Round window membrane intracochlear drug delivery enhanced by induced advection

Abstract

Delivery of therapeutic compounds to the inner ear via absorption through the round window membrane (RWM) has advantages over direct intracochlear infusions; specifically, minimizing impact upon functional hearing measures. However, previous reports show that significant basal-to-apical concentration gradients occur, with the potential to impact treatment efficacy. Here we present a new approach to inner ear drug delivery with induced advection aiding distribution of compounds throughout the inner ear in the murine cochlea. Polyimide microtubing was placed near the RWM niche through a bullaostomy into the middle ear cavity allowing directed delivery of compounds to the RWM. We hypothesized that a posterior semicircular canalostomy would induce apical flow from the patent cochlear aqueduct to the canalostomy due to influx of cerebral spinal fluid. To test this hypothesis, young adult CBA/CaJ mice were divided into two groups: bullaostomy approach only (BA) and bullaostomy+canalostomy (B+C). Cochlear function was evaluated by distortion product otoacoustic emission (DPOAE) and auditory brainstem response (ABR) thresholds during and after middle ear infusion of salicylate in artificial perilymph (AP), applied near the RWM. The mice recovered for 1week, and were re-tested. The results demonstrate there was no significant impact on auditory function utilizing the RWM surgical procedure with or without the canalostomy, and DPOAE thresholds were elevated reversibly during the salicylate infusion. Comparing the threshold shifts for both methods, the B+C approach had more of a physiological effect than the BA approach, including at lower frequencies representing more apical cochlear locations. Unlike mouse cochleostomies, there was no deleterious auditory functional impact after 1week recovery from surgery. The B+C approach had more drug efficacy at lower frequencies, underscoring potential benefits for more precise control of delivery of inner ear therapeutic compounds.

Keywords: Cochlea; Concentration gradient; Drug delivery; Infusion; Inner ear; Mouse.

Figure 1

Illustration of the mouse inner ear depicting the hypothesized advection flow induced by creation of a fluidic exit via canalostomy (c) in the posterior semicircular canal. Drug is delivered to the round window (rw) via polyimide microtubing inserted through a bullaostomy (b) into the middle ear (m) and positioned at the RWM niche. The drug is absorbed through the RWM into scala tympani (st). Diffusion is shown in red as dotted lines. Induced advection flow due to CSF influx from the cochlear aqueduct (ca) is shown as solid lines in blue. This flow carries the drug throughout the cochlea via a flow path through st, to the helicotrema at the apex, through scala vestibuli (sv) and into the vestibular system to the canalostomy (c). Adapted from [14].

Figure 2

Illustration depicting the experimental infusion setup. Polyimide microtubing was preloaded with 50 mM salicylate solution. The setup was attached to a syringe pump to precisely control infusion rate. The infusion tubing was inserted through the bullaostomy and positioned at the RWM niche. Adapted from [14].

Figure 3

Ventral surgical approach exposing the tympanic bulla with middle ear access for blind alignment to the RWM niche. (SMG) submandibular gland, (LEC) left ear canal, (DM) digastric muscle, (IT) infusion tubing, (BA) bullaostomy, (LB) left bulla. Inset: Explanted left cochlea highlighting the location of the bullaostomy relative to the round window. (TM) tympanic membrane, (OW) oval window, (RW) round window.

Figure 4

DPOAE threshold shifts versus F2 frequency for the BA (red traces) and B+C (blue traces) approaches during delivery of 50 mM salicylate to the RWM niche. Surgery had an insignificant impact on cochlear function. Clear threshold shifts are observed during delivery for both approaches, demonstrating transfer of the salicylate across the intact RWM. Asterisks indicate statistically significant main effects between methods. Data are plotted as mean ± SEM (n=6 animals for each approach).

Figure 5

DPOAE threshold shifts for the most basal F2 frequency over time. After infusion is stopped, there is a reduction of approximately 0.3dB per minute for the B+C approach suggesting a decline in basal salicylate concentration. This rapid decline may enable concentration control via flow control at the RWM niche. Data are plotted as mean ± SEM (n=6 animals for each approach).

Figure 6

In stark contrast to cochleostomy procedures in mice, DPOAE (left) and ABR (right) threshold shifts are insignificant as compared to pre-surgery baselines after one week of recovery. There is no difference between the BA (red) and B+C (blue) responses, and shifts from baseline are not significant, suggesting no impact to cochlear function with either approach. Data shown as mean ± SEM (n=6 animals for each approach).