Chapter 19: Antibacterial Agents PDF

 Patrick: An Introduction to Medicinal Chemistry 6e Chapter 19 ANTIBACTERIAL AGENTS  Antibacterial Overview Bacterial infection is still a major cause of death in the developing world World health Organization 2019 – Tuberculosis responsible for 1.4 deaths worldwide...

 Patrick: An Introduction to Medicinal Chemistry 6e Chapter 19 ANTIBACTERIAL AGENTS  Antibacterial Overview Bacterial infection is still a major cause of death in the developing world World health Organization 2019 – Tuberculosis responsible for 1.4 deaths worldwide (10 million cases) – Gut infections 8th leading cause of death with >1.5 million – 2.5 million antibiotic resistant bacteria and fungu infections in US in 2019 (35,000 deaths)  History of Antibacterial Agents Antibacterial herbs and or potions have been used for centuries – Chinese used moldy soybean curd for infections – Greek used wine, myrah, and inorganic salts – Honey was sometimes used to prevent infection on wounds Bacteria were first identified by van Leeuwnehoek in the 1670s  History of Antibacterial Agents Not until the 19th century that bacteria’s link with disease was noticed A European surgeon, Joesph Lister, was an advocate of “germ theory of disease” and introduced carbolic acid as an antiseptic and sterilizing agent. His surgery survival rates increased significantly.  History of Antibacterial Agents Paul Ehrlich an immunologist that proposed the use of chemicals that could directly interfere with microorganisms at levels that did not harm the host. In 1910 Ehrlich developed the first synthetic antimicrobial drug, the arsenic-containing salvarsan (effective against sleeping sickness and syphilis)  History of Antibacterial Agents Proflavine was developed in WWII in 1934 against bacterial infections in wounds. Too toxic for systematic use. In 1935 prontosil was discovered. It was the first antibacterial that could be used in vivo based on a red dye and was later further developed into the class of antibacterial the sulpha drugs  History of Antibacterial Agents Penicillin was discovered in 1928 but not able to be effectively isolated until the 1940s. Revolutionized the fight against bacterial infections – Not effective against all types of infections – Spurred scientists to search other microbial cultures for other compounds  Bacterial Cell Antibacterial treatments owe much of their success to the fact that they can act selectivity against bacterial cells rather than animal cells Bacterial (prokaryotic) and animal (eukaryotic) cells differ in their function and biosynthetic pathways. Prokaryotic Eukaryotic No nucleus Defined nucleus No organelles Organelles Synthesize vitamins from precursors Must obtain many vitamins from external sources Cell wall and cell membrane Cell membrane only  Mechanisms of Antibacterial Action Five main mechanisms by which antibacterial agents act – Inhibition of cell metabolism – Inhibition of bacterial cell wall synthesis – Interactions with the plasma membrane – Disruption of protein synthesis – Inhibition of nucleic acid transcription and replication  Agents That Inhibition Cell metabolism Antibacterial agents which inhibit cell metabolism are called antimetabolites and inhibit the metabolism of the microorganism but not those of the host. Sulphonamides In 1935 a red dye called prontosil was discovered to have antibacterial properties in laboratory animals It did not appear to kill the bacteria grown in a test tube  Sulphonamides Sulphonamides Metabolized by bacteria in the small intestine to produce a sulphanilamide Early example of a prodrug Sulphanilamide was the first antibacterial agent active against a wide range of infections Developments led to a range of sulphonamides active against gram- positive organisms  Sulphonamides SAR Studies Para-amino group is essential and must be unsubsititued (R1) Aromatic ring and sulphonamide functional group are both required Sulphonamide and amino group must be directly attached to the ring The aromatic ring must be para substituted only (steric reasons) Sulphonamide nitrogen must be primary or secondary R2 is the only possible site of diversity  Sulphonamides Analogues R2 is the only site that can be varied R1 can be H or an acyl group Alternative heterocycles or aromatic rings Can affect how the drug binds to plasma protein (stronger binding released slower) and thus affect the blood levels and lifetime of the drug Varying can also affect solubility These variations affect the pharmakinetics rather than the mechanism of action  Sulphonamides Applications Drug of choice before penicillin to treat infectious diseases. Winston Churchill survived an infection he developed after attending the Casablanca conference in WWII thanks to the new sulfonamide drugs of the time. After penicillin was widely available sulphonamides were used less often New development in longer lasting sulfonamides such as sulohadoxine (can be taken once a week) have shown a reinterest in use. Used currently as a treatment for urinary track infections and eye lotions  Sulphonamides Mechanism Act as competitive enzyme inhibitors of dihydropteroate synthetase Block the biosynthesis of tetrahydrofolate in bacterial cells Important as a precursor to DNA synthesis  Sulphonamides Mechanism Do not actively kill the bacteria but prevent them from growing and multiplying and allow host to kill them off Not recommended for patient with weakened immune systems Mimics p-aminobenzoic acid (PABA) one of the normal substrate for dihydropteroate synthetase Prevents native substrate from binding and stops synthesis  Sulphonamides Mechanism Inhibition is reversible and some organisms can acquire resistance by increasing PABA synthesis (allows it to out compete the drug) Bacteria can also evolve to modify the target enzyme and prevent drugs from binding Humans synthesize tetrahydrofolate through a different path and do not need or have dihydropteroate synthetase – Therefor no dihydropteroate synthetase enzyme in the host to inhibit and stop synthesis Folic acid (from diet) acts as the precursor of tetrahydrofolate Agents That Inhibition Cell Wall Synthesis Major class based of penicillin and analogues of penicillin Bacteria have cell walls to survive in a range of conditions such as varying pH, temperature, and osmatic pressure. Cell wall prevents the cell from swelling and bursting Animal cells do not have a cell wall so it makes an ideal drug target for antibacterials Wall is made up of peptide and sugar units About 30 enyzmes involved in cell wall synthesis The final cross-linking reaction, catalyzed by transpeptidase, is inhibited by penicillin The nonlinked cell wall is then too fragile and can no longer prevent the cell from bursting Agents That Inhibition Cell Wall Synthesis Proposed that penicillin has a conformation similar to the transition state conformation of the natural substrate of the enzyme The enzyme acts on the ring to attempt to cleave it the same way as it would a peptide bond, but penicillin is cyclic, so it is not cleaved into two.  Penicillin SAR of penicillins Strain β-lactam ring is essential Free carboxylic acid is essential Bicyclic ring contributes to the strain of the β-lactam ring, increases activity 6-acylamino side chain is essential Sulfur is usual but not essential Stereochemistry is important  Penicillin Resistance To affect the cell wall a drug must pass through the cell wall to the outer membrane where the enzyme is located Gram positive bacteria have thicker walls yet highly porous, allowing small molecules like penicillin to pass Gram negative have an outer lipopolysaccharide membrane which polar molecules cannot pass (penicillin is less effective against) – Porins can allow through in some bacteria  Penicillin Resistance β-Lactamases – Enzyme that was mutated from transpeptidases. – Hydrolyzes the β-lactam ring of penicillin and analogues. – Released by bacteria in presence of penicillin to intercept it before it can even reach the cell wall. – Development of β-Lactamase inhibitors that can be given in conjunction with penicillin and analogues Some bacteria can produce high levels of transpeptidase to prevent penicillin from inhibiting the excess Some bacteria have proteins that can selectively pump penicillin out of the cell  Mechanisms of Antibacterial Action Five main mechanisms by which antibacterial agents act – Inhibition of cell metabolism – Inhibition of bacterial cell wall synthesis – Interactions with the plasma membrane – Disruption of protein synthesis – Inhibition of nucleic acid transcription and replication Agents That Act on the Plasma Membrane Interact with the plasma membrane of bacterial cells to affect membrane permeability Valinomycin and gramicidin A (peptides) act as an ion carriers, allowing the passage of potassium ions to the outside of the cell (disrupting ionic equilibrium) Polymyxin B has a similar action but allows the passage of small molecules (nucleosides) outside of cell Such agents may not be selective to bacteria (animal cells have plasma membranes too) so are more often used as topical treatments  Agents That Impair Protein Synthesis Bind to ribosomes and inhibit different stages of the translation process Selectiveness for bacteria can arise from different drug diffusion rates or different ribosomal targets. Ribosomes of eukaryotic cells are larger and sufficiently different from prokaryotic that some drugs can distinguish between them. Several types: aminoglycosides, tetracyclines, macrolides, oxazolidinones  Agents That Impair Protein Synthesis Aminoglycosides Carbohydrate structure that contains basic amine groups Streptomycin – Isolated from a soil mixture in 1944 Work best in slightly alkaline conditions, creates a positive charge that can add in absorption (interact with membranes and the lipopolysaccharides of Gram-negative bacteria to produce pores that drug can pass through)  Agents That Impair Protein Synthesis Aminoglycosides Polar functional groups (alcohols and amines) present in aminoglycosides form hydrogen bonds with the sugar-phosphate backbone of RNA Resistance Can arise from a range of enzymes that modify the structure of aminoglycosides to weaken interactions by: – Acetylation of amino groups – Phosphorylation or addition of ADP to hydroxyl groups Mutations in target binding site Structural changes in outer membrane  Agents That Impair Protein Synthesis Tetracyclines Most widely prescribed antibiotic after penicillins Chlortetracycline was isolated in 1948 from a mud growing microorganism Act by binding the bacterial ribosome and inhibiting protein translation Prevent tRNA from binding to ribosome and stops further addition of amino acids to the protein chain  Agents That Impair Protein Synthesis Tetracyclines SAR Substituents at positions 6 and 7 can be varied or removed Few other alterations are tolerated Epimerizing chiral centers is detrimental Variations should be concentrated at C or D ring  Agents That Impair Protein Synthesis Tetracyclines Binding H-bonds to sugar-phosphate backbone of RNA Interactions with a magnesium ion bridge Pi-pi stacking interactions with ring D  Agents That Impair Protein Synthesis Tetracyclines Complexity makes economic synthesis difficult Recently developments have allowed a series of novel tetracyclines to be accessible Resistance has arisen from overuse and their use in farming Selectivity is a result in the tetracyclines being concentrated more quickly in bacterial than animal cells  Agents That Impair Protein Synthesis Macrolides 14 membraned macrocyclic lactone ring with sugar and an amino sugar attached Best known is erythromycin a microbial metabolite isolated from soil in 1952 Inhibits translocation Unstable to stomach acids but can be administered in a coated tablet that protects drug from stomach acids but dissolves in intestines.  Agents That Impair Protein Synthesis Macrolides Alcohol and tertiary amine groups on amino sugar are crucial to activity Macrocyclic rings forms van der Waals interaction Desosamine hydroxyl group forms hydrogen bond to an adenine base  Agents That Impair Protein Synthesis Oxazolidinones Relatively new class that inhibit protein synthesis at a much earlier stage of protein synthesis Prevents the two subunits of ribosome from combining and as a result translation cannot start. Binds to ribosome through van der Waals and pi-pi stacking interactions Hydrogen bond between amide and a phosphate group in RNA sugar phosphate backbone Agents Against Nucleic Acid Transcription and Replication Prevents cell division and /or synthesis of essential proteins Quinolones and fluoroquinolones Inhibit the replication and transcription of bacterial DNA by stabilizing the bacteria specific DNA topisomerases (IV) Aminoacridines Interact directly with DNA by intercalation (only used topical) Rifamycins Binds non-covalently to (prokaryotic) DNA-dependent RNA polymerase and inhibiting the start of RNA synthesis Many others  Drug Resistance Still urgent need for novel antibiotic treatments Bacteria have an ability to acquire resistance Multiply at a rapid rate, much more mutations possible if population is treated and not eradicated those that develop immunity can then multiply Bacteria can also pass genetic information through transduction and conjugation Transduction involves small segments of genetic information (plasmids) is transferred by bacteriophages (bacteria viruses) Conjugation involves bacterial cells passing genetic material directly to one each other through a bridge of sex pili Drug Resistance  Drug Resistance Still urgent need for novel antibiotic treatments Bacteria have an ability to acquire resistance Multiply at a rapid rate, much more mutations possible if population is treated and not eradicated those that develop immunity can then multiply Bacteria can also pass genetic information through transduction and conjugation Transduction involves small segments of genetic information (plasmids) is transferred by bacteriophages (bacteria viruses) Conjugation involves bacterial cells passing genetic material directly to one each other through a bridge of sex pili  Drug Resistance More useful a drug the more often it is prescribed and the great chance of resistance forming Use in animal feeding to increase weight Ease of different bacteria to develop resistance varies S aureus undergoes transduction and can quickly develop resistance Syphilis microorganism is seemingly incapable and still susceptible to original drugs Pharmaceutical companies are reluctant to develop New initiatives are developed to encourage research  Drug Resistance Designing drugs to hit multiple targets New targets: aminoacyl tRNA synthetases – Attaching amino groups to tRNA – Evolved early and so divergent enzymes from bacterial and humans – Mupirocin: topical antibiotic active against MRSA – Novel inhibitors are being developed Alter existing drugs to combat resistance – Kanamycin is inactivated by phosphorylation – Alcohol replaced with ketone can combat this  Drug Resistance Design drugs with self destruct mechanism after excretion – Cephalosporin containing a protected hydrazine group – When it is excreted light removes protecting group and the hydrazine reacts with beta lactam to deactivate the molecule Drug combinations