Injectable: 40 mg/ml solution for i.v., i.m., s.c. injection (human preparation), 100 mg/ml solution for i.v., i.m., s.c. injection (Genta).
Ophthalmic/aural solution: 0.5% solution (Tiacil); 0.3% ophthalmic gel (Clinagel). Gentamicin is a component of some topical ear preparations.
Action: Aminoglycosides inhibit bacterial protein synthesis and require an oxygen-rich environment to be effective, thus they are ineffective in low-oxygen sites (abscesses, exudates), making all obligate anaerobic bacteria resistant. They are bactericidal and their mechanism of killing is concentration-dependent, leading to a marked post-antibiotic effect, allowing pulse-dosing regimens which may limit toxicity.
Use: Active against Gram-negative bacteria, but some staphylococcal and streptococcal (Streptococcus faecalis) species are also sensitive. All obligate anaerobic bacteria and many haemolytic streptococci are resistant. Use in domestic animals is limited by nephrotoxicity and, more rarely, ototoxicity and neuromuscular blockade. Microbial resistance is a concern, although many bacteria resistant to gentamicin may be susceptible to amikacin. Consider specific glomerular filtration rate measurements to assess risk prior to initiating therapy. The trough serum level should be allowed to fall below 2 µg/ml. When used for ‘blind’ therapy of undiagnosed serious infections, gentamicin is usually given in conjunction with a penicillin and/or metronidazole. Aminoglycosides are more active in an alkaline environment. Geriatric animals or those with reduced renal function should only be given this drug systemically when absolutely necessary, although the move to dosing q24h should reduce the likelihood of nephrotoxicity. Use with caution in rabbits, birds and reptiles. Fluid therapy is essential during treatment of reptiles; monitor uric acid levels in birds and reptiles. Used for the treatment of bacterial disease in fish.