Novel In Vivo Imaging Analysis of an Inner Ear Drug Delivery System in Mice: Comparison of Inner Ear Drug Concentrations over Time after Transtympanic and Systemic Injections

Sho Kanzaki,Masaya Nakamura,Hirotaka James Okano,Kaoru Ogawa,Masato Fujioka,Shinsuke Shibata,Hideyuki Okano,Akimasa Yasuda,Demetrios Vavvas

doi:10.1371/journal.pone.0048480

Abstract

ObjectiveSystemic steroid injections are used to treat idiopathic sudden-onset sensorineural hearing loss (ISSHL) and some inner ear disorders. Recent studies show that transtympanic (TT) steroid injections are effective for treating ISSHL. As in vivo monitoring of drug delivery dynamics for inner ear is lacking, its time course and dispersion of drugs is unknown. Here, we used a new in vivo imaging system to monitor drug delivery in live mice and to compare drug concentrations over time after TT and systemic injections.MethodsLuciferin delivered into the inner ears of GFAP-Luc transgenic mice reacted with luciferase in GFAP-expressing cells in the cochlear spiral ganglion, resulting in photon bioluminescence. We used the Xenogen IVIS® imaging system to measure how long photons continued to be emitted in the inner ear after TT or systemic injections of luciferin, and then compared the associated drug dynamics.ResultsThe response to TT and IP injections differed significantly. Photons were detected five minutes after TT injection, peaking at ∼20 minutes. By contrast, photons were first detected 30 minutes after i.p. injection. TT and i.p. drug delivery time differed considerably. With TT injections, photons were detected earlier than with IP injections. Photon bioluminescence also disappeared sooner. Delivery time varied with TT injections.ConclusionsWe speculate that the drug might enter the Eustachian tube from the middle ear. We conclude that inner-ear drug concentration can be maintained longer if the two injection routes are combined. As the size of luciferin differs from that of therapeutics like dexamethasone, combining drugs with luciferin may advance our understanding of in vivo drug delivery dynamics in the inner ear.

Highlights

Sensorineural hearing loss is mostly caused by inner ear disorders
We speculate that the drug might enter the Eustachian tube from the middle ear
We conclude that inner-ear drug concentration can be maintained longer if the two injection routes are combined

Summary

Introduction

Sensorineural hearing loss is mostly caused by inner ear disorders. Systemic injection of steroids is generally used in inner ear diseases, especially idiopathic sensorineural hearing loss. Recent studies show that an approach for drug delivery through the tympanic membrane to the round window (RW) into the inner ear is as effective as systemic injections [1]. There are a few studies examining the pharmacokinetics of inner ear drug delivery, and the time course remains elusive. Patients with hearing loss exhibit a significant rate of RW obstruction, suggesting that drugs may not infiltrate the cochlea in these cases [2] [3]. When the RW is covered with connective tissue, we expect drug delivery into the inner ear can be facilitated

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