Options for Study Design
The definitive data in an ototoxicity study is generated by correlating the auditory brainstem response with otic microscopy, including specific cytocochleogram analysis. As the assessments for middle ear histology and cytocochleogram each require one intact set of auditory structures, integrating these assessments into the study design can be problematic. To enable assessment of both of these post-life endpoints, and to maintain statistical power, there are two approaches to designing an ototoxicity study which requires direct middle ear exposure.
The first option is to use direct bilateral administration to the middle ears of each animal. This can be accomplished via transtympanic administration or via middle ear catheters implanted into both bullae. Using this technique, one ear may be used for middle ear histopathology and the other for the cytocochleogram analysis. This approach reduces the number of animals required for study and may allow for better correlation of middle ear histopathological and inner ear cytocochleogram analysis by reducing the inter-animal differences in drug distribution and metabolism.
The second option is to use direct administration to one ear (e.g., unilaterally) of each animal. This can also be accomplished via transtympanic administration or via middle ear catheters implanted into one bulla of each animal. To allow evaluation of both the cytocochleogram and the histopathology of relevant otic structures using this study design, twice as many animals are required for the assessment. Although most studies use one of the above alternative procedures for direct administration to the middle ear, in addition to this specialized route, systemic administration is a less complicated option relevant to some drug classes.
Auditory Brainstem Response Evaluation
Hearing thresholds are determined by repetitive recordings of the auditory brain stem responses, (ABR). The ABR procedure is conducted by evaluating the global neuronal activity within the auditory pathway from cochlea to cortex following stimulus presentations at specified frequencies and amplitudes. Determination of the response engendered in the auditory pathway at each frequency is used to interpolate the stimulus characteristics evoking the minimum hearing threshold. As this evaluation employs a quantitative EEG waveform to determine auditory thresholds, it is an ideal assessment which reduces behavioral variability and potential interpretational issues. Our test-retest data and interrater reliability with expert electrophysiologists in the field demonstrate our high level of expertise in auditory brainstem response data interpretation.
MPI Research has conducted multiple studies to validate our ABR methods for sensitivity, reliability, and predictive validity in several relevant species. The following graphs are representative data from these studies.
In addition to otic micrsocopy, cytocochleograms are required on auditory safety studies to support or refute any potential test article related ABR changes. Cytocochleograms are graphical illustrations of the number of missing auditory hair cells over the length of the cochlea. Drug-induced hair cell toxicity produces permanent hearing loss making the cytocochleogram a critical endpoint on auditory safety studies.
MPI Research characterizes middle ear toxicity by targeting those major structures associated with amplification/conduction of the sound pressure waves representing auditory stimuli. The structures evaluated are as follows: tympanum, middle ear ossicles (malleus, incus, stapes), tensor tympani, round window, cochlea, and the middle ear mucosa. If required, longitudinal sections of the Eustachian tube may also be included when possible. These procedures may be modified to include additional structures, as necessary.