Name: slow-release drug delivery through elvax 40w to the rat retina: implications for the treatment of chronic conditions
Uploaded: 2014-09-17T13:00:00
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This research was supported by the Australian Research Council through the ARC Centre of Excellence in Vision Science (CE0561903), the National Health and Medical Research Council (1049990), and Progetto regionale speciale multiasse 'Reti per l'Alta Formazione' - PO FSE Abruzzo 2007-2013 - Azione 4.II.iii.

All experiments conducted were in accordance with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research, and with the approval of the University of L&#8217;Aquila Animal Ethics Committee and the Australian National University Animal Experimentation Ethics Committee. Adult rats (P100-200) were used throughout this study.
1. Prepare the Copolymer Resin Pellets
<ol>
	<li>Place 20 Elvax 40W pellets into a small glass beaker in the fume cupboard.</li>
	<li>Fill the beaker wi...

<ol>
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This paper demonstrated the use of a drug-resin complex for the slow-release delivery of drugs to the retina.&#160;We aimed to present a method which is relatively inexpensive and easy to apply in a small animal model.
Given that the function of APB is to act as a glutamate analogue, it will block the retinal response in the eye. The results show that APB caused the blockage of the retinal response by one week postimplantation. This indicates that APB was successfully released into the vitreou...

The treatment of chronic diseases such as diabetes and high blood pressure presents many challenges, as these diseases generally require treatment for extended periods of time, often for life. This has called for the development of slow release drug delivery systems, which reduce the need for frequent dosing. The effectiveness of these slow release methods has been demonstrated through the development of insulin pumps, to reduce the number of insulin injections needed to treat diabetes. Chronic diseases of the eye, particularly those affecting its inner layers, require the frequent administering of drugs through invasive procedures. One such chronic disease affecting the human eye is age related macular degeneration (AMD). It affects the central retina, which is a layer of neural tissue situated at the back of the eye responsible for initiating vision. AMD is the leading cause of blindness in the Western World1. A particular challenge with treating retinal diseases is that the drug is required to reach this deep layer in the eye, which often requires invasive methods of delivery. Drugs are usually administered to the vitreous chamber and retina using intravitreal injections. However, with each injection there is a risk of postinjection complications, including endophthalmitis, retinal detachment, cataract and vitreous hemorrhaging2. This risk is multiplied with every reinjection of the drug.
Reducing the need for multiple injections would be a major benefit in the treatment of AMD. To treat the wet form of AMD, where new vessel growth is a hallmark, the established therapeutic strategy is to target the endothelial growth factors (VEGF) using VEGF inhibitors3. At present, these are delivered through repeated intravitreal injections. Similarly, in the treatment of macular oedema, a common complication of diabetic retinopathy, corticosteroids are delivered through repeated injections4. Delivery of these drugs through slow release methods could indeed reduce the risk of postinjection complications.
The notion of a slow release drug delivery system was first described using a silicone rubber vehicle to deliver small molecules into animal tissues5. Since then, other slow release methods have been developed to deliver larger molecules, several of which have been tested in the eye. Particle carriers, such as biodegradable microspheres, poly-lactide-co-glycolide (PLGA) nanoparticles and phospholipid vesicles (liposomes) can be useful as delivery vehicles6,7. PLGA nanoparticles and liposomes have been compared in an in vitro environment for their ability to deliver anti-cancer agents across the sclera over time7. Both vehicles were effective in slowly releasing the drugs. However, the study was only conducted in an in vitro environment. Bochot et al. (2002)8 tested the efficacy of liposomes to deliver molecules to the retina in vivo. They have demonstrated that liposomes successfully deliver small oligonucleotides to the rabbit retina. The authors suggested that liposomes could be beneficial in the treatment of retinal diseases8. However, the nature of these vesicles to float in the vitreous means that they will likely blur or impair vision9.
Okabe et al. (2003)10 used non biodegradable polymer discs made up of 33% ethylene-vinyl acetate to apply beta-methasone in rabbits. They implanted discs into a scleral pocket and demonstrated the effective release of the drug into the vitreous and retina for up to one month10. However, in this particular protocol, the implant was relatively large and rigid, and required a more complex surgical procedure including a large scleral incision and suturing.
An earlier study investigated the tissue response to various polymer vehicles by implanting them into the cornea of rabbit eyes, and found that ethylene-vinyl acetate copolymers washed in alcohol did not cause inflammation or irritation. These complexes were shown to sustain the delivery of larger compounds into animal tissues for extended periods of time, some exceeding up to 100 days dependent on the drug11. One such type of copolymer resin has been developed industrially in the form of Elvax 40W (40% by weight ethylene-vinyl acetate comonomer content with a &#8216;W&#8217; amide additive to improve pellet handling). This copolymer resin is an inert substance which is stable at both room and body temperature. It has not been shown to cause allergies or toxicity in biological tissue. This resin has effectively delivered a large variety of drugs in different experimental models that investigate the functions of various systems, such as the mammary ductal system12, the primary auditory cortex13,14, and the frog visual system15. This resin has also been used in the eye to deliver drugs to developing turtle16,17, chicken embryo18,19, and adult ferret retinas20. In the rat central nervous system, the resin has only been used in the brain21-23, but its use in the rat eye has not been documented.
The advantages of using this copolymer resin to deliver drugs slowly to the retina over other methods, is that it is a stable compound that does not cause inflammation or irritation in the eye. Unlike particle carriers, the drug-resin complex would not impair vision after implantation, as it usually remains at the site of delivery instead of floating in the vitreous. It would only require a simple implantation process into the vitreous cavity close to the limbus of the eye, and would not require suturing after implantation. Recently, there has been an emergence in several novel delivery systems, such as encapsulated cell technology (ECT)24,25, hydrogels26, and microfilms27. However, the method used in the current study for preparing and delivering the drug-resin complex is both easy to follow and inexpensive, thereby being more advantageous for use in a basic research environment. The challenge of using this complex to deliver long-term drug treatments is to determine the optimal concentration of the drug that will maximize the therapeutic benefits of having fewer intravitreal injections.
This paper aims to demonstrate the use of the drug-resin complex for long-term treatment of the adult rat retina. The efficacy of this mode of delivery is tested using the glutamate analogue 2-amino-4-phosphonobutyrate (APB) as the drug. APB blocks the light response of the ON bipolar cells by mimicking glutamate, an endogenous neurotransmitter in the retina28. When APB competes with glutamate for its receptor, it blocks the light response. APB has been used in physiological studies to control retinal function and measure its effect using electrophysiological methods such as electroretinography (ERG). In previous studies, APB has been used for both short-term29 and long-term treatment of the developing retina; the latter involved giving a single dose via intraocular injection daily for 30 days30. The same amount of APB (0.092 mg in sterile saline, at a concentration of 50 &#181;M) was used for all injections as suggested in previous work28,29,31. We chose APB to test the drug-resin complex as a slow-release vehicle to deliver drugs into the eye. The methods outlined in this study are similar to previously described methods involving the preparation of the drug-resin complex16,32; however we also detail its use specifically in the adult rat eye. After surgical implantation of the APB-loaded resin into the eye, ERG was performed to establish whether APB abolishes the retinal light response, and therefore whether APB has been successfully released into the vitreous and retina.

<table border="0" cellpadding="0" cellspacing="0">
	<tbody>
		<tr>
			<td>Elvax 40W Pellets</td>
			<td>Du Pont, DE, USA</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Dimethyl sulfoxide (DMSO)</td>
			<td>Sigma-Aldrich Co. LLC., MO, USA</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>2-amino-4-phosphonobutyric acid (APB)</td>
			<td>Sigma-Aldrich Co. LLC., MO, USA</td>
			<td>A1910</td>
			<td></td>
		</tr>
		<tr>
			<td>Dichloromethane</td>
			<td>Sigma-Aldrich Co. LLC., MO, USA</td>
			<td>34856</td>
			<td></td>
		</tr>
		<tr>
			<td>Fast Green FCF</td>
			<td>Sigma-Aldrich Co. LLC., MO, USA</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Drierite, calcium sulfate</td>
			<td>Sigma-Aldrich Co. LLC., MO, USA</td>
			<td>238910</td>
			<td></td>
		</tr>
		<tr>
			<td>Ketamine, Ilium Ketamil</td>
			<td>Troy Laboratories Pty. Ltd., NSW, Australia</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Xylazine, Ilium Xylazil-20</td>
			<td>Troy Laboratories Pty. Ltd., NSW, Australia</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Atropine Sulphate, Minims eye drops</td>
			<td>Bausch &#38; Lomb Pty. Ltd., NSW, Australia</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Tetracaine Hydrochloride, Minims eye drops</td>
			<td>Bausch &#38; Lomb Pty. Ltd., NSW, Australia</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Chloramphenicol, Chlorsig ointment</td>
			<td>Aspen Pharma Pty. Ltd., NSW, Australia</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Pentobarbital, Lethabarb</td>
			<td>Virbac Australia Pty. Ltd., NSW, Australia</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Lignocaine Hydrochloride, Ilium Lignocaine-20</td>
			<td>Troy Laboratories Pty. Ltd., NSW, Australia</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Uni-Core Punch Tool</td>
			<td>World Precision Instruments Inc., FL, USA</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Curved Forceps</td>
			<td>World Precision Instruments Inc., FL, USA</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Operating Microscope, Zeiss OPMI 99</td>
			<td>Zeiss, West Germany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>25G Insulin Needle</td>
			<td>Terumo Corp., Tokyo, Japan</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Dumont Tweezers</td>
			<td>World Precision Instruments Inc., FL, USA</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

slow-release drug delivery through elvax 40w to the rat retina: implications for the treatment of chronic conditions

Diseases of the retina are difficult to treat as the retina lies deep within the eye. Invasive methods of drug delivery are often needed to treat these diseases. Chronic retinal diseases such as retinal oedema or neovascularization usually require multiple intraocular injections to effectively treat the condition. However, the risks associated with these injections increase with repeated delivery of the drug. Therefore, alternative delivery methods need to be established in order to minimize the risks of reinjection. Several other investigations have developed methods to deliver drugs over extended time, through materials capable of releasing chemicals slowly into the eye. In this investigation, we outline the use of Elvax 40W, a copolymer resin, to act as a vehicle for drug delivery to the adult rat retina. The resin is made and loaded with the drug. The drug-resin complex is then implanted into the vitreous cavity, where it will slowly release the drug over time. This method was tested using 2-amino-4-phosphonobutyrate (APB), a glutamate analogue that blocks the light response of the retina. It was demonstrated that the APB was slowly released from the resin, and was able to block the retinal response by 7 days after implantation. This indicates that slow-release drug delivery using this copolymer resin is effective for treating the retina, and could be used therapeutically with further testing.

Full-field electroretinography (ERG) was used to detect the effect of APB on the retina. For details on how the ERG was performed, refer to the following studies33,34. In brief, after anesthetizing the animal, a gold electrode was placed on the cornea of the eye, and the reference electrode was placed in the anterior scalp between the eyes, to record the electrical activity of the retina. Stimuli were generated using an electronic flash unit and the intensity ranged from 10 to 106 photoisomerization...

Watch this Scientific Journal Video about Slow-release Drug Delivery through Elvax 40W to the Rat Retina: Implications for the Treatment of Chronic Conditions at JoVE.com

Slow-release Drug Delivery through Elvax 40W to the Rat Retina: Implications for the Treatment of Chronic Conditions

This paper details how Elvax 40W can be used as a slow-release method for drug delivery to the adult rat retina. The protocol for preparing, loading, and delivering the drug-resin complex to the eye is described.

Diseases of the retina are difficult to treat as the retina lies deep within the eye. Invasive methods of drug delivery are often needed to treat these ...

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