Гентамицин

Gentamicin is a mixture of several antibiotic components produced by fermentation of Mi cromonospora purpurea and other related soil microorganisms (hence its name is spelled with an “i” instead of a “ y”). Gentamicins C-1, C-2, and C-1a are most prominent. Gentamicin is the most important of the aminoglycoside antibiotics still in use. Gentamicin was, for example, one of the first antibiotics to have significant activity against Pseudomonas aeruginosa infections. This water-loving, opportunistic pathogen frequently is encountered in burns, pneumonias, and urinary tract infections.

Amorphous solid. Freely soluble in water, pyridine, acid solutions; moderately soluble in methanol, ethanol, and acetone; practically insoluble in benzene and halogenated hydrocarbons.

antibacterial

This antibiotic is a combination of three related aminoglycoside agents obtained from cultures of Micromonospora purpurea and acts by interfering with the bacterial synthesis of protein. It prevents bacterial protein synthesis by irreversibly binding to 30S ribosomal subunits. Its antibiotic spectrum is similar to that of neomycin, and cross-resistance does occur. Gentamicin is active against gram-negative organisms including Escherichia coli and a high percentage of strains of Pseudomonas species and other gram-negative bacteria. Proteus organisms show a variable degree of sensitivity. Some gram-positive organisms, including S. aureus and group A β-hemolytic streptococci, are also affected.
In general, higher concentrations are needed to inhibit streptococci than those needed to inhibit staphylococci and many gram-negative bacteria. It is inactive against fungi, viruses, and most anaerobic bacteria. The most important use of gentamicin is in the treatment of systemic gram-negative infections, particularly those due to Pseudomonas organisms. Widespread use is unwarranted not only because equally effective drugs are available but also because of the risk of increasing the background of gentamicin-resistant organisms. Allergic reactions to gentamicin are unusual but may occur with prolonged use. Cross-reactivity with neomycin may occur.

Gentamicin has been used inin situ preparations for the treatment of minor infections. Antibiotics that are also available for systemic use are not considered acceptable for topical use because of the risk of development of resistance. Neomycin is the topical aminoglycoside listed in the WHO Model List of Essential Drugs.

It is active against staphylococci, but streptococci are at least moderately resistant. Gram-positive bacilli, including Actinomyces and Listeria spp., are moderately susceptible, but clostridia and other obligate anaerobes are resistant. There is no clinically useful activity against mycobacteria. It is active against most enterobacteria, including Citrobacter, Enterobacter, Proteus, Serratia and Yersinia spp., and against some other aerobic Gram-negative bacilli including Acinetobacter, Brucella, Francisella and Legionella spp., although its in-vitro activity against intracellular parasites such as Brucella spp. is of doubtful usefulness. It is active against Ps. aeruginosa and other members of the fluorescens group, but other pseudomonads are often resistant and Flavobacterium spp. are always resistant.
The MIC for susceptible strains of Ps. aeruginosa can vary more than 300-fold with the Mg2+ content of the medium. Activity against Ps. aeruginosa is also significantly lower in serum or sputum than in ion-depleted broth, as a result both of binding (more in sputum than in serum) and antagonism by ions.
The action is bactericidal and increases with pH, but to different degrees against different bacterial species. Marked bactericidal synergy is commonly demonstrable with β-lactam antibiotics, notably with ampicillin or benzylpenicillin against E. faecalis, and with vancomycin against streptococci and staphylococci. Bactericidal synergy with β-lactam antibiotics can also be demonstrated in vitro against many Gram-negative rods, including Ps. aeruginosa. Antagonism with chloramphenicol occurs in vitro, but this is of doubtful clinical significance. Like other aminoglycosides, gentamicin is degraded in the presence of high concentrations of some β-lactam agents.

Resistant strains of staphylococci, enterobacteria, Pseudomonas and Acinetobacter spp. have been reported from many centers, often from burns and intensive care units where the agent has been used extensively. Overall prevalence rates of resistance in various countries range from 3% to around 50% for Gramnegative organisms. Countries in which control of the prescription of antibiotics is lax often have very high rates.
Acquired resistance in Gram-negative organisms is usually caused by aminoglycoside-modifying enzymes. The prevalence of the different enzymes varies geographically. ANT(2″) is most common in the USA, but in Europe various forms of AAC(3), particularly AAC(3)-II, are common. ANT(2″) is also common in the Far East, usually accompanied by AAC(6′). Strains that owe their resistance to a non-specific decrease in uptake of aminoglycosides have been involved in outbreaks of hospital-acquired infection, and are cross- resistant to all aminoglycosides.
Resistance in staphylococci and high-level resistance in enterococci is usually caused by the bifunctional APH(2″)- AAC(6′) enzyme. Other aminoglycoside-modifying enzymes do not contribute greatly to gentamicin resistance. Gentamicinresistant staphylococci began to emerge in the mid-1970s. Rates of resistance in the UK are around 2.5% in methicillin-sensitive Staph. aureus, 9% in MRSA and 23–73% in coagulase-negative staphylococci depending on methicillin susceptibility.
High-level resistance to gentamicin (MIC >2000 mg/L) in E. faecalis is widespread, accounting for around one-third of blood culture isolates in some places. Penicillin does not exert synergistic bactericidal activity against such strains, although the combination of penicillin with streptomycin may remain active. High-level gentamicin resistance in E. faecium is much less common, but has been reported in the UK, the USA and Asia.

In severe sepsis of unknown origin, gentamicin has been traditionally combined with other agents. However, monotherapy has been shown to be as effective as combination therapy. In systemic Ps. aeruginosa infections it is advisable to combine gentamicin with an antipseudomonal penicillin or cephalosporin, owing to likelihood of gentamicin resistance.
Suspected or documented Gram-negative septicemia, particularly when shock or hypotension is present
Enterococcal endocarditis (with a penicillin)
Respiratory tract infection caused by Gram-negative bacilli
Urinary tract infection
Bone and soft-tissue infections, including peritonitis, burns complicated by sepsis and infected surgical and traumatic wounds
Serious staphylococcal infection when other conventional antimicrobial therapy is inappropriate
Gentamicin drops are used for conjunctival infections and for infections of the external ear. The drug is also used in orthopedic surgery in bone cements. In these applications systemic concentrations achieved are negligible and toxicities are restricted to local effects.
In the elderly and those with renal impairment the dosage must be suitably modified.

Poison by intravenous, intraperitoneal, intramuscular, and subcuta neous routes. Mildly toxic by ingestion. Ex perimental teratogenic and reproductive effects. Mutation data reported. Human systemic effects: change in motor activity, changes in vestibular functions, dlstorted perceptions, eye hemorrhage, hallucinations, hdney changes, motor activity changes, trigeminal nerve sensory changes, vestibular function changes, visual field changes. Af fects the peripheral nervous system by intra venous route. An antibiotic. When heated to decomposition it emits acrid smoke and irritating fumes. See also other gentamycin entries.