aminoglycoside and macrolide antibiotics Flashcards
aminoglycoside MOA
The aminoglycosides inhibit protein biosynthesis by binding to the 30S ribosomal subunit. They bind to the 16S rRNA forming the A site. This interferes with formation of the initiation complex, blocks further translation, and elicits premature termination. It also causes impairment of the proofreading function of the ribosome and formation of “nonsense proteins” resulting from selection of the wrong amino acids during translation. The “nonsense proteins” impair bacterial cell wall function. The damaged membranes have altered permeability characteristics and actually allow transport of larger amount of aminoglycoside, and protein synthesis ceases altogether. Ultimately the aminoglycosides lead to leakage of ions and disruption of the cytoplasmic membrane, resulting in cell death.
bacterial aminoglycoside uptake
1) The initial entry of the positively charged aminoglycosides through the outer membrane involves the displacement of Mg++ and Ca++ ions that form salt bridges with phosphates of the phospholipids in the membrane. This makes the membrane more permeable to the aminoglycosides.
2) Passage through the cytoplasmic membrane is an active transport process
aminoglycoside resistance
1) Metabolism: Bacteria inactivate aminoglycosides by acetylation, adenylation, and phosphorylation. This can occur at multiple places within 1 antibiotic structure. Clinical isolates worldwide exhibit different patterns of enzyme production. Many of the metabolizing enzymes have cross-resistance specificity, so resistance can emerge to aminoglycosides in general. Note: the aminoglycosides are not metabolized by humans (as opposed to bacteria) to any appreciable extent, and are excreted largely unchanged in the urine.
2) Altered Ribosomes: The 16S rRNA binding site can be altered through point mutations. This has been observed clinically with Mycobacterium tuberculosis.
3) Altered Aminoglycoside Uptake: The rate of emergence is far less than resistance due to metabolism, and the phenotype reverts after the drug is removed.
aminoglycoside toxicity
All aminoglycosides are ototoxic* (irreversible) and nephrotoxic* (reversible). The irreversible ototoxicity can result initially in tinnitus and high-frequency hearing loss, or in vestibular damage resulting in vertigo, loss of balance, and ataxia. Serial audiograms can be obtained where feasible, particularly in high risk patients. Risk factors include concurrent use of other ototoxic drugs (e.g. loop diuretics or vancomycin), compromised renal function, or genetic vulnerability.
Concurrent use with loop diuretics (ethacrynic acid and furosamide) or other nephrotoxic antimicrobial drugs (vancomycin or amphotericin) can potentiate nephrotoxicity** and should be avoided. Creatinine clearance can be monitored and the dose decreased in order to avoid nephrotoxicity. Evidence of ototoxicity** (dizziness, vertigo, tinnitus, roaring in the ears, and hearing loss) or nephrotoxicity requires discontinuation of the drug or dosage adjustment.
Curare-like effects are less common but can result from large doses of aminoglycosides. Respiratory paralysis** can usually be reversed by neostigmine or calcium gluconate, but mechanical respiratory assistance may be necessary.
The likelihood of aminoglycoside toxicity increases with the treatment period. It is more likely to occur if treatment is extended more than 5 days, in the elderly, if renal function is impaired, and in higher doses.
Since more effective and less toxic alternatives to aminoglycosides are usually available, they should be used sparingly and only for specific indications. If an aminoglycoside has to be used, the duration of therapy should be minimized and serum concentrations should be monitored.
therapeutic use of aminoglycosides
Although the aminoglycosides have broad spectrum antibiotic activity against both Gram-(+) and Gram-(–) bacteria, in practice their use is almost always reserved for treatment of Gram-(–) bacteria. They are often used in combination with penicillins in order to take advantage of the synergism between these two classes of antibiotics. However, when penicillins are given with aminoglycosides, they should be administered in different compartments** (typically one in each arm) in order to avoid a chemical reaction between the two classes.
Therefore, aminoglycosides and penicillins should not be mixed together in the same solution.
Penicillin/aminoglycoside combinations are used to treat bacterial endocarditis**.
Streptomycin is most often used to treat tuberculosis.
Gentamicin is actually a mixture of related compounds and is the most important of the aminoglycosides. It is used for urinary tract infections, burns, some pneumonias, and joint and bone infections caused by susceptible Gram-(–) infections.
Aminoglycoside-resistant strains are common in hospitals. Fortunately, amikacin* has retained antibacterial activity against these resistant strains and is therefore still useful to treat nosocomial infections.
kanamycin
Kanamycin is a mixture of at least three compounds (A, B, and C, with A predominating) isolated from Streptomyces kanamyceticus. It is chemically very stable, but it is inactivated by R-factor enzymes. It is used parenterally against some Gram-(–) bacteria, but anaerobes and Pseudomonas aeruginosa are usually resistant***. It is used parenterally against some mycobacteria
(M. kansasii, M. marinum, and M. intracellulare). Injections of kanamycin A are painful enough to require use of a local anesthetic.
Like the other aminoglycoside antibiotics, kanamycin can affect the 30S ribosomal subunit to cause a frame shift. This means that instead of a codon CCG (for example in sequence CCGU), a codon CGU is read by the aminoacyl tRNA, which carries a different amino acid, because the anticodon on the aa-tRNA is different. This results in the formation of altered proteins. Note: CGU codes for Arg.
amikacin
Amikacin is synthesized from kanamycin A. The presence of the L-hydroxyaminobuteryl amide moiety inhibits bacterial metabolism by R-factors, so amikacin is more potent than kanamycin. Amikacin is used competitively with gentamicin for treatment of Mycobacterium tuberculosis, Francisella tularensis, and severe Pseudomonas aeruginosa infections resistant to other agents. Amikacin is also used to treat aminoglycoside-resistant nosocomial infections in hospitals.
Tobramycin
Tobramycin is produced by fermentation of Streptomyces tenebrarius. It lacks a 3’-hydroxyl group and cannot be phosphorylated at that position. However, it is adenylated at C-2’ and acetylated at C-3. Because tobramycin has superior activity vs. Pseudomonas aeruginosa, it is widely used parenterally to treat gentamicin-resistant Pseudomonas aeruginosa infections**, as well as other difficult infections.
Tobramycin has antimicrobial activity, pharmacokinetic properties, and a toxicity profile that are very similar to gentamicin. The indications for use of tobramycin are the same as form gentamicin.
gentamicin
Gentamicin is a mixture of several antibiotics produced by fermentation of Micromonospora purpurea and other related soil microorganisms. It is the most important of the aminoglycoside antibiotics still in use because of its low cost and reliable activity against** all but the most resistant Gram-(–) aerobes. It is used to treat urinary tract infections, and joint and bone infections caused by Gram-(–) bacteria. Gentamicin topical preparations are used to treat infections of the skin and in dressings used to treat burn patients. Gentamicin ophthalmic preparations are also used to treat eye infections.
orally used aminoglycosides
Although aminoglycosides are not absorbed from the GI tract, neomycin, paromomycin, and kanamycin are used clinically to suppress gut flora in travelers diarrhea* and prophylactically prior to GI surgery* to decrease the incidence of peritonitis. Neomycin is also used as a topical ointment, and paromomycin is used to treat amoebic dysentery as well as dwarf tapeworm and beef tapeworm.
streptomycin
Streptomycin is administered by deep IM injections that are often painful. It is presently used in combination with other agents for treatment of tuberculosis. It is the least used of the first-line agents. Streptomycin is also useful against bubonic plague, leprosy, and tularemia.
structure and biosynthesis of macrolides
1) The macrolide antibiotics are macrocyclic lactones. They are usually 14-membered lactone rings.
2) The unusual deoxyhexose sugars are essential for biological activity.
3) Macrolide antibiotics are polyketides because they are produced by sequential addition of proprionate groups to a growing chain. This results in methyl groups on alternate carbon atoms in the macrolide ring.
This class of secondary metabolites contains many diverse structures. Most are produced by Streptomyces strains for defense. Fermentation provides a source of erythronolide that can be used as a starting material for semisynthetic derivatives.
4) The pKa of the amine in erythromycin is 8.8. The amine can form salts that are more soluble.
macrolide MOA
Recall that during peptide bond formation, the polypeptide attached to the tRNA in the P site of the ribosome is transferred to the amino group of the aminoacyl-tRNA in the A site (transpeptidation). The ribosome then moves to the next codon. The empty tRNA is ejected and the peptidyl-tRNA is shifted from the A site to the P site (translocation). A new aminoacyltRNA then binds to the A site.
Macrolides inhibit bacterial protein synthesis by binding reversibly to the P site of the bacterial ribosome, thereby inhibiting translocation of peptidyl-tRNA from the A site to the P site. Some macrolides also appear to bind between the P and A sites and obstruct peptide bond formation. Macrolide binding mainly involves the bacterial 23S RNA and not the protein. Macrolide antibiotic action is mainly bacteriostatic, but can also be bactericidal in high concentrations. Macrolides tend to accumulate within leukocytes, and are therefore actually transported into the site of infection.
macrolide MOR
1) Lactone ester hydrolase induced to degrade the macrolides by hydrolysis of the macrocycle.
2) Drug-induced production of an RNA methylase. A specific adenine base (A2058) on the 23S RNA molecule of the 50S ribosomal subunit is methylated. This inhibits the binding of macrolides to the 50S subunit.
Note: the erythromycin-producing organism uses the same ribosomal methylation technique to protect itself from the effects of its own toxic metabolite. This leads to the hypothesis that antibiotic resistance may originate in the producing organism and then be transferred to other bacteria.
3) Mutation of adenine to guanine at the specific site A2058. This results in a 10,000-fold reduction in binding of erythromycin and clarithromycin to the 23 S ribosomal RNA.
4) An efflux pump ejects drugs from the cell by an active transport process.
chemical reactivity of macrolides
The parent molecule can be inactivated under acidic conditions by a process involving the 6-OH group. The reaction is an intramolecular acid-catalyzed ketal formation. The ketal reaction product is inactive. Oral erythromycin is therefore administered as enteric coated tablets or as more stable salts or esters.
Notes:
1) Acid stability can be achieved with the 6-OCH3 derivative, which enhances oral absorption. This ether derivative blocks ketal formation at low pH. The resulting antibiotic is Clarithromycin.
2) An amine analog (Azithromycin) is acid stable and has reliable absorption. In this analog an N-methylated methyleneamino moiety replaces the C-9 ketone, so ketal formation is no longer possible.
