Polymyxin B (Polymyxin b Sulfate) - Description and Clinical Pharmacology

DESCRIPTION

Polymyxin B for Injection is one of a group of basic polypeptide antibiotics derived from B polymyxa (B aerosporous). Polymyxin B sulfate is the sulfate salt of Polymyxins B₁ and B₂, which are produced by the growth of Bacillus polymyxa (Prazmowski) Migula (Fam. Bacillacea). It has a potency of not less than 6000 polymyxin B units per mg, calculated on the anhydrous basis. The structural formulae are:

Each vial contains 500,000 polymyxin B units for parenteral or ophthalmic administration.

Polymyxin B for Injection is in powder form suitable for preparation of sterile solutions for intramuscular, intravenous drip, intrathecal, or ophthalmic use.

In the medical literature, dosages have frequently been given in terms of equivalent weights of pure polymyxin B base. Each milligram of pure polymyxin B base is equivalent to 10,000 units of polymyxin B and each microgram of pure polymyxin B base is equivalent to 10 units of polymyxin B.

Aqueous solutions of polymyxin B sulfate may be stored up to 12 months without significant loss of potency if kept under refrigeration. In the interest of safety, solutions for parenteral use should be stored under refrigeration and any unused portion should be discarded after 72 hours. Polymyxin B sulfate should not be stored in alkaline solutions since they are less stable.

CLINICAL PHARMACOLOGY

Polymyxin B sulfate has a bactericidal action against almost all gram-negative bacilli except the Proteus group. Polymyxins increase the permeability of bacterial cell wall membranes. All grampositive bacteria, fungi, and the gram-negative cocci, N gonorrhoeae and N meningitidis, are resistant.

Susceptibility plate testing: If the Kirby-Bauer method of disc susceptibility testing is used, a 300-unit polymyxin B disc should give a zone of over 11 mm when tested against a polymyxin B susceptible bacterial strain.

Polymyxin B sulfate is not absorbed from the normal alimentary tract. Since the drug loses 50 percent of its activity in the presence of serum, active blood levels are low. Repeated injections may give a cumulative effect. Levels tend to be higher in infants and children. The drug is excreted slowly by the kidneys. Tissue diffusion is poor and the drug does not pass the blood brain barrier into the cerebrospinal fluid. In therapeutic dosage, polymyxin B sulfate causes some nephrotoxicity with tubule damage to a slight degree.