COMBINATION ANTIBIOTIC THERAPIES

Abstract

The invention provides combination antibiotic therapies for treating an A. baumannii infection in a subject; the combination therapies include rifabutin and a second antibiotic, such as colistin and cefiderocol.

Description

COMBINATION ANTIBIOTIC THERAPIES RELATED APPLICATIONS This application claims the benefit and priority of U.S. Provisional Patent Applications Serial Nos. 62 / 902,019, filed September 18, 2019, 62 / 899,257, filed September 12, 2019, 62 / 941,160, filed November 27, 2019, and 62 / 977,659, filed February 17, 2020, the contents of which are incorporated herein in their entirety. FIELD OF INVENTION The invention relates in general to rifabutin combination therapies for treating A. baumann infections. BACKGROUND OF THE INVENTION The emergence of multidrug-resistant (MDR) and extensively drug-resistant (XDR) bacterial strains in recent decades has made bacterial infections an increasingly serious public health problem. One of the bacterial species that poses the greatest threat to health is Acinetobacter baumannii, which can cause pneumonia, meningitis, and infections of the blood, urinary tract, and skin. Because A. baumannii cells can survive on artificial surfaces for extended periods, the bacterium is easily transmitted in a hospital setting, and most A. baumannii infections are acquired through hospitalization. For example, many soldiers in the Middle East have been infected with A. baumannii while receiving treatment for combat injuries, and multidrug-resistant strains of the bacterium represent a major complication in the rehabilitation of wounded soldiers.Treating A. baumannii infections presents a challenge. Through the use of transposable genetic elements, A. baumannii strains have developed resistance to antibiotics from several different classes, including aminoglycosides, aminocyclitols, tetracyclines, chloramphenicol, and carbapenems. Polymyxins, such as colistin, are typically used as a last resort due to their serious side effects, but some A. baumannii strains are also resistant to colistin (Zubair et al., 2015). Consequently, current tools for treating and preventing disease caused by this bacterium are inadequate for many patients. Significant efforts have been made to find a solution for treating this nosocomial pathogen, one of which is combination therapy (Levin et al., 1999; Wood et al., 2003).Combinations of two antibiotics have shown different effects on each other, and in many cases the effect is synergistic or potentiating, but in some cases, antagonism is observed (Montero et al., 2004; Tripodi et al., 2007). Rifampicin (an antibiotic belonging to the rifamycin class, like rifabutin) targets the B subunit of bacterial DNA-dependent RNA polymerase (rpoB) and is an antibiotic frequently used in combination with other antibiotics. Rifampicin (also known as rifampin) has shown synergy with colistin against A. baumannii; however, the outcome of this combination depends on the MICs of rifampin (Giannouli et al., 2012). In particular, no synergistic effect of rifampicin and colistin was observed in A. baumann isolates in which the high MICs of rifampicin were due to mutations in the rpoB target gene. BRIEF DESCRIPTION OF THE INVENTION The invention provides combination therapies comprising rifabutin and a second antibiotic, such as a polymyxin (e.g., colistin, polymyxin B, polymyxin B nonapeptide; polymyxin analogues as exemplified by MRX-8), other cationic antimicrobial peptides (e.g., SPR741; chimeric peptidomimetic antibiotics exemplified by POL7306; octapeptin cyclic peptides), or cefiderocol, for treating A. baumannii infections. The invention is based on the finding that rifabutin acts synergistically with some other antibiotics to inhibit the growth of A. baumannii cells. The antibiotic combinations exhibit synergy with a broad range of A. baumannii strains. This synergy greatly increases the susceptibility of A. baumannii cells.baumannü to rifabutin and colistin, with some strains showing a more than 500-fold increase in sensitivity to one of these antibiotics when used in combination with the other compared to when that antibiotic is used alone. Furthermore, and unexpectedly, rifabutin and colistin combinations act synergistically to inhibit the growth of strains that are resistant to both antibiotics when administered individually, thus making these strains susceptible to combination therapy even when the elevated MICs to rifabutin and rifampin are due to mutations in the rpoB gene. This observation contrasts sharply with that observed with the rifampin and colistin combination, where strains with mutations remain resistant to the combination.In this way, the invention reveals the therapeutic potential of antibiotics in environments where they are otherwise powerless and provides effective therapies for the treatment of serious A. baumann infections. In one aspect, the invention provides methods for treating an A. baumannii infection in a subject by administering rifabutin and a second antibiotic to the infected subject. The second antibiotic can be a polymyxin (e.g., colistin, polymyxin B, polymyxin B nonapeptide) or cefiderocol. The individual may be infected with a strain of A. baumannii that is resistant to one or more antibiotics. The strain may be resistant to one or more of the following: aminocyclitol, aminoglycoside, beta-lactam, beta-lactamase inhibitor, carbapenem, cephalosporin, polymyxin, quinolone, rifamycin, sulfonamide, minocycline, eravacycline, sulbactam, and tetracycline. The strain may also be resistant to one or more of the following: amikacin, trimethoprim-sulfamethoxazole, cefepime, cefiderocol, ceftazidime, chloramphenicol, ciprofloxacin, colistin, polymyxin B, doripenem, gentamicin, imipenem, levofloxacin, meropenem, penicillin, piperacillin, rifabutin, rifampicin, tazobactam, and tigecycline. Each antibiotic can be administered via a different route. Two or more antibiotics can be administered via the same route. Each antibiotic can be administered independently via intravenous, oral, parenteral, subcutaneous, inhalation, injection, and / or infusion routes. Each antibiotic can be administered in a separate formulation. Two or more antibiotics can be administered in a single formulation. The antibiotics can be administered according to the same dosing regimen, or two or more antibiotics can be administered according to different dosing regimens. The dosing regimen may include one or more doses, a dosing frequency, or an interval between doses. The subject may be a human being. The subject may be a pediatric subject, a newborn, a neonate, an infant, a child, an adolescent, a preadolescent, an adolescent between 13 and 19 years of age, an adult, or an elderly person. The subject may be in critical care, intensive care, neonatal intensive care, pediatric intensive care, coronary care, cardiothoracic care, surgical intensive care, medical intensive care, long-term intensive care, an operating room, an ambulance, a field hospital, or an out-of-hospital field setting. The method may include administering one or more additional antibiotics besides the first two antibiotics, e.g., rifabutin and either colistin or cefiderocol. The additional antibiotics may be an aminocyclitol, an aminoglycoside, a beta-lactam, a beta-lactamase inhibitor, a carbapenem, a cephalosporin, a polymyxin, a quinolone, a rifamycin, a sulfonamide, a minocycline, an eravacycline, a sulbactam, or a tetracycline. The additional antibiotics may include amikacin, trimethoprim-sulfamethoxazole, cefepime, cefiderocol, ceftazidime, chloramphenicol, ciprofloxacin, colistin, doripenem, gentamicin, imipenem, levofloxacin, meropenem, or penicillin. ΜΛ / lOOu piperacillin, polymyxin B, rifabutin, rifampicin, tazobactam and tigecycline. In another aspect, the invention provides combination therapies that include rifabutin and a second antibiotic in a therapeutically effective amount to treat an A. baumannii infection in a subject. The second antibiotic may be a polymyxin (e.g., colistin, polymyxin B, polymyxin B nonapeptide) or cefiderocol. The subject may be infected with a strain of A. baumannii that is resistant to one or more antibiotics, such as any of those described above. Each antibiotic can be administered via a different route. Two or more antibiotics can be administered via the same route. Each antibiotic can be administered independently via intravenous, oral, parenteral, subcutaneous, inhalation, injection, and / or infusion routes. Each antibiotic can be administered in a separate formulation. Two or more antibiotics can be administered in a single formulation. The antibiotics can be administered according to the same dosing regimen, or two or more antibiotics can be administered according to different dosing regimens. The dosing regimen may include one or more doses, a dosing frequency, or an interval between doses. The subject can be a human or a class of humans, such as any of those described above. Combination therapy may include administering, in a therapeutically effective amount, one or more additional antibiotics, such as any of those described above. In another aspect, the invention provides uses of combinations including rifabutin and a second antibiotic to manufacture one or more medications for treating a baumannii infection in a subject. The second antibiotic may be colistin or cefiderocol. In the modalities of use, the subject may be infected with a strain of A. baumannii that is resistant to one or more antibiotics, such as any of those described above. In the usage modalities, each antibiotic is administered via a different route. In the usage modalities, two or more antibiotics are administered via the same route. In the usage modalities, each antibiotic is administered independently via intravenous, oral, parenteral, subcutaneous, inhalation, injection, and / or infusion routes. In some usage modes, each antibiotic is administered in a separate formulation. In some usage modes, two or more antibiotics are administered in a single formulation. In some usage modes, the antibiotics are administered according to the same dosing schedule. In some usage modes, two or more antibiotics are administered according to different dosing schedules. The dosing schedule may include one or more doses, a dosing frequency, or an interval between doses. In the modalities of use, the subject is a human or a member of a class of humans, such as any of those described above. In terms of usage, the combination includes one or more additional antibiotics, such as any of those described above. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an image of a checkerboard plate of 96 wells of A. baumann cells cultured in various concentrations of rifabutin and colistin. FIG. 2 is an image of a checkerboard plate of 96 wells of A. baumann cells cultured in various concentrations of rifabutin and cefiderocol. DETAILED DESCRIPTION OF THE INVENTION The invention provides combination therapies for treating an A. baumannii infection in a subject. The combination therapies are based on the finding that rifabutin acts synergistically with antibiotics such as colistin and cefiderocol to inhibit the growth of A. baumannii cells. Therefore, the use of rifabutin in combination with either colistin or cefiderocol is more effective than the use of either of these antibiotics alone in treating A. baumannii infections. Furthermore, and unexpectedly, the combination of rifabutin and colistin is even effective against strains of A. baumannii that are resistant to both antibiotics when given individually, but become susceptible to treatment when given in combination. Combination therapies The combination therapies of the invention include two antibiotics that act synergistically to inhibit the growth of A. baumannii cells. The synergy between the antibiotics, such as rifabutin and colistin, can be determined by any suitable method. One method includes determining the minimum inhibitory concentration (MIC) for each antibiotic individually and in combination and calculating a Fractional Inhibitory Concentration Index (FICI) as follows: MA / lOOu ICIF = { CIMA combination A+Bó I^¡ antibiotic A ) + (' CIΜBcomtination A+B f / M3ntibiotic B The antibiotic pair is characterized as acting synergistically or not based on the ICIF according to the following criteria: synergy (ICIF <0.5); indifferent (ICIF >0.50 and <4); antagonistic (ICIF >4). The determination of antibiotic synergy based on the ICIF is described, for example, in Jenkins, SG & Schuetz, AN Current Concepts in Laboratory Testing to Guide Antimicrobial Therapy. Mayo Clin. Proc. 87, 290-308 (2012), the content of which is incorporated herein by reference. In the combination therapies of the invention, one of the antibiotics is rifabutin. The combination therapies include a second antibiotic that acts synergistically with rifabutin. The second antibiotic may be a polymyxin, such as colistin, or a cephalosporin, such as cefiderocol. Colistin may be supplied as colistimethate sodium or colistin sulfate. The combination therapies may include additional antibiotics, e.g., they may include 3, 4, 5, or more different antibiotics. Each antibiotic may independently be an aminocyclitol, an aminoglycoside, a beta-lactam, a beta-lactamase inhibitor, a carbapenem, a cephalosporin, a polymyxin, a quinolone, a rifamycin, a sulfonamide, a minocycline, an eravacycline, a sulbactam, or a tetracycline.Each antibiotic can be independently amikacin, trimethoprim-sulfamethoxazole, cefepime, cefiderocol, ceftazidime, chloramphenicol, ciprofloxacin, colistin, doripenem, gentamicin, imipenem, levofloxacin, meropenem, penicillin, piperacillin, polymyxin B, rifabutin, rifampicin, tazobactam, or tigecycline. Rifabutin is a dark reddish-purple powder, has a molecular formula of C46H62NO11, a molecular weight of 847.02 and the following structure: ).. . -x. AA .NH / γ Ό,' v- > p J ] A ° 'f A ,NHb-i-Á b....®.. : O / \ Rifabutin has a broad spectrum of antimicrobial activity. It is considerably more active than rifampin against MAC, M. tuberculosis, and M. leprae. It is also active against most atypical mycobacteria, including M. kansasir; however, M. chemonae is relatively resistant. Rifabutin is also active against staphylococci, group A streptococci, N. gonorrhoeae, N. meningitidis, H. inguinae, H. ducreyi, C. jejuni, H. pyiori, C. trachomatis, T. gondii, and A. baumannii. Each antibiotic can be administered by any suitable route. For example, and without limitation, each antibiotic can be administered independently by intravenous, oral, parenteral, subcutaneous, inhalation, injection, and / or infusion routes. One or more antibiotics in combination therapies may be administered with the same dosing regimen. One or more antibiotics may be administered according to different dosing regimens. A dosing regimen may include a dose, an administration schedule, or both. A dose may be described by an absolute amount of drug (e.g., mg) or by a relative amount of drug for the subject (e.g., mg / kg). An administration schedule may be described by the interval between doses. For example, and without limitation, the interval between doses may be approximately 1 hour, approximately 2 hours, approximately 3 hours, approximately 4 hours, approximately 6 hours, approximately 8 hours, approximately 12 hours, approximately 24 hours, approximately 36 hours, approximately 48 hours, approximately 3 days, approximately 4 days, approximately 5 days, approximately 6 days, approximately 7 days, or more. Formulations One or more of the antibiotics may be supplied in a single formulation. One or more antibiotics may be supplied in separate formulations. Each formulation may be prepared for delivery by a particular route of administration, such as intravenous, oral, parenteral, subcutaneous, inhalation, injection, and / or infusion. Antibiotics can be provided as pharmaceutically acceptable salts, such as non-toxic acid addition salts, which are salts of an amino group formed with inorganic acids such as, but not limited to, hydrochloric acid, hydrobromic acid, phosphoric acid, sulfuric acid and perchloric acid or with organic acids such as, but not limited to, acetic acid, maleic acid, tartaric acid, citric acid, succinic acid, methanesulfonic acid, glucuronic acid, melic acid, gluconic acid, lactic acid, aspartic acid or malonic acid. The formulation can be administered by injection, infusion, implantation (intravenous, intramuscular, subcutaneous, or similar), or inhalation in dosage forms, formulations, or via suitable delivery devices or implants containing conventional, non-toxic, pharmaceutically acceptable carriers, solvents, diluents, and adjuvants. The formulation and preparation of such compositions are well known to those skilled in pharmaceutical formulation. Parenteral formulations can be provided in unit dosage forms (e.g., single-dose ampoules and vials), in vials containing multiple doses to which a suitable preservative can be added (see below), in pre-filled syringes, or in pre-filled intravenous bags. The pharmaceutical compositions described herein may be in the form suitable for sterile injection. The formulations may include solutions containing rifabutin. Rifabutin solutions and methods of manufacturing rifabutin solutions are described in U.S. Application No. 62 / 902,019, currently pending and owned by the applicant, the contents of which are incorporated herein by reference. Depending on the patient's needs and clinical conditions, intravenous administration of the composition may be favored over oral administration because it allows for rapid introduction of the antibiotic into the systemic circulation, provides complete bioavailability, allows for better control of the pharmacokinetic parameters that drive pharmacological efficacy, and avoids problems of stability in the gastrointestinal tract and absorption. The typical dose of rifabutin is that capable of achieving plasma or local levels where the Cmax of rifabutin is >2 mg / L but <50 mg / L and the AUC is 10 mg*h / L <200 mg*h / L The formulations can be formulated for parenteral administration, such as by injection or infusion. The injection or infusion can be subcutaneous or intravenous. Treatment of A. baumannii infections The combination therapies of the invention are useful for treating an A. baumannii infection in a subject. The subject may be a human. The subject may be a pediatric subject, a newborn, a neonate, an infant, a child, an adolescent, a preadolescent, an adolescent between 13 and 19 years of age, an adult, or an elderly person. The subject may be in critical care, intensive care, neonatal intensive care, pediatric intensive care, coronary care, cardiothoracic care, surgical intensive care, medical intensive care, long-term intensive care, an operating room, an ambulance, a field hospital, or an out-of-hospital field setting. The individual may have an antibiotic-resistant A. baumannii infection. For example, and without limitation, A. baumannii infection may be resistant to one or more of the following: aminocyclitol, aminoglycoside, beta-lactam, beta-lactamase inhibitor, carbapenem, cephalosporin, polymyxin, quinolone, rifamycin, sulfonamide, tetracycline, amikacin, trimethoprim-sulfamethoxazole, cefepime, cefiderocol, ceftazidime, chloramphenicol, ciprofloxacin, colistin, doripenem, gentamicin, imipenem, levofloxacin, meropenem, penicillin, piperacillin, rifabutin, rifampicin, tazobactam, and tigecycline. A. baumannii infection may be resistant to rifabutin, colistin, or both. A. baumannii infection may be resistant to rifabutin, cefiderocol, or both. Antibiotics in combination therapy can be administered simultaneously or sequentially. Sequential or alternating administration may involve administering each antibiotic exclusively for a period of time. Sequential administration may include an overlap period during which the patient receives both the IV formulation containing rifabutin and the formulation containing the other therapeutic. The exclusivity and overlap periods may be, independently, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 1 day, 2 days, 3 days, 4 days, 5 days, 6 days, 1 week, or 2 weeks. EXAMPLES Summary The objective of this study was to identify standard-of-care (SoC) antibiotics that synergize with rifabutin against Acinetobacter baumannii. Synergy was assessed using the checkerboard minimum inhibitory concentration (MIC) in multiple clinical isolates of A. baumannii. Rifabutin synergy was first identified with colistin and cefiderocol tested on the LAC-4 strain. When tested on a panel of strains, rifabutin / cefiderocol synergy was observed in 100% of the strains, with a >4-fold decrease in the cefiderocol MIC. Rifabutin / colistin synergy was observed in 100% of the strains; this synergy was strong and independent of the initial resistance level to either rifabutin or colistin.Rifabutin activity was superior to that of rifampicin when combined with colistin, as synergy with rifampicin depended on the initial level of rifampicin resistance and the presence or absence of rpoB mutations, as described in the literature. Unexpectedly, the rifabutin / colistin combination was active against strains resistant to rifabutin (including isolates with rpoB mutations) and / or colistin, indicating that the combination overcomes both resistances. In conclusion, rifabutin has the ability to enhance the antibacterial activity of cefiderocol and colistin against A. baumannii strains. Antibacterial agents BV-015-3219-001-02 (rifabutin, (lot no. 17008MR89D)) was manufactured by Olon SpA and 10 g / L stock solutions were prepared in DMSO. Stock solutions of rifampicin (Sigma R3501) and cefiderocol (Synnovator SYNNAAX397783) were prepared at 10 mg / mL in DMSO. Stock solutions of colistin sulfate (Sigma C4461), meropenem (Sigma M2578), cefotaxime (Acras 45495), tobramycin (Sigma T1783), eravacycline (MedChemExpress HY-16980A), minocycline (Sigma M9511), and SPR741 (Spero Therapeutics, FullReg P0271508-1) were prepared at 10 mg / mL in water. Finally, stock solutions of ciprofloxacin (Sigma 17850) were prepared at 10 mg / mL in 0.1 N NaOH. The stock solutions were stored at -20 °C until use. Bacterial strains The clinical isolates of A. baumannii used in this study were obtained from the BioVersys strain collection. The strains were stored at -80 °C as 20% (v / v) glycerol stock cultures. Antimicrobial susceptibility and synergy testing The synergy of rifabutin with SoC antibiotics was assessed using the broth microdilution checkerboard method. Checkerboards were prepared according to CLSI parameters used for MIC5 by microbroth dilution. CA-MHB, iron-depleted CAMHB (ID-CA-MHB), or RPMI supplemented with 10% FCS medium was used, as specified, and ID-CA-MHB was prepared according to CLSI guidelines. Rifabutin was serially diluted along the abscissa, and the combination antibiotic was diluted along the ordinate. This setup allows the combination of rifabutin and another antibiotic at increasing concentrations to provide a final classification of the combination based on a Fractional Inhibitory Concentration (FIC) index as follows: synergy (ICIF <0.5); indifferent (ICIF >0.50 and <4); antagonistic (ICIF >4). The ICIF is calculated as follows: CIMAcombination A+BCIMBcombinationICIF= (—7777-----------)+(---7777----------) ^11^antibiotic A ¿antibiotic B Consequently, synergy is defined when there is a decrease of at least 4 times in the MIC of the tested antibiotics in combination compared to the MIC of the tested antibiotics alone. EXAMPLE 1 Rifabutin acts synergistically with colistin and cefiderocol against the LAC-4 strain of A. baumannii The synergy of rifabutin with SoC antibiotics was demonstrated using a checkerboard assay in the LAC-4 strain of A. baumannii. Checkerboard assays were performed in CA-MHB or ID-CA-MHB for cefiderocol. The results are shown in Table 1. TABLE 1 Second antibiotic ICIF Interpretation Colistin 0.254 synergy Meropenem 1 indifferent Cefotaxime 1.5 indifferent Ciprofloxacin 1.5 indifferent Tobramycin 1.5 indifferent Cefiderocol* 0.5 synergy Eravacycline 1 indifferent Minocycline 1 indifferent *Cefiderocol was tested in ID-CA-MHB Of the 8 SoC antibiotics tested, only colistin and cefiderocol showed synergy with rifabutin, while the remaining 6 were indifferent. Figure 1 is a checkerboard pattern of 96 wells of A. baumannii cells activated at various concentrations of rifabutin and colistin. The wells used to determine the MICs of the antibiotics alone are circled in green, the wells for the combination MICs are circled in blue, and the well used to calculate the ICIF is circled in red. Figure 2 is a checkerboard pattern of 96 wells of A. baumannii cells incubated in various concentrations of rifabutin and cefiderocol. The wells used to determine the MICs of the antibiotics alone are circled in green, the wells for the combination MICs are circled in blue, and the well used to calculate the ICIF is circled in red. EXAMPLE 2 Rifabutin decreases the MICs of cefiderocol by >4 times against tested A. baumannii strains To further investigate the synergy between rifabutin and cefiderocol against A. baumannii, checkerboard patterns were performed on a panel of 16 MDR clinical isolates of A. baumannii, including 5 isolates with high MICs (>32 mg / L) to rifabutin (and rifampicin) that had mutations in the rpoB gene. To describe the level of synergy in more detail, Table 2 presents the MICs of rifabutin and cefiderocol, alone and in combination, along with the variation associated with these MICs for each strain. As expected, cefiderocol in combination with rifabutin had little or no effect on the activity of rifabutin against isolates with rpoB gene mutations. Unexpectedly, rifabutin produced at least a 4-fold decrease in the MIC of cefiderocol against all Acinetobacter baumannii strains tested. IVIA / IOOO TABLE 2 Strain Rifabutin MIC (mg / l) Cefiderocol MIC (mg / l) alone combined variation only combined variation HUMC1 4 4 1 2 0.5 4 UNT091-1 8 2 4 4 1 4 IHMA690517 4 4 1 0.25 0.06 4 IHMA863866 4 0.125 16 4 1 4 IHMA919656 4 4 1 0.125 0.03 4 IHMA1013816 4 4 1 0.06 0.016 4 LAC-4** 4 1 4 1 0.25 4 UNT238-1** 4 4 1 4 1 4 UNT191-1** 8 0.5 16 16 4 4 UNT239-1** 4 1 4 4 1 4 UNT087-1 8 4 2 0.5 0.125 4 402292-17 >128 128 >1 >32 8 >4 402608-17 128 32 4 8 2 4 IHMA867231 >128 128 >1 1 0.25 4 401046-18** 32 16 2 128 32 4 401255-18 32 16 2 16 4 4 ** Synergy determined in ID-CA-MHB. Overall, the data indicate that combining rifabutin with cefiderocol may improve the outcome of treatment for A. baumann infections. EXAMPLE 3 Rifabutin shows strong synergy with colistin in 100% of the A. baumannii strains tested. The same exercise was performed to assess the synergy of rifabutin with colistin against A. baumannii. Checkerboard patterns were created using a panel of 16 MDR clinical isolates of A. baumannii, including 5 isolates with high MICs (>32 mg / L) to rifabutin (and rifampicin) that had mutations in the rpoB gene and 5 colistin-resistant strains (MIC >4 mg / L). Synergy was tested in CA-MHB medium, the approved medium for testing colistin MICs. The results are shown in Table 3. Unexpectedly, colistin in combination with rifabutin had a pronounced effect on rifabutin activity against isolates with a mutation in the rpoB gene. In these cases, the MIC variation of rifabutin was >32-fold when combined with colistin. TABLE 3 Strain Synergy in CA-MHB CIM of Rifabutin (mq / l) CIM of Co istine (mg / l) single combined variation single combined variation HUMC1 4 0.125 32 0.5 0.125 4 UNT091-1 8 <0.03 >256 0.125 IHMA 8 0.06 128 16 0.5 32 IHMA863866 4 0.06 64 >512 1 >512 IHMA919656 4 0.03 128 512 1 512 IHMA1013816 8 0.125 16 LAC-54 >64. 0.016 256 0.5 0.125 4 UNT238-1 8 2 4 0.25 0.06 4 UNT191-1 16 <0.03 >512 2 0.5 4 UNT239-1 4 1 4 1 0.25 4 UNT239-1 151 367. 0.25 4 402292-17 >128 4 32 1 0.25 4 402608-17 128 2 64 1 0.25 4 401255-18 128 2 64 0.25 0.06 4 IHMA 87 23 24 24 >1 8 401046-18 64 1 64 0.5 0.125 4 The synergy of rifabutin with colistin was observed in 100% of strains when tested in CA-MHB. Remarkably, the synergy was independent of the original levels of resistance to either rifabutin or colistin, as exemplified in CA-MHB, where colistin MICs were reduced by at least 16-fold in colistin-resistant strains, rendering all but one susceptible to colistin. These results indicate that the combination of rifabutin with colistin has the potential to overcome both rifabutin and colistin resistance in clinical isolates of A. baumannii. These observations could indicate that colistin acts as a cell permeabilizer, synergistically interacting with rifabutin. To assess whether colistin solely acts as a permeabilizer, the synergy of rifabutin was tested in combination with the colistin derivative SPR741, which retains permeabilizing activity but loses antibacterial activity. The results are shown in Table 4. TABLE 4 Strain Rifabutin / SPR741 Synergy Rifabutin MIC (mg / l) SPR741 MIC in combination (mg / l) alone combined variation HUMC1 4 0.25 16 4 UNT091-1 0.25 0.06 4 2 IHMA690517 4 1 4 16 IHMA863866 4 2 2 16 IHMA919656 8 1 8 8 IHMA1013816 8 2 4 16 ACC00535 8 0.5 16 4 LAC-4 4 0.06 64 4 UNT238-1 4 0.125 32 4 UNT191-1 8 0.5 16 4 UNT239-1 4 0.25 16 4 UNT087-1 8 0.25 32 8 402292-17 >32 2 >16 32 402608-17 >32 2 >16 16 IHMA867231 >32 >32 1 32 401046-18 >32 2 >16 8 401255-18 >32 2 >16 4 The synergy of rifabutin with SPR741 was observed in 88% of the strains. However, the synergy was less pronounced compared to that with colistin, since, for most strains, the MICs of the rifabutin combination remained in the range of 0.125–2 mg / L. Furthermore, the concentration of SPR741 required to achieve synergy with rifabutin was at least 8 times higher than that of colistin. These results indicate that the intrinsic antibacterial activity of colistin is necessary to achieve strong synergy with rifabutin. EXAMPLE 4 Comparison of the synergy of rifampicin in combination with colistin against A. baumann isolates As a comparator, the synergy of rifampicin with colistin was determined in the panel of A. baumann strains. The results are shown in Table 5. TABLE 5 Strain Rifampicin / colistin synergy Rifampicin MIC (mg / l) Co istine MIC (mg / l) alone combined variation alone combined variation HUMC1 4 1 4 0.5 0.125 4 UNT091-1 4 0.125 32 0.5 0.125 4 IHMA690517 4 0.06 64 16 <0.5 >32 IHMA863866 2 0.125 16 512 0.5 1024 IHMA919656 2 0.03 64 512 1 512 IHMA1013816 2 0.06 32 32 <0.5 >64 LAC-4 2 0.06 32 0.5 0.125 4 UNT238-1 2 0.125 16 0.5 0.125 4 UNT191-1 4 0.25 16 1 0.25 4 UNT239-1 2 0.25 8 1 0.25 4 UNT087-1 4 4 1 0.5 0.125 4 402292-17 512 256 2 0.25 0.125 4 402608-17 512 64 8 1 0.25 4 IHMA867231 512 32 16 32 4 8 401046-18 512 64 8 0.25 0.06 4 401255-18 512 128 4 0.25 0.06 4 Synergy was observed in 88% of the strains. As with rifabutin, synergy was independent of the level of colistin resistance, as demonstrated by the synergy observed in most colistin-resistant strains. However, in contrast to rifabutin, and as expected from the literature, rifampicin / colistin synergy was weak in isolates with mutations in the rpoB gene. For these isolates, the MICs of the rifampicin combination remained high (>32 mg / L). In general, colistin was shown to enhance the activity of rifabutin, and vice versa, against A. baumannii strains that have high MICs to rifabutin, colistin, or both. However, unlike rifampicin, there was unexpected activity of rifabutin in combination with colistin against isolates with mutations in the rpoB gene that would otherwise be resistant to antibiotics. Recruitment by Reference Throughout this disclosure, references and citations have been made to other documents, such as patents, patent applications, patent publications, journals, books, articles, and web content. All of these documents are incorporated herein by reference in their entirety for all purposes. Equivalents Several modifications of the invention and many additional embodiments thereof, besides those shown and described herein, will become apparent to those skilled in the art from the complete contents of this document, including the references to the scientific and patent literature cited herein. The subject matter of this document contains important information, examples, and guidance that can be adapted to the practice of this invention in its various embodiments and equivalents.

Claims

NOVELTY OF THE INVENTION CLAIMS 1.- The use of rifabutin and a second antibiotic in a therapeutically effective amount for the preparation of a medicament to treat an A. baumannii infection in a subject infected with A. baumannii, wherein the second antibiotic is selected from the group consisting of a polymyxin and cefiderocol.

2. The use as claimed in claim 1, wherein polymyxin is a derivative or analogue of polymyxin.

3. The use as claimed in claim 1, wherein the polymyxin is polymyxin B.

4. The use as claimed in claim 1, wherein the polymyxin is colistin.

5. The use as claimed in claim 1, wherein the second antibiotic is cefiderocol. 6.- The use as claimed is suitable for intravenous administration. 7.- The use as claimed is suitable for administration by inhalation.

8. The use as claimed is suitable for oral administration. in claim 1, where rifabutin is 9.- The use as claimed in claim 1, wherein the rifadutin and the second antibiotic are supplied in a single formulation.

10. The use as claimed in claim 1, wherein the rifadutine and the second antidioxant are supplied separately. 11.- The use as claimed in claim 1, wherein the A. baumannii comprises an rpoB mutation. 12.- The use as claimed in claim 1, wherein the subject is infected with a strain of A. baumannii that is resistant to rifadutin. 13.- The use as claimed in claim 1, wherein the subject is infected with a strain of A. baumannii that is resistant to the second antidiox. 14.- Rifadutin and a second antidioxant in a therapeutically effective amount for use in treating an A. baumannii infection in a subject infected with A. baumannii, wherein the second antidioxant is selected from the group consisting of a polymyxin and cefiderocol. 15.- Rifabutin and a second antibiotic for use as claimed in claim 14, wherein polymyxin is a derivative or analogue of polymyxin. 16.- Rifabutin and a second antibiotic for use as claimed in claim 14, wherein the polymyxin is polymyxin B. 17.- Rifabutin and a second antibiotic for use as claimed in claim 14, wherein the polymyxin is colistin. 18.- Rifabutin and a second antibiotic for use as claimed in claim 14, wherein the second antibiotic is cefiderocol. 19.- Rifabutin and a second antibiotic for use as claimed in claim 14, wherein the rifabutin is adapted to be administered intravenously. 20.- Rifabutin and a second antibiotic for use as claimed in claim 14, wherein the rifabutin is adapted to be administered by inhalation. 21.- Rifabutin and a second antibiotic for use as claimed in claim 14, wherein the rifabutin is adapted to be administered orally. 22.- Rifadutin and a second antidiotic for use as claimed in claim 14, wherein the rifadutin and the second antidiotic are supplied in a single formulation.

23. Rifadutin and a second antidiotic for use as claimed in claim 14, wherein the rifadutin and the second antidiotic are supplied separately. 24.- Rifadutin and a second antidiotic to be used as claimed in claim 14, wherein the A. baumannü comprises an rpoB mutation. 25 - Rifadutin and a second antidote for use as claimed in claim 14, wherein the subject is infected with a strain of A. baumann that is resistant to rifadutin.

26. Rifadutin and a second antidioxant for use as claimed in claim 14, wherein the subject is infected with a strain of A. baumannüque that is resistant to the second antidioxant.