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Polymyxin B

Immune Support

Cyclic lipopeptide antibiotic

Polymyxin B is a cyclic lipopeptide antibiotic discovered in the late 1940s that works by binding to.

§Dosing at a glance

1 protocol · from the research
What it's forDoseHow oftenHowFor how long
Systemic into a vein therapy (MDR Gram-negative infections)0.4 mg/kgDaily—2 days

Approximate values pulled from the research — double-check before dosing.

§01Summary

Polymyxin B is a cyclic lipopeptide antibiotic discovered in the late 1940s that works by binding to and disrupting the outer membrane of Gram-negative bacteria, making it one of the few agents still active against many extensively drug-resistant (XDR) and carbapenem-resistant pathogens19. It has been used topically for decades and experienced a major clinical revival in the 2000s as infections caused by multidrug-resistant organisms such as carbapenem-resistant Acinetobacter baumannii and Pseudomonas aeruginosa became increasingly common worldwide19,20.

In its systemic intravenous form, polymyxin B serves as a last-resort treatment for life-threatening infections when other antibiotics have failed3. Global surveillance data show it retains strong activity against key problem pathogens, with resistance rates remaining low even among multidrug-resistant strains9. Polymyxin B is also being investigated in a hemoperfusion format — where blood is passed over a polymyxin B-coated cartridge to remove circulating bacterial toxins (endotoxins) in septic shock — though large randomized trials have not demonstrated a consistent survival benefit with this approach1,2. Several combination regimens pairing polymyxin B with newer agents are actively being developed to address the most resistant infections4,11,18.

This is the layperson summary. Mechanism, dosing, the evidence base, and the published literature are in the sections below — every claim links to its source.

§02In depth

Polymyxin B is a cyclic lipopeptide antibiotic consisting of a cyclic heptapeptide ring with a tripeptide side chain and a fatty acid tail. Its primary bactericidal mechanism involves high-affinity electrostatic binding to the lipid A portion of lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria. The positively charged amino groups of polymyxin B displace the divalent cations (Ca²⁺ and Mg²⁺) that normally stabilize the outer membrane, leading to membrane disruption, increased permeability, leakage of cellular contents, and ultimately bacterial cell death20. This membrane-targeted mechanism confers activity against many drug-resistant organisms because it does not rely on intracellular enzyme targets susceptible to conventional resistance mutations9,20.

Polymyxin B's LPS-binding affinity also underlies its application in hemoperfusion, where the drug immobilized on a fiber cartridge directly adsorbs circulating endotoxin from the bloodstream. The biological rationale for this approach is that neutralizing LPS may attenuate the downstream inflammatory cascade — including release of pro-inflammatory cytokines such as IL-6 and activation of the complement and coagulation systems — that drives the hemodynamic instability and organ failure of septic shock1,5. Hemodynamic improvements including increased cardiac index and reduced vasopressor requirements have been observed in pilot studies using this approach13, potentially reflecting removal of mediators beyond measurable endotoxin.

Resistance to polymyxin B arises through modifications of the lipid A target, most importantly through addition of cationic groups (e.g., phosphoethanolamine or 4-aminoarabinose) that reduce the net negative charge of the outer membrane and decrease polymyxin B binding affinity. Such modifications can be chromosomally encoded (via two-component regulatory systems such as PmrAB and PhoPQ) or, more alarmingly, encoded on transferable plasmids via the MCR-1 determinant, enabling horizontal spread of resistance across bacterial species and clinical environments20. The global dissemination of MCR-1, driven in part by heavy veterinary use of polymyxins, represents an active public health concern20.

Pharmacodynamically, polymyxin B exhibits concentration-dependent killing. A key pharmacokinetic distinction from its congener colistin is that polymyxin B is administered as the active drug directly (not as an inactive prodrug), resulting in more predictable plasma exposures. Polymyxin B is not renally cleared to the same extent as colistin, and standard dosing does not typically require adjustment for renal impairment — though the drug itself is nephrotoxic, making renal monitoring essential3. Pharmacokinetic characterization across renal function strata is an area of active clinical investigation16. Combination therapy strategies pairing polymyxin B with rifamycin derivatives (such as BV100), carbapenems, or newer beta-lactam/beta-lactamase inhibitor combinations are being actively explored to exploit synergistic membrane-disruption and intracellular-target effects against carbapenem-resistant Acinetobacter baumannii and other XDR pathogens4,11,18.

§04Evidence & efficacy

Evidence base
294Studies
121Human
34Animal

Polymyxin B retains broad in vitro antibacterial activity against key problem pathogens including Pseudomonas aeruginosa and Acinetobacter baumannii (MIC90 of 2 mg/L, resistance rates of 1.3% and 2.1% respectively), with consistently low resistance rates even among multidrug-resistant and carbapenem-resistant strains9. It is recognized internationally as a critical salvage antibiotic for otherwise untreatable MDR Gram-negative infections3,19,20. The intrinsic resistance of Proteus spp. and Serratia spp. to polymyxin B is a clinically relevant limitation that should inform empiric therapy decisions9.

For IV treatment of XDR Gram-negative infections, polymyxin B is accepted as last-resort therapy, with international consensus guidelines providing PK/PD-optimized dosing frameworks to enhance clinical outcomes3. Combination regimens pairing polymyxin B with carbapenems or novel agents are in active clinical investigation4,8,11,18.

In the context of polymyxin B hemoperfusion for septic shock and endotoxemia, the evidence base shows a complex picture. An early small RCT reported significant improvements in hemodynamic parameters, vasopressor requirements, SOFA scores, and 28-day mortality (32% vs. 53%)5, and a pilot study demonstrated significant improvements in cardiac index and oxygen delivery13. However, the larger and more definitive EUPHRATES RCT (n=450) found no statistically significant reduction in 28-day mortality (37.7% vs. 34.5%)1, and the ABDOMIX multicenter RCT in peritonitis-induced septic shock showed a non-significant trend toward higher mortality in the PMX-HP group at both 28 and 90 days2. The more recent TIGRIS trial using a Bayesian framework may suggest a mortality reduction in a narrowly defined endotoxic septic shock population7, with active research ongoing in this area.

Topical polymyxin B as part of an oropharyngeal decontamination regimen has been reported to substantially reduce ventilator-associated pneumonia incidence (from 78% to 16%), though without a statistically significant mortality benefit in the available trial6.

§05Safety

The safety profile of polymyxin B differs substantially by route of administration. When administered systemically by IV, nephrotoxicity and neurotoxicity are the primary concerns — effects that historically led to abandonment of parenteral polymyxin use for approximately two decades19. More recent clinical experience and consensus guidelines suggest that the severity of these adverse effects may have been overestimated in older literature, and that with careful monitoring and optimized dosing, systemic polymyxin B is used in critically ill patients as a last resort3,19. International consensus guidelines explicitly include recommendations for prevention of renal failure and toxicity monitoring protocols3.

In the hemoperfusion format, where polymyxin B is immobilized on a cartridge and not systemically delivered, the drug's direct toxicity concerns are largely avoided13. Pilot studies reported good tolerability and no significant adverse effects with the hemoperfusion procedure13. However, in the large EUPHRATES RCT, the treatment group experienced a higher rate of serious adverse events (65.1%) compared to sham (57.3%), with worsening sepsis occurring in 10.8% versus 9.1% of patients respectively, suggesting a tolerability concern at the procedure level in critically ill patients1. The TIGRIS trial reported serious adverse events in 30% of the polymyxin B hemoperfusion group versus 22% of controls, with only 2% of serious adverse events considered treatment-related7.

Topical oropharyngeal application of polymyxin B as part of a combination decontamination regimen was well tolerated, and no resistant microorganisms emerged during the study period in a placebo-controlled trial6. Pharmacokinetic studies in renally impaired patients are ongoing to further define dose-safety relationships in vulnerable populations16.

§06History

Polymyxin B was first isolated in the late 1940s from Bacillus polymyxa, a soil bacterium, as part of broad screening programs for new antibacterial agents in the post-penicillin era. It entered clinical use in the 1950s for treatment of Gram-negative infections, notably before the establishment of contemporary drug development and regulatory procedures — a historical reality that meant formal pharmacokinetic characterization, optimized dosing protocols, and rigorous safety profiling were never completed at the time of initial approval3.

Due to reports of significant nephrotoxicity and neurotoxicity with parenteral administration, clinical use of systemic polymyxin B was largely abandoned during the 1970s and 1980s as safer broad-spectrum antibiotics became available19. However, the rapid global emergence of multidrug-resistant Gram-negative pathogens — particularly carbapenem-resistant Acinetobacter baumannii, Pseudomonas aeruginosa, and Klebsiella pneumoniae — beginning in the 1990s and accelerating through the 2000s prompted a major clinical revival of the polymyxin class19. A landmark 2005 review in Clinical Infectious Diseases, ultimately accumulating over 1,700 citations, was highly influential in repositioning polymyxins as acceptable last-resort agents for otherwise untreatable infections19.

The 2010s brought intense investigation of polymyxin B hemoperfusion for septic shock, culminating in high-profile RCTs including EUPHAS (2009)5, ABDOMIX (2015)2, and EUPHRATES (2018)1. In 2019, the first international consensus guidelines for optimal polymyxin use — endorsed by six major professional societies — were published, providing the field's first formal evidence-based dosing framework3. The 2020s have seen novel formulation development, combination strategies with agents such as BV1004,11,18, and continued refinement of patient selection for hemoperfusion therapy7.

§07References