Measurement of Ototoxicity Following Intracochlear Bisphosphonate Delivery

Abstract

Hypothesis: Assessing the maximum safe dose for local bisphosphonate delivery to the cochlea enables efficient delivery without ototoxicity.

Background: Otosclerosis is a disease of abnormal bone metabolism affecting the otic capsule, which can cause conductive hearing loss. Larger otosclerotic lesions involving the cochlear endosteum and spiral ligament can result in sensorineural hearing loss. Bisphosphonates are used to treat patients with metabolic bone diseases, including otosclerosis. Local delivery is the most efficient way of delivery to the cochlea while avoiding systemic side effects. To attain intracochlear bisphosphonate delivery without ototoxicity, the maximum safe dose of bisphosphonates requires definition. In the present study, we tested increasing concentrations of zoledronate, a third-generation bisphosphonate in an intracochlear delivery system. We measured ototoxicity by monitoring distortion product otoacoustic emissions and compound action potentials.

Methods: Artificial perilymph and increasing molar concentrations of zoledronate were administered to the cochlea in guinea pigs via a cochleostomy. Hearing was measured at multiple time points. A fluorescently labeled zoledronate derivative (6-FAM-ZOL) was coadministered as an internal control for drug delivery. Specimens embedded in the resin blocks were ground to a mid-modiolar section and fluorescent photomicrographs were taken.

Results: No significant shift in hearing was observed in animals treated either with artificial perilymph or with 4% of the human systemic zoledronate dose. However, compound action potentials thresholds increased during infusion of 8% of the human systemic zoledronate dose, improved 4 hours later, and then increased again 4 weeks later. Using fluorescent photomicrography, intracochlear bisphosphonate delivery up to the apical cochlear turn was confirmed by visualizing 6-FAM-ZOL.

Conclusion: These findings provide reference values for intracochlear bisphosphonate delivery in the treatment of cochlear otosclerosis and describe a useful method for tracking cochlear drug delivery.

Figures

Fig. 1. Fluorescent photomicrographs following direct intracochlear infusion of artificial perilymph, 0.04× solution, and 0.08× solution

Fluorescent photomicrographs taken at mid-modiolar sections of the cochlea are shown for cochleas treated with artificial perilymph (A and D), cochleas treated with 0.04× solution (B and E), and cochleas treated with 0.08× solution (C and F). The upper row shows fluorescent photomicrographs of guinea pigs sacrificed 4 hours after infusion, and the lower row those of guinea pigs sacrificed 4 weeks after infusion. 0.04× and 0.08× solution, respectively, indicate 4% and 8% of the human systemic zoledronate dose mixed with a small amount of 6-FAM-ZOL at corresponding molar concentrations.

Fig. 2. DPOAE and CAP measurements following cochlear infusion

The upper row indicates distortion product otoacoustic emissions (DPOAE) and the lower row compound action potentials (CAP). The chronic experiments are graphed along the curve labeled 4 weeks, while the other curves are from acute experiments. Averages from independent experiments are shown. 0.04× and 0.08× solution indicate 4% and 8% of human systemic zoledronate dose, respectively, mixed with a small amount of 6-FAM-ZOL at corresponding molar concentrations. No significant DPOAE shift was observed during the course of the experiments. The CAP shift in the 0.08× solution group occurred at 30 minutes between 8 kHz and 24 kHz, partially normalized at 260 minutes, and increased again four weeks later.

Fig. 3. Threshold shift of compound action potentials (CAP) following infusion of 0.08× solution at the frequencies of 24 (A), 16 (B), 12 (C), and 8 kHz (D)

The graphs present the average values generated from independent experiments of either acute or chronic experiments. The acute experiments were performed in one day, while the chronic experiment animals were kept alive for 4 weeks after surgery. The vertical and the horizontal standard error bars are shown for CAP threshold shift and the time point when the CAP was measured, respectively. The rectangular grey-shaded areas indicate the time period over which drug was infused. CAP shift in 0.08× solution peaked between 12 and 16 kHz. 0.08× solution indicates 8% of the human systemic zoledronate dose mixed with a small amount of 6-FAM-ZOL at corresponding molar concentrations.