Colistin


Colistin, also known as polymyxin E, is an antibiotic medication used as a last-resort treatment for multidrug-resistant Gram-negative infections including pneumonia. These may involve bacteria such as Pseudomonas aeruginosa, carbapenem-resistant Klebsiella pneumoniae, or Acinetobacter. It comes in two forms: colistimethate sodium can be injected into a vein, injected into a muscle, or inhaled, and colistin sulfate is mainly applied to the skin or taken by mouth. Colistimethate sodium is a prodrug; it is produced by the reaction of colistin with formaldehyde and sodium bisulfite, which leads to the addition of a sulfomethyl group to the primary amines of colistin. Colistimethate sodium is less toxic than colistin when administered parenterally. In aqueous solutions, it undergoes hydrolysis to form a complex mixture of partially sulfomethylated derivatives, as well as colistin. Resistance to colistin began to appear as of 2015.
Common side effects of the injectable form include kidney problems and neurological problems. Other serious side effects may include anaphylaxis, muscle weakness, and Clostridioides difficile-associated diarrhea. The inhaled form may result in constriction of the bronchioles. It is unclear if use during pregnancy is safe for the fetus. Colistin is in the polymyxin class of medications. It works by breaking down the cytoplasmic membrane, which generally results in bacterial cell death.
Colistin was discovered in 1947 and colistimethate sodium was approved for medical use in the United States in 1970. It is on the World Health Organization's List of Essential Medicines. The World Health Organization classifies colistin as critically important for human medicine. It is available as a generic medication. It is derived from bacteria of the genus Paenibacillus.

Medical uses

Antibacterial spectrum

Colistin has been effective in treating infections caused by Pseudomonas, Escherichia, and Klebsiella species. The following represents minimum inhibitory concentration susceptibility data for a few medically significant microorganisms:
  • Escherichia coli: 0.12–128 μg/mL
  • Klebsiella pneumoniae: 0.25–128 μg/mL
  • Pseudomonas aeruginosa: ≤0.06–16 μg/mL
For example, colistin in combination with other drugs is used to attack P. aeruginosa biofilm infection in lungs of patients with cystic fibrosis. Biofilms have a low-oxygen environment below the surface where bacteria are metabolically inactive, and colistin is highly effective in this environment. However, P. aeruginosa reside in the top layers of the biofilm, where they remain metabolically active. This is because surviving tolerant cells migrate to the top of the biofilm via pili and form new aggregates via quorum sensing.

Administration and dosage

Forms

Two forms of colistin are available commercially: colistin sulfate and colistimethate sodium. Colistin sulfate is cationic; colistimethate sodium is anionic. Colistin sulfate is stable, whereas colistimethate sodium is readily hydrolysed to a variety of methanesulfonated derivatives. Colistin sulfate and colistimethate sodium are eliminated from the body by different routes. With respect to Pseudomonas aeruginosa, colistimethate is the inactive prodrug of colistin. The two drugs are not interchangeable.
  • Colistimethate sodium may be used to treat Pseudomonas aeruginosa infections in patients with cystic fibrosis, and it has come into recent use for treating multidrug-resistant Acinetobacter infection, although resistant forms have been reported. Colistimethate sodium has also been given intrathecally and intraventricularly in Acinetobacter baumannii and Pseudomonas aeruginosa meningitis and ventriculitis Some studies have indicated that colistin may be useful for treating infections caused by carbapenem-resistant isolates of Acinetobacter baumannii.
  • Colistin sulfate may be used to treat intestinal infections, or to suppress colonic flora. Colistin sulfate is also used in topical creams, powders, and otic solutions.
  • Colistin A and colistin B can be purified individually to research and study their effects and potencies as separate compounds.

    Dosage

Colistin sulfate and colistimethate sodium may both be given intravenously, but the dosing is complicated. The different labeling of the parenteral products of colistin methanesulfonate in different parts of the world was noted by Li et al. Colistimethate sodium manufactured by Xellia is prescribed in international units, whereas colistimethate sodium manufactured by Parkdale Pharmaceuticals is prescribed in milligrams of colistin base:
  • Colomycin 1,000,000 units is 80 mg colistimethate;
  • Coly-mycin M 150 mg colistin base is 360 mg colistimethate or 4,500,000 units.
Because colistin was introduced into clinical practice over 50 years ago, it was never subject to the regulations that modern drugs are subject to, and therefore there is no standardised dosing of colistin and no detailed trials on pharmacology or pharmacokinetics. The optimal dosing of colistin for most infections is therefore unknown. Colomycin has a recommended intravenous dose of 1 to 2 million units three times daily for patients weighing 60 kg or more with normal renal function. Coly-Mycin has a recommended dose of 2.5 to 5 mg/kg colistin base a day, which is equivalent to 6 to 12 mg/kg colistimethate sodium per day. For a 60 kg man, therefore, the recommended dose for Colomycin is 240 to 480 mg of colistimethate sodium, yet the recommended dose for Coly-Mycin is 360 to 720 mg of colistimethate sodium. Likewise, the recommended "maximum" dose for each preparation is different. Each country has different generic preparations of colistin, and the recommended dose depends on the manufacturer. This complete absence of any regulation or standardisation of dose makes intravenous colistin dosing difficult for the physician.
Colistin has been used in combination with rifampicin; evidence of in vitro synergy exists, and the combination has been used successfully in patients. There is also in vitro evidence of synergy for colistimethate sodium used in combination with other antipseudomonal antibiotics.
Colistimethate sodium aerosol is used to treat pulmonary infections, especially in cystic fibrosis. In the UK, the recommended adult dose is 1–2 million units nebulised colistimethate twice daily. Nebulized colistin has also been used to decrease severe exacerbations in patients with chronic obstructive pulmonary disease and infection with Pseudomonas aeruginosa.

Resistance

Resistance to colistin is rare, but has been described., no agreement exists about how to define colistin resistance. The uses a MIC cut-off of 2 mg/L, whereas the British Society for Antimicrobial Chemotherapy sets a MIC cutoff of 4 mg/L or less as sensitive, and 8 mg/L or more as resistant. No standards for describing colistin sensitivity are given in the United States.
The first known colistin-resistance gene in a plasmid which can be transferred between bacterial strains is mcr-1. It was found in 2011 in China on a pig farm where colistin is routinely used and became publicly known in November 2015. The presence of this plasmid-borne gene was confirmed starting December 2015 in South-East Asia, several European countries, and the United States. It is found in certain strains of the bacteria Paenibacillus polymyxa.
India reported the first detailed colistin-resistance study, which mapped 13 colistin-resistant infections recorded over 18 months. It concluded that pan-drug-resistant infections, particularly those in the bloodstream, have a higher mortality. Multiple other cases were reported from other Indian hospitals. Although resistance to polymyxins is generally less than 10%, it is more frequent in the Mediterranean and South-East Asia, where colistin resistance rates are increasing. Colistin-resistant E. coli was identified in the United States in May 2016.
A recent review from 2016 to 2021 fount that E. coli is the dominant species harbouring mcr genes. Plasmid - mediated colistin resistance is also conferred upon other species that carry different genes resistant to antibiotics. The emergence of the mcr-9 gene is quite remarkable.
Use of colistin to treat Acinetobacter baumannii infections has led to the development of resistant bacterial strains. They have also developed resistance to the antimicrobial compounds LL-37 and lysozyme, produced by the human immune system. This cross-resistance is caused by gain-of-function mutations to the pmrB gene, which controls the expression of lipid A phosphoethanolamine transferases located on the bacterial chromosome. Similar results have been obtained with mcr-1 positive E. coli, which became better at surviving a mixture of animal antimicrobial peptides in vitro and more effective at killing infected caterpillars.
Not all resistance to colistin and some other antibiotics is due to the presence of resistance genes. Heteroresistance, the phenomenon wherein apparently genetically identical microbes exhibit a range of resistance to an antibiotic, has been observed in some species of Enterobacter since at least 2016 and was observed in some strains of Klebsiella pneumoniae in 2017–2018. In some cases this phenomenon has significant clinical consequences.

Inherently resistant

  • Brucella
  • Burkholderia cepacia
  • Chryseobacterium indologenes
  • Edwardsiella
  • Elizabethkingia meningoseptica
  • Francisella tularensis spp.
  • Gram-negative cocci
  • Helicobacter pylori
  • Moraxella catarrhalis
  • Morganella spp.
  • Neisseria gonorrhoeae and Neisseria meningitidis
  • Proteus
  • Providencia
  • Serratia
  • Some strains of ''Stenotrophomonas maltophilia''