Polymyxin B is a lipopeptide antibiotic that kills Gram-negative bacteria by ripping apart their outer membranes, and it has become one of the last reliable options against infections that resist virtually everything else. Produced naturally by the soil bacterium Paenibacillus polymyxa, it was largely shelved for decades because of kidney and nerve toxicity, only to be pulled back into clinical use as carbapenem-resistant organisms became a global crisis. Its pharmacokinetics are unusual, its dosing is tricky, and the line between a therapeutic dose and a toxic one is thin. Understanding how it works, how the body processes it, and what drugs interact with it is essential for anyone involved in its use.
How Polymyxin B Kills Bacteria
Polymyxin B targets lipid A, the anchor component of lipopolysaccharide (LPS) embedded in the outer membrane of Gram-negative bacteria. The drug’s positively charged amino groups are attracted to the negatively charged phosphate groups on lipid A. Once it binds, polymyxin B displaces calcium and magnesium ions that normally stabilize the membrane, causing it to buckle and leak. The result is rapid cell death as the bacterium’s contents spill out.
The binding process involves a specific molecular recognition step mediated by pyrophosphate groups on LPS. Laboratory experiments using fluorescence techniques have shown that the presence of rough-type LPS dramatically enhances polymyxin B’s ability to disrupt membranes, lowering the minimum drug-to-lipid ratio needed for leakage by roughly a thousandfold compared to membranes lacking LPS.1PubMed Central. Interactions of polymyxin B with lipopolysaccharide-containing membranes Stereochemistry matters too: when researchers tested mirror-image versions of polymyxin B, those enantiomeric compounds showed reduced LPS binding, weaker outer membrane permeabilization, and diminished antibacterial activity.2PubMed Central. Polymyxin Stereochemistry and Its Role in Antibacterial Activity and Outer Membrane Disruption In other words, the three-dimensional shape of the molecule is not incidental; it is central to how polymyxin B recognizes and destroys its target.
What Polymyxin B Works Against, and What Resists It
Polymyxin B has strong bactericidal activity against a broad range of Gram-negative bacteria, including many of the most dangerous hospital-acquired pathogens: Pseudomonas aeruginosa, Acinetobacter baumannii, Klebsiella pneumoniae, and Escherichia coli. It does not work against Gram-positive bacteria (which lack the outer membrane LPS layer it targets) or against anaerobes.
Some Gram-negative species are naturally resistant. Neisseria meningitidis, Proteus mirabilis, and Burkholderia species all shrug off polymyxin B because their LPS already contains modifications that reduce the negative charge on lipid A.3PubMed. Mechanisms of Polymyxin Resistance Among organisms that are normally susceptible, resistance can develop. In P. aeruginosa, mutations in LPS-related genes alter the outer membrane’s structure, and some mutants lose the aminoarabinose-modified lipid A species that the wild type produces in low-magnesium environments.4PubMed Central. Characterization of the polymyxin B resistome of Pseudomonas aeruginosa
The most alarming development in polymyxin resistance is the spread of mobile colistin resistance (mcr) genes, which can jump between bacterial species on plasmids. In MCR-1-producing E. coli strains, lipid A analysis reveals additions of phosphoethanolamine and aminoarabinose that reduce the membrane’s net negative charge and weaken polymyxin binding.5The Microbe. Exploring the complex interplay of polymyxin resistance mechanisms, lipid A variations, and virulence factors in Escherichia coli and Klebsiella pneumoniae At least ten main lineages of mcr genes have been identified, and their spread across clinical pathogens is a major concern.6PubMed Central. A Review of Resistance to Polymyxins and Evolving Mobile Colistin Resistance Gene (mcr) among Pathogens of Clinical Significance
How the Body Handles Polymyxin B
Polymyxin B has some quirky pharmacokinetics that set it apart from most antibiotics. After intravenous infusion, it distributes into tissues with a volume of distribution around 0.3 to 0.5 L/kg and binds to plasma proteins substantially, with binding measured at roughly 50 to 75 percent in critically ill patients on renal replacement therapy.7Journal of Antimicrobial Chemotherapy. Pharmacokinetics of polymyxin B in patients on continuous venovenous haemodialysis
The elimination route is what really stands out. Only about 1.5 to 1.7 percent of a dose appears as polymyxin B in urine within 24 hours, and renal clearance rates are far lower than nonrenal clearance rates in both healthy people and those with mild kidney impairment.8PubMed Central. Pharmacokinetic study of polymyxin B in healthy subjects and subjects with renal insufficiency This means that, unlike many antibiotics, polymyxin B is eliminated primarily through nonrenal pathways. The exact mechanisms of this nonrenal clearance are not fully mapped out, but the practical consequence is significant: dose adjustments based on kidney function alone are unreliable. This is the opposite of what most clinicians are trained to expect with nephrotoxic drugs.
Dosing Strategy and the Therapeutic Window
Getting the dose right with polymyxin B is a balancing act. The drug’s killing activity is best predicted by the ratio of the area under the concentration-time curve to the minimum inhibitory concentration (the AUC/MIC ratio). Different clinical studies have converged on a target steady-state AUC over 24 hours somewhere around 50 to 100 mg·h/L, though the exact cutoff varies by infection type.
For carbapenem-resistant Klebsiella pneumoniae infections, simulations targeting an AUC/MIC of 50 found that a fixed dose of 100 mg every 12 hours achieved good drug exposure regardless of kidney function, but the risk of nephrotoxicity was high.9PubMed Central. Evaluation of polymyxin B AUC/MIC ratio for dose optimization in patients with carbapenem-resistant Klebsiella pneumoniae infection For nosocomial pneumonia caused by carbapenem-resistant organisms, a prospective study found that a maintenance dose of 75 mg or 100 mg every 12 hours could achieve adequate drug levels when the bacterial MIC was 0.5 or 1 mg/L, respectively, with an AUC/MIC cutoff of about 67 being the best predictor of clinical success in patients receiving combination therapy.10PubMed Central. Optimization of polymyxin B regimens for the treatment of carbapenem-resistant organism nosocomial pneumonia
The practical takeaway is that polymyxin B dosing is not one-size-fits-all. Infection site, bacterial susceptibility, and concomitant therapies all influence the target. This is why therapeutic drug monitoring has become increasingly central to polymyxin B use.
Therapeutic Drug Monitoring
Because the gap between an effective and a toxic dose is so narrow, many centers now measure polymyxin B blood levels and adjust doses accordingly. In one randomized clinical trial, clinicians drew blood samples after the drug reached steady state (typically on day four), calculated the AUC using a limited sampling strategy, and then titrated the dose in 25 mg increments to keep the daily AUC within a target window of 50 to 100 mg·h/L.11PubMed Central. Polymyxin B therapy based on therapeutic drug monitoring in carbapenem-resistant organisms sepsis Doses were adjusted on the same day or the next, and rechecked after four more doses. This approach is labor-intensive, but it represents the best current strategy for maximizing efficacy while limiting kidney damage.
Dosing in Patients on Dialysis or With Obesity
Critically ill patients often need renal replacement therapy, and the question of whether to reduce polymyxin B doses for them is common. Studies in patients on continuous venovenous hemodialysis found that recommended doses should not be reduced; these patients still need standard or even higher dosing.7Journal of Antimicrobial Chemotherapy. Pharmacokinetics of polymyxin B in patients on continuous venovenous haemodialysis For patients on continuous venovenous hemofiltration specifically, simulations suggest a loading dose of 200 mg followed by 150 mg every 12 hours to hit the right AUC targets, along with ongoing dose adjustments guided by blood-level monitoring.12International Journal of Antimicrobial Agents. Population pharmacokinetics of polymyxin B in critically ill patients receiving continuous venovenous haemofiltration
Obesity complicates things further. A population pharmacokinetic study found that total body weight did not significantly change how the body handles polymyxin B, but dosing based on adjusted body weight (a calculation that splits the difference between actual and ideal weight) had the best probability of landing in the therapeutic range without excessive toxicity. Fixed doses of 125 or 150 mg every 12 hours carried a high toxicity risk in obese patients, so weight-adjusted regimens capped at a daily total under 250 mg are generally preferred for those with MIC values of 0.5 mg/L or below.13PubMed Central. Population Pharmacokinetics of Polymyxin B in Obese Patients for Resistant Gram-Negative Infections
Kidney Toxicity
Nephrotoxicity is the most common and most feared adverse effect of polymyxin B. The drug is taken up by kidney proximal tubular cells, where it triggers programmed cell death through multiple pathways. Laboratory work has shown that polymyxin B activates caspase enzymes, damages mitochondria, and generates oxidative stress inside tubular cells.14PubMed Central. Major pathways of polymyxin-induced apoptosis in rat kidney proximal tubular cells One pathway involves the Nrf2/NQO1 antioxidant defense system: polymyxin B suppresses this system, leaving cells unable to neutralize the reactive oxygen species the drug generates, which amplifies kidney cell death.15PubMed. Polymyxin B exerts nephrotoxicity effects via inhibition of the Nrf2/NQO1 pathway-mediated antioxidant response
In clinical practice, the rate of acute kidney injury (AKI) among patients receiving polymyxin B is high. A multicentre prospective cohort study found that doses of 150 mg/day or more roughly doubled the risk of AKI, with higher weight and older age independently raising the danger as well. The risk increased significantly at doses between 150 and 199 mg/day regardless of patient weight, with no further significant increase at even higher doses, suggesting a threshold rather than a simple linear relationship.16Journal of Antimicrobial Chemotherapy. Risk factors for acute kidney injury (AKI) in patients treated with polymyxin B and influence of AKI on mortality A retrospective ICU study confirmed that duration of polymyxin B use, higher BMI, and advanced age were the most important independent risk factors for developing AKI.17PubMed Central. Risks of Polymyxin B Nephrotoxicity and Its Precursors in the Intensive Care Unit
Neurotoxicity
Nerve-related side effects from polymyxin B are rarer than kidney damage but potentially life-threatening. Polymyxin B can cause neuromuscular blockade, meaning it interferes with the signal from nerve to muscle, producing weakness that in severe cases leads to respiratory failure. Most historical reports of this complication date from the 1960s and 1970s, when the drug was used more casually and at higher doses. A modern case report described a patient who developed respiratory failure from neuromuscular dysfunction after starting polymyxin B for a multidrug-resistant E. coli infection; after the drug was stopped, the patient regained full strength and was successfully taken off the ventilator.18PubMed Central. Polymyxin-induced neuromuscular weakness: a case report The condition is reversible, but only if clinicians recognize what is happening and stop the drug in time.
Drug Interactions That Increase Toxicity
The most clinically important drug interaction involves vancomycin, which is frequently prescribed alongside polymyxin B in critically ill patients with mixed infections. A study comparing ICU patients who received polymyxin B plus vancomycin versus polymyxin B alone found a strikingly higher incidence of AKI in the combination group: about 63 percent compared with roughly 29 percent. This was true even though patients receiving the combination actually had more favorable baseline clinical profiles and better kidney function at the start.19PubMed Central. Polymyxin-B and vancomycin-associated acute kidney injury in critically ill patients Any other nephrotoxic drug, including aminoglycosides, amphotericin B, and radiocontrast agents, should be expected to compound kidney risk in a similar way, though dedicated studies for each combination are less robust.
Neuromuscular blocking agents deserve caution too. Because polymyxin B itself can block neuromuscular junctions, stacking it with anesthetic paralytics or aminoglycosides (which share a neuromuscular blocking effect) raises the chance of prolonged weakness or respiratory depression.
Synergistic Combinations for Better Killing
When polymyxin B is combined with certain other antibiotics, the antimicrobial effect can be far greater than either drug alone. The best-studied synergistic partner class is the rifamycins. In lab testing against multidrug-resistant A. baumannii, polymyxin B combined with rifabutin showed synergy in 90 percent of isolates, with rifampicin achieving synergy in about 60 percent. At half the usual minimum inhibitory concentration, these rifamycin-polymyxin B combinations could completely clear the bacteria.20PubMed. Comparison of in vitro synergy between polymyxin B or colistin in combination with 16 antimicrobial agents against multidrug-resistant Acinetobacter baumannii isolates
Polymyxin B combined with rifampicin showed bactericidal activity in about 42 percent of carbapenem-resistant A. baumannii isolates, the highest rate among combinations tested in a Singapore study.21PLoS ONE. In-Vitro Activity of Polymyxin B, Rifampicin, Tigecycline Alone and in Combination against Carbapenem-Resistant Acinetobacter baumannii in Singapore Triple combinations of polymyxin B, imipenem, and rifampin have also been tested and were bactericidal against all eight multidrug-resistant A. baumannii isolates within 24 hours in an earlier study.22PubMed Central. In vitro double and triple synergistic activities of Polymyxin B, imipenem, and rifampin against multidrug-resistant Acinetobacter baumannii These are all in vitro results, so the clinical translation is not guaranteed, but they point to combination therapy as the standard approach for serious polymyxin B-treated infections rather than monotherapy.
Alternative Routes of Administration
Polymyxin B is typically given intravenously, but some infections require alternative delivery. For respiratory tract infections, aerosolized polymyxin B has been studied in animal models, where it achieved far higher drug concentrations in lung fluid than intravenous administration could, while also reducing lung inflammation and preserving tissue integrity.23PubMed Central. Aerosolized Polymyxin B for Treatment of Respiratory Tract Infections This route potentially sidesteps much of the systemic toxicity, though clinical evidence in humans is still catching up.
For central nervous system infections caused by carbapenem-resistant organisms, polymyxin B can be delivered directly into the cerebrospinal fluid via intrathecal or intraventricular injection. A retrospective study of patients with hospital-acquired CNS infections treated with intrathecal or intraventricular polymyxin B reported a clinical cure rate of about 77 percent and a bacterial clearance rate of roughly 86 percent, with no adverse effects attributed to the local drug administration.24PubMed. Effectiveness of combination therapy with intrathecal or intraventricular administration of polymyxin B for hospital-acquired central nervous system infections caused by carbapenem-resistant Acinetobacter baumannii A prospective observational study in neurocritical care patients confirmed the efficacy and safety of this approach when combined with systemic antibiotics.25PubMed. Efficacy analysis of intraventricular/intrathecal polymyxin B sulphate injections in the treatment of Post-neurosurgical CNS infections in neurocritical ill patients
How Polymyxin B Compares to Colistin
Colistin (polymyxin E) is the other polymyxin used clinically, and the two drugs are sometimes discussed interchangeably. That is a mistake. While they have nearly identical activity in the lab, they behave very differently in the body. Colistin is given intravenously as an inactive prodrug, colistin methanesulfonate (CMS), which must convert to active colistin in the body. That conversion is slow and incomplete, making blood levels harder to predict and optimize.26PubMed Central. Colistin and polymyxin B: peas in a pod, or chalk and cheese? Polymyxin B, by contrast, is administered as the active compound, so therapeutic levels are reached faster and more reliably.27PubMed Central. Pharmacokinetics/pharmacodynamics of colistin and polymyxin B: are we there yet?
When it comes to toxicity, one comparison study found that kidney injury rates were similar between the two (about 45 percent with polymyxin B and 40 percent with CMS), but polymyxin B caused more neurological side effects, mostly paresthesias (tingling and numbness), occurring in 13 of 77 patients versus 1 of 70 with CMS. All neurological events resolved after stopping the drug. Interestingly, CMS was withdrawn more often after kidney injury occurred, and kidney recovery was slightly more frequent after CMS discontinuation than after stopping polymyxin B.28PubMed Central. Polymyxin B vs. colistin: the comparison of neurotoxic and nephrotoxic effects of the two polymyxins The overall clinical pharmacology picture tends to favor polymyxin B for its more predictable drug levels, but the choice between the two often comes down to regional availability and institutional preference.
Endotoxin Neutralization Beyond Antibiotic Use
Polymyxin B has a second clinical life unrelated to its antibiotic function. Because it binds and neutralizes endotoxin (the LPS released from dying Gram-negative bacteria), it has been harnessed for extracorporeal blood purification in sepsis. A device called Toraymyxin uses polymyxin B attached to polystyrene fibers inside a hemoperfusion column. The patient’s blood is run through the column, and circulating endotoxin sticks to the polymyxin B without the drug entering the patient’s bloodstream, avoiding the nephrotoxicity and neurotoxicity of systemic dosing. This device was approved for clinical use in Japan in 1994 for treating endotoxemia and septic shock.29PubMed Central. Anti-endotoxin Properties of Polymyxin B-immobilized Fibers The evidence for its benefit in sepsis outcomes has been debated in subsequent large trials, but the device illustrates just how tightly polymyxin B binds LPS, a property central to both its antibiotic action and its endotoxin-neutralizing capacity.30PubMed. Mapping residue-specific contacts of polymyxin B with lipopolysaccharide by saturation transfer difference NMR