Polymyxins

CLASS

Chemical Structure

Polymyxins are a group of polypeptide antibiotics which were discovered in 1947 (185). They are produced by fermentation in different species of Paenibacillus polymyxa (previously Bacillus polymyxa) by nonribosomal peptide synthetase enzyme complexes (26, 82). They are pentacationic polypeptides consisting of a cyclic heptapeptide, a linear tripeptide and a fatty acid tail linked to the N-terminal of the tripeptide (86). They are formed by both cationic and hydrophobic amino acids (82).  The five L-diaminobutyric acid (L-Dab) molecules are positively charged in positions 1,3,5,8 and 9.

Polymyxins included historically 5 main different chemical compounds (polymyxin A, B, C, D, and E). The main differences between these compounds are found in amino acid sequences and fatty acid side chains. The major representatives of polymyxins that have been used in clinical practice are polymyxin B and polymyxin E (colistin). The main difference between these two molecules is that polymyxin B contains phenylalanine in position 6 (starting from the N-terminal), while colistin contains D-leucine (Figure 1a and 1b) (100, 185,136).  Polymyxin B preparations consist mainly of two major components, polymyxin B1 and B2 in addition to a small amount of other polymyxins B. Similarly, colistin consists mainly of polymyxin E1 (colistin A) and polymyxin E2 (colistin B) and fewer amounts of other polymyxins E (82).  The proportion of these components may vary between different pharmaceutical preparations. At least 30 different components have been detected in colistin (36, 144).  These components differ only in the fatty acid side chain (106).

Polymyxin B was developed from P. polymyxa. Its tripeptide side chain is linked to the fatty acid residue, which has been identified as 6-methyl-octan-oic acid (polymyxin B1) or 6-methyl-eptanoic acid (polymyxin B2). Colistin was developed from P. polymyxa subsp. Colistinus (95, 96). Its tripeptide side chain is linked to the fatty acid residue, which has been identified as 6-methyl-octan-oic acid (colistin A) or 6-methyl-eptanoic acid (colistin B) (185).  Besides the 5 main compounds, several other naturally produced polymyxins have been discovered. The main representatives are polymyxin M (179) polymyxin S, and polymyxin T (122, 177, 178).  They are produced by different species and differ from polymyxin B and colistin in the amino acids included in their molecule; in addition, polymyxin M and T are also active against Gram positive bacteria.

Novel polymyxins have been developed by enzymatic cleavage of the fatty acid and the first L-α-γ-diaminobutyric acid (nonapeptides) or the entire linear tripeptide (heptapeptides), introduction of benzyl groups, or reduction of the positively charged Dab molecules from five to three (207, 209).  In derivatives by enzymatic cleavage, although binding to lipopolysacharides (LPS) was preserved, the loss of the fatty acid resulted in lesser toxicity, suggesting a key role of the fatty acid for the antibacterial activity of polymyxins (143, 169).  Derivatives with longer (nine or more carbons) or shorter (six or less carbons) fatty acid chains had in general lesser activity against Gram negative bacteria in additional experiments.  Nonapeptides were also associated with fewer adverse events than their parent molecules. The introduction of benzyl groups enhanced antimicrobial activity against Gram positive bacteria, but reduced the activity against Escherichia coli(210).  The reduction of the cationic Dab components resulted in molecules with different or no antibacterial activity, which was correlated with the location of the positive charges (200, 201, 202,203).

ANTIMICROBIAL ACTIVITY

Spectrum

Most GNB are susceptible to polymyxins, including multi-drug resistant (MDR) Acinetobacter baumannii and Pseudomonas aeruginosa, Klebsiella spp.>, Enterobacterspp., Escherichia coli, Salmonella spp., Shigella spp., Citrobacter spp., Yersinia pseudotuberculosis, and Haemophilus influenzae (55, 77, 139).  Increase in inoculum size decreases the antibacterial activity of colistin; its effects are not altered in the presence of plasma. Using colistin sulfate, Wright and Welch (1959) demonstrated a greater degree of activity of this compound than of polymyxin B against Pseudomonas, Salmonella, Shigella, and members of the coli-aerogenes group (211).   Colistin has also been shown to possess a considerable in vitro activity againstStenotrophomonas maltophilia strains (83-88% of the tested isolates were sensitive to colistin in two studies) (55, 77, 139).   It has also been reported that colistin is potentially active against several mycobacterial species including M. xenopi, M. intracellulare, M. tuberculosis, M. fortuitum, M. phlei, and M. smegmatis (159, 130, 161). Several pathogens such as Proteus spp.,Providencia spp., Morganella spp., and most isolates of Serratia spp., isolates of Brucella spp., Neisseria spp., Chromobacterium spp., and Burkholderia spp. exhibit intrinsic resistance to the polymyxins. Finally, polymyxin B was found to be active against Cryptococcus neoformans (215).

In Vitro Colistin Susceptibility

The disk diffusion method (DDM) using 10 micrograms of colistin sulfate disk (Oxoid, Basingstoke, Hants, England) or 30 micrograms polymyxin B disk, has been a common method for susceptibility testing of colistin and polymyxin B, respectively. Isolates are considered sensitive if the inhibition zone is ≥ 11 mm. However, recent studies showed that the Etest, broth microdilution method, agar dilution and VITEK 2 are preferable tests compared to the DDM for the determination of in vitro colistin susceptibility (54, 166, 181).  Two studies reported that there is general agreement in the results obtained from agar dilution and broth microdilution methods regarding testing of polymyxins (189, 205).  Reports showing an 87% agreement between agar dilution and Etest and 82% between agar dilution and Vitek 2 suggest that both methods require at least confirmation by a standard MIC susceptibility testing method.

When DDM was compared with six other methods of colistin susceptibility testing (agar dilution on Mueller-Hinton [MH] and Isosensitest agar, Etest on MH and Isosensitest agar, broth microdilution, and VITEK 2) on clinical isolates from intensive care unit patients, it was found to be an unreliable method to measure susceptibility to colistin (114).  High error rates and low levels of reproducibility were observed in the DDM as well. On the contrary, the colistin Etest, agar dilution, and the VITEK 2 showed a high level of agreement with the broth microdilution reference method. Heteroresistant to colistin populations of Enterobacter cloacae and A. baumannii were reported in this study; all methods besides VITEK 2 were able to detect heteroresistant subpopulations of E. cloacae (114).

Both the Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) provide breakpoints for colistin susceptibility for P. aeruginosa, A. baumannii, and Enterobacteriaceae. Susceptibility break points refer to colistin sulfate. Only CLSI provides breakpoints for polymyxin B, which are the same as for colistin. Several reports has shown nearly complete agreement of susceptibility results for colistin and polymyxin B (55, 205).  The general MIC breakpoint to identify bacteria susceptible to polymyxins is≤ 2 μg/ml (27, 46).  The main difference is that CLSI defines as susceptible those P. aeruginosa strains with an MIC ≤2 μg/ml, while EUCAST those with an MIC ≤ 4μg/ml. Table 1 shows the current breakpoints provided from the CLSI and EUCAST for the aforementioned bacteria.

Currently,  A. baumannii, P. aeruginosa and Enterobacteriaceae (those not inherently resistant to polymyxins) remain susceptible to polymyxins. K. pneumoniae may be an exception in a few areas of the globe (Asia-Pacific, South America, Mediterranean countries).

Mechanisms of Action and Structure-Activity Relationship

Polymyxin B and colistin share the same mechanism of action. The L-Dab molecules contained in polymyxins are positively charged, while the lipopolysaccharides (LPS) present in the cell wall of Gram-negative bacteria, are negatively charged. Polymyxins possess a higher affinity for LPS molecules of the outer cell membrane of Gram-negative bacteria than do divalent cations such as magnesium (Mg+2) and calcium (Ca+2) which normally stabilize the LPS molecules (35, 136, 175).   Polymyxins bind to the Gram-negative bacterial cell membrane phospholipids, producing a disruptive physicochemical effect, which leads to cell-membrane permeability changes and ultimately cell death (47). The initial association of polymyxins with the bacterial membrane occurs through electrostatic interactions between the cationic polypeptide and anionic LPS molecules in the outer membrane of the Gram-negative bacteria. Polymyxins displace magnesium (Mg+2) and calcium (Ca+2), from the negatively charged LPS, leading to a local disturbance of the outer membrane. The result of this process causes an increase in the permeability of the cell envelope consisting of the cell wall and the cytoplasmic membrane, leakage of cell contents, and subsequently cell death (35, 136).  However, high concentrations of calcium and/or magnesium can antagonize the binding of polymyxins with LPS molecules (35, 68).  It has been suggested that this detergent effect may render Gram negative bacteria more susceptible to other antimicrobials following exposure to polymyxins.

A secondary target site for polymyxins is the type II NADH-quinone oxydoreductase respiratory enzyme of the inner bacterial membrane. These enzymes are an integral part of the bacterial electron transport pathway. Newer studies show that polymyxins inhibit these enzymes in both Gram positive and Gram negative bacteria (39).

In addition, polymyxins exert anti-endotoxin activity by binding to and neutralizing endotoxins of Gram negative bacteria. The significance of this mechanism (namely the prevention of the endotoxin's ability to induce shock through the release of cytokines) for in vivo antimicrobial action is not clear, since plasma endotoxin is immediately bound by LPS-binding protein and the complex quickly bound to cell-surface CD14 (61). 

MecHANISMS OF RESISTANCE

Organisms Inherently Resistant

Proteus spp., Neisseria spp., Serratia spp., Morganella spp., and Providencia spp. are resistant to polymyxins. Gram-positive bacteria are, in the main, also unaffected. Pseudomonas mallei,Burkholderia cepacia, Edwardsiella spp., and Brucella spp. are all resistant to polymyxins. In addition, polymyxins are not active against all anaerobes, fungi, and parasites (25, 185).  Most of the data regarding intrinsic resistance come from studies in Proteus spp, which possess a cell envelope that prevents polymyxins from accessing the susceptible lipid target sites or changes in lipid A that account for reduced binding (100, 101, 142, 186).   Ιncreased polymyxin susceptibility in B. cepacia and P. mirabilis, has been associated with defects in UDP-glucose dehydrogenase and UDP-glucose phosphorylase, enzymes involved in the biosynthesis of the LPS precursor, UDP-glucose (85).

In addition, sterol-like compounds called hopanoids in the outer membrane of Burkholderia multivorans create a barrier, and they have been associated with intrinsic polymyxin resistance (117).   Finally, the periplasmic proteases found in B. cenocepacia and the multidrug efflux pump NorM in Burkholderia vietnamien have been shown to contribute to polymyxin resistance (51, 115,116).

Mechanisms of Acquired Resistance

De novo resistance to polymyxins in Gram-negative bacteria can develop through either mutation or adaptation mechanisms. Mutation is inherited, low-level, and independent of the continuous presence of the antibiotic, whereas adaptation is the opposite (non-inherited, high level, and requires the continuous presence of the antibiotic). Almost complete cross-resistance exists between colistin and polymyxin B (62, 65,133).

Modifications of the LPS moiety, in addition to further alterations of the outer bacterial membrane, are probably the most important mechanism conferring resistance to polymyxins (22, 89, 133). These modifications are a result of a gene mediated transcription pathway leading to the production of 4-amino-4-deoxy-l-arabinose (LAra4N) and its addition to lipid A. This causes an increase of the absolute charge of lipid A that lowers the affinity for polymyxins (89).  The major gene implicated in this pathway is called arn (formerly known as pmr – polymyxin resistance operon) and the whole process is mediated by the PmrA/PmrB and PhoP/PhoQ regulatory systems with two components: a sensor cytoplasmic membrane kinase (PmrB or PhoQ) which upon activation phosphorylates the regulator component (PmrB or PhoP) (163). These in turn promote arn transcription (49, 66, 124, 134).  These genes were first studied in S. enterica serotype Typhimuriumand E. coli (66).  However, similar pathways, that may implicate more regulatory genes, exist in other gram-negative bacteria, such as P. aeruginosa, B. cepacia, Salmonella spp., Yersinia pestis and K. pneumonia (21, 23). These mechanisms, although common in gram-negative bacteria, exhibit interspecies variation.

It has been suggested that another mechanism of resistance in P. aeruginosa is that the outer membrane protein OprH blocks the self-promoted uptake pathway of polymyxins by replacing Mg+2on the LPS molecule. Thus, the overexpression of OprH caused by mutation or as a result of adaptation to a Mg+2-deficient medium can be associated with resistance to polymyxins (137).   Furthermore, increased levels of the outer membrane protein H1 inhibits the action of polymyxins by replacing Mg2+ or Ca2+ at the binding sites on LPS (11, 138).   Cell surface changes were also related with the development of resistance. These include alterations of the outer membrane of the bacteria cell (reduction of LPS, reduced levels of specific outer membrane proteins, reduction in cell envelope Mg+2 and Ca+2 contents, and lipid alterations). Specifically, the absence of 2-hydroxylaurate or an increase in the palmitate content of lipid A has been associated with resistance of P. aeruginosa to polymyxins (38).  In biofilms of P. aeruginosa, colistin resistance coincided with the over expression of the mexAB–oprM efflux pump system (148).

In A. baumannii, resistance mechanisms are not well understood. However, mutations in the PmrA/PmrB encoding genes are linked to polymyxin resistance (1). Another surprising polymyxin resistance mechanism reported in A. baumannii involves complete loss of LPS production (130).  In order to compensate for the decreased outer membrane integrity due to the LPS loss, polymyxin-resistant A. baumannii strains upregulated the expression of genes responsible for phospholipid, lipoprotein and poly-β-1,6-N-acetylglucosamine production (75).

In K. pneumoniae strains the increased production of capsule polysaccharide (CPS) limits the interaction of polymyxins with their target sites. Thus, upregulation of CPS production contributes to increased polymyxin resistance (22). In K. pneumoniae, a deficiency in the outer membrane protein OmpA, which mediates adhesion to eukaryotic cells, has been also associated with an increased susceptibility to polymyxin B (113). Moreover, the AcrAB–TolC efflux pump has been associated with polymyxin resistance in both K. pneumoniae and E. coli (147, 208).

An efflux pump/potassium system has been found to be associated with the mediation of resistance to cationic antimicrobial peptides in general, including polymyxin B in Yersinia spp.

Methods to Overcome or Prevent Resistance

Treatment with a combination of antibiotics has been long proposed as a mechanism to reduce resistance. A few in vitro studies on the synergistic activity of carbapenems and polymyxins provided evidence that this can be achieved (218). However, this data require confirmation in clinical studies. To date, no studies have focused on the effect of the dosage and/or duration of treatment with polymyxins on the development of resistance. However, studies had shown that low concentrations of colistin have been associated with amplification of the resistant subpopulations in hetero-resistant strains (13, 41, 157).

Ecological Considerations

Colistin resistance among Gram-negative microorganisms has been reported in several in vitro studies, as well as in surveillance studies for antimicrobial resistance (58, 154).  In most of these studies the isolated strains of P. aeruginosa, K. pneumoniae, and A. baumannii exhibited a greater than 98% susceptibility to colistin or polymyxin B (168). There are only few clinical reports in the current literature regarding infections due to P. aeruginosa with in vitro resistance to colistin (38, 188).  Colistin resistant P. aeruginosa was found in 5 of 150 children with cystic fibrosis over a 5-year period of follow up (188).  However, reports of polymyxin-resistant K. pneumoniae infections are increasingly published (24, 45,123).

Isolation of colistin-resistant gram negative bacteria has been associated with colistin use in several studies (92, 123).  Recently, reports have been published regarding the potential selection of strains inherently resistant to polymyxins following the increasing consumption of polymyxins worldwide. These included strains of B. cepacia in Czech Republic, P. stuartii in the USA, and Enterobacteriaceae inherently resistant to colistin in Greece (69, 72, 92, 172).Breakthrough infections due to such organisms were also reported (40).

PHARMACOKINETICS  AND PHARMACODYNAMICS

Following the worldwide spread of extensively drug resistant (XDR) Gram negative bacteria in the early years of the 21st century, for which limited treatment options were available, the pharmacokinetic (PK) and pharmacodynamic (PD) properties of polymyxins were studied using more rigorous scientific methods. It was only recently that their PK properties were properly delineated. The following data refer to parenteral administration, since polymyxins are not absorbed through the gastrointestinal tract or the intact skin. Since colistin is the most widely used antibiotic, most data refer to it. The known differences between polymyxin B and colistin will be then shortly described (Table 2).

Distribution, Protein Binding, and Elimination

It is important to note that colistin is administered in its inactive prodrug form, colistimethate sodium (CMS). Following intravenous administration, CMS displays linear dose dependent PK properties, and approximately 60% of the dose is cleared through the kidneys. Colistin on the other hand is mainly (>99%) non-renally excreted by means of mechanisms not yet fully understood (110) while the evidence suggests extensive renal tubular re-absorption (107).   Despite that, colistin urinary concentration are high due to conversion of CMS to colistin in the urine. Both CMS and colistin are eliminated by venovenous hemofiltration and hemodialysis (29, 118).

In vitro 31.2% of CMS was hydrolyzed to colistin in 4 hours at 37oC in human plasma (108) while in vivo a smaller proportion of CMS is hydrolyzed to colistin (109, 119). This arises mainly from the fact that CMS renal clearance is higher than conversion clearance to colistin, especially in patients with normal renal function. Thus it is estimated that in such patients only up to 25% of CMS is converted to active colistin (135).  Furthermore, there is inter-individual variability in conversion of CMS to colistin at a given creatinine clearance (a ten-fold range in the steady state of plasma colistin concentration has been observed), that arises from the inefficiency of CMS as a pro-drug and the probability of batch-to-batch variability in the composition of the administered formulation (135). In renal failure, the renal excretion of CMS is decreased resulting in a greater conversion to colistin (67).

Thus, following intravenous administration, colistin plasma concentration increases slowly due to the slow conversion of CMS to colistin. In ICU patients with variable renal function treated with colistin 3x106 IU or 240mg every 8 hours, approximately 7 hours were required to reach peak plasma concentrations after the first dose. The steady state mean plasma concentration was 2μg/ml, however at least 3 doses were required to achieve these values. Fluctuation of mean plasma concentrations was limited as a result of the long half-life and the selected dosing interval. The elimination half-life was 14.4 hours (2.3 hours for CMS) (155).  The administration of a loading dose (6x106 IU or 480 mg) resulted in faster achievement of bactericidal concentrations in critically ill patients compared to patients who did not receive a loading dose (131).  Still, this took several hours (4 hours). It is suggested that a CMS loading dose according to body weight (the lower of either actual or ideal body weight) should be considered but should not exceed 10x106 IU or 800mg. Similarly, maintenance daily dose should not exceed 10x106 IU (29).

Tissue binding is a dominant factor in the distribution and elimination of polymyxins. Old experimental studies have shown that polymyxins accumulate in liver, lung, kidney, heart, and muscles (97). At 24h, less than 50 % of the doses of polymyxins were bound to tissues; the largest amount was in the skeletal muscle (31).  A plasma protein binding of 55% was reported for colistin in experimental animal studies following intravenous administration (3, 107).  Additionally, higher protein binding for colistin A compared with colistin B was observed, probably associated with the different fatty acid residue (107).

Intravenous colistin achieves low concentrations to the cerebrospinal fluid and in general, meningeal or systemic inflammation does not enhance their central nervous system (CNS) penetration to adequate therapeutic levels (76, 196).  In both experimental and human studies in patients with CNS infections, the cerebrospinal fluid (CSF) to plasma ratio was very low (around 5%), although steady in time, suggesting probably inadequate bactericidal concentrations in the CSF (8, 80, 120).  The reasons for that could be the higher than optimal molecular weight of colistin to cross the blood-brain barrier, chemical properties of colistin that do not favor passive permeability, and the presence of active efflux pumps (P-glycoproteins) (80, 81).  When CMS was administered intrathecally/intraventricularly, the colistin trough levels were consistently above 2μg/ml if the administered daily dose was over 65000 IU, but there was still great inter-patient variability (8, 78).   If CMS was administered both intravenously and intraventricularly, mean CSF colistin concentrations were higher than intravenous or intraventricular alone (217).

Polymyxin B is administered as its sulfate salt which is the active antibiotic. Thus conversion is not required. It is eliminated mainly by non-renal pathways. As a result, polymyxin B concentration in the urine is low (214). The total body clearance appears to be relatively insensitive to renal function (173).  Inter-individual variability in plasma concentration is low (173).  Its serum half-life depends on the age of the patient and ranges from 3.1 (in neonates) to 13.6 hours (47, 98, 99, 170, 214).  The unbound fraction in plasma is approximately 40%.  A loading dose seems to improve significantly its PK parameters (173).  Recently, the liposomal form of polymyxin B has been developed, which was found to achieve higher concentrations in the epithelial lining fluid and better effectiveness than aqueous formulations in neutropenic mice models (73).

Pharmacodynamic Effects

Colistin shows concentration dependent, rapid bactericidal activity against P. aeruginosa, K. pneumoniae and A. baumannii in in vitro static kill studies (17, 145, 157).  However, regrowth occurs even at the highest concentrations (64μg/ml), most likely due to heteroresistance. In addition, bactericidal activity diminishes at higher inocula (15, 16, 17).   Dynamic kill studies in P. aeruginosa simulating once, twice and three times daily administration of the same colistin dose showed that, all regimens rapidly killed bacteria, regrowth occurred in all, but the thrice daily regimen delayed the development of resistance (17, 187).  Similar findings were observed for polymyxin B (13, 17).  However, in A. baumannii models, no difference was observed between the colistin regimens in development of resistance (17, 190).

Colistin produces a moderate post-antibiotic effect (PAE) against P. aeruginosa and K. pneumoniae strains at high concentrations (2-3 hours) (106, 157).   In A. baumannii studies, both negative and significant (up to 8h) PAE has been reported based upon the exposure time to colistin (145, 146, 156).  The clinical utility of the colistin PAE in is unclear, since colistin has a relatively long half-life compared to the current dosing schedules and the fluctuation of its plasma concentrations is low (17).

The PK/PD parameter that best predicts the activity of colistin is the free AUC/MIC. In an in vitro model against P. aeruginosa colistin activity was best correlated with the AUC/MIC ratio of total and unbound colistin. This index seemed to be superior to Cmax/MIC ratio. Following this finding, it has been suggested that time-averaged exposure to colistin is a more important target than the achievement of high peak concentrations (14). Accordingly, in vivo lung and murine models showed that colistin activity best correlated with free AUC/MIC against A. baumannii and P. aeruginosa (41, 42).

In Vitro Synergistic Activity

As already stated, polymyxins act by increasing the permeability of the bacterial cell membrane, thus they could be synergistic with other antimicrobial agents by facilitating their entrance into the bacterial cell. Therefore, the possibility of synergistic activity of polymyxins with other antimicrobial agents against Gram-negative bacteria has been explored in several studies. Although the synergy testing methods are not standardized and their results are not readily reproducible or comparable (191), several studies showed that synergism seems to exist but not in all isolates; this depends on bacterial species, resistance pattern and the combination of studied antibiotics. In general, time-kill studies showed the higher synergistic activity, followed by Etest and microdilution methods (218).   Combinations showed synergistic activity against resistant strains to at least one of the studied antibiotics or the antibiotic with the higher MICs, but not in susceptible strains. In a systematic review regarding synergy of carbapenems with polymyxins, synergy was higher for A. baumannii (77%), followed by P. aeruginosa (50%) and K. pneumoniae (44%). In addition, doripenem showed high synergy for all bacteria, while meropenem was more synergistic for A. baumannii and imipenem for P. aeruginosa (218).

Synergistic activity has been shown for KPC producing K. pneumoniae strains with rifampicin, tigecycline, fosfomycin, vancomycin and carbapenems for polymyxin resistant strains, and with carbapenems and fosfomycin for polymyxin susceptible strains (53, 103, 149, 171, 182, 206).   On the contrary, in VIM producing strains antagonism with carbapenems was shown in colistin-resistant strains and synergism for colistin susceptible strains (183).  Finally, in a study with NDM Enterobacteriaceae colistin showed minimal or no synergistic activity with fosfomycin and tigecycline for colistin susceptible and colistin resistant strains, respectively (10).

Synergistic activity of colistin with ceftazidime was also noted in an in vitro study performed in two MDR P. aeruginosa strains (64).  Carbapenems, fosfomycin and rifampin have also shown in vitro synergistic activity with polymyxins to a greater or lesser extent for colistin susceptible or resistant strains of P. aeruginosa (74, 171, 150, 192).  The combination of colistin, rifampin, and amikacin was found to be synergistic in vitro (20) and led to treatment success in an immunosuppressed patient with multiple abscesses of the lungs, perineum, and gluteus due to MDRPseudomonas aeruginosa (193).  On the other hand, vancomycin and trimethoprim-sulfamethoxazole did not show synergy with colistin for P. aeruginosa (206).

The addition of rifampin to colistin was also found to have in vitro synergistic bactericidal activity against MDR strains A. baumannii  (59, 191, 151). In addition, synergy was reported between polymyxin B and imipenem, polymyxin B and rifampin, and polymyxin B, imipenem, and rifampin against MDR A. baumannii strains (213). Synergistic effectiveness of colistin with meropenem and sulbactam was also shown (104).  In this study bacterial regrowth at 24 hours was observed when colistin was combined with meropenem, but not when it was combined with sulbactam or both of them.

Regarding MDR S. maltophilia strains, in vitro synergy of colistin with rifampin and to a lesser extent of colistin with trimethoprim/sulfamethoxazole was documented (60). The combination of polymyxin B with neomycin demonstrated, in vitro, a synergistic effect against a Salmonella enteritidis strain (63).

Overall, these studies provided valuable data rearding the polymyxin containing combination regimens. First, regrowth consistently seen in both colistin and polymyxin B monotherapy was eliminated by combination. This could reduce the possibility for selection of resistant subpopulations or de novo development of resistance. Second, synergy was more evident in polymyxin-resistant strains. Third, bactericidal activity was observed even at subinhibitory colistin concentrations in susceptible strains (100).

DosAGE

Adults

Two different forms of colistin are available for clinical use: 1) colistin sulfate which is administered orally for bowel decontamination and topically as a powder for the treatment of bacterial skin infections, and 2) colistimethate sodium (CMS) (also called colistin methanesulfate, pentasodium colistimethanesulfate, and colistin sulfonyl methate) for parenteral (intravenous, intramuscular, aerosolized and intrathecal/intraventricular) therapy. CMS has been less toxic and with fewer adverse events than colistin sulfate (7, 18).  Commercially polymyxin B is available as the sulfate salt. Polymyxin B sulfate is available for parenteral (intravenous and intramuscular), topical (ophthalmic and otic instillation), and intrathecal use.

CMS is available commercially in several formulations that contain different amounts of colistin A, colistin B and other components.  In general, 1 mg of CMS equals to 12.500 IU CMS; this means that 80 mg is equal to 1 million IU CMS. Furthermore, 1 mg of colistin base is contained in 2.4 mg or 30.000 IU of CMS. The intravenous preparations of colistin contain only the dry antibiotic powder, which is dissolved in 0.9% sodium chloride 5% or dextrose in water solution or water for injection before injection. The solution can be administered slowly over 3-5 minutes, 15-30 minutes or continuously over 24 hours (126).

The currently recommended intravenous doses for CMS in adult patients with normal renal function differ in Europe and USA. The manufacturers of European CMS products recommend in general 4-6mg/kg or 50.000-75.000 IU/kg daily of CMS in 2-3 divided doses, i.e. for a 70 kg adult patient a dose of 90-140 mg or 1.2-1.75 million IU every 8 hours. Greece, France and Austria are the exceptions. In Greece the manufacturer recommends 6-9 million IU daily, divided in 3 doses (9 million IU recommended for ICU patients). In France and Austria up to 12mg/kg (150000 IU/kg) daily is recommended (195).  In the USA, colistin products contain 150 mg colistin base (equivalent to 360 mg or 4.5 million IU of CMS) and the recommended dose is 2.5-5 mg/kg colistin base daily divided in 2-4 doses, depending on the severity of the infection (equivalent to 6-12 mg/kg or 75000-150000 IU/kg of CMS, i.e. for a 70 kg adult patient a dose of 140-280mg or 1.75-3.5 million IU every 8 hours).

It is evident that at least on PK/PD grounds discussed previously, most of the currently recommended doses are not sufficient for the treatment of patients. In the same grounds, sustainable concentrations would be achieved even with once daily administration. However, other issues including the higher possibility of development of resistance in studies simulating once or twice daily administration and lack of comparative data from clinical studies preclude such proposals. Currently, the most accurate method to calculate the dosing scheme of colistin base (in mg) in order to achieve adequate concentrations for an MIC target is by using the formula (57):

Loading dose: colistin Css,avg target x 2.0 x body weight

Maintenance daily dose: colistin Css,avg targetx (1.50 x CrCL + 30),

where colistin Css,avg target is the desirable mean colistin concentration at steady state expressed in mg/l, which depends on MIC of the pathogen, site, and severity of infection. The lower of ideal or actual body weight is used, expressed in kg.

In addition, colistin can be administered intramuscularly at doses similar to the recommended intravenous ones. However, intramuscular administration is not commonly used in clinical practice due to the severe pain caused at the injection site. The combination of CMS with 0.5% lidocaine may be used to treat eye infections. Colistin sulfate could be administered orally to infants and children with diarrhea caused by bacteria susceptible to the drug; the dose is 3-5 mg/kg daily, in three divided doses. Colistin sulfate is also used as part of selective bowel decontamination regimens, however in an era of advanced bacterial resistance and lack of definitive data regarding the usefulness of bowel decontamination in institutions with MDR bacteria, this approach should be discouraged.

When CMS is given by inhalation (nebulized), the dosage recommended by the manufacturers in the United Kingdom is 40 mg (500.000 IU) every 12 hours for patients with body-weight ≤ 40 kg and 80 mg (1 million IU) every 12 hours for patients with body-weight > 40 kg. For recurrent pulmonary infections and for the treatment of ventilator-associated pneumonia in critically ill patients the dosage of aerosolized CMS can be increased to 160 mg (2 million IU) every 8-12 hours (94, 128).  In spontaneously breathing patients CMS can be administered as follows: 1 million IU (80mg) of CMS is added to 4ml of normal saline, swirled slowly to mix, and the solution is nebulized with 8 liters/min oxygen flow and inhaled via a face mask. In patients undergoing mechanical ventilation aerosolized CMS can be delivered by means of most ventilators. In addition, inhaled colistin can be administered through jet or ultrasonic nebulizers. More recently, CMS has been formulated as a dry powder to be administered via a hand-held inhaler in cystic fibrosis patients ≥6 years of age with chronic P. aeruginosa infections. Compared with nebulized CMS, the CMS dry powder for inhalation (DPI) formulation reduces treatment time and improves patient convenience (28).

As previously described, penetration of colistin into the CSF is inadequate and therefore intravenous colistin alone is not recommended for the treatment of post-neurosurgical meningitis or ventriculitis due to MDR or XDR Gram negative bacilli. Intrathecal or intraventricular administration of CMS has been described. The IDSA guidelines proposed a daily dose of 10mg (125000 IU) for intrathecal administration (198).  A systematic review of patients with MDR A. baumannii (mainly), P. aeruginosa, and K. pneumoniae CNS infections reported that the dosing of CMS in case reports was mainly empirical and ranged from 1.6-40mg (20000-500000 IU) (8).

Commercial formulations of polymyxin B are not currently available in most areas of the globe. In general, 1 mg of polymyxin B is equal to 10.000 IU. The dosage of intravenous polymyxin B recommended by the manufacturer is 1.5-2.5 mg/kg/day (15.000-25.000 IU/kg/day), divided to 2 equal doses for adults and children older than 2 years, with normal renal function. Polymyxin B may also be administered intramuscularly, although this mode is not routinely recommended due to the severe pain caused at the injection site. The recommended dosage for intramuscular administration is 2.5-3 mg/kg/day (25.000-30.000 IU/kg/day) divided to 4 or 6 equal doses for adults and children older than 2 years (9).

Polymyxin B has been used intrathecally in cases of MDR Gram-negative meningitis. The dosage recommended for intrathecal use for adults and children under 2 years of age is 5 mg (50.000 IU) once daily for 3 to 4 days and then 5 mg (50.000 IU) once every other day for at least 2 weeks after the cultures of the cerebrospinal fluid are negative and sugar levels of the cerebrospinal fluid are normal. For children under 2 years of age the dosage of intrathecal polymyxin B should be 2 mg (20.000 IU) once daily for 3 to 4 days, then 2.5 mg (25.000 IU) once every other day until the previous mentioned conditions are met (9).

When polymyxin B is used to treat eye infections due to P. aeruginosa a concentration of 0.1-0.25% (10.000 IU to 25.000 IU/ml) is administered in 1 to 3 drops every hour. Subconjuctival injection of up to 10 mg/day (100.000 IU/day) may also be used for the treatment of Pseudomonas aeruginosa infections of the cornea and conjunctiva (9).  In addition, polymyxin B with a caine-type local anesthetic is an FDA permitted combination, which is used in intramuscular administration of the medication, in ear drops, and ointments.

Children

The currently available data for intravenous dosing recommendations in children outside the cystic fibrosis population are limited, untraceable or non-existing. Most dosing schemes in case reports or case series are empirical. Therefore, the definite dose for children has not been defined (195).  For colistin, the adult dosing is recommended for children in the manufacturers’ recommendations. For polymyxin B, it is suggested that infants with normal renal function may receive up to 4 mg/kg/day (40.000 IU/kg/day) in cases of life-threatening infections.

Renal Failure

According to the manufacturers, the dose of polymyxins must be reduced in patients with impaired renal function. However, the recommended doses differ substantially between USA and several European countries (Table 3) (195).  In general, the data from PK studies show that for polymyxin B dose adjustment is not required for patients with any degree of renal impairment, including patients in dialysis (173, 174).   However, manufacturers and the FDA recommend dose adjustment with declining renal function, without specification of the dosages. For colistin, the situation is far more complicated. As already mentioned, colistin is not removed by the kidneys, but CMS is. In USA, dose adjustment is recommended even for mild renal impairment. In Europe, recommendations differ between countries, from adjustment even in mild renal impairment to adjustment only for severe renal impairment. Table 3 provides a summary of recommendations for colistin adjustment. Since both CMS and colistin are removed in dialysis, the dose should be adjusted accordingly.

Recent PK studies showed that an independent of renal function loading colistin dose followed by dose adjustment according to renal function is required. The aforementioned formulas for patients with normal renal function apply also for ICU patients with renal impairment (57).   Recommended dosage intervals based on creatinine clearance (CrCl) are as follows (colistin base activity, CBA):

CrCl 10-70 ml/min/1.73 m2: every 12 (or 8) hours

CrCl <10 ml/min/1.73 m2: every 12 hours

Receiving intermittent hemodialysis: Daily dose of CBA on a non-hemodialysis day to achieve each 1.0-mg/liter colistin Css,avg target is 30 mg. On a hemodialysis day, a supplemental dose of colistin should be added.  If the supplemental dose is administered during the last hour of the session, 50% of the daily dose should be added. If it is administered after the session, 30% of the daily dose should be added. Twice-daily dosing is suggested.

Receiving continuous renal replacement therapy: Daily dose of CBA to achieve each 1 mg/liter colistin Css,avg target is 192 mg. Doses may be given every 8-12 hours.

For patients in continuous ambulatory peritoneal dialysis no formal recommendations exist from the manufacturers. PK studies showed that the dose of CSM should not be increased since clearance for both CMS and colistin was minimal. A loading dose of 300mg colistin base followed by 150-200mg of colistin base every day was sufficient to achieve steady-state plasma concentration of 2.5μg/ml (93).

Hepatic Failure

There is no need for dosage modification of polymyxins in patients with liver failure.

Body Composition (Obesity, Wasting, Various Body Builds)

In obese patients dosage of colistin should be based on ideal body weight (132).

Pregnancy

Currently,  polymyxins are listed in the FDA’s pregnancy category C. Subsequently, they should be used only for cases where the potential benefit is higher than the potential risk. There are no adequate and well-controlled studies assessing the safety of polymyxin B and colistin in women during pregnancy. In a small study, the authors reported that the risk for congenital abnormalities in newborns whose mothers were treated with polymyxin B during pregnancy was minimal but could not be ruled out (87).

ADVERSE EVENTS

Nephrotoxicity

The mechanism of polymyxins associated neprotoxicity at the molecular level remains unknown. Recent studies showed that it involves the mitochondrial, death receptor, and endoplasmic reticulum pathways (33, 44).  The final result is induction of apoptosis of renal cells and acute tubular necrosis manifested as elevation in serum creatinine and urea levels. It seems that nephrotoxicity depends on both the concentration and length of exposure to polymyxins (6, 48).

The reported frequency of nephrotoxicity of polymyxins in studies published in the last 15 years varied from as low as 0% up to 55% (50, 70, 121, 125, 127).  The definitions of nephrotoxicity, the characteristics and number of enrolled patients and the doses of administered polymyxins were variable. Polymyxins were administered mainly to patients with severe infections and co-morbidity and concurrent use of other potentially toxic drugs, including antibiotics might have also contributed to nephrotoxicity. Adjustment for these factors was not performed in the majority of the studies. Although definitive conclusions cannot be made, it seems that the frequency of nephrotoxicity was higher in patients with previous renal impairment, in older individuals and those who received higher doses (125, 176).

One study reported that although nephrotoxicity was observed during colistin administration, a decrease in the mean (±SD)/median serum creatinine level of 0.2 (±1.3)/0.1 mg/dl was noticed at the end of CMS treatment compared to the baseline levels (84).  Interestingly, in patients who received intravenous CMS for more than 4 weeks for the treatment of MDR Gram-negative infections, the median creatinine value increased by 0.25 mg/dl during treatment compared to the baseline, and returned to baseline levels at the end of treatment (48). Another study in cystic fibrosis patients showed that renal dysfunction was potentiated by the co-administration of aminoglycosides; however, colistin alone or in combination with other antibiotics did not appear to be highly nephrotoxic (4).

Recent experimental studies also showed that polymyxin B is 3 times more cytotoxic to human kidney proximal tubular cells than colistin (203).  In contrast to that and to older clinical studies (102,141), recent ones using the more strict RIFLE criteria for definition of kidney injury reported a lower frequency of nephrotoxicity among polymyxin B than colistin treated patients (2, 152, 199).

Neurotoxicity

Neurological toxicity may be manifested with dizziness, weakness, facial and peripheral paresthesia, vertigo, confusion, ataxia, visual and speech disturbances, and neuromuscular blockade, which can lead to respiratory failure or apnea. Partial deafness and severe ataxia have been reported in patients with excessively high plasma levels of colistin. The incidence of colistin-associated neurotoxicity reported in earlier literature was approximately 7%, with paresthesias comprising the main manifestation (90, 180). The neurotoxic events related to colistin therapy appears to occur more frequently in cystic fibrosis (19, 162) than in non-cystic fibrosis patients (56, 105,121). Intraventricular administration of polymyxins may lead to convulsions (88).

Other Adverse Events

Pruritus, dermatoses, drug fever, gastrointestinal disturbances and superinfections may develop infrequently during the course of therapy with polymyxins.  Leukopenia and granulocytopenia may be possibly associated with the use of colistin. Pain at the site of intramuscular injection or superficial thrombophlebitis may occur. Furthermore, the development of Clostridium difficile infection represents an additional, although rare, potential adverse event of polymyxin treatment (90).

Treatment with aerosolized colistin may further be complicated with bronchoconstriction and chest tightness. Treatment with inhaled β2-agonists before the initiation of aerosolized colistin could prevent the development of bronchoconstriction (32).

Treatment and Avoidance

Both, renal and neurological toxicity associated with the use of polymyxins, are considered to be dose-dependent and reversible after early discontinuation of the drugs. However, there are scarce published reports of irreversible nephrotoxicity after the cessation of colistin treatment (90).  Supportive treatment to patients treated with polymyxins, close monitoring of renal function, restriction of factors and drugs that may precipitate renal impairment when polymyxins are administered may prevent the development of adverse events.

Overdoses

A fatal case of a previously healthy child who received colistin therapy for intra-abdominal infection due to P. aeruginosa and E. coli strains has been reported. This patient developed both acute tubular necrosis, confirmed by autopsy and apnea requiring mechanical ventilation. However, the dose of CMS used was 120 mg/kg/day, which is 10 times above the currently upper recommended dose (167).  Although colistin and CMS are removed at a higher extent than polymyxin B with dialysis, there are no data regarding the effectiveness of hemodialysis or other treatment strategies for the management of patients with polymyxin overdose.

MoniTORING REQUIREMENTS

Therapeutic Drug Monitoring

There are no conclusive data about the usefulness of therapeutic drug monitoring for adjustment of CMS dose. Recent studies on CMS PK properties suggest that even though the concentration of active colistin at steady state does not fluctuate, the 2 μg/ml target is not achieved in many patients, mainly due to insufficient dosing, especially in patients with mild to moderate renal dysfunction. In addition, some of them will have plasma concentrations up to 10μg/ml due to unknown enzymatic activities in the tissues or drug to drug interactions. If such an approach was to be applied, it would be advisable to collect blood samples before the administration of the next dose, in order to minimize overestimation of the results due to the conversion of CMS to colistin after collection (29).

Other Laboratory Monitoring

Clinicians should be alert for the possibility of development of polymyxins-related adverse reactions, mainly nephrotoxicity and neurotoxicity. Subsequently, monitoring of renal function by measuring serum creatinine every two days is advised, until more data on the value of other tests for patient monitoring become available.

DRUG INTERACTIONS

Other antibiotics, such as aminoglycosides, can increase the potential of polymyxins for nephrotoxicity and neurotoxicity and they should be co-administered with the greatest caution with polymyxins. The concurrent use of polymyxins with a curariform muscle relaxant and other neurotoxic drugs such as ether, tubocurarine, succinylcholine, gallamine, decamethonium, and sodium citrate should be used with great caution also, since these agents may potentiate the development of neuromuscular blockade (37). Co-administration of sodium cephalothin and colistin may enhance the development of nephrotoxicity, so this medication should be avoided (90).

CLINICAL INDICATIONS  

The intravenous formulations of polymyxins were used therapeutically against Gram negative infections from the early 1950s for approximately two decades (164). They were gradually abandoned worldwide in the early 1970s because of the reported severe toxicities, mainly nephrotoxicity (52, 129, 165).  However, the emergence of MDR and XDR Gram-negative bacteria in combination with the lack of development of new antimicrobial agents led to the revival of polymyxins as “salvage” therapy for the treatment of nosocomial infections due to MDR Gram negative bacteria. Nowadays, polymyxins should be considered for the treatment of infections caused by Gram negative bacteria resistant to other available antimicrobial agents, confirmed by appropriate in vitro susceptibility testing. In addition, polymyxins appear a valuable option in patients with infections due to Gram-negative bacteria, which are in vitro susceptible to other antimicrobial agents, when the treatment with these agents has been clinically ineffective.

Several, mainly retrospective studies have been published regarding the effectiveness of intravenous polymyxins, alone or in combination, in several dosing regimens, for the treatment of ICU or hospitalized patients with MDR or XDR A. baumannii, P. aeruginosa and lately K. pneumoniae infections. Most of them concluded that polymyxins are an adequate treatment option for such infections based on the acceptable mortality and clinical cure rates, given the critical condition and co-morbidity of the enrolled patients (212).  Polymyxins were mostly administered with other active or inactive antibiotics. In studies that compared a polymyxin alone against the combination of a polymyxin with one or more antibiotics, there was no difference in mortality or clinical cure in multivariate analyses between the compared groups. Only one non-comparative preliminary study regarding the application of a 9 million IU loading dose has been published. The cure rate was similar to the cure rates reported in other studies (34). Small, inconclusive studies have been also published regarding the continuous administration of polymyxins (114).

Following the publication of data regarding the synergism of rifampin with colistin against A. baumannii, two RCTs examined the effectiveness of the combination against colistin monotherapy. No difference was observed in all cause or infection-related mortality and the duration of hospitalization between the compared groups. Combination therapy was associated with faster and higher microbiological eradication (5, 43).

There is extensive experience with the use of aerosolized colistin in patients with cystic fibrosis (140).  Intravenous and aerosolized colistin is provided in these patients in an attempt to eradicate P. aeruginosa colonization of the respiratory tract and to treat exacerbations mainly caused by the same microorganism. A 3-week course of CMS inhalation in combination with oral ciprofloxacin led to promising results in eradicating early P. aeruginosa colonization. Concerns about rapid development of P. aeruginosa strains resistant to colistin or the emergence of lung infections due to microorganisms with inherited resistance to colistin were not confirmed after more than 10 years of published experience in patients with cystic fibrosis. Compared with tobramycin, the rate of development of resistance to colistin was slower (112).

Besides cystic fibrosis, inhaled polymyxins have been increasingly used for the treatment of patients with pneumonia, and especially ventilator-associated pneumonia (56, 111, 128).  Thus far the data support a higher clinical and microbiological cure rate and fewer days under mechanical ventilation in patients receiving intravenous and nebulised colistin compared to intravenous colistin alone. A difference in mortality was not observed (9, 197).  Aerosolized polymyxins has been used also to reduce the exacerbations in patients with non-cystic fibrosis bronchiectasis, but the results are not as promising as in patients with cystic fibrosis (71).

Since penetration of polymyxins in the CSF is inadequate, intraventricular and intrathecal administration has been used for neurosurgical patients with meningitis or ventriculitis due to MDR Gram negative bacteria. Most of the published evidence comes from case reports due to A. baumannii infections. Reviews on the issue reported successful treatment in the majority of patients (8, 83,153).   Polymyxins were administered alone or in combination with other antibiotics, including rifampin. Comparative data regarding monotherapy and combination regimens are not available.

Reports published in the old literature suggested that the penetration of colistin into bones is not adequate, thus it was used only as an irrigation solution (166).   However, there is limited promising experience with the use of intravenous colistin for the treatment of bone infections (related to prosthetic materials or not) (184, 204).

Finally, due to its adsorptive properties, polymyxin B has been used as a substrate for endotoxin removal in patients with septic shock. Most of the data come from small trials in Japan. A systematic review and meta-analysis concluded that blood purification was associated with lower mortality in patients with septic shock. Trials studying hemoperfusion with polymyxin B were the ones that primarily influenced the results of the meta-analysis (216).

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Tables

Figure 1 (a,b). Structure of colistin and polymyxin B. The fatty acid molecule is 6-methyloctanoic acid for colistin A and Polymyxin B1, and 6-methylheptanoic acid for colistin B and polymyxin B2. (α,γ)L-Dab. Diaminobutyric acid, α and γ indicate the respective –NH2 involved in the peptide linkage.

Table 1. Minimum inhibitory concentrations for polymyxin B and colistin according to CLSI and EUCAST.

	CLSI 2014			EUCAST 2014
Micro-organism	Susceptible	Intermediate	Resistant	Susceptible	Resistant
Enterobacteriaceae*	≤2 μg/ml	4 μg/ml	≥8 μg/ml	≤2 μg/ml	>2 μg/ml
P. aeruginosa	≤2 μg/ml	4 μg/ml	≥8 μg/ml	≤4 μg/ml	>4 μg/ml
A. baummanni	≤2 μg/ml	---	≥4 μg/ml	≤2 μg/ml	>2 μg/ml

* Does not refer to isolates inherently resistant to polymyxins

EUCAST does not provide breakpoints for polymyxin B.

Abbreviations: CLSI - Clinical and Laboratory Standards Institute, EUCAST - European Committee on Antimicrobial Susceptibility Testing

Table 2. Major differences between colistin and polymyxin B, which may affect clinical outcomes.

COLISTIN	POLYMYXIN B
Administered as pro-drug (CMS), requires conversion to colistin	Administered as the active antibiotic, no conversion is required
Considerable inter-individual variation in plasma steady-state concentration exists	Low inter-individual variation in plasma concentrations
CMS excreted in urine, conversion results to high colistin urine concentration	It is not excreted in the urine, low urine concentration
CMS is converted slowly to colistin, its plasma concentration increases slowly*	Plasma concentration increases sufficiently
Loading dose required on PK grounds, improves time to bactericidal concentration	Loading dose required on PK grounds, improves rapidly time to bactericidal concentration
Adjustment for renal function is mandatory	No adjustment is required
Removed by hemodialysis, dose adjustment is required	Removed to a minor extent, dose adjustment is not required
Therapeutic drug monitoring requires stringent procedures in the collection, storage and transportation of the sample to prevent conversion of CMS to colistin after collection	Difficulties not present

* Potential for inadequate treatment for several hours/days (it is unknown if this is associated with increased mortality and development of resistance, both are possible).

Abbreviations: CMS - colistimethate sodium, PK - pharmacokinetic

Table 3. Recommended maintenance dose for intravenous polymyxins in Europe and USA, as provided by the manufacturers and PK studies.

Recommendation by	Antibiotic	Degree of renal impairment
		Normal (≥80 ml/min)	Mild (CrCl 50-79 ml/min)	Moderate (CrCl 30-49 ml/min)	Severe (CrCl 10-29 ml/min)	Renal replacement
FDA USA, manufacturers#	Colistin base*	2.5-5 mg/kg (divided in 2-4 doses)	2.5-3.8 mg/kg (divided in 2 doses)	2.5 mg/kg (in 1 or 2 doses)	1.5 mg/kg (in 1 dose every 36h)	---
Manufacturers Europe	CMS*	4-10 mg/kg (in 2-3 divided doses)	2.3-10 mg/kg (in 2-3 divided doses)	1-6 mg/kg (in 1-3 divided doses)	1-6 mg/kg (divided in doses every 8-36 h)	---
PK studies	Colistin base*	2.5-5 mg/kg (in 2-3 divided doses)	Dose adjustment according to the formula: colistin Css,avg targetx (1.50 x CrCL + 30)			CVVHD: 192 mg for every 1μg//ml Css,avg
Manufacturer	Polymyxin B	1.5-2.5 mg/kg (divided in 2 doses)	The dose should be reduced from 1.5 mg/kg downwards according to renal impairment			---
PK studies	Polymyxin B	1.5-2.5 mg/kg (divided in 2 doses)	Dose adjustment is not required

* 1 mg of colistin base is equal to 2.4 mg of CMS, 1 mg of CMS is 12500 IU

# % of urea clearance is used rather than creatinine clearance in the FDA and most manufacturers; the provided values approximate those in FDA

Abbreviations: CMS -colistimethate sodium, CrCl - creatinine clearance, CVVHD - continuous venovenous hemodialysis, Css, avg - mean colistin steady state concentration

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Bard JD, et al.  Rationale for Eliminating Staphylococcus Breakpoints for β-lactam Agents Other Than Penicillin, Oxacillin or Cefoxitin, and Ceftaroline. Clin Infect Dis 2014;58:1287-1296.

Sandri AM, et al.  Population Pharmacokinetics of Intravenous Polymyxin B in Critically Ill Patients: Implications for Selection of Dosage Regimens.  Clin Infect Dis 2013;57:524.

Akajagbor DS, et al.   Higher Incidence of Acute Kidney Injury with Intravenous Colistimethate Sodium Compared with Polymyxin B in Critically Ill Patients at a Tertiary Care Medical Center.  Clin Infect is 2013;57:1300.

Ziaka M, et al. Combined Intravenous and Intraventricular Administration of Colistin Methanesulfonate in Critically Ill Patients with Central Nervous System Infection.   Antimicrobial Agents and Chemotherapy 2013;57:1938-1940.

Gauthier TP, et al. Incidence and Predictors of Nephrotoxicity Associated with Intravenous Colistin in Overweight and Obese Patients. Antimicrob Agents and Chemotherapy.2012;56:2392-6.

Kwa ALH, et al. Pharmacokinetics of Polymyxin B in a Patient with Renal Insufficiency: A Case Report. Clinical Infectious Diseases. 2011;52:1280-1.  

Colistin vs. Colistimethate: serious dosing error [http://www.ashp.org/DocLibrary/Policy/PatientSafety/NANAlert-Colistimethatesodium.aspx]

FDA Warning: Colistin in Nebulizer Solution.

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