i. antibiotics: overview - southern methodist...

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I. Antibiotics: Overview Commonly Encountered Problem Bacteria: - Resistant Staphylococcal infections (mortality rates are 25% to 63%) - MRSA: Methicillin Resistant Staph aureus - MRSE: Methicillin Resistant Staph epidermis - Vancomycin Resistant Enterococcal infections (mortality rates are 42% to 81%) - VRE Antibiotics that act on bacterial cell wall biosynthesis: - β-Lactam antibiotics - Penicillins - Cephalosporins - Glycopeptide antibiotics that bind un-cross-linked peptide strands and block transpeptidation - Vancomycin Review: Bacterial Cell Walls - Bacteria (prokaryotes) have cell walls, while mammals have only cell membranes - Bacteria come in two types: - Gram positive bacteria: Staphylococci, Streptococci, and Enterococci - Gram-negative bacteria: Escherichia coli, Slamonella, Pseudomonas, and Yersinia - Both Gram-positive and Gram-negative bacteria have cell walls (composed of peptidoglycan) and also an inner, cytoplasmic membrane. - However, Gram-negative bacteria also have an outer membrane, and thus also an enclosed area, between the outer membrane and the cell wall. This enclosed area is called the periplamic space.

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- Peptidoglycan - The peptidoglycan layer consists of orthogonal peptide and glycan strands - The peptide strands cross link the glycan strands, adding mechanical strength and a mechanical barrier to osmotic pressure forces. - The joining of two glycan strands (to form a longer glycan strand) is catalyzed by an enzyme called a "transglycosidase"

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- The joining of two "dangling" peptide strands to form a peptide bridge (between two glycan strands) is catalyzed by an enzyme called a "transpeptidase"

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- Note from the diagrams below, that while Gram-positive bacteria have a thicker PG layer, that the Gram-negative bacteria have an additional outer membrane

- Note the different actions of penicillin and vancomycin on Gram positive and Gram negative bacteria - Both penicillin and vancomycin interact directly with the peptidoglycan layer in Gram positive bacteria - However, Vancomycin cannot penetrate the outer membrane of Gram negative bacteria - By contrast, some penicillins can access the periplasmic space (often through porins) and, therefore, the peptidoglycan layer of Gram negative bacteria.

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One key target in the formation (cross linking) of the glycan strands, is the cleavage between two unusual D-ala residues (see below)

- This D-Ala must be prepared by the bacterial cell (see below)

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β-Lactam Antibiotics

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Penicillins

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Cephalosporins

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II. Penicillin Resistance due to β-Lactamase A. What is β-lactamase? B. Why is it a problem?

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III. Penicillins: Part II A. Methicillin: A drug designed to be resistant to b-lactamase (previously called penicillinase). 1. Structure of methicillin 2. Notice "steric shield" on side chain to protect b-lactam from hydrolysis 3. Biological activity & pharmacokinetics of methicillin a. Has to be administer parenterally, since it has no electron withdrawing group on the side chain b. Inactive against gram negative bacteria

B. Oxacillin 1. Still resistant to b-lactamases 2. More acid stability than methicillin

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C. Better Gram-negative activity: Ampicillin and Amoxicillin 1. Attaching a hydrophilic group to the side chain seemed to give the drug better Gram negative activity 2. This was achieved by employing an amino substituent directly adjacent to carbonyl of side chain 3. Still inactive agains Pseudomonas aeruginosa, a particularly challenging pathogen 4. Sometimes administered as prodrugs (esters) due to poor absorption through the gut (show pivampicillin structure)

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D. Best activity against Gram-negative organisms, including Pseudomonas aeuruginosa: carbenicillin II. Cephalosporins: A. History 1. First isolated from fungus found in sewer line on island of Sardina in 1948 2. Structure wasn't elucidated until 1961 B. Prototypical Early cephalosporin: Cephalothin 1. Less antibiotic activity than Penicillin G against Gram positive bacteria 2. More activity than Pen G against Gram negative bacteria 3. Can be used on patients who are allergic to penicillin 4. Side chain acetoxy group is hot point for metabolic inactivation

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C. Cephaloridine 1. Better leaving group in form of positively charged pyridinium group will "activate" system 2. Avoids metabolic inactivation 3. Note that compound is "zwitterion"

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D. Ceftazidime 1. Combines activation of ceftazidime with steric shielding of b-lactam to protect it from hydrolysis by b-lactamase 2. Note additional hydrophilic groups on side chain further improve activity against gram negative strains.

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Glycopeptide Antibiotics Two glycopeptide antibiotics (shown below) have been approved for human clinical use. Vancomycin is produced by Streptococcus orientalis, an actinomycete isolated from soil samples obtained in indonesia and India. Vancomycin is primarily active against gram-positive bacteria, particularly including Staph aureus.

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Mechanism of Action of Vancomycin Vancomycin is an unusual drug in that it does not target an enzyme or a receptor, but instead binds and sequesters the substrate. This particular substrate is the D-Ala-D-Ala terminus of cell wall precursor units as shown below.

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Clinical Use of Vancomycin: Vancomycin is poorly absorbed orally and is typically administered intravenously. Vancomycin hydrochloride is marketed as Vancocin.

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Antibiotics that Block Bacterial Protein Synthesis Overview: The bacteria ribosome (which is, of course, responsible for protein synthesis0 has two subunits, a 30S subunit and a 50S subunit. In the past 50 years, a number of antibiotics were discovered (through random screenings) and later shown to have their mode of activity at the bacterial ribosome. The ribosome is about one third protein and two thirds RNA. It has been shown that the RNA in the ribosome is catalytically active (thus the RNA is acting as an enzyme, the same as a protein-based enzyme). The RNA is also responsible for actually binding the antibiotics (as has very recently been shown by x-ray crystsllography).

There are three sites on the ribosome: A Site: "Acceptor" or "Aminoacyl" site. This is where the aminoacyl t-RNA binds. P Site: "Peptidyl" site. This is where the growing peptide sits. E Site: "Exit" site: This is where the "empty" t-RNA goes to leave the ribosome after it has completed its job of delivering its attached amino acid.

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Macrolide Antibiotics:

The macrolide antibiotics are so called because they contain a large ring (a cyclic ester, called a "lactone"). This is a 14 membered ring for erythromycin and clarithromycin and a 15-membered ring for azithromycin. They contain two sugars as side chains, an aminosugar known as β-D-desosamine and also α-L-cladinose. Erythromycin was discovered in 1952 in the metabolic products of a strain of Streptomyces erythreus, found in the Philippines. Clarithromycin and Azithromycin are synthetic derivatives of erythromycin. Erythromycin is usually bacteriostatic, but can be bacteriocidal at high dosages. It is most effective against Gram-positive cocci and bacilli. Their relative inactivity against Gram-negative microorganisms, particularly including E. coli and Pseudomonas, is viewed as a major shortcoming. These drugs are used to treat respiratory and soft tissue infections. Erythromycin and the other macrolide antibiotics bind to the 50S subunit of sensitive microorganisms. The binding place is known to be near the binding spot of chloramphenicol, between the A and the P site. It is believed that these antibiotics prohibit the peptide chain from the A to the P site.

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Tetracyclines Tetracycline antibiotics were discovered by systematic screen of soil specimens for antibiotic activity. These compounds have been known since 1948. They are active against a number of Gram-positive and Gram-negative bacteria and are known as "broad spectrum" antibiotics. Tetracyclines bind to the 30S ribosomal subunit and prevent access of aminoacyl tRNA to the "A" site. Decades of use of these drugs has resulted in increased bacterial resistance and these drugs are now rarely used as first line therapy.

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Aminoglycoside Antibiotics The aminoglycoside antibiotics contain amino sugars linked to a central aminocyclitol by glycosidic bonds. Due to their many amino groups, they are polycationic. These drugs are used to treat infections caused by Gram-negative microorganisms, particularly including Pseudomonas aeruginosa, and are bacteriocidal. A limiting constraint is their toxicity, particularly their ototoxicity. Permanent damage can occur before the patient is aware of it. Streptomycin, the first such drug to be used was discovered in 1943, due to a systematic screening of soil specimens.

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