|Trade names||Poly-Rx, others|
|Topical, intramuscular, intravenous, intrathecal, eye drops|
|CompTox Dashboard (EPA)|
|Chemical and physical data|
|Molar mass||1301.57 g·mol−1|
|(what is this?)|
Polymyxin B, sold under the brand name Poly-Rx among others, is an antibiotic used to treat meningitis, pneumonia, sepsis, and urinary tract infections. While it is useful for many Gram negative infections, it is not useful for Gram positive infections. It can be given by injection into a vein, muscle, or cerebrospinal fluid or inhaled. The injectable form is generally only used if other options are not available. It is also available as the combinations bacitracin/polymyxin B and neomycin/polymyxin B/bacitracin for use on the skin.
Common side effects when given by injection include kidney problems, neurological problems, fever, itchiness, and rash. Injections into muscle may result in significant pain. Other serious side effects may include fungal infections, anaphylaxis, and muscle weakness. It is unclear if use during pregnancy is safe for the baby. Polymyxin B works by breaking down the cytoplasmic membrane which generally results in bacterial cell death.
Polymyxin B was approved for medical use in the United States in 1964. It is on the World Health Organization's List of Essential Medicines, the safest and most effective medicines needed in a health system. It is available as a generic medication. In Europe it is only approved to be applied to the skin as of 2015. It is derived from the bacterium Paenibacillus polymyxa also known as Bacillus polymyxa.
Spectrum of susceptibility
Polymyxin B has been used to treat urinary tract infections and meningitis caused by Pseudomonas aeruginosa and Haemophilus influenzae, respectively. The following represents MIC susceptibility data for a few medically significant microorganisms.
- Haemophilus influenzae: ≥0.8 μg/ml
- Pseudomonas aeruginosa: 0.25 μg/ml – 1 μg/ml
An endotoxin removal cartridge (Toraymyxin) is a blood purification medical device and it uses polymyxin B as immobilized adsorbent.
Mechanism of action
- Alters bacterial outer membrane permeability by binding to a negatively charged site in the lipopolysaccharide layer, which has an electrostatic attraction for the positively charged amino groups in the cyclic peptide portion  (this site normally is a binding site for calcium and magnesium counter ions); the result is a destabilized outer membrane
- Fatty acid portion dissolves in hydrophobic region of cytoplasmic membrane and disrupts membrane integrity
- Leakage of cellular molecules, inhibition of cellular respiration
- Binds and inactivates endotoxin
- Relative absence of selective toxicity: nonspecific for cell membranes of any type, highly toxic.
Polymyxin B is composed of polymyxins B1, B1-I, B2, B3, and B6. Polymyxins B1 and B2 are considered major components. These related components are structurally identical with the exception of a variable fatty acid group on each fraction. Results from in vitro studies have shown marginal differences in MIC data when comparing the fractions.
Polymyxin B is also used to induce envelope stress in order to study the organisms response to such stress. Polymyxin envelope stress assays such as this have been used for the study of small RNA (sRNA) responses in Salmonella enterica.
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