http://www.nature.com/scibx/journal/v2/n17/full/scibx.2009.688.html
Stepping toward a therapy
Developing a therapeutic to target miRNA-96 will likely pose two main challenges: delivery to cells in the inner ear and avoidance of side effects associated with interfering with non–hearing-related genes potentially regulated by the miRNA.
Before either of those issues can be addressed, however, 2131e Feinstein, CSO of Quark Pharmaceuticals Inc., said a key next step is determining whether the hearing loss phenotype is caused by a loss-of-function mutation, in which the miRNA-96 mutant cannot bind its original targets, or a gain-of-function mutation, in which the miRNA-96 mutant binds a new target instead of or in addition to its original targets.
In addition, Quark president and CEO Daniel Zurr said controlled, local delivery of an miRNA-96-targeting therapeutic to the hair cells will be essential. "Because miRNAs can affect the expression of many target genes, there exists the potential for unwanted modulation of off-target genes in healthy tissue if delivery is not highly specific," he said.
Quark has a small interfering RNA molecule in preclinical development against an undisclosed target to treat acute hearing loss associated with ototoxicity and acoustic trauma.
Jonathan Kil, president and CEO of Sound Pharmaceuticals Inc., thinks targeting a single miRNA such as miRNA-96 to treat age-related hearing loss could be a tall order. "Age-related hearing loss is a multi-factorial disease with a complex underlying biology, and it's not clear that targeting a single miRNA like miRNA-96 is going to be sufficient to reduce or reverse disease," he said.
Sound Pharmaceuticals has two orally available small molecules, both inducers of glutathione peroxidase (GPX) antioxidant activity, in Phase II testing: SPI-1005 to treat noise-induced hearing loss and SPI-3005 to treat chemotherapy-induced hearing loss.
The company's SPI-5557, a locally injected siRNA against cyclin-dependent kinase inhibitor 1B (CDKN1B; p27; Kip1), is in preclinical development to induce regeneration of cochlear cells and treat deafness. Kil and colleagues have shown in mice that knocking out p27, a growth inhibitory protein, can induce cell proliferation and hair cell regeneration in the inner ear.
Regardless of therapeutic strategy, Thomas Meyer, founder and managing director of Auris Medical AG, said the indication itself poses multiple challenges. Age-related hearing is a slowly progressing condition that generally lacks an identifiable therapeutic window, he noted.
"Because age-related hearing loss usually progresses at an average rate of only one to two decibels per year, the patient is often unaware there's a problem until the disease has advanced so far that any type of treatment may be too late," Meyer said. "In such a situation, a preventive therapeutic strategy might a priori be more promising. However, it's unclear whether people would accept long-term treatment for a problem that would materialize only in the distant future and not with certainty," he added.
Auris is developing therapies for acute hearing loss. "Our focus is on acute hearing loss that results from acoustic trauma or other cochlear stress factors like viral infections and vascular disturbances. Those conditions are generally rarer than age-related hearing loss but potentially more amenable to treatment, especially in an emergency setting," said Meyer.
The company's AM-111, a cell-permeable peptide that inhibits the c-jun N-terminal kinase (JNK) pathway and is delivered by injection through the eardrum, is in a Phase IIb trial in Europe to treat acute hearing loss. Targeting the JNK pathway has an antiapoptotic effect and thus potentially prevents damaged hair cells from dying.
Steel noted that she isn't married to any one therapeutic approach to modulating miRNA-96. "Our future work to elucidate the targets of miRNA-96 could provide us with genes and proteins that might be targeted directly by small molecules to trigger regeneration and treat some forms of hearing loss," she said. "In any case, the therapeutic goal remains to stop progressive degeneration of hair cells that can cause age-related and environmental hearing impairment."