Round Window Membrane and Delivery of Biologically Active Agents into the Cochlea

Establishing efficient methods for local administration of drugs to the inner ear has great clinical relevance for the management of inner ear disorders. However, the administration route remains a critical issue. The most feasible approach for a non invasive drug delivery to the inner ear is application of medication to the middle ear cavity on the promise that it will diffuse through the thin round window membrane (RWM) separating the inner ear from the middle ear cavity. Gene therapy represents a promising future in otology and offers an exciting therapeutic alternative as it could be used in prevention or management of cochlear disorders. Also for a gene therapy approach, RWM application seems most feasible administration route. Exploring and characterizing the RWM route of administration is thus a fundamentally important area of research for the development of future treatment of inner ear disorders. The objectives of the thesis were to evaluate the efficacy of two drug and gene delivering vehicles to the inner ear, sodium hyaluronate (HYA) and chitosans, which can be applied to the cochlea. Ultimate aim is to establish an efficient drug delivery system and gene transfection for the inner ear. HYA and chitosans loaded with the ototoxic drug neomycin as tracer for drug release have been instilled into the middle ear of the guinea pigs. Effects on RWM and cochlear hair cells were evaluated after a single instillation of HYA (day 7 and 28), chitosans and saline solution (day 7). The hearing organ was analysed for hair cell loss and the thickness and ultrastructural properties of the RWM were analysed by light and transmission electron microscopy. The in vitro transfection efficiency of chitosan was tested by exposing organotypic cultures of the organ of Corti, prepared from postnatal day 2 rats, to chitosan carrying plasmid DNA (pDNA). The in vivo transfection efficiency was tested at one day or seven days after infusing chitosan/pDNA polyplexes with the use of osmotic pumps into the cochlea of adult guinea pigs. Tissue analysis was made by immunohistochemsitry and RT-PCR. HYA and chitosans, especially glycosylated derivative, are safe vehicles that can be used for drug transport into the inner ear through the RWM. Both vehicles successfully released the loaded neomycin, which exerted toxic effects on cochlear hair cells in a degree depending on the concentrations used. The vehicles per se had no noxious effect on the cochlear hair cells but they provoked a comparable effect on the thickness and morphology of the RWM. The thickness of the RWM returned to normal 4 weeks after exposure to HYA. Chitosan as a carrier for inner ear transfection, was associated with inconsistent transfection in vitro and in vivo. The importance of the RWM as a portal for local therapy of inner ear disorders is highlighted in this thesis by focusing on efficiency and effects of the vehicles, applied to the RWM for delivering biologically active agents into the cochlea. The difficulties and variability associated with applying substances to the RWM were explored. The results of this thesis add new knowledge concerning mechanisms of passage of biologically active agents through the RWM and may help us to better understand the role of RWM in the local cochlear therapy and problems of local treatment of inner ear diseases.