Polymyxin B

Polymyxin B is a cyclic polypeptide antibacterial with activity against selected gram-negative organisms. It is used mainly in local antimicrobial preparations, with specialist systemic use for some resistant infections. A separate veterinary application, particularly in horses, exploits binding of bacterial endotoxin to reduce some endotoxin-mediated inflammatory effects.

Group

Polypeptide antibacterial and endotoxin-binding agent

Mechanism of Action

Polymyxin B binds negatively charged components of the gram-negative outer membrane and disrupts membrane organization and permeability, causing bacterial death. Binding to the lipid A region of lipopolysaccharide also reduces interaction of accessible endotoxin with host inflammatory pathways. The antibacterial and endotoxin-binding actions overlap chemically but provide distinct clinical rationales.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Bacterial infection	15000–25000 IU/kg	IV	Every 12 h	Reported dilution: 500000 IU in 300-500 mL 5% dextrose for infusion.
Bacterial infection	30000 IU/kg	IM	Every 24 h	Daily IM amount; reported dissolves 500000 IU in 2 mL suitable diluent. Painful injection.
Endotoxemia	1 mg/kg	IV		Dilute in 10 mL saline and infuse. Milligram dose is polymyxin B base; 1 mg base equals 10000 IU.

Cats

Use / stage	Dose	Route	Schedule	Instructions
Bacterial infection	15000–25000 IU/kg	IV	Every 12 h	Reported dilution: 500000 IU in 300-500 mL 5% dextrose for infusion.
Bacterial infection	30000 IU/kg	IM	Every 24 h	Daily IM amount; reported dissolves 500000 IU in 2 mL suitable diluent. Painful injection.
Endotoxemia	1 mg/kg	IV		Dilute in 10 mL saline and infuse. Milligram dose is polymyxin B base; 1 mg base equals 10000 IU.

Horses

Use / stage	Dose	Route	Schedule	Instructions
Endotoxemia	1000–10000 IU/kg	IV	Every 8 h	Usually 6000 IU/kg. Infuse in 1 L saline over 15 minutes; reported regimen describes five treatments.
Endotoxemia	6000 IU/kg	IV	Every 8 h	Typical regimen; 15-minute infusion, five treatments described.

Indications

Local treatment of susceptible superficial, ocular or otic bacterial infections with a corresponding preparation.
Specialist treatment of selected serious polymyxin-susceptible resistant gram-negative infections, with limited veterinary outcome evidence.
Adjunctive treatment of selected equine endotoxemia to bind accessible circulating endotoxin.

Contraindications

Renal disease: avoid or use with particular caution.

Interactions

Nephrotoxins increase renal risk; neuromuscular blockers and other neurotoxic drugs increase respiratory risk.
Calcium, magnesium, strong acids/alkalis and anionic detergents can be incompatible.

Adverse effects

Nephrotoxicity, injection pain, allergy and respiratory depression at high systemic doses.

Preparations

Preparation	Form	Strength	Route
Polymyxin B 500000 IU vial	Injection	500000 IU / vial	IV, IM
Terramycin oxytetracycline/polymyxin B ophthalmic ointment Sterile ocular ointment contains oxytetracycline HCl 5 mg and polymyxin B sulfate activity 10000 IU per gram; no single-component calculator opt-in.	Other	polymyxin b 10000 IU / g + oxytetracycline hydrochloride 5 mg / g	TOPICAL

Monitoring and Administration

Maintain renal perfusion and hydration; monitor BUN, creatinine and electrolytes. Use microbiology to justify systemic antibacterial treatment.

Intrathecal meningitis instructions conflict with a simultaneous oral-route statement: 500000 IU in 10 mL saline, 20000 IU daily then 25000 IU every other day. This local procedure requires route clarification and is not represented as IV or oral dosing.

Ophthalmic polymyxin B provides gram-negative activity and is combined with other antibacterials for broader coverage. Local hypersensitivity is possible; no numeric ocular application schedule is specified here.