Polymyxin B Renal Dosing
For patients with normal renal function, administer a loading dose of 2-2.5 mg/kg followed by a maintenance dose of 1.5-3 mg/kg/day divided into 2 doses; critically, no dose adjustment is necessary for patients with renal impairment or those on continuous renal replacement therapy (CRRT), as polymyxin B plasma concentrations are not influenced by renal function. 1
Dosing by Renal Function Status
Normal Renal Function
- Loading dose: 2-2.5 mg/kg administered once 1
- Maintenance dose: 1.5-3 mg/kg/day divided into 2 doses (every 12 hours) 1
- Maximum daily dose: Do not exceed 25,000 units/kg/day (approximately 3 mg/kg/day) 2
- Continuous infusion may be suitable as an alternative administration method 1
Moderate to Severe Renal Impairment
- Loading dose: 2-2.5 mg/kg (same as normal renal function—always administer full loading dose) 1
- Maintenance dose: 1.5-3 mg/kg/day divided into 2 doses 1
- Critical distinction: Unlike colistin, polymyxin B dosing is calculated based on body weight and plasma concentrations are NOT influenced by renal function 1
- Recent pharmacokinetic studies demonstrate decreased clearance in renal insufficiency (2.1 L/h vs 3.9 L/h in normal function), suggesting potential for toxicity accumulation 3, 4
- Practical recommendation: For patients with creatinine clearance <80 mL/min, consider a fixed maintenance dose of 60 mg every 12 hours (approximately 100 mg/day total) to balance efficacy with reduced nephrotoxicity risk 5
Continuous Renal Replacement Therapy (CRRT)
- Loading dose: 2-2.5 mg/kg 1
- Maintenance dose: 1.5-3 mg/kg/day divided into 2 doses—no dose adjustment necessary 1
- Dialysis clearance accounts for only 5.6-12.2% of total body clearance 6
- A fixed maintenance dose of 100 mg every 12 hours is optimal for patients on CVVHD 3, 7
- The incidence of renal failure appears lower with polymyxin B compared to colistin 1
Intermittent Hemodialysis
- Dosing: Standard dosing regimen should be maintained 2, 8
- Administer after dialysis session when possible 8
- A case report successfully used 2.5 mg/kg loading dose followed by 1 mg/kg on days 4 and 8, then 0.8 mg/kg daily 8
Key Pharmacokinetic Principles
Why Polymyxin B Differs from Colistin
- Polymyxin B is administered as the active drug, NOT as an inactive prodrug like colistimethate sodium (CMS) 1
- Pharmacokinetic findings for colistin/CMS cannot be extrapolated to polymyxin B 1
- Renal function has minimal impact on polymyxin B plasma concentrations, unlike colistin which requires significant dose adjustments 1
Loading Dose Rationale
- A loading dose is essential to achieve optimal plasma levels on the first day of therapy 1
- This applies to ALL patients regardless of renal function status 1
Safety Considerations and Monitoring
Nephrotoxicity Risk
- Polymyxin B demonstrates lower incidence of renal failure compared to colistin 1
- In elderly patients, keeping plasma concentrations below specific thresholds reduces AKI risk: 9
- Cmin <2.94 mg/L
- Css,avg <4.14 mg/L
- AUCss,24h <99.35 mg·h/L
- Patients with renal insufficiency show decreased clearance and are at higher risk for toxicity 3, 4
Therapeutic Drug Monitoring
- Consider TDM in patients with extremes of renal function to optimize dosing and minimize toxicity 3, 4
- Target AUC0-24h of 50-100 mg·h/L for optimal efficacy while minimizing nephrotoxicity 5, 9
Common Pitfalls to Avoid
- Do not reduce polymyxin B doses in renal impairment using the same approach as colistin—this is a critical error based on different pharmacokinetics 1
- Do not omit the loading dose in any patient population, including those with renal dysfunction 1
- Do not increase doses for patients on CRRT—standard dosing is appropriate 1
- Be aware that FDA labeling recommends dose reduction for renal impairment 2, but this contradicts current pharmacokinetic evidence showing renal function does not significantly affect polymyxin B concentrations 1