The microsphere method for studies of inner ear blood flow

Poly-Lactic Acid-Based Biopolymer Formulations Are Safe for Sustained Intratympanic Dexamethasone Delivery

HYPOTHESIS AND BACKGROUND

The clinical treatment of sudden sensorineural hearing loss currently relies on the administration of steroids, either systemically or via intratympanic injections. Intratympanic injections bypass the hemato-cochlear barrier, reducing its systemic side effects. The efficacy of the injections is limited through rapid drug clearance via the Eustachian tube, and through nonoptimal properties of slow-release drug carriers. A new slow-release drug delivery vehicle based on hexyl-substituted-poly-lactic-acid (HexPLA), with the highest possible safety profile and complete bio-degradability, has been evaluated for safety and efficacy in a standardized guinea pig model of intratympanic injection.

METHODS

A total of 83 animals received through retrobullar injection either empty Nile-red-colored HexPLA vehicle, 5%-dexamethasone-HexPLA, 5%-dexamethasone suspension, or a sham operation. Long-term residence time of vehicle, biocompatibility, click- and pure-tone hearing thresholds, and dexamethasone levels in the perilymph were prospectively assessed.

RESULTS

At 1 week after injection, HexPLA vehicle was morphologically present in the middle ear and perilymph levels in the 5%-dexamethasone-HexPLA were on average 2 to 3 μg/ml and one order of magnitude higher compared with those of the 5%-dexamethasone suspension group. No significant postoperative morphological or functional changes were observed up to 3 months postdelivery.

CONCLUSIONS

HexPLA is safe, fully biocompatible, and efficient for sustained high-dose, intratympanic delivery of dexamethasone at least for 1 week and therefore of high interest for the treatment of sudden sensorineural hearing loss and other acute inner ear diseases. Due to the favorable chemical properties, a wide range of other drugs can be loaded into the vehicle further increasing its potential value for otological applications.

Enhanced local bioavailability of single or compound drugs delivery to the inner ear through application of PLGA nanoparticles via round window administration

In this paper, the potential of poly(D,L-lactide-co-glycolide acid) (PLGA) nanoparticles (NPs) for carrying single or compound drugs traversing the round window membrane (RWM) was examined after the round window (RW) administration of different NPs to guinea pigs. First, coumarin-6 was incorporated into PLGA NPs as a fluorescent probe to investigate its ability to cross the RWM. Then, PLGA NPs with salvianolic acid B (Sal B), tanshinone IIA (TS IIA), and total panax notoginsenoside (PNS) including notoginsenoside R1 (R1), ginsenoside Rg1 (Rg1), and ginsenoside Rb1 (Rb1) were developed to evaluate whether NPs loaded with compound drugs would pass through the RWM and improve the local bioavailability of these agents. PLGA NPs loaded with single or compound drugs were prepared by the emulsification solvent evaporation method, and their particle size distribution, particle morphology, and encapsulation efficiency were characterized. In vitro release study showed sustained-release profiles of Sal B, TS IIA, and PNS from the NPs. The pharmacokinetic results showed that NPs applied to the RWM significantly improved drug distribution within the inner ear. The AUC0-t of coumarin-6 in the perilymph (PL) following RW administration of NPs was 4.7-fold higher than that of coumarin-6 solution, and the Cmax was 10.9-fold higher. Furthermore, the AUC(0-t) of R1, Rg1, and Rb1 were 4.0-, 3.1-, and 7.1-fold greater, respectively, after the application of NPs compared to the compound solution, and the Cmax were, respectively, 14.4-, 10.0-, and 16.7-fold higher. These findings suggest that PLGA NPs with unique properties at the nanoscale dimensions have a powerful ability to transport single or compound drugs into the PL through the RWM and remarkably enhance the local bioavailability of the encapsulated drugs in the inner ear. The use of PLGA NPs as nanoscale delivery vehicles to carry drugs across the RWM may be a promising strategy for the treatment of inner ear diseases.

Functional mapping of the auditory tract in rodent tinnitus model using manganese-enhanced magnetic resonance imaging

Animal models of salicylate-induced tinnitus have demonstrated that salicylate modulates neuronal activity in several brain structures leading to neuronal hyperactivity in auditory and non-auditory brain areas. In addition, these animal tinnitus models indicate that tinnitus can be a perceptual consequence of altered spontaneous neural activity along the auditory pathway. Peripheral and/or central effects of salicylate can account for neuronal activity changes in salicylate-induced tinnitus. Because of this ambiguity, an in vivo imaging study would be able to address the peripheral and/or central involvement of salicylate-induced tinnitus. Therefore, in the present study, we developed a novel manganese-enhanced magnetic resonance imaging (MEMRI) method to map the in vivo functional auditory tract in a salicylate-induced tinnitus animal model by administrating manganese through the round window. We found that acute salicylate-induced tinnitus resulted in higher manganese uptake in the cochlea and in the central auditory structures. Furthermore, serial MRI scans demonstrated that the manganese signal increased in an anterograde fashion from the cochlea to the cochlear nucleus. Therefore, our in vivo MEMRI data suggest that acute salicylate-induced tinnitus is associated with higher spontaneous neural activity both in peripheral and central auditory pathways.

Thin and open vessel windows for intra-vital fluorescence imaging of murine cochlear blood flow

Normal microvessel structure and function in the cochlea is essential for maintaining the ionic and metabolic homeostasis required for hearing function. Abnormal cochlear microcirculation has long been considered an etiologic factor in hearing disorders. A better understanding of cochlear blood flow (CoBF) will enable more effective amelioration of hearing disorders that result from aberrant blood flow. However, establishing the direct relationship between CoBF and other cellular events in the lateral wall and response to physio-pathological stress remains a challenge due to the lack of feasible interrogation methods and difficulty in accessing the inner ear. Here we report on new methods for studying the CoBF in a mouse model using a thin or open vessel-window in combination with fluorescence intra-vital microscopy (IVM). An open vessel-window enables investigation of vascular cell biology and blood flow permeability, including pericyte (PC) contractility, bone marrow cell migration, and endothelial barrier leakage, in wild type and fluorescent protein-labeled transgenic mouse models with high spatial and temporal resolution. Alternatively, the thin vessel-window method minimizes disruption of the homeostatic balance in the lateral wall and enables study CoBF under relatively intact physiological conditions. A thin vessel-window method can also be used for time-based studies of physiological and pathological processes. Although the small size of the mouse cochlea makes surgery difficult, the methods are sufficiently developed for studying the structural and functional changes in CoBF under normal and pathological conditions.

Local drug delivery to the inner ear using biodegradable materials

The lack of an effective method of drug delivery has been a considerable obstacle in the development of novel therapeutics for inner ear diseases. However, several strategies have been investigated to achieve drug delivery to the inner ear, particularly for local application. Here, we review recent advances in the development of inner ear drug-delivery systems, focusing on biodegradable materials. Both synthetic and natural biodegradable materials have shown efficacy for inner ear drug delivery, resulting in an attenuation of hearing loss in animal models. We expect the further development of such drug-delivery systems to help translate the findings of experimental studies to clinical applications.

Insulin-like growth factor 1 treatment via hydrogels rescues cochlear hair cells from ischemic injury

This study was designed to investigate the protective effects of recombinant human insulin-like growth factor 1 (rhIGF1), applied locally via a hydrogel, against ischemic damage of the cochleae in gerbils. A hydrogel was immersed in rhIGF1 or saline and was applied on the round window membrane 30 min after the ischemia. Local rhIGF1 treatment significantly reduced the elevation of auditory brain responses thresholds at a frequency of 8 kHz on days 1, 4, and 7 after ischemia. A histological analysis revealed increased survival of inner hair cells in the animals treated with rhIGF1 via the hydrogel 7 days after ischemia. These findings showed that local rhIGF1 application using a hydrogel has the potential to protect the cochleae from ischemic injury.

Novel Therapy for Hearing Loss

HYPOTHESIS

Local application of recombinant human insulin-like growth factor 1 (rhIGF-1) via a biodegradable hydrogel after onset of noise-induced hearing loss (NIHL) can attenuate functional and histologic damage.

BACKGROUND

The biodegradable gelatin hydrogel makes a complex with drugs by static electric charges and releases drugs by degradation of gelatin polymers. We previously demonstrated the efficacy of local rhIGF-1 application via hydrogels before noise exposure for prevention of NIHL.

METHODS

First, we used an enzyme-linked immunosorbent assay to measure human IGF-1 concentrations in the cochlear fluid after placing a hydrogel containing rhIGF-1 onto the round window membrane of guinea pigs. Second, the functionality and the histology of guinea pig cochleae treated with local rhIGF-1 application at different concentrations after noise exposure were examined. Control animals were treated with a hydrogel immersed in physiologic saline alone.

RESULTS

The results revealed sustained delivery of rhIGF-1 into the cochlear fluid via the hydrogel. The measurement of auditory brainstem responses demonstrated that local rhIGF-1 treatment significantly reduced the threshold elevation from noise. Histologic analysis exhibited increased survival of outer hair cells by local rhIGF-1 application through the hydrogel.

CONCLUSION

These findings indicate that local rhIGF-1 treatment via gelatin hydrogels is effective for treatment of NIHL.

Drug delivery to the cochlea using PLGA nanoparticles

OBJECTIVES

This study aimed to investigate the efficacy of encapsulating therapeutic molecules in poly lactic/glycolic acid (PLGA) nanoparticles for drug delivery to the cochlea.

STUDY DESIGN

An experimental study.

METHODS

We examined the distribution of rhodamine, a fluorescent dye, in the cochlea, liver, and kidney of guinea pigs. Intravenous injection of rhodamine or rhodamine-encapsulated PLGA nanoparticles was used to target the fluorescent dye systemically to the liver, kidney, and cochlea, and these molecules were applied locally to the round window membrane (RWM) of the cochlea. The localization of rhodamine fluorescence in each region was quantitatively analyzed.

RESULTS

After systemic application of rhodamine nanoparticles, fluorescence was identified in the liver, kidney, and cochlea. The systemic application of nanoparticles had a significant effect on targeted and sustained delivery of rhodamine to the liver but not the kidney or cochlea. Rhodamine nanoparticles placed on the RWM were identified in the scala tympani as nanoparticles, indicating that the PLGA nanoparticles can permeate through the RWM. Furthermore, the local application of rhodamine nanoparticles to the RWM was more effective in targeted delivery to the cochlea than systemic application.

CONCLUSIONS

These findings indicate that PLGA nanoparticles can be an useful drug carrier to the cochlea via local application.

Disorders of cochlear blood flow

The cochlea is principally supplied from the inner ear artery (labyrinthine artery), which is usually a branch of the anterior inferior cerebellar artery. Cochlear blood flow is a function of cochlear perfusion pressure, which is calculated as the difference between mean arterial blood pressure and inner ear fluid pressure. Many otologic disorders such as noise-induced hearing loss, endolymphatic hydrops and presbycusis are suspected of being related to alterations in cochlear blood flow. However, the human cochlea is not easily accessible for investigation because this delicate sensory organ is hidden deep in the temporal bone. In patients with sensorineural hearing loss, magnetic resonance imaging, laser-Doppler flowmetry and ultrasonography have been used to investigate the status of cochlear blood flow. There have been many reports of hearing loss that were considered to be caused by blood flow disturbance in the cochlea. However, direct evidence of blood flow disturbance in the cochlea is still lacking in most of the cases.

Blood flow measurements in the ears of patients receiving cochlear implants

We measured cochlear blood flow in 12 patients who received cochlear implants, using a laser-Doppler probe with an outer diameter of 0.8 mm. The subjects had congenital deafness, idiopathic progressive sensorineural hearing loss, Waardenburg's syndrome, narrow internal auditory canal, or sudden deafness. Putting the probe tip to the site of drilling for cochlear implantation, we measured blood flow before, during, and after the cochlear bony wall was opened. The laser-Doppler output was confirmed even after the tip of the probe was inserted into the perilymphatic space in all cases. Our results revealed that blood flow was maintained in all cochleas, although there was a probability of reduction in blood flow volume. We conclude that laser-Doppler flowmetry is both relatively safe and useful for measuring blood flow in the ears during cochlear implantation procedures.

Changes in serum osmolarity influence the function of outer hair cells

Fast motility of outer hair cells (OHC) is thought to be based on a hydromechanic principle. In vitro, the function of OHCs can be disturbed by a change in the osmolarity of the culture medium. Whether changes in the serum osmolarity in vivo can also interfere with OHC motility has not been investigated as yet. Serum osmolarity of New Zealand White rabbits (n = 18) was elevated by a continuous infusion of glucose 40%, decreased by an infusion of aqua dest, or kept constant by an infusion of saline. OHC function was monitored using distortion products of otoacoustic emissions (DPOAE). Input output curves were established between 2 and 5 kHz (geometric mean of f2) with primaries of levels between 35 and 55 dB SPL. Cochlear perfusion was measured using a fluorescence microsphere method. Elevation of the serum osmolarity from 306 +/- 17 mosm/l to 365 +/- 23 induced a decrease of DPOAE between 3 and 12 dB SPL. Cochlear blood flow increased from 0.11 +/- 0.09 to 0.15 +/- 0.10 ml/min/g. When decreasing the serum osmolarity from 303 +/- 9 to 281 +/- 8 mosm/l, only slight changes of the DPOAE could be verified. As in the control group, cochlear perfusion was almost unchanged. In the control group, neither serum osmolarity nor DPOAE changed. Comparable to findings in vitro, increasing the serum osmolarity can lead to a disturbance of OHC function. In patients suffering from sudden hearing loss. dehydration due to physical or mental stress is often observed. This new and promising pathophysiological concept needs further clinical evaluation.

Osmotic water permeability of capillaries from the isolated spiral ligament: new in-vitro techniques for the study of vascular permeability and diameter

Perilymph is separated from blood by a barrier called the blood-labyrinth or blood-perilymph barrier in analogy to the blood-brain or blood-cerebrospinal fluid barrier. These barriers consist mainly of vascular endothelial cells. To characterize the blood-labyrinth barrier we developed in vitro techniques for the quantitative determination of the osmotic water permeability and for the determination of changes in the diameter of isolated inner ear capillaries. Both techniques rely on measurement of the velocity of marker red cells trapped in the lumen of capillaries. The velocity of marker red cells is a measure for the capillary permeability when a water flux across the capillary wall is induced by an osmotic gradient or a measure for a change in the capillary diameter. With these techniques the osmotic water permeability coefficient (Pf) and the pH sensitivity of isolated capillaries from the spiral ligament of the inner ear was determined. Pf at 23 degrees C was (1.49 +/- 0.17) 10(-3) cm/s at pH 7.4 and (1.61 +/- 0.23) 10(-3) cm/s at pH 6.8 (n = 12: mean +/- SEM: n = number of tissues). Pf at 37 degrees C was (2.26 +/- 0.23) 10(-3) cm/s at pH 7.4 and (2.35 +/- 0.17) 10(-3) cm/s at pH 6.8 (n = 13). No change in capillary diameter was observed when the pH of the interstitial fluid was lowered from pH 7.4 to 6.8. These data demonstrate that Pf and the capillary diameter of spiral ligament capillaries are pH independent and suggest that water crosses the blood-labyrinth barrier via an aqueous pathway. Further, these data suggest that the relatively low Pf is another characteristic shared by the blood-labyrinth and the blood-brain barrier.

The cochlear blood flow: a comparison between the laser Doppler and the microsphere surface methods

Since the introduction in the early 1980s, of the laser Doppler (LD) method for measuring cochlear blood flow (CBF) it has been debated whether the measured changes reflect the total or regional blood flow and whether the method per se influences the CBF. In order to answer those questions, the effect of one vasodilating drug, sodium nitroprusside, was investigated after topical application on the round window membrane (RWM) with respect to its influence on CBF. Two different techniques, the microspheres surface method and the LD method, were used. Untreated animals and animals which received saline or nicotine acid on the RWM were used as controls. The effects on CBF and blood pressure (BP) were continuously registered with LD. When a maximal flow had stabilized, 6 x 10(6) microspheres were injected into the left side of the heart. After the microspheres had been distributed within the body, the animals were killed. Both cochleae were microdissected and the microspheres counted turn by turn in the lateral wall. The number of spheres in the two ears was compared and the difference was recorded as the increase caused by the drug. The percentage change in CBF measured using the LD was compared with that obtained by using the microsphere surface method (MSM). No change in CBF measured by the two techniques was registered in the untreated animals, or after saline or nicotinic acid, while sodium nitroprusside induced a substantial increase in CBF. The mean percentage change of CBF measured with the LD method was compared with the calculated mean percentage change of microspheres for all turns in the cochlea, and in the first turn. Student's t-test and the linear correlation coefficient were calculated.(ABSTRACT TRUNCATED AT 250 WORDS)