Polymyxin B Ophthalmic Concentration: Both Are Equivalent
The recommended concentration is 10,000 units/mL, which is exactly equivalent to 1 mg/mL, since 1 mg of polymyxin B sulfate equals 10,000 units. 1, 2
Understanding the Conversion
- The critical conversion factor is: 1 mg polymyxin B sulfate = 10,000 units 1, 2
- This means that 10,000 units/mL and 1 mg/mL represent the identical concentration—they are simply different ways of expressing the same amount 1
- This conversion is specific to polymyxin B and differs significantly from colistin (polymyxin E), where 1 million IU = approximately 80 mg of colistimethate sodium 1, 2
FDA-Approved Ophthalmic Formulation
- The FDA-approved polymyxin B ophthalmic solution contains polymyxin B sulfate at 10,000 units/mL (equivalent to 1 mg/mL) combined with trimethoprim 1 mg/mL 3
- This formulation is preserved with benzalkonium chloride 0.04 mg/mL and has a pH of 4.0-6.2 3
- The osmolality is 270-310 mOsm/kg, making it suitable for topical ocular use 3
Clinical Context
- When prescribing or preparing polymyxin B eye drops, either notation (10,000 units/mL or 1 mg/mL) is correct and refers to the same concentration 3
- The common pitfall is confusing polymyxin B unit conversions with colistin conversions—these are completely different antibiotics with different conversion factors 1, 2
- For ophthalmic use, polymyxin B demonstrates bactericidal activity against gram-negative organisms, particularly Pseudomonas aeruginosa, E. coli, Klebsiella pneumoniae, and Haemophilus influenzae 3