Cyclic Lipopeptides and Polymyxins

Membrane Destroyer Agents

Daptomycin and colistin are agents with physical bactericidal action that destroy the cell membrane of multidrug-resistant bacteria. Its therapeutic range requires caution in its usual clinical dosage.

Mechanism

💊 Generic and Commercial Names

Lipopeptides: daptomycin (Cubicin). Polymyxins: colistin / colistimethate sodium (Coly-Mycin, Promixin).

🔬 Pharmacological Group

Cyclic lipopeptides (Gram-positive) and polymyxins or lipid surfactants (Gram-negative). Membrane-disrupting agents.

Mechanism of Action

Its target physically destroys the electrochemical gradient of the cell membrane:

Membrane Depolarization by Daptomycin (Gram-Positive Bacteria): It binds specifically to the bacterial cytoplasmic membrane through a cation-dependent electrostatic interaction. calcium (Ca²⁺). By capturing calcium, its lipid tail is inserted into the lipid membrane of Gram-positive bacteria, inducing the aggregation of the drug in oligomers that form ionic flow channels through which a massive and uncontrolled release of potassium (K⁺). This causes depolarization and rapid bacterial death without cell lysis.
Detergent Effect of Colistin (Polymyxin E, Gram-Negative Bacteria): Acts as a cationic surfactant. It binds electrostatically to the negatively charged phosphates of the Lipid A of lipopolysaccharide (LPS) of the Gram-negative outer membrane, displacing the stabilizing calcium and magnesium cations. The hydrophobic lipid portion of colistin then penetrates the outer and inner membrane, causing a detergent-like structural disruption with loss of cellular integrity and subsequent bacterial osmotic lysis.

Pharmacokinetics

Key Pharmacokinetics

Parameter	Daptomycin	Colistimethate Sodium (CMS - Prodrug) / Colistin
Routes of Administration	Exclusively rapid IV or 30 minute infusion.	IV (in the form of CMS), Inhaled (for Pseudomonas in cystic fibrosis).
Distribution and Tissues	Very low (0.10 L/kg). Predominantly confined to the circulatory space; excellent penetration into fatty tissues; negligible CSF penetration.	Low (0.25 - 0.35 L/kg). Poor penetration into CSF, intra-articular and pleural space.
Protein Binding	Extremely high (~90-93% bound to albumin).	Low (~50% bound to proteins).
Metabolism	Does not undergo classic hepatic metabolism; it is cleared through the urine in an unchanged form (50%).	CMS is slowly hydrolyzed non-enzymatically in plasma to active colistin with mixed excretion.
Excretion and Half-Life	Renal glomerular excretion unchanged at 50-60%. Elimination half-life of 8.0 to 9.0 hours.	Majority renal glomerular excretion for CMS; active colistin is cleared non-renally. Half-life of 14.0 hours for the active fraction.

Antimicrobial Spectrum

Daptomycin: Excellent against multidrug-resistant Gram-positive cocci, including MRSA, VRE (Enterococcus faecalis and Enterococcus faecium resistant to vancomycin), resistant pneumococcus and severe nosocomial staphylococci.
Colistin: Last-line ultra-limited nosocomial Gram-negative spectrum. Active against carbapenemase- and MBL-producing Enterobacteriaceae, with excellent coverage against Pseudomonas aeruginosa and Acinetobacter baumannii with a multidrug-resistant course. Does not innately cover Proteus spp., Serratia spp. or Providencia spp.

Indicators and dose

Dosage and Adjustment

Daptomycin: 6 to 8 mg/kg every 24 hours by IV route (in severe VRE bacteremia or MRSA endocarditis, the recommended dose is increased to 10 to 12 mg/kg every 24 hours).
Colistimethate Sodium (CMS): Loading dose of 9 MIU IV, followed by a usual maintenance dose of 4.5 MIU every 12 hours IV (equivalent to 150 mg CBA every 12 hours).
Adjustment of Daptomycin in Renal Failure:
- Cl_cr < 30 mL/min: Extend the daptomycin dosing interval to every 48 hours (for both 6 and 8 mg/kg doses), with serum CK monitoring every 48 hours.

Security

Contraindications and ADRs

Contraindications: Demonstrated hypersensitivity to colistin or daptomycin.
Adverse Effects (ADR):
- Inactivation by Pulmonary Surfactant of Daptomycin: Daptomycin binds irreversibly and with high affinity to the pulmonary surfactant of the alveolus, immediately inactivating its analgesic lipid tail. For this reason, it is absolutely contraindicated for the management of pneumonia of any type, since it does not reach effective levels in the lung parenchyma.
- Daptomycin Myopathy and Rhabdomyolysis: Elevation of plasma creatine kinase (CK), diffuse proximal muscle pain and risk of severe rhabdomyolysis, with a reversible course after withdrawing the drug. Requires weekly CK monitoring.
- Nephrotoxicity of Colistin (Very common - 30-50%): Acute renal tubular necrosis due to increased permeability of tubular cells. It is associated with high accumulated doses.
- Neurotoxicity of Colistin: Perioral and distal paresthesias, neuromuscular weakness, mental confusion, seizures. Reversible after discontinuation of the dose.

Colistimethate Sodium (CMS) Dosing Alert

The prescription of Colistin is associated with a high risk of serious errors due to the confusing use of different nomenclatures worldwide: Millions of International Units (MUI) versus Milligrams of Colistin Base Activity (CBA). One milligram of CBA is equivalent to 2.4 MIU of CMS (approximately 30,000 IU of colistimethate). The nomenclature recommended by the local manufacturer must be strictly specified to prevent lethal overdoses due to acute renal failure or ineffective subtherapeutic doses.

Epistemis is educational review material. It is not a medical device, does not diagnose or prescribe treatment, and does not replace formal medical training, current clinical guidelines, or professional clinical judgment.

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System: Antimicrobial and Infectious
Group: Membrane Destroyer Agents

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