Course Coordinator Jamaluddin Shaikh, Ph.D. Protein Synthesis Inhibitors: Aminoglycosides Course Coordinator Jamaluddin Shaikh, Ph.D. School of Pharmacy, University of Nizwa Lecture 4 September 26, 2011

Aminoglycosides It was the main treatment of serious infections due to aerobic gram-negative bacilli However, because their use is associated with serious toxicities, they have been replaced to some extent by safer antibiotics, such as the third- and fourth-generation cephalosporins, and carbapenems Bactericidal in nature An aerobic organism or aerobe is an organism that can survive and grow in an oxygenated environment

It was made in the nucleus by transcription from a DNA molecule. This is a molecule of messenger RNA. It was made in the nucleus by transcription from a DNA molecule. codon A U G G G C U U A A A G C A G U G C A C G U U mRNA molecule

A ribosome on the rough endoplasmic reticulum attaches to the mRNA molecule. A U G G G C U U A A A G C A G U G C A C G U U

Amino acid U A C tRNA molecule A transfer RNA molecule arrives. It brings an amino acid to the first three bases (codon) on the mRNA. anticodon The three unpaired bases (anticodon) on the tRNA link up with the codon. A U G G G C U U A A A G C A G U G C A C G U U

U A C C C G Another tRNA molecule comes into place, bringing a second amino acid. Its anticodon links up with the second codon on the mRNA. A U G G G C U U A A A G C A G U G C A C G U U

U A C C C G Peptide bond A peptide bond forms between the two amino acids. A U G G G C U U A A A G C A G U G C A C G U U

U A C C C G The first tRNA molecule releases its amino acid and moves off into the cytoplasm. A U G G G C U U A A A G C A G U G C A C G U U

C C G The ribosome moves along the mRNA to the next codon. A U G G G C U U A A A G C A G U G C A C G U U

C C G A A U Another tRNA molecule brings the next amino acid into place. A U G G G C U U A A A G C A G U G C A C G U U

C C G C C G A peptide bond joins the second and third amino acids to form a polypeptide chain. A U G G G C U U A A A G C A G U G C A C G U U

A C G G U C The process continues. The polypeptide chain gets longer. This continues until a termination (stop) codon is reached. The polypeptide is then complete. A U G G G C U U A A A G C A G U G C A C G U U

Inhibition of Protein Synthesis Steps in protein synthesis: 1. Initiation 2. Elongation 3. Translocation 4. Termination 80S vs. 70 S But similar to mitochondrial ribosomes which may account for some toxicity Different aspects of protein synthesis are impacted on based on the target.

Aminoglycosides Gentamicin Netilmicin Tobramycin Amikacin

Aminoglycosides: Mechanism of action Diffuse through porin channel of gram –ve bacteria and enter periplasmic space Following oxygen-dependent transport across the cytoplasmic membrane, they bind to polysomes and interfere with protein synthesis by causing misreading and premature termination of translation of mRNA Transported into cells and block bacterial protein synthesis by binding to 30S ribosome To initiate translation, a 30S ribosomal subunit first binds to a sequence on the mRNA called the ribosome binding site found just prior to the start codon AUG. Then a tRNA having a complementary anticodon (UAC) and carrying the amino acid f-methionine (fMet) forms H-bonds with the AUG start codon. This forms the initiation complex for translation. Next, a 50S ribosomal subunit joins the complex. Antibiotics known as aminoglycosides bind to the 30S subunit of bacterial ribosomes and block the attachment of the 50S subunit to the initiation complex.

Aminoglycosides: Antibacterial Spectrum Effective in the empirical treatment of infections suspected of being due to aerobic gram –ve bacilli, including Pseudomonas aeruginosa Effective against aerobes only because aminoglycosides need oxygen-dependent active transport inside bacteria To achieve an additive or synergistic effect, often combined with a β-lactam antibiotic, or vancomycin, or a drug active against anaerobic bacteria Empirical data is data produced by an experiment or observation

Pharmacokinetics: Aminoglycosides Poorly absorbed from the gut due to their highly polar, polycationic structure Given by intramuscular or intravenous injections Poorly protein bound (30%) and excreted renally Half-lives are short, usually 2 hours Three times daily administration is usually adequate In patients with renal dysfunction, dose reduction and/or increased dose interval is required

Adverse Effects: Aminoglycosides Ototoxicity: Vestibular (dizziness and loss of balance) and cochlear (deafness) nerve damage Nephrotoxicity: Retention of the aminoglycosides by the proximal tubular cells disrupts calcium-mediated transport processes, and this results in kidney damage Neuromuscular paralysis: Most often occurs after direct intraperitoneal or intrapleural application of large doses of aminoglycosides. Patients with myasthenia gravis are particularly at risk. Allergic reactions: Contact dermatitis is a common reaction to topically applied neomycin.

Drug Interactions: Aminoglycosides Aminoglycosides enhance neuromuscular blockade of non-depolarizing neuromuscular antagonists Loop diuretics potentiate their nephrotoxicity and ototoxicity

Mechanism of Resistance: Aminoglycosides Resistance can be caused by decreased uptake of drug when the oxygen-dependent transport system for aminoglycosides or porin channels are absent plasmid-associated synthesis of enzymes (for example, acetyl transferases, nucleotidyltransferases, and phosphotransferases) that modify and inactivate aminoglycoside antibiotics