Living Textbook MC610

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Aminoglycosides

They were introduced in 1944 by Waksman capping a few years of search for antibiotics by screening streptomyces sp. They were the first agents to show effectiveness against Tuberculosis.

Chemically they are aminosugars linked together by glycosidic linkages. They all share a common pharmacophore, a 1,3-diaminoinositol.

Mostly are isolated from Streptomyces (-mycin) or Micromonospora sp. (-micin). They are polycationic at physiological pH, and are thus used as sulfate salts. They are highly water soluble with very little metabolism, very well distributed through body fluids, but show very poor oral absorption (less than 1%), poor distribution to bones, CNS and fatty tissues. They have high affinity to some tissues such as the kidneys and sensory cells of the inner ear.

They have a very broad spectrum against aerobic Gram positive and Gram negative bacteria (better against the latter), including pseudomonas and enterobacter sp. They are also effective against mycobacterium and some protozoa such as entamoebae.

Mechanism of Action:

Although several mechanisms may be involved such as altering membrane activity, their main activity comes from binding irreversibly to the 30S subunit of the ribosome and will interfere with protein synthesis via two mechanisms:

  • At low concentration, they will cause misreading of amino acid codons by tRNA and impairing proofreading and thus incorporates incorrect amino acid and gives rise to nonsense proteins.
  • At high concentration, they will interfere with proper amino acid polymerization and elongation, and cause complete cessation of protein biosynthesis.

The importance of each mechanism may be dependent on the specific agent, but has been shown to be a function of the intracellular concentration of Aminoglycosides. The production of the faulty proteins is the rapid effect, while cessation of protein biosynthesis is responsible for the more prolonged effect of Aminoglycosides.

It has also been shown, that some of the faulty proteins produced via the first mechanism, may get incorporated in the cytoplasmic membrane, leading to the disruption of the membrane, allowing higher influx of these agents into the bacterial cell, and thus the second mechanism becomes the more relevant mechanism of action.

These agents are bactericidal. This inability of the bacterial cells to synthesize proteins in an irreversible fashion, plus to a lesser degree, the production of faulty membrane proteins that causes disruption of the normal functions of the cell wall, lead to cell death.

Uptake:

Small amounts of Aminoglycosides enter the cell by binding to the negatively charged phospholipids and entering via an electron transport-linked system. This method can be antagonized by divalent cations such as Calcium and Magnesium. Membrane disruption can then lead to higher influx of these agents into the bacterial cell.

Toxicity:

Ototoxicity due to neurotoxicity to the 8th cranial nerve can lead to vertigo and irreversible deafness. Nephrotoxicity due to kidney tubular necrosis may also arise. The reason for these toxicities is the affinity of Aminoglycosides to these tissues and their long t1/2 within these tissues. The half-life is 4-fold the serum half-life in the inner ear, and can reach hundreds of hours in the kidneys. The onset of these effects is delayed and is related to blood levels.

A less common side effect is a neuromuscular blockade effect, by competitively inhibiting Calcium dependent release of AcetylCholine at the NMJ, which may lead to muscle weakness upon higher than usual blood levels or in Myasthenia Gravis and Parkinson's disease patients.

These side effects are now well known, and in practice patients are monitored and the dose adjusted to avoid such problems.

Resistance:

The most important mechanism involves the production of inactivating enzymes that will cause N-acetylation of amine groups, phosphorylation and adenylation of the hydroxyl groups. There are at least nine different "transferases" depending on the position that is inactivated.

Two lesser important mechanisms involve ribosomal mutations and decreasing permeability to these agents.

Notes:

  1. Synergism that has been reported with ß-lactams may be due to better penetration of aminoglycosides through compromised cell wall of the bacteria, but remember that they are chemically incompatible, due to the basic nature of the aminoglycosides.
  2. These agents have the same clinical applications as Quinolones in many cases, and so as the Quinolones' popularity increases, use of aminoglycosides decreases.
  3. These agents have a broad spectrum against a wide variety of bacterial species. They are however ineffective against anaerobes.

Structure Activity Relationship:

Ring I, Crucial for broad-spectrum activity. It is also the first target of inactivating enzymes. The two hydroxyl groups are not essential for activity. Methylation of the amines will retain activity and will lower susceptibility to transferases. All substitutions must be equatorial.

Ring II , Many modifications are possible, for example the 1-amine can be substituted or acylated.

Ring III , The amine can be changed to a hydroxyl group, but its removal will abolish activity.

Modification:

Improve margin of safety. Not very successful.

Lower resistance by forming derivatives less or not affected by the inactivating enzymes. This was accomplished by either removing the affected functional group or add a steric group to protect the affected region.

Individual Agents:

Streptomycin is used parenterally in TB as adjunct agent, and in other infections such as bubonic plague and occasionally locally in GIT infections. It is unstable in acids and high temperature probably due to the formyl group. It is also highly susceptible to inactivating enzymes.

Neomycin is used in dermatological preparation in combination with other agents. It can also be used for preoperative bowel sanitation. Although it is poorly absorbed orally, monitoring of the patient's blood level of the drug is advisable, as there have been reports of toxicity after oral use. It has also been shown to lower serum cholesterol levels. It is more stable and is less susceptible to resistance.

Paromomycin differs from Neomycin only in ring I. Used for intestinal amoebic dysentery.

Kanamycin was introduced in 1957, shows better stability. Kanamycin injections are very painful, usually administered with a local anesthetic. It is very susceptible to bacterial resistance due to presence of all possible functional groups that are targets of the inactivating enzymes.

Amikacin retains about half the activity of Kanamycin, but resists attacks by inactivating enzymes on both rings II and III, possibly due to reduced binding. It shows lower ototoxicity than Kanamycin, but this is offset by the higher dose needed.

Gentamicin was introduced in 1958, and is effective against most Gram positive and negative bacteria, including pseudomonas. Used topically for skin infections and parenterally for pseudomonal infections due to burns and in bone and joint infections. It is sometimes used in combination with ß-lactams such as Carbenicillin. Inactivation by enzymes is minimal due to chemical modifications, but resistant strains are emerging.

Tobramycin introduced in 1976, and showed higher activity against pseudomonas compared to Gentamicin and is widely used in difficult infections, especially those caused by Gentamicin resistant pseudomonal infections. It can be used by inhalation through nabulizers to treat pseudomonas aeruginosa infections in cystic fibrosis patients.

Netilmicin is similar to Gentamicin, but shows better resistance to the inactivating enzymes compared to both Gentamicin and Tobramycin and is thus used in infections due to resistant strains.

Spectinomycin is not a typical Aminoglycoside, but is closely related to these agents. It has a broad spectrum, but is bacteriostatic possibly due to the fact that they do not cause misreading of the code, but only inhibits protein synthesis elongation. It is mostly used against ß-lactamase producing gonococcal infections (over 90% cure) as a single dose bolus injection, but its use is limited as it is ineffective against other sexually transmitted diseases such as Syphilis and Chlamydia. It may cause loss of balance and dizziness similar to neuromuscular junction blockers, but apparently doesn't cause the other side effects associated with Aminoglycosides.