Aminoglycosides are important antibacterials used primarily to treat infections caused by susceptible aerobic gram-negative bacteria. Unfortunately, they have a narrow margin of safety producing characteristic lesions in kidney, cochlea, and vestibular apparatus within the therapeutic dose range. Because they are polycations, they cross membranes very poorly.
Aminoglycosides are water soluble weak bases that are polycations at body pH. They are chemically similar in that they have one of two bases to which is attached two or three aminosugars. The aminosugars are linked to the by glycosidic bonds, hence the group name. The base in streptomycin is streptidine, but all of the others have 2-deoxystreptamine so that most of the members differ in the number and nature of the aminosugars attached to the 2-deoxystreptamine.
Identification of subgroups is less marked than, e.g., within the penicillins, but reference is sometimes made to the streptomycin (streptomycin and dihydrostreptomycin), kanamycin (kanamycin, amikacin, and tobramycin), gentamicin (gentamicin and netilmicin), and neomycin (neomycin) groups. Apramycin, a veterinary drug, belongs to none of these groups. With the exception of the gentamicin group which originates in Micromonospora actinomyctes, these drugs are derived from Streptomyces spp. Note the difference in spelling, "-micin" versus "-mycin", depending on source.
The aminoglycosides are cidal making them unique among the antibacterials that act by inhibiting protein synthesis. They act by tightly binding to a structural component of the 30S ribosomal subunit to inhibit protein synthesis. This binding is much stronger than that of other protein sysnthesis inhibitors possibly accounting for the fact that these are the only "cidal" protein synthesis inhibitors. At other concentrations, some aminoglycosides cause an alteration in the codon:anticodon matching. The resulting misreading of the mRNA allows an incorrect charged tRNA to bind to the "acceptor" (A) site on the ribosome. The protein produced has an incorrect primary amino acid sequence making it a defective protein that fails to function correctly. Other effects on protein synthesis have been noted, as well. The drugs are most effective on growing organisms.
Isolated ribosomes of most bacterial species are sensitive to the aminoglycosides. However, their antibacterial spectrum is much narrower because the aminoglycoside concentration varies widely among species. As might be expected from their polycationic nature, their intracellular concentration is dependent upon a transport system located in the cell membrane. The oxygen-dependent system ordinarily transports polyamines and is absent in anaerobes. Therefore, the aminoglycosides are only clinically useful against organisms growing in aerobic conditions.
Whereas the primary susceptibility to aminoglycosides can be predicted from considerations just discussed, actual sensitivity is highly dependent upon the presence or absence of plasmids that code for enzymes that biotransform the drugs. The cell membranes of resistant organisms have one or more enzymes from three classes: phosphorylases, acetylases, and adenyl transferases. Note the use of "plural" with respect to the enzymes. There are families of each with differing target specificities and frequency of occurrence.
Three of many possible examples of inactivating enzymes, their frequency of occurrence, and susceptible drugs will be given here. A 3'-O-phosphotransferase is very commonly found in Pseudomonas, Klebsiella, E. coli, and Staphylococcus aureus. Kanamycin is inactivated by being phosphorylated, but gentamicin, tobramycin, and amikacin are resistant to this enzyme. Pseudomonas organisms occasionally have a plasmid coding for a 3'N-acetyltransferase that acetylates and inactivates gentamicin, but not amikacin or tobramycin. Staphylococcus aureus organisms uncommonly have a plasmid that codes for 4'-O-adenylransferase that links an adenine-ribosyl group to amikacin, tobramycin, and kanamycin. Gentamicin is resistant to this enzyme. Sites that are blocked or open to specific enzymes are marked on the gentamicin, tobramycin, kanamycin, and amikacin structures in the accompanying figure. Note that amikacin, a specifically designed semisynthetic derivative of kanamycin, is resistant to most of the enzymes.
Fortunately, each of aminoglycosides met few resistant organisms when they were first introduced. Heavy use has increased the frequency of resistant organisms so that streptomycin is now largely useless except for a few isolated indications, such as a secondary agent for tuberculosis and in combination with a tetracycline for treating brucellosis and plague. Kanamycin has met a similar fate in many environments and gentamicin is increasingly met with resistant organisms. Cessation of use of a specific aminoglycoside for some period of time like 6 months to a year results in an increase in proportion of isolates sensitive to the specific drug. But, resistance quickly returns when the drug is re-introduced into the local environment. For this reason, experts plead with clinicians to use amikacin sparingly, only when absolutely needed, to slow the selection for organisms resistant to it.
Drug Therapeutic Maximum Peak Maximum Trough
Concentration Concentration Concentration
(mcg/mL) (mcg/mL) (mcg/mL)
Amikacin 15-25 35 5
Gentamicin 4-10 10 2
Kanamycin 15-30 30-35 5
Netilmicin 6-12 16 2
Streptomycin -- 20-25* --
Tobramycin 4-10 10 2
* In patients with renal damage. Peak concentrations greater than 50 mcg
per mL are associated with increased risk of toxicity. [USPDI95,71]
One-step mutation leading to resistant or dependent organisms has been well characterized for streptomycin. Replacement of a single amino acid of a ribosomal subunit is responsible for this interesting, but not major source of resistance. Most resistance is due to plasmid-borne enzymes as discussed earlier. Subunit S12 normally has a LYS in position 42. If this is replaced with ASPn the organisms are resistant to streptomycin. If the replacement is GLUn, the organism is dependent on streptomycin.
The pharmacokinetics of the aminoglycosides are dominated by their polycationic nature which makes them very water soluble and poorly lipid soluble. In the absence of a carrier mediated transport system or filtration, they cross biological membranes at such a slow rate that they do not reach therapeutic concentrations. Lack of penetration of these drugs into milk (Ziv, et. al.) is shown by a comparison of milk:plasma concentration ratio after long term IV infusions of gentamicin, kanamycin, and neomycin into cows. Ultrafiltrates of each were used so that only free drug concentrations were compared. Although it was predicted from ion trapping considerations that concentration in milk would be 7 to 10 times that of plasma, the experimentally observed ratio was 0.2 to 0.4. In contrast, tylosin had a predicted ratio of 5 and an observed ratio of 1 to 5. This clearly indicates that the aminoglycosides were unable to penetrate the epithelial cell layer of the mammary gland parenchyma.
No slowly absorbed dose forms have been developed for these drugs and all are given as water soluble sulfate salts. Most of an IM or SC injected dose is rapidly absorbed, but because of strong tissue binding residual amounts can be found at the injection site for long periods of time. For this reason, drug should not be injected into meat cuts in food producing animals. The amount that is locally bound is insignificant otherwise.
Aminoglycosides are minimally protein bound (ca. 10%) and do not penetrate into the CNS or eye, but they may cross the placenta. It is not clear to GLC how the different types of placentae in various species of animals affects the rate of penetration. Therapeutic concentrations are produced only in extracellular fluids. However, tissue concentrations that are relevant for food residues and toxicity are significant. Thus, relatively high concentrations of the drugs have been found in kidney, the cochlea, and vestibular apparatus. The elimination half life from these tissues is much longer (up to 100 hours?) than that from plasma, an important point toxicologically.
Over 90% of injected drug is eliminated from the kidney via glomerular filtration. Serum elimination half-life is on the order of 2-3 hours when renal function is normal. Reduced renal function can lead to a marked slowing of elimination, necessitating dosage adjustment. Dosage adjustments can be made initially from some estimate of renal function, e.g., creatinine clearance. Therapeutic drug monitoring procedures are used for aminoglycosides because of their narrow safety margin, so after the initial doses and serum sampling, one can use pharmacokinetic principles to adjust dosage.
Adjustment of a dosage regimen can be done by either lowering the dose given or by increasing the interval or both in severe cases. Riviere and Coppoc obtained evidence from studies on gentamicin in canine models, that it may be best to adjust dose by lengthening the interval so that the drug concentration can drop below 1 ug/ml at the trough. This apparently increases the gradient from kidney, cochlea, and vestibular apparatus to plasma sufficiently that their concentration does not build to strongly toxic concentrations too quickly. Courses of therapy longer than five days should not be undertaken without some means of monitoring function of all three of these sites as well as the plasma drug concentration.
Many schemes have been devised for adjusting aminoglycoside dosage in renally compromised patients. The accompanying Nomogram for Aminoglycoside Dosage Adjustment typifies these schemes and one should understand its use. Creatinine clearance is on the X-axis. The left-hand Y-axis is the "full-dose interval multiplier" and the right-hand Y-axis is the "half-dose interval multiplier." Assuming the patient had a creatinine clearance of 2 ml/min/kg, one would give the normal dose at twice the normal interval. If the clearance were 0.5 to 0.6 ml/min/kg, one should give half the normal dose at four times the normal dose interval. If renal function is severely compromised, one must lower the dose as well as increase the interval. Otherwise, the dose interval becomes so long that there will be prolonged times during which the plasma concentration is below the MIC for most target organisms. Of course, the first dose should be in the normal range following the concept of a loading dose.
The aminoglycosides can produce four types of dose-related adverse effects: (1) proximal tubular cell damage, (2) destruction of sensory cells in the cochlea, (3) destruction of sensory cells in the vestibular apparatus, and (4) neuromuscular paralysis.
Aminoglycosides progressively damage the sensory cells of the cochlea and vestibular apparatus. Depending on the organ affected and the degree of damage, the result may be loss of hearing, vertigo, ataxia, and loss of balance. Killed sensory cells do not regenerate so loss may be permanent if severe. This toxicity depends on the area under the curve (AUC) once a threshold concentration has been reached. That is, a high concentration will cause toxicosis within a short duration of therapy whereas long durations of therapy will produce it at lower concentrations. Increased serum concentrations, as might be caused by undetected renal dysfunction, will especially predispose patients to this risk.
All of the aminoglycosides may damage sensory cells of both structures. There is evidence that vestibular damage is more likely with gentamicin and streptomycin. Cochlear damage may be more likely with kanamycin, amikacin, and neomycin. Short term use may produce adverse effects on hearing and balance in 1 to 3% of human patients, depending on the drug used. No data is available for incidence of these effects in clinical treatment of animals. Long term therapy of tuberculosis with streptomycin presents obvious potential for deafness.
Aminoglycosides cause proximal tubular necrosis when exposure is sufficiently long and/or concentration is high enough. It is advised that serum gentamicin concentrations not be allowed to exceed 10 ug/ml and that trough values be below 2 ug/ml. Courses of therapy longer than 5 days should be undertaken only with great caution and close monitoring of renal function.
Aminoglycosides, especially those that are more polycationic like gentamicin and neomycin, enter proximal tubular cells by pinocytosis. Because the drugs inhibit lysosomal enzymes, the vesicles accumulate and take on a whorl-like appearance. These are called cytosegresomes. Excessive numbers of these apparently kill the cells, producing severe toxicity. If exposure stops before the cells are killed, the toxicity is completely reversible.
Onset of toxicosis is indicated by proteinuria and cylindruria (casts in urine). Pre-existing renal pathology is a serious predisposing factor. Renal function should be monitored closely and, where available, therapeutic drug monitoring should be performed. Proximal tubular damage causes the whole nephron to fail, leading to increased serum aminoglycoside concentrations. This can, in turn, increase damage to other tubules and to the cochlea and vestibular apparatus.
Neuromuscular paralysis results from inhibition of calcium movement into the nerve terminal upon depolarization. Calcium is required for exocytotic release of the acetylcholine. Weakness can be produced at doses just a bit higher than those normally recommended, but may be observed at routine doses if muscular blocking agents have been used. Respiratory paralysis was produced by use of intrapleural or intraperitoneal lavages with streptomycin during the early days of its clinical use. Other members of the group would also cause it, but wise persons have avoided such contraindicated use because of the experience with streptomycin. Injections of calcium antagonize the paralysis.
The aminoglycosides are used topically, orally (for local gastrointestinal effect [especially in newborn pigs for diarrhea]), systemically, and intrathecally (into CSF). Amikacin, gentamicin, and tobramycin can be used for respiratory infections via aerosol nebulization [USPDI94]. Although on occasion they may be used for serious urinary tract infections, their primary parenteral use is for serious septicemias caused by aerobic gram-negative organisms. Despite their effectiveness against such gram-positive organisms as Staphylococcus aureus, they are seldom used for these because they are more toxic than other drugs that are equally effective. Newer penicillin and cephalosporin derivatives have reduced the heavy dependence on these drugs, but they are still mainstays of practice. Sensitive organisms are defined as those that are inhibited by 4-8 ug/ml of gentamicin and tobramycin. The values are 8-16 ug/ml for kanamycin and amikacin which are both slightly less potent (dose) and less toxic so the higher concentrations can be tolerated.
Streptomycin is 20 to 30 X more active versus gram negative bacilli at pH 8 than at 5.8. Therefore, it is generally recommended that one alkalinize urine when using aminoglycosides. BM6th,1988. p823)
Anitacterial spectrum: Aminoglycosides are particularly effective against Pasteurella, Brucella, Hemophilus, Salmonella, Klebsiella, Shigella, & Mycobacterium spp. (some aminoglycosides).
Synergy is often observed when one combines an aminoglycoside with a beta-lactam derivative. Apparently, a disrupting effect on the bacterial cell permeability barrier (cell membrane / wall) increases entry of drug into the cell.
Infections for which aminoglycosides are commonly used include: biliary tract infections, bone and joint infections, central nervous system infections (including meningitis and ventriculitis), intra-abdominal infections (including peritonitis), pneumonia (gram-negative), septicemia, skin and soft tissue infections (including burn wound infections), and urinary tract infections (complicated, recurring). Streptomycin is used for the treatment of brucellosis, granuloma inguinale, plague, tuberculosis, and tularemia. [USPDI94]
In dogs, primary topical application is for otitis externa. Must be sure ear drum is intact. If ruptured, direct access to middle ear produces ototoxicity.
Apramycin, used in veterinary medicine only, is contraindicated in cats. [Brander5th,1991]
Apramycin is mainly indicated for reatment of colibacillosis and salmonellosis in calves and pigs. Can be used p.o. or parenterally. [Brander5th,1991]
Spectinomycin resembles the "classical" aminoglycosides in that it is technically an aminocyclitol. It is derived from Streptomyces spectabilis. It inhibits protein synthesis of gram-negative bacteria by binding to the 30S ribosomal subunit, but does not cause misreading of the mRNA codes, therefore, it is not cidal. Resistance develops easily by mutation. This is one of the main problems with spectinomycin and limits its clinical usefulness.
The spectrum of specytinomycin includes some gram negative bacteria, but it is inferior to other drugs. Its only approved use for human medicine is for Neisseria gonococcus organisms that are resistant to the penicillins. It is effective against these at concentrations of 7 to 20 ug/ml, the concentration produced by recommended doses.
It is much less toxic than the aminoglycosides; as much as 400 mg/kg IV can be tolerated. There are few important side effects including no otoxocity or nephrotoxicity. There are however, pain at the injection site; dizziness, nausea, and insomnia; and urticaria, chills, and fever.
It is used only IM in humans, but there are feed-based preparations for animals. It is rapidly absorbed from IM injection. A 2 gram IM dose (ca. 30 mg/kg), produces a peak serum concentration at 1 hour of 100 ug/ml. Eight hours later the concentration is 15 ug/ml. Active drug is eliminated in the urine.
In veterinary medicine, spectinomycin is used as a feed additive and for pneumonias. Most commonly used with Lincosin (L-550) - to provide broad spectrum coverage for gram +/- infection. Approved for use in water in turkeys. Spectinomycin has been used to treat coliform diarrheas in piglets, although not as effective as oral gentamicin or MecadoxR.