Intratympanic (IT) delivery of drugs to the ear is increasingly used for both clinical and research purposes. One limitation of IT delivery is that drugs are rapidly lost from the middle ear by a number of processes, so that prolonged delivery of drug is technically difficult. In the present study, the delivery characteristics of a poloxamer hydrogel formulation containing dexamethasone (dex) were evaluated. The gel is liquid at room temperature, allowing IT injection, but transitions to a gel at body temperature, providing a prolonged residence time in the middle ear. A 50-µl volume of control or dex-containing gel (dex-gel) was injected through the tympanic membrane of guinea pigs. Cochlear function was assessed with cochlear action potential and acoustic emission thresholds measured immediately, 6 or 24 h after IT gel injection. After 6- or 24-hour treatment with dex-gel, perilymph drug gradients along the cochlea were assessed by taking samples sequentially from the apex, and endolymph was sampled from the basal turn. Control gel injections caused small changes in sound field calibrations and functional measures for low-frequency stimuli, consistent with an induced conductive loss. Within 24 h, responses returned to normal. Twenty-four hours after dex-gel injection, low-frequency changes remained as the dex-gel was retained better in the middle ear, but there was no indication of high-frequency loss. While perilymph sample data showed that dex gradients were substantially lower than after single injections of dex solution, quantitative analysis of this result suggests that some dex may have entered the perilymph through the thin bone in the apical region of the cochlea. Endolymph levels of dex remained lower than those in the perilymph. This study confirms that a poloxamer hydrogel-based dex formulation provides an effective method for a prolonged delivery, providing a more uniform distribution of drug in the inner ear.

Introduction Drugs applied intratympanically in humans are believed to enter the cochlea primarily through the round window membrane (RWM). Local drug treatments of the ear are commonly evaluated in rodent models. The otic capsule is much thinner at the cochlear apex in rodents than in humans. We therefore investigated whether drugs applied to the middle ear could enter perilymph through the otic capsule as well as through the RWM. Methods The distribution of gentamicin and the marker trimethylphenylammonium (TMPA) along the guinea pig cochlea was assessed with sequential apical perilymph sampling following two delivery paradigms that included i) completely filling the tympanic bulla with solution, and ii) applying the solution to the RWM only. In addition, TMPA entry into perilymph of the third turn was measured with ion-selective electrodes while the bulla was filled with TMPA solution. Results In application protocols that allowed drug to contact the otic capsule (by completely filling the bulla) markedly higher drug concentrations were found in the apical, low-frequency regions of the cochlea compared with drug applications to the RWM only. Conclusions Gentamicin and TMPA can enter perilymph of guinea pigs through the RWM and simultaneously through the bony otic capsule. Drug distribution along the cochlea following intratympanic applications will therefore be dramatically different in rodents and humans. Results obtained from intratympanic drug treatments of animals, in which the bulla is filled with solution and contacts the bony capsule of the cochlea, do not provide a good model for the human situation.

Introduction-Intratympanic drug delivery has become widely used in the clinic but little is known about how clinically-utilized drug preparations affect round window membrane permeability or how much drug is actually delivered to the cochlea. This study evaluated the effect of clinically relevant carrier solutions and of suction near the round window membrane (RWM) on the permeability properties of the RWM.

The blood vessels that supply the inner ear form a barrier between the blood and the inner ear fluids to control the exchange of solutes, protein, and water. This barrier, called the blood-labyrinth barrier (BLB) is analogous to the blood-brain barrier (BBB), which plays a critical role in limiting the entry of inflammatory and infectious agents into the central nervous system. We have developed an in vivo method to assess the functional integrity of the BLB by injecting sodium fluorescein into the systemic circulation of mice and measuring the amount of fluorescein that enters perilymph in live animals. In these experiments, perilymph was collected from control and experimental mice in sequential samples taken from the posterior semicircular canal approximately 30 min after systemic fluorescein administration. Perilymph fluorescein concentrations in control mice were compared with perilymph fluorescein concentrations after lipopolysaccharide (LPS) treatment (1 mg/kg IP daily for 2 days). The concentration of perilymphatic fluorescein, normalized to serum fluorescein, was significantly higher in LPS-treated mice compared to controls. In order to assess the contributions of perilymph and endolymph in our inner ear fluid samples, sodium ion concentration of the inner ear fluid was measured using ion-selective electrodes. The sampled fluid from the posterior semicircular canal demonstrated an average sodium concentration of 145 mM, consistent with perilymph. These experiments establish a novel technique to assess the functional integrity of the BLB using quantitative methods and to provide a comparison of the BLB to the BBB.

It has been widely believed that drug entry from the middle ear into perilymph occurs primarily via the round window (RW) membrane. Entry into scala vestibuli (SV) was thought to be dominated by local, inter-scala communication between scala tympani (ST) and SV through permeable tissues such as the spiral ligament. In the present study, the distribution of the ionic marker trimethylphenylammonium (TMPA) was compared following intracochlear injections or applications to the RW niche, with or without occlusion of the RW membrane or stapes area. Perilymph TMPA concentrations were monitored either in real time with TMPA-selective microelectrodes sealed into ST and SV, or by the collection of sequential perilymph samples from the lateral semi-circular canal. Local inter-scala communication of TMPA was confirmed by measuring SV and ST concentrations following direct injections into perilymph of ST. Application of TMPA to the RW niche also showed a predominant entry into ST, with distribution to SV presumed to occur secondarily. When the RW membrane was occluded by a silicone plug, RW niche irrigation produced higher concentrations in SV compared to ST, confirming direct TMPA entry into the vestibule in the region of the stapes. The proportion of TMPA entering by the two routes was quantified by perilymph sampling from the lateral semi-circular canal. The TMPA levels of initial samples (originating from the vestibule) were markedly lower when the stapes area was occluded with silicone. These data were interpreted using a simulation program that incorporates all the major fluid and tissue compartments of the cochlea and vestibular systems. From this analysis it was estimated that 65 % of total TMPA entered through the RW membrane and 35% entered the vestibule directly in the vicinity of the stapes. Direct entry of drugs into the vestibule is relevant to inner ear fluid pharmacokinetics and to the growing field of intratympanic drug delivery.