Patent Publication Number: US-2019175644-A1

Title: Methods for preventing or treating infectious diseases caused by extracellular microorganisms, including antimicrobial-resistant strains thereof, using gallium compounds

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation of co-pending U.S. patent application Ser. No. 15/782,860, filed Oct. 13, 2017, which is a Continuation of U.S. patent application Ser. No. 15/259,100, filed Sep. 8, 2016, which is a Continuation of U.S. patent application Ser. No. 14/829,675, filed Aug. 19, 2015, which is a Continuation of U.S. patent application Ser. No. 14/530,766, filed Nov. 2, 2014, which was a Continuation of U.S. patent application Ser. No. 12/058,484, filed Mar. 28, 2008, now issued as U.S. Pat. No. 8,895,077, which claimed the benefit of U.S. Provisional Application No. 60/909,658, filed Apr. 2, 2007, the foregoing applications are incorporated herein by reference in their entirely. 
    
    
     1. INTRODUCTION 
     Field 
     The present invention relates to methods of preventing or treating infectious diseases caused by various extracellular microorganisms, including bacteria and fungi, using gallium compounds. In particular, the invention relates to the pharmaceutical use of gallium compounds in preventing or treating infectious diseases by systemic administration thereof, such as oral administration, intravenous administration, intramuscular administration, subcutaneous administration, and the like. Infectious diseases that are preventable or treatable by the present invention include those caused by microorganisms that are known to be resistant to conventional antibiotics and/or drugs. 
     2. BACKGROUND 
     The emergence of an increasing number of deadly pathogenic microorganisms that are resistant to conventional antibiotics and other antimicrobial agents has become a great concern to public health worldwide. The over-prescription/over-use of antibiotics in humans and farm animals has contributed to a rapid development of antibiotics-resistant strains of various microorganisms. For example,  Staphylococcus aureus , known to be a common cause of hospital-acquired (“nosocomial”) infections that can spread to the heart, bones, lungs, and bloodstream with fatal consequences if not treated, was well controlled by penicillin in the early 1940s. However, by the late 1960s, more than 80 percent of  Staphylococcus aureus  had developed resistance against penicillin and, by 1972, 2 percent of  Staphylococcus aureus  were found to be methicillin-resistant. The percentage of methicillin-resistant bacteria continued to rise to 57.1 percent by 2002 (“Bad Bugs, No Drugs” by Infectious Diseases Society of America (“IDSA”), July 2004, based on Centers for Disease Control (“CDC”) National Nosocomial Infections Surveillance System, August 2003). 
     Similarly, the percentage of enterococci, an important cause of endocarditis, as well as other nosocomial infections, including urinary tract and wound infections and bacteremia, that are resistant to vancomycin (VRE) has increased since the late 1980s and, in 2002, more than 27 percent of the tested enterococci samples from intensive care units were resistant to vancomycin (by IDSA, 2004, supra). Other bacteria known to have developed antibiotics resistance include methicillin resistant, coagulase-negative staphylococci (“CNS”), ceftazidime resistant  Pseudomonas aeruginosa , amipicillin resistant  Escherichia coli , ceftazidime resistant  Klebsiella pneumoniae , penicillin resistant  Streptococcus pneumoniae , and the like. In addition, drug-resistance is no longer limited to hospital-acquired infections, but has spread to community-acquired infections, as evidenced by, for example, a total of 12,000 cases of community-acquired methicillin-resistant  Staphylococcus aureus  (MRSA) infections found in correctional facilities in Georgia, California, and Texas between 2001 and 2003 (2004, IDSA, supra). 
     Antibiotic-resistant microorganisms cause an enormous economic burden to society. Infectious diseases caused by drug-resistant microorganisms require longer hospitalizations, higher costs for alternative medications, more lost work days and so forth, and often result in death. According to the report by the Institute of Medicine (“TOM”) (1998 , Antimicrobial Resistance: Issues and Options ), infections caused by MRSA cost an average of $31,400 per case to treat. The total cost to U.S. society of drug-resistant microorganisms is said to be at least $4 billion to $5 billion annually. 
     Despite the urgent need for new drugs to control antimicrobial resistance, development of new antibiotics has slowed considerably in recent years as the focus of product development in the pharmaceutical fields has increasingly shifted toward chronic diseases, rather than to acute illness, such as acute bacterial infections, mainly due to higher profitability associated with the treatment of the former (March 2004, by U.S. Food and Drug Administration (“FDA”),  Innovation/Stagnation: Challenge and Opportunity on the Critical Path to New Medical Products ; and December 2003, by Sellers, L. J., “Big Pharma bails on anti-infectives research”,  Pharmaceutical Executive  22). According to 10M and FDA, only two new classes of antibiotics have been developed in the past 30 years: oxazolidinones in 2000 and lipopeptides in 2003, and resistance to oxazolidinones have already been reported. 
     Gallium is a group Ma semi-metallic element that has been used for many years for diagnosing neoplasms and inflammation in the field of nuclear medicine. Gallium has also shown some efficacy in the treatment of cancers (Adamson et al., 1975 , Cancer Chemothe. Rept  59:599-610; Foster et al., 1986 , Cancer Treat Rep  70:1311-1319; Chitambar et al., 1997 , Am J Clin Oncol  20:173-178), symptomatic cancer-related hypercalcemia (Warrell et al., 1989, in “Gallium in the treatment of hypercalcemia and bone metastasis”,  Important Advances in Oncology , pp. 205-220, J. B. Lippincott, Philadelphia; Bockman et al., 1994 , Semin Arthritis Rheum  23:268-269), bone resorption (Warrell et al., 1984 , J Clin Invest  73:1487-1490; Warrell et al., 1989, supra), autoimmune diseases and allograft rejection (Matkovic et al., 1991 , Curr Ther Res  50:255-267; Whitacre et al., 1992 , J Newuro immunol  39:175-182; Orosz C. G. et al., 1996 , Transplantation  61:783-791; Lobanoff M. C. et al., 1997 , Exp Eye Res  65:797-801), stimulating wound healing and tissue repair (Bockman et al., U.S. Pat. No. 5,556,645; Bockman et al., U.S. Pat. No. 6,287,606) and certain infections, such as syphilis (Levaditi C. et al., 1931 , C R Hebd Seances Acad Sci Ser D Sci Nat  192:1142-1143), intracellular bacterial, fungal or parasitic infections, such as tuberculosis, histoplasmosis, and leishmaniasis, respectively (Olakanmi et al., 1997 , J. Invest. Med.  45:234 A; Schlesinger et al., U.S. Pat. No. 6,203,822; Bernstein, et al., International Patent Application Publication No. WO 03/053347),  Pseudomonas aeruginosa  infection (Schlesinger et al., U.S. Pat. No. 6,203,822), and trypanosomiasis (Levaditi C. et al. supra). 
     Although the exact mechanism of gallium&#39;s activity against bone resorption and hypercalcemia is not well known, its antiproliferative properties against cancer cells and antimicrobial activities are said to be likely due to its competition with ferric iron (i.e., Fe 3+ ) for uptake by cancer cells or microorganisms (Bernstein, 1998 , Pharmacol Reviews  50(4):665-682). Iron is an essential element for most living organisms, including many pathogens, and is required for DNA synthesis and various oxidation-reduction reactions (Byers et al., 1998 , Metal Ions Bio syst  35:37-66; Guerinot et al., 1994 , Annu Rev Microbiol  48:743-772; Howard, 1999 , Clin Micobiol Reviews  12(3):394-404). Ga 3+  is known to have solution- and coordination-chemistries similar to those of Fe 3+  (Shannon, 1976 , Acta Crystallographica  A32:751-767; Huheey et al., 1993, In  Inorganic Chemistry: Principles of Structure and Reactivity I, ed.  4, Harper Collins, NY; Hancock et al., 1980, In Org Chem 19:2709-2714) and behaves very similarly to Fe 3+  in vivo by binding to the iron-transport protein transferrin (Clausen et al., 1974 , Cancer Res  34:1931-1937; Vallabhajosula et al., 1980 , J Nucl Med  21:650-656). It is speculated that gallium enters microorganisms via their iron transport mechanisms and interferes with their DNA and protein synthesis. 
     U.S. Pat. No. 5,997,912 discloses a method for inhibiting growth of  Pseudomonas aeruginosa  by administering gallium compounds intravenously, orally or by aerosol and U.S. Patent Application Publication No. 2006/0018945 discloses a method of preventing or inhibiting biofilm growth formation using gallium compounds. 
     U.S. Pat. No. 6,203,822 and International Patent Application No. WO 03/053347 disclose methods for treating patients infected with intracellular bacteria, in particular, species of the genus  Mycobacterium , by intravenously or orally administering gallium compounds to patients infected by this class of bacteria (also see Olakanmi et al., 2000 , Infection and Immunity  68(10):5619-5627). These organisms primarily infect macrophages, which are known to store large amounts of iron and overexpress transferrin receptors. Parenterally or orally administered gallium compounds are readily taken up by macrophages through transferrin receptors and then, within these cells, are taken up by the infecting organisms, thereby interfering with the organisms&#39; metabolism. 
     The antimicrobial activities of gallium against microorganisms other than intracellular organisms have thus far not been explored to a great extent. 
     Furthermore, the use of gallium compounds against the ever increasing number of multi-antibiotic resistant microorganisms has not been explored. 
     3. SUMMARY 
     This invention is based upon the inventors&#39; finding that gallium compounds are effective in controlling the growth of a variety of pathogenic, extracellular microorganisms, including those which are known to be resistant to conventional antibiotics and/or drugs. 
     Accordingly, the present invention provides methods for preventing and/or treating infectious diseases caused by extracellular microorganisms, said method comprising systemically administering to a subject in need thereof a prophylactically or therapeutically effective amount of a gallium compound. In a preferred embodiment, such extracellular microorganisms exclude  Pseudomonas aeruginosa  and  Legionella  spp., but include other iron-dependent, extracellular, pathogenic microorganisms. Such microorganisms may be bacteria or fungi, which infect host organisms, including mammals and birds, and most notably humans. Those microorganisms include, but are not limited to, bacteria within the genera,  Staphylococcus, Enterococcus, Escherichia, Streptococcus, Campylobacter, Salmonella, Helicobacter, Bacillus, Clostridium, Corynebacterium, Chlamydia, Coxilla, Ehrlichia, Francisella, Legionella, Pasteurella, Brucella, Proteus, Klebsiella, Enterobacter, Tropheryma, Acinetobacter, Aeromonas, Alcaligenes, Capnocytophaga, Erysipelothrix, Listeria, Yersinia , and the like; and fungi, such as  Candida albicans, Microsporum canis, Sporothrix schenckii, Trichophyton rubrum, Trichophyton mentagrophytes, Malassezia furfur, Pityriasis versicolor, Exophiala werneckii, Trichosporon beigelii, Coccidioides immitis, Blastomyces dermatitidis, Aspergillus fumigatus, Epidermophyton  spp.,  Fusarium  spp.,  Zygomyces  spp.,  Rhizopus  spp.,  Mucor  spp., and so forth. 
     In another preferred embodiment, the microorganisms targeted by the present invention are resistant to at least one antibiotic or antimicrobial compound other than gallium compounds. In a specific embodiment, such drug-resistant microorganisms include, but are not limited to, methicillin-resistant  Staphylococcus aureus  (MRSA), vancomycin-resistant enterococci (VRE), ampicillin-resistant  E. coli  (e.g.,  E. coli  O157:H7), fluoroquinolone-resistant  Salmonella typhi , ceftazidime-resistant  Klebsiella pneumoniae , and fluoroquinolone-resistant  Neisseria gonorrhoeae , and the like. 
     In another aspect, the present invention provides a method for preventing and/or treating infectious diseases caused by extracellular microorganisms, other than  Pseudomonas aeruginosa  and  Legionella  spp., said method comprising systemically co-administering to a subject in need thereof a prophylactically or therapeutically effective amount of a gallium compound and at least one additional antimicrobial agent. Such additional antimicrobial agents include, but are not limited to, antibacterial agents, such as conventional antibiotics, antifungal agents, and other naturally or synthetically derived agents with antimicrobial activities. 
     3.1 Definitions 
     The term “subject” as used herein refers to an animal, including a fowl (e.g., chickens, turkeys, and the like), and a mammal such as a non-primate (e.g., cows, pigs, horses, cats, dogs, rats, etc.) and a primate (e.g., monkey and human), most preferably a human. 
     The term “systemic” or “systemically” as used herein refers to an administration of gallium compounds to a subject in a manner whereby the compound is distributed throughout the entire body of the subject mainly through the circulatory system, such as the cardiovascular system including the heart and blood vessels and the lymphatic system including lymph nodes, lymph vessels and ducts. Thus, systemic administrations of the gallium compound include oral administration and parenteral administration, including intravenous, intramuscular, subcutaneous, and intraperitoneal administrations, as well as suppositories, and the like. In certain situations, systemic administration can also provide an advantage of the direct contact of the compound with causative organisms, without going through the circulatory systems. For example, the gallium compound that is orally administered would exert its effect not only via systemic distribution but also via direct contact with the microorganisms that infect the digestive tracts. 
     The term “prophylactically effective amount” as used herein refers to that amount of the gallium compounds sufficient to prevent a disease or disorder associated with pathogenic microorganisms. A prophylactically effective amount can refer to the amount of the gallium compounds sufficient to prevent or suppress the growth of pathogenic microorganisms or kill pathogenic microorganisms in a subject. 
     The term “therapeutically effective amount” as used herein refers to that amount of the gallium compounds sufficient to treat, manage or ameliorate a disease or disorder caused by pathogenic organisms in a subject. A therapeutically effective amount can refer to the amount of the gallium compounds sufficient to reduce the number of pathogenic microorganisms, to suppress the growth of pathogenic microorganisms (i.e., stasis), or to kill pathogenic microorganisms at the affected sites or in the bloodstream of a subject. Further, a therapeutically effective amount of the gallium compounds means that the amount of the gallium compounds alone, or in combination with other therapies and/or other drugs, that provides a therapeutic benefit in the treatment, management, or amelioration of a disease or disorder. 
    
    
     4. DETAILED DESCRIPTION 
     4.1 Iron Transport and Gallium 
     Most microorganisms, with a few exceptions (e.g.,  Lactobacillus  spp.—see Archibald, 1983 , FEMS Microbiol Lett  19:29-32; Weinberg, 1997 , Perspectives in Biology and Medicine  40(4):578-583; and  Borrelia burgdorferi —see Posey et al., 2000 , Science  288:1651-1653), require iron for their survival (Weinberg, 1978 , Microbiol Rev  42:45-66; Neilands, 1972 , Struct Bond  11:145-170). Despite the fact that iron is one of the most abundant metals, its availability to microorganisms is limited due to its existence as insoluble compounds (oxides-hydroxides) in aerobic environments (Guerinot, 1994, supra; Spiro, et al., 1966 , J Am Chem Soc  88:2721-2725; Vander Helm et al., 1994, In  Metal ions in fungi vol.  11, pp. 39-98, Marcel Dekker, Inc. New York, N.Y.). Accordingly, microorganisms, such as bacteria and fungi, have developed various mechanisms for acquiring iron in the face of its limited availability in the environment (Howard, 1999, supra). 
     One such mechanism is the synthesis of potent iron-chelating compounds called siderophores. Microorganisms produce siderophores, which bind Fe 3+  in the environment and are transported into the cells of the microorganisms via specific transport systems, where Fe.sup 3+  is released as Fe 2+  and then stored. Known siderophores include hydroxamates, such as rhodotorulic acid, coprogens, ferrichromes, and fusarinines; polycarboxylates; phenolates-catecholates and desferioxamine (Howard, 1999, supra). Other mechanisms include direct internalization of iron complexed with siderophores or host iron transporters (e.g., transferrin and lactoferrin), membrane-associated reductase mechanisms, and receptor-mediated mechanisms as well as membrane-mediated direct-transfer mechanisms (Howard, 1999, supra; Crosa, 1997 , Microbiol Mol Biol Rev  61:319-336; Payne, 1994 , Methods Enzymol  235:329-344). The availability of iron through these mechanisms is closely linked to the virulence of microorganisms (Litwin et al., 1993 , Clin Microbiol Reviews  6(2):137-149), and each organism may have multiple alternative mechanisms for obtaining iron from iron-scarce environments to support its growth and survival (for example, see Spatafora et al., 2001 , Microbiology  147:1599-1610). 
     It has been reported that gallium ion (Ga 3+ ) and ferric ion (Fe 3+ ) have strong biochemical similarities, in particular, with regard to their binding to proteins and chelators. These similarities are mainly attributed to their comparable ionic radii and the degrees of ionic (electrostatic) versus covalent contributions to bonding (for review, see Bernstein, 1998, supra). Because of these similarities, Ga 3+  can mimic Fe 3+  in various biological processes. For example, Ga 3+  binds to transferrin (see, for example, Clausen et al., 1974 , Cancer Res  34:1931-1937; Vallabhajosula et al., 1980 , J Nucl Med  21:650-656) and is transported into the cell via transferrin-mediated endocytosis (Chitambar, 1987 , Cancer Res  47:3929-3934). 
     Without intending to be bound by theory, it is believed that Ga 3+  can competitively bind to siderophores and be easily taken up by microorganisms, where it can disrupt DNA and protein syntheses or bind to bacterial proteins and impair the growth of the microorganisms, thereby eventually leading to the stasis or death of the organisms. Alternatively, it is possible that Ga 3+  may occupy membrane-reductases of the microorganisms and prevent Fe 3+  from binding to the reductases to be reduced to Fe 2+ , which would be more bioavailable than Fe.sup 3+ . Since the uptake of gallium does not immediately kill the microorganisms but rather leads to an initial stasis (i.e., a state where the growth or multiplication of microorganisms is inhibited), it has a reduced risk for generating resistant microorganisms. Furthermore, because iron is an essential element for pathogenic microorganisms for their survival and the biochemical similarities between iron and gallium are so close, it is additionally less likely for the microorganisms to be able to develop mechanisms that can discriminate iron from gallium and become resistant to gallium. Gallium may also prevent a microorganism from producing toxins by interfering with its toxin enzyme production. 
     The present invention takes advantage of these characteristics of gallium compounds and provides methods for preventing or treating infectious diseases caused by such pathogens, including those that are resistant to at least one antimicrobial agent other than gallium. 
     4.2 Gallium Compounds 
     Gallium compounds suitable for use in the present invention include any gallium-containing compounds that are pharmaceutically acceptable and safe for animal use, such as avian and mammalian use, in particular, for human use. Gallium compounds have been used diagnostically and therapeutically in humans and are known to be safe for human use (see Foster et al., 1986, supra; Todd et al., 1991 , Drugs  42:261-273; Johnkoff et al., 1993 , Br J Cancer  67:693-700). 
     Pharmaceutically acceptable gallium compounds suitable for use in the present invention include, but not by way of limitation, gallium nitrate, gallium maltolate, gallium citrate, gallium phosphate, gallium chloride, gallium fluoride, gallium carbonate, gallium formate, gallium acetate, gallium sulfate, gallium tartrate, gallium oxalate, gallium oxide, and any other gallium compounds which can safely provide effective levels of element gallium in various applications. Furthermore, gallium complexes, such as gallium pyrones, gallium pyridones, and gallium oximes, as well as gallium bound to proteins, such as transferrin and lactoferrin, or gallium bound to siderophores, such as hydroxamates, polycarboxylates, and phenolates-catecholates, desferioxamine and other iron-chelators, such as cysteine, .alpha.-keto acids, hydroxy acids and pyridoxal isonicotinyl hydrazone class (Richardson et al., 1997 , Antimicrobial Agents and Chemotherapy  41(9):2061-2063) and the like are also suitable for use in the present invention. 
     4.3 Pharmaceutical Use of Gallium Compounds 
     The present invention is directed to methods for preventing or treating infectious diseases by systemically administering to a subject in need thereof a prophylactically or therapeutically effective amount of gallium compounds. 
     Examples of infectious diseases treatable by the present invention are those as to which the subject to be treated can benefit from a systemic administration of gallium compounds and include, but are not limited to, those caused by extracellular bacteria of the species of  Staphylococcus , such as  Staphylococcus aureus, Staphylococcus epidermidis , and the like; of  Enterococcus , such as  Enterococcus faecalis, Enterococcus faecium , and the like; of  Salmonella , such as  Salmonella typhi, Salmonella typhimurium, Salmonella enterica , and the like; of  Escherichia , such as  Escherichia coli , and the like; of  Streptococcus , such as  Streptococcus pneumoniae, Streptococcus pyogenes, Streptococcus agalactiae , and the like; of  Helicobacter , such as  Helicobacter pylori , and the like; of  Campylobacter , such as  Campylobacter jejuni , and the like; as well as the species of genera,  Yersinia, Chlamydia, Coxilla, Ehrlichia, Francisella, Legionella, Pasteurella, Brucella, Proteus, Klebsiella, Enterobacter, Tropheryma, Acinetobacter, Aeromonas, Alcaligenes, Capnocytophaga, Bacillus, Clostridium, Corynebacterium, Erysipelothrix, Listeria  and the like. Examples of infectious diseases treatable by the present invention also include infections caused by fungi, such as  Candida albicans, Microsporum canis, Sporothrix schenckii, Trichophyton rubrum, Trichophyton mentagrophytes, Malassezia furfur, Pityriasis versicolor, Exophiala werneckii, Trichosporon beigelii, Coccidioides immitis, Blastomyces dermatitidis, Aspergillus fumigatus, Epidermophyton  spp.,  Fusarium  spp.,  Zygomyces  spp.,  Rhizopus  spp.  Mucor  spp., and so forth. 
     Gallium compounds can be administered by any methods that result in systemic distribution or delivery of the gallium compounds and include oral administration and parenteral administration, such as intravenous administration, intramuscular administration, subcutaneous administration, intraperitoneal administration, and the like. In certain infections, oral administration of gallium compounds provides not only systemic distribution/delivery of the gallium compounds to the affected area but also a direct contact of the compounds with the causative microorganisms in the affected area, such as within the digestive tracts. Thus, oral administration of the gallium compounds is especially useful in preventing or treating digestive tract infections caused by various microorganisms, including, but not limited to,  Staphylococcus aureus, Enterococcus faecalis, Enterococcus faecium, Salmonella typhi, Salmonella typhimurium, Salmonella enterica, Escherichia coli, Campylobacter jejuni, Clostridium difficile, Clostridium perfringens , and the like.  Helicobacter pylori  that causes gastric and duodenal ulcers, gastritis, duodenitis, and gastric cancer, is also a good target for the methods of the present invention. 
     Furthermore, the methods of the present invention can be applied to preventing or treating infectious diseases caused by microorganisms that are resistant to at least one antimicrobial agent other than gallium compounds. The term “antimicrobial agent” used herein refers to any naturally or synthetically derived agent that kills microorganisms or inhibits the growth thereof, directly or indirectly, and includes conventional antibiotics as well as synthetic chemotherapeutic agents, such as sulfonamides, isoniazid, ethambutol, AZT, synthetic peptide antibiotics, and the like. Thus, in a specific embodiment, the infectious diseases preventable or treatable by the present invention are caused by antimicrobial-resistant strains of microorganisms mentioned above, in particular, of  Staphylococcus aureus, Enterococcus faecium, Enterococcus faecalis, E. coli, Salmonella typhi, Campylobacter jejuni, Klebsiella pneumoniae, Neisseria gonorrhoeae, Candida albicans , and the like. More specifically, such antimicrobial-resistant organisms include methicillin-resistant  Staphylococcus aureus  (MRSA), vancomycin-resistant enterococci (VRE), ampicillin-resistant  E. coli  (e.g.,  E. coli  O157:H7), fluoroquinolone-resistant  Salmonella thyphi , ceftazidime-resistant  Klebsiella pneumoniae , fluoroquinolone-resistant  Neisseria gonorrhoeae , and the like. The methods of the present invention can be applied to any other pathogenic microorganisms which have become resistant to antimicrobial agents other than gallium, as far as they are dependent on iron for their growth and survival. 
     Gallium compounds to be used in the present invention can be formulated in conventional manner using one or more pharmaceutically acceptable carriers or excipients. As used herein the phrase “pharmaceutically acceptable carriers or excipients” is intended to include any and all solvents, dispersion media, coatings, isotonic and absorption delaying agents, and the like, which are compatible with pharmaceutical administration. The use of various pharmaceutically acceptable carriers or excipients for pharmaceutically active substances is well known in the art. With regard to gallium compounds, an injectable formula of gallium nitrate (Ganite™) is commercially available from Genta Inc (Berkeley Heights, N.J.). Ganite™ is an aqueous solution of Ga(NO 3 ).9H 2 O and sodium citrate dehydrate. An oral formula of gallium maltolate developed by Titan Pharmaceuticals, Inc. (San Francisco, Calif.) is currently in Phase II clinical testing in patients with metastatic prostate cancer and refractory multiple myeloma. 
     The therapeutically effective amount (i.e., dosage) of a gallium compound can vary based on the nature and severity of the infection to be treated, the types of etiologic microorganism, the location of the affected area, the method of administration, the age and immunological background of a subject, the types of gallium compound used, as well as other factors apparent to those skilled in the art. Typically, a therapeutically effective amount of a gallium compound can be that amount which gives a gallium concentration at the affected area of the body or in blood plasma, of at least about 1 μM, at least about 50 μM, at least about 100 μM, at least about 500 μM, at least about 1 mM, at least about 10 mM, at least about 50 mM, at least about 100 mM, at least about 200 mM, up to about 500 mM. Due to gallium&#39;s low toxicity, the amount may be liberally increased to more than 500 mM but less than that amount which causes any toxicity. For reference, it has been reported that healthy adults can tolerate at least about 200 mg/m.sup.2/day gallium nitrate intravenous infusion for at least 7 days (see U.S. Pat. No. 6,203,822, supra). Also, an oral administration of 100 mg to 1400 mg per 24 hours as a single agent did not cause major toxicity in ovarian cancer patients and lung cancer patients (see Collery et al., 2002, “Gallium in cancer treatment”,  Oncology/Hematology  42:283-296). Thus, for the methods of the present invention, what is contemplated is administration of the gallium compounds at dosages of, at least about 10 mg/m 2 /day, at least about 50 mg/m 2 /day, at least about 100 mg/m 2 /day, at least about 200 mg/m 2 /day, at least about 300 mg/m 2 /day, at least about 500 mg/m 2 /day, at least about 600 mg/m 2 /day, at least about 700 mg/m 2 /day, or at least about 800 mg/m 2 /day, but less than that dosage which causes any toxicity. 
     The prophylactically effective amount of a gallium compound may be that amount sufficient to prevent a disease or disorder associated with pathogenic microorganisms and may vary based on the location of the affected area, the types and the number of the pathogenic organisms in the area, the types of gallium compound to be used, as well as on the methods of application and other factors apparent to those skilled in the art. Typically, the prophylactically effective amount of a gallium compound may be that amount which gives a gallium concentration at the affected area of the body or in blood plasma, of at least about 0.1 μM, at least about 50 μM, at least about 100 μM, at least about 500 μM, at least about 1 mM, at least about 10 mM, at least about 50 mM, at least about 100 mM, up to about 200 mM. Again, the amount of a gallium compound for prophylactic purposes may be liberally increased to more than 200 mM but less than the amount that causes any toxicity. 
     In another aspect, the present invention provides a method for preventing and/or treating infectious diseases caused by extracellular microorganisms, said method comprising co-administering to a subject in need thereof prophylactically or therapeutically effective amounts, individually or collectively, of a gallium compound and at least one additional antimicrobial agent. The term “co-administration” or “co-administering” used herein refers to the administration of gallium compound and at least one additional antimicrobial agent either sequentially in any order or simultaneously, by the same administration method or a combination of different administration methods, for example, by an intravenous administration of the gallium compound and an oral administration of the additional antimicrobial agent, or vice versa. Such co-administration of one or more additional antimicrobial agents together with the gallium compound is especially beneficial because the drugs attack the causative organisms by non-overlapping, completely different mechanisms, and/or because the development of antimicrobial resistance in the organisms may involve different mechanisms for the different antimicrobial agents, thereby causing nearly complete eradication of the organisms, by the drugs themselves or in combination with the actions by the host&#39;s own immune system and reducing or eliminating the chance for the causative organisms to develop resistance to the drugs. Furthermore, thanks to the low toxicity of gallium, by increasing the dosage of gallium, a combination therapy can reduce the dosage of an additional antimicrobial agent to an amount less than that required when the latter is used alone, thereby reducing adverse effects of the latter. Moreover, co-administration of a gallium compound and an additional antimicrobial agent may result in a synergistic effect and, thus, require less dosages than those required when each is used alone. 
     Additional antimicrobial agents that can be co-administered with gallium compounds can be antibacterial agents or antifungal agents, depending on the type of the causative organisms. Examples of antibacterial agents include, but not by way of limitation, those in the classes of penicillins, including amipicillin, flucloxacillin, dicloxacillin, methicillin, ticarcillin, piperacillin, carbapenems, mecillinams, and the like; cephems, including cephalosporin and cephamycins; sulfonamides; aminoglycosides, including amikacin, gentamicin, kanamycin, neomycin, netilmicin, paromomycin, streptomycin, tobramycin, apramycin, and the like; chloramphenicol; tetracyclines, including chlortetracycline, oxytetracycline, demeclocycline, doxycycline, lymecycline, meclocycline, methacycline, minocycline, rolitetracycline, and the like; macrolides, including erythromycin, azithromycin, clarithromycin, dirithromycin, roxithromycin, carbomycin A, josamycin, iktasamycin, oleandomycin, spiramycin, troleandomycin, tylosin/tylocine, telithromycin, cethromycin, ansamycin, and the like; lincosamides, including lincomycin, clindamycin, and the like; streptogramins, including mikamycins, pristinamycins, oestreomycins, virginiamycins, and the like; glycopeptides, including acanthomycin, actaplanin, avoparcin, balhimycin, bleomycin B (copper bleomycin), chloroorienticin, chloropolysporin, demethylvancomycin, enduracidin, galacardin, guanidylfungin, hachimycin, demethylvancomycin, N-nonanoyl-teicoplanin, phleomycin, platomycin, ristocetin, staphylocidin, talisomycin, teicoplanin, vancomycin, victomycin, xylocandin, zorbamycin, and the like; rifamycins, including rifampicin, rifabutin, rifapentine, and the like; nitroimidazoles, including metronidazole, nitrothiazoles, and the like; quinolones, including nalidixic acid, cinoxacin, flumequine, oxolinic acid, piromidic acid, pipemidic acid, ciprofloxacin, enoxacin, fleroxacin, lomefloxacin, nadifloxacin, norfloxacin, ofloxacin, pefloxacin rufloxacin, balofloxacin, grepafloxacin, levofloxacin, pazufloxacin mesilate, sparfloxacin, temafloxacin, tosufloxacin, clinafloxacin, gemifloxacin, moxifloxacin, gatifloxacin, sitafloxacin, trovafloxacin, and the like; dihydrofolate reductase inhibitors, including trimethoprim; oxazolidinones, including linezolid, eperezolid, and the like; lipopeptides, including gramicidins, polymyxins, surfactin, and the like; and analogs, salts and derivatives thereof. Examples of antifungal agents include, but are not limited to, polyenes, such as amphotericin, nystatin, pimaricin, and the like; azole drugs, such as fluconazole, itraconazole, ketoco, and the like; allylamine and morpholine drugs, such as naftifine, terbinafine, amorolfine, and the like; antimetabolite antifungal drugs, such as 5-fluorocytosine, and the like; and analogs, salts and derivatives thereof. 
     Which antimicrobial agent should be used in combination with the gallium compounds in any given infection can be determined by various simple and routine methods known to one skilled in the art. For example, an infectious microorganism isolated from a patient can be tested for its sensitivity to various antimicrobial agents using a standardized disk-diffusion method (e.g., Kirby-Bauer disk-diffusion method). Briefly, in this method, an appropriate agar plate is uniformly inoculated with the test organism and paper disks impregnated with predetermined concentrations of different antibiotics are placed on the agar surface. After incubation, the diameter of a circular zone, around the disks, in which the growth of the organism is inhibited is measured. The diameter of the inhibition zone is a function of the amount of the antibiotic in the disk as well as the susceptibility of the organism to the antibiotic. The antibiotics to which the organism shows susceptibility can be used for a combination treatment with the gallium compounds. Other examples of antibiotic susceptibility tests include, but are not limited to, a broth tube dilution method for determining Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) of a given antimicrobial agent against a given organism. These methods are described in Section 6.1, infra. Thus, in a specific embodiment, an infection caused by MRSA can be treated by co-administration of gallium compound and vancomycin or linezolid (e.g., ZyVox™ by Pfizer, NY) to a subject in need thereof. Vancomycin and Zyvox™, respectively, are currently used as the antibiotics of choice to treat MRSA infections. Likewise, in another specific embodiment, an infection caused by VRE can be treated by co-administration of a gallium compound and linezolid. In yet another specific embodiment, an infection or a disease/disorder (e.g., peptic ulcers, gastritis, duodenitis, gastric cancer, and the like) caused by  Helicobacter pylori  can be treated by co-administration of a gallium compound and clarithromycin, amoxicillin and/or metronidazole. Other agents that directly or indirectly inhibit or suppress the growth of  Helicobacter pylori  can be also co-administered with the gallium compound. Such agents include, but are not limited to, proton pump inhibitors, such as omeprazole that is currently used together with clarithromycin and amoxicillin in triple therapy for peptic ulcers; and urease inhibitors, such as fluorofamide, acetohydroxamic acid, certain divalent metal ions, including Zn, Cu, Co, and Mn, and the like; as well as other agents, such as bismuth compounds (e.g., bismuth subsalicylate) that not only protect the stomach lining by coating the latter, but also suppress  H. pylori  growth (S. Wagner et al., 1992, “Bismuth subsalicylate in the treatment of H2 blocker resistant duodenal ulcers: role of  Helicobacter pylori ”, Gut 33:179-183). 
     In another aspect, the present invention provides a kit comprising one or more vials containing a gallium compound and one or more additional antimicrobial agents. 
     5. EXAMPLES 
     The following examples are provided to further illustrate the current invention but are not intended to in any way limit the scope of the current invention. 
     5.1. In Vitro Study: Susceptibility of Microorganisms to Gallium 
     Example 1 
     Susceptibility of various microorganisms to gallium was tested by determining the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) for each microorganism using gallium nitrate. In general, MIC is determined by (i) mixing a series of broths, each containing a standard number of microorganisms, with serially diluted solutions of the gallium compound; and (ii) determining the MIC, after incubation, that is the lowest concentration of the gallium compound that inhibits the growth of the microorganism. The lower the MIC, the more susceptible the organism is. The MBC is determined by subculturing an aliquot of each sample from the MIC test on an appropriate agar plate containing no gallium compound. After incubation, the MBC is determined to be the lowest concentration of the gallium compound at which no growth is observed. 
     Specifically, in the present experiment, two grams of gallium nitrate powder were dissolved in 10 ml of filter-sterilized deionized water and the resulting 20% (w/v) (i.e., 200 mg/ml) solution was once again filter-sterilized. Two-fold serial dilutions were prepared in sterile deionized water down to 0.156% (i.e., 1.56 mg/ml) for the tests for most of the organisms, except for the test for  Candida albicans , in which 10%, 1%, 0.5%, 0.1%, 0.05%, 0.01%, 0.005% and 0.001% of gallium nitrate solutions were prepared. 
     Table 1 shows the list of microorganisms tested for MIC and MBC. All organisms were obtained from the American Type Culture Collection (ATCC), Manassas, Va. Each microorganism was picked from the seed culture (see Table 1) and inoculated in an appropriate type of broth to obtain a 0.5 McFarland turbidity standard. The standard suspension of the microorganism was then diluted to 1:100 with the broth and used for the tests. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 TEST ORGANISM 
                 ATCC # 
                 SEED CULTURE 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 
                   Candida albicans 
                 
                 10231 
                 On Sabouraud dextrose 
               
               
                   
                   
                 agar, at 25-30° C. for 24-48 
               
               
                   
                   
                 hours 
               
               
                 Methicillin-resistant 
                 33592 
                 On tryptic soy agar with 
               
               
                 
                   Staphylococcus aureus 
                 
                   
                   Staphylococcus aureus  5% 
               
               
                 (MRSA) 
                   
                 sheep blood (BAP), at 35- 
               
               
                 Vancomycin-resistant 
                 51575 
                 37° C. for 24-48 hours 
               
               
                 
                   Enterococcus faecalis 
                 
                   
                   
               
               
                 (VRE) 
                   
                   
               
               
                   Escherichia coli  O157:H7 
                 35150 
                   
               
               
                 
                   Salmonella typhi 
                 
                 6539 
                   
               
               
                 
                   Campylobacter jejuni 
                 
                 29428 
                 On  Brucella  agar with 5% 
               
               
                   
                   
                 sheep blood, at 35-37° C. 
               
               
                   
                   
                 for 48 hours under 
               
               
                   
                   
                 microaerophilic conditions 
               
               
                   
               
               
                   a Antibiotics resistance of the organism was confirmed by CLSI (Clinical Laboratory Standards Institute) Oxacillin disk-diffusion test. The zone of inhibition was 6 mm (CLSI Oxacillin resistant range: ≤10 mm). 
               
               
                   b Antibiotics resistance of the organism was confirmed by CLSI Vancomycin disk-diffusion test. The zone of inhibition was 10 mm (CLSI Vancomycin resistant range: ≤14 mm). 
               
            
           
         
       
     
     Each microorganism was tested in duplicate by either a microdilution broth method in 96-well plates (i.e., 0.1 ml of the gallium nitrate solution mixed with 0.1 ml of the microorganism suspension) or a macrodilution broth method in test tubes (i.e., 1 ml of the gallium nitrate solution mixed with 1 ml of the microorganism suspension) as follows: 
     Microdilution broth method:  Candida albicans; Escherichia coli  O157:H7; and  Campylobacter jejuni.    
     Macrodilution broth method: Methicillin-resistant  Staphylococcus aureus  (MRSA); Vancomycin-resistant  Enterococcus faecalis  (VRE); and  Salmonella typhi.    
     The growth of the microorganisms were determined by visual observation of turbidity in the samples. 
     The following controls were incubated together with the test samples: 
     Viability control: A mixture of equal volumes of deionized water and an appropriate broth inoculated with a test microorganism but without gallium nitrate; and 
     Sterility control: A mixture of equal volumes of deionized water and an appropriate broth without either microorganisms or gallium nitrate. 
     Purity of each microorganism was confirmed by streaking an appropriately diluted suspension of the microorganism onto an appropriate agar plate to obtain isolated colonies and observing colony morphology. 
     The concentrations of microorganisms in the suspension used in MIC test were determined by inoculating serial dilutions of the suspensions onto appropriate agar plates and counting the number of colonies. 
     To determine MBC, 10 μl of each sample used in MIC were inoculated onto an appropriate agar plate and incubated. The lowest concentration of the gallium nitrate that showed no growth was determined to be the MBC. 
     The results are shown in Table 2 below. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Final Conc 
                   
                   
               
               
                   
                 Broth 
                 Agar Plate 
                 Incubation 
                 of Organism 
                 MIC 
                 MBC 
               
               
                   
                 (MIC) 
                 (MBC) 
                 Condition 
                 (CFU/ml) 
                 (mg/ml) 
                 (mg/ml) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 
                   Candida 
                 
                 Sabouraud 
                 Sabouraud dextrose 
                 At 27° for 
                 9.75 × 10 5   
                 10 
                 &gt;100 
               
               
                 
                   albicans 
                 
                 Dextrose 
                 agar 
                 48 hours 
               
               
                 Methicillin- 
                 Muller 
                 Tryptic soy agar 
                 At 36° for 
                 5.2 × 10 5   
                  ND* 
                 12.5 
               
               
                 resistant 
                 Hinton 
                 with 5% sheep blood 
                 48 hours 
               
               
                 
                   Staphylococcus 
                 
               
               
                   aureus  (MRSA) 
               
               
                 Vancomycin- 
                 Muller 
                 Tryptic soy agar 
                 At 36° for 
                 3.9 × 10 5   
                 ND 
                 25 
               
               
                 resistant 
                 Hinton 
                 with 5% sheep blood 
                 48 hours 
               
               
                 
                   Enterococcus 
                 
               
               
                   faecalis  (VRE) 
               
               
                 
                   Escherichia 
                 
                 Muller 
                 Tryptic soy agar 
                 At 36° for 
                 1.38 × 10 6   
                 ND 
                 6.25 
               
               
                   coli  O157:H7 
                 Hinton 
                 with 5% sheep blood 
                 48 hours 
               
               
                 
                   Salmonella 
                 
                 Muller 
                 Tryptic soy agar 
                 At 36° for 
                 8.3 × 10 5   
                 ND 
                 6.25 
               
               
                 
                   typhi 
                 
                 Hinton 
                 with 5% sheep blood 
                 48 hours 
               
               
                 
                   Campylobacter 
                 
                 Muller 
                 Tryptic soy agar 
                 At 36° for 
                 4.9 × 10 5   
                 ND 
                 &lt;0.78 
               
               
                 
                   jejuni 
                 
                 Hinton 
                 with 5% sheep blood 
                 48 hours 
               
               
                   
               
               
                 *ND: Not determined due to non-specific turbidity caused by the precipitation of gallium nitrate at some dilutions. 
               
            
           
         
       
     
     5.2. In Vivo Study: Effect of Gallium Nitrate in Animal Models 
     Example 2 
     Methicillin-Resistant  Staphylococcus aureus  (MRSA) 
     Adult BALBc mice are inoculated with 1×10 6  CFU/mouse of  Staphylococcus aureus -MRSA strain (e.g., ATCC 33592) by intraperitoneal injection. Following bacterial injections (approximately 8 hours post-inoculation), each mouse receives a single intravenous injection of one of the following: 0.9% saline (control), 30 mg/kg, 45 mg/kg, or 60 mg/kg of gallium nitrate, 200 mg/kg of vancomycin, or 45 mg/kg of gallium nitrate and 200 mg/kg of vancomycin, all in 0.9% saline. Initially, there are 5 mice in each of the six groups. Following inoculation, the mice are monitored twice daily for morbidity. Body temperature is obtained twice daily and a mouse whose body temperature decreases by 4° C. or greater will be considered moribund and euthanized. Body weights are taken once daily for the duration of the study. On Day 5, all remaining animals are euthanized. Spleen, lymph nodes and kidneys are collected, homogenized in sterile PBS and serially diluted for bacterial quantitation. 
     Example 3 
     Vancomycin-Resistant  Enterococcus faecalis  (VRE) 
     Adult CF1 mice are caged individually and total counts of native enterococci and possible VRE in colony forming unit (CFU) per gram of feces are determined as a baseline for each mouse. On Day 1, each mouse receives 0.5 ml (about 10 9  CFU/ml) of VRE (e.g., ATCC 51575) suspension in Muller-Hinton broth (MHB), or MHB alone (control), via gavage with a stainless steel feeding tube. At specified intervals thereafter (e.g., 1, 7, 14 days and so on), 2 fresh fecal pellets from each mouse are collected, weighed, and emulsified in MHB and the numbers of CFU of VRE, enterococci, and gram-negative enteric bacilli per gram of feces are determined by standard serial dilution and plating techniques. For example, total enterococcal counts can be measured with bile-esculin agar, counts of enteric bacilli with MacConkey agar, and counts of VRE with Muller-Hinton II agar containing vancomycin (50 μg/ml), streptomycin (100 μg/ml), polymyxin (100 μg/ml) and nystatin (2 μg/ml) (see M. S. Whitman et al., 1996, “Gastrointestinal tract colonization with vancomycin-resistant  Enterococcus faecium  in an animal model”,  Antimicrobial Agents and Chemotherapy  40(6):1526-1530). Groups of mice (at least 5 mice/group) are assigned to receive daily either sterile drinking water (control), or drinking water containing 100 μg/ml, 200 μg/ml or 300 μg/ml of gallium citrate, 250 μg/ml of vancomycin, 250 μg/ml of linezolid, or 200 μg/ml of gallium citrate and 250 μg/ml of linezolid, starting 24 hours after the inoculation of the mice up to 10 days. Counts of VRE and total enterococci in feces are determined for each group at specified intervals up until 40 days after the inoculation and compared with the baseline counts. 
     Example 4 
     
       Helicobacter pylori  
     
     C57BL/6 mice are inoculated with the mouse-adapted  Helicobacter pylori  SS1 strain (Lee A, O&#39;Rouke et al., 1997, “A standardized mouse model of  Helicobacter pylori  infection: introducing Sydney strain”,  Gastroenterology  112:1386-97) by intragastric delivery of 0.1 ml of the bacterial suspension (approximately 1-2×10 9  bacteria/ml) in an appropriate medium (e.g., brucella broth). Control mice are given 0.1 ml of the medium without the bacteria. Mice are left for 1-3 weeks for bacterial colonization to become established. Groups of mice (at least 5 mice/group) are assigned to receive daily, via intragastric gavage, either sterile saline (control), or 60 mg/kg, 80 mg/kg or 100 mg/kg of gallium maltolate with or without 15 mg/kg of omeprazole in saline solution for 14 days. Mice are euthanized 24 hours after the completion of the treatment. A longitudinal section of gastric tissue is removed, fixed in formalin solution, embedded in paraffin and cut at 8μ. to produce histologic sections. The sections are prepared with Giemsa stain and examined microscopically for  Helicobacter pylori  colonization of the gastric mucosa. A second longitudinal section of gastric tissue is removed, weighed and homogenized in 1 ml brucella broth. The homogenate is diluted in phosphate-buffered saline and an aliquot is plated, in duplicate, on a selective medium (e.g., blood agar supplemented with 5% defibrinated sheep blood, 100 μg/ml vancomycin, 3.3 μg/ml polymixin B, 200 g/ml bacitracin, 10.7 μg/ml nalidixic acid and 50 μg/ml amphotericin B (see J. I. Keena et al., 2004, “The effect of  Helicobacter pylori  infection and dietary iron deficiency on host iron homeostasis: A study in mice”,  Helicobacter  9(6):643-650). Growth of  Helicobacter pylori  is confirmed based on Gram staining, morphology and urease production. The numbers of colony forming unit (CFU) per gram of tissue are determined and compared among the groups. 
     6. EQUIVALENTS 
     Those skilled in the art to which the present invention is related will recognize, or be able to ascertain, many equivalents to the specific embodiments of the invention described herein using no more than routine experimentation. Such equivalents are intended to be encompassed by the following claims. 
     All publications, patents and published patent applications mentioned in this specification are herein incorporated by reference into the specification. 
     Citation or discussion of a reference herein shall not be construed as an admission that such is prior art to the present invention.