Antibiotics

• Polymyxins (Colistin)

They are group of polypeptide antibiotics, polymyxin B and polymyxin E, or colistin, are most commonly used topically and PO.

 

Polymyxins are bactericidal; they interact strongly with phospholipids in bacterial cell membranes and radically disrupt their permeability and function.

The polymyxins are more effective against gram-negative than gram-positive bacteria. Their rather narrow spectrum includes Enterobacter, Klebsiella, Salmonella, Pasteurella, Bordetella, and Shigella spp, and Escherichia coli.

 

Polymyxins act synergistically when combined with potentiated sulfonamides, tetracyclines, and some other antibacterials; they also reduce the activity of endotoxins in body fluids and may be beneficial in endotoxemia. Their action is inhibited by divalent cations, unsaturated fatty acids, and quaternary ammonium compounds.

 

Polymyxins are not absorbed after PO or topical administration; plasma concentrations peak ~2 hr after parenteral administration Blood concentrations usually are low because polymyxins bind to cell membranes as well as tissue debris and purulent exudates

The polymixins undergo renal elimination mostly as degradation products

They are notably nephrotoxic and neurotoxic. Neuromuscular blockade can be seen at higher concentrations Intense pain at sites of injection and hypersensitivity reactions also can be expected

 

The main indication for parenteral use of polymyxins is life-threatening infection due to gram-negative bacilli or Pseudomonas spp that are resistant to other drugs. Polymyxins are also used PO against susceptible intestinal infections. Anti-endotoxin binding activity is an additional therapy via slow IV bolus. Topical application is common, eg, for otitis externa. Recommended dose rates for polymyxins vary considerably.

 

20.000 U/kg, PO, bid;

5.000 U/kg, IM, bid;

50,000-100,000 U by intramammary infusion;

100.000      U intrauterine in cattle.

IV administration of polymyxins is potentially dangerous.