Overview of Translation (2) Protein Synthesis Inhibitors • Macrolides - Lincosamides Initiation – tRNA + AA binds translation • Aminoglycosides elongation factor • Tetracyclines – Enters ribosome and attaches at the A site • Chloramphenicol • Streptogramins • Oxazolidinones Lecture Outline Overview of Translation (3) • Description of protein synthesis Amino Acid Transfer • Antibiotics – Petidyltransferase on – Structure - function - classification 50S ribosome attaches – Mechanism(s) of action the next AA to the – Mechanism(s) of resistance polypeptide – Spectrum of activity/Indications for use – Met added to Leu at A – Pharmacology site – Toxicity • Clinical examples Overview of Translation (1) Overview of Translation (4) Initiation: 30S binds RBS of mRNA AA binds tRNA using Elongation aminoacyl-tRNA synthetase tRNA moved to P site by IF2 and fmet-tRNA binds EF-G creating room at A 30S at P site site for next tRNA 50S binds complex 70S Translation termination Occurs at nonsense codon sites e.g. UAA Release factors Ribosome dissociates 1 Mechanisms of Action of Protein Macrolides Synthesis Inhibitors (i.e. Translation) • Inhibition of formation of the initiation complex - • Broad spectrum antibiotics linezolid (linezolid-oxazolidinone) • Inhibition of peptide bond formation -chloramphenicol, • Original agent: erythromycin macrolides • Azalides: azithromycin and • Inhibition of translocation - macrolides, lincosamides • Inhibition of binding to the tRNA site - tetracyclines clarithromycin • Misreading and premature termination of peptide chains – selected antimicrobial and pharmacokinetic - aminoglycosides advantages Large 14 member macrolactone Mechanisms of Action - Protein ring with one or more deoxy Synthesis Inhibitors sugars attached. Inhibits formation of 50S ribosome blocking trans- peptidation or translocation. Large 14 member lactone ring with modification of C6 to a methoxy group. Azithromycin has a 15 membered lactone ring Macrolides - Mechanisms of Mnemonics (Such as they are) Resistance • Mechanisms differ for different bacterial species 30S: aminoglycoside-tetracycline • Decreased permeability of envelope (e.g., the enterobacteriaceae) TAT • Mutation of the 23S ribosomal RNA of the 50S ribosomal subunit (alters binding site) 50S: chloramphenicol-erythromycin-lincosamide (clindamycin)-linezolid-streptogramins – Can be chromosomal, plasmid or on a transposon – Can confer resistance to macrolides, lincomycins and streptogramins (mls) CELLS • Active efflux of antibiotic (plasmid-mediated) - mostly with Gram positive bacteria 2 Macrolides - Spectrum of Activity Macrolides - Pharmacology • Erythromycin: • High concentrations in alveolar cells and – Gram positives: pneumococci, viridans polymorphonuclear leukocytes, especially azalides streptococci, Group A streptococci, methicillin sensitive staphylococci • Most of drug is concentrated in the liver and – Gram negatives: bordetella, neisseria, excreted in the bile. Some is inactivated in the campylobacter, ± hemophilus liver by demethylation. – Miscellaneous: mycoplasma, legionella, chlamydia, treponemes Azalides - Spectrum of Activity Macrolides - Indications for Use • Spectrum similar to erythromycin • Community acquired pneumonia: • Increased activity against hemophilus, mycoplasma, legionella, chlamydia Mycobacterium avium intracellulare, • Pertussis toxoplasma • Campylobacter jejuni gastroenteritis • Azithromycin > Gram negative activity • MAC (azalides) • Clarithromycin > Gram positive activity • Alternative agents for: group A,C,G streptococcal infections, rheumatic fever prophylaxis, C. trachomatis urethritis, anthrax Macrolides - Pharmacology Macrolides - Indications for Use • Can be administered orally or parenterally • Novel indication for use – Well absorbed - especially azalides – Potential antibacterial (vs. anti-inflammatory) • t1/2 erythromycin - 1.4h effects in the treatment of P. aeruginosa • Azalides have long t1/2 infections in Cystic Fibrosis – Clarithromycin 3-7h – Azithromycin 2-4 days • Well distributed, CNS penetration limited except with inflammation 3 Macrolides - Toxicity Aminoglycosides • Well tolerated Polycationic molecule with at • Gastrointestinal symptoms - cramps, diarrhea least 2 aminosugars linked by secondary to motility stimulating effects of antibiotic. glycosidic bonds to an amino- Motilin receptor agonist cyclitol ring • Cholestatic hepatitis (rare) Removal of amino or hydroxyl • Drug interactions - erythromycin > clarithromycin groups correlates with loss of interferes with the cytochrome P450 enzymes leading antibacterial activity and toxicity Water soluble - limited ability to to increased levels of other drugs e.g. dilantin, cross lipid membranes warfarin, cyclosporine Aminoglycosides - Mechanism of Clindamycin (Lincosamide) Action • MOA similar to macrolides • Diffuses through porin channels in outer membrane of Gram negative bacteria • Bacteristatic activity against Gram positive • Binds to and alters bacterial cell membrane causing bacteria and anaerobes - also toxoplasma leakage of the outer Gram negative membrane and • Pharmacology - high bone concentrations disruption of the cell wall • Toxicity - diarrhea, allergy • *Interferes with mRNA translational accuracy primarily at the 30S ribosome causing misreading and • Indications - penicillin-resistant anaerobic premature chain termination infections • Bactericidal activity appears to be multifactorial Aminoglycosides - Mechanism of Aminoglycosides Resistance • Complex sugars with glycosidic linkages • *Enzymatic modification of the aminoglycoside by • Bactericidal antibiotics with activity primarily adenylation, phosphorylation or acetylation directed against aerobic Gram negative – Usually found on plasmids or transposons bacteria • Anaerobes are resistant because they lack an O2 • Narrow therapeutic window with significant dependent transport system toxicity • Chromosomal mutations can also cause alterations • Primarily used as a second therapeutic agent in in binding and uptake e.g. S. aureus the treatment of serious Gram negative or enterococcal infections 4 Antibacterial Spectrum Aminoglycosides - Toxicity • Nephrotoxicity: incidence 5-25% • Aerobic gram negative bacilli • Pseudomonas aeruginosa – Damages proximal tubular cells, • Gram positive bacteria (used for synergy) • Ototoxicity: Cochlear 3-14%, Vestibular 4-6% Staphylococcus spp., Enterococcal spp. – Long otic fluid t1/2 • Selected aminoglycosides have activity – Cochlear damage to the outer hair cells of the organ against: Mycobacteria spp., Yersinia pestis of Corti • No activity against: hemophilus, anaerobes, – Vestibular damage to type 1 hair cell of the summit pneumococcus, neisseria of the ampullar cristae Aminoglycosides - Indications for Aminoglycosides - Toxicity Use • Empiric therapy: life-threatening infections that • Neuromuscular blockade: rare require broad spectrum coverage – usually occurs following concomitant use of other • Specific therapy: synergistic antimicrobial activity blocking agents – Enterococcal endocarditis – Interferes with both presynaptic release of – Pseudomonas infections acetylcholine and blockade of postsynaptic • Monotherapy: rarely used, inhalational therapy for receptors CF patients with pseudomonal pneumonia Aminoglycosides - Pharmacology Tetracyclines • Minimal absorption after oral • Broad spectrum bacteristatic agents administration • Grouped based on differences in t1/2 - short, • Limited tissue distribution due to polarity intermediate and long-acting • Not used for treatment of staphylococcal or Gram • Not metabolized, excreted by the kidney negative bacterial infections because of the rapid • Rapid absorption after IM administration emergence of resistance 5 Tetracyclines Tetracyclines - Indications for Use Basic structure consists of 4 fused 6 carbon rings - • Treatment of chlamydia, mycoplasma, hydronaphthacene nucleus brucella, vibrio, helicobacter, rickettsia, with modifications at borrelia, ehrlichia (anaplasma) infections selected positions • Mycobacterium marinum infections Binds to the 30S portion of ribosome, prevents access of •Acne aminoacyl tRNA molecules • Rarely the first drug of choice to the mRNA ribosome- peptide complex Tetracyclines - Mechanism of Tetracyclines - Pharmacology Resistance • Primarily oral agents • Common in both Gram positive and negative bacteria • Cations Ca2+, Mg2+ interfere with absorption by • Generally, but not exclusively, plasmid-mediated chelating tetracyclines, e.g., dairy products • *Decreased uptake and increased excretion of the drug • t1/2 varies with agent as does extent of excretion by (pump) the kidney. • Resistance is conferred to all tetracyclines – Doxycycline excreted in the feces • Has been associated with the extensive use of – Minocycline metabolized in the liver tetracyclines in animal food • In general excellent tissue distribution • Concentrated in the bile, achieves levels of 10-26% of serum in CSF Tetracyclines - Antimicrobial Tetracyclines - Toxicity Spectrum • Gram positives: S. pneumoniae, S. pyogenes, S. • Gastrointestinal symptoms: common agalactiae, enterococci • Photosensitivity • Gram negatives: E. coli, Neisseria spp., • Discoloration of teeth due to binding to calcium - Hemophilus spp., Shigella spp. not reversible • Miscellaneous: Spirochetes - Borrelia, Rickettsiae, • Hypersensitivity reactions: rash, urticaria, Chlamydiae, Mycoplasma, Legionella, Pasteurella, anaphylaxis (rare) Ehrlichia (Anaplasma) • Hepatotoxicity - especially during pregnancy 6 Chloramphenicol Linezolid - Oxazolidinone