The most common form of glucan is yeast glucan or Saccharomyces cerevisiae glucan. An additional form of glucan is lentinan from the Japanese mushroom, Lentines edodes.
Yeast glucan is isolated from the cell walls of Saccharomyces cerevisiae, and is a relatively high molecular weight (about 240,000) polysaccharide (Manners et al., Biochem. J. 135, 19 (1973)). Glucan is responsible for cell rigidity, and under a scanning microscope retains many of the morphological characteristics of yeast; i.e. it is a spheroid approximately 5 .mu.m in diameter with a bud scar on each unit (Baxter et al., Rad. Res. 51, 540 (1972)). Glucan has a high proportion of .beta.-(1-6)-glucosidic linkages (Manners et al., Biochem. J. 135, 31 (1973)). Glucan, in its natural state, is water insoluble, and x-ray diffraction studies have shown that glucan exists in the form of a triple-stranded helix (Sarko et al., Biochem. Soc. Trans. 11, 139 (1983)). Upon hydrolysis, glucan yields predominantly or completely, the monosaccharide, D-glucose (Manners et al., Biochem. J. 135, 19 (1973)).
Although naturally occurring glucan is water insoluble, at least two forms of water soluble glucan have been prepared. One form of water soluble glucan is a phosphorylated glucan described in U.S. Pat. Nos. 4,739,046 and 4,761,402. The other form of soluble glucan is prepared using the method of Sasaki et al., Gann 67, 191 (1976) as modified by DiLuzio et al., Int. J. Cancer 24, 773 (1979). These soluble glucans have the same general therapeutic effects of naturally occurring glucans, but are easier to use as injectable agents. Neither water soluble nor water insoluble glucans has been found to have good gelling properties. Therefore, glucans have not been used alone to make formed compositions.
Glucan has been recognized, therapeutically, as a macrophage activator (Wolk et al., Medical Biology 63, 73 (1985); Leibovich et al., J. Reticuloendothel. Soc. 27, 1 (1980); Williams et al., Surgery 93, 448 (1983); Burgaleta et al., Cancer Res. 37, 1739 (1977); Mansell et al., J. Nat'l. Can. Inst. 54, 571 (1975)). It has also been reported that glucan increases vascular permeability in the skin (West, Int. Arch. Allergy Appl. Immun. 56, 380 (1978)) and accelerates wound healing through the release of fibroblast-stimulating activity from monocytic macrophages (Kenyon, Am. J. Vet. Res. 44, 652 (1983)). Further reports of therapeutic uses for glucan include its administration to increase ganulocyte and macrophage production (Burgaleta et al., supra; Lotzova et al., J. Immunol. 123, 607 (1979)), its enhancement of interleukin-1, interleukin-2 and lymphokine production (Sherwood et al., Int. J. Immunopharmac. 9, 261 (1987)) and its anti-tumor and anti-staphylococcal activity (DiLuzio et al., Int. J. Cancer 24, 773 (1979).
Additional reports describe glucan's enhancement of: phagocytic function (Wooles et al., J. Reticuloendothel. Soc. 1, 160 (1964)); humoral immunity (Williams et al., J. Reticuloendothal. Soc. 23, 479 (1978)); cell-mediated immunity (Wooles et al., Science 142, 1078 (1963)); serum complement levels (Haendchen et al., Fed. Proc. 40, 1151 (1981)); and macrophage secretory function (Kokoshis et al., J. Reticuloendothal. Soc. 25, 85 (1979)). Administration of glucan has also been shown to enhance hemopoietic activity including granulopoiesis, monocytopoiesis and erythropoiesis (Patchen, Suru. Immunol. Res. 2, 237 (1983)). Glucans have not been used for ophthalmic purposes or in ophthalmic preparations.
Like glucan, collagen is a relatively well-known composition. Collagen is a polypeptide substance of molecular weight about 130,000. Collagen comprises about one-third of the total protein in mammalian organisms, and is the main constituent of skin and connective tissue, and the organic substance of bone and teeth.
Although there are different types of collagen, all collagens are composed of molecules which contain three polypeptide chains (.alpha. chains) arranged in a triple helical configuration. The amino acid sequence of the .alpha. chains is mostly a repeating structure with glycine at every third position and proline or 4-hydroxyproline frequency preceding the glycine residues.
Collagen is differentiated from accompanying fibrous proteins (i.e. elastin and reticulin) by: (1) its content of proline, hydroxyproline and hydroxylysine; (2) the absence of tryptophan and its low tyrosine and sulfur contents; and (3) its high content of polar groups originating from the difunctional amino acids. These polar groups are responsible for swelling properties which lead to the dispersion of collagen in dilute acid. Denaturation of collagen comprises the conversion of the rigidly coiled helix to a random coil referred to as gelatin.
Collagen has been used as the fibers in sutures, and has been used in leather substitutes. In addition, collagen has been used as a gel in photographic emulsions, and has been used in coatings and in food casings.
Collagen has also been used in ophthalmic preparations. U.S. Pat. No. 4,713,446 describes a viscoelastic collagen solution of particular usefulness in ophthalmic surgery such as intracapsular and extracapsular cataract lens extraction, intraocular lens implantation, corneal transplant surgery and retinal detachment surgery. This viscoelastic collagen solution may also be used as a vitreous replacement.
Furthermore, collagen has been used ophthalmically in the form of collagen sponges or collagen shields. Kay et al., Ophth. Surgery 17, 626 (1986) describes the sub-tenon placement of a collagen sponge to deliver .sup.57 Cobleomycin after glaucoma filtration surgery in rabbits. Sheets et al., Ann. Ophthalmol. 18, 297 (1986) describes the use of freeze-dried collagen eye patches in the treatment of inflammatory eye conditions.
Numerous authors describe the ophthalmic use of collagen shields with or without therapeutic agents. Kaufman, J. Cataract Refract. Surg. 14, 487 (1988) introduces a series of articles relating to collagen eye shields by describing the shields as similar to contact lenses. The shields are generally made of porcine scleral collagen which can be cross-linked in variable amounts in order to effect their rate of dissolution after insertion. When put into the eye and hydrated, the collagen shields conform to the shape of the eye, but do not "suck on" the way vision corrective contact lenses do. As the collagen shields dissolve, they provide a layer of biologically compatible collagen solution which lubricates the surface of the eye, thereby minimizing rubbing of the eyelids on the cornea and fostering epithalial healing.
Kaufman, supra further reports that the collagen fibrils of the shields can trap, within their interstices, molecules of drug which are then held on the cornea by the shield. Both water soluble and water-insoluble drugs are suggested for use with collagen shields.
Some of the drugs which have actually been used with collagen shields are pilocarpine (Rubin et al., J. Clin. Pharmacol. August-September, 309 (1973); Aquauella et al., J. Cataract Refract. Surg. 14, 492 (1988); U.S. Pat. No. 4,164,559), tobramycin (Unterman et al., Invest. Ophthalmol. Vis. Sci. 29 (Suppl), 52 (1988); Unterman et al. J. Cataract Refract. Surg. 14, 500 (1988); O'Brien et al., J. Cataract Refract. Surg. 14, 505 (1988); Sawush et al., Invest. Ophthalmol Vis. Sci. 29 (Suppl), 228 (1988); Poland et al., J. Cataract Refract. Surg. 14, 489 (1988); Aquavella et al., supra), gentamicin (Aquavella et al., supra; U.S. Pat. No. 4,164,559) and flurbiprofen sodium (Aquavella et al., supra).
The drugs which have ben used with collagen eye shields are small, low molecular weight chemical entities. Large, high molecular weight substances, such as polysaccharides were not believed capable of incorporation in collagen, particularly for purposes of drug delivery. O'Brien et al., supra, have explicitly recognized that the uptake and release of a drug by a collagen shield depends upon the molecular weight and chemical structure of the drug. High molecular weight polysaccharides such as glucan were not believed to be capable of incorporation into, and delivery from a therapeutic collagen shield.