What is the appropriate indication, dosage, and treatment duration for Polymyxin B (10,000 IU) and Trimethoprim (1 mg) eye drops in bacterial conjunctivitis?

Polymyxin B and Trimethoprim Eye Drops for Bacterial Conjunctivitis

Polymyxin B (10,000 IU) and Trimethoprim (1 mg) eye drops are FDA-approved for treating surface ocular bacterial infections including acute bacterial conjunctivitis and blepharoconjunctivitis, dosed as 1 drop every 3 hours (maximum 6 doses daily) for 7-10 days in mild to moderate infections. 1, 2

Indications

Polymyxin B/Trimethoprim ophthalmic solution is specifically indicated for bacterial infections caused by susceptible organisms including 1, 2:

• Staphylococcus aureus
• Staphylococcus epidermidis
• Streptococcus pneumoniae
• Streptococcus viridans
• Haemophilus influenzae
• Pseudomonas aeruginosa (though efficacy studied in fewer than 10 infections)

The combination provides broad-spectrum coverage for the most common pathogens causing bacterial conjunctivitis. 3

Dosing Regimen

Adults

• 1 drop in affected eye(s) every 3 hours (maximum 6 doses per day)
• Duration: 7-10 days for mild to moderate infections 2

Pediatric Patients

• Same dosing as adults for children over 2 months of age
• Clinical studies have demonstrated safety and efficacy in this age group 2

Comparative Efficacy Considerations

While Polymyxin B/Trimethoprim remains effective, recent evidence shows important comparative limitations:

Speed of Clinical Resolution

Newer fluoroquinolones achieve faster clinical cure rates. A multicenter pediatric study found that at 48 hours, moxifloxacin achieved complete resolution in 81% of patients versus only 44% with Polymyxin B/Trimethoprim (P=0.001). 4 However, by days 7-10, clinical cure rates were equivalent (95-96%), demonstrating that Polymyxin B/Trimethoprim is ultimately effective but slower-acting. 5

Cost-Effectiveness

Polymyxin B/Trimethoprim offers significant cost savings compared to fluoroquinolones while maintaining comparable final clinical cure rates, making it a reasonable first-line option when rapid resolution is not critical. 5

Bacterial Eradication

The combination demonstrates effective microbiological cure against common pathogens, with Haemophilus influenzae being the most prevalent organism isolated in clinical trials. 3 No statistically significant differences in bacteriologic cure rates were found compared to chloramphenicol or fluoroquinolones at treatment completion. 6, 7, 5

Clinical Context and Limitations

When to Consider Alternatives

Fluoroquinolones should be preferred over Polymyxin B/Trimethoprim when 8:

• Rapid symptom resolution is needed to minimize disease transmission
• Contact lens-related infections are suspected (broader Pseudomonas coverage)
• Severe or vision-threatening infections are present

The American Academy of Pediatrics guidelines list topical fluoroquinolones (besifloxacin, levofloxacin, gatifloxacin, ciprofloxacin, moxifloxacin, ofloxacin) as preferred agents for bacterial conjunctivitis, though they note systemic toxicity is not a concern with topical therapy and selection should be based on suspected pathogens, efficacy, tolerability, and cost. 8

Antibiotic Resistance Concerns

Increasing fluoroquinolone resistance has been documented from 2005-2015, particularly in MRSA and Pseudomonas aeruginosa. 8 This makes older combination agents like Polymyxin B/Trimethoprim potentially valuable for antibiotic stewardship, as they avoid contributing to fluoroquinolone resistance patterns. 9

Treatment Expectations

Natural History Context

Approximately 55% of bacterial conjunctivitis cases resolve spontaneously without antibiotic treatment by days 4-9. 9 Antibiotic use increases clinical cure rates by approximately 26% compared to placebo (RR 1.26,95% CI 1.09-1.46), with patients 27% less likely to have persistent clinical infection. 9

Follow-Up Recommendations

• Patients should return in 3-4 days if no improvement is noted 8
• Treatment incompletion is less likely with antibiotics versus placebo (RR 0.64) 9
• No serious systemic side effects have been reported with topical Polymyxin B/Trimethoprim 9

Safety Profile

The combination is well-tolerated with no adverse reactions reported in multiple double-blind trials. 6, 7 Ocular side effects appear less frequent with fluoroquinolones compared to non-fluoroquinolone combinations, though evidence certainty is very low. 9