What is the Difference Between Polymyxin B and Colistin?

Polymyxin B and colistin (polymyxin E) are both polypeptide antibiotics with indistinguishable microbiological activity in vitro, but they differ in their administration and pharmacological properties. The main differences between polymyxin B and colistin are:

1. Form of administration: Polymyxin B is administered directly as the active antibiotic, while colistin is administered as the inactive prodrug, colistin methanesulfonate (CMS). CMS is converted in vivo to the active form, with variable and slow (hours) conversion.
2. Pharmacokinetics: Polymyxin B achieves therapeutic concentrations rapidly, provides more predictable serum concentrations, and does not require renal dose adjustments. In contrast, colistin's peak serum concentration may not be reached for up to 7 hours, serum concentrations are unpredictable due to significant interpatient variability, and renal dose adjustments must be considered.
3. Urinary excretion: Colistin is excreted in the urine as the prodrug CMS, which is readily converted to its active form in urine. Polymyxin B is minimally excreted in urine.
4. Nephrotoxicity: Polymyxin B may be less nephrotoxic than colistin.
5. Clinical usage: Colistin is preferred for urinary tract infections due to its ready conversion to the active form in urine. Polymyxin B is preferred for non-urinary infections due to its more predictable pharmacokinetics and fewer renal side effects.

In summary, while polymyxin B and colistin have similar antibacterial activities, they differ in their administration, pharmacokinetics, urinary excretion, and nephrotoxicity. These differences have implications for the choice of antibiotic depending on the type of infection and patient-specific factors.

Comparative Table: Polymyxin B vs Colistin

Polymyxin B and colistin are both antibiotics used to treat infections caused by multidrug-resistant bacteria. They have similar structures and microbiological activities in vitro, but there are differences in their administration and pharmacokinetics. Here is a comparison table of the two antibiotics:

Due to their similarities in structure, polymyxin B and colistin have been regarded as being equivalent, but differences in their pharmacokinetics have significant clinical implications for their use. Based on their kinetics and toxicity profiles, polymyxin B may be the preferred agent for treatment of systemic infections, while colistin is preferred for infections limited to the urinary tract.