Mechanisms Involved in Ototoxicity

Murine cochlear damage models in the context of hair cell regeneration research

Understanding the complex pathologies associated with hearing loss is a significant motivation for conducting inner ear research. Lifelong exposure to loud noise, ototoxic drugs, genetic diversity, sex, and aging collectively contribute to human hearing loss. Replicating this pathology in research animals is challenging because hearing impairment has varied causes and different manifestations. A central aspect, however, is the loss of sensory hair cells and the inability of the mammalian cochlea to replace them. Researching therapeutic strategies to rekindle regenerative cochlear capacity, therefore, requires the generation of animal models in which cochlear hair cells are eliminated. This review discusses different approaches to ablate cochlear hair cells in adult mice. We inventoried the cochlear cyto- and histo-pathology caused by acoustic overstimulation, systemic and locally applied drugs, and various genetic tools. The focus is not to prescribe a perfect damage model but to highlight the limitations and advantages of existing approaches and identify areas for further refinement of damage models for use in regenerative studies.

Effects of bumetanide on neonatal seizures: A systematic review of animal and human studies

BACKGROUND

Bumetanide, an inhibitor of the sodium-potassium-chloride cotransporter-1, has been suggested as an adjunct to phenobarbital for treating neonatal seizures.

METHODS

A systematic review of animal and human studies was conducted to evaluate the efficacy and safety of bumetanide for neonatal seizures. PubMed, Embase, CINAHL and Cochrane databases were searched in March 2023.

RESULTS

26 animal (rat or mice) studies describing 38 experiments (28 in-vivo and ten in-vitro) and two human studies (one RCT and one open-label dose-finding) were included. The study designs, methods to induce seizures, bumetanide dose, and outcome measures were heterogeneous, with only 4/38 experiments being in animal hypoxia/ischaemia models. Among 38 animal experiments, bumetanide was reported to have antiseizure effects in 21, pro-seizure in six and ineffective in 11. The two human studies (n = 57) did not show the benefits of bumetanide as an add-on agent to phenobarbital in their primary analyses, but one study reported benefit on post-hoc analysis. Overall, hearing impairment was detected in 5/37 surviving infants in the bumetanide group vs. 0/13 in controls. Four of the five infants with hearing impairment had received aminoglycosides concurrently. Other adverse effects reported were diuresis, mild-to-moderate dehydration, hypotension, and electrolyte disturbances. The studies did not report on long-term neurodevelopment. The certainty of the evidence was very low.

CONCLUSION

Animal data suggest that bumetanide has inconsistent effects as an antiseizure medication in neonates. Data from human studies are scarce and raise some concerns regarding ototoxicity when given with aminoglycosides. Well conducted studies in animal models of hypoxic-ischaemic encephalopathy are urgently needed. Future RCTs, if conducted in human neonates, should have an adequate sample size, assess neurodevelopment, minimize using aminoglycosides, be transparent about the potential ototoxicity in the parent information sheet, conduct early hearing tests and have trial-stopping rules that include hearing impairment as an outcome.

DNA methylation in the mouse cochlea promotes maturation of supporting cells and contributes to the failure of hair cell regeneration

Mammalian hair cells do not functionally regenerate in adulthood but can regenerate at embryonic and neonatal stages in mice by direct transdifferentiation of neighboring supporting cells into new hair cells. Previous work showed loss of transdifferentiation potential of supporting cells is in part due to H3K4me1 enhancer decommissioning of the hair cell gene regulatory network during the first postnatal week. However, inhibiting this decommissioning only partially preserves transdifferentiation potential. Therefore, we explored other repressive epigenetic modifications that may be responsible for this loss of plasticity. We find supporting cells progressively accumulate DNA methylation at promoters of developmentally regulated hair cell genes. Specifically, DNA methylation overlaps with binding sites of Atoh1, a key transcription factor for hair cell fate. We further show that DNA hypermethylation replaces H3K27me3-mediated repression of hair cell genes in mature supporting cells, and is accompanied by progressive loss of chromatin accessibility, suggestive of facultative heterochromatin formation. Another subset of hair cell loci is hypermethylated in supporting cells, but not in hair cells. Ten-eleven translocation (TET) enzyme-mediated demethylation of these hypermethylated sites is necessary for neonatal supporting cells to transdifferentiate into hair cells. We also observe changes in chromatin accessibility of supporting cell subtypes at the single-cell level with increasing age: Gene programs promoting sensory epithelium development loses chromatin accessibility, in favor of gene programs that promote physiological maturation and function of the cochlea. We also find chromatin accessibility is partially recovered in a chronically deafened mouse model, which holds promise for future translational efforts in hearing restoration.

Extended High-Frequency Audiometry for Ototoxicity Monitoring: A Longitudinal Evaluation of Drug-Resistant Tuberculosis Treatment

PURPOSE

The aim of this study was to describe extended high-frequency (EHF) pure-tone audiometry monitoring of ototoxicity in a longitudinal treatment program for drug-resistant tuberculosis (DRTB).

METHOD

This was a retrospective record review of longitudinal conventional (0.25-8 kHz) and EHF (9-16 kHz) audiometry for ototoxicity monitoring of DRTB patients undergoing treatment at community-based clinics between 2013 and 2017. Data from 69 patients with an average age of 37.9 years (SD = 11.2, range: 16.0-63.8 years) were included. Patients were assessed by primary health care audiologists (87%) or community health workers (13%) using portable audiological equipment. The average length of time between initial and exit assessments was 84.6 days (SD = 74.2, range: 2-335 days).

RESULTS

EHF ototoxicity of a mild or greater degree of hearing loss (> 25 dB HL in one or both ears across frequencies) was evident in 85.5% of patients' posttreatment, compared with 47.8% of patients across conventional frequencies. EHF audiometry demonstrated an ototoxic shift (American Speech-Language-Hearing Association criteria) in 56.5% of cases compared with 31.9% when only conventional audiometry was considered. Mean hearing deterioration for patients was significant across EHFs (9-16 kHz) bilaterally (p < .05). Absent EHF thresholds at the initial assessment, owing to maximum output limits, was a limitation that occurred most frequently at 16 kHz (17.4%, 24/138).

CONCLUSIONS

EHF audiometry is most sensitive for the early detection of ototoxicity and should be included in monitoring programs. Clinical ototoxicity monitoring protocols should consider shortened assessment approaches that target frequencies most sensitive to ototoxicity, including EHFs.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.21651242.

Prevention of Severe Vestibular Hypofunction after Systemic Gentamicin

The importance of early evaluation by a neurotologist in patients with infective endocarditis treated with systemic gentamicin and its impact on the patients’ quality of life was evaluated. This is a longitudinal retrospective cohort study of 29 patients who received intravenous gentamicin for the treatment of infective endocarditis. Patients were classified into two groups: group A, before a neurotologist was included in the treatment protocol, and group B, after the inclusion of a neurotologist. The frequency of the different symptoms in each group was measured, and the gain of the vestibulo-ocular reflex (VOR) and its relationship with the presence of oscillopsia. In total, 13 and 16 patients were assigned to groups A and B, respectively. The mean gain of the VOR measured using the video head impulse test in group A was 0.44 in the best side and 0.39 in the worst side. In group B, the mean gain was 0.71 (best side) and 0.64 (worst side) (p < 0.0001). The patients who complained about oscillopsia had a main gain of 0.41 in the best side and 0.35 in the worst side. Evaluation of vestibular function should be included in the infective endocarditis treatment protocol, including the adverse effects of systemic gentamicin.

A Pilot Randomized, Controlled, Double-Blind Trial of Bumetanide to Treat Neonatal Seizures

OBJECTIVE

In the absence of controlled trials, treatment of neonatal seizures has changed minimally despite poor drug efficacy. We tested bumetanide added to phenobarbital to treat neonatal seizures in the first trial to include a standard-therapy control group.

METHODS

A randomized, double-blind, dose-escalation design was employed. Neonates with postmenstrual age 33 to 44 weeks at risk of or with seizures were eligible. Subjects with electroencephalography (EEG)-confirmed seizures after ≥20 and <40mg/kg phenobarbital were randomized to receive additional phenobarbital with either placebo (control) or 0.1, 0.2, or 0.3mg/kg bumetanide (treatment). Continuous EEG monitoring data from ≥2 hours before to ≥48 hours after study drug administration (SDA) were analyzed for seizures.

RESULTS

Subjects were randomized to treatment (n = 27) and control (n = 16) groups. Pharmacokinetics were highly variable among subjects and altered by hypothermia. The only statistically significant adverse event was diuresis in treated subjects (48% vs 13%, p = 0.02). One treated (4%) and 3 control subjects died (19%, p = 0.14). Among survivors, 2 of 26 treated subjects (8%) and 0 of 13 control subjects had hearing impairment, as did 1 nonrandomized subject. Total seizure burden varied widely, with much higher seizure burden in treatment versus control groups (median = 3.1 vs 1.2 min/h, p = 0.006). There was significantly greater reduction in seizure burden 0 to 4 hours and 2 to 4 hours post-SDA (both p < 0.01) compared with 2-hour baseline in treatment versus control groups with adjustment for seizure burden.

INTERPRETATION

Although definitive proof of efficacy awaits an appropriately powered phase 3 trial, this randomized, controlled, multicenter trial demonstrated an additional reduction in seizure burden attributable to bumetanide over phenobarbital without increased serious adverse effects. Future trials of bumetanide and other drugs should include a control group and balance seizure severity. ANN NEUROL 2021;89:327-340.

[Drug-induced hearing loss as a manifestation of drug-induced ototoxicity]

The ability of drugs to have an ototoxic effect has been studied for a long time, however, the true prevalence of this undesirable phenomenon is unknown, which is due to the use of various audiological protocols, a wide range of reactions to drugs in different ethnic groups, and most importantly, the lack of caution with regard to otological symptoms due to their reversibility or lack of immediate threat to life. Drug-induced ototoxicity is a functional disorder of the inner ear (cochlea and/or vestibular apparatus) or eighth pair of cranial nerves. Pharmacotherapy, associated with the development of ototoxic drug reactions, may remain undervalued for a long time, often until irreversible hearing impairment is formed. The most frequently prescribed drugs that cause ototoxic phenomena include anticancer drugs, antibacterial drugs of the aminoglycoside group, loop diuretics, calcium channel blockers, non-steroidal anti-inflammatory drugs, antimalarial drugs, salicylates, etc. Monitoring the degree of hearing impairment before and during therapy is important in preventing the development of drug-induced ototoxicity and makes it possible to consider alternative treatment regimens in a timely manner. It is in this connection that the role of participation in the appointment of rational pharmacotherapy to patients with a potential risk of developing otological phenomena of a clinical pharmacologist and audiologist undoubtedly increases.

Pharmacogenetics of drug-induced ototoxicity caused by aminoglycosides and cisplatin

Aminoglycosides and the anticancer drug cisplatin can cause permanent hearing loss, which impacts patients' quality of life and results in considerable subsequent costs. Since patients' individual susceptibility to aminoglycoside- and cisplatin-induced ototoxicity varies considerably, strategies are needed to identify patients at risk, who may require alternative treatments or specific protection strategies. For both drugs, various genetic variants were linked to an increased or decreased risk for ototoxicity. Except for the association between the A1555G mitochondrial DNA mutation and aminoglycoside ototoxicity, their evidence is considered low because study cohorts were often small and replication studies either missing or contradictory. This review summarizes the pharmacogenetic markers linked to aminoglycoside- or cisplatin-induced ototoxicity and discusses reasons for replication failure and future perspective.

Assessment of nutrient supplement to reduce gentamicin-induced ototoxicity

Gentamicin is an aminoglycoside antibiotic used to treat gram-negative bacterial infections. Treatment with this antibiotic carries the potential for adverse side effects, including ototoxicity and nephrotoxicity. Ototoxic effects are at least in part a consequence of oxidative stress, and various antioxidants have been used to attenuate gentamicin-induced hair cell death and hearing loss. Here, a combination of nutrients previously shown to reduce oxidative stress in the hair cells and attenuate hearing loss after other insults was evaluated for potential protection against gentamicin-induced ototoxicity. Guinea pigs were maintained on a nutritionally complete standard laboratory animal diet or a diet supplemented with β-carotene, vitamins C and E, and magnesium. Three diets with iterative increases in nutrient levels were screened; the final diet selected for study use was one that produced statistically reliable increases in plasma levels of vitamins C and E and magnesium. In two separate studies, significant decreases in gentamicin-induced hearing loss at frequencies including 12 kHz and below were observed, with less benefit at the higher frequencies. Consistent with the functional protection, robust protection of both the inner and outer hair cell populations was observed, with protection largely in the upper half of the cochlea. Protection was independently assessed in two different laboratories, using two different strains of guinea pigs. Additional in vitro tests did not reveal any decrease in antimicrobial activity with nutrient additives. Currently, there are no FDA-approved treatments for the prevention of gentamicin-induced ototoxicity. The current data provide a rationale for continued investigations regarding translation to human patients.