Heavy Metal Salts of Hyaluronic Acid and Their Use as Antimicrobial Agents

Throughout this application various publications are referenced by arabic numerals with parentheses. Full citations for these references may be found at the end of the specification immediately preceding the claims. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of art as known to those skilled therein as of the date of the invention described and claimed herein.

This invention concerns the heavy metal salts of hyaluronic acid.

Hyaluronic acid is present in various connective tissues of animals, such as skin and cartilage. Some organs are specifically rich in hyaluronic acid, such as the umbilical cord, synovial fluid, the vitreous humor and rooster combs. In addition, hyaluronic acid is produced by various microorganisms, such as streptococci Type A and C.

In skin and cartilage, the role of hyaluronic acid is to bind water and retain the tonicity and elasticity of the tissue. In joint fluids, the viscous hyaluronic acid solution serves as a lubricant to provide protective environment to the cells. A solution of ultrapure hyaluronic acid from rooster combs has been in use for several years as a supportive medium in ophthalmic surgery, see U.S. Pat. No. 4,141,973 of E. A. Balazs (1979). A similar preparation has been shown to be beneficial in the treatment of inflamed knee joints of race horses. Another use of hyaluronic acid results from its highly hydrophilic nature, making it an ideal constituent of moisturization lotions for cosmetic use, U.S. Pat. No. 4,303,676 of E. Balazs (1981).

Hyaluronic acid has been isolated from the various biological sources, as described above, including microbial broth. The isolation and characterization of hyaluronic acid has been described by Meyer et al., J. Biol. Chem. 107,629 (1934); J. Biol. Chem. 114,689 (1936), and has recently been reviewed in Methods in Enzymol. 28, 73 (1972). The structure of hyaluronic acid was elucidated by Weissman et al., J. Am. Chem. Soc. 76, 1753 (1954) and Meyer, Fed. Proc. 17, 1075 (1958). Other publications such as, U.S. Pat. No. 4,141,973, Feb. 27, 1979 by E. A. Balazs, concerned the production and purification of hyaluronic acid from the sources such as animal connective tissue.

Radioactively labelled hyaluronic acid and sodium salt thereof has been produced by growing streptococcus in fermentation broth containing radioactively labelled glucose. (3)

Numerous silver compounds having a wide range of uses are known. These include silver acetate and silver chlorate which may be used as oxidizing agents; silver bromide and silver oxalate which are used in photography; silver difluoride for use in the flourination of hydrocarbons; silver chloride and silver cyanide for use in silver plating; and silver chromate (VI) and silver oxide which may be employed as catalysts, to name just a few.

The silver ion has also been shown to be an effective antimicrobial agent. It is not associated with significant side effects, is not an allergen, and is only rarely associated with the induction of resistant strains of bacteria. Many silver salts are useful as topical anti-infectives or as antiseptics. These include: silver fluoride, silver iodide, silver lactate, mild silver protein and silver nitrate. Silver lactate and silver nitrate may also be employed as astringents and silver picrate and silver sulfadiazine may be used as antimicrobial or antibacterial agents.

It is believed that silver compounds produce their antimicrobial effects by the time-dependent release of silver ions and their effectiveness is direclty related to the constant presence of the free ions in the tissues. The use of simple silver salts, such as silver nitrate, as an antibacterial agent has been limited by the requirement of frequent applications to achieve effective concentrations of the silver ions (10-20 .mu.g/ml).

Although the use of silver sulfadiazine, a more complex silver compound, results in a more sustained release of the silver metal ions, the use of both silver nitrate and silver sulfadiazine is often accompanied by adverse side-effects due to the anion part of the salts, namely nitrate and sulfadiazine. For example, sulfadiazine has such side effects as leucopenia and immunosuppression activity. Other silver preparations, such as foils and silver nylon, have recently been offered for use, but their release of silver ions is limited and do not result in high enough concentrations.

Thus, the need for a silver compound providing sustained release of the silver metal ion, but having no adverse side effects is apparent.

Rheumatoid arthritis is characterized by severe inflammation of the joints which is followed by the appearance of degraded hyaluronic acid in the joint fluid. In severe cases, anti-inflammatory agents such as corticosteroids and gold salts, e.g. gold sodium thiomalate or gold sodium thiosulfate are administered intra-articularly. However these agents are active for only a short duration, and there is a need to sustain their action. Thus, the invention of an anti-inflammatory agent having sustained action would meet a long felt need and be a significant advance in the anti-inflammatory art.

The present invention is directed to heavy metal salts of hyaluronic acid, including silver hyaluronate, gold hyaluronate, cerium hyaluronate, and tungsten hyaluronate.

The invention is also directed to methods of inhibiting microbial growth utilizing these heavy metal salts.

The present invention concerns methods of producing silver hyaluronate, compositions containing silver hyaluronate, and the treatment of wounds, burns and infections, especially soft-tissue infections and gonoccocal opthalmalogical infections, with silver hyaluronate.

The invention also concerns treating arthritis and joint inflammation with gold hyaluronate.

Finally, this invention is directed to compositions containing radioactively labelled heavy metal salts e.g. .sup.14 C AgHA, which compositions may be used for diagnostic purposes.

The invention concerns heavy metal salts of hyaluronic acid, in particular, silver hyaluronate, gold hyaluronate, cerium hyaluronate, and tungsten hyaluronate.

The invention also concerns methods of making the silver hyaluronate by mixing aqueous sodium hyaluronate (NaHA) solution with a molar excess of aqueous silver nitrate (AgNO.sub.3) solution to form aqueous silver hyaluronate (AgHA) solution, precipitating the silver hyaluronate from the solution and recovering the precipitated silver hyaluronate.

These heavy metal salts of hyaluronic acid may be incorporated into compositions, such as pharmaceutical compositions containing an effective amount of the heavy metal salt, e.g., silver hyaluronate or gold hyaluronate, and a pharmaceutically acceptable carrier.

These heavy metal salts of hyaluronic acid are useful as antimicrobial agents. In particular, microbial growth may be inhibited by contacting the microbes with an effective amount of silver hyaluronate. Silver hyaluronate may also be used to inhibit microbial growth in infections, by topically applying an effective amount of the silver hyaluronate to the infection.

Gold hyaluronate may be used to treat arthritis and joint inflammation by administering an effective amount of the gold hyaluronate intra-articularly to the afflicted subject.

This invention also concerns compositions containing heavy metal salts of radioactively labelled hyaluronic acid.

FIG. 1: Slow Release of Ag Ions from AgHA

To test the Ag ion release properties of AgHA, a sample containing 1800 .mu.g of Ag ion was dialyzed against distilled water at room temperature. At points indicated in FIG. 1, the dialysis bag containing AgHA was transferred to 500 ml fresh distilled water. At each point a sample of the water was removed and the concentration of Ag ions determined by atomic absorption. The results in FIG. 1 show that Ag ions are released in a slow manner, as 70% of the Ag ion concentration was retained in the dialysis bag after 48 h.

This invention concerns heavy metal salts of hyaluronic acid. More particularly, the salts encompassed by this invention include the silver, gold, cerium, and tungsten salts of hyaluronic acid.

The invention also concerns methods of making the silver salt of hyaluronic acid by mixing aqueous sodium hyaluronate (NaHA) solution with a molar excess of aqueous silver nitrate (AgNO.sub.3) solution to form aqueous silver hyaluronate (AgHA) solution, precipitating the silver hyaluronate from the solution and recovering the precipitated silver hyaluronate. The silver hyaluronate precipitate may be recovered by separating the silver hyaluronate precipitate from the aqueous silver hyaluronate solution, e.g., by centrifugation, washing the separated precipitate with ethanol and drying the washed precipitate over nitrogen.

Preferably, the silver hyaluronate is prepared in the absence of light and the aqueous sodium hyaluronate and aqueous silver nitrate mixture is shaken for a sufficient period of time, ordinarily several hours, to form aqueous silver hyaluronate solution. The aqueous silver hyaluronate solution is then treated to effect precipitation of the silver hyaluronate which may be recovered by rinsing the silver hyaluronate precipitate with ethanol, drying the rinsed precipitate with nitrogen and further drying the nitrogen-dried precipitate by high vacuum drying.

The other heavy metal salts of hyaluronic acid may be prepared by methods analogous to the preparation of silver hyaluronate.

These heavy metal salts of hyaluronic acid may be combined with carriers to form compositions. The carrier or carriers may be any suitable carrier known to those of ordinary skill in the art. These compositions may be pharmaceutical compositions containing an effective amount of the heavy metal salt of hyaluronic acid, for example, silver hyaluronate, or gold hyaluronate and a pharmaceutically acceptable carrier.

The heavy metal salts of hyaluronic acid and compositions containing same are useful as antimicrobial agents. In particular, microbial growth may be inhibited by contacting the microbes with an effective amount of silver hyaluronate. Silver hyaluronate may also be used for treating burns, wounds, soft tissue infections, for example, gonoccocal opthalmalogical infections or sepsis by topically applying an effective amount of the silver hyaluronate to the site of the burn, wound, soft tissue infection or infection from which the sepsis stems.

Gold hyaluronate may also be used for treating arthritis, rheumatoid arthritis, and joint inflammation in a subject by administering an effective amount of the gold hyaluronate intra-articularly to the subject.

This invention also concerns compositions containing the radioactively labelled heavy metal salts of hyaluronic acid. These include salts wherein the hyaluronate moiety is radioactively labelled, as well as salts wherein the heavy metal is in isotope form. Preferably, the radioactively labelled heavy metal salt is a heavy metal salt of radioactively labelled hyaluronic acid. More preferably, the hyaluronate moiety is radioactively labelled with .sup.14 C. Preferably the heavy metal is silver. Most preferably the radioactively labelled heavy metal salt is .sup.14 C radioactively labelled silver hyaluronate. These radioactively labelled heavy metal salts and compositions containing them may be used for diagnostic purposes.

As used throughout this application the term "heavy metal" includes any metal in Period 5, 6, or 7 or the 4f (Lanthanide) or 5f (Actinide) series of the Periodic Table.

Hyaluronic acid (HA) is a major constitutent of connective tissue, body fluids and skin, and hence is absolutely non-immunogenic. Due to their large size (M.W. 1.5.times.10.sup.6 Daltons), HA salts form viscous solutions, and after injection into the tissues the polysaccharide diffuses extremely slowly from the injection site.

Naturally occuring hyaluronic acid is a glycosaminoglycan consisting of a linear polymer of molecular weight of 50,000-13,000,000 daltons. It is a polysaccharide made of a repeating units of glucuronic acid and N-acetyl-glucosamine, bound by alternating 1.fwdarw.3 and 1.fwdarw.4 bonds.

As employed throughout this application the term "hyaluronic acid" includes substantially pure naturally occurring or synthetic hyaluronic acid, hyaluronic acid derivatives, hyaluronic acid cross-linked with itself, and hyaluronic acid cross linked with other substances such as collagen. See WO 86/00079 and WO 86/00912.

Silver hyaluronate has been prepared as follows:

A. One volume of sodium hyaluronate solution in water (0.5% of purified bacterially-derived sodium hyaluronate, M.W. 2.7.times.10.sup.6 daltons) was mixed with 1 volume of 0.5M AgNO.sub.3 in water. Two volumes of ethanol were then added, and the precipitated silver salt was centrifuged, washed with ethanol and dried over nitrogen. The silver content of this hyaluronate salt was found by atomic absorption to be 20% w/w, i.e., corresponding to approximately 92% substitution of the carboxylic ions.

This preparation was found to effectively kill Staphylococci, Pseudomonas, Candida albicans and Candida tropicans when applied as a 0.1% solution (1:1, v/v) to cultures containing 10.sup.6 microorganisms per ml.

B. Preliminary studies have indicated that removal of traces of chloride ions is essential for obtaining a relatively stable, pure and clear AgHA product. This has been achieved by ethanolic precipitation of sodium HA from a sodium nitrate solution. The procedure for the production of AgHA, starting with 1 gram of pure clinical grade sodium hyaluronate, is as follows:

1. Pharmaceutical grade NaHA (1 gram) is dissolved in 200 ml of 0.5M NaNO.sub.3.

2. 300 ml of ethanol is added, and the precipitated NaHA collected and washed thoroughly with 96% ethanol.

3. The precipitate is redissolved in 200 ml of double distilled water.

4. 200 ml of 0.5M AgNO.sub.3 is added in the dark and the mixture shaken for several hours. All subsequent operations are performed in relative darkness.

5. Ethanol (600 ml) is added, and the precipitated AgHA is rinsed with 96% ethanol, dried with nitrogen, then followed by high vacuum drying.

Portions of the final material are taken and redissolved in sterile, double-distilled water to give a working solution. A 1% solution gives a viscous, clear liquid with a brownish color.

Several batches of this material have been prepared.

Silver Content:

Analysis of the silver content, as determined by atomic absorption, was 82 grams per mole, which is 76% of the theoretical stoichiometrical value, while the concentration of residual sodium ions was 0.5% of the theoretical value.

Stability Studies:

Some preliminary observations of the stability of the 1% AgHA solutions have been made. Storage in the dark under refrigeration had no effect on the appearance of the material: no changes in viscosity or darkening of the clear solution have developed during 3-4 months. However, exposure of the solution to room temperature and light resulted in gradual browning, development of turbidity and a drop in viscosity within a week to two weeks.

Molecular Weight:

The various batches of AgHA were prepared from NaHA of M.W. of 3-3.5.times.10.sup.6. In order to measure the M.W. of AgHA, the material had to be converted to the Na form. This was done by ethanol precipitation from a concentrated (1M) solution of NaNO.sub.3. The M.W., as deduced from limiting viscosity measurements with these precipitates, was found to be 1.7-2.times.10.sup.6 daltons immediately after preparation of the AgHA solutions. On the shelf, at room temperature and lighting, the M.W. dropped to:

1.27.times.10.sup.6 daltons after 7 days;

0.96.times.10.sup.6 daltons after 29 days.

Slow release of Ag ions is mandatory for a long-lasting effect of the antimicrobial in the form of Ag complex. To test the Ag ion release properties of AgHA, a sample containing 1800 .mu.g of Ag ion was dialyzed against distilled water at room temperature. At points indicated in FIG. 1, the dialysis bag containing AgHA was transferred to 500 ml fresh distilled water. At each point a sample of the water was removed and the concentration of Ag ions determined by atomic absorption. The results in FIG. 1 show that Ag ions are released in a slow manner, as 70% of the Ag ion concentration was retained in the dialysis bag after 48 h. The calculated dissociation constant of AgHA, assuming 100% dissociation of free HA, is 6.7.times.10.sup.-5 M.

A. Minimal Inhibitory Concentration. Minimal inhibitory concentration was tested on eight species of bacteria isolated from hospitalized patients. The eight species are E. coli; Seratia; Pseudomonas; Proteus; K. pneumoniae; Acinobacter; C. albicans; and M. morgani.

AgHA was diluted in 0.25% NaHA to yield final concentration in growth media of 100, 50, 40, 20, 10 .mu.g/ml of silver ion equivalent. The growth media was L broth containing no NaCl and supplemented with NaHA to a final concentration of 0.25%. The latter NaHA concentration was chosen as standard concentration for growth in liquid media because some bacteria did not grow as well at concentration of 0.5% (Table 1). Concentrations of 0.5% NaHA and above slowed the growth rate. This is attributed to the high viscosity of the media which apparently reduced oxygen solubility.

2 ml cultures were inoculated with approximately 10.sup.5 cells and grown on a rotary shaker at 37.degree. C. for 72 h. After 24, 48 and 72 hours, cultures were scored for growth. A slight turbidity observed in test tubes was scored as growth and marked in tables with the notation + (plus) (Table 2).

To compare the efficacy of AgHA to that of Ag-Sulfadiazine, L broth media containing concentrations of silver ions identical to those used for AgHA were prepared by serial diluents. Growth and scoring methodology were the same as described previously (Table 3). For a control experiment, L broth containing silver nitrate at identical concentrations of silver ions to those used for AgHA and Ag-Sulfadiazine were used (Table 4). The results of these experiments are presented in Table 3 and Table 4. The data obtained indicate that except for C. abicans and K. pneumoniae, all other clinical isolates were inhibited by a concentration of 20 .mu.g/ml silver ions. A concentration of about 40 .mu.g/ml was needed to inhibit growth of C. albicans and K. pneumoniae. It is clear that AgHA inhibits microbial growth.

B. Double Agar-Layer Test. Ag compounds are most commonly used as ointments. Ag ions have to be released and reach the target by diffusion. The methodology used to test this aspect of AgHA was adopted from Dettch et al. (2) and modified by using petri plates. AgHA and Ag-Sulfadiazine were mixed with agar separately to yield a calculated concentration of 40 .mu.g/ml silver ions. A layer of 2.2 mm silver containing agar was first poured and after solidification a layer of L-agar of 2.2 or 2.0 mm was poured on top. The plates were aged for 24, 48, 72, and 96 h prior to use. Cultures of bacteria were grown overnight and diluted in L broth media to yield about 10.sup.5 -10.sup.6 cells. 0.1 ml of diluted cultures were applied to each plate to yield a final concentration of 10.sup.4 -10.sup.5 cells per plate. No Ag ions were used in the control experiments when determining total viable cells per plate. Table 5 and Table 6 show the results obtained for AgHA and Ag-Sulfadiazine. The results are shown in fraction form as the number of cells which grew on the silver ion plates (numerator) over the number of cells which grew on the control plates containing no silver (denominator). The fraction is also expressed as a percent. In Table 5 the results correspond to the experiment where the upper layer of L agar was 2.0 mm. Except for C. albicans, more than 99% of the population of the different species were fully inhibited. The surviving fraction was higher in the experiment shown in Table 6 than that shown in Table 5. This is attributed to the fact that the upper L-agar layer was 2.2 mm. Dettch found that each increase of 2 mm in height of the upper agar layer increased survival number tenfold. The increase of 0.2 mm in height of L-agar in our experiments is in agreement with the above finding, as the number of survivals did not exceed 2-3%. No significant difference is observed between plates aged for 24-96 h, indicating that by 24 h the minimum concentration of silver ions has reached the L-agar surface.

Antimicrobial activity was tested against Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa and Candida albicans.

The tests were performed using two different methods, a Minimum Inhibitory Concentration (MIC) test (1) where the Ag-hyaluronate was added to a liquid growth medium and an Agar Cup Test (2) where the test substance was filled into small wells punched in agar plates.

The results indicates that a concentration of 1% of the Ag-hyaluronate preparation inhibits the growth of the four test strains. In liquid medium P. aeruginosa was inhibited at a concentration of 0.1%.

The Ag-hyaluronate preparation which was used in the tests contained 16 mg/ml of the silver salt.

Materials and Methods: Ag-hyaluronate.

Test strains: Escherichia coli ATCC 8739; Pseudomonas aeruginosa ATCC 9027; Staphylococcus aureus ATCC 6538; Candida albicans ATCC 102 31.

The microorganisms were grown on Trypticase Soy Broth (TSB) at 35.degree.-37.degree. C. for 18-24 hours. Dilutions were made in physiological saline solution.

Ag-hyaluronate was added to TSB to a final concentration of 1.0%. Solutions with Ag-hyaluronate concentrations of 0.1% and 001% were made by further dilutions in TSB. Test organisms were added to the medium to a concentration of approximately 10.sup.3 org/ml. The test tubes were then incubated at 35.degree.-37.degree. C. and inspected for visible signs of growth during a period of one week. A turbid medium was recorded as growth. For Candida albicans the presence of a white-colored button at the bottom of the test tube was recorded as growth.

Overnight cultures of the test strains were diluted in sterile physiological saline. The suspensions were flushed over 14 cm diameter Trypticase Soy Agar plates and excess liquid was removed.

The plates were dried in a laminar air flow unit for 30 minutes. 5 mm diameter wells were punched in the agar. The test solutions were filled in the wells. The plates were incubated at 35.degree.-37.degree. C. and were inspected after overnight incubation. Clear zones surrounding the wells indicated antimicrobial activity. The diameter of the zones were measured.

The following solutions were tested:

Ag-hyaluronate (undiluted)

Ag-hyaluronate diluted in distilled water 1:1

Ag-hyaluronate diluted in horse serum (SBL) 1:1

0.01% Thiomersal Reference

Results are shown in Tables 10, 11 and 12:

The results from the MIC-test in liquid medium indicates that a 1% solution of the test substance in Trypticase Soy Broth inhibits the growth of S. aureus, C. albicans and E. coli. P. aeruginosa is inhibited at a concentration of 0.1%, A 1% solution contains 0.16 mg/ml of Ag-hyaluronate.

The tests with agar cup methodology confirms the results from the tests in liquid medium, though the lowest concentration for inhibition of P. aeruginosa was 1% Ag-hyaluronate.

When the Ag-hyaluronate was mixed with serum before testing with the agar cup method no significant difference could be measured as compared with Ag-hyaluronate mixed with distilled water.

A white-colored precipitate, probably AgCl, appears immediately when the Ag-hyaluronate comes in contact with culture media or serum.

The biosynthesis of HA by the bacterial involves the utilization of exogeneous glucose. The sugar is used mainly as an energy source, while a small portion of it is converted into glucoronic acid and N-acetylglucosamine. These are then incorporated into the HA molecule. Thus, the incorporation of radioactivity can be achieved by feeding carbon-14 labelled glucose into the fermentation broth.

Various experiments were performed, firstly in shake flasks, then in a specially-made pH- and temperature-controlled vessel. It has been found that fermentation in flasks was not efficient resulting both in very low incorporation ratios and low molecular weights. A "mini-fermentor" has been constructed, with a volume of 5 mL of medium, and studies were performed in this system on the rate of glucose incorporation and utilization. It has been found that at the optimal conditions for HA production, about 5% of the exogeneous glucose is incorporated into HA. The molecular weight of the HA produced in the mini-fermentor was about 2.times.10.sup.6 daltons. The system was then used for labelling the HA with .sup.14 C-glucose, and in one fermentation about 20 mg of crude .sup.14 C-HA was obtained. This material was then purified. The yield of this procedure was about 50%, resulting in a final yield of 10 mg of HA of specific activity of 1 .mu.Ci of .sup.14 C per mg and a molecular weight of 2.2.times.10.sup.6 daltons. The .sup.14 C radioactively labelled NaHA so purified may be converted into radioactively labelled AgHA following the procedures described in Example 1.

1. Carr, H. S., T. J. Wlodkowski and H. S. Rozenkrantz, "Silver Sulfadiazine: In Vitro Antibacterial Activity", Antimicrob. Agents and Chemoterhapy 4: 585-587 (1973).

2. Dettch, E. A., A. A. Marino, T. E. Gillespie and J. A. Albright, "Silver-nylon: A New Antimicrobial Agent", Antimicrob. Agents and Chemotherapy 23: 356-359 (1982).

3. Roseman, S. et al, "Biosynthesis of Hyaluronic Acid By Group A Streptococcus", Biol. Chem. 203, 212-225 (1953).