Chapter 19: Drug Therapy With Aminoglycosides and Fluoroquinolones

Overview

Aminoglycosides and fluoroquinolones are two important classes of antimicrobial drugs used in the treatment of various bacterial infections. Their mechanisms of action, pharmacokinetics, indications, and toxicity profiles are essential knowledge for microbiology and clinical practice.

Aminoglycosides

Pharmacologic Characteristics

• Bactericidal agents with similar pharmacologic, antimicrobial, and toxicologic characteristics.

• Primarily used to treat serious gram-negative infections.

• Prototype: Gentamicin.

• Poorly absorbed from the gastrointestinal (GI) tract; oral administration exerts local effects only.

• Well absorbed via intramuscular (IM) and intravenous (IV) routes.

• Not typically used as monotherapy; often combined with other antibiotics (e.g., beta-lactams).

Distribution and Excretion

• Parenteral administration leads to wide distribution in extracellular fluids.

• Therapeutic levels achieved in blood, urine, bone, inflamed joints, pleural and ascitic fluids.

• Accumulate in high concentrations in renal tubules (risk of nephrotoxicity) and inner ear (risk of ototoxicity).

• Poor distribution in central nervous system (CNS), intraocular fluids, and respiratory tract secretions.

• Injected aminoglycosides are not metabolized; excreted unchanged in urine.

• Oral ingestion leads to excretion in feces.

Mechanism of Action

• Penetrate bacterial cell walls and inhibit protein synthesis by binding to the 30S ribosomal subunit.

• Disrupts bacterial function and replication, leading to cell death.

Indications for Use

• Serious systemic infections caused by susceptible aerobic gram-negative organisms.

• Specific conditions include septicemia, respiratory tract infections, intra-abdominal infections, urinary tract infections, osteomyelitis, meningitis, endocarditis (often in combination with beta-lactam antibiotics).

• Suppression of intestinal bacteria (pre-operative bowel surgery, hepatic coma treatment).

• Topical use for eye and skin infections (limited).

Principles of Therapy

• Drug selection and dosage depend on the specific organism causing infection.

• Dosage must be carefully monitored based on serum concentration.

• Conventional dosing: multiple daily doses; single daily dose regimens are also used.

• Monitoring of peak and trough serum levels is essential to optimize efficacy and minimize toxicity.

Reducing Aminoglycoside Toxicity

• Identify patients at high risk for adverse effects (e.g., those with renal impairment).

• Maintain adequate hydration.

• Use caution with concurrent administration of diuretics.

• Limit duration of therapy to no longer than 10 days unless absolutely necessary.

• Detect adverse effects early; reduce dosage or discontinue medication as needed.

Special Population Use

• Children, older adults, patients with abnormal kidney function, hepatic impairment, critical illness, diabetes mellitus, and those receiving home care require special consideration.

Fluoroquinolones

Pharmacologic Characteristics

• Synthetic bactericidal agents active against both gram-negative and gram-positive organisms.

• Prototype: Ciprofloxacin.

• Well absorbed when administered orally; achieve therapeutic concentrations in most body fluids.

• Metabolized mainly in the kidneys, with some hepatic metabolism.

• 30% to 60% of oral dosing is excreted unchanged in urine.

Mechanism of Action

• Interfere with bacterial DNA enzyme synthesis, specifically inhibiting DNA gyrase and topoisomerase IV.

• Disrupts bacterial growth and development, leading to cell death.

Indications for Use

• Treatment of various infections caused by aerobic gram-negative and other organisms.

• Effective for infections of the respiratory, genitourinary, gastrointestinal tracts, bone, joints, skin, and soft tissues.

Contraindications

• Hypersensitivity to fluoroquinolones.

• Concurrent use with tizanidine.

• Children under 18 years of age.

• Pregnant or lactating females.

• Drug selection depends on the specific organism causing infection.

• Resistance is associated with overuse, particularly with Pseudomonas aeruginosa and Staphylococcus aureus.

Nursing Implications

Clinical Considerations

• Preventing drug interactions.

• Proper administration of medication.

• Assessment for therapeutic effects and adverse effects.

• Patient education regarding drug use, side effects, and adherence.

Comparison Table: Aminoglycosides vs. Fluoroquinolones

Key Definitions

• Bactericidal: Refers to agents that kill bacteria, as opposed to bacteriostatic agents which inhibit bacterial growth.

• Nephrotoxicity: Toxicity to the kidneys, often a concern with aminoglycosides.

• Ototoxicity: Toxicity to the inner ear, leading to hearing loss or balance issues.

• DNA Gyrase: An enzyme targeted by fluoroquinolones, essential for bacterial DNA replication.

Relevant Equations

Serum Drug Monitoring:

• Peak and trough levels are measured to ensure therapeutic efficacy and minimize toxicity.

Example Equation for Drug Clearance:

Example Equation for Creatinine Clearance (for dosing adjustments):

For females, multiply the result by 0.85.

Examples and Applications

• Gentamicin is used in combination with ampicillin for the treatment of endocarditis caused by Enterococcus species.

• Ciprofloxacin is commonly prescribed for urinary tract infections and certain types of gastroenteritis.

Additional info: Expanded explanations of mechanisms, toxicity, and clinical applications were added for academic completeness.