Permeability Of Round Window Membrane Research Articles

A prestin-targeting peptide-guided drug delivery system rearranging concentration gradient in the inner ear: An improved strategy against hearing loss

Hearing loss is mainly due to outer hair cell (OHC) damage in three cochlear turns. Local administration via the round window membrane (RWM) has considerable otological clinical potential in bypassing the blood–labyrinth barrier. However, insufficient drug distribution in the apical and middle cochlear turns results in unsatisfactory efficacy. We functionalized poly (lactic-co-glycolic acid) nanoparticles (PLGA NPs) with targeting peptide A665, which specifically bound to prestin, a protein uniquely expressed in OHCs. The modification facilitated the cellular uptake and RWM permeability of NPs. Notably, the guide of A665 towards OHCs enabled more NPs perfusion in the apical and middle cochlear turns without decreasing accumulation in the basal cochlear turn. Subsequently, curcumin (CUR), an appealing anti-ototoxic drug, was encapsulated in NPs. In aminoglycoside-treated guinea pigs with the worst hearing level, CUR/A665-PLGA NPs, with superior performance to CUR/PLGA NPs, almost completely preserved the OHCs in three cochlear turns. The lack of increased low-frequencies hearing thresholds further confirmed that the delivery system with prestin affinity mediated cochlear distribution rearrangement. Good inner ear biocompatibility and little or no embryonic zebrafish toxicity were observed throughout the treatment. Overall, A665-PLGA NPs act as desirable tools with sufficient inner ear delivery for improved efficacy against severe hearing loss.

Assessment of drug permeability through an ex vivo porcine round window membrane model

Delivery of pharmaceutical therapeutics to the inner ear to treat and prevent hearing loss is challenging. Systemic delivery is not effective as only a small fraction of the therapeutic agent reaches the inner ear. Invasive surgeries to inject through the round window membrane (RWM) or cochleostomy may cause damage to the inner ear. An alternative approach is to administer drugs into the middle ear using an intratympanic injection, with the drugs primarily passing through the RWM to the inner ear. However, the RWM is a barrier, only permeable to a small number of molecules. To study and enhance the RWM permeability, we developed an exvivo porcine RWM model, similar in structure and thickness to the human RWM. The model is viable for days, and drug passage can be measured at multiple time points. This model provides a straightforward approach to developing effective and non-invasive delivery methods to the inner ear.

Ultrasound Microbubbles Enhance the Efficacy of Insulin-Like Growth Factor-1 Therapy for the Treatment of Noise-Induced Hearing Loss.

The application of insulin-like growth factor 1 (IGF-1) to the round window membrane (RWM) is an emerging treatment for inner ear diseases. RWM permeability is the key factor for efficient IGF-1 delivery. Ultrasound microbubbles (USMBs) can increase drug permeation through the RWM. In the present study, the enhancing effect of USMBs on the efficacy of IGF-1 application and the treatment effect of USMB-mediated IGF-1 delivery for noise-induced hearing loss (NIHL) were investigated. Forty-seven guinea pigs were assigned to three groups: the USM group, which received local application of recombinant human IGF-1 (rhIGF-1, 10 µg/µL) following application of USMBs to the RWM; the RWS group, which received IGF-1 application alone; and the saline-treated group. The perilymphatic concentration of rhIGF-1 in the USM group was 1.95- and 1.67- fold of that in the RWS group, 2 and 24 h after treatment, respectively. After 5 h of 118 dB SPL noise exposure, the USM group had the lowest threshold shift in auditory brainstem response, least loss of cochlear outer hair cells, and least reduction in the number of synaptic ribbons on postexposure day 28 among the three groups. The combination of USMB and IGF-1 led to a better therapeutic response to NIHL. Two hours after treatment, the USM group had significantly higher levels of Akt1 and Mapk3 gene expression than the other two groups. The most intense immunostaining for phosphor-AKT and phospho-ERK1/2 was detected in the cochlea in the USM group. These results suggested that USMB can be applied to enhance the efficacy of IGF-1 therapy in the treatment of inner ear diseases.

Development of thermosensitive poloxamer 407-based microbubble gel with ultrasound mediation for inner ear drug delivery

ABSTRACTS Our previous study first investigated feasibility of applying ultrasound (US) and microbubbles (MBs) via external auditory canal to facilitate drug delivery into inner ear. However, most drugs are in aqueous formulae and eliminated via Eustachian tubes after drug application. In this study, feasibility of sustained release of thermosensitive poloxamer 407 (P407)-based MB gel for US mediation-enhanced inner ear drug (dexamethasone, DEX) delivery was investigated. The sol-to-gel transition temperature showed that mixture of DEX and only 10% and 12.5% P407 in MBs can be used for in vitro and in vivo drug delivery experiments. In in vitro Franz diffusion experiments, the release rates of 12.5% P407-MBs + US groups in the model using DEX as the delivered reagent at 3 h resulted in values 1.52 times greater than those of 12.5% P407-MBs groups. In guinea pigs, by filling tympanic bulla with DEX in 12.5% P407-MBs (DEX-P407-MBs), USMB applied at post-treatment days 1 and 7 induced 109.13% and 66.67% increases in DEX delivery efficiencies, respectively, compared to the group without US. On the 28th day after US-mediated P407-MB treatment, the safety assessment showed no significant changes in the hearing thresholds and no damage to the integrity of cochlea or middle ear. These are the first results to demonstrate feasibility of US-modified liquid form DEX-P407-MB cavitation for enhancing permeability of round window membrane. Then, a gel form of DEX-P407-MBs was generated and thus prolonged the release of DEX in middle ear to maintain the therapeutic DEX level in inner ear for at least 7 days.

Characterization of the Sheep Round Window Membrane.

Intratympanic injection is a clinically used approach to locally deliver therapeutic molecules to the inner ear. Drug diffusion, at least in part, is presumed to occur through the round window membrane (RWM), one of the two openings to the inner ear. Previous studies in human temporal bones have identified a three-layered structure of the RWM with a thickness of 70-100μm. This is considerably thicker than the RWM in rodents, which are mostly used to model RWM permeability and assess drug uptake. The sheep has been suggested as a large animal model for inner ear research given the similarities in structure and frequency range for hearing. Here, we report the structure of the sheep RWM. The RWM is anchored within the round window niche (average vertical diameter of 2.1 ± 0.3mm and horizontal diameter of 2.3 ± 0.4mm) and has a curvature that leans towards the scala tympani. The centre of the RWM is the thinnest (55-71μm), with increasing thickness towards the edges (< 171μm), where the RWM forms tight attachments to the surrounding bony niche. The layered RWM structure, including an outer epithelial layer, middle connective tissue and inner epithelial layer, was identified with cellular features such as wavy fibre bundles, melanocytes and blood vessels. An attached "meshwork structure" which extends over the cochlear aqueduct was seen, as in humans. The striking anatomical similarities between sheep and human RWM suggest that sheep may be evaluated as a more appropriate system to predict RWM permeability and drug delivery in humans than rodent models.

Ultrastructural Changes Associated With the Enhanced Permeability of the Round Window Membrane Mediated by Ultrasound Microbubbles.

The round window membrane (RWM) is the most common entryway for local drug and gene delivery into the inner ear, but its permeability can change the treatment outcome. We previously demonstrated a feasible and highly efficient approach using ultrasound-aided microbubble (USMB) cavitation to enhance the permeability of the RWM. Here, we investigated the safety of USMB exposure and the association between temporal changes in RWM permeability and ultrastructure. Experimental guinea pigs were divided into two treatment groups: a control group receiving round window soaking (RWS) with MBs and treatment (USM) groups undergoing 3 (USM-3) or 5 (USM-5) consecutive USMB exposures (1 min/exposure) at an acoustic intensity of 3 W/cm2 and 1 MHz frequency. The trans-RWM delivery efficiency of biotin-fluorescein isothiocyanate conjugates, used as permeability tracers, revealed a greater than 7-fold higher delivery efficiency for the USM groups immediately after 3 or 5 exposures than for the RWS group. After 24 h, the delivery efficiency was 2.4-fold higher for the USM-3 group but was 6.6-fold higher for the USM-5 group (and 3.7-fold higher after 48 h), when compared to the RWS group. Scanning electron microscopy images of the RWM ultrastructure revealed USMB-induced sonoporation effects that could include the formation of heterogeneous pore-like openings with perforation diameters from 100 nm to several micrometers, disruption of the continuity of the outer epithelial surface layer, and loss of microvilli. These ultrastructural features were associated with differential permeability changes that depended on the USMB exposure course. Fourteen days after treatment, the pore-like openings had significantly decreased in number and the epithelial defects were healed either by cell expansion or by repair by newly migrated epithelial cells. The auditory brainstem response recordings of the animals following the 5-exposure USMB treatment indicated no deterioration in the hearing thresholds at a 2-month follow-up and no significant hair cell damage or apoptosis, based on scanning electron microscopy, surface preparations, and TUNEL assays. USMBs therefore appear to be safe and effective for inner ear drug delivery. The mechanism of enhanced permeability may involve a disruption of the continuity of the outer RWM epithelial layer, which controls transmembrane transport of various substances.

The Application of Theranostic Microbubble Ultrasound System for the Minimally Invasive Surgery

Sensorineural hearing loss and balance dysfunction are common symptoms of inner ear disorders. Therapeutic strategies may include medication, surgical intervention, sound amplification, or physical therapy. Methods for local delivery can include intratympanic or intracochlear approaches. Intracochlear drug delivery is achieved via a cochleostomy through the round window membrane (RWM) or directly through the otic capsule, and a variety of devices, including syringe, osmotic pumps, microinjector, and a reciprocating drug delivery system, have been employed. Although intracochlear application can achieve greater bioavailability of drugs entering the inner ear than the intratympanic approach, the former usually requires invasive surgical manipulation to interrupt the inner ear structure, and may increase the risk of deafness. Active intratympanic drug delivery was developed in an attempt to overcome the shortcomings of direct transtympanic injection. Potential utilization for such a topical application of medicine to the inner ear will be further explored in the future in view of its convenience in application, fewer side effects, and minimal invasiveness. Recently, ultrasound (US) combined with a contrast agent, microbubbles (MBs), has been used to target or control drug release to tissues and cells. MBs, which are small gas-filled microspheres that are used as contrast agents in diagnostic ultrasound-based imaging, are now being intensively investigated for ultrasound-mediated gene and drug delivery as cavitation nuclei. In this presentation, the feasibility of US combined with the most suitable particle size MBs to enhance the drug delivery by theranostic MBs-US system in inner ear was firstly demonstrated. The practical application of optimal MBs-US on increasing the RWM permeability for facilitating drug or medication delivery to the inner ear. Biotin–fluorescein isothiocyanate conjugates (biotin–FITC) are as delivery agents, and guinea pig animal models simulate the clinical case. Finally, the efficiency and feasibility of theranostic MBs-US system in clinical purpose will be assessed.

Microperforations significantly enhance diffusion across round window membrane.

Introduction of microperforations in round window membrane (RWM) will allow reliable and predictable intracochlear delivery of pharmaceutical, molecular, or cellular therapeutic agents. Reliable delivery of medications into the inner ear remains a formidable challenge. The RWM is an attractive target for intracochlear delivery. However, simple diffusion across intact RWM is limited by what material can be delivered, size of material to be delivered, difficulty with precise dosing, timing, and precision of delivery over time. Further, absence of reliable methods for measuring diffusion across RWM in vitro is a significant experimental impediment. A novel model for measuring diffusion across guinea pig RWM, with and without microperforation, was developed and tested: cochleae, sparing the RWM, were embedded in 3D-printed acrylic holders using hybrid dental composite and light cured to adapt the round window niche to 3 ml Franz diffusion cells. Perforations were created with 12.5-μm-diameter needles and examined with light microscopy. Diffusion of 1 mM Rhodamine B across RWM in static diffusion cells was measured via fluorescence microscopy. The diffusion cell apparatus provided reliable and replicable measurements of diffusion across RWM. The permeability of Rhodamine B across intact RWM was 5.1 × 10(9-) m/s. Manual application of microperforation with a 12.5-μm-diameter tip produced an elliptical tear removing 0.22 ± 0.07% of the membrane and was associated with a 35× enhancement in diffusion (P < 0.05). Diffusion cells can be applied to the study of RWM permeability in vitro. Microperforation in RWM is an effective means of increasing diffusion across the RWM.

Contrast perilymphatic MRI findings of inner ear in sudden hearing loss with vertigo

Objective To explore the contrast perilymphatic MRI characteristics of inner ears with sudden hearing loss with vertigo. Methods Forty three patients with sudden hearing loss with vertigo and 35 patients with unilateral tinnitus diagnosed by the department of Otolaryngology-Head and Neck Surgery were retrospectively included. Forty eight ears (38 ears with unilateral sudden hearing loss and 10 ears in 5 cases with bilateral sudden hearing loss) were regarded as sudden hearing loss group, 35 asymptomatic ears (the opposite ears of the unilateral tinnitus ears) as control group. Thirty eight opposite ears of the 38 unilateral sudden hearing loss ears and 35 ears of the control group showed normal in the pure tone audiometry exam. One hundred and twenty one ears (48 hearing loss ears, 38 opposite ears of the 38 unilateral sudden hearing loss ears and the 35 control group's ears) underwent contrast perilymphatic MR exams, 0.4 to 0.5 ml gadolinium hydrate diluted with saline was injected through the tympanic membrane, twenty four hours later, three-dimensional fluid attenuated inversion recovery (3D-FLAIR) sequence was performed using a 3.0 T MRI scanner. After the contrast perilymphatic MR exam, success in the exam was judged through the raw image. The signal intensity ratio (SIR) of cochlea basal turn and homolateral brain stem was quantitatively calculated. The SIR difference of 48 ears with sudden hearing loss and 35 asymptomatic ears in control group was analyzed by t test. The SIR difference of the 38 unilateral sudden hearing loss ears and the contralateral asymptomatic ears was also analyzed. The presence of endolymphatic hydrops was judged. Results The contrast perilymphatic MRI of 121 inner ears with intratympanic gadolinium injection were all successful. Endolymphatic hydrops were observed in 12 sudden hearing loss ears with the occurrence rate of 25%(12/48). The SIR of basal turn and homolateral brain stem of the sudden hearing loss ears (2.062±0.907) were lower than the SIR of the control groups' asymptomatic ears (2.703± 0.640) with significant difference between them (t=3.619,P=0.001), the SIR of basal turn and homolateral brain stem of the unilateral sudden hearing loss ears (1.941±0.860) were also lower than the SIR of the contralateral asymptomatic ears (2.411±0.670) with statistical difference between them (t =3.270,P=0.002). Conclusions Endolymphatic hydrops were observed in sudden hearing loss with vertigo. The SIR of the cochlea in sudden hearing loss ears were lower than that of the asymptomatic ears, indicating the abnormal permeability of the round window membrane. Key words: Hearing loss; Sudden; Magnetic resonance imaging

The effect of surface charge of glycerol monooleate-based nanoparticles on the round window membrane permeability and cochlear distribution

The aim of this study is to elucidate the impact of surface charge of glycerol monooleate-based nanoparticles (NPs) on the cellular uptake and its distribution in the cochlea. These NPs are modified using varied concentration of anionic or cationic lipid. Upon dilution, these lipid mixtures self-assemble to form a series of cubic NPs with various surface charges, but with similar particle size. Positively charged NPs exhibited dose-dependent cytotoxicities against L929 cells proportional to the concentration of cationic lipid; whereas negatively charged NPs did not show obvious cytotoxic properties as compared to unmodified NPs. Meanwhile, confocal microscopy and flow cytometry results suggested that NPs with high positive surface charge were taken up more efficiently by L929 cells. The permeability of round window membrane (RWM) was high for highly positively charged NPs, which is likely due to their highly cellular uptake efficiency and consequently high concentration gradient between RWM and cochlear fluid. More importantly, 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) modified NPs greatly facilitated broadly distribution in cochlea, favoring the treatment of hearing loss of low frequencies. Taken together, these findings about charge-dependent of NPs on RWM permeability and cochlear distribution could serve as guideline in the rational design of NP for drug and gene delivery to inner ear.

Round window membrane permeability to golimumab in guinea pigs: A pilot study

Autoimmune inner ear disorder is one of a few types of sensorineural hearing loss that is treatable and potentially reversible. Treatment involves oral steroids and methotrexate. Other treatment modalities have been tried with variable success. All such treatments are systemic, with inherent side effects limiting their effectiveness. Recently, tumor necrosis factor (TNF)-α blockers have been suggested as a modality of treatment. The objective of this study was to assess the round window membrane permeability to golimumab, a TNF-α blocker. This study is the first to look at the feasibility of local golimumab delivery into the inner ear, which may allow for targeted immune modulation of autoimmune inner ear disorders without the consequences of systemic treatment. This is a single-blinded, placebo-controlled, pilot study using guinea pigs to assess round window membrane permeability to golimumab. Golimumab was instilled into the guinea pigs' middle ear. Inner ear fluid was sampled through the round window membrane after approximately 30 minutes of drug exposure. Golimumab presence in the inner ear was assessed by enzyme-linked immunosorbent assay in both drug-treated and control ears. Higher concentrations of golimumab were detected in the inner ear fluid samples of golimumab-exposed ears than in the control ears. The difference was statistically significant (P < .001). Golimumab crosses the round window membrane and is detected in measurable concentrations in the inner ear fluid after 30 minutes of exposure to the membrane. Further studies are needed to learn its pharmacokinetics and the time needed to reach optimal concentration in the inner ear. NA. Laryngoscope, 123:2840-2844, 2013.

Ultrasound-aided microbubbles facilitate the delivery of drugs to the inner ear via the round window membrane

The round window membrane (RWM) acts as a barrier between the middle ear and cochlea and can serve as a crucial route for therapeutic medications entering the inner ear via middle ear applications. In this study, we targeted the practical application of microbubbles (MBs) ultrasound on increasing the RWM permeability for facilitating drug or medication delivery to the inner ear. Using biotin–fluorescein isothiocyanate conjugates (biotin–FITC) as delivery agents and guinea pig animal models, we showed that MB ultrasound exposure can improve the inner ear system use of biotin–FITC delivery via the RWM by approximately 3.5 to 38 times that of solely soaking biotin–FITC around the RWM for spontaneous diffusion. We also showed that there was significant enhancement of hair cell uptake of gentamicin in animals whose tympanic bullas were soaked with MB-mixed gentamicin–Texas Red or gentamicin and exposed to ultrasound. Furthermore, increased permeability of the RWM from acoustic cavitation of MBs could also be visualized immediately following ultrasound exposure by using Alexa Fluor 488-conjugated phalloidin as a tracer. Most importantly, such applications had no resulting damage to the integrity of the RWM or deterioration of the hearing thresholds assessed by auditory brainstem responses. We herein provide a basis for MB ultrasound-mediated techniques with therapeutic medication delivery to the inner ear for future application in humans.

Efficient cochlear gene transfection in guinea-pigs with adeno-associated viral vectors by partial digestion of round window membrane

The auditory portion of the inner ear, the cochlea, is an ideal organ for local gene transfection owing to its relative isolation. Various carriers have been tested for cochlear gene transfection. To date, viral vectors appear to have much higher transfection efficacy than non-viral mechanisms. Among these vectors, recombinant adeno-associated virus (rAAV) vectors have several advantages such as being non-pathogenic and the ability to produce prolonged gene expression in various cell types. However, rAAV vectors cannot pass through the intact round window membrane (RWM), otherwise a very attractive approach to access the human inner ear. In this study, performed in guinea-pigs, we describe a method to increase the permeability of RWM to rAAV vectors by partial digestion with collagenase solution. Elevated delivery of rAAV across the partially digested RWM increased transfection efficacy to a satisfactory level, even though it was still lower than that achieved by direct cochleostomy injection. Functional tests (auditory brainstem responses) showed that this enzymatic manipulation did not cause permanent hearing loss if applied appropriately. Morphological observations suggested that the damage to RWM caused by partial digestion healed within four weeks. Taken together, these findings suggest that partial digestion of the RWM is a safe and effective method for increasing the transfection of cochlear sensory cells with rAAV.

Round Window Fibrous Plugs

*Hospital de Bellvitge, USP Institut Universitari Dexeus, Barcelona, Spain; and †House Ear Institute, Los Angeles, California, U.S.A. Address correspondence and reprint requests to Fred H. Linthicum, Jr., M.D., Histopathology Department, House Ear Institute, 2100 West Third Street, Fifth Floor, Los Angeles, CA 90057; E-mail: [email protected]

Permeability of the Round Window Membrane Is Influenced by the Composition of Applied Drug Solutions and by Common Surgical Procedures

Intratympanic drug delivery has become widely used in the clinic, but little is known regarding how clinically used drug preparations affect round window membrane (RWM) 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 RWM on the permeability properties of the RWM. RWM permeability was assessed by perfusion of the marker trimethylphenylammonium into the round window niche while monitoring entry into perilymph using trimethylphenylammonium-selective electrodes sealed into scala tympani. High-osmolarity solution increased RWM permeability by a factor of 2 to 3, benzyl alcohol (a preservative used in some drug formulations) increased permeability by a factor of 3 to 5, and suctioning near the RWM increased permeability by a factor of 10 to 15. Variations in available drug formulations can potentially alter RWM permeability properties and affect the amount of drug delivered to the inner ear. Drug solution osmolarity, benzyl alcohol content, and possible drying of the RWM during suctioning the middle ear can all have a substantial influence of the perilymph levels of drug achieved.

In vitro permeability of round window membrane to transforming dexamethasone with delivery vehicles — a dosage estimation

In recent years the interest of sustained drug delivery into inner ear is promising, at the same time a great deal of novel oral drugs using biodegradable vehicles have been produced to achieve sustained drug release. The aim of this study was to use biodegradable vehicles to release dexamethasone in the round window membrane application. Dexamethasone gels composed of alginate and chitin were prepared and the release-permeating profiles were studied using a reproducible in vitro apparatus. A longer-period time course was simulated using the parameters acquired in this study. The data obtained in this study was compared with those of other studies in intratympanic drug delivery, and an appropriate initial dosage was extrapolated. The combination of alginate and chitin could efficiently restrict dexamethasone diffusion and the time course suggested a sustained drug concentration within 24 hours. A higher initial dosage was estimated to achieve a stable therapeutic concentration in vivo. The combination of alginate and chitin could be used as vehicle for sustained release of dexamethasone in intratympanic application.

Concentration Gradient Along the Scala Tympani After Local Application of Gentamicin to the Round Window Membrane

The distribution of gentamicin along the fluid spaces of the cochlea after local applications has never previously been demonstrated. Computer simulations have predicted that significant basal-apical concentration gradients might be expected, and histologic studies indicate that hair cell damage is greater at the base than at the apex after local gentamicin application. In the present study, gradients of gentamicin along the cochlea were measured. A recently developed method of sampling perilymph from the cochlear apex of guinea pigs was used in which the samples represent fluid originating from different regions along the scala tympani. Gentamicin concentration was determined in sequential apical samples that were taken after up to 3 hours of local application to the round window niche. Substantial gradients of gentamicin along the length of the scala tympani were demonstrated and quantified, averaging more than 4,000 times greater concentration at the base compared with the apex at the time of sampling. Peak concentrations and gradients for gentamicin varied considerably between animals, likely resulting from variations in round window membrane permeability and rates of perilymph flow. The large gradients for gentamicin demonstrated here in guinea pigs account for how it is possible to suppress vestibular function in some patients with a local application of gentamicin without damaging auditory function. Variations in round window membrane permeability and in perilymph flow could account for why hearing losses are observed in some patients.

Cochlear microdialysis for quantification of dexamethasone and fluorescein entry into scala tympani during round window administration

Before new drugs for the treatment of inner ear disorders can be studied in controlled clinical trials, it is important that their pharmacokinetics be established in inner ear fluids. Microdialysis allows drug levels to be measured in perilymph without the volume disturbances and potential cerebrospinal fluid contamination associated with fluid sampling. The aims of this study were to show: (i) that despite low recovery rates from miniature dialysis probes, significant amounts of drug are removed from small fluid compartments, (ii) that dialysis sampling artifacts can be accounted for using computer simulations and (iii) that microdialysis allows quantification of the entry rates through the round window membrane (RWM) into scala tympani (ST). Initial experiments used microdialysis probes in small compartments in vitro containing sodium fluorescein. Stable concentrations were observed in large compartments (1000 μl) but significant concentration declines were observed in smaller compartments (100, 10 and 5.6 μl) comparable to the size of the inner ear. Computer simulations of these experiments closely approximated the experimental data. In in vivo experiments, sodium fluorescein 10 mg/ml and dexamethasone–dihydrogen–phosphate disodium salt 8 mg/ml were simultaneously applied to the RWM of guinea pigs. Perilymph concentration in the basal turn of ST was monitored using microdialysis. The fluorescein concentration reached after 200 min application (585 ± 527 μg/ml) was approximately twice that of dexamethasone phosphate (291 ± 369 μg/ml). Substantial variation in concentrations was found between animals by approximately a factor of 34 for fluorescein and at least 41 for dexamethasone phosphate. This is, to a large extent, thought to be the result of the RWM permeability varying in different animals. It was not caused by substance analysis variations, because two different analytic methods were used and the concentration ratio between the two substances remained nearly constant across the experiments and because differences were apparent for the repeated samples obtained in each animal. Interpretation of the results using computer simulations allowed RWM permeability to be quantified. It also demonstrated, however, that cochlear clearance values could not be reliably obtained with microdialysis because of the significant contribution of dialysis to clearance. The observed interanimal variation, e.g., in RWM permeability, is likely to be clinically relevant to the local application of drugs in patients.

The effects of Streptolysin-O and sodium hyaluronate on the permeability of the round window membrane in guinea pigs--an electrophysiologic study

The round window membrane (RWM) acts like a barrier for topically applied substances into the middle ear preventing diffusion into the perilymphatic fluid compartment. In an animal model, modulation of the permeability of the RWM was attempted using Streptolysin-O (SLO) in various concentrations and sodium hyaluronate. Thereafter, the effect of intratympanically applied Lidocain 2 % on hearing threshold measured by auditory brainstem response was tested for Lidocain 2 % and Lidocain 2 % in combination with SLO or sodium hyaluronate. The results show that both, SLO and sodium hyaluronate, influence the effect of Lidocain 2 % on hearing thresholds as an indirect sign for changes of the permeability of the RWM. However SLO by itself in low concentrations decreased auditory thresholds in some animals while this was never observed with sodium hyaluronate. Our findings preclude SLO and favor sodium hyaluronate as a modulator of RWM permeability in therapeutic trials.

Analysis of gentamicin kinetics in fluids of the inner ear with round window administration.

That a theoretical basis for quantifying drug distribution in the inner ear with local applications can be established. As methods of local drug delivery to the inner ear gain wider clinical acceptance it becomes important to establish how drugs are distributed in the ear as a function of time and for different delivery methods. The time course of gentamicin concentration in the inner ear fluids was simulated with a program that considered general pharmacokinetic principles and incorporated inner ear dimensions and drug dispersal processes, including diffusion, clearance, and intercompartmental exchange. Cochlear fluid space dimensions of the chinchilla were derived from three-dimensional magnetic resonance images and were incorporated into the simulator. The published time course of gentamicin in vestibular perilymph of chinchillas was closely approximated by the adjustment of parameters defining round window membrane permeability, clearance, and interscala exchange. To simulate the time course, it was necessary for drug entry into the vestibule to be dominated by interscala exchange rather than longitudinal spread through the helicotrema. The effects of different round window delivery methods were also calculated. Perilymph drug levels and spatial distribution in the ear were shown to be markedly influenced by the time the applied drug remained in the middle ear. The development of local inner ear drug application strategies requires consideration of inner ear pharmacokinetic characteristics, delivery methods, and therapeutic range of the drug.