside antibiotics||Preclinical|
- C. Askew et al., “Tmc gene therapy restores auditory function in deaf mice,” Sci Trans Med, 295:295ra108, 2015.
- B.M. Ryals, E.W. Rubel, “Hair cell regeneration after acoustic trauma in adult Coturnix quail,” Science, 240:1774-76, 1988.
- K. Mizutari et al., “Notch inhibition induces cochlear hair cell regeneration and recovery of hearing after acoustic trauma,” Neuron, 77:58-69, 2013.
- S.G. Kuzawa, M.C. Liberman, “Adding insult to injury: cochlear nerve degeneration after ‘temporary’ noise-induced hearing loss,” J Neurosci, 29:14077-85, 2009.
- G. Wan et al., “Neurotrophin-3 regulates ribbon synapse density in the cochlea and induces synapse regeneration after acoustic trauma,” eLIFE, 3:e03564, 2014.
- W. Chen et al., “Restoration of auditory evoked responses by human ES-cell-derived otic progenitors,” Nature, 490:278-82, 2012.