Antibiotics Lecture 04

Bio 319: Antibiotics

Lecture Four

TopicInhibitors of Nucleic acid synthesisInhibitors of synthesis of essential metabolitesIntroduction to antibiotics resistance

Lecturer: Dr. G. Kattam Maiyoh

03/21/13 GKM/BIO319:Antibiotics/Sem2/2013 1

Reminder

03/21/13 2GKM/BIO319:Antibiotics/Sem2/2013

The Action of Antimicrobial Drugs


Inhibitors of Cell Wall Synthesis


Protein synthesis inhibitors– Aminoglycosides

– Tetracyclins

- Spectinomycin

– Macrolides

– Chloramphenicol

– Clindamycin


Inhibitors of Nucleic Acid Synthesis

~Inhibitors of DNA synthesis~Inhibitors of RNA synthesis


Inhibitors of RNA SynthesisFor this class of antibiotics, selectivity is due to differences between prokaryotic and eukaryotic RNA polymerase i.e.

All RNA polymerases are multi-protein complexes, however, the number of proteins that are assembled to form the active enzyme is much larger in eukaryotes;

•The basic catalytic core is made up of 12 subunits.

•By comparison, bacterial RNAP has 5 subunits.


Rifampin, Rifamycin, Rifampicin, Rifabutin (bactericidal)

• Mode of action - These antimicrobials bind to DNA-dependent RNA polymerase and inhibit initiation of mRNA synthesis.

• Spectrum of activity - Broad spectrum but is used most commonly in the treatment of tuberculosis

• Resistance – Common– For example; – Resistance to rifampicin develops quickly during treatment, so

monotherapy should not be used to treat these infections — it should be used in combination with other antibiotics.

– Resistance to rifampicin arises from mutations that alter residues of the rifampicin binding site on RNA polymerase, resulting in decreased affinity for rifampicin.

• Combination therapy - Since resistance is common, rifampin is usually used in combination therapy.


Inhibitors of DNA SynthesisFor this class of antibiotics, selectivity due to differences between prokaryotic and eukaryotic DNA replication enzymes

Consider the following:

Eukaryotic replication is more complicated. •First, they have multiple ori per chromosome that allow for bidirectional synthesis of the linear chromosome. •They also use several DNA polymerases (I, II, III), and ligase (seals the nicks in the DNA strand), and RNA primer (gives a 3' end for the DNA polymerase to start synthesis).

•As for other differences in their synthesis of DNA is their speed, prokaryotes can replicate their chromosome at about 1,000 bp/sec, while eukaryotes can replicate their chromosomes at about 100 bp/sec. •They also differ in the number of ori, eukaryotes (as it was stated above) has multiple ori, while prokaryotes have only one.


The Quinolones

• Current drugs are fluoridated 4-quinolones

• Broad coverage (some broader than others)

• Targets DNA gyrase (G-) and topoisomerase IV (G+)

• Resistance due to efflux and mutations in targets


Pharmacological attributes of Quinolones• Favorable pharmacological attributes

– Orally administered, quickly absorbed, even with a full stomach

– Excellent bioavailability in a wide range of tissues and body fluids (including inside cells)

• Mostly cleared by the kidneys– Exceptions are pefloxacin and

moxifloxacin which are metabolized by liver

• Ciprofloxacin, ofloxacin, and pefloxacin are excreted in breast milk

““Got Cipro?”Got Cipro?”


Therapeutic Uses of Quinolones

• Urinary tract infections

• Prostatitis• STD’s– Chlamydia– Chancroid - painful

sores on the genitalia– Not syphilis or

gonorrhea (due to increased resistance)



• GI and abdominal – Travelers diarrhea– Shigellosis– Typhoid fever

• Respiratory tract– New agents for strep. pneumonia



• Bone, joint, soft tissue– Ideal for chronic

osteomylitis - infection of the bone or bone marrow• Resistance developing in

S. aureus, P. aeruginosa, and S. marcesens

– Good against polymicrobial infections like diabetic foot ulcers



• Ciprofloxacin for anthrax and tuleremia (rabbit fever, deer fly fever, Ohara's fever)

• Combined with other drugs, useful for atypical Mycobacterium sp.

Pulmonary AnthraxPulmonary Anthrax


Toxicity/Contraindications of Quinolones• Nausea, vomiting, abdominal discomfort (common)• Diarrhea and antibiotic-associated colitis (uncommon to rare)• CNS side effects

– Mild headache and dizziness (common to rare)– Hallucinations, delirium, and seizures (rare)

• Arthropy in immature animals (common)– Quinolones not given to children unless benefits outweigh the risks

• Leukopenia, eosinophila, heart arythmias (rare)

delirium - disorder involving incoherent speech, hallucinations, etc., caused by intoxication, fever, etc


Antimetabolite Antimicrobials


Inhibitors of Folic Acid Synthesis

• Basis of Selectivity• Review of Folic

Acid Metabolism

p-aminobenzoic acid + Pteridine

Dihydropteroic acid

Dihydrofolic acid

Tetrahydrofolic acid

Pteridine synthetase

Dihydrofolate synthetase

Dihydrofolate reductase

ThymidinePurines

Methionine

Trimethoprim

Sulfonamides


Sulfonamides, Sulfones (bacteriostatic)

• Mode of action - These antimicrobials are analogues of para-aminobenzoic acid and competitively inhibit formation of dihydropteroic acid.

• Spectrum of activity - Broad range activity against gram-positive and gram-negative bacteria; used primarily in urinary tract and Nocardia infections.

• Resistance - Common

• Combination therapy - The sulfonamides are used in combination with trimethoprim; this combination blocks two distinct steps in folic acid metabolism and prevents the emergence of resistant strains.


Trimethoprim, Methotrexate, Pyrimethamine (bacteriostatic)

• Mode of action - These antimicrobials binds to dihydrofolate reductase and inhibit formation of tetrahydrofolic acid.

• Spectrum of activity - Broad range activity against gram-positive and gram-negative bacteria; used primarily in urinary tract and Nocardia infections.

• Resistance - Common

• Combination therapy - These antimicrobials are used in combination with the sulfonamides; this combination blocks two distinct steps in folic acid metabolism and prevents the emergence of resistant strains.


Sulfonamides

• Analogues of para-aminobenzoic acid

• Broad spectrum• Competitive inhibitors of

dihydropteroate synthase – needed for folic acid synthesis

Gerhard Domagk gets a Nobel Gerhard Domagk gets a Nobel for Medicine, 1939.for Medicine, 1939.


Sulfonamides• Mostly absorbed from GI tract• Binds variably to serum albumin • Wide tissue distribution, including

transplacentally• Variably inactivated in liver by

acetylation and then excreted in urine

• Some agents can precipitate in acid urine


1. Rapidly Absorbed and Eliminated Sulfonamides

• Sulfisoxazole, sulfamethoxazole, sulfadiazine

• Bind extensively to plasma proteins

• Highly concentrated in urine (cidal)

• Sulfamethoxazole combined with trimethoprim (Bactrim) is widely used to treat a variety of infections (esp. UTI)


2. Poorly Absorbed Sulfonamides

• E.g. Sulfasalazine• Poorly absorbed in GI tract• Used to treat ulcerative colitis

and irritable bowel syndrome• Gut flora metabolize drug into

2 compounds, 1 toxic- sulfapyridine, 1 therapeutic (5-aminosalicylate)

• Hence is a prodrug – effective after breakdown

Ulcerative Colitis


Sulfonamides for Topical Use

• E.g. 1.Sulfacetamide– Good penetration in eye

– Non-irritating

• 2. Silver sulfadiazine– Prevention and treatment of

burn wound infections

Bacterial corneal infectionBacterial corneal infection


Long Acting Sulfonamide

• Serum half-life is measured in days rather than minutes or hours

• E.g. Sulfadoxine

• Combined with pyirethamine to treat malaria

Plasmodium vivaxPlasmodium vivax


Therapeutic Uses of Sulfonamides

• Urinary tract infections• Nocardiosis -serious infection

caused by a fungus-like bacterium that begins in the lungs and can spread to the brain.

• Toxoplasmosis - caused by protozoan (avoid using in pregnant women)

Nocardia asteroidesNocardia asteroides


Toxicity/Contraindications of Sulfonamides - UT

• Crystallization in acid urine– Common to uncommon

depending on drug.

– Alkalize urine or increase hydration


Toxicity/Contraindications of Sulfonamides - blood

• Acute hemolytic anemia – Rare to extremely rare

– Associated with glucose-6-phosphate dehydrogenase activity in RBC

• Agranulocytosis - failure of the bone marrow to make enough white

blood cells (neutrophils). (extremely rare)

• Aplastic anemia - marrow doesn't make enough new blood cells.

(extremely rare)


Toxicity/Contraindications of Sulfonamides - immune

• Hypersensitivity reactions (common to uncommon)– Skin and mucous membrane

manifestations (rashes)– Serum sickness - type III hypersensitivity

reaction that results from the injection of heterologous or foreign protein or serum

– Focal or diffuse necrosis of the liver (rare)


Toxic Epidermal Necrolysis (TEN)


Toxicity/Contraindications of Sulfonamides - miscellaneous

• Nausea, anorexia, vomiting (common)

• Kernicterus; damage caused by excessive jaundice– Displacement of bilirubin from

plasma albumin to brain resulting in encephalopathy(disease of the brain)

– Never give sulfa drugs to a pregnant or lactating woman

• Potentiation of oral coagulants, sulfonylurea hypoglycemic drugs, and hydrantoin anticonvulsants

Bilirubin deposits in neonatal Bilirubin deposits in neonatal brainbrain


INJURY TO THE PLASMA MEMBRANE - Brief

• All cells are bound by a cell membrane. • And although the membranes of all cells are quite

similar, those of bacteria and fungi differ from eukaryotic cells.

• These slight differences allow for selective action of some antimicrobial agents.

• Certain antibiotics, like polymyxins, act as detergents to dissolve bacterial cell membranes by binding to phospholipids present in the membranes.


Antibiotic Resistance• A variety of mutations can lead to antibiotic resistance

• Mechanisms of antibiotic resistance

1. Enzymatic destruction of drug

2. Prevention of penetration of drug

3. Alteration of drug's target site

4. Rapid ejection of the drug

• Resistance genes are often on plasmids or transposons that can be transferred between bacteria.


Transmission of drug resistance

• Transmission of drug resistance – Bacterial plasmids – Transposons – Bacteriophages


Resistance to Antibiotics


Role of “misuse”

• Misuse of antibiotics selects for resistance mutants.

• Misuse includes;– Using outdated or weakened antibiotics– Using antibiotics for the common cold and other

inappropriate conditions– Using antibiotics in animal feed– Failing complete the prescribed regimen– Using someone else's leftover prescription


Microbe Library

American Society for Microbiology

www.microbelibrary.org


Antimicrobial Drug ResistanceMechanisms

• Altered permeability– Altered efflux

• tetracycline

Microbe Library





• Inactivation– β-lactamase– Chloramphenicol

acetyl transferase

Microbe Library





• Altered target site– Penicillin binding proteins

(penicillins)– RNA polymerase (rifampin)– 30S ribosome

(streptomycin)

Microbe Library




Antimicrobial Drug Resistance Mechanisms

• Replacement of a sensitive pathway– Acquisition of a resistant enzyme

(sulfonamides, trimethoprim)


Antibiotics Lecture 04

Documents

Transcript of Antibiotics Lecture 04