Thermal Irreversible Gel Corneal Contact Lens Formed in Situ

1. Field of the Invention

This invention relates to ophthalmic corneal protective devices comprising an aqueous gel.

2. Description of the Prior Art

Corneal protective devices are needed in cases in which corneal injury occurs and the immobilization of the eye using an eye patch is not resorted to. Molded collagen shields have been developed for this use. These are often not satisfactory because they lack sufficient flexibility to conform to the individual corneal curvature. The clinical uses of collagen shields are disclosed by Poland et al in Journal of Cataract Refractive Surgery, Volume 14, September 1988, pages 489-491. The author describes the use of collagen shields immersed in tobramycin solution in order to rehydrate the collagen prior to use. These are described as useful following cataract extraction or in patients having nonsurgical epithelial healing problems. More rapid healing of epithelial defects after surgery resulted from the use of the collagen shield. Collagen shields have also been utilized as agents for the delivery of drugs to the cornea as disclosed in Reidy et al Cornea, in press, 1989 the Raven Press, Ltd., New York and Shofner et al, Opthalmology Clinics of North America, Vol. 2, No. 1, March 1989, pages 15-23.

Over the years, methods have been developed to achieve the efficient delivery of a therapeutic drug to a mammalian body part requiring pharmaceutical treatment. Use of an aqueous liquid which can be applied at room temperature as a liquid but which forms a semisolid gel, when warmed to body temperature, which readily conforms to corneal curvature, has been utilized as a vehicle for drug delivery since such a system combines ease of application, improved patient tolerance, and greater retention at the site requiring treatment than would be the case if the aqueous composition were not converted to a gel as it is warmed to mammalian body temperature. In U.S. Pat. No. 4,188,373, PLURONIC.RTM. polyols are used in aqueous compositions to provide thermally gelling aqueous systems. Adjusting the concentration of the polymer provides the desired sol-gel transition temperature, that is, the lower the concentration of polymer, the higher the sol-gel transition temperature, after crossing a critical concentration minimum, below which a gel will not form. Other polyoxyalkylene gel compositions are disclosed in U.S. Pat. No. 4,810,503 and U.S. Pat. No. 4,879,109.

In U.S. Pat. Nos. 4,474,751; '752; '753; and 4,478,822 drug delivery systems are described which utilize thermosetting gels; the unique feature of these systems is that both the gel transition temperature and/or the rigidity of the gel can be modified by adjustment of the pH and/or the ionic strength, as well as by the concentration of the polymer.

Other patents disclosing pharmaceutical compositions which rely upon an aqueous gel composition as a vehicle for the application of the drug are U.S. Pat. Nos. 4,883,660; 4,767,619; 4,511,563; and 4,861,760. Thermosetting gel systems are also disclosed for application to injured mammalian tissues of the thoracic or peritoneal cavities in U.S. Pat. No. 4,911,926.

Ionic polysaccharides have been used in the application of drugs by controlled release. Such ionic polysaccharides as chitosan or sodium alginate are disclosed as useful in providing spherical agglomerates of water-insoluble drugs in the Journal of Pharmaceutical Sciences volume 78, number 11, November 1989, Bodmeier et al. Alginates have also been used as a depot substance in active immunization, as disclosed in the Journal of Pathology and Bacteriology volume 77, (1959), C. R. Amies. Calcium alginate gel formulations have also found use as a matrix material for the controlled release of herbicides, as disclosed in the Journal of Controlled Release, 3 (1986) pages 229 -233, Pfister et al. Alginates have also been used to form hydrogel foam wound dressings, as disclosed in U.S. Pat. No. 4,948,575.

In U.S. Pat. No. 3,640,741, a molded plastic mass composed of the reaction product of a hydrophilic colloid and a cross-linking agent such as a liquid polyol, also containing an organic liquid medium such as glycerin, is disclosed as useful in the controlled release of medication or other additives. The hydrophilic colloid can be carboxymethyl cellulose gum or a natural alginate gum which is cross-linked with a polyol. The cross-linking reaction is accelerated in the presence of aluminum and calcium salts.

In U.S. Pat. No. 4,895,724, compositions are disclosed for the controlled release of pharmacological macromolecular compounds contained in a matrix of chitosan. Chitosan can be cross-linked utilizing aldehydes, epichlorohydrin, benzoquinone, etc.

In U.S. Pat. No. 4,795,642, there are disclosed gelatin-encapsulated, controlled-release compositions for release of pharmaceutical compositions, wherein the gelatin encloses a solid matrix formed by the cation-assisted gellation of a liquid filling composition incorporating a vegetable gum together with a pharmaceutically-active compound. The vegetable gums are disclosed as polysaccharide gums such as alginates which can be gelled utilizing a cationic gelling agent such as an alkaline earth metal cation.

While the prior art is silent with respect to aqueous corneal protective compositions, the alginate hydrogel foam wound dressings disclosed in U.S. Pat. No. 4,948,575, cited above, are of interest in disclosing compositions which absorb wound exudate without appreciable swelling. These compositions contain a water soluble alginate, an effervescent compound which effervesces upon reaction with an acid, a water soluble acid, and a water insoluble di- or trivalent metal salt. U.S. Pat No. 4,255,415 is also of interest in disclosing a polyvinyl alcohol based ophthalmic gel for drug delivery. Osmotic drug delivery systems are disclosed in U.S. Pat. No. 4,439,196 which utilize a multi-chamber compartment for holding osmotic agents, adjuvants, enzymes, drugs, pro-drugs, pesticides, and the like. These materials are enclosed by semipermeable membranes so as to allow the fluids within the chambers to diffuse into the environment into which the osmotic drug delivery system is in contact. The drug delivery device can be sized for oral ingestion, implantation, rectal, vaginal, or occular insertion for delivery of a drug or other beneficial substance. Since this drug delivery device relies on the permeability of the semipermeable membranes to control the rate of delivery of the drug, the drugs or other pharmaceutical preparations, by definition, are not isotonic with mammalian blood.

Compositions and a process are disclosed for corneal protective devices. The compositions can be formed in situ and are compositions useful in protecting the cornea subsequent to injury, surgical or otherwise. The compositions in one embodiment of the invention provide a physiologically acceptable aqueous media which has a buffered pH and is osmotically balanced, preferably, so as to provide an isotonic mixture which is iso-osmotic with body fluids and has a pH similar to bodily fluids, such as lacrimal tears. The pH and osmotic pressure of lacrimal tears is about PH 7.4 and 290 mOsm/kg. In addition, the compositions are, optionally, sterilized so as to insure that the protective compositions of the invention do not provide a source of infection.

Polyphase systems are also useful and may contain non-aqueous solutes, non-aqueous solvents, and other non-aqueous additives. Homogeneous, polyphase systems can contain such additives as water insoluble high molecular weight fatty acids and alcohols, fixed oils, volatile oils and waxes, mono-, di-, and triglycerides, and synthetic, water insoluble polymers without altering the functionality of the system.

The compositions of the invention in one embodiment comprise aqueous mixtures of a polyoxyalkylene polymer and an ionic polysaccharide, optionally containing a latent counter-ion to gel the polysaccharide upon release of the counter-ion and to render the gelled mixture thermally irreversible so that it remains a gel upon cooling and thus is rendered durable at ambient temperature. The counter-ion can be microencapsulated in a heat sensitive medium, for instance, the walls of the microcapsule can be made of mono-, di-, or tri-glycerides or other natural or synthetic heat sensitive polymer medium. Alternatively, ion exchange resins can be incorporated in the compositions of the invention so as to release the desired counter-ion upon contact with an environment opposite in pH to the pH of the ion exchange resin. The aqueous mixture can be delivered to the cornea of the mammalian body as a low viscosity liquid at ambient temperatures which, upon contact with the higher temperature mammalian body, forms a semi-solid gel having a very high viscosity. Release of the counter-ion further strengthens the gel and renders it irreversible upon cooling to ambient temperature. Alternatively, a two part system can be used in which the counter-ion can be separately applied to the semi-solid gel formed by the polyoxyalkylene polymer upon contact with the cornea. This further strengthens the gel and renders it irreversible upon cooling. Because the preferred compositions of the invention are low viscosity liquids at ambient temperatures, they easily coat the cornea insuring maximum contact between exposed tissue and the composition of the invention. The gel compositions of the invention can be peeled away subsequent to application, if desired. The gels are gradually weakened upon exposure to mammalian body pH conditions but provide a durable protection for injured corneal tissue against eyelid abrasion.

A wide variety of polyoxyalkylene polymers are suitable for the preparation of the pharmaceutical compositions of the invention. Generally, it is necessary to adjust the polymer concentration in aqueous solution so as to obtain the desired sol-gel transition temperature in order that the compositions can be provided as low viscosity liquids at ambient temperature, yet form semi-solid gels at mammalian body temperatures. In addition to the concentration of the polymer other suitable excipients must be added so as to provide the desired pH and isotonic, iso-osmotic properties.

The useful polymers which provide the sol-gel characteristics of the pharmaceutical compositions of the invention are, preferably, polyoxyalkylene block copolymers.

The ionic polysaccharides are natural polymers such as chitosan or alginates. Aqueous solutions of alginate ionic polysaccharides form gels upon contact with aqueous solutions of counter-ions such as calcium, strontium, aluminum, etc. Aqueous solutions of chitosan form gels upon contact with a metal tripolyphosphate counter-ion. The discovery forming the basis of this application is that when ionic polysaccharides are present in aqueous solutions in admixture with certain polyoxyalkylene block copolymers and a counter-ion, that such mixtures form thermally-irreversible gels instead of the thermo-reversible gels known to form with aqueous solutions of certain polyoxyalkylene block copolymers.

The drawing provides a curve showing the penetration, as measured by a Precision Universal Penetrometer, of a 20mm thickness aqueous gel comprising poloxamer 407 and an alginate prepared in accordance with the procedure of Example 1. The scale at the left side of the plot indicates the depth of penetration, while the scale on the bottom of the plot indicates the temperature of the composition when tested. The arrow in the plot indicates the point at which an aqueous solution of calcium ions at a concentration of 0.137 molar is made to contact the gelled poloxamer 407/alginate solution so as to render thermally irreversible the gelled mixture and prevent it from becoming fluid at ambient temperature.

It has been found that aqueous vehicles containing a polyoxyalkylene block copolymer, which have the unique feature of being liquid at ambient temperatures and transitioning at mammalian body temperatures to a semi-solid gel, can be rendered thermally irreversible (no longer a liquid at ambient temperature) and resistant to shear thinning. Upon contacting the mixture with a counter-ion, the polyoxyalkylene polymer aqueous gel becomes more resistant to penetration by the inclusion of a polysaccharide in admixture with the polyoxyalkylene polymer. The compositions can be made isotonic or iso-osmotic and adjusted to the pH of mammalian body fluids, such as lacrimal tears. The pH and osmotic pressure of such bodily fluids are 7.4 and 290 mOsm/kg, respectively. It is advantageous to provide a protective corneal contact lens formed in situ in order to protect injured corneal tissue. It is desirable to provide such protective corneal contact lens in an aqueous media having a pH and osmotic pressure which match those of bodily fluids. Optionally, the compositions of the invention can be provided in a sterile condition.

The block copolymer compositions of the invention comprise: at least one polyoxyalkylene block copolymer of the formula

wherein A is a polyoxyalkylene moiety having an oxygen/carbon atom ratio of less than 0.5, x is at least 2, Y is derived from water or an organic compound containing x reactive hydrogen atoms, E is a polyoxyalkylene moiety constituting at least about 60% by weight of the copolymer, n has a value such that the average molecular weight of A is at least about 500 to about 900, as determined by the hydroxyl number of a hydrophobe base intermediate,

and the total average molecular weight of the copolymer is at least about 5,000.

Determination of the average molecular weight via hydroxyl number is an end group analysis which reflects the number of molecules per unit weight of the polymer mixture. The number of molecules is determined by counting the number of terminal hydroxyl groups in a sample. Any method known in the art may be used to count the number of hydroxyl groups, such as reaction with a titratable reagent (Ogg, et al. Ind. Eng. Chem. Anal. Ed. 17:394-397(1945); Conix, Makromol. Chem., 26:226-235 (1958)), or use of infrared spectroscopy (Ward, Trans. Faraday Soc., 53:1406-1414 (1957); Daniels, et al., J. Polymer Sci., 33:161-170 (1958)).

Based on the number of terminal hydroxyl groups per molecule (e.g., two per molecule in the case of poloxamers), the number of molecules is calculated, preferably expressed as a molar quantity. The average molecular weight is then determined by dividing the weight of the sample by the number of moles of the polymer.

Generally, the polyoxybutylene-based block copolymers useful in the compositions of the invention are prepared by first condensing 1,2 butylene oxide with a water soluble organic compound initiator containing 1 to about 6 carbon atoms, such as, 1,4 butylene glycol or propylene glycol and at least 2 reactive hydrogen atoms to prepare a polyoxyalkylene polymer hydrophobe of at least about 500, preferably, at least about 1000, most preferably, at least about 1500 average molecular weight. Subsequently, the hydrophobe is capped with an ethylene oxide residue. Specific methods for preparing these compounds are described in U.S. Pat. No. 2,828,345 and British Patent No. 722,746, both of which are hereby incorporated by reference.

Useful polyoxybutylene based block copolymers conform to the following generic formula:

wherein a and b are integers such that the hydrophobe base represented by (C.sub.4 H.sub.8 O) has a molecular weight of at least about 500, preferably, at least about 1000 and most preferably, at least about 3000, as determined by hydroxyl number, the polyoxyethylene chain constituting at least about 60%, preferably, at least about 70% by weight of the copolymer and the copolymer having a total average molecular weight of at least about 5000, preferably, at least about 10,000, and most preferably, at least about 15,000.

The copolymer is characterized in that all the hydrophobic oxybutylene groups are present in chains bonded to an organic radical at the former site of a reactive hydrogen atom thereby constituting a polyoxybutylene base copolymer. The hydrophilic oxyethylene groups are used to cap the polyoxybutylene base polymer.

Polyoxyethylene-polyoxypropylene block copolymers which can be used to form aqueous gels can be represented by the following formula:

wherein a and b are integers such that the hydrophobe base represented by (C.sub.3 H.sub.6 O)a has a molecular weight of at least about 900, preferably, at least about 2500, most preferably, at least about 4000 average molecular weight, as determined by hydroxyl number; the polyoxyethylene chain constituting at least about 60%, preferably, at least about 70% by weight of the copolymer and the copolymer having a total average molecular weight of at least about 5000, preferably, at least about 10,000, and most preferably, at least about 15,000.

Polyoxyethylene-polyoxypropylene block copolymer adducts of ethylene diamine which can be used may be represented by the following formula: ##STR1## wherein a and b are integers such that the copolymer may have (1) a hydrophobe base molecular weight of at least about 2000, preferably, at least about 3000, and most preferably, at least about 4500, (2) a hydrophile content of at least about 60%, preferably, at least about 70% by weight, and (3) a total average molecular weight of at least about 5000, preferably, at least about 10,000 and most preferably, at least about 15,000.

The hydrophobe base of the copolymer of formula V is prepared by adding propylene oxide for reaction at the site of the four reactive hydrogen atoms on the amine groups of ethylene diamine. An ethylene oxide residue is used to cap the hydrophobe base. The hydrophile polyoxyethylene groups are controlled so as to constitute at least about 60%, preferably, at least about 70% by weight, and most preferably, at least about 80% by weight of the copolymer.

The procedure used to prepare aqueous solutions which form gels of the polyoxyalkylene block copolymers is well known. Either a hot or cold process for forming the solutions can be used. A cold technique involves the steps of dissolving the polyoxyalkylene block copolymer at a temperature of about 5.degree. to about 10.degree. C. in water. When solution is complete the system is brought to room temperature whereupon it forms a gel. If the hot process of forming the gel is used the polymer is added to water heated to a temperature of about 75.degree. C. to about 85.degree. C. with slow stirring until a clear homogenous solution is obtained. Upon cooling, a clear gel is formed. Block copolymer gels containing polyoxybutylene hydrophobes must be prepared by the above hot process, since these will not liquify at low temperatures.

As used herein, the term "gel" is defined as a solid or semisolid colloid containing a certain quantity of water. The colloidal solution with water is often called a "hydrosol".

The organic compound initiator which is utilized in the process for the preparation of the polyoxyalkylene block copolymers generally is water or an organic compound and can contain a plurality of reactive hydrogen atoms. Preferably, Y in formulas I and II above is defined as derived from a water soluble organic compound having 1 to about 6 carbon atoms and containing x reactive hydrogen atoms where x has a value generally, of at least 1, preferably, a value of at least 2. Falling within the scope of the compounds from which Y is derived from water soluble organic compounds having at least two reactive hydrogen atoms are water soluble organic compounds such as propylene glycol, glycerin, pentaerythritol, trimethylolpropane, ethylene diamine, and mixtures thereof and the like.

The oxypropylene chains can optionally contain small amounts of at least one of oxyethylene or oxybutylene groups. Oxyethylene chains can optionally contain small amounts of at least one of oxypropylene or oxybutylene groups. Oxybutylene chains can optionally contain small amounts of at least one of oxyethylene or oxypropylene groups. The physical form of the polyoxyalkylene block copolymers can be a viscous liquid, a paste, or a solid granular material depending upon the molecular weight of the polymer. Useful polyoxyalkylene block copolymers generally have a total average molecular weight of about 5,000 to about 50,000, preferably, about 5,000 to about 35,000 and most preferably, about 10,000 to about 25,000.

In addition to those polyoxyalkylene block copolymers referred to above, which are suitable in the formation of the pharmaceutical compositions of the invention, other polyoxyalkylene polymers which form gels at low concentrations in water are suitable. One such polymer is described in U.S. Pat. No. 4,810,503, incorporated herein by reference. These polymers are prepared by capping conventional polyether polyols with an alpha-olefin epoxide having an average of about 20 to about 45 carbon atoms, or mixtures thereof. Aqueous solutions of these polymers gel in combination with surfactants, which can be ionic or nonionic. The combination of the capped polyether polymers and the surfactants provide aqueous gels at low concentrations of the capped polymer and surfactant, which generally do not exceed 10% by weight total. Detailed methods of preparing these aqueous gels are disclosed in U.S. Pat. No. 4,810,503. Preparation of said aqueous gels is generally described below. Preferred surfactants for use in preparing these gels are also disclosed in said patent.

A conventional copolymer polyether polyol is prepared by preparing block or heteric intermediate polymers of ethylene oxide and at least one lower alkylene oxide having 3 to 4 carbon atoms as intermediates. These are then capped with the alpha-olefin epoxide to prepare the polymers. Ethylene oxide homopolymers capped with said alpha-olefin oxides are also useful as intermediates.

The heteric copolymer intermediate is prepared by mixing ethylene oxide and at least one lower alkylene oxide having 3 to 4 carbon atoms with a low molecular weight active hydrogen-containing compound initiator having at least two active hydrogens and preferably, 2 to 6 active hydrogen atoms such as a polyhydric alcohol, containing from 2 to 10 carbon atoms and from 2 to 6 hydroxyl groups, heating said mixture to a temperature in the range of about 50.degree. C. to 150.degree. C., preferably from 80.degree. C. to 130.degree. C., under an inert gas pressure preferably from about 30 psig to 90 psig.

A block copolymer intermediate is prepared by reacting either the ethylene oxide or said alkylene oxide having 3 to 4 carbon atoms with said active hydrogen-containing compound followed by reaction with the other alkylene oxide.

The ethylene oxide and the alkylene oxides having from 3 to 4 carbon atoms are used in said intermediates in amounts so that the resulting polyether product will contain at least 10 percent by weight, preferably about 70 percent to about 90 percent by weight, ethylene oxide residue. The ethylene oxide homopolymer intermediate is prepared by reacting ethylene oxide with said active hydrogen-containing compound. The reaction conditions for preparing the block copolymer and ethylene oxide homopolymer intermediates are similar to those for the heteric copolymer intermediate. The temperature and pressure are maintained in the above ranges for a period of about one hour to ten hours, preferably one to three hours.

The alpha-olefin oxides which are utilized to modify the conventional polyether intermediate of the prior art are those oxides and the commercially available mixtures thereof generally containing an average of about 20 to 45, preferably about 20 to 30, carbon atoms. The amount of alpha-olefin required to obtain the more efficient capped polyethers is generally about 0.3 to 10 percent, preferably about 4 to 8 percent, of the total weight of the polyethers.

Since the preparation of heteric and block copolymers of alkylene oxides and ethylene oxide homopolymers are well known in the art, further description of the preparation of said polymers is unnecessary. Further details of the preparation of heteric copolymers of lower alkylene oxide can be obtained in U.S. Pat. No. 3,829,506, incorporated herein by reference. Further information on the preparation of block copolymers of lower alkylene oxides can be obtained in U.S. Pat Nos. 3,535,307; 3,036,118; 2,979,578; 2,677,700; and 2,675,619 incorporated herein by reference.

The surfactants may be ionic or non-ionic and many surfactants and types of surfactants may be employed. While all surfactants may not be effective in the preparation of the isotonic gels of the instant invention, the fact that many are effective makes it a simple matter for one skilled in the art to select such surfactant with a minimum of trial and error.

The amounts of capped polyether polymer and surfactant may be as little as 1.0 percent by weight or less of each depending on the type and amount of the other component. There appears to be no maximum amount of either component than that dictated by economic considerations. However, the total amount of capped polymer and surfactant would generally not exceed 10 percent by weight.

The ionic polysaccharides found useful in the present invention are hydrophilic colloidal materials and include the natural gums such as alginate gums, i.e., the ammonium and alkali metal salts of alginic acid and mixtures thereof as well as chitosan, which is a common name for the deacetylated form of chitin. Chitin is a natural product comprising poly-(N-acetyl-D-glucosamine). The alginates are available as dry powders from Protan, Inc., Commack, N.Y. and from Kelco Company, San Diego, Calif.

Generally, the alginates can be any of the water-soluble alginates including the alkali metal alginates, such as sodium, potassium, lithium, rubidium and cesium salts of alginic acid, as well as the ammonium salt, as well as the soluble alginates of an organic base such as mono-, di-, or tri-ethanolamine, aniline and the like. Generally, about 0.2% to about 2.5% by weight and, preferably, about 0.5% to about 1.5% by weight of alginate or chitosan ionic polysaccharides, based upon the total weight of the composition, are used to obtain the thermo-irreversible compositions of the invention. In general, the drug delivery composition of the invention will contain about 0.01% to about 60% by weight of medicament or pharmaceutical, about 10% to about 50% by weight of the polyoxyalkylene polymer, and about 80% to about 20% by weight of water together with the above amounts of ionic polysaccharide. In special situations, these amounts may be varied to increase or decrease the dosage or gel properties.

Useful counter-ions for thermo-irreversibly gelling the alginate ionic polysaccharide in combination with the polyoxyalkylene polymer compositions of the invention are cationic gelling agents, preferably, comprising a divalent or trivalent cation. Useful divalent cations include the alkaline earth metals, preferably, selected from the group consisting of calcium and strontium. Useful trivalent cations include aluminum. The most preferred counter-ions for gelling an alginate are contained in ionic compounds selected from pharmaceutically-acceptable gluconates, flourides, citrates, phosphates, tartrates, sulfates, acetates, borates, chlorides, and the like having alkaline earth metal cations such as calcium and strontium. Especially perferred counter-ion containing inorganic salts for use as ionic polysaccharide gelling agents include such inorganic salts as the chloride salts, such as strontium chloride, calcium chloride, and mixtures thereof. Generally, a molar ratio of counter-ion to chitosan or alginate of about 1:1 to about 10:1, preferably, about 2:1 to about 5:1, and, most preferably, about 3:1 to about 5:1 is used to render the compositions of the invention thermally-irreversibly gelled.

While the counter-ion, such as calcium or other counter-ions may be obtained by contact with bodily fluids, it is preferred that the counter-ion in latent form be added to the alginate ionic polysaccharide and polyoxyalkylene polymer compositions of the invention. Alternatively, a counter-ion can be added to the alginate ionic polysaccharide and polyoxyalkaline polymer compositions of the invention utilizing a two part system in which the counter-ion is topically applied to an aqueous solution of the thermally reversible gel polyoxyalkylene compositions of the invention subsequent to the topical application of such compositions to the cornea. Incorporation of the counter-ion in a latent form together with the alginate ionic polysaccharide and polyoxyalkylene polymer compositions of the invention may be accomplished by either encapsulating an aqueous solution of one of the counter-ion gelling agents, previously described above or by the incorporation of the counter-ion gelling agent into a matrix which provides for the controlled, slow-release of the gelling agent. For instance, the gelatin-encapsulated controlled-release compositions disclosed in U.S. Pat. No. 4,795,642, incorporated herein by reference, disclose the preparation of a gelatin shell encapsulating a controlled-release formulation in which the gelatin composition includes calcium chloride as the gelling agent. Alternatively, the counter-ion can be incorporated as an aqueous solution of a cationic gelling agent encapsulated in a vesicle composed, for instance, of alpha-tocopherol, as disclosed in U.S. Pat. No. 4,861,580, incorporated herein by reference.

Generally, aqueous solutions of chitosan can be gelled with multivalent, anion gelling agents, preferably, comprising a metal polyphosphate, such as an alkali metal or ammonium polyphosphates, pyrophosphates, or metaphosphates. Representative metaphosphate, pyrophosphate, and polyphosphate gelling agents include sodium and potassium, polyphosphates, sodium and potassium pyrophosphates, sodium and potassium metaphosphates, and sodium and ammonium (mono-, di-, tri-) phosphates.

Drug delivery systems which are liquid at room temperature and assume a semi-solid form at human body temperature have been proposed, such as those described in U.S. Pat. No. 4,188,373, which disclose the use of PLURONIC.RTM. polyols. In U.S. Pat. No. 4,861,760 and U.S. Pat. No. 4,474,751, ophthalmic drug delivery systems are disclosed which are characterized by liquid-gel phase transitions. In the '751 patent, polymers are disclosed which are tetra substituted derivatives of ethylenediamine, propylenediamine, butylenediamine, pentylenediamine, or hexylenediamine. These are described as block copolymers of poly(oxypropylene) and poly(oxyethylene) of various chain lengths. These polymers are utilized as aqueous drug delivery vehicles which contain from 10% to 50% by weight of polymer based on the weight of the total drug delivery vehicle. In the '760 patent, the liquid-gel phase transition compositions for ophthalmological use contain polymers which form gels at concentrations 10-100 fold lower than those used in systems such as the '751 patent, involving thermogellation. Accordingly, the drug delivery systems of the '760 patent are said to be very well tolerated by the eye. The polymers utilized in the drug delivery vehicles of the '760 patent are described as polysaccharides obtained by fermentation of a microorganism.

The corneal protective compositions of the invention are an improvement over these prior art ophthalmological drug delivery systems in that the compositions can be not only optimized for physiological tolerance in the eye by formulating the compositions so as to have isotonic characteristics, but are made more durable because of increased resistance to shear thinning, as the result of higher gel strength. These latter advantages are obtained by the incorporation of an ionic polysaccharide in admixture with a polyalkylene polymer. By matching the osmolality of the drug delivery compositions of the invention to those of the lacrimal fluid of the eye, it is possible to eliminate burning or other discomfort upon application of the drug delivery systems of the invention to the eye. The higher gel strength compostions upon contact with a counter ion for the ionic polysaccharide allow retention of the gel as a protective in situ formed contact lens for long intervals.

If desired, the protective contact lens of the invention may also contain preservatives, cosolvents, suspending agents, viscosity enhancing agents, ionic-strength and osmolality adjustors and other excipients in addition to the polyoxyalkylene polymer and ionic polysaccharide. Suitable water soluble preservatives which may be employed are sodium bisulfite, sodium thiosulfate, ascorbate, benzalkonium chloride, chlorabutanol, thimerosal, phenylmercuric borate, parabens, benzylalcohol phenylethanol and others. These agents may be present, generally, in amounts of about 0.001% to about 5% by weight and, preferably, in the amount of about 0.01 to about 2% by weight.

Suitable water soluble buffering agents are alkali or alkaline earth metal carbonates, phosphates, bicarbonates, citrates, borates, acetates, succinates and the like, such as sodium phosphate, citrate, borate, acetate, bicarbonate, carbonate and tromethamine (TRIS). These agents are present in amounts sufficient to maintain the pH of the system at 7.4.+-.0.2 and preferably, 7.4. As such the buffering agent can be as much as 5% on a weight basis of the total composition.

Representative buffering agents or salts useful in maintaining the pH at about 7.4.+-.0.2 are alkali or alkali earth carbonates, chlorides, sulfates, phosphates, bicarbonates, citrates, borates, acetates and succinates. Representative preservatives are sodium bisulfite, sodium thiosulfate, ascorbate, benzalkonium chloride, chlorobutanol, thimerosal, phenylmercuric borate, parabens, benzylalcohol and phenylethanol.

The preparation of the corneal contact lens compositions of the invention are described below. The Examples which follow are prepared according with the following preparation procedure. Since the polyoxyalkylenes dissolve more completely at reduced temperatures, the preferred methods of solubilization are to add the required amount of polymer to the amount of water to be used. Generally after wetting the polymer by shaking, the mixture is capped and placed in a cold chamber or in a thermostatic container at about 0.degree. C. to 10.degree. C. in order to dissolve the polymer. The mixture can be stirred or shaken to bring about a more rapid solution of the polymer. The various additives such as buffers, salts, and preservatives can subsequently be added and dissolved. The desired pH of 7.4.+-.0.2 is obtained by the addition of appropriate buffering agents.

The following Examples illustrate the various aspects of the invention but are not intended to limit its scope. Where not otherwise specified throughout this specification and claims, temperatures are given in degrees centigrade and parts, percentages, and proportions are by weight.

This Example describes a composition of the invention for ophthalmic use as a protective corneal contact lens. The composition prepared was characterized as iso-osmotic, sterile, and having a pH of 7.4.+-.0.2. An aqueous solution was made of a polyoxyethylene-polyoxypropylene block copolymer having the structure generically shown above as Formula IV and having a polyoxypropylene hydrophobe base average molecular weight of about 4000, a total average molecular weight of about 11,500, and containing oxyethylene groups in the amount of about 70% by weight of the total weight of copolymer. This copolymer (Formula VI below) is sold under the trademark PLURONIC.RTM. F-127 (also known as Poloxamer 407) by the BASF Corporation, Parsippany, N.J. A solution in TRIS hydrochloride buffer was made by dissolving said polymer and sodium alginate in cold (4.degree. C.) buffer to give a concentration of 19% by weight polyoxyalkylene and 1% by weight sodium alginate. More specific solution procedures are described in "Artificial Skin I Preparation and Properties of PLURONIC F-127 Gels For Treatment of Burns", Journal of Biomedical Material Research 6, 527, 1972, incorporated herein by reference. The block copolymer has the formula: ##STR2## This formulation forms the basis for the Figure in which the curve shows the penetration of a 20 mm thickness aqueous gel at various temperatures. After contact of the gel with calcium ions, as indicated by the vertical line at 40.degree. C., the gel strength is not reduced or the composition rendered fluid by lowering the temperature to 25.degree. C.

These examples describe pH balanced, thermo-sensitive aqueous systems which are suitable for forming a thermally reversible corneal contact lens in situ. Both examples will result in the formation of thermally irreversible systems upon exposure to an aqueous solution of 2% to 10% by weight calcium chloride. The formulations are:

The formulations are prepared by the "Hot Method", "BWC surfactants in gel cosmetics", I. R. Schmolka, Cosmetics and Toiletries, vol 92, July 1977, pages 77-79. The procedure is as follows:

1. The poloxamer blocks (BASF Corp) are melted at 65.degree. C. in a water jacketed mixing bowl. The mixer used is a Stephan UMC5 mixer-blender (Stephan Machinery, Columbus, Ohio).

2. A weighed amount of buffer is placed in a one liter beaker. Weighed amounts of Glycerin (JT Baker) and Sodium Alginate (Protonal SF120, Protan, Inc.) are added to dissolve and mix.

3. This solution is added to the molten poloxamer and mixed at 65.degree. C. for 15 minutes in a nitrogen atmosphere.

4. The temperature is gradually dropped to 25.degree. C. and then to 15.degree. C. by the circulation of ice-cold water.

5. The final product is stored overnight at 4.degree. C. in a glass beaker.

6. The next day the following tests were done and the results were as follows:

Examples 2 and 3 are repeated substituting for poloxamer 407, 2% by weight of polymer #2, as described in U.S. Pat. No. 4,810,503 and 4% by weight of surfactant #1, as described therein. The balance of the percentage of Poloxamer 407 used in Examples 2 and 3 is made up with a borate buffer or a phosphate borate buffer, respectively. These formulations form soft gels at room temperature which are usefully stiffened upon exposure to a 2% by weight aqueous solution of calcium chloride. Substantially similar pH and osmolality results are obtained.

Ion exchange resin beads sold under the tradename Duolite were treated so as to incorporate calcium by first treating a 30 gram sample of the ion exchange resin with a solution of 0.1 molar hydrochloric acid so as to allow for the exchange of protons for sodium. After three washings with 0.1 molar hydrochloric acid, the beads were washed with water and then washed twice with a 2% aqueous solution of calcium chloride. Each of the washing steps took place over a period of 16 hours (overnight). The beads were thereafter filtered and washed with water utilizing coarse filter paper and a Buchner glass filter assembly. The beads were then left overnight in a desiccator to dry. The dried beads of ion exchange resin which were obtained are utilized in the amount of 2 grams to fill a first compartment (close to the needle of the syringe) of a glass syringe utilized to apply liquids and dry materials. The syringe is sold under the tradename Hypak. Into the second compartment of the syringe, there is placed the solution of Example 2. Pushing the plunger of the syringe forward results in mixing the solution of Example 2 with the ion exchange beads. After 5 to 10 minutes subsequent to mixing, the mixture is expelled from the syringe. After an additional 15 minutes the expelled material forms a thermo-irreversible film on the substrate on which it is expelled.

While this invention has been described with reference to certain specific embodiments, it will be recognized by those skilled in the art that many variations are possible without departing from the scope and spirit of the invention, and it will be understood that it is intended to cover all changes and modifications of the invention, disclosed herein for the purposes of illustration, which do not constitute departures from the spirit and scope of the invention.