Gentamicin ototoxicity: histological and ultrastructural alterations after transtympanic administration

Adverse outcome pathway for aminoglycoside ototoxicity in drug-resistant tuberculosis treatment

Individuals treated for multidrug-resistant tuberculosis (MDR-TB) with aminoglycosides (AGs) in resource-limited settings often experience permanent hearing loss. However, AG ototoxicity has never been conceptually integrated or causally linked to MDR-TB patients' pre-treatment health condition. We sought to develop a framework that examines the relationships between pre-treatment conditions and AG-induced hearing loss among MDR-TB-infected individuals in sub-Saharan Africa. The adverse outcome pathway (AOP) approach was used to develop a framework linking key events (KEs) within a biological pathway that results in adverse outcomes (AO), which are associated with chemical perturbation of a molecular initiating event (MIE). This AOP describes pathways initiating from AG accumulation in hair cells, sound transducers of the inner ear immediately after AG administration. After administration, the drug catalyzes cellular oxidative stress due to overproduction of reactive oxygen species. Since oxidative stress inhibits mitochondrial protein synthesis, hair cells undergo apoptotic cell death, resulting in irreversible hearing loss (AO). We identified the following pre-treatment conditions that worsen the causal linkage between MIE and AO: HIV, malnutrition, aging, noise, smoking, and alcohol use. The KEs are: (1) nephrotoxicity, pre-existing hearing loss, and hypoalbuminemia that catalyzes AG accumulation; (2) immunodeficiency and antioxidant deficiency that trigger oxidative stress pathways; and (3) co-administration of mitochondrial toxic drugs that hinder mitochondrial protein synthesis, causing apoptosis. This AOP clearly warrants the development of personalized interventions for patients undergoing MDR-TB treatment. Such interventions (i.e., choosing less ototoxic drugs, scheduling frequent monitoring, modifying nutritional status, avoiding poly-pharmacy) will be required to limit the burden of AG ototoxicity.

Changes in cytochemistry of sensory and nonsensory cells in gentamicin-treated cochleas

Effects of a single local dose of gentamicin upon sensory and nonsensory cells throughout the cochlea were assessed by changes in immunostaining patterns for a broad array of functionally important proteins. Cytochemical changes in hair cells, spiral ganglion cells, and cells of the stria vascularis, spiral ligament, and spiral limbus were found beginning 4 days post administration. The extent of changes in immunostaining varied with survival time and with cell type and was not always commensurate with the degree to which individual cell types accumulated gentamicin. Outer hair cells, types I and II fibrocytes of the spiral ligament, and fibrocytes in the spiral limbus showed marked decreases in immunostaining for a number of constituents. In contrast, inner hair cells, type III fibrocytes and root cells of the spiral ligament, cells of the stria vascularis, and interdental cells in the spiral limbus showed less dramatic decreases, and in some cases they showed increases in immunostaining. Results indicate that, in addition to damaging sensory cells, local application of gentamicin results in widespread and disparate disruptions of a variety of cochlear cell types. Only in the case of ganglion cells was it apparent that the changes in nonsensory cells were secondary to loss or damage of hair cells. These results indicate that malfunction of the ear following gentamicin treatment is widespread and far more complex than simple loss of sensory elements. The results have implications for efforts directed toward detecting, preventing, and treating toxic effects of aminoglycosides upon the inner ear.

Transtympanic versus sustained-release administration of gentamicin: kinetics, morphology, and function

OBJECTIVES/HYPOTHESIS

Transtympanic gentamicin therapy has become a popular treatment modality for Meniere's disease, but questions regarding the ideal dose of medicine, the best administration paradigm, and the safest treatment end-point remain unanswered. The goal of this study is to examine the inner ear kinetics of transtympanic gentamicin and compare this with the kinetics of sustained-release delivery in a basic science model. In addition, we plan to examine the relationship of these kinetics curves to the effect of the two treatment modalities on inner ear function and morphology. It is hoped that this analysis will help clinicians to better apply local medical therapy to the ear.

STUDY DESIGN

The study is a basic science project designed to examine perilymph gentamicin concentrations, hearing results, and inner ear morphology in an animal model.

METHODS

Gentamicin was applied to the right ear of chinchillas either through a transtympanic approach or in a sustained-release device. The left ear remained untreated as an internal control. At set time points the animals' hearing and balance function was studied and the perilymph was harvested, after which the animal was killed and preserved for histological evaluation. Kinetics curves were constructed for each of the two treatment paradigms and compared with histological and functional outcomes.

RESULTS

The two groups yielded dramatically different kinetics curves. The transtympanic curve had a high peak level at 24 hours with rapid fall-off and almost total elimination by 48 hours, whereas the sustained-release curve was characterized by a long, flat plateau phase with a peak that was approximately one-third that of the transtympanic curve. In addition, the variability seen in perilymph concentrations was significantly higher in the transtympanic group than in the sustained-release group. Immunohistochemical analysis using antibodies against cleaved caspase-3 and cleaved caspase-7 demonstrated early damage in the spiral ganglion of both groups, before any obvious morphological change in the hair cells. The staining was significantly more dense in animals with transtympanic delivery. Cochlear and vestibular hair cell damage was seen at late time points in animals from both groups. Hearing loss (HL) progressed in an orderly fashion in the sustained-release group of animals, with no HL seen in the early time points and universal significant threshold shifts present by 72 hours. In the transtympanic group, the HL was more variable, with significant threshold shifts occurring as early as 4 hours after treatment, but with some animals demonstrating preserved hearing at the 72-hour time point. All animals demonstrated profound HL at the 6-day time point.

CONCLUSIONS

There is a significant difference in the shape and variability of the perilymph kinetics curve when comparing sustained-release delivery to transtympanic delivery of gentamicin. High early peak levels of gentamicin seen with transtympanic therapy may have a profound effect on the spiral ganglion and produce early HL before obvious hair cell damage. Sustained delivery of gentamicin produces universal HL at 72 hours. The reliability of sustained-release delivery to the ear reduces functional and morphological variations between animals.

Intraotic administration of gentamicin: a new method to study ototoxicity in the crista ampullaris of the bullfrog

A new method of local gentamicin administration was tested in the bullfrog inner ear to achieve ototoxic-induced hair cell destruction. Gelfoam pledgets soaked with known amounts of gentamicin were inserted into the perilymphatic cisterna of the bullfrog through a ventral surgical approach. A dose of 1.20 mg gentamicin, consistent with a perilymphatic concentration of 65 microg/ml, resulted in the desired ototoxic-induced hair cell damage, that is, complete hair cell destruction with minimal disruption of other components of the sensory epithelium. This study demonstrates that this is a useful and simple method to investigate the process of vestibular ototoxicity and hair cell regeneration, including aspects of hair cell destruction and repair.

Neurotrophin-4/5, brain-derived neurotrophic factor, and neurotrophin-3 promote survival of cultured vestibular ganglion neurons and protect them against neurotoxicity of ototoxins

The ability of neurotrophin-4/5 (NT-4/5), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and nerve growth factor (NGF) to promote survival of postnatal rat vestibular ganglion neurons (VGNs) was examined in dissociated cell cultures. Of the four neurotrophins, NT-4/5 and BDNF were equally effective but more potent than NT-3 in promoting the survival of VGNs. In contrast, NGF showed no detectable effects. As expected, TrkB-IgG (a fusion protein of extracellular domain of TrkB and Fc domain of human immunoglobulin G) specifically inhibited the survival-promoting, effects by NT-4/5 or BDNF and TrkC-IgG fusion protein completely blocked that of NT-3. Immunohistochemistry with TrkB, TrkA, and p75 antisera revealed that VGNs made TrkB and p75 proteins, but not TrkA protein. Ototoxic therapeutic drugs such as cisplatin and gentamicin often induce degeneration of hair cells and ganglion neurons in both auditory and vestibular systems that leads to impairment of hearing and balance. When cisplatin and gentamicin were added to the dissociated VGN culture in which the hair cells were absent, additional cell death of VGNs was induced, suggesting that the two ototoxins may have a direct neurotoxic effect on ganglion neurons in addition to their known toxicity on hair cells. However, if the cultures were co-treated with neurotrophins, NT-4/5, BDNF, and NT-3, but not NGF, prevented or reduced the neurotoxicity of the two ototoxins. Thus, the three neurotrophins are survival factors for VGNs and are implicated in the therapeutic prevention of VGN loss caused by injury and ototoxins.

Histological evidence for hair cell regeneration after ototoxic cell destruction with local application of gentamicin in the chinchilla crista ampullaris

Two experiments were conducted to study the ototoxic effects of local gentamicin (GM) administration and the subsequent hair cell (HC) regeneration process in the chinchilla cristae ampullares (CA). In the first experiment, 3 different doses of GM (0.1, 0.2 and 1.2 mg) were administered by surgical implantation of GM-soaked Gelfoam pledgets in the perilymphatic space in the otic capsule of the left superior semicircular canal. The CA was histologically processed for light-microscopic examination. In the second experiment, 6 groups of 2 chinchillas each were treated with 0.1 mg of GM. To document cell proliferation and HC regeneration, Alzet micro-osmotic pumps were implanted in each chinchilla to deliver bromodeoxyuridine (BrdU) at 125 micrograms/h for 1 week. Chinchillas were subsequently killed at 1 and 4 days and 1, 2, 4 and 8 weeks post-treatment (PT). The CA was processed for light microscopy and BrdU immunocytochemistry. In the first experiment the smallest dose produced damage restricted to HCs alone, while the medium and large doses produced severe damage in the sensory epithelium, including supporting cells and HCs. Results in the second experiment demonstrated that at 1 and 4 days PT the HCs showed extensive damage, including clumping of nuclear material. By 4 days PT the supporting cell nuclei lost their monolayer configuration. Calyceal terminals appeared empty, and vacuolized remnants of nerve calyces were evident in the basal portion. At 1 week PT complete disappearance of HCs from the sensory epithelium was evident, and there was cytoplasmic extrusion into the endolymphatic space. At 2 weeks PT there was complete HC loss, the supporting cell nuclei were scattered randomly in the crista, and the nerve fibers were retracted from the sensory epithelium. At 4 weeks PT there was evidence of sensory epithelium repair and HC regeneration. Short cells resembling type-II HCs were evident in the surface of the sensory epithelium. At 8 weeks PT the number of HCs increased in a uniform fashion on the surface of the sensory epithelium, and the supporting cell nuclei were realigned on the basal membrane. Nerve fibers with growth cones penetrated the basal membrane. Supporting cell proliferation was evident by the presence of mitotic figures and BrdU immunoreactivity in the chromatin material of dividing cells at 2 weeks PT. The labeling was more evident in newly formed cells at 4 and 8 weeks PT. These results demonstrate that in chinchillas the vestibular organs have the capacity of self-repair and the process includes HC regeneration after local administration of GM. The overall process involves changes in different cells in the sensory epithelium and neural elements, all of which show modifications with an orderly pattern.