Microneedles Facilitate Small-Volume Intracochlear Delivery Without Physiologic Injury in Guinea Pigs

HypothesisMicroneedle-mediated intracochlear injection through the round window membrane (RWM) will facilitate intracochlear delivery, not affect hearing, and allow for full reconstitution of the RWM within 48 hours.BackgroundWe have developed polymeric microneedles that allow for in vivo perforation of the guinea pig RWM and aspiration of perilymph for diagnostic analysis, with full reconstitution of the RWM within 48 to 72 hours. In this study, we investigate the ability of microneedles to deliver precise volumes of therapeutics into the cochlea and assess the subsequent consequences on hearing.MethodsVolumes of 1.0, 2.5, or 5.0 mu L of artificial perilymph were injected into the cochlea at a rate of 1 mu L/min. Compound action potential (CAP) and distortion product otoacoustic emission were performed to assess for hearing loss (HL), and confocal microscopy was used to evaluate the RWM for residual scarring or inflammation. To evaluate the distribution of agents within the cochlea after microneedle-mediated injection, 1.0 mu L of FM 1-43 FX was injected into the cochlea, followed by whole mount cochlear dissection and confocal microscopy.ResultsDirect intracochlear injection of 1.0 mu L of artificial perilymph in vivo, corresponding to about 20% of the scala tympani volume, was safe and did not result in HL. However, injection of 2.5 or 5.0 mu L of artificial perilymph into the cochlea produced statistically significant high-frequency HL persisting 48 hours postperforation. Assessment of RWMs 48 hours after perforation revealed no inflammatory changes or residual scarring. FM 1-43 FX injection resulted in distribution of the agent predominantly in the basal and middle turns.ConclusionMicroneedle-mediated intracochlear delivery of small volumes relative to the volume of the scala tympani is feasible, safe, and does not cause HL in guinea pigs; however, injection of large volumes induces high-frequency HL. Injection of small volumes of a fluorescent agent across the RWM resulted in significant distribution within the basal turn, less distribution in the middle turn, and almost none in the apical turn. Microneedle-mediated intracochlear injection, along with our previously developed intracochlear aspiration, opens the pathway for precision inner ear medicine.