Anatomical basis of drug delivery to the inner ear

The isolated anatomical position and blood-labyrinth barrier hampers systemic drug delivery to the mammalian inner ear. Intratympanic placement of drugs and permeation via the round- and oval window are established methods for local pharmaceutical treatment. Mechanisms of drug uptake and pathways for distribution within the inner ear are hard to predict. The complex microanatomy with fluid-filled spaces separated by tight- and leaky barriers compose various compartments that connect via active and passive transport mechanisms. Here we provide a review on the inner ear architecture at light- and electron microscopy level, relevant for drug delivery. Focus is laid on the human inner ear architecture. Some new data add information on the human inner ear fluid spaces generated with high resolution microcomputed tomography at 15 μm resolution. Perilymphatic spaces are connected with the central modiolus by active transport mechanisms of mesothelial cells that provide access to spiral ganglion neurons. Reports on leaky barriers between scala tympani and the so-called cortilymph compartment likely open the best path for hair cell targeting. The complex barrier system of tight junction proteins such as occludins, claudins and tricellulin isolates the endolymphatic space for most drugs. Comparison of relevant differences of barriers, target cells and cell types involved in drug spread between main animal models and humans shall provide some translational aspects for inner ear drug applications.