6-10 Aminoglycosides Pharmacology and Clinical Use

Comprehensive practice flashcards reviewing the mechanism of action, antimicrobial spectrum, pharmacokinetics, adverse effects, dosing strategies, and guideline updates for aminoglycoside antibiotics.

What ribosomal subunit do aminoglycosides bind to, and what is their primary mechanism of bactericidal action?

Aminoglycosides bind irreversibly to the $30\text{S}$ ribosomal subunit, interfering with protein synthesis initiation and causing misreading of mRNA codons, which results in the incorporation of incorrect amino acids into bacterial proteins.

How do aminoglycosides cross bacterial membranes to access the cytoplasm?

They enter the periplasm by passive diffusion through outer membrane porins and are subsequently transported across the cytoplasmic membrane via an active transport mechanism.

Which aminoglycoside antibiotic is shown in the chemical structure below?

Streptomycin

Why must aminoglycosides be administered parenterally for the treatment of systemic infections?

Aminoglycosides consist of amino sugars linked by glycosidic bonds whose amino groups become protonated and polycationic in physiological fluids, resulting in poor oral bioavailability and GI absorption.

From which microorganism and in what year was streptomycin first isolated?

Streptomycin was isolated from Streptomyces griseus in $1944$.

What are the three main mechanisms of bacterial resistance against aminoglycosides, and which is the most frequent?

1. Enzymatic modification by transferases (the main mechanism)
2. Target site modification via mutation or deletion of the $30\text{S}$ ribosomal subunit protein
3. Impaired intracellular entry due to mutation or deletion of outer membrane porin proteins

What pharmacodynamic features provide the rationale for once-daily (extended-interval) dosing of aminoglycosides?

Concentration-dependent killing (greater bactericidal efficacy at higher peak concentrations) and a prolonged postantibiotic effect (continued suppression of bacterial growth after serum drug concentrations fall below the MIC).

What is the normal serum elimination half-life of aminoglycosides, and how long can it extend during renal dysfunction?

The normal half-life is $2\text{--}3\,\text{hours}$, which increases exponentially to $24\text{--}48\,\text{hours}$ in patients with renal failure.

Why are aminoglycosides considered ineffective as monotherapy for pneumonia?

They penetrate lung tissue and respiratory secretions poorly, and their antibacterial potency is markedly diminished in acidic pH and low oxygen tension environments.

Which specific aminoglycosides pose the highest risk of vestibulotoxicity versus ototoxicity (auditory toxicity)?

Streptomycin and gentamicin are the most vestibulotoxic (causing vertigo and ataxia), while neomycin, kanamycin, and amikacin are the most ototoxic (auditory damage).

What is the earliest laboratory indicator of aminoglycoside-induced nephrotoxicity?

An increase in the serum concentration of the aminoglycoside itself (which precedes an elevation in serum creatinine or a drop in creatinine clearance).

How can aminoglycoside-induced neuromuscular blockade be reversed if respiratory depression or muscle weakness occurs?

Administration of neostigmine or intravenous ($\text{IV}$) calcium.

Why are aminoglycosides deemed compatible with breastfeeding despite their known toxicity profile?

Although aminoglycosides enter breast milk, they are poorly absorbed through the infant gastrointestinal tract.

What is the CDC 2025 recommended first-line aminoglycoside regimen for severe tularemia?

Gentamicin administered at $6\,\text{mg/kg}$ every $24\,\text{hours}$ $\text{IV/IM}$ for $10\,\text{days}$, adjusted using serum drug levels and renal function.

Why has gentamicin supplanted streptomycin as the preferred aminoglycoside for tularemia in recent CDC clinical updates?

Streptomycin carries a higher risk of irreversible hearing loss and suffers from limited clinical availability in the United States.

What target peak and trough serum concentrations should be maintained for patients receiving gentamicin every $8\,\text{hours}$?

The maximum peak concentration should reach $5\text{--}10\,\mu\text{g/mL}$, while the trough level must remain $< 1\text{--}2\,\mu\text{g/mL}$ (levels $> 2\,\mu\text{g/mL}$ signal drug accumulation and imminent toxicity).

Which aminoglycoside demonstrates the greatest activity against Pseudomonas aeruginosa?

Tobramycin

What specific formulation and route of tobramycin is used for chronic Pseudomonas aeruginosa infection in cystic fibrosis patients?

An inhalation solution of $300\,\text{mg} / 5\,\text{mL}$.

How do Enterococcus faecalis and Enterococcus faecium differ in susceptibility to tobramycin?

Enterococcus faecalis is susceptible to both gentamicin and tobramycin, whereas Enterococcus faecium is resistant to tobramycin.

Why is amikacin effective against many bacterial strains that have developed resistance to gentamicin and tobramycin?

Amikacin is a semisynthetic derivative of kanamycin whose structure is resistant to degradation by most bacterial aminoglycoside-inactivating enzymes.

For what two gastrointestinal indications are neomycin or kanamycin administered orally?

1. Preoperative bowel decontamination before elective colorectal surgery ($1\,\text{g}$ neomycin plus $1\,\text{g}$ erythromycin every $6\text{--}8\,\text{hours}$ for $1\text{--}2\,\text{days}$)
2. Hepatic coma ($1\,\text{g}$ every $6\text{--}8\,\text{hours}$ to suppress coliform intestinal flora and reduce ammonia production)

What is the primary FDA-approved clinical indication for plazomicin, and what bacterial resistance mechanism limits its efficacy?

Plazomicin is indicated for adults ($\ge 18\,\text{years}$) with complicated urinary tract infections ($\text{cUTIs}$), including pyelonephritis. It is inactive against isolates that overproduce $16\text{S}$ ribosomal RNA ($\text{rRNA}$) methyltransferases or upregulate drug efflux pumps.

What is spectinomycin, how does its pharmacodynamics differ from standard aminoglycosides, and what is its main clinical indication?

Spectinomycin is an aminocyclitol antibiotic structurally related to aminoglycosides that acts bacteriostatically (not bactericidally); it is administered intramuscularly ($40\,\text{mg/kg}$ up to $> 2\,\text{g}$) for drug-resistant gonorrhea or in penicillin-allergic patients.

Why is gentamicin not recommended for the management of newborn meningitis?

Gentamicin is ineffective and toxic in neonatal meningitis; third-generation cephalosporins are used instead.

What are the first-line antibiotic options for the clinical management of plague (Yersinia pestis) according to current CDC guidance?

Gentamicin and fluoroquinolones are considered primary first-line therapies; streptomycin remains an accepted FDA-approved alternative, but its use is restricted by limited availability.