What are the recommended dosing regimens, administration routes, and monitoring parameters for colistimethate sodium (colistin) and polymyxin B in treating serious multidrug‑resistant Gram‑negative infections?

Polymyxin Dosing and Administration for Multidrug-Resistant Gram-Negative Infections

For colistimethate sodium (colistin), administer a loading dose of 6-9 million IU followed by maintenance doses of 4.5 million IU every 12 hours in critically ill patients with creatinine clearance above 50 mL/min; for polymyxin B, use a loading dose of 2-2.5 mg/kg followed by 1.5-3 mg/kg/day divided into 2 doses. 1, 2

Colistimethate Sodium (Colistin) Dosing

Loading Dose - Critical for Optimal Therapy

• 6-9 million IU administered to all patients, including those with renal dysfunction 1
• The loading dose is essential because colistin displays a relatively long half-life, and without it, plasma concentrations remain sub-optimal for 2-3 days before reaching steady state 1
• Pharmacokinetic studies demonstrate that intravenous CMS administration without loading doses leads to suboptimal plasma concentrations associated with higher mortality 1

Maintenance Dosing

For patients with normal renal function (CrCl >50 mL/min):

• 4.5 million IU every 12 hours (total 9 million IU/day in 2-3 divided doses) 1, 2
• Alternative formulation: 2.5 mg/kg (equivalent to 5 mg/kg loading) followed by maintenance dose calculated as: 2.5 mg × (1.5 × CrCl + 30) twice daily 2

Renal dose adjustments:

• CrCl 50-79 mL/min: 2.5-3.8 mg/kg divided into 2 doses daily 3
• CrCl 30-49 mL/min: 2.5 mg/kg once daily or divided into 2 doses 3
• CrCl 10-29 mL/min: 1.5 mg/kg every 36 hours 3

Renal replacement therapy:

• Continuous renal replacement therapy (CRRT): At least 9 million IU/day with normal loading dose 1
• Intermittent hemodialysis: 2 million IU every 12 hours with normal loading dose; perform dialysis toward the end of dosing interval 1

Critical Dosing Unit Conversion

Pay close attention to avoid medication errors - colistin has multiple dosing unit conventions 1, 4:

• 1 million IU = 80 mg CMS (mass) = 33 mg colistin base activity (CBA)
• Colistin sulfate: 1 mg = 22,700 IU

Polymyxin B Dosing

Key Pharmacokinetic Differences

Polymyxin B is administered as the active drug (not a prodrug like colistin), and its dosing is weight-based with plasma concentrations not influenced by renal function 1. This represents a significant advantage in critically ill patients with fluctuating renal function.

Dosing Regimen

• Loading dose: 2-2.5 mg/kg 1
• Maintenance: 1.5-3 mg/kg/day divided into 2 doses 1
• Continuous infusion may be suitable as an alternative administration method 1

Renal replacement therapy:

• No dose adjustment necessary for patients on CRRT - use same doses as normal renal function 1

Safety Advantage

Evidence suggests polymyxin B has a lower incidence of renal failure compared to colistin 1, making it a potentially preferable alternative when both agents are available and active in vitro.

Administration Routes and Methods

Intravenous Administration of Colistin

Two acceptable methods 3:

1. Direct intermittent administration: Inject one-half of total daily dose over 3-5 minutes every 12 hours

2. Continuous infusion (preferred for critically ill):

   • Inject one-half of total daily dose over 3-5 minutes
   • Add remaining half to compatible IV solution (0.9% NaCl, D5W, D5 0.9% NaCl, D5 0.45% NaCl, D5 0.225% NaCl, Lactated Ringer's, or 10% invert sugar)
   • Infuse over 22-23 hours starting 1-2 hours after initial dose
   • Prepare fresh solutions; use within 24 hours 3

Intramuscular Administration

• Deep IM injection into large muscle mass (gluteal or lateral thigh)
• Store reconstituted solution refrigerated (2-8°C) or at room temperature (20-25°C); use within 7 days 3

Monitoring Parameters

Mandatory Renal Function Monitoring

Monitor renal function closely during colistin therapy - this is a strong recommendation given that acute kidney injury is one of the most important factors related to clinical failure and mortality 2. The guidelines emphasize:

• Baseline creatinine clearance before initiation
• Serial monitoring during therapy (frequency based on clinical status)
• Avoid concomitant nephrotoxic and ototoxic drugs 4

Therapeutic Drug Monitoring (TDM)

While not universally available, TDM is warranted where possible due to high interpatient variability in plasma colistin concentrations 5. This is particularly important in critically ill patients with:

• Augmented renal clearance
• Fluctuating renal function
• Infections with MIC ≥1 mg/L

Combination Therapy Considerations

Evidence for Combination Therapy

The evidence on combination therapy remains controversial 2. However, based on available data:

• Colistin combination therapy may be considered over monotherapy, particularly when:

  • The pathogen has in vitro susceptibility to additional agents 2
  • MIC is >1 mg/L (increasing colistin dose alone may not be feasible due to nephrotoxicity risk) 5
  • Treating severe infections (pneumonia, bacteremia) 4

• Specific combinations with evidence:

  • Colistin + carbapenem (if meropenem MIC ≤8 mg/L for CRE or ≤32 mg/L for CRAB, using extended 3-hour infusion) 4
  • Colistin + fosfomycin
  • Colistin + rifampicin
  • Colistin + sulbactam

• For polymyxin B: Limited evidence suggests combination therapy may reduce mortality compared to monotherapy, even when the combined agent lacks in vitro activity 2

When Newer Agents Are Available

If new beta-lactam/beta-lactamase inhibitors (ceftazidime-avibactam, meropenem-vaborbactam, ceftolozane-tazobactam) are active in vitro, combination therapy with these agents is NOT recommended 6. Reserve polymyxins for situations where these newer agents are unavailable or inactive.

Clinical Pitfalls and Caveats

1. Dosing unit confusion: The multiple conventions for expressing colistin doses (IU, mg CMS, mg CBA) create high risk for medication errors. Always verify which unit system your institution uses 1, 4.

2. Delayed therapeutic effect: Without a loading dose, therapeutic colistin concentrations are not achieved for 2-3 days, potentially leading to treatment failure in severe infections 1.

3. Suboptimal lung penetration: Colistin may be undetectable in bronchoalveolar lavage at standard dosing, particularly problematic for ventilator-associated pneumonia 1. Consider adjunctive inhaled therapy for pulmonary infections.

4. Renal dosing errors: Unlike colistin, polymyxin B does NOT require renal dose adjustment, which is frequently misunderstood 1.

5. Nephrotoxicity risk stratification: Elderly patients and those with pre-existing chronic kidney disease have higher risk of colistin-induced nephrotoxicity; carefully weigh benefits versus harms 2.

6. Resistance concerns: Reserve polymyxins for carbapenem-resistant organisms or when all other options (beta-lactams, fluoroquinolones, tigecycline) show resistance 1.