Glucan is a generic term referring to an oligo- or polysaccharide composed predominantly or wholly of the monosaccharide D-glucose. Glucans are widely distributed in nature with many thousands of forms possible as a result of the highly variable manner in which the individual glucose units can be joined (glucosidic linkages) as well as the overall steric shape of the parent molecule.
The glucan referred to in this invention typically is a linear chain of multiple glucopyranose units with a variable number of side-branches of relatively short length. The glucosidic linkages are predominantly (not less than 90%) .beta.-1,3 type with a lower number (not greater than 10%) of .beta.-1,6 type linkages; the .beta.-1,3 linkages form the bulk of the backbone of the molecule, while the .beta.-1,6 linkages occur predominantly in the side-branches. The chemical name of this form of glucan is poly-(1,3)-.beta.-D-glucopyranosyl-(1,6)-.beta.-D-glucopyranose. Glucan is well described molecule.
The form of glucan is found principally in the cell wall of most fungi (including yeasts and moulds) and in some bacteria. Glucan, in combination with other polysaccharides such as mannan and chitin, is responsible for the shape and mechanical strength of the cell wall. The glucan typically accounts for approximately 40% to 50% of the weight of the cell wall in these cells.
The chemical structure of fungal cell wall glucan has been studied in detail, with the following sentinel articles being incorporated herein by reference--Bacon et al (1969); Manners et al (1973).
Fungal cell wall glucans have long been used in industry, particularly the food industry, usually in a semi-purified form. Their uses have included use as stabilizers, binders, thickeners and surface active materials.
It also has been known for some forty years that fungal cell wall glucans are biologically active, exerting a number of effects on the reticuloendothelial and immune systems of animals. The outstanding biological effect in this regard is their ability to stimulate non specifically the activity of the body's primary defence cells--the macrophage and the neutrophil. This is thought to be due to receptors to .beta.-1,3 glucan displayed on the surface of these cells (Czop and Austen. 1985). The interaction between glucan and its receptor producing such stimulatory effects as enhanced phagocytosis (Riggi and Di Luzio, 1961), increased cell size (Patchen and Lotzova, 1980), enhanced cell proliferation (Deimann and Fahimi, 1979), enhanced adherence and chemotactic activity (Niskanen et al, 1978), and production of a wide range of cytokines and leukotrienes (Sherwood et al. 1986, 1987).
The aforementioned biological responses to fungal cell wall glucan have been reported to result in a number of clinical effects including: enhanced resistance to infections with fungi (Williams et al. 1978), bacteria (Williams et al, 1983), viruses (Williams and Di Luzio, 1985), protozoa (Cook et al, 1979) following systemic application: enhanced antitumour activity following systemic application (Williams et al. 1985) or intralesional application (Mansell et al. 1975); and enhanced immune responsiveness following systemic application (Maeda and Chihara, 1973). It will be readily seen that these clinical effects are highly beneficial and important and represent an opportunity to develop novel pharmaceutics based on fungal cell wall glucans, such pharmaceutics having potentially wide application in both veterinary and human medicine.
Of the various fungal cell wall glucans tested, that from the yeast Saccharomyces cerevisiae has proven to be acceptable in terms of efficacy and safety as an immune stimulant in animals and humans. Hereinafter this will be referred to as Saccharomyces cerevisiae ("Sc")-glucan. Predominantly or wholly .beta.-1,3 glucans from other fungi, bacteria or plants from the Graminaceae family have been shown to be immunostimulatory in animals but compared to Sc-glucan either are not as potent or if they do have comparable or greater potency then that is usually associated with a higher level of undesirable side-effects.
Sc-glucan has been shown to be biologically active as an immune stimulant in animals in various forms. These include (a) a large molecular weight (typically greater than 3.times.10.sup.6 d), water-insoluble, microparticulate form, or (b) smaller molecular weight (typically less than 500,000 d) forms which are dispersible or soluble in water. Water-solubility is described as being achieved either through cleavage of the large microparticulate glucan form to smaller molecules using processes such as enzymatic digestion or vigorous pH adjustments, or by complexing to salts such as amines, sulphates and phosphates. The principal advantage of the smaller, water-soluble form vs the larger microparticulate form is that it is safer when given by parenteral routes of administration such as intravenously. Also, it is likely that the smaller sized molecules are more bio-available on a molar basis.
To date it has neither been technically possible nor economically feasible to synthesise glucan on a commercial basis. Thus preparation of commercial quantities of .beta.-1,3 glucan for therapeutic uses requires that it be extracted from fungi, bacteria, algae or cereal grains.