AM-111 protects against permanent hearing loss from impulse noise trauma

State-of-the-art mechanisms of intracochlear drug delivery

PURPOSE OF REVIEW

Treatment of auditory and vestibular dysfunction has become increasingly dependent on inner ear drug delivery. Recent advances in molecular therapy and nanotechnology have pushed development of alternate delivery methodologies involving both transtympanic and direct intracochlear infusions. This review examines recent developments in the field relevant to both clinical and animal research environments.

RECENT FINDINGS

Transtympanic delivery of gentamicin and corticosteroids for the treatment of Meniere's disease and sudden sensorineural hearing loss continues to be clinically relevant, with understanding of pharmacokinetics becoming more closely studied. Stabilizing matrices placed on the round window membrane for sustained passive delivery of compounds offer more controlled dosing profiles than transtympanic injections. Nanoparticles are capable of traversing the round window membrane and cochlear membranous partitions, and may become useful gene delivery platforms. Cochlear and vestibular hair cell regeneration has been demonstrated by vector delivery to the inner ear, offering promise for future advanced therapies.

SUMMARY

Optimal methods of inner ear drug delivery will depend on toxicity, therapeutic dose range, and characteristics of the agent to be delivered. Advanced therapy development will likely require direct intracochlear delivery with detailed understanding of associated pharmacokinetics.

Inner ear drug delivery for auditory applications

Many inner ear disorders cannot be adequately treated by systemic drug delivery. A blood-cochlear barrier exists, similar physiologically to the blood-brain barrier, which limits the concentration and size of molecules able to leave the circulation and gain access to the cells of the inner ear. However, research in novel therapeutics and delivery systems has led to significant progress in the development of local methods of drug delivery to the inner ear. Intratympanic approaches, which deliver therapeutics to the middle ear, rely on permeation through tissue for access to the structures of the inner ear, whereas intracochlear methods are able to directly insert drugs into the inner ear. Innovative drug delivery systems to treat various inner ear ailments such as ototoxicity, sudden sensorineural hearing loss, autoimmune inner ear disease, and for preserving neurons and regenerating sensory cells are being explored.

Up-regulation of prestin mRNA expression in the organs of Corti of guinea pigs and rats following unilateral impulse noise exposure

Prestin is the motor protein of the outer hair cells (OHCs) and is required for both their electromotility and for cochlear amplification. We investigated the prestin mRNA expression in guinea pigs and rats in relation to the degree of noise-induced hearing loss (NIHL) induced by unilateral impulse noise exposure (167dB peak SPL) for 2.5-5 min. Distortion product otoacoustic emissions (DPOAE) and auditory brainstem responses were recorded before and one week post exposure. Prestin mRNA was examined by quantitative reverse transcription-polymerase chain reaction. Either the whole organs of Corti or the apical, middle and basal parts were examined separately. The specimens were pooled and grouped according to the degree of NIHL measured in the exposed ears. In rats, the number of hair cells was counted. A clear base-to-apex gradient in the prestin mRNA expression was found to exist in guinea pig and rat controls. In both species, there was an increase in the number of prestin RNA transcripts at a mean NIHL of about 15-25 dB indicating an up-regulation in the remaining intact cells. In rats, this degree of NIHL corresponded to an OHC loss of about 40%. Interestingly, the contralateral ears also revealed an up-regulation of prestin mRNA accompanied by significant DPOAE improvements.