Injectable polyethylene oxide gel implant and method for production

IS 2 145 293 T3

DESCRIPTION

Injectable polyethylene oxide gel implant and its production procedure. Background of the invention

This invention relates generally to a method for the production of a polyethylene oxide implant and, in particular, to a method for the production of a biocompatible, cross-linked polyethylene oxide gel, which can be injected into the human body for replacement or increased tissue.

It is well known that hydrogels have been used in various biomedical applications, since they can be made non-toxic and tissue compatible. United States Patents Nos. 4,983,181 and 4,994,081, which were published in 1991 in the name of Civerchia, describe a method of polymerization of a hydrogel in the presence of a cross-linking agent to form a three-dimensional polymeric mesh, which has controlled spacings between its molecules to bind macromolecules having a known size and ensure that the micro molecules are dispersed substantially uniformly within the polymeric network of the polymerized hydrophilic monomer. The hydrogel crosslinking step can be carried out with a crosslinking agent that can be external, such as ultraviolet radiation, or a crosslinking agent added to the hydrogel clear viscous monomer solution, the crosslinking agent of which can be, for example, ethylene glycol dimethacrylate. The hydrogel described in these patents is a transparent collagen hydrogel that is capable of promoting epithelial cell growth.

Some of the drawbacks to using collagen gels are that they typically biodegrade in three to six months, and they are well known for their infectious and immunological reactions. Additionally, collagen implants are, in turn, colonized by recipient cells and vessels.

US 3,264,202 describes a method for crosslinking polyethylene oxide using ionizing radiation. Additionally, document WO94 / 17851 describes a biocompatible material, which is useful for implantation in the cornea, comprising a cross-linked polyethylene oxide hydrogel, which can be obtained by irradiating an aqueous solution of polyethylene oxide in a suitable vessel. with gamma radiation.

Notwithstanding any other statements made in this specification, the present invention relates to a method for producing a cross-linked polyethylene oxide gel, having a modulus of elasticity less than about 4 x 10⁴ newtons / m² and a refractive ondx of between 1.3 and 1.52 for implantation in the cornea of ​​one eye, the method of which includes the stages of:

dissolve a sample of polyethylene oxide in a Balanced Salt Solution;

transfer said solution to a sealed container;

remove free oxygen from the container; replacing the oxygen within said container with an inert gas;

and irradiating said container to crosslink said polyethylene oxide.

Another type of substance commonly used in biomedical applications is a silicone gel. However, silicone gels are also known to elicit immunological reactions and tend to migrate away from the implantation site. Additionally, silicone implants are encapsulated by dense fibrous tissues created by cellular reactions to a foreign substance implanted in the tissue. Finally, although silicone gels allow efficient diffusion of oxygen, there is insufficient nutrient transport through the space occupied by the implants.

Summary of the present invention

Therefore, an object of the present invention is to provide a process for producing a gel implant that is biocompatible and non-erodible in the body.

Another object of the present invention is to provide an implant that can be easily removed from the body, if desired.

It is also an object of the present invention to provide a biocompatible gel that is injectable in the body and does not cause infectious, inflammatory or immunological reactions after implantation.

A further object of the present invention is to provide an injectable biocompatible gel, which does not migrate outside the injection site, and allows both oxygen and nutrient support.

A still further object of the present invention is to provide a polyethylene oxide gel, which can crack after gelling, but before entering the body or during the actual injection process.

These and other objects are achieved in the present invention using a novel process to create a polyethylene oxide (PEO) gel that can be injected into the body as an implant. In accordance with the present invention, a method is described for producing a removed polyethylene oxide gel , having a modulus of elasticity less than about 4 x 10⁴newtons / m² and a refractive onyx of between 1.3 and 1.52 to implant in the cornea of ​​an eye, the method of which includes the steps of:

dissolve a polyethylene oxide sample in a Balanced Salt Solution;

transfer said solution to a sealed container;

remove free oxygen from the container; replacing the oxygen within said container with an inert gas;

and irradiating said container to crosslink said polyethylene oxide.

Advantageously, the gel is suitable for use as permanent soft implants for tissue replacement and augmentation, which is useful in plastic and reconstructive surgery, in ophthalmic procedures such as corneal surgery, in retinal detachment and oculoplastic surgery.

Using this new process, the PEO gel is biocompatible and its characteristics can

ES 2 145 293 T3 be designed by modulating the PEO-water concentration and radiation dose (to control its transparency and hardness) and modulating the electrolyte concentration (to control the volume expansion and final water content) to adjust a specific medical requirement. . The gel is injectable through small gauge needles (for example, 50 mm (25 ga)), and is biocompatible intra-stromal and subcutaneously. The gel is not colonized by cells and vessels, and it is easily separable by flooding using saline solutions (preferably hypertonic). The configuration of the implants composed of this PEO gel is moldable by digital massage of the tissue that surrounds the implant.

Brief description of the drawings

Figure 1 graphically illustrates a single PEO molecule.

Figure 2 is a graphical representation showing the influence of molecular weight after gelling dose.

Figure 3 is a graphical representation showing the percentage of light transmission through both a human cornea and a PEO gel implant prepared by (a) the present process.

Figures 4A and 4B graphically illustrate the reflection of light from an implant within a cornea.

Figure 5 is a graphical representation showing the percentage of light reflection from a cornea with an implant with respect to the refractive ondx of the implant; Y

Figure 6 is a schematic view of the cornea illustrating both the transverse and radial directions, in which the modulus of elasticity is measured.

Detailed description of the preferred embodiment

Polyethylene oxide (PEO) and polyethylene glycol (PEG) are manufactured by two different methods, but they generally refer to the same synthetic polymeric product that has the formula:

- (- CH2-CH2-0-) nThe difference between these two polyomers lies in the use of respective molecular weight. PEGs have a molecular weight below a few thousand daltons, while PEOs have molecular weights that start from several thousand to several million daltons.

PEO is soluble in benzene, freon, chloroform, and tetrahydrofuran, and is also soluble in water at all temperatures except near the boiling point. PEO is soluble also in saline solutions.

Since PEO polymer is very soluble in water, to use it as a biocompatible material, it is necessary to decrease its solubility. This can be done by creating an insoluble crosslinked network, as can be seen in figure 1. Each crosslink is indicated by a union, as shown at 1 in figure 1. This network has the advantage of being hydrophilic, and as a consequence, it swelled. in water.

One method for the production of crosslinked PEO is by final crosslinking of the network with a chemical reaction using, for example, hexamethylene diisocyanate as the crosslinking agent and a derivatizing agent such as mannitol, pentaerythritol or 1,2,6-hexametriol. However, because toxic chemical reagents (in the same concentration range as PEO) are used during crosslinking, an additional purification step can be employed to remove any remaining traces of the reagents.

Another way to create this network is to expose PEO to gamma radiation. However, although pure PEO can be cross-linked with gamma rays without water, the process requires a very high radiation dose (greater than 100 Mrad), making it not very practical. Using a PEO-water solution, crosslinking can be achieved using a much lower radiation dose (approximately 1 Mrad). This crosslinking is indirect and involves water molecules:

^γ

H2O ------------- H + OH

The radicals produced react on the PEO polymer chain to produce:

HO ........- CH2-CH2-O -....... OH

HO ........- CH2-CH2-O -....... OH

The crosslinked PEO chain has a much higher molecular weight than the base PEO used in the reaction. If a single bond occurs between two 200,000 dalton chains, a 400,000 dalton molecule is obtained. A bond can occur between two carbon moieties of either of two different PEO molecules, as shown in the formula above. Gelling occurs when at least one crosslinker per polymer chain was initially present.

The gelation depends on several parameters: the PEO concentration, the molecular weight, and the radiation dose. The influence can be represented in the diagram shown in figure 2, which shows the radiation dose with respect to the PEO concentration in aqueous solution for different molecular weights, where MW1> MW2> MW3> MW4. As can be seen in Figure 2, at a given concentration, the higher the molecular weight, the lower the dose of radiation required to form a gel. However, gelation may not occur, since the dissolved oxygen in the solution acted as a gamma ray scavenger and therefore cooled the crosslinking process.

To avoid this, the PEO solution should be carefully degassed. The solution is evacuated until no more gas bubbles appear in the solution, then the vacuum or is replaced by argon or another inert gas. This procedure can be repeated several times in order to decrease the residual amount of oxygen remaining in the solution.

In the preferred embodiment, a 0.8% to 8% PEO solution by weight was prepared by dissolving a PEO preparation (eg, 200,000 daltons) in a saline solution.

IS 2 145 293 T3

The solution used, a Balanced Saline Solution (BSS), was selected to the extent that it best suits the intended medical application. Other solutions can be used, depending on the intended use of the gel. The composition of BSS, available from Alcon, Inc., is listed below in Table I.

TABLE I

<td>Solute</td><td>Percentage (by weight)</td>

<td>Sodium chloride</td><td> 0,64</td>

<td>Potassium chloride</td><td> 0,075</td>

<td>Calcium chloride</td><td> 0,048</td>

<td>Magnesium chloride</td><td> 0,03</td>

<td>Sodium Acetate</td><td> 0,009</td>

<td>Sodium Citrate Dihydrate</td><td> 0,17</td>

Free oxygen was removed from the solution by placing the solution in a sealed container which was evacuated using vacuum and then filled with pure Argon gas (&gt; 99.999%) to prevent gaseous contamination of the surrounding atmosphere. The container was then irradiated by exposing it to a gamma ray source (Cobalt 60) for a dose of between 2.5 and 25 Mrad to cross-link the PEO. To obtain a uniform gel (Isotrope), the solution can be continuously stirred, even during radiation (using a rocking platform shaker). Aseptic and contamination-free transfer of the PEO gel for sterile syringes was performed in a UV-radiation presterilized laminar flow hood for use in the experimental procedures described.

It has been observed that PEO hydrogel of a specific electrolyte concentration swells when immersed in a saline solution with a lower electrolyte content, while it contracts if it is immersed in a saline solution with a higher electrolyte concentration. Therefore, implantation of a cross-linked PEO gel in a saline solution, which has an electrolyte concentration different from the surrounding tissue, will result in a postoperative change in implant volume. Although this phenomenon can lead to postoperative complications in certain medical applications, it may be advantageous in applications such as polymeric vitreous replacement and retinal detachment surgery, where controlled tissue-to-tissue compression is required.

For a given PEO solute concentration, the higher the irradiation dose, the higher the crosslinking density. Using a 0.8% PEO solution, the irradiation dose was varied from 0.8 Mrads to approximately 13 Mrads. It appeared that 0.8 Mrads is the minimum dose required to obtain gelation without gravitational collapse of the polymer, while any dose above 9 Mrad appears to have very little effect on the physical properties of PEO.

A minimum dose of 2.5 Mrad was selected for the irradiation dose, corresponding to the minimum dose required for gamma ray sterilization. Using a higher dose, it is possible to cross-link and simultaneously sterilize the PEO gel implant.

Referring again to Figure 2, it can be seen that for a given crosslinking density, the higher the PEO solute concentration, the lower the required irradiation dose. The initial test carried out with a PEO of approximately 200,000 daltons indicated that, below 0.5%, it is difficult to obtain gelation, even at a higher radiation dose. Therefore, a solute concentration that varied between 0.8% and 8.0% was selected.

With a 0.8% PEO solution, 200,000 daltons irradiated at 5 Mrads, the cross-linked gel is transparent and can be used in ophthalmology for corneal tissue augmentation procedures, such as Gel Injection Adjustable Keratoplasty (GLAK), which described in US Patent No.^° 5.090.955.

The visibility of the gel within the eye is a cosmetic and therapeutic issue related to the GLAK procedure. The visibility of the gel is directly related to both the flexibility and the absorption of the gel used. Therefore, at any visible wavelength, the percentage of light transmission through the implant should be at least as great as through the cornea. Figure 3 shows a graph illustrating the transmission through both the cornea and the implant prepared according to the present invention as a percentage of transmission of light through the cornea as a function of the wavelength of light. The graph of light transmission through the gel is a dotted line designated 2, while the graph of light transmission through the cornea is a solid line designated 4. As can be seen in figure 3, for the spectrum of visible light (from 400 nanometers to 800 nanometers), the percentage of light transmission through the gel reaches 100%. Therefore, the implant of the present invention is optically transparent to the light that passes through the implant. Figure 3 also shows that the implant transmits more light in the near ultraviolet, visible and near infrared range than the normal cornea (wavelengths 300-1350 nm).

Since the eye can detect approximately 10% difference in reflection, it is important that the refractive ondx of the gel differs no more than ± 10% from the refractive ondx of the cornea. Figure 4A shows a beam of light passing through an implant that has been placed within the cornea of ​​an eye. A beam 10 passes through the anterior section of the cornea 12 and strikes the anterior surface 14a of implant 14, where it is partially reflected, as shown at 16. As beam 10 continues through implant 14, it strikes on the posterior surface 14b of implant 14, and is partially reflected as shown at 18.

Referring now to Figure 4B, the reflection properties of the cornea are taken into consideration, unless the beam passes through a cornea containing an implant. As the beam 10 'strikes the anterior surface 20a of the tear film 20 of the cornea 12', it is partially reflected, as shown at 22. The beam

IS 2 145 293 T3

10 'continues through tear film 20 and is partially reflected on anterior surface 12a' of cornea 12 ', as shown at 24. Beam 10' continues on cornea 12 ', where it is partially reflected on surface anterior 14a 'of implant 14', as shown at 26. Posterior surface 14b 'partially reflects beam 10' as it passes through posterior surface 14b ', shown at 28. Finally, beam 10 'is reflected as it strikes the posterior surface 12b' of cornea 12 ', as shown at 32.

Figure 5 illustrates the percentage of light as a function of the refractive onyx of the implant produced using the method of the present invention. The curve designated 36 shows the percentage of light reflected by the cornea and the implant together as a function of the refractive onyx of the implant. As can be seen from Figure 5, if the refractive ondx of the implant is equal to the ondx of refraction of the cornea (i.e., 1.376), the percentage of incident light that is reflected is in the mononym, which is approximately 4%. Since it is desirable that the total reflection of the cornea and the implant together does not differ from the total reflection of the cornea alone by more than approximately 10%, the total reflection of the implant plus the cornea should not be greater than 4.4% . If we find the point on lone 36 that provides a total reflection of 4.4%, it can be seen that it corresponds to a refractive ondx for the implant of approximately 1.52. Since a hydrogel is mostly water and the refractive onyx of water is about 1.3, the refractive onyx of the implant should be at least 1.3.

Therefore, it is more desirable that the gel used in GLAK surgery have a refractive ondx greater than 1.3 and less than 1.52.

It is also essential that the absorption of the injected gel closely matches the absorption of the cornea. This will be important if procedures need to be performed on the eye later. If the gel has different absorption characteristics, laser eye surgery and photocoagulation may not be possible, since the light energy will not have a uniform effect on the gel and the cornea.

Another important characteristic of the injected gel that will affect its performance in the eye is its modulus of elasticity. This issue is described in an article entitled "Keratoprosthesis: Engineering and Safety Assessment," which was published in the May / June 1993 issue of Refractive and Corneal Surgery. If the injected implant is stiffer than the cornea, the cornea is deformed, whereas if the cornea is stiffer than the implant, the implant is deformed. For example, a keratoprosthesis that is composed of glass or polymethylmethacrylate (PMMA) is extruded from the cornea, since these relatively hard materials have a much greater elastic modulus than that of the cornea. Therefore, to prevent extrusion of the gel from the cornea, its modulus of elasticity must be less than that of the cornea. Figure 6 shows a representation of a cornea in order to locate the site for the selection of the appropriate modulus of elasticity both in the transverse and radial directions. The cornea 40 was composed of a plurality of layers of loamines 42 that form the stroma 44. The corneal surface is indicated at 46, while the anterior chamber of the eye is indicated at 48. At the incision site in the cornea for this procedure (approximately 2.5 mm from the center of the cornea), the thickness of the cornea is between 550 and 650 microns (μm). At the level at which the annular canal is formed, which is indicated at 50 in FIG. 6, the cornea has both a radial elaostic module and a transverse elaostic module. The radial modulus is directed along a plane designated 52, while the transverse modulus is directed along a plane designated 54. The transverse modulus is between 219x10⁴ and 4.12x10⁴ newtons / m² , while the radial module is between 2x10⁶ and 5x10⁶ newtons / m² . To avoid problems with extrusion, the gel should have a lower elastic modulus than both radial and transverse corneal modules and preferably less than approximately 4x10.⁴ newtons / m² .

Other necessary characteristics of an injectable gel for this procedure include: the prevention of the migration of cells within the implant that impaired their elimination (if it is necessary to readjust the corneal curvature); and the transmission of oxygen and other essential nutrients through the gel in all parts of the eye.

In an experiment, using the procedure shown in the aforementioned patent, the sterile cross-linked gel was injected into an annular intrastomal canal formed between the lamellar layers in the cornea of ​​a mouse at a spaced distance from the central corneal region. After the canal was formed in the cornea, the gel was injected into the canal using a 19-25 gauge needle (1mm to 0.50mm). It was shown that the PEO gel was not toxic to the mouse cornea with excellent corneal transparency, no surface opacification, no extrusion, and no migration.

Histologically, no giant coells were found, nor necrosis, and a normal keratocyte population close to the implant. Additionally, the PEO gel was optically transparent in the visible spectrum and its refractive ondx (1,334) was relatively close to the refractive ondx of the cornea (1,376). The modulus of elasticity of the gel was estimated with a penetroometer at 1.7x10³ newtons / m². The gel produced by the method of the present invention was shown to remain stable for 22 months in the mouse cornea. By using a solution during PEO gel preparation that approximates the electrolyte concentration or osmootic activity of the cornea, it would be possible to reduce any change in implant volume to a minimum.

Other potential uses are for vitreous replacement and keratophagy lentholes. Increasing the PEO concentration increases the mechanical resistance of the gel, while decreasing the transparency. For example, a 1% PEO solution irradiated at 5 Mrads produced a more tenacious gel that can be used for subcutaneous tissue augmentation procedures performed5

ES 2 145 293 T3 two in plastic and reconstruction surgery, oculoplasty, or other procedures where transparency is not necessary. Several in vivo procedures have been carried out to demonstrate the biocompatibility of this PEO gel when injected subcutaneously. Six mice received subcutaneous injection of a PEO gel prepared according to the present invention in the dorsal area and in the ears. The results showed a good tolerance to this material and no evident degradation of the product after two months.

The process of gamma ray crosslinking of PEO solutions produces an excess amount of free water (synaeresis). Water can be unwanted in certain surgeries and must be removed before transferring the gel from the container to the syringe. To perform this task, the container was equipped with a second chamber separated from the first by a fine mesh screen. After the irradiation process, the container will be inverted and the excess water will be drained into the final container, while maintaining the cross-linked PEO in a sterile atmosphere.

In certain cases, it can be difficult to predict at the time of PEO manufacture what exact configuration and size are required for a particular implant. In these situations, the PEO gel can break into smaller pieces (i.e. cracked) with an average particle size ranging from several microns (μ) (for use in filling the biolagic space with greater precision) up to about 1 cm for cases where large volumes of gel are required. The cracking process can be done before implantation or during the implantation process.