Murine intracochlear drug delivery: Reducing concentration gradients within the cochlea

Abstract

Direct delivery of compounds to the mammalian inner ear is most commonly achieved by absorption or direct injection through the round window membrane (RWM), or infusion through a basal turn cochleostomy. These methods provide direct access to cochlear structures, but with a strong basal-to-apical concentration gradient consistent with a diffusion-driven distribution. This gradient limits the efficacy of therapeutic approaches for apical structures, and puts constraints on practical therapeutic dose ranges. A surgical approach involving both a basal turn cochleostomy and a posterior semicircular canal canalostomy provides opportunities for facilitated perfusion of cochlear structures to reduce concentration gradients. Infusion of fixed volumes of artificial perilymph (AP) and sodium salicylate were used to evaluate two surgical approaches in the mouse: cochleostomy-only (CO), or cochleostomy-plus-canalostomy (C+C). Cochlear function was evaluated via closed-system distortion product otoacoustic emissions (DPOAE) threshold level measurements from 8-49 kHz. AP infusion confirmed no surgical impact to auditory function, while shifts in DPOAE thresholds were measured during infusion of salicylate and AP (washout). Frequency dependent shifts were compared for the CO and C+C approaches. Computer simulations modeling diffusion, volume flow, interscala transport, and clearance mechanisms provided estimates of drug concentration as a function of cochlear position. Simulated concentration profiles were compared to frequency-dependent shifts in measured auditory responses using a cochlear tonotopic map. The impact of flow rate on frequency dependent DPOAE threshold shifts was also evaluated for both surgical approaches. Both the C+C approach and a flow rate increase were found to provide enhanced response for lower frequencies, with evidence suggesting the C+C approach reduces concentration gradients within the cochlea.

Round window membrane intracochlear drug delivery enhanced by induced advection

“…A concentration of 8 mg/ml (50 mM) at 50 nl/min flow rate with 1000 nl total delivery yielded measurable responses within the acute timeframe of the planned experiments. This concentration is similar to that employed with dexamethasone delivery to the RWM in guinea pigs (10 mg/ml) [21], with both concentration and flow rate higher than those used in our past studies with direct intracochlear infusion into scala tympani (1.6 mg/ml (10mM), 16nl/min) [14] as expected with diffusion through the RWM.…”

“…The animal was positioned on a heated operative plane on its back under aseptic conditions. Surgery was performed on the left (ipsilateral) ear, following procedures outlined by Borkholder and colleagues [14]. The tympanic bulla was exposed by a ventral approach as shown in Figure 3.…”

“…The surrounding tissue was removed to expose the inferior-medial aspect of the bulla which was carefully cleaned and dried. A bullaostomy was drilled by hand at a location approximately 1.5 mm laterally below the stapedial artery using a 175 µm diameter carbide micro drill bit modified to include insertion stops [14]. Sequentially longer insertion-depth 175 µm bits were used (300 and 350 µm), with bulla entry determined by a subtle change in mechanical resistance and visualization of the middle ear cavity.…”

“…Drug delivery is generally accomplished via a syringe pump [14] or osmotic pump [15] for continuous infusion, or a reciprocating pump for zero net volume delivery [16]. While approaches that directly enter the perilymphatic space can preserve auditory function acutely, reports of successful recovery and chronic retention of hearing are rare, particularly in small rodents.…”

“…We previously discovered that perfusion from a basal turn, scala tympani cochleostomy to a fluidic exit created by a posterior semicircular canal canalostomy reduced concentration gradients in the mouse cochlea [14]. While this approach had no impact on acute hearing function, recovery surgeries evidenced loss of hearing as measured by both distortion product otoacoustic emissions (DPOAEs) and auditory brainstem responses (ABRs) one week post-surgery.…”