Aminoglycosides: Bactericidal Aminoglycosides: Bactericidal Inhibitors of Protein SynthesisInhibitors of Protein Synthesis

Aminoglycosides

Most commonly employed agents Gentamicin, tobramycin, amikacin

Narrow-spectrum antibiotics Bactericidal Use – aerobic gram-negative bacilli Can cause serious injury to inner ear and

kidney Not absorbed from the GI tract Microbial resistance

Fig. 86-1. Structural formulas of the major aminoglycosides.

Fig. 86-2. Mechanism of action of aminoglycosides.A, Protein synthesis begins with binding of the 50S and 30S ribosomal subunits to messenger RNA (mRNA), followed by attachment of the first amino acid of the new protein to the 50S subunit. As the ribosome moves down the mRNA strand, additional amino acids are added to the growing peptide chain. When the new protein is complete, it separates from the ribosome and the ribosomal subunits separate from the mRNA. B, Aminoglycosides bind to the 30S ribosomal subunit and can thereby (1) block initiation, (2) terminate synthesis before the new protein is complete, and (3) cause misreading of the genetic code, which causes synthesis of faulty proteins.

Bacterial resistance

Bacterial resistance to aminoglycosides occurs via one of three mechanisms that prevent the normal binding of the antibiotic to its ribosomal target:

(1)Efflux pumps prevent accumulation of the aminoglycoside in the cytosol of the bacterium.

(2) Modification of the aminoglycoside prevents binding to the ribosome.

(3)Mutations within the ribosome prevent aminoglycoside binding.

Gentamicin (Garamycin)Gentamicin (Garamycin)

Used to treat serious infections caused by aerobic gram-negative bacilli Pseudomonas aeruginosa Escherichia coli Klebsiella Serratia Proteus mirabilis

Adverse effects Nephrotoxicity Ototoxicity

Other AminoglycosidesOther Aminoglycosides

Tobramycin (Nebcin)Tobramycin (Nebcin) Amikacin (Amikin)Amikacin (Amikin) NeomycinNeomycin Kanamycin (Kantrex)Kanamycin (Kantrex) StreptomycinStreptomycin Paromomycin (Humatin)Paromomycin (Humatin)

Quinolones

Introduction

The quinolones have a number of advantages over other classes of antibacterial agents.

They are effective against many organisms, well-absorbed orally, well-distributed in tissues, and they have relatively long serum half-lives and minimal toxicity.

Because of deep-tissue and cell penetration, they are useful for urinary tract infections, prostatitis, infections of the skin and bones, and penicillin-resistant sexually transmitted diseases.

The quinolone antimicrobials comprise a group of synthetic substance possessing in common an N-alkylated-3-carboxypyrid-4-one ring. The discovery of quinolone is an epoch-making events. Since 1962 the first quinolone, Nalidixic acid was developed, more than 100,000 quinolone compounds have been synthesized and screened their pharmacological activities. Currently, there are more than 20 quinolones used in clinic. The advantages of this kind of drugs are their lower cost in synthesis together with the excellent activities.

X N

O

COOHR1

R2

R

Brief History and Overview

The history can be traced to the discovery of an antibacterial by-product formed during the synthesis of antimalarial agent Chloroquine, an isomer of key intermediate, 7-chloro-1-ethyl-1,4-dihydro-4-oxo-3-quinolinecarboxylic acid (1).

NCl

HNN

Chloroquine

N

O

COOH

Cl

1

Brief History and Overview

In 1962, Lesher et al. described the 1-ethyl-7-methyl-4-oxo-1,4-dihydro-1,8-naphthyridine-3-carboxylic acid (2), also known as Nalidixic acid. It was the first commercially available compound of this class and was approved for treatment of urinary tract infections in 1964.

N N

O

COOH

H3C

2

Nalidixic acid

N

O

COOH

Cl

1

The first-generation quinolones

Norfloxacin, a fluoroquinolone with a broad spectrum of antibacterial activity, was patented in

1978 .

Between 1978 and 1982, many new fluoroquinolones were prepared and patented. These fluoroquinolones now classified as second-generation quinolones.

N

O

OH

O

NHN

F

The second-generation quinolones

The third-generation quinolones A third advance was made in early 1990s. All third-

generation fluoroquinolones have significantly improved activity against gram-positive bacteria, notably streptococcus pneumoniae. Some of them have good activity against anaerobes and atypical pathogens.

*withdrawn from the market in 1999

Generation Drug Names Spectrum

1stnalidixic acid

cinoxacin

Gram- but not Pseudomonas species

2nd

norfloxacin

ciprofloxacin

enoxacin

ofloxacin

Gram- (including Pseudomonas species), some Gram+ (S. aureus) and some atypicals

3rd

levofloxacin

sparfloxacin

moxifloxacin

gemifloxacin

Same as 2nd generation with extended Gram+ and atypical coverage

4th

*trovafloxacin (withdrawn from the market in 1999)

Same as 3rd generation with broad anaerobic coverage

Mechanism of action

Quinolones enter the cell by passive diffuse. Intracellularly, they inhibit the synthesis of bacterial DNA by interfering with the action of DNA gyrase ， topoisomerase .Ⅳ

The chromosome of bacteria is composed of helical double-stranded DNA and contains 60 to 70 spatial regions of organisation, termed domains of supercoiling. Each domain is attached to an RNA core and is organised by supercoiling which occurs quite independently of the DNA coiling in any other domain. Supercoiling is controlled by the enzyme DNA gyrase, which introduces transient breaks into both DNA strands of each domain, removes about 400 turns from its DNA helix, then reseals the DNA so locking in the supercoiling. This supercoiled state is essential to the well-being of bacteria as it enables them to accommodate their chromosome within the confines of their cell envelope.The target site of action of the quinolone antibacterial agents is DNA gyrase and its inhibition by them sets off a complex series of events which ultimately causes bacteria to die.

Mechanism of actionModel of the formation of negative DNA supercoils

by DNA gyrase.

(1) A node of positive is created for(+) superhelix.(2) The enzyme introduces a double-strand break in the DNA and passes the front segment through the break. (3)The break is then resealed, creating a negative (-) supercoil. Quinolones inhibit both the nicking and closing activity of the gyrase.

Mechanism of action

A A

BB

• •

Binding sites of quinolones

ATP-ATP-

Gram-positive bacteria

Some Staphylococcus aureus, Streptococcus pyogenes, Virdans group streptococci, Streptococcus pneumoniae

Gram-negative bacteria

Neisseria spp. Haemophilus influenzae

Many Enterobacteriaceae, Some Pseudomonas aeruginosa

Anaerobic bacteria Some clostridia spp, Some Bacteroides spp.

Atypical bacteria Chlamydia and Chlamydophilia, Mycoplasma pneumoniae, Legionella spp

Mycobacteria Mycobacterium tuberculosis, Mycobacterium avium complex, Mycobacterium leprae

Antimicrobial Activity of the Quinolones (oral)

Resistance Mechanisms

Mutations that enhance antibiotic efflux capability Bacterial chromosomal mutations for genes that

encode for bacterial DNA gyrase and Topo IV Mutations in outer membrane porins (Gram-)

MECHANISMS OF RESISTANCE TO QUINOLONESMECHANISMS OF RESISTANCE TO QUINOLONES

• Changes in the protein targets.• DNA gyrase• Topoisomerase IV.

• Reduction in the accumulation of the quinolone. - Decrease in permeability.

- Increase in active efflux system(s).

• DNA gyrase and topoisomerase IV protection - qnr gene