Gene Therapy / Stem Cell Therapy

Gene Therapy / Stem Cell Therapy Accompany with Other Stimuli
A multichannel scala tympani electrode array incorporating a drug delivery system for chronic intracochlear infusion
Robert K. Shepherd, Jin Xu
Hearing Research 172 (2002) 92-98
Cochlear is a tiny microenvironment with high electrical stimuli, thus, electrical stimuli should be a very important component when considering to regenerate hair cells or SGNs by either gene therapy or stem cell therapy. Several studies already showed that cell differentiation of development of cell phenotypes are highly dependent on the environment which cultured them. So right now, let's see the effect brought from additional electrical stimulation. The authors developed a novel scala tympani electrode array suitable for use in experimental animals. A unique feature of this array is its ability to chronically deliver pharmacological agents to the scala tympani. The design of the electrode array is described in detail. Experimental studies performed in guinea pigs confirm that this array can successfully deliver various drugs to the cochlea while chronically stimulating the auditory nerve.
*The multichannel electrode array*
Figure 1 shows the multichannel electrode array. The array consists of three major parts : (1) an intracochlear electrode array, (2) a connector and (3) a stainless steel leadwire assembly (Fig. 1). The intracochlear electrode array is manufactured using injection-molding techniques. It consists of three Pt rings, each 0.3 mm in width, on a silicone rubber carrier. The array tapers from a diameter of 0.40 mm at the tip to 0.50 mm 5 mm from the tip. A ‘dummy’ Pt ring is located at this point as a guide to insertion depth during surgery. The three Pt electrodes are located near the tip of the array with an inter-electrode separation of 0.45 mm. Teflon-insulated Pt-Ir (90: 10) wire (25 Wm diameter) is welded to each Pt electrode. Prior to injection molding, a length of polyimide tubing is placed longitudinally within the central core of the array. After the injected silicone has cured, any protruding polyimide tubing at the apical tip of the array is removed. The opposite end of this polyimide tubing exits the leadwire and is connected to an osmotic pump. Each 25 Wm diameter Pt-Ir wire from the electrode array is connected to a Teflon-insulated multistranded stainless steel leadwire. The connector is encapsulated by silicone rubber, and the three stainless steel leadwires are protected within a silicone tube (Fig. 1). The polyimide delivery tube passes through the connector and exits the leadwire proximal to the connector. Approximately 60mm of single lumen polyvinyl chloride (PVC) tubing is placed over the polyimide delivery tube and glued to the leadwire using silicone adhesive. Particular care is taken to ensure that the polyimide tube is sealed within the PVC tube.
Figure 1 Diagram illustrating the novel electrode assembly. A polyimide microtube provides a path to deliver pharmacological agents from an osmotic pump into the scala tympani via the tip of the electrode array. All dimensions are in mm. The inset is a micrograph of the array showing the three Pt ring electrodes and the polyimide tube (arrow). Each Pt electrode is 0.3 mm wide.
*Results*
Patency of the electrode assemblies All electrode arrays exhibited a patent drug delivery system both prior to implantation and following the 28-day implantation period. No assembly showed evidence of a blockage, and apart from the cannula at the tip of the array, there was no evidence of leaks when fluid was delivered to the PVC tube. Moreover, inspection of the osmotic pumps revealed little, if any, residual fluid following the 28-day implantation period. Status of the electrodes All electrode arrays remained functionally viable for the duration of the implant period. Electrode impedances varied from 2 to 8 k6 during the course of the implantation period; this range is typical for electrodes of this design. EABRs of normal morphology for this species were readily evoked over the duration of the implant period (Fig. 2), and confirm the functional status of the electrode assembly.
Figure 2. EABRs recorded from GP_N02 immediately (A) and 28 days (B) following implantation of a bipolar electrode array that chronically perfused the cochlea with neomycin. EABRs were evoked using 100 Ws/phase biphasic current pulses at current amplitudes indicated on the right of this graph. Two responses were recorded at each current level.
Cochlear histopathology Chronic neomycin infusion in prior normal cochleae Fig. 3 illustrates representative histology showing the chronic neomycin-infused and unimplanted control cochleae from both neomycin-treated animals. Qualitatively, both control cochleae appeared normal, exhibiting near normal organ of Corti throughout all turns, and no evidence of loss of SGNs or their peripheral processes. In contrast, both implanted cochleae showed no surviving hair cells ; a collapsed or completely absent organ of Corti; and a moderate to severe loss of neural elements. This pathology was not restricted to the basal turn but extended throughout all turns of both cochleae. Finally, both implanted cochleae exhibited a minimal fibrous tissue reaction in response to the chronically implanted electrode array. Chronic infusion of artificial perilymph in deafened cochleae Fig. 4 illustrates the upper basal turn of two of our implanted/stimulated animals, showing the absence of the sensory epithelium, and reduced SGN survival as a result of the KA/FU deafening procedure. Importantly for the evaluation of this electrode array, there is a minimal tissue response to the implanted array following chronic electrical stimulation and delivery of sterile Ringer’s solution. The minimal tissue response observed in these cochleae is consistent with that observed following chronic neomycin infusion.		Figure 3 Photomicrographs illustrating the extent of cochlear histopathology associated with the chronic neomycin infusion experiments. (A)Upper middle turn (UMT) of control cochlea GP_001R. (B) UMT of deafened cochlea GP_001L (C) UMT of control cochlea GP_002R. (D) UMT of deafened cochlea GP_002L. (E) Midmodiolar view of normal cochlea GP_002R. (F) Midmodiolar view of deafened cochlea GP_002L. Note the loss of the organ of Corti and SGNs throughout the deafened cochleae (right column) compared to the control (left column). Scale bar=100 Wm (A^D); 1 mm (E,F).
Conclusion	Figure 4 Photomicrographs of the basal turn of two cochleae following chronic infusion of Ringer’s solution. Both animals had been deafened for 33 days, their left cochleae (illustrated) implanted with an electrode array and drug delivery assembly for 28 days. During this period the cochlea was electrically stimulated V6 h/day and 200 Wl of sterile Ringers solution was delivered to the scala tympani. Note the loss of the organ of Corti (S), the partial loss of SGNs (arrow), and the minimal tissue response to the electrode assembly within the scala tympani. Scale bar=100 Wm.
Drug delivered by multichannel electrode array system accompany with osmotic pumping system provides longer-distance distribution of the delivering drugs The electrode is a biocompatible assembly: a minimal tissue response, implying that a tissue capsule - which may affect drug distribution - is not a common occurrencein this model. This multichannel electrode array system would be applicable for many other animal models with only minimum revision: Theoretically, this concept could also have clinical application although the mechanical characteristics of the electrode array and the limited pump life would need to be addressed, as would important safety considerations associated with the administration of drugs into the cochlea The design of the system is not restrict to the use of a single pump. Delivering a pharmacological cocktail to the cochlea with several pumps simultaneously is possible. Osmotic pumps with different flow rates could provide additional control over such experiments. This report demonstrates the feasibility of achronic drug delivery system coupled to an intracochlear electrode array. Such an array will allow controlled depolarization of SGNs while chronically administering various pharmaceutical agents to the scala tympani.