BIOL2049 Antibiotics Lecture 3 PDF

BIOL2049 Pharmacology Semester 2 ANTIBIOTICS - Lecture 3 Inhibitors of Nucleic Acid Synthesis Dr. Fatima Pereira [email protected]  Aims Provide information on the selectivity a...

BIOL2049 Pharmacology Semester 2 ANTIBIOTICS - Lecture 3 Inhibitors of Nucleic Acid Synthesis Dr. Fatima Pereira [email protected]  Aims Provide information on the selectivity and mechanism of action of antibiotics targeting bacterial nucleic acid synthesis How do bacteria acquire resistance to this type of antibiotics Inhibitors of nucleic acid synthesis  Learning objectives 1. Describe the mechanism of action of rifampicin 2. Explain why rifampicin does not kill human cells 3. Identify the two enzymes inhibited by fluoroquinolones 4. Explain why are fluoroquinolones bactericidal 5. Identify a mechanism of fluoroquinolone and rifampicin resistance  Inhibitors of nucleic acid synthesis: Rifampicin 10.1038/s41429-021-00462-x Rifampicin (Rifampin/Rifamycin) Member of the ansamycin group Bactericidal Mainly active against Gram positive bacteria Especially useful against tuberculosis, leprosy and Legionnaires' disease Inducer of P450 Intracellular Legionella pneumophila  Rifampicin: mechanism of action Inhibitor of RNA polymerase Binds to the -subunit (rpoB gene) Inhibits initiation, not elongation of mRNA synthesis: STOP Does not block formation of the first phosphodiester bond, but prevents further additions  Rifampicin: inhibitor of mRNA synthesis Prokaryotic transcription: Catalyzed by a single RNA polymerase RNAP E.coli : 5 subunits, 450 kDa Eukaryotic transcription: Catalyzed by 3 different RNA polymerases RNAP II: 12 subunits, 550 kDa High affinity of rifampicin for the prokaryotic RNA polymerase. The selectivity of the rifampicin depends on the fact that they have a very poor affinity for the analogous mammalian enzyme.  Rifampicin: Resistance and side effects Frequency of resistance mutations in the rpoB gene from Mycobacterium tuberculosis: Resistance rapidly develops from a single point mutation (Ser531Leu) in the -subunit. Should not be used in monotherapy!!!  Inhibitors of DNA replication: Quinolones Nalidixic acid STOP Bicyclic ring Active against Gram positive and few Gram negative Bacteriostatic at low concentrations, bactericidal at high concentrations Used almost exclusively for urinary tract infections  Quinolones: Mechanism of action Interfere with DNA gyrase: A2B2 structure Unwinds double stranded DNA and so introduces negative supercoils into circular DNA. ATP-dependent 10.1136/bmjophth-2022-001002 Quinolones: interfere with DNA gyrase action Gyrase- Quinolone- STOP DNA Stable complex! +2 ATP ★★ Resistance develops from a single point mutation in gyrA https://basicmedicalkey.com/quinolones Fluoroquinolones Fluoroquinolones Gram positive and Gram negative Inhibit DNA gyrase and topoisomerase IV  Fluoroquinolones: Mechanism of action Inhibition of DNA gyrase Inhibition of topoisomerase IV Fluoroquinolones interact with the enzyme- bound DNA complex to create conformational changes that result in the inhibition of normal enzyme activity  Fluoroquinolones: action & Resistance Stabilized by fluoroquinolones: Gyrase-Fluoroquinolone-DNA Complex Topoisomerase IV-Fluoroquinolone-DNA Complex Protein-stabilized DNA breaks and ultimately cell death: cidal effect 10.1136/bmjophth-2022-001002  Fluoroquinolones: Resistance and side effects Mechanisms contributing to resistance:  single or multiple chromosomal mutations that alter the topoisomerases in places where it binds the quinolone: quinolone-resistance determining region (QRDR) in gyrA  Gram negative: Multidrug resistant (MDR) membrane- associated efflux pumps  Gram negative: Decreased levels of porins Inhibitor of cytochrome P450 – CYP2C9 and CYP3A4  (Fluoro)quinolones: advice on use  Fluoroquinolones should not be prescribed for treatment of mild to moderate infections unless no other antibiotic options are available  These serious side effects include: inflamed or torn tendon muscle pain or weakness joint pain or swelling etc.  Other antibiotics that affect nucleic acids Nitrofurantoin Metronidazole Reduction in vivo -> reactive compounds -> mutagenesis and nucleic acid breaks  Additional Literature Gualerzi, C.O., Brandi, L., Fabbretti, A. and Pon, C.L. (2013) Antibiotics: Targets, Mechanisms and Resistance. Wiley-VCH; 1st edition (5 Sept. 2013) Kohanski, M.A., Dwyer, D.J., and Collins, J.J. (2010) How antibiotics kill bacteria: from targets to networks. Nat Rev Microbiol 8: 423–435. DOI: 10.1038/nrmicro2333 Adams, R.A., Leon, G., Miller, N.M., Reyes, S.P., Thantrong, C.H., Thokkadam, A.M., et al. (2021) Rifamycin antibiotics and the mechanisms of their failure. J Antibiot 74: 786–798. doi: 10.1038/s41429-021-00462-x Blondeau, J.M. (2004) Fluoroquinolones: mechanism of action, classification, and development of resistance. Survey of Ophthalmology 49: S73–S78. doi: 10.1016/j.survophthal.2004.01.005