BMS2037 Antibiotics - Part 1 PDF

BMS2037 A brief history of antimicrobial chemotherapy Antibiotic drug classes – mechanisms of action and resistance Dr Alison Cottell  Overview of lecture: Understanding of early treatments against infectious disease Define “selective to...

BMS2037 A brief history of antimicrobial chemotherapy Antibiotic drug classes – mechanisms of action and resistance Dr Alison Cottell  Overview of lecture: Understanding of early treatments against infectious disease Define “selective toxicity” Define different broad categories of antimicrobials Understand phenotypic methods for determining antibiotic susceptibility in vitro Understand recent and predicted trends in antimicrobial consumption, and the impact of resistance. Review some treatment plans for a range of bacterial infections  Early treatment for infections Oils and crude plant extracts as anti-helminths Quinine for malaria Chenopodium Mercury for syphilis ambrosioides(wormseed goosefoot): “vermifuge” used Chaulmoogra oil for leprosy by ancient Greeks against tapeworms and roundworms Lister used carbolic acid to treat wound infections Carbolic acid - low efficacy and damaging to healthy tissue 1900s: arsenicals as antiprotozoal agents, and for schistosomiasis and syphilis WWI soldier following treatment for an infected bullet wound to chest Sulfa drugs of the 1930s – limited spectrum of activity  Biocides: disinfectants and antiseptics 70% IMS / isopropyl alcohol / rubbing alcohol Bleach Denatures proteins and Broad spectrum. Triclosan dissolves lipids. Chlorine denatures Bisphenol. Ineffective against spores proteins Multiple targets of membrane and cytoplasm. and some viruses. Found in dentifrices Incorporated with plastic / fabrics  Principles of selective toxicity Antibiotics work by: 1. Affecting targets found in bacterial cells and not in mammalian eukaryotic cells. 2. Becoming concentrated in the bacterial cell and not in the host cell. Antibiotics vary in the extent to which they fulfil these criteria. Side-effects vary between specific antibiotic agents Bactericidal: bacteria are killed Bacteriostatic: bacteria are inhibited: host defences usually take over An antibiotic is given to mean a broad range of microbes can be killed: hence salvarsan is not a true antibiotic Paul Ehrlich 1912: Ehrlich developed Staphylococci / pneumococci arsphenamine (Neosalvarsan): activity in Gram-stained sputum limited to spirochaetes The discovery of clinically useful antibiotics Penicillium chrysogenum is the species used for drug production The original species discovered by Flemming (Penicillium rubens) was not effective enough for pharmaceutical use Significant efforts to find species and strains that were the most prolific producers of antimicrobial substances Mary Hunt in the lab, and with mouldy cantaloupe The discovery of clinically useful antibiotics During the 1930s antimicrobial components were produced in sufficient quantities and purity for clinical use. difficult to purify difficult to produce in clinically useful amounts 1942: one of the first lives to be saved by penicillium. Anne Miller, died 1999. Selman Waksman: discovery of streptomycin, the first drug to treat tuberculosis Discovered streptomycin from Streptomyces griseus in 1944 First effective antibiotic for TB; won the Nobel Prize in 1952 Drug discovered from systematic screening of soil microbes Contrasts with Flemmings’ serendipitous discovery  Summary of mechanisms of action Nucleic acid Cell Wall / membrane Quinolones Beta-lactams Metronidazole?? How infections are established: Vancomycin Isoniazid 1. Attachment to epithelial surface Polymyxin 2. Penetration of epithelial surface Daptomycin 3. Interference / avoidance of host defences 4. Multiplication in host tissue Anti-metabolites 5. Damage of host tissue Sulphonamides Most antibiotics target stage 4 by killing Trimethoprim the organisms or slowing growth to the Protein synthesis point where the host defences are Protein synthesis usually able to regain control. Chloramphenicol Aminoglycosides Macrolides Tetracyclines Linezolid Inhibitors of cell wall synthesis  Cell walls of Gram-positive & Gram-negative bacteria +ve -ve Thicker layer of peptidoglycan Thinner layer of peptidoglycan Teichoic acids Outer membrane is an intractable barrier to many antibiotics  Inhibitors of cell wall synthesis NAM: N-acetylmuramic acid; NAG: N-acetylglucosamine (NAM is made of NAG + lactic acid and 3 amino acids) Peptidoglycan = NAM:NAG linked by peptide bridges Cell wall synthesis with site of action of antibiotics L-ala x 2 D-ala x 2 Cycloserine Amino acids D-ala-D-ala NAM NAM- Beta-lactams pentapeptide Glycopeptides Fosfomycin Transfer to + Lipid carrier Cross- NAG NAG (membrane) peptidoglycan linking Bacitracin Dephosphorylation of lipid NAG = N-acetlyglucosamine NAM = N-acetylmuramic acid Adapted from Greenwood et al. Mechanism of action of glycopeptides 1 Normal cell wall 2 Transpeptidase enzymes bind NAM-NAG monomers 3 Vancomycin binds NAM-NAG 4 Autolysins break peptide bonds but monomers: TP inhibited no further bonds are created  Inhibitors of peptidoglycan synthesis Glycopeptide antibiotics include vancomycin and teicoplanin. Glycopeptides bind to the growing peptidoglycan chain and prevent transglycosylation Vancomycin is often regarded as an antibiotic of last resort: Vancomycin- resistant enterococci (VRE) Vancomycin- intermediate Staph. aureus (VISA) Resistant organisms modify cell-wall precursors e.g. replacing D-alanine with D- lactate  Mechanism of action of Beta-lactam antibiotics Penicillin (and all ß-lactams) bind to penicillin-binding proteins (PBPs) in the growing cell wall. Cross-links between peptidoglycan chains are inhibited and cell lysis results Bactericidal, although only growing cells are killed Cephalosporins have beta-lactam Structure of ring with dihydrothiazine ring penicillin molecule Variable group Beta-lactam ring Pentapeptide Different classes of ß-lactams Mechanism of action of beta-lactamase enzyme Lots of different classes of these enzymes e.g: Penicillinases Extended-spectrum (ESBLs) Beta-lactamase hydrolysis Metallo Cephalosporinases ESBLs What are ESBLs? Beta-lactam Extended spectrum beta-lactam Beta-lactamase Extended spectrum beta-lactamase Penicillin Harboured by Gram-negative organisms: E.coli, Klebsiella, Enterobacter Pseudomonas Haemophilus, N. gonorrhoeae Linked with infections such as: UTIs, hospital acquired pneumonia Line infections & sepsis Abdominal infections Ceftazidime  Beta-lactamase inhibitors Beta-lactamase inhibitors have a lactam ring, but are weak antibiotics. To be effective: Beta-lactamase must have higher affinity for inhibitor than drug Inhibitor must bind irreversibly to beta-lactamase I D Clavulanate - Amoxicillin inhibitor - drug  B-lactamase inhibitors B-lactamase must have higher affinity for inhibitor than drug Inhibitor must bind irreversibly to B-lactamase Bacterial beta- lactamase inhibitor I D Active drug D D component (amoxicillin) I Inhibitor I component D (clavulanate) I I D Difference between beta-lactams and vancomycin 4 Autolysins break peptide bonds but no further bonds are created Transpeptidase enzymes bind Stage four occurs normally if a NAM-NAG monomers beta-lactam is present P Beta-lactams bind to transpeptidase, rather than the short peptide portion of the monomer  Inhibitors of cell membrane function and synthesis Mechanism of action of polymyxins: Disrupts inner and outer cell membranes, causing cell lysis Acts Gram-negatives only Polymyxin B; Polymyxin E (colistin) Domingues et. al., 2012  Colistin metabolism in person with renal function disorder: Early use - nephrotoxicity and neurotoxicity side-effects in some patients: polymyxin use declined as less toxic antibiotics were introduced. Polymyxins are effective Renal against multi-drug resistant: clearance Acinetobacter baumannii Pseudomonas aeruginosa Klebsiella pneumoniae Colistimethane Outcome: most of the colistimethane is converted to the more active colistin Non-renal clearance (hydrolysis) Colistin Conclusion: In healthy people, majority of colistimethane is removed via the kidneys In renal patients this doesn’t happen. Most = majority of drug = minority of drug colistimethane converted to colistin in the bloodstream. More side effects / toxicity  Widespread publicity following emergence of colistin resistance (Nov, 2015) and first clinical infection caused by the strain in the US (May 2016) May, 2016 Nov, 2015  Estimates of mcr-1 clinical resistance in China: Impact of colistin resistance 2016 1.4% in E. coli Difficult to monitor: difficult to measure 0.7% in Klebsiella pneumoniae phenotypically Newer molecular measures are more reliable – Survey of isolates from pork meat sold in China – detection of mcr-1 studies found up to 20% of meat samples contained resistant bacteria Widespread geographical spread 2022 Colistin-resistant infection rates are low 4% of faecal E. coli samples from clinical setting are Mcr-1 positive The most recently developed antibiotics include: Quinupristin-dalfopristin (targets Gram- positives) Colistin is effective against: Linezolid (targets Gram-positives) Tigecycline: effective against some Gram- Acinetobacter baumannii negatives, but not Ps. aeruginosa. Pseudomonas aeruginosa Klebsiella pneumoniae Inhibitors of protein synthesis  Inhibitors of protein synthesis Bacterial ribosome Eukaryotic ribosome 70S (Svedberg units) 80S (Svedberg units) mRNA enters via the small subunit nascent polypeptide emerges from large subunit 30S subunit 50S subunit 40S subunit 60S subunit Aminoglycosides: Clinical applications: Binding and distortion of ribosome – perturbation of Life-threatening Gram –ve translocation prevents protein synthesis initiation. infections Severe skin; bone; tissue Examples: infection Gentamicin, amikacin and tobramycin Complicated urinary tract Streptomycin - antibiotic for TB treatment infection Neomycin: topical (ears & skin) Sepsis Peritonitis Spectrum of activity: Severe pelvic inflammatory Gram-negatives and staphylococci. disease Inactive against streptococci and anaerobes Mycobacterial infection Neonatal sepsis  Inhibitors of protein synthesis Macrolides Bind to 50S and cause dissociation of peptidyl- tRNA Used to treat Gram-positive bacteria (limited activity against Gram- negatives) Broad spectrum: Staphylococci; streptococci; Mycoplasma; Legionella; Chlamydia; Campylobacter spp. Include the following agents: Erythromycin – a product of Streptomyces erythreus Azithromycin – derivative; improved bioavailability & half life Clarithromycin – as above; also better absorbtion Inhibitors of nucleic acid synthesis Inhibition of nucleic acid synthesis Three “interventions” in nucleic acid synthesis: Some have been developed as Interruption of nucleotide metabolism and synthesis anti-cancer drugs. Perturbation of DNA as a template (complex with Not clinically useful, due to a molecule; structural alteration) lack of selective toxicity. Inhibition of enzymatic processes Quinolones bind to unpaired DNA bases There is a complex interaction between gyrase, DNA and the quinolone molecules The presence of gyrase stimulates the binding of quinolone to DNA Broad spectrum of activity  Inhibition of nucleic acid synthesis Quinolones 18 gene products involved in bacterial chromosome replication Topoisomerases untwist supercoiled DNA by ‘nicking’ one of the strands causing the DNA to become loose 1st generation quinolones: E.g. Nalidixic acid: have limited antibacterial spectrum of activity. 2nd generation quinolones (ciprofloxacin, norfloxacin, ofloxacin) have greater potency. 3rd generation quinolones include lomefloxacin 4th generation includes trovafloxacin Further Reading: General module textbook (i.e. via Bibliu) has sufficient information. General Textbooks: Antimicrobial Chemotherapy, Fifth Edition, By Greenwood et al. ( Open University Press, ISBN: 0198570163). [Any recent edition] Antibiotics and Chemotherapy: anti-infective agents and their use in therapy, Eighth Edition, By Finch et al.: Churchill Livingstone, ISBN: 0443071292). [Any recent edition]