IDENTIFICATION AND USE: Gentamicin sulfate is an aminoglycoside antibiotic. Gentamicin is widely used in the treatment of severe infections. It is active against many strains of Gram-negative bacteria and Streptococus aureus. It is inactive against anaerobes and poorly active against Streptococus hemolyticus and Pneumococcus. HUMAN EXPOSURE AND TOXICITY: Main risks and target organs: The main toxic effects are vestibular damage, deafness and renal dysfunction. The damage on the vestibular portion of the eighth cranial nerve appears to be greater than that on the cochlear portion. The main target organs are the eighth cranial nerves and the kidneys. Damage to eighth cranial nerve (both divisions) resulting in tinnitus, deafness, nausea, vomiting, vertigo, dizziness and nystagmus, and nephrotoxicity causing acute tubular necrosis resulting in renal failure. Loss of hearing, dizziness, vertigo, ataxia, nausea, vomiting and renal impairment developing in a patient on gentamicin therapy suggests a diagnosis of gentamicin toxicity. Other toxic features are muscular paralysis and respiratory depression. As gentamicin accumulates in the renal cortex, a critical concentration is reached when the concentrating ability of the kidney becomes impaired. Nephrotoxicity appears to be related to the duration for which the trough serum concentration exceeds 2 ug/ml. The exact mechanism of toxicity is unknown. Ototoxicity and vestibular toxicity seem most highly correlated with elevated peak concentrations (greater than 10 ug/mL) of gentamicin. Gentamicin accumulates in endolymph and perilymph and progressive destruction of ventricular and cochlear cells occurs. Repeated courses of gentamicin may produce progressive destruction of cells leading to deafness. Gentamicin appears to damage the vestibular portion more than the cochlear portion. Neuromuscular blockade with acute muscular paralysis and apnea may occur rarely. Most episodes have occurred in association with anesthesia or administration of other neuromuscular blockers but may also occur after intrapleural or intraperitoneal instillation of large doses of gentamicin or other aminoglycosides. This phenomenon may occur after intravenous or intramuscular administration. ANIMAL STUDIES: Clinical signs of intoxication in rodents included convulsions, prostration, hypoactivity, polydipsia, dyspnoea and ataxia. Dogs exhibited muscle tremors, salivation, and anorexia. Histopathological examination of kidneys from dogs that died up to 13 days after dosing revealed necrosis of the proximal convoluted tubule. Groups of 3 female Rhesus monkeys were injected i.m. with doses of 0, 6 or 30 mg/kg bw/day gentamicin in an aqueous vehicle for 3 weeks. Adverse clinical signs were limited to the 30 mg/kg bw/day group, which included pronounced facial paling and ptosis, markedly disturbed equilibrium from day 20, and depressed food intake and body- weight gain from week 2 onwards. Electron microscopy of renal tubules from the 30 mg/kg bw/day monkeys revealed myeloid bodies present in both tubular cells and lumen, increased phagosomes, disappearance of brush borders and sloughing of epithelial cells from the basement membrane. Groups of beagle dogs (4/sex/group) were administered oral doses of 0, 2, 10, or 60 mg/kg bw/day gentamicin in capsules for 14 weeks. Emesis and diarrhoea were observed occasionally in treated dogs. The only postmortem change was interstitial nephritis observed in 2 animals in the high-dose group. Gentamicin had negative effects on sperm parameters and testis apoptosis in rats. No treatment-related changes in pregnancy rate, litter size and weight, prenatal mortality or fetal abnormalities were reported in 2 generation study in rats. Gentamicin was tested in vitro for its ability to induce forward gene mutation in Chinese hamster ovary cells at concentrations of 128-5000 ug/mL and chromosomal aberrations in these cells at concentrations of 800-5000 ug/mL, both with and without metabolic activation. It was also tested in vivo for its ability to induce nuclear anomalies in mouse bone-marrow cells at intravenous doses of 20-80 mg/kg bw, the highest dose being the maximum tolerated dose. There was no indication of mutagenic activity.
Intravenous and intramuscular therapy with gentamicin has been linked to mild and asymptomatic elevations in serum alkaline phosphatase levels, but rarely affects aminotransferase levels or bilirubin, and changes resolve rapidly once gentamicin is stopped. Only isolated case reports of acute liver injury with jaundice have been associated with aminoglycoside therapy including gentamicin, most of which are not very convincing. The hepatic injury described in these reports is typically mixed but can evolve into a cholestatic hepatitis. The latency to onset is rapid, occurring within 1 to 3 weeks and is typically associated with skin rash, fever and sometimes eosinophilia. Recovery typically occurs within 1 to 2 months and chronic injury has not been described. Aminoglycosides are not listed or mentioned in large case series of drug induced liver disease and acute liver failure; thus, hepatic injury due to gentamicin is rare if it occurs at all.
Gentamicin appears to be more readily inactivated by antipseudomonal penicillins (eg, ticarcillin) than amikacin both in vitro and in vivo in patients with renal failure.
Concomitant and/or sequential use of an aminoglycoside and other systemic, oral, or topical drugs that have neurotoxic, ototoxic, or nephrotoxic effects (e.g., other aminoglycosides, acyclovir, amphotericin B, bacitracin, capreomycin, certain cephalosporins, colistin, cisplatin, methoxyflurane, polymyxin B, vancomycin) may result in additive toxicity and should be avoided, if possible. /Aminoglycosides/
Gentamicin is distributed into cerebrospinal fluid (CSF) in low concentrations following IM or IV administration. CSF concentrations of gentamicin following intrathecal administration depend on the dose administered, the site of injection, the volume in which the dose is diluted, and the presence or absence of obstruction to CSF flow. There may be considerable interpatient variation in concentrations achieved. In one study, intrathecal administration of 4 mg of gentamicin resulted in CSF concentrations of the drug of 19-46 ug/mL for 8 hours and less than 3 ug/mL at 20 hours. Gentamicin crosses the placenta.
Following parenteral administration of usual dosages of gentamicin, the drug can be detected in lymph, subcutaneous tissue, lung, sputum, and bronchial, pleural, pericardial, synovial, ascitic, and peritoneal fluids. Concentrations in bile may be low, suggesting minimal biliary excretion. In patients with ventilator-associated pneumonia receiving IV gentamicin (240 mg once daily), drug concentrations in alveolar lining fluid were 32% of serum concentrations and averaged 4.24 ug/mL 2 hours after a dose. Only minimal concentrations of gentamicin are attained in ocular tissue following IM or IV administration.
Accumulation of gentamicin does not appear to occur in patients with normal renal function receiving 1-mg/kg doses every 8 hours for 7-10 days. However, accumulation may occur with higher doses and/or when the drug is given for prolonged periods, especially in patients with renal impairment.
