WO2018172733A1

WO2018172733A1 - Combinations of polymyxins

Info

Publication number: WO2018172733A1
Application number: PCT/GB2018/050577
Authority: WO
Inventors: Professor Anthony COATES; Yanmin Hu
Original assignee: Helperby Therapeutics Ltd
Current assignee: Helperby Therapeutics Ltd
Priority date: 2017-03-23
Filing date: 2018-03-07
Publication date: 2018-09-27
Anticipated expiration: 2019-09-23
Also published as: GB201704620D0

Abstract

The present invention relates to a combination comprising a polymyxin derivative of formula (I) defined herein wherein R1 is Dab; R2 is Thr; R3 is DThr; R4 is Dab; R5 is Dab; R6 is DPhe; R7 is Leu; R8 is Abu; R9 is Dab; R10 is Thr; and R(FA) is octanoyl; or wherein R1 is absent; R2 is Thr; R3 is DSer; R4 is Dab; R5 is Dab; R6 is DPhe; R7 is Leu; R8 is Dab; R9 is Dab; R10 is Thr; and R(FA) is octanoyl; or pharmaceutically acceptable salts thereof; and at least one antibiotic resistance breaker selected from the group consisting of zidovudine, suloctidil, mefloquine, phenoxybenzamine or a pharmaceutically acceptable derivative thereof. The invention also provides the use of this combination in the treatment of bacterial infections, preferably Gram-negative bacterial infections, particularly drug-resistant Gram-negative bacterial infections.

Description

COMBINATIONS OF POLYMYXINS

Field of the invention

The present invention relates to the use of at least one antibiotic resistance breaker selected from zidovudine, mefloquine, suloctidil, phenoxybenzamine or a pharmaceutically acceptable derivative thereof in combination with a polymyxin derivative of formula (I) as defined herein or pharmaceutically acceptable salts thereof for treating bacterial infections.

Background of the invention

Before the introduction of antibiotics, patients suffering from acute microbial infections (e.g. tuberculosis or pneumonia) had a low chance of survival. For example, mortality from tuberculosis was around 50%. Although the introduction of antimicrobial agents in the 1940s and 1950s rapidly changed this picture, bacteria have responded by progressively gaining resistance to commonly used antibiotics. Now, every country in the world has antibiotic- resistant bacteria.

Indeed, more than 70% of bacteria that give rise to hospital acquired infections in the USA resist at least one of the main antimicrobial agents that are typically used to fight infection (Nature Reviews, Drug Discovery, 1 , 895-910 (2002)). In its 2014 report of global antimicrobial resistance, the World Health Organization focussed on the high levels of antibiotic resistance in the bacteria that cause common infections.

The global problem of advancing antimicrobial resistance has also led to a renewed interest in polymyxins. Polymyxins are a group of closely related antibiotic substances with a general structure consisting of a cyclic peptide and a hydrophobic tail. They are produced by non- ribosomal peptide synthetase systems in Gram-positive bacteria such as Paenibacillus polymyxa, and are selectively toxic for Gram-negative bacteria due to their specificity for the lipopolysaccharide molecule that exists within many Gram-negative outer membranes. Polymyxins B and E are used in the treatment of Gram-negative bacterial infections.

There have, however, been reports which show that even polymyxin E (colistin) may be losing its effectiveness in antibacterial therapy. The U.S. Military HIV Research Program has for instance reported colistin resistance in a human E. coli infection

(www sc;enced8ily.com/re;e¾¾es/20 ;50526lS2033.hiiTi ;. In order to develop more long-term solutions to the problem of bacterial resistance, it is clear that alternative approaches are required. One such approach is to co-administer another drug with the failing antibiotic so as to restore sufficient antibacterial activity. These compounds are often called "antibiotic resistance breakers" (ARBs), and their use to restore antibiotics is exemplified by the successful co-administration of β-lactamase inhibitors, such as clavulanic acid, with β-lactam antibiotics, such as amoxicillin (White, A. R, et a/,, J. Antimicrob. Chemother. 53 (Suppl. 1), i3-i20 (2004); Prabhudesai, P. P. et al., J. Indian Med. Assoc. 109, 124-127 (2011)). Existing β-ΐ3θί3Γη/β-ΐ3θί3Γη33β inhibitor combinations include Tazocin® (piperacillin/tazobactam), Avycaz® (ceftazidime/avibactam) and Carbavance® (meropenem/vaborbactam).

The Applicant's International Patent Application published as WO2012032360 discloses a combination comprising phenoxybenzamine or a pharmaceutically acceptable derivative thereof and a polymyxin selected from polymyxin E and polymyxin B or a pharmaceutically acceptable derivative thereof, and its use in treating a microbial infection.

WO2014147405 then discloses the use of colistin (polymyxin E) in combination with zidovudine for treating a microbial infection. WO2016097754 discloses a combination comprising suloctidil or a pharmaceutically acceptable derivative or prodrug thereof, and a polymyxin selected from polymyxin E and polymyxin B or a pharmaceutically acceptable derivative thereof, and its use in treating a microbial infection.

Bacterial resistance to these combinations is, however, inevitable.

Consequently there is an ongoing need in the art for new combinations which have activity against bacterial infections, particularly Gram-negative bacterial infections. There is also an urgent need for combinations which are effective against multi-resistant Gram-negative bacteria.

This need is met with the present invention because a polymyxin derivative of formula (I) defined herein or a pharmaceutically acceptable salt thereof is combined with at least one antibiotic resistance breaker selected from zidovudine, phenoxybenzamine, mefloquine, suloctidil or a pharmaceutically acceptable derivative thereof.

The inventors surprising found that these combinations exhibit synergistic antibacterial activity against bacteria, particularly against Gram-negative bacteria. In other words, the combinations were unexpectedly found to have a greater biological activity than the expected additive effect of each agent at the stated dosage level. The surprising biological activity of the combinations of the present invention offers the opportunity to shorten chemotherapy regimens and may result in a reduction in the emergence of microbial resistance associated with the use of such combinations. Advantageously, and as described below, the combinations of the present invention have also been demonstrated to be particularly effective against drug- resistant bacteria, particularly drug-resistant Gram-negative bacteria. This opens the way for the combinations to be administered both to drug-resistant strains and in said strains before drug-resistance is built up, i.e. as a first line treatment.

Synergy in the context of antimicrobials drugs is measured in a number of ways that conform to the generally accepted opinion that "synergy" is an effect greater than additive. One of the ways to assess whether synergy has been observed is to use the "chequerboard" technique. This is a well-accepted method that leads to the generation of a value called the fractional inhibitory concentration index (FICI).

Orhan et al J. Clin. Microbiol. 2005, 43(1): 140 for instance describes the chequerboard method and analysis in the paragraph bridging pages 140-141. This document explains that the FICI value is a ratio of the sum of the MIC (Minimum Inhibitory Concentration) level of each individual component alone and in the mixture. The combination is considered synergistic when the∑FIC is≤0.5, indifferent when the∑FIC is >0.5 to <2, and antagonistic when the∑FIC is≥2.

Another accepted test for ascertaining the presence or absence of synergy is to use time-kill methods where the dynamic effect of a drug combination is compared to each drug alone when assessing the effect on bacterial log or stationary-growth over time. Again, the possible results are synergy, indifference, or antagonism.

Summary of the Invention

Thus, in one embodiment the present invention provides a combination comprising a polymyxin derivative of formula (I):

;_ii 3

(I) wherein: R1 is Dab;

R2 is Thr;

R3 is DThr;

R4 is Dab;

R5 is Dab;

R6 is DPhe;

R7 is Leu;

R8 is Abu;

R9 is Dab;

R10 is Thr; and

R(FA) is octanoyl;

or wherein: R1 is absent;

R2 is Thr;

R3 is DSer;

R4 is Dab;

R5 is Dab;

R6 is DPhe;

R7 is Leu;

R8 is Dab;

R9 is Dab;

R10 is Thr; and

R(FA) is octanoyl;

or pharmaceutically acceptable salts thereof; and at least one antibiotic resistance breaker selected from the group consisting of zidovudine, suloctidil, mefloquine, phenoxybenzamine or a pharmaceutically acceptable derivative thereof.

In another embodiment, the present invention provides the use of a polymyxin derivative of formula (I) as defined hereinabove or a pharmaceutically acceptable salt thereof, and at least one antibiotic resistance breaker selected from the group consisting of zidovudine, suloctidil, mefloquine, phenoxybenzamine or a pharmaceutically acceptable derivative thereof, in the manufacture of a medicament for treating a bacterial infection.

Additionally the present invention provides the combination of a polymyxin derivative of formula (I) as defined hereinabove or a pharmaceutically acceptable salt thereof, and at least one antibiotic resistance breaker selected from the group consisting of zidovudine, suloctidil, mefloquine, phenoxybenzamine or a pharmaceutically acceptable derivative thereof, for use in the treatment of a bacterial infection.

In a further embodiment, the invention provides a method of treating a bacterial infection which comprises administering the combination of a polymyxin derivative of formula (I) as defined hereinabove or a pharmaceutically acceptable salt thereof, and at least one antibiotic resistance breaker selected from the group consisting of zidovudine, suloctidil, mefloquine, phenoxybenzamine or a pharmaceutically acceptable derivative thereof, to a patient in need thereof. There is also provided a pharmaceutical composition comprising a polymyxin derivative of formula (I) as defined hereinabove or a pharmaceutically acceptable salt thereof, and at least one antibiotic resistance breaker selected from the group consisting of zidovudine, suloctidil, mefloquine, phenoxybenzamine or a pharmaceutically acceptable derivative thereof, and a pharmaceutically acceptable adjuvant, diluent or carrier. In one embodiment, the pharmaceutical composition is for use in treating a bacterial infection.

In a further embodiment, the invention relates to a product comprising a polymyxin derivative of formula (I) or a pharmaceutically acceptable salt thereof as defined hereinabove, and at least one antibiotic resistance breaker selected from the group consisting of zidovudine, suloctidil, mefloquine, phenoxybenzamine or a pharmaceutically acceptable derivative thereof, as a combined preparation for simultaneous, separate or sequential use in treating a bacterial infection.

Detailed Description of the Invention

Abbreviations used herein: cy or cyclo refers to the cyclic part of the peptide, enclosed within brackets; Dab or Dbu refers to α,γ-diamino-n-butyryl (i.e. 2,4-diaminobutyryl); Abu refers to 2- aminobutyryl; Thr refers to L-threonine; DThr refers to D-threonine; DPhe refers to D- phenylamine; Leu refers to L-leucine; DSer refers to D-serine and OA refers to octanoyl.

As used herein, "physiological pH" refers to a pH value of more than 7.0 and below 7.6, such as a pH value in the range of from 7.1 to 7.5, for example in the range of from 7.2 to 7.4. As used herein, the term "antibiotic resistance breaker" refers to non-antibiotic compounds that, when combined with existing antibiotics, act to block resistance or enhance antibacterial activity. These compounds are also known in the art as "antibiotic adjuvants", "resistance breakers" and "antibiotic potentiators".

The antibiotic resistance breakers in the present invention are selected from the group consisting of zidovudine, suloctidil, mefloquine, phenoxybenzamine or pharmaceutically acceptable derivatives thereof. All of these compounds are known non-antibiotic drugs.

As used herein, the term "in combination with" covers both separate and sequential administration of the polymyxin derivative and antibiotic resistance breaker (ARB).

When the polymyxin derivative and ARB are administered sequentially, either the polymyxin derivative or ARB may be administered first. When administration is simultaneous, the polymyxin derivative or ARB may be administered either in the same or a different pharmaceutical composition. Adjunctive therapy, i.e. where one agent is used as a primary treatment and the other agent is used to assist that primary treatment, is also an embodiment of the present invention.

The combinations of the present invention may be used to treat bacterial infections. In particular they may be used to kill multiplying, non-multiplying and/or clinically latent bacteria associated with bacterial infections, preferably multiplying or non-multiplying bacteria, more preferably multiplying bacteria, e.g. multiplying bacteria associated with Gram-negative bacterial infections.

References herein to the treatment of a bacterial infection include killing multiplying, non- multiplying and/or clinically latent bacteria associated with such infections.

As used herein, "kill" means a loss of viability as assessed by a lack of metabolic activity.

As used herein, "clinically latent bacteria" means a bacterium that is metabolically active but has a growth rate that is below the threshold of infectious disease expression. The threshold of infectious disease expression refers to the growth rate threshold below which symptoms of infectious disease in a host are absent.

The metabolic activity of clinically latent bacteria can be determined by several methods known to those skilled in the art; for example, by measuring mRNA levels in the bacterium or by determining their rate of uridine uptake. In this respect, clinically latent bacteria, when compared to bacteria under logarithmic growth conditions (in vitro or in vivo), possess reduced but still significant levels of:

(I) mRNA (e.g. from 0.0001 to 50%, such as from 1 to 30, 5 to 25 or 10 to 20%, of the level of mRNA); and/or

(II) uridine (e.g. [³H]uridine) uptake (e.g. from 0.0005 to 50%, such as from 1 to 40, 15 to 35 or 20 to 30% of the level of [³H]uridine uptake). Clinically latent bacteria typically possess a number of identifiable characteristics. For example, they may be viable but non-culturable; i.e. they cannot typically be detected by standard culture techniques, but are detectable and quantifiable by techniques such as broth dilution counting, microscopy, or molecular techniques such as polymerase chain reaction. In addition, clinically latent bacteria are typically phenotypically tolerant, and as such are sensitive (in log phase) to the biostatic effects of conventional antimicrobial agents (i.e. bacteria for which the minimum inhibitory concentration (MIC) of a conventional antimicrobial is substantially unchanged); but possess drastically decreased susceptibility to drug-induced killing (e.g. bacteria for which, with any given conventional antimicrobial agent, the ratio of minimum microbiocidal concentration (e.g. minimum bactericidal concentration, MBC) to MIC is 10 or more).

As used herein, the term "bacteria" (and derivatives thereof, such as "bacterial infection") includes, but is not limited to, references to organisms (or infections due to organisms) of the following classes and specific types:

Gram-positive cocci, such as Staphylococci (e.g. Staph, aureus, Staph, epidermidis, Staph, saprophyticus, Staph, auricularis, Staph, capitis capitis, Staph, c. ureolyticus, Staph, caprae, Staph, cohnii cohnii, Staph, c. urealyticus, Staph, equorum, Staph, gallinarum, Staph, haemolyticus, Staph, hominis hominis, Staph, h. novobiosepticius, Staph, hyicus, Staph. intermedius, Staph, lugdunensis, Staph, pasteuri, Staph, saccharolyticus, Staph, schleiferi schleiferi, Staph, s. coagulans, Staph, sciuri, Staph, simulans, Staph, warneri and Staph, xylosus);

Streptococci (e.g.beta-haemolytic, pyogenic streptococci (such as Strept. agalactiae, Strept. canis, Strept. dysgalactiae dysgalactiae, Strept. dysgalactiae equisimilis, Strept. equi equi, Strept. equi zooepidemicus, Strept. iniae, Strept. porcinus and Strept. pyogenes),

microaerophilic, pyogenic streptococci (Streptococcus "milleri", such as Strept. anginosus, Strept. constellatus constellatus, Strept. constellatus pharyngidis and Strept. intermedius), oral streptococci of the "mitis" (alpha-haemolytic - Streptococcus "viridans", such as Strept. mitis, Strept. oralis, Strept. sanguinis, Strept. cristatus, Strept. gordonii and Strept. parasanguinis), "salivarius" (non-haemolytic, such as Strept. salivarius and Strept. vestibularis) and "mutans" (tooth-surface streptococci, such as Strept. criceti, Strept. mutans, Strept. ratti and Strept. sobrinus) groups, Strept. acidominimus, Strept. bovis, Strept. faecalis, Strept. equinus, Strept. pneumoniae and Strept. suis, or Streptococci alternatively classified as Group A, B, C, D, E, G, L, P, U or V Streptococcus);

Gram-negative cocci, such as Neisseria gonorrhoeae, Neisseria meningitidis, Neisseria cinerea, Neisseria elongata, Neisseria flavescens, Neisseria lactamica, Neisseria mucosa, Neisseria sicca, Neisseria subflava and Neisseria weaveri;

Bacillaceae, such as Bacillus anthracis, Bacillus subtilis, Bacillus thuringiensis, Bacillus stearothermophilus and Bacillus cereus;

Enterobacteriaceae, such as Escherichia coli,

Enterobacter (e.g. Enterobacter aerogenes, Enterobacter agglomerans and Enterobacter cloacae), Citrobacter (such as Citrob. freundii and Citrob. divernis), Hafnia (e.g. Hafnia alvei), Erwinia (e.g. Erwinia persicinus), Morganella morganii, Salmonella (Salmonella enterica and Salmonella typhi), Shigella (e.g. Shigella dysenteriae, Shigella flexneri, Shigella boydii and Shigella sonnei), Klebsiella (e.g. Klebs. pneumoniae, Klebs. oxytoca, Klebs. ornitholytica, Klebs. planticola, Klebs. ozaenae, Klebs. terrigena, Klebs. granulomatis (Calymmatobacterium granulomatis) and Klebs. rhinoscleromatis), Proteus (e.g. Pr. mirabilis, Pr. rettgeri and Pr. vulgaris), Providencia (e.g. Providencia alcalifaciens, Providencia rettgeri and Providencia stuartii), Serratia (e.g. Serratia marcescens and Serratia liquifaciens), and Yersinia (e.g. Yersinia enterocolitica, Yersinia pestis and Yersinia pseudotuberculosis);

Enterococci (e.g. Enterococcus avium, Enterococcus casseliflavus, Enterococcus cecorum, Enterococcus dispar, Enterococcus durans, Enterococcus faecalis, Enterococcus faecium, Enterococcus flavescens, Enterococcus gallinarum, Enterococcus hirae, Enterococcus malodoratus, Enterococcus mundtii, Enterococcus pseudoavium, Enterococcus raffinosus and Enterococcus solitarius);

Helicobacter (e.g. Helicobacter pylori, Helicobacter cinaedi and Helicobacter fennelliae) ; Acinetobacter (e.g. A. baumanii, A. calcoaceticus, A. haemolyticus, A. johnsonii, A. junii, A. Iwoffi and A. radioresistens);

Pseudomonas (e.g. Ps. aeruginosa, Ps. maltophilia (Stenotrophomonas maltophilia), Ps. alcaligenes, Ps. chlororaphis, Ps. fluorescens, Ps. luteola. Ps. mendocina, Ps. monteilii, Ps. oryzihabitans, Ps. pertocinogena, Ps. pseudalcaligenes, Ps. putida and Ps. stutzeri);

Bacteriodes fragilis;

Peptococcus (e.g. Peptococcus niger); Peptostreptococcus;

Clostridium (e.g. C. perfringens, C. difficile, C. botulinum, C. tetani, C. absonum, C. argentinense, C. baratii, C. bifermentans, C. beijerinckii, C. butyricum, C. cadaveris, C. carnis, C. celatum, C. clostridioforme, C. cochlearium, C. cocleatum, C. fallax, C. ghonii, C. glycolicum, C. haemolyticum, C. hastiforme, C. histolyticum, C. indolis, C. innocuum, C. irregulare, C. leptum, C. limosum, C. malenominatum, C. novyi, C. oroticum, C. paraputrificum, C. piliforme, C. putrefasciens, C. ramosum, C. septicum, C. sordelii, C. sphenoides, C. sporogenes, C. subterminale, C. symbiosum and C. tertium);

Mycoplasma (e.g. M. pneumoniae, M. hominis, M. genitalium and M. urealyticum);

Mycobacteria (e.g. Mycobacterium tuberculosis, Mycobacterium avium, Mycobacterium fortuitum, Mycobacterium marinum, Mycobacterium kansasii, Mycobacterium chelonae, Mycobacterium abscessus, Mycobacterium leprae, Mycobacterium smegmitis,

Mycobacterium africanum, Mycobacterium alvei, Mycobacterium asiaticum, Mycobacterium aurum, Mycobacterium bohemicum, Mycobacterium bovis, Mycobacterium branderi,

Mycobacterium brumae, Mycobacterium celatum, Mycobacterium chubense, Mycobacterium confluentis, Mycobacterium conspicuum, Mycobacterium cookii, Mycobacterium flavescens, Mycobacterium gadium, Mycobacterium gastri, Mycobacterium genavense, Mycobacterium gordonae, Mycobacterium goodii, Mycobacterium haemophilum, Mycobacterium hassicum, Mycobacterium intracellular, Mycobacterium interjectum, Mycobacterium heidelberense, Mycobacterium lentiflavum, Mycobacterium malmoense, Mycobacterium microgenicum, Mycobacterium microti, Mycobacterium mucogenicum, Mycobacterium neoaurum, Mycobacterium nonchromogenicum, Mycobacterium peregrinum, Mycobacterium phlei, Mycobacterium scrofulaceum, Mycobacterium shimoidei, Mycobacterium simiae, Mycobacterium szulgai, Mycobacterium terrae, Mycobacterium thermoresistabile, Mycobacterium triplex, Mycobacterium triviale, Mycobacterium tusciae, Mycobacterium ulcerans, Mycobacterium vaccae, Mycobacterium wolinskyi and Mycobacterium xenopi); Haemophilus (e.g. Haemophilus influenzae, Haemophilus ducreyi, Haemophilus aegyptius, Haemophilus parainfluenzae, Haemophilus haemolyticus and Haemophilus parahaemolyticus) ;

Actinobacillus (e.g. Actinobacillus actinomycetemcomitans, Actinobacillus equuli, Actinobacillus hominis, Actinobacillus lignieresii, Actinobacillus suis and Actinobacillus ureae);

Actinomyces (e.g. Actinomyces israelii);

Brucella (e.g. Brucella abortus, Brucella canis, Brucella melintensis and Brucella suis);

Campylobacter (e.g. Campylobacter jejuni, Campylobacter coli, Campylobacter lari and Campylobacter fetus) ; Listeria monocytogenes;

Vibrio (e.g. Vibrio cholerae and Vibrio parahaemolyticus, Vibrio alginolyticus, Vibrio carchariae, Vibrio fluvialis, Vibrio furnissii, Vibrio hollisae, Vibrio metschnikovii, Vibrio mimicus and Vibrio vulnificus) ;

Erysipelothrix rhusopathiae;

Corynebacteriaceae (e.g. Corynebacterium diphtheriae, Corynebacterium jeikeum and Corynebacterium urealyticum);

Spirochaetaceae, such as Borrelia (e.g. Borrelia recurrentis, Borrelia burgdorferi, Borrelia afzelii, Borrelia andersonii, Borrelia bissettii, Borrelia garinii, Borrelia japonica, Borrelia lusitaniae, Borrelia tanukii, Borrelia turdi, Borrelia valaisiana, Borrelia caucasica, Borrelia crocidurae, Borrelia duttoni, Borrelia graingeri, Borrelia hermsii, Borrelia hispanica, Borrelia latyschewii, Borrelia mazzottii, Borrelia parkeri, Borrelia persica, Borrelia turicatae and Borrelia venezuelensis) and Treponema (Treponema pallidum ssp. pallidum, Treponema pallidum ssp. endemicum, Treponema pallidum ssp. pertenue and Treponema carateum); Pasteurella (e.g. Pasteurella aerogenes, Pasteurella bettyae, Pasteurella canis, Pasteurella dagmatis, Pasteurella gallinarum, Pasteurella haemolytica, Pasteurella multocida multocida, Pasteurella multocida gallicida, Pasteurella multocida septica, Pasteurella pneumotropica and Pasteurella stomatis); Bordetella (e.g. Bordetella bronchiseptica, Bordetella hinzii, Bordetella holmseii, Bordetella parapertussis, Bordetella pertussis and Bordetella trematum);

Nocardiaceae, such as Nocardia (e.g. Nocardia asteroides and Nocardia brasiliensis);

Rickettsia (e.g. Ricksettsii or Coxiella burnetii);

Legionella (e.g. Legionalla anisa, Legionalla birminghamensis, Legionalla bozemanii, Legionalla cincinnatiensis, Legionalla dumoffii, Legionalla feeleii, Legionalla gormanii, Legionalla hackeliae, Legionalla israelensis, Legionalla jordanis, Legionalla lansingensis, Legionalla longbeachae, Legionalla maceachernii, Legionalla micdadei, Legionalla oakridgensis, Legionalla pneumophila, Legionalla sainthelensi, Legionalla tucsonensis and Legionalla wadsworthii);

Moraxella catarrhalis;

Cyclospora cayetanensis; Entamoeba histolytica; Giardia lamblia; Trichomonas vaginalis; Toxoplasma gondii; Stenotrophomonas maltophilia; Burkholderia cepacia; Burkholderia mallei and Burkholderia pseudomallei; Francisella tularensis; Gardnerella (e.g. Gardneralla vaginalis and Gardneralla mobiluncus); Streptobacillus moniliformis; Flavobacteriaceae, such as Capnocytophaga (e.g. Capnocytophaga canimorsus, Capnocytophaga cynodegmi, Capnocytophaga gingivalis, Capnocytophaga granulosa, Capnocytophaga haemolytica, Capnocytophaga ochracea and Capnocytophaga sputigena);

Bartonella {Bartonella bacilliformis, Bartonella clarridgeiae, Bartonella elizabethae, Bartonella henselae, Bartonella quintana and Bartonella vinsonii arupensis);

Leptospira (e.g. Leptospira biflexa, Leptospira borgpetersenii, Leptospira inadai, Leptospira interrogans, Leptospira kirschneri, Leptospira noguchii, Leptospira santarosai and Leptospira weilii);

Spirillium (e.g. Spirillum minus);

Baceteroides (e.g. Bacteroides caccae, Bacteroides capillosus, Bacteroides coagulans, Bacteroides distasonis, Bacteroides eggerthii, Bacteroides forsythus, Bacteroides fragilis, Bacteroides merdae, Bacteroides ovatus, Bacteroides putredinis, Bacteroides pyogenes, Bacteroides splanchinicus, Bacteroides stercoris, Bacteroides tectus, Bacteroides thetaiotaomicron, Bacteroides uniformis, Bacteroides ureolyticus and Bacteroides vulgatus); Prevotella (e.g. Prevotella bivia, Prevotella buccae, Prevotella corporis, Prevotella dentalis (Mitsuokella dentalis), Prevotella denticola, Prevotella disiens, Prevotella enoeca, Prevotella heparinolytica, Prevotella intermedia, Prevotella loeschii, Prevotella melaninogenica, Prevotella nigrescens, Prevotella oralis, Prevotella oris, Prevotella oulora, Prevotella tannerae, Prevotella venoralis and Prevotella zoogleoformans);

Porphyromonas (e.g. Porphyromonas asaccharolytica, Porphyromonas cangingivalis, Porphyromonas canoris, Porphyromonas cansulci, Porphyromonas catoniae, Porphyromonas circumdentaria, Porphyromonas crevioricanis, Porphyromonas endodontalis, Porphyromonas gingivalis, Porphyromonas gingivicanis, Porphyromonas levii and Porphyromonas macacae);

Fusobacterium (e.g. F. gonadiaformans, F. mortiferum, F. naviforme, F. necrogenes, F. necrophorum necrophorum, F. necrophorum fundiliforme, F. nucleatum nucleatum, F. nucleatum fusiforme, F. nucleatum polymorphum, F. nucleatum vincentii, F. periodonticum, F. russii, F. ulcerans and F. varium);

Chlamydia (e.g. Chlamydia trachomatis); Cryptosporidium (e.g. C. parvum, C. hominis, C. canis, C. felis, C. meleagridis and C. muris); Chlamydophila (e.g. Chlamydophila abortus (Chlamydia psittaci), Chlamydophila pneumoniae (Chlamydia pneumoniae) and Chlamydophila psittaci (Chlamydia psittaci));

Leuconostoc (e.g. Leuconostoc citreum, Leuconostoc cremoris, Leuconostoc dextranicum, Leuconostoc lactis, Leuconostoc mesenteroides and Leuconostoc pseudomesenteroides); Gemella (e.g. Gemella bergeri, Gemella haemolysans, Gemella morbillorum and Gemella sanguinis); and Ureaplasma (e.g. Ureaplasma parvum and Ureaplasma urealyticum).

Preferably, the bacterial infections treated by the combinations described herein are Gram- negative bacterial infections. Particular Gram-negative bacteria that may be treated using a combination of the invention include:

Enterobacteriaceae, such as Escherichia coli, Klebsiella (e.g. Klebs. pneumoniae and Klebs. oxytoca) and Proteus (e.g. Pr. mirabilis, Pr. rettgeri and Pr. vulgaris);

Haemophilis influenzae;

Mycobacteria, such as Mycobacterium tuberculosis; and

Enterobacter (e.g. Enterobacter cloacae) .

Preferably, the bacteria are Enterobacteriaceae, such as Escherichia coli, Klebsiella (e.g. Klebs. pneumoniae and Klebs. oxytoca) and Enterobacter (e.g. Enterobacter cloacae) and Acinetobacter spp.

Particularly preferred are Escherichia coli, Klebsiella and Enterobacter, e.g. Escherichia coli, Klebs. pneumoniae, Enterobacter cloacae and Acinetobacter spp, (e.g. Acinetobacter baumannii). The combination of the present invention is particularly beneficial in treating (multi)-drug- resistant ((M)DR) bacteria. With respect to Enterobacteriaceae, drug resistance most often builds up to carbapenemase i.e. carbapenemase-resistant strains and "extended spectrum β-lactamase" (ESBL) strains for example New Delhi Metallo-beta-lactamase-1 (NDM-1) resistant Klebs. Pneumonia, and NDM-1 E.coli.

It should be kept in mind that although a combination such as that claimed may initially be demonstrated to be functional in treating (M)DR strains, they can then be used in treating non-resistant strains. This is especially valuable in the context of the presently claimed combination where the primary therapy for Enterobacteriaceae, such as Escherichia coli, Klebsiella (e.g. Klebs. pneumoniae and Klebs. oxytoca) and Enterobacter (e.g. Enterobacter cloacae) are anti-microbial drugs that are expensive due to prevailing patent protection. The replacement of such "ethical" drugs by a combination of "generic" antibiotics is thought to be beneficial from a therapeutic perspective as well as financial/economic perspective in times where governments are seeking to reduce the cost of healthcare.

The combinations of the present invention may be used to treat infections associated with any of the above-mentioned bacterial organisms, and in particular they may be used for killing multiplying and/or clinically latent microorganisms associated with such an infection, e.g. a Gram-negative bacterial infection.

Particular conditions which may be treated using the combination of the present invention include those which are caused by Gram-negative bacteria such as abscesses, asthma, bacilliary dysentry, bacterial conjunctivitis, bacterial keratitis, bacterial vaginosis, bone and joint infections, bronchitis (acute or chronic), brucellosis, burn wounds, cat scratch fever, cellulitis, chancroid, cholangitis, cholecystitis, cystic fibrosis, cystitis, nephritis, diffuse panbronchiolitis, dental caries, diseases of the upper respiratory tract, empymea, endocarditis, endometritis, enteric fever, enteritis, epididymitis, epiglottitis, eye infections, furuncles, gardnerella vaginitis, gastrointestinal infections (gastroenteritis), genital infections, gingivitis, gonorrhoea, granuloma inguinale, Haverhill fever, infected burns, infections following dental operations, infections in the oral region, infections associated with prostheses, intraabdominal abscesses, Legionnaire's disease, leptospirosis, listeriosis, liver abscesses, Lyme disease, lymphogranuloma venerium, mastitis, mastoiditis, meningitis and infections of the nervous system, non-specific urethritis, opthalmia (e.g. opthalmia neonatorum), osteomyelitis, otitis (e.g. otitis externa and otitis media), orchitis, pancreatitis, paronychia, pelveoperitonitis, peritonitis, peritonitis with appendicitis, pharyngitis, pleural effusion, pneumonia, postoperative wound infections, postoperative gas gangrene, prostatitis, pseudo-membranous colitis, psittacosis, pyelonephritis, Q fever, rat-bite fever, Ritter's disease, salmonellosis, salpingitis, septic arthritis, septic infections, septicameia, systemic infections, tonsillitis, trachoma, typhoid, urethritis, urinary tract infections, wound infections; or infections with, Escherichia coli, Klebs. pneumoniae, Klebs. oxytoca, Pr. mirabilis, Pr. rettgeri, Pr. vulgaris, Haemophilis influenzae, Enterococcus faecalis, Enterococcus faecium, and Enterobacter cloacae.

It will be appreciated that references herein to "treatment" extend to prophylaxis as well as the treatment of established diseases or symptoms.

The polymyxin derivative is a compound of formula (I):

(I)

wherein: R1 is Dab;

R2 is Thr;

R3 is DThr;

R4 is Dab;

R5 is Dab;

R6 is DPhe;

R7 is Leu;

R8 is Abu;

R9 is Dab;

R10 is Thr; and

R(FA) is octanoyl;

or wherein: R1 is absent;

R2 is Thr;

R3 is DSer;

R4 is Dab;

R5 is Dab;

R6 is DPhe;

R7 is Leu;

R8 is Dab;

R9 is Dab;

R10 is Thr; and R(FA) is octanoyl; or pharmaceutically acceptable salts thereof.

The polymyxin derivatives of formula (I) are known compounds developed by Northern Antibiotics Oy.

The polymyxin derivative of formula (I) wherein R1 is Dab; R2 is Thr; R3 is DThr; R4 is Dab; R5 is Dab; R6 is DPhe; R7 is Leu; R8 is Abu; R9 is Dab; R10 is Thr; and R(FA) is octanoyl; was first disclosed in WO2016113470 (A1) as NAB815. Example 1 of WO2016113470 (A1) discloses the synthesis of NAB815. The disclosure of this reference insofar as it relates to NAB815 is incorporated herein by reference.

The polymyxin derivative of formula (I) wherein R1 is absent (i.e. replaced by a covalent bond); R2 is Thr; R3 is DSer; R4 is Dab; R5 is Dab; R6 is DPhe; R7 is Leu; R8 is Dab; R9 is Dab; R10 is Thr; and R(FA) is octanoyl; was first disclosed in WO2008017734 (A1) as NAB739. Example 1 of WO2008017734 (A1) discloses the synthesis of NAB739. The disclosure of this reference insofar as it relates to NAB739 is incorporated herein by reference.

Preferably the polymyxin derivative of formula (I) is NAB 815, wherein R1 is Dab; R2 is Thr; R3 is DThr; R4 is Dab; R5 is Dab; R6 is DPhe; R7 is Leu; R8 is Abu; R9 is Dab; R10 is Thr; and R(FA) is octanoyl. In one embodiment the polymyxin derivative of formula (I) has R1-R10 representing an amino acid sequence Thr-DSer-cy[Dab-Dab-DPhe-Leu-Dab-Dab-Thr-]. In one embodiment the polymyxin derivative of formula (I) has R1-R10 representing SEQ ID NO. 2 defined herein.

SEQ ID NO.2 is Thr-DSer-cy[Dab-Dab-DPhe-Leu-Dab-Dab-Thr-]. SEQ ID NO.2 can also be written as Thr-DSer-[cyclo-Dbu-Dbu-DPhe-Leu-Dbu-Dbu-Thr] where Thr refers to Threonine; DSer refers to D-Serine; Dbu refers to 2,4-diaminobutyric acid; DPhe refers to D-Phenylalanine; Leu refers to Leucine and residues 3-9 form the cyclic heptapeptide portion.

In another embodiment the polymyxin derivative of formula (I) has R1-R10 representing the amino acid sequence Dab-Thr-DThr-cy[Dab-Dab-DPhe-Leu-Abu-Dab-Thr-]. In one embodiment the polymyxin derivative of formula (I) has R1-R10 representing SEQ ID NO. 1 defined herein.

Preferably the polymyxin derivative of formula (I) has R1-R10 representing SEQ ID N0.1 defined herein. SEQ ID N0.1 is Dab-Thr-DThr-cy[Dab-Dab-DPhe-Leu-Abu-Dab-Thr-].

SEQ ID N0.1 can also be written as Dbu-Thr-DThr-[cyclo -Dbu-Dbu-DPhe-Leu-Abu-Dbu-Thr] where Thr refers to Threonine; Dbu refers to 2,4-diaminobutyric acid; DThr refers to D- Threonine; DPhe refers to D-Phenylalanine; Leu refers to Leucine; Abu refers to 2- aminobutyric acid; and residues 4-10 form the cyclic heptapeptide portion. The polymyxin derivative of formula (I) can also be represented as OA-Thr-DSer-cy[Dab- Dab-DPhe-Leu-Dab-Dab-Thr-], i.e. OA-SEQ ID NO. 2 or as OA-Dab-Thr-DThr-cy[Dab-Dab- DPhe-Leu-Abu-Dab-Thr-], i.e. OA-SEQ ID NO. 1. OA-SEQ ID NO. 1 is preferred.

Both NAB 815 and NAB 739 have a total number of positive charges at physiological pH of 3. For NAB 739 these positive charges are located at positions R5, R8 and R9 of formula (I), i.e. within the heptapeptide ring. For NAB 815 two of these positive charges are located within the heptapeptide ring at positions R5 and R9, and one is located on the hydrophobic tail at position R1.

As used herein the term "pharmaceutically acceptable salt" refers to salts with acids and bases which are known to be non-toxic and are commonly used in the pharmaceutical literature.

Examples of such salts are acid addition salts including carboxylate salts (e.g. formate, acetate, trifluoroacetate, propionate, isobutyrate, heptanoate, decanoate, caprate, caprylate, stearate, acrylate, caproate, propiolate, ascorbate, citrate, glucuronate, glutamate, glycolate, a-hydroxybutyrate, lactate, tartrate, phenylacetate, mandelate, phenylpropionate, phenylbutyrate, benzoate, chlorobenzoate, methylbenzoate, hydroxybenzoate, methoxybenzoate, dinitrobenzoate, o-acetoxybenzoate, salicylate, nicotinate, isonicotinate, cinnamate, oxalate, malonate, succinate, suberate, sebacate, fumarate, malate, maleate, hydroxymaleate, hippurate, phthalate or terephthalate salts), halide salts (e.g. chloride, bromide or iodide salts), sulfonate salts (e.g. benzenesulfonate, methyl-, bromo- or chloro- benzenesulfonate, xylenesulfonate, methanesulfonate, ethanesulfonate, propanesulfonate, hydroxyethanesulfonate, 1- or 2- naphthalene-sulfonate or 1 ,5-naphthalenedisulfonate salts) or sulfate, pyrosulfate, bisulfate, sulfite, bisulfite, phosphate, monohydrogenphosphate, dihydrogenphosphate, metaphosphate, pyrophosphate or nitrate salts, and the like.

A typically used acid for formation of the pharmaceutically acceptable salt of the polymyxin derivative of formula (I) is sulfuric acid. As used herein, the term "pharmaceutically acceptable derivative" means:

(a) pharmaceutically acceptable salts (as defined above); and/or

(b) solvates (including hydrates).

The at least one antibiotic resistance breaker (ARB) used in combination with the polymyxin derivative or pharmaceutically acceptable thereof is also a known compound and is typically commercially available or can be prepared by synthesis methods known in the art. For example, mefloquine, mefloquine hydrochloride, zidovudine, suloctidil and phenoxybenzamine hydrochloride are commercially available from Sigma Aldrich Limited. Suitable dosages and formulations are also known in the art.

The at least one antibiotic resistance breaker combined with the polymyxin derivative of formula (I) or a pharmaceutically acceptable salt thereof is preferably zidovudine or a pharmaceutically acceptable derivative thereof.

Zidovudine is for example known as azidothymidine (AZT) and is an antiretroviral medication used to prevent and treat HIV/AIDs. It is generally recommended for use with other antiretrovirals and has the following chemical structure:

Its lUPAC name is l-^/i^S.SSJ-^Azido-S-ihydroxymethy oxolan^-yll-S-methylpyrimidine- 2,4-dione, and it is available by prescription only under the trade name Retrovir®. Zidovudine is preferably used in its non-salt form.

Suloctidil is a sulfur-containing aminoalcohol that was brought to market in the early 1970s as a vasodilator. It has the following chemical structure:

Suloctidil is preferably used in its non-salt form.

Mefloquine is an orally administered medication used in the prevention and treatment of malaria. It is commercially available in Europe under the trade name Lariam® in tablet form. Lariam® tablets include the hydrochloride salt of mefloquine and are indicated for the therapy and prophylaxis of malaria.

Mefloquine is a chiral molecule with two asymmetric carbon centres, which means that it has four different stereoisomers. Mefloquine is commercially available as a racemate of the (R,S)- and (S,R)-enantiomers

In all aspects of the invention, the term "mefloquine" includes all enantiomers, tautomers and stereoisomers thereof. The corresponding enantiomers and/or tautomers may also be isolated/prepared by methods known in the art.

A preferred salt of mefloquine is the hydrochloride salt thereof, i.e. mefloquine hydrochloride.

Phenoxybenzamine is a non-selective, irreversible alpha blocker which is marketed under the trade name Dibenzyline®. It is used in the treatment of hypertension and has the following chemical structure:

Phenoxybenzamine is preferably used as the hydrochloride salt.

The compounds used in the present invention may be administered as the raw material but the active ingredients are preferably provided in the form of pharmaceutical compositions. The active ingredients may be used either as separate compositions or as a single combined composition. When combined in the same composition it will be appreciated that the two compounds must be stable and compatible with each other and the other components of the composition. Pharmaceutical compositions of the invention include those suitable for parenteral (including subcutaneous e.g. by injection or by depot tablet, intravenous, intradermal, intrathecal, intramuscular e.g. by depot and intravenous), rectal, vaginal and topical (including dermal, buccal and sublingual) or in a form suitable for administration by inhalation or insufflation administration. The most suitable route of administration may depend upon the condition and disorder of the patient.

These forms of administration comprise solutions or emulsions which may be injected intravenously, intraarterially, intrathecally, subcutaneously, intradermally, intraperitoneally or intramuscularly, and which are prepared from sterile or sterilisable solutions. The pharmaceutical compositions of the present invention may also be in form of suppositories, pessaries, suspensions, emulsions, lotions, ointments, creams, gels, sprays, solutions or dusting powders.

An alternative means of transdermal administration is by use of a skin patch. For example, the active ingredients can be incorporated into a cream consisting of an aqueous emulsion of polyethylene glycols or liquid paraffin. The active ingredients can also be incorporated, at a concentration of between 1 and 10% by weight, into an ointment consisting of a white wax or white soft paraffin base together with such stabilisers and preservatives as may be required.

Preferably, the compositions of the invention are formulated for intravenous administration.

The pharmaceutical compositions may conveniently be presented in unit dosage form and may be prepared by any of the methods well known in the art of pharmacy e.g. as described in "Remington: The Science and Practice of Pharmacy", Lippincott Williams and Wlkins, 21^st Edition, (2005).

Suitable methods include the step of bringing into association the active ingredients with a carrier which constitutes one or more excipients. In general, compositions are prepared by uniformly and intimately bringing into association the active ingredients with liquid carriers or finely divided solid carriers or both and then, if necessary, shaping the product into the desired formulation. It will be appreciated that when the two active ingredients are administered independently, each may be administered by a different means. When formulated with excipients, the active ingredients may be present in a concentration from 0.1 to 99.5% (such as from 0.5 to 95%) by weight of the total mixture; conveniently from 30 to 95% for tablets and capsules and 0.01 to 50% (such as from 3 to 50%) for liquid preparations. The amount of active ingredients required for use in treatment will vary with the nature of the condition being treated and the age and condition of the patient, and will ultimately be at the discretion of the attendant physician or veterinarian. In general however, doses employed for adult human treatment will typically be in the range of 0.02 to 5000 mg per day, preferably 1 to 1500 mg per day. The desired dose may conveniently be presented in a single dose or as divided doses administered at appropriate intervals, e.g. as two, three, four or more sub-doses per day.

Biological Tests

Test procedures that may be employed to determine the biological (e.g. bactericidal or antimicrobial) activity of the active ingredients include those known to persons skilled in the art for determining:

(a) bactericidal activity against clinically latent bacteria; and

(b) antimicrobial activity against log phase bacteria.

In relation to (a) above, methods for determining activity against clinically latent bacteria include a determination, under conditions known to those skilled in the art (such as those described in Nature Reviews, Drug Discovery 1 , 895-910 (2002), the disclosures of which are hereby incorporated by reference), of Minimum Stationary-cidal Concentration ("MSC") or Minimum Dormicidal Concentration ("MDC") for a test compound.

By way of example, WO2000028074 describes a suitable method of screening compounds to determine their ability to kill clinically latent bacteria. A typical method may include the following steps:

(1) growing a bacterial culture to stationary phase;

(2) treating the stationary phase culture with one or more antimicrobial agents at a concentration and or time sufficient to kill growing bacteria, thereby selecting a phenotypically resistant sub-population;

(3) incubating a sample of the phenotypically resistant subpopulation with one or more test compounds or agents; and (4) assessing any antimicrobial effects against the phenotypically resistant subpopulation.

According to this method, the phenotypically resistant sub-population may be seen as representative of clinically latent bacteria which remain metabolically active in vivo and which can result in relapse or onset of disease.

In relation to (b) above, methods for determining activity against log phase bacteria include a determination, under standard conditions (i.e. conditions known to those skilled in the art, such as those described in WO 2005014585, the disclosures of which document are hereby incorporated by reference), of Minimum Inhibitory Concentration ("MIC") or Minimum Bactericidal Concentration ("MBC") for a test compound.

Specific examples of such methods are described below.

Examples

Example 1 : In vitro activity of zidovudine combined together with NAB739 and NAB815 against various Gram-negative bacteria using the chequerboard method The chequerboard method described in Antimicrob Chemo (2013) 68, 374-384 was carried out in Example 1 to investigate the in vitro activity of zidovudine combined together with NAB739 or NAB815 against various Gram-negative bacteria.

Log phase growth of various Gram-negative bacterial strains was carried out as known in the art. The strains were as follows:

- E.coli (ESBL)

K.pneumoniae (ESBL)

MDR P. aeruginosa

NDM-1 strains of Enterbacter cloacae (1 strain); Escherichia coli (2 strains); and Klebsiella pneumoniae (4 strains);

- MCR-1 E.coli.

Zidovudine was obtained from a commercial source.

NAB815 was synthesized according to the method disclosed in Example 1 of WO20161 13470 (A1)

NAB739 was synthesized according to the method disclosed in Example 1 of WO2008017734 (A1). The log phase bacterial cultures were incubated with each of the combinations overnight by diluting the cultures with nutrient broth (Oxoid) to 10⁷ CFU/ml, and adding 280 μΙ of each culture to each well to make a final concentration of 300 μΙ. Incubation of the combinations with the bacterial suspensions was carried out for 24 hours. The zidovudine concentration ranged from 64 mg/L to 1 mg/L and the compound concentrations ranged from 16 mg/L to 0.02 mg/L.

The effects of each combination of the present invention against the above bacteria strains were examined by calculating the fractional inhibitory concentration index (FICI) of each combination, as follows: (MIC of drug A, tested in combination)/(MIC of drug A, tested alone)+(MIC of drug B, tested in combination)/(MIC of drug B, tested alone).

The interaction of the combination was defined as showing synergy if the FICI was≤0.5, no interaction if the FICI was >0.5 but <4.0 and antagonism if the FICI was >4.0.

The results are shown in the table below.

Tota l no. (%) of strains when Z combined with

Combination

Strains FICI NAB815 NAB739

Activity

E. coli (ESBL) Synergy <0.5 15 (65%) 19 (83%)

Indifferent >0.5 <4 8 (35%) 4 (17%)

Antagonism >4 0 0

K. pneumoniae

Synergy <0.5

(ESBL) 17 (55%) 19 (61%)

Indifferent >0.5 <4 14 (45%) 12 (39%)

Antagonism >4 0 0

MDR P. aeruginosa Synergy <0.5 2 (67%) 3 (100%)

Indifferent >0.5 <4 1 (33%) 0

Antagonism >4 0

NDM-1 strains Synergy <0.5 7 (100%) 7 (100%)

Indifferent >0.5 <4 0 0

Antagonism >4 0 0 mcr-1 E. coli Synergy <0.5 11 (85%) 12 (92%)

Indifferent >0.5 <4 2 (15%) 1 (8%)

Antagonism >4 0 0

As noted hereinabove, the NDM-1 strains were Enterobacter cloacae (1 strain), E.coli (2 strains) and K.pneumoniae (4 strains). The combination of zidovudine with NAB815 or NAB739 surprisingly exhibited synergistic activity against each of these drug-resistant strains.

Synergy was also observed for the majority of combinations of zidovudine and NAB815 against the E.coli (ESBL) strain (65%), the K.pneumoniae (ESBL) strain (55%), the MDR P. aeruginosa strain (67%) and the MCR-1 E.coli strain (85%). Importantly, there were no combinations which showed antagonism against any of the drug- resistant Gram-negative bacterial strains.

A similar pattern can be seen for the combination of zidovudine and NAB739. In fact, slightly higher percentages for synergy are seen for this combination: 83% against the E.coli (ESBL) strain, 61 % against the K.pneumoniae (ESBL) strain, 100% against the MDR P. aeruginosa strain and 92% against the MCR-1 E.coli strain.

As would be appreciated by the skilled person, there is no way of predicting what will happen when two drugs are co-administered. Synergy is not an expected or guaranteed result. The synergy seen between an antibiotic resistance breaker such as zidovudine and the polymyxin derivative of formula (I) as defined hereinabove is therefore both a surprising and advantageous technical effect and means that the combinations of the present invention could provide a useful solution to the problem of antimicrobial resistance.

Claims

1. A combination comprising a polymyxin derivative of formula (I):

_: 7 t*)R«

wherein: R1 is Dab;

R2 is Thr;

R3 is DThr;

R4 is Dab;

R5 is Dab;

R6 is DPhe;

R7 is Leu;

R8 is Abu;

R9 is Dab;

R10 is Thr; and

R(FA) is octanoyi; or wherein: R1 is absent;

R2 is Thr;

R3 is DSer;

R4 is Dab;

R5 is Dab;

R6 is DPhe;

R7 is Leu;

R8 is Dab;

R9 is Dab;

R10 is Thr; and

R(FA) is octanoyi; or pharmaceutically acceptable salts thereof; and at least one antibiotic resistance breaker selected from the group consisting of zidovudine, suloctidil, mefloquine, phenoxybenzamine or a pharmaceutically acceptable derivative thereof.

2. The combination of claim 1 , wherein R1-R10 is SEQ ID NO 1.

3. The combination of claim 1 , wherein R1-R10 is SEQ ID NO 2.

4. The combination of claim 1 , wherein for the polymyxin derivative of formula (I) or a pharmaceutically acceptable salt thereof, R1 is absent; R2 is Thr; R3 is DSer; R4 is Dab; R5 is Dab; R6 is DPhe; R7 is Leu; R8 is Dab; R9 is Dab; R10 is Thr; and R(FA) is octanoyl.

5. The combination of claim 1 , wherein for the polymyxin derivative of formula (I) or a pharmaceutically acceptable salt thereof, R1 is Dab; R2 is Thr; R3 is DThr; R4 is Dab; R5 is Dab; R6 is DPhe; R7 is Leu; R8 is Abu; R9 is Dab; R10 is Thr; and R(FA) is octanoyl.

6. The combination of any one of claims 1 to 5, wherein the at least one antibiotic resistance breaker is zidovudine or a pharmaceutically acceptable derivative thereof.

7. The combination of any one of claims 1 to 5, wherein the at least one antibiotic resistance breaker is suloctidil or a pharmaceutically acceptable derivative thereof.

8. The combination of any one of claims 1 to 5, wherein the at least one antibiotic resistance breaker is mefloquine or a pharmaceutically acceptable derivative thereof.

9. The combination of any one of claims 1 to 5, wherein the at least one antibiotic resistance breaker is phenoxybenzamine or a pharmaceutically acceptable derivative thereof.

10. The combination of claim 9, wherein the antibiotic resistance breaker is phenoxybenzamine hydrochloride.

11. A pharmaceutical composition comprising an effective amount of a polymyxin derivative of formula (I) as defined in any one of claims 1 to 5, at least one antibiotic resistance breaker as defined in any one of claims 1 and 6 to 10, and at least one pharmaceutically acceptable adjuvant, diluent or carrier.

12. The combination as defined in any one of claims 1 to 10 or the pharmaceutical composition as defined in claim 1 1 for use in treating a bacterial infection.

13. The combination for use or the pharmaceutical composition for use according to claim 12, wherein the bacterial infection is caused by Gram-negative bacteria.

14. The combination for use or the pharmaceutical composition for use according to claim 13, wherein the Gram-negative bacteria are drug-resistant, preferably colistin-resistant.

15. A method of treating a bacterial infection, comprising administering the combination according to any one of claims 1 to 10, or the pharmaceutical composition according to claim 1 1 , to a subject in need thereof.

16. The method according to claim 15, wherein the bacterial infection is caused by Gram negative bacteria.

17. The method according to claim 16, wherein the Gram-negative bacteria are drug- resistant, preferably colistin-resistant.

18. Use of the combination according to any one of claims 1 to 10, or the pharmaceutical composition according to claim 11 for the manufacture of a medicament for the treatment of a bacterial infection.

19. The use according to claim 18, wherein the bacterial infection is caused by Gram- negative bacteria.

20. The use according to claim 19, wherein the Gram-negative bacteria are drug-resistant, preferably colistin-resistant.