Abstract:
Hyaluronic acid, a polysaccharide, is prepared in high yield from streptococcus bacteria by fermenting the bacteria under anaerobic conditions in a CO 2  -enriched growth medium, separating the bacteria from the resulting broth and isolating the hyaluronic acid from the remaining constituents of the broth. The bacteria may be grown free of endotoxins by filtering all ingredients through a 10K Millipore® filter prior to inoculation of the medium and subsequently maintaining pyrogen-free conditions. Separation of the microorganisms from the polysaccharide is facilitated by killing the bacteria with trichloroacetic acid. After removal of the bacterial cells and concentration of the higher molecular weight fermentation products, the hyaluronic acid is isolated and purified by precipitation, resuspension and reprecipitation.

Description:
BACKGROUND OF THE INVENTION 
     Hyaluronic acid is a mucoid polysaccharide of biological origin. The sodium salt, sodium hyaluronate, in buffered physiological saline solution, has found significant use as a vitreous replacement in optical surgery and in other medical applications. Some of these applications are described in U.S. Pat. Nos. 4,141,973 and 4,328,803. For such medical purposes, a pyrogen-free, highly purified sodium hyaluronate having a molecular weight in excess of 750,000 has heretofore been used. A commercial product known as HEALON™, manufactured by Pharmacia, Inc., Piscataway, N.J., is a one-percent solution of sodium hyaluronate sold for such purposes. For example, dilute HEALON™ solution (0.1-0.2% sodium hyaluronate) has been reported to be useful as an eye drop for the treatment of patients with keratitis sicca syndrome. 
     Hyaluronic acid has also been used as an ingredient for in vitro culture of leprosy baccili and as a component for cosmetic formulations. Cosmetic formulations, which are described in U.S. Pat. No. 4,303,676, include both a low molecular weight fraction (about 10-50,000) and a higher molecular weight fraction (in excess of 1×10 6 ). 
     The source of hyaluronic acid for all of the foregoing uses have been rooster combs, human umbilical cords or other vertebrate tissue. Extraction and purification of hyaluronic acid from such tissue is a relatively complex process which results in a very expensive product. 
     Hyaluronic acid can be produced by Group A and C strains of Streptococcus bacteria. One use reported for the bacterial product appears to be as a reagent for determination of anti-streptococcal hyaluronidase in human serum samples, Kjems and Lebech, Acta Path. microbiol scand., Section B, 84: 162-164 (1976). In that paper, the authors describe a defined media for growing Group A streptococci and isolating hyaluronic acid, reporting a yield of 0.3 grams per liter of culture broth. However, hyaluronic acid produced by bacteria has not found substantial use because it is of a low molecular weight range. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides hyaluronic acid from bacterial sources of a preferred higher molecular weight range than is reported in the prior art as being obtained from bacterial sources. It also provides a method for producing hyaluronic acid from bacterial sources in much higher yields than has previously been reported. The method additionally produces hyaluronic acid which has a purity comparable to or better than any presently available for medical applications. Although the hyaluronic acid produced by the present method typically has an average molecular weight of about 55,000, it has potentially significant use as an eye drop ingredient and as an ingredient of cosmetic formulations. The high yield, high purity and low cost of the hyaluronic acid produced by the inventive method also permits it to be used in ways not previously described or contemplated for hyaluronic acid obtained from mammalian or low yield bacterial sources. For instance, hyaluronic acid might be used in food preparations as a humectant, in other applications as a lubricant, and in post-surgical applications for reducing complications due to fibrotic response and/or adhesion formation. The material might also be used in tertiary oil recovery as a substitute for polyacrylamide, similar synthetic polymers or biologically-produced polymers. 
     The inventive method in its preferred form comprises growing a culture of a hyaluronic acid-producing streptococcus strain under anaerobic conditions in a CO 2  -enriched growth medium, which includes those raw materials necessary for the production of the hyaluronic acid by the bacteria, preferably although not necessarily killing the bacteria, separating the bacteria from the growth medium and isolating the hyaluronic acid. Preferably, growth is accomplished by fermentation in a broth culture. Other growth techniques and media may be used. For example, an agar culture may be used. Hence, the term &#34;growth medium&#34; herein is to be taken broadly as meaning liquid or solid media or combinations thereof and other types of growth in addition to fermentation, as are all well known in the art. 
     Although the preferred form of the invention contemplates the growth of bacteria directly in the culture medium in which the hyaluronic acid is to be produced, it is also possible to grow the bacteria in other growth media, separate the bacteria from the medium, resuspend the bacteria in a buffered suspension medium or distilled water and add the appropriate raw materials to the suspension for the production of the hyaluronic acid by the already grown bacteria. This is considered to be an art equivalent to the preferred method and merely involves the use of a resting cell suspension. Consequently, such terminology herein as &#34;growing a culture&#34; and the like is to be taken as including both approaches within its purview. 
     Unlike prior methods of hyaluronic acid production, endotoxins can be excluded from the system initially by filtering all ingredients through a 10,000 (10K) nominal molecular weight limit (NMWL) cutoff filter, such as the Millipore® Pellicon® cassette tangential flow filtration system, prior to inoculation and subsequently maintaining pyrogen-free anaerobic growth conditions. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the preferred embodiment of the present invention, a semi-defined growth medium such as the following is used: 
     1. Casein hydrolysate, enzymatic: 20.0 g 
     2. Potassium chloride: 3.0 g 
     3. Sodium phosphate, dibasic: 2.8 g 
     4. Magnesium sulfate (7H 2  O): 0.5 g 
     5. Calcium chloride (2H 2  O): 10.0 mg 
     6. Glucose: 20.0 g 
     7. Vitamin solutions: 
     (a) d-Biotin: 1.0 mg 
     (b) D-Calcium Pantothenate: 1.0 mg 
     (c) Choline chloride: 1.0 mg 
     (d) Folic acid: 1.0 mg 
     (e) i-Inositol: 2.0 mg 
     (f) Nicotinamide: 1.0 mg 
     (g) Pyridoxal HCl: 1.0 mg 
     (h) Riboflavin: 0.1 mg 
     (i) Thiamine HCl: 1.0 mg 
     The medium is made up to one liter with reverse osmosis pyrogen-free water. Of course, other growth media suitable for this purpose may also be used. 
     A 100 ml culture of Streptococcus pyogenes type is grown anaerobically for six hours in the medium at 37±1° C. The other hyaluronic acid-producing bacteria referred to hereinabove may be used, but the type 18 is preferred. Five liters of the same medium are inoculated with this six-hour culture and grown to a high visible density, preferably to at least 2×10 8  cells per ml and typically to 5×10 8  cells per ml. The five liter inoculum is then used to inoculate 160 liters of medium in a 200 liter fermentor to begin a production run. 
     During the production run, the culture is grown with continuous agitation while infusing CO 2  gas at a rate of flow sufficient to maintain a dissolved level of CO 2  as determined by a CO 2  monitoring probe. A 5-10% level of dissolved CO 2  is preferred. The gas is preferably infused as an N 2  /CO 2  mixture. An 85/15 ratio is preferred, but not critical. The CO 2  level may range up to about 50%±10%. The gas is filter-sterilized as it is introduced into the growth chamber. Temperature is preferably controlled at about 37±1° C. The pH is preferably controlled to a substantially constant value ±0.1 within the range of about 6.5 to 7.5 by monitoring with a pH probe/controller and by the addition of KOH as called for by the controller. Fermentation is considered complete when the pH of the culture stops dropping (no more KOH is called for to maintain pH within the set limitation), or when the cell density reaches the high visible density, typically 1-5×10 9  cells per ml. At this point the fermentation is terminated by the addition of 100% saturated solution of aqueous trichloroacetic acid to make the fermentation mixture up to a final trichloroacetic acid concentration of about 5%. This may vary. 
     The addition of trichloroacetic acid to the fermentation broth not only terminates growth by killing the bacteria, but also makes separation of the cells from the broth substantially easier by contributing to flocculation of the cells. The mixture without trichloroacetic acid is very difficult to separate without causing severe disruption of the integrity of both components. Microorganisms and the polysaccharide ie., the hyaluronic acid, do not readily separate by centrifugation or filtration without the trichloroacetic acid addition. Thus, while it is possible to terminate growth of the culture by other means, for instance heat treatment, trichloroacetic acid treatment has the advantage of facilitating subsequent separation of the hyaluronic acid. 
     The fermentation mixture is pumped from the fermenter through a 0.22 micrometer pore-size Durapore® filtration cassette using the aforementioned Millipore® tangential flow filtration system. This step concentrates the cells from 160 liters to approximately five liters. The filtrate is retained and diafiltered against greater than 10 mega-ohm conductivity reverse osmosis water using a 30,000 nominal molecular weight cut-off Millipore® Pellicon® cassette system until the filtrate, which is continuously discarded, reaches a conductivity of approximately 0.5 mega-ohms. Diafiltering is a powered dialysis technique, such as is disclosed in Catalog Number OM029, March 1981, entitled Pellicon® Cassette System, of Millipore Corporation, Bedford, Mass. 01730, as opposed to conventional passive dialysis techniques. The hyaluronic acid is then concentrated by continuing the filtration process without further input of water. 
     The concentrate is then treated with reagent grade ethanol, preferably in a 3:1 ratio. Other alcohols, acetone, chloroform or other organic solvents as well as certain organic salts such as CETAB, a mixed trimethylammonium bromide, may be used to precipitate the hyaluronic acid or sodium hyaluronate from the aqueous solution. This should be done without any mixing other than occurs in the act of pumping the hyaluronic acid into the solvent. Stirring during alcohol treatment has been found to reduce the process yield of hyaluronic acid. The precipitate at this stage can be stored indefinitely in the dark at 4° C. 
     As is seen from the procedure described, a unique approach is found in the isolation of the hyaluronic acid from the broth by a two-step process in which a molecular weight separation step is carried out by diafiltration to separate the acid from substantially all of the lower molecular weight components of the broth, and then the acid is separated from any remaining broth constituents by precipitation. 
     The precipitated hyaluronic acid can be dewatered (removal of the bulk of the water/alcohol solution) by a number of conventional techniques and then resuspended in reverse osmosis water or a 0.15M NaCl solution. The resuspended material is then lyophilized (freeze-dried), spray-dried, vacuum-dried or diafiltered to remove the last traces of alcohol. Further purification is performed by making a 0.05M Borate buffer solution, pH 8.0, with approximately a 10 mg/ml sodium hyaluronate concentration. 0.32% CETAB, is then added to the solution and the mixture stirred at 4° C. overnight to yield a precipitate, sodium hyaluronate. Other precipitating agents may be used, such as cetyl pyridium chloride or related salts. The precipitate is recovered by coarse filtration and resuspended in a 1M NaCl solution made with reverse osmosis water. The resuspended hyaluronic acid is then diafiltered and concentrated as above. The resultant hyaluronic acid can then be filter-sterilized and used or converted to sodium hyaluronate and then be filter-sterilized and used. 
     Conventional dewatering techniques include pressing, centrifugation, chemical addition and the like. The particular technique selected will depend on the subsequent intended use of the precipitate. 
     If a medical grade pyrogen-free product is desired, a pyrogen-free filtered growth medium is used and all operations of the process, including the isolation and processing of the hyaluronic acid/sodium hyaluronate are performed under conditions of a class 100 clean room using pyrogen-free containers. If the material is to be used only for chemical grade application, the cleanliness of the room and collection containers is not critical with respect to pyrogens. 
     The inventive method which emphasizes growing cells under non-aerated conditions prevents the streptococcus from producing its normal complement of end products, primarily the pyrogenic exotoxins for which the microbe is so well known. The described growth conditions also give a much higher yield of hyaluronic acid than has been previously reported. A minimum of 2 grams of hyaluronic acid per liter of culture broth has been obtained using the preferred cell growth and isolation conditions described above. The high yield under the non-aerated conditions is unexpected since one of the proposed functions of hyaluronic acid is thought to be that of providing an oxygen barrier for the cell. Thus, its production would only be expected to be maximized under conditions of exposure to oxygen. 
     The hyaluronic acid/sodium hyaluronate prepared as described has an average molecular weight of about 55,000± about 20% within a molecular weight range of from about 10,000-2,000,000 as determined by gel filtration or by quasi-elastic light scattering techniques. These techniques are well known as are the variations in measurement and the results obtained with them due to biological variation. The product also has a protein content of between 0.3% and 0.03% depending on method of analysis. The UV absorption of the 0.1% solution is 0.314 at 260 nm and 0.169 at 280 nm. Viscosity of a 1% solution is approximately 300 centistokes. 
     A 0.5-1.5 percent solution of the pyrogen-free NaHy produced by the inventive method may be used as an eye drop composition in place of the very dilute solutions of high molecular weight rooster comb derived hyaluronic acid presently used for treatment of keratitis sicca. 
     Other hyaluronic acid-producing streptococci in the Group A and Group C strains may be used in the invention. Additionally, variations in the growth medium and conditions of growth, as well as variations in the isolation procedures, may be made without departing from the invention which is set forth in the following claims.