Colistin

Colistin, also known as polymyxin E, is an antibiotic produced by certain strains of the bacteria Paenibacillus polymyxa. Colistin is a mixture of the cyclic polypeptides colistin A and B and belongs to the class of polypeptide antibiotics known as polymyxins. Colistin is effective against most Gram-negative bacilli. Colistin, also known as polymyxin E, is an antibiotic produced by certain strains of the bacteria Paenibacillus polymyxa. Colistin is a mixture of the cyclic polypeptides colistin A and B and belongs to the class of polypeptide antibiotics known as polymyxins. Colistin is effective against most Gram-negative bacilli. Colistin is a decades-old drug that fell out of favor in human medicine due to its kidney toxicity. It remains one of the last-resort antibiotics for multidrug-resistant Pseudomonas aeruginosa, Klebsiella pneumoniae, and Acinetobacter. NDM-1 metallo-β-lactamase multidrug-resistant Enterobacteriaceae have also shown susceptibility to colistin. Resistance to colistin in human pathogens is rare. The first colistin-resistance gene in a plasmid which can be transferred between bacterial strains was found in 2011 in China and became publicly known in November 2015. The presence of this plasmid-borne mcr-1 gene was confirmed starting December 2015 in South-East Asia, several European countries and the United States. Colistin has been effective in treating infections caused by Pseudomonas, Escherichia, and Klebsiella species. The following represents MIC susceptibility data for a few medically significant microorganisms: For example, colistin in combination with other drugs are used to attack P. aeruginosa biofilm infection in lungs of CF patients. 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 motility and form new aggregates via quorum sensing. Two forms of colistin are available commercially: colistin sulfate and colistimethate sodium (colistin methanesulfonate sodium, colistin sulfomethate sodium). Colistin sulfate is cationic; colistimethate sodium is anionic. Colistin sulfate is stable, but 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 . Colistin sulfate and colistimethate sodium may both be given intravenously, but the dosing is complicated. The very different labeling of the parenteral products of colistin methanesulfonate in different parts of the world was first revealed by Li et al. Colistimethate sodium manufactured by Xellia (Colomycin injection) is prescribed in international units, but colistimethate sodium manufactured by Parkdale Pharmaceuticals (Coly-Mycin M Parenteral) is prescribed in milligrams of colistin base: 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 (480 mg for Colomycin and 720 mg for Coly-Mycin). 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 any physician. Colistin has been used in combination with rifampicin, and 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.