PHARMACOLOGY OF COMMON ANTIBIOTICS USED FOR LOWER RESPIRATORY TRACT INFECTIONS Dr. Suleiman Al-Sabah Department of Pharmacology and Toxicology Faculty of Medicine, Kuwait University Room 67, Ext:

Pharmacology of common antibiotics used for lower respiratory tract infections (ID#7629) WLO: Understand the mechanisms of action of common antibiotics used in the management of LRTIs Specific Objectives: 1. To understand the mechanisms of action of the main antibacterials used in treating lower LRTIs 2. To understand the main adverse reactions for antibacterials used in treating LRTIs

Lower Respiratory Tract (LRT) infections: Antibiotic use a)first line treatment for LRT infections but not indicated in viral infections. b)antibiotic selection (single or multiple antibiotics) is based on the infecting organism. c)Full course of therapy needs to be completed to minimize resistance. d)change therapy with the evolving nature of LRT infections and the emerging resistance to conventional therapies.

Lower Respiratory Tract (LRT) infections Most common infections are bronchitis and pneumonia. Pneumonia - serious infection of the small bronchioles and alveoli that can involve the pleura. - either community or hospital acquired - life-threatening in the elderly or in immuno-compromised patients - most common treatment is antibiotics but choice depends on severity of disease, nature of infecting organism, resistance, etc Bronchitis - acute or chronic inflammation of the trachea and the bronchi. - mostly caused by viruses - only use antibiotics in bacterial infections Examples in use: trimethoprim/sulfamethoxazole, azithromycin and clarithromycin. Children under the age of eight are usually given amoxicillin.

Selective Toxicity Resistance

Classes of antibiotics commonly used for Lower respiratory tract infections: Anti-folates (e.g. sulfanilamide, trimethoprim) Beta lactams (e.g. penicillins, cephalosporins etc.) Glycopeptides (e.g. vancomycin) Tetracyclines (e.g. doxycycline) Aminogylcosides (e.g. gentamicin) Amphenicols (e.g. chloramphenicol) Macrolides (e.g. erythromycin) Lincosamides (e.g. clindamycin) Oxazolidinines (e.g. linezolid) Quinolones (e.g. ciprofloxacin) Nitroimidazoles (e.g. metronidazole)

Classes of antibiotics commonly used for Lower respiratory tract infections: Anti-folates (e.g. sulfanilamide, trimethoprim) Beta lactams (e.g. penicillins, cephalosporins etc.) Glycopeptides (e.g. vancomycin) Tetracyclines (e.g. doxycycline) Aminogylcosides (e.g. gentamicin) Amphenicols (e.g. chloramphenicol) Macrolides (e.g. erythromycin) Lincosamides (e.g. clindamycin) Oxazolidinines (e.g. linezolid) Quinolones (e.g. ciprofloxacin) Nitroimidazoles (e.g. metronidazole) folate synthesis & action

Classes of antibiotics commonly used for Lower respiratory tract infections: Anti-folates (e.g. sulfanilamide, trimethoprim) Beta lactams (e.g. penicillins, cephalosporins etc.) Glycopeptides (e.g. vancomycin) Tetracyclines (e.g. doxycycline) Aminogylcosides (e.g. gentamicin) Amphenicols (e.g. chloramphenicol) Macrolides (e.g. erythromycin) Lincosamides (e.g. clindamycin) Oxazolidinines (e.g. linezolid) Quinolones (e.g. ciprofloxacin) Nitroimidazoles (e.g. metronidazole) folate synthesis & action cell wall synthesis

Classes of antibiotics commonly used for Lower respiratory tract infections: Anti-folates (e.g. sulfanilamide, trimethoprim) Beta lactams (e.g. penicillins, cephalosporins etc.) Glycopeptides (e.g. vancomycin) Tetracyclines (e.g. doxycycline) Aminogylcosides (e.g. gentamicin) Amphenicols (e.g. chloramphenicol) Macrolides (e.g. erythromycin) Lincosamides (e.g. clindamycin) Oxazolidinines (e.g. linezolid) Quinolones (e.g. ciprofloxacin) Nitroimidazoles (e.g. metronidazole) folate synthesis & action cell wall synthesis protein synthesis

Classes of antibiotics commonly used for Lower respiratory tract infections: Anti-folates (e.g. sulfanilamide, trimethoprim) Beta lactams (e.g. penicillins, cephalosporins etc.) Glycopeptides (e.g. vancomycin) Tetracyclines (e.g. doxycycline) Aminogylcosides (e.g. gentamicin) Amphenicols (e.g. chloramphenicol) Macrolides (e.g. erythromycin) Lincosamides (e.g. clindamycin) Oxazolidinines (e.g. linezolid) Quinolones (e.g. ciprofloxacin) Nitroimidazoles (e.g. metronidazole) folate synthesis & action cell wall synthesis protein synthesis DNA synthesis

Folic Acid Antagonists Or Anti-folates First commercially available antibiotic was prontosil, patented Gerhard Domagk received the 1939 Nobel prize for medicine for his work on prontosil. The red dye was active in vivo but not in vitro. Prontosil was a prodrug that was metabolised to sulfanilamide.

Folic Acid Antagonists Or Anti-folates Folate is required for DNA synthesis in both humans and bacteria. Sulofonamides (e.g. sulphanilamide) are structural analogues of para ˗ aminobenzoic acid (PABA) and inhibit the enzyme dihydropteroate synthetase an essential step in folic acid synthesis. Selective toxicity arises because human’s do not synthesis folic acid and have evolved specific uptake mechanisms to transport folic acid into cells. Most bacteria lack these mechanisms and must synthesis their own folate.

Folic Acid Antagonists Or Anti-folates Trimethoprime inhibits the enzyme dihydrofolate reductase and hence tetrahydrofolic acid synthesis an important cofactor in thymidylate (and hence DNA) synthesis. The bacterial form of dihydrofolate reductase is many times more sensitive to trimethoprime than the human form.

Action Of Sulfonamides & Trimethoprim On Bacterial Folate Synthesis PABA Folate Tetrahydorfolate Synthesis of thymidylate etc DNA Dihydropteroate synthetase Dihydrofolate reductase Sulfonamides Trimethoprim

Co-trimoxazole (sulfamethoxazole & trimethoprim) Bacteriostatic Used to treat infection Pneumocystis carinii infection inpatients with AIDS. Unwanted effects of sulfonamides include nausea and vomiting, headache and mental depression, more serious adverse effects include hepatitis, hypersensitivity reactions, bone marrow depression and crystallurina. Unwanted effects of trimethoprim include nausea, vomiting, blood disorders and skin rashes. Folate deficiency can be prevented by giving folinic acid.

The cell wall of bacteria contains peptidoglycan which is not found in eukaryotic cells. β-lactam antibiotics (e.g. penicillin, amoxicillin) inhibit the bacterial transpeptidase enzymes responsible for cross-linking peptide chains of peptidoglycan and hence cell wall synthesis. β-lactam antibiotics also inactivate inhibitors of bacterial autolytic enzymes in the cell wall leading to cell lysis. Glycopeptide Antibiotics (e.g. vancomycin) inhibits bacterial cell wall synthesis by binding to the D-Ala-D-Ala region of the peptidoglycan subunits inhibiting their release from the cell membrane carrier and preventing cross-linking of subunits that are released.

Cytoplasm Cell Membrane M UDP P C 55 M=N-acetylmuranic acid

Cytoplasm Cell Membrane M UDP M P C 55 P UMP P C 55

Cytoplasm Cell Membrane M UDP M P C 55 P UMP G UDP M P C 55 P G P UDP G=N-acetylglucosamine

Cytoplasm Cell Membrane M UDP M P C 55 P UMP G UDP M P C 55 P G M P P G P UDP

Cytoplasm Cell Membrane M UDP M P C 55 P UMP G UDP M P C 55 P G M P P G M G P P UDP Vancomycin

Cytoplasm Cell Membrane M UDP M P C 55 P UMP G UDP M P C 55 P G M P P G P P UDP M G

Cytoplasm Cell Membrane M UDP M P C 55 P UMP G UDP M P C 55 P G M P P G P P UDP M G

Cytoplasm Cell Membrane M UDP M P C 55 P UMP G UDP M P C 55 P G M P P G M G P P UDP M G M G β-Lactam

preferentially bind to specific penicillin-binding proteins (PBPs) located inside the bacterial cell wall (transpeptidases) inhibits peptidoglycan cross-link formation in bacterial cell wall β-lactam antibiotics also inactivate inhibitors of bacterial autolytic enzymes in the cell wall leading to cell lysis. Mechanism of Action Cell Lysis (bactericidal) Penicillins (e.g. penicillin (discovered by Alexander Fleming in 1928) methicilin & amoxycillin) Cephalosporins (e.g. Cefotaxime) β-Lactams Inhibitors Of Bacterial Cell Wall Synthesis

The intrinsic activity of β-lactam antibiotics against a particular organism depends on its ability to gain access to and bind with the necessary PBP. They have broad spectrum of activity and usually a wide therapeutic index (safe to use) Adverse reactions of β-lactam antibiotics Mainly Hypersensitivity reactions (range from a benign rash to anaphylaxis). Nausea/vomiting and diarrhea Exhibit cross-sensitivity or cross alergenicity (Allergy to other penicillins or cephalosporins)

Mechanism of Bacterial Resistance To Penicillins & Cephalosporins  Inactivation of the drug by β-lactamases (most common)  Structural differences in the PBPs that are the targets of these drugs.  Presence of permeability barrier preventing penetration of antibiotic to the target site. (Occurs mainly with gram (-) organisms which have an outer membrane that limits the penetration of hydrophillic antibiotics. Alone may not be a sufficient cause for resistance but become important in the presence of a β-lactamase which hydrolyses antibiotic as it slowly penetrates the cell.)

O Major Mechanism of Bacterial Resistance to Penicillins and Cephalsporins  Inactivation of the drug by β-lactamases R1R1 C O N H B N S COOH CH 3 A Penicillin Nucleus Beta-lactamase hydrolyses the β -lactam ring Therefore given together with beta-lactamase inhibitors

Beta-Lactamase Inhibitors e.g. Clavulanic acid B N O COOH CHCH 2 OH O Clavulanic acid  Resemble β-lactams in structure  Have no antibacterial activity  Inhibits many β-lactamases.  Protect hydrolyzable β-lactam antibiotics from inactivation by the β-lactamases  Given in combination with the hydrolyzable β-lactams E.g. Co-Amoxiclav (with amoxycillin)

β-Lactams Resistant to β-Lactamases R1R1 C O N H B N O SO 3 H CH 3 Monobactams (β-lactamase resistant) H3CH3C CH B N HO S R1R1 COOH Carbapenems (high resistance to β-lactamases) (e.g. Aztreonam) (e.g. Meropenem)

Glycopeptide Antibiotics (e.g. Vancomycin) Mechanism of Action  Vancomycin is bactericidal  Binds to precursors of cell wall synthesis and inhibits peptidoglycan elongation  This binding occurs at a different site of action from that of penicillin (D-alanine residues)  The net result is an alteration of bacterial cell wall permeability.  In addition, RNA synthesis is inhibited. Adverse Reactions Ototoxicity Nephrotoxicity

Glycopeptide Antibiotics: Inhibitors of cell wall synthesis (e.g. Vancomycin, teicoplanin) Therapeutic Uses Vancomycin is particularly useful against penicillin- and methicillin-resistant S. aureus (MRSA) and S. epidermidis infections and for treating gram-positive infections in penicillin-allergic patients. Not absorbed orally and given parenterally (i.v) Orally it is given for antibiotic-associated C. difficile colitis (not absorbed but for local effect in G.I. tract) Caution: Vancomycin resistance strains of MRSA and enterococci have emerged, so should be used judiciously!

Inhibitors Of Protein Synthesis Protein synthesis takes place in ribosomes. Selective toxicity of antibacterial agents that target ribosomes arises as eukaryotic and prokaryotic ribosomes differ. Bacterial ribosomes are made up of a 50S and 30S subunit, whereas mammalian ribosomes are made up of 60S and 40S.

mRNA Direction of ribosome travel 30S tRNA 50S Bacterial Ribosome A E P

mRNA Direction of ribosome travel 30S tRNA 50S Bacterial Ribosome A E P tRNA Tetracycline: Bind 30S subunit inhibiting tRNA binding to mRNA

mRNA Direction of ribosome travel 30S tRNA 50S Bacterial Ribosome A E P tRNA Aminoglycosides: Reversibly bind 30S subunit causing misreading of the message

mRNA Direction of ribosome travel 30S tRNA 50S Bacterial Ribosome A E P tRNA Chloramphenicol: Binds to 50S inhibiting transpeptidation

mRNA Direction of ribosome travel 30S tRNA Bacterial Ribosome A E P tRNA Macrolides: Bind 50S Preventing translocation

mRNA Direction of ribosome travel 30S tRNA Bacterial Ribosome A E P tRNA Oxazolidines: Inhibit the formation of the 30S and 50S ribosomal subunit assembly

mRNA Direction of ribosome travel 30S tRNA 50S Macrolides: Bind 50S Preventing translocation Lincosamides Tetracycline: Bind 30S subunit inhibiting tRNA binding to mRNA Aminoglycosides: Reversibly bind 30S subunit causing misreading of the message Chloramphenicol: Binds to 50S inhibiting formation of peptide bond Oxazolidines: Inhibit the formation of the 30S and 50S ribosomal subunit assembly Bacterial Ribosome

Mechanism of Action They reversibly bind to 30S ribosomal subunit inhibiting bacterial protein synthesis and are therefore generally bacteriostatic [at high concentrations tetracyclines can be bactericidal] Tetracyclines : Inhibitors of Protein Synthesis (e.g. Doxycycline)

Adverse Effects  Diarrhea, nausea/vomiting, abdominal pain, anorexia  Hepatotoxicity  Teratogenic effects  Serious effect on the dentin and enamel of developing teeth, causing permanent yellow or brown discoloration Tetracyclines

Aminoglycosides: Inhibitors of Protein Synthesis (e.g. Gentamicin, streptomycin) Mechanism of Action  Rapidly bactericidal (even though inhibitors of protein synthesis are usually bacteriostatic)  Bacterial killing is concentration-dependent i.e. The higher the concentration, the greater is the rate at which bacteria are killed.  Exhibit post-antibiotic effect (PAE) i.e. suppression of bacterial growth continues after the antibiotic concentration falls below the bacterial minimum inhibitory concentration (MIC). The post-antibiotic effect can be bacteria specific, as well as drug specific [short for most gram-positive organisms (< 2 hours) and longer for gram-negative organisms (2—7 hours)]

These properties probably account for the efficacy of once-daily dosing regimens of aminoglycosides. Recent studies with aminoglycosides administered as a single daily dose versus three evenly spaced doses revealed less severe nephrotoxicity in the single-dose group with no difference in efficacy.

Bind irreversibly to the 30S ribosomal subunit inhibits bacterial protein synthesis (Exact mechanism of cell death is not known) Aminoglycosides Mechanism of Action They enter bacterial cells by diffusing through aqueous channels formed by porin proteins in the outer membrane of gram-negative bacteria or via an oxygen-dependent transporter system and then inhibit bacterial protein synthesis. Gram (-) cell wall Anaerobic bacteria are not susceptible to aminoglycosides due, at least in part, to a lack of an oxygen-dependent active transport mechanism for aminoglycoside uptake.

Aminoglycosides Pharmacokinetics Gentamicin is not absorbed orally. Thus, IV infusion over 15—30 minutes is the usual mode of administration. Adverse Effects Nephrotoxicity, Ototoxicity, Neuromuscular blockade Have Low Therapeutic Index (should monitor plasma levels during therapy)

Macrolides : Inhibitors of Protein Synthesis (e.g. Erythromycin, azithromycin, clarithromycin) inhibiting bacterial protein synthesis mainly bacteriostatic Mechanism Of Action

a)Erythromycin can inhibit the hepatic metabolism of other drugs such as warfarin and theophylline b)Interaction with digoxin whereby macrolides can destroy gut flora that usually inactivates digoxin leading to its greater reabsorption from enterohepatic circulation and higher levels in plasma. Macrolides: Drug interactions

Adverse Effects  Hypersensitivity reactions  GI disturbances  Cholestatic Jaundice  Reversible ototoxicity Macrolides

Lincosamides: Inhibitors of Protein Synthesis (e.g. Clindamycin) Mechanism of Action Binds to the 50 S subunit of bacterial ribosomes and inhibits protein synthesis. Clindamycin is either bacteriostatic or bactericidal, depending on its concentration at the site of action & on the specific susceptibility of the organism being treated Adverse Effect: Limited use due to serious side effects of antibiotic-associated colitis.

Chloramphenicol: Inhibitor of Protein Synthesis Clinical use limited due to serious adverse effects. Systemic Therapy must be limited to life-threatening infections (e.g. H. influenzae) for which the benefits of the drug outweigh the risks of the potential toxicities. Mechanism of Action: Binds to the 50 S subunit of bacterial ribosomes at the same sites of erythromycin & clindamycin resulting in an inhibition of mitochondrial protein synthesis in bacterial cells. Adverse Effects: Hematological effects (serious; may be attributed to their ability to inhibit mitochondrial protein synthesis in mammalian cell.) Gray baby syndrome (fatal; occurs in neonate)

Oxazolidinones (e.g. Linezolid) First of a new line of antibiotics - the oxazolidinones Bacteriostatic. Inhibitor of protein synthesis by interfering with 30S and 50S ribosomal subunit assembly Treats skin and skin structure infections, nosocomial pneumonia (MRSA and non-MRSA caused) Given orally or IV. Risk of bone marrow toxicity (uncommon). Designated for treatment of Methicillin-Resistant S. aureus Shows promise for treatment of vancomycin-resistant gram positive infections.

Enter the bacterium by passive diffusion through porins in the outer membrane Inhibit DNA gyrase (topoisomerase II) & topoisomerase IV Mechanism of Action Inhibit replication of DNA Cell death Fluoroquinolones: Inhibitors of DNA synthesis (e.g. Ciprofloxacin )

Mechanism of Action of Quinolones

Selectivity Although human cells also contain DNA topoisomerase enzyme that functions in the same manner, it is structurally different. This mammalian enzyme is NOT affected by bactericidal concentrations of quinolones. PAE Like Aminoglycosides, Fluoroquinolones exhibit concentration- dependent bacterial killing and a prolonged post-antibiotic effect (PAE). Adverse Reactions In general, well tolerated Fluoroquinolones

Nitroimidazoles: Inhibitors of DNA synthesis (e.g Metronidazole) Mechanism of Action Metronidazole is bactericidal. It is one of the most effective drugs available against anaerobic bacterial infections. Its selectivity for anaerobic bacteria is a result of the ability of these organisms to reduce metronidazole to its active form intracellularly (aerobes lack the required nitroreductase enzyme). The reduced form of metronidazole binds DNA and inhibit its synthesis in anaerobes. Resistance to metronidazole is almost nonexistent.

Metronidazole Active form Reduction DNA DNA Fragmented Bacterium Cell death Mechanism of Action

Drug List Co-trimoxazole Amoxycillin/Amoxicillin Cefotaxime Clavulanic Acid Vancomycin Tetracycline Streptomycin Chloramphenicol Erythromycin Linezolid Ciprofloxacin