Principles of Antimicrobial Chemotherapy

Question 1

Chemotherapy

Answer 1

Applied to the use of chemicals (either natural or synthetic) to inhibit the growth/replication of ‘invading organisms’ or cancerous cells within the body.

Question 2

Erhlichs discoveries

Answer 2

Discovered and established the concept of selective toxicity.

• Gram staining technique

Question 3

Erhlichs discovery about trypanosomes

Answer 3

Trypanosomes were killed by Salvarsan

Trypanosomes could become resistant

Trypanosomes resistant to one agent, remained susceptible to others.

Question 4

Domagk and rational development of antibiotics

Answer 4

He pursued a series of dyes as potential antibacterials.

Discovered PRONTOSIL : a red dye that inhibited bacteria

Jacques and Therese Trefouel found that prontosil was metabolised to sulphanilamide which was not a dye, but still active against bacteria.

Question 5

Domagk’s achievement

Answer 5

He developed the concept of rational design of anti-bacterials.

He went on to develop anti-tuberculosis agents.

Question 6

Penicillin

Answer 6

Was a chance discovery by Alexander Fleming.

He was unable to make enough material to make treatment possible.

So, he published the discovery as a method to select bacteria on agar.

He was aware of the potential threat of AMR.

Question 7

Florey and Chain

Answer 7

Recognised the value of penicillin as potential treatment of bacterial infection.

Used large fermenters to create enough starting material to allow extraction of penicillin.

They worked in great secrecy during the war, later in America.

The urine of treated patients was collected and penicillin re-extracted.

Question 8

Penicillin G

Answer 8

Semi-synthetic

Question 9

Penicillin nucleus

Answer 9

Doesn’t exist in nature

Question 10

Ampicillin

Answer 10

Semi-synthetic

Question 11

Pre-1946 treatments

Answer 11

Pelargonium roots
Cod liver oil
Gold

Question 12

Waksman and Schatz

Answer 12

Antibiotic discovery based on a theory that soil organisms may have produced agents to kill mycobacteria which were derived from soil organisms.

Thousands of soil samples were screened for the presence of factors that inhibit mycobacteria.

Streptomycin discovered.

Question 13

Streptomycin

Answer 13

Single most effective drug against TB

Question 14

The streptomycin trial

Answer 14

Treatment groups :

• Bed rest : 52 patients
• Streptomycin : 55 patients

Question 15

The streptomycin trial short term results of deaths assessed

Answer 15

6 months

Bed rest : 27%
Streptomycin : 7%

P = 0.01

Question 16

The streptomycin trial long term results of deaths assessed

Answer 16

5 years

Bed rest : 62%
Streptomycin : 58%

Question 17

How many of the streptomycin patients that were tested have developed resistance ?

Answer 17

35 out of 41

Question 18

Describe how antibiotics can be developed

Answer 18

Based on a theory of activity - even though it is subsequently proved to be false

As a result of chance occurrences

Question 19

Describe how antibiotics can be discovered

Answer 19

On the basis of systematic screening of natural products.

Question 20

Selective toxicity

Answer 20

Reduces damage to the host/normal cells [PATIENT]

Maximises damage to invading organism / cancerous cells.

Question 21

Principles of selective toxicity

Answer 21

1. Central to the use of chemotherapeutic agents is
   the concept of SELECTIVE TOXICITY. These drugs are
   intended to be toxic to the invading organism or
   cancerous cell but be relatively harmless to the host
   or normal cells.
2. This approach depends upon the existence of
   biochemical differences between the target group of
   cells and the host.

Question 22

Chemotherapeutic agents

Answer 22

These drugs are intended to be toxic to the invading organism or cancerous cell but be relatively harmless to the host or normal cells.

Question 23

Examples of different selective toxicity

Answer 23

Penicillins
Amino glycosides

Question 24

Penicillins (example of selective toxicity)

Answer 24

In the absence of allergy, they have a very low toxicity and high doses can be used.

Question 25

Amino-glycosides (example of selective toxicity)

Answer 25

They have a narrow therapeutic index thus the dose that causes toxicity is very close to the therapeutic dose.

Question 26

Therapeutic index

Answer 26

Lethal dose 50% / Effective dose 50%

Question 27

Anti-tuberculosis drugs

Answer 27

Isoniazid and Pyrazinamide A number of patients will develop hepatoxicity that is not dose related and may require treatment to be stopped.

Question 28

Main mechanisms of action of the main antibiotic classes

Answer 28

Class 1-3 reactions

Question 29

Class 1 reactions

Answer 29

Result in the synthesis of the precursor molecules necessary for Class 2 reactions. Precursor molecules = NH4+ and SO4 2-

Question 30

Class 2 reactions

Answer 30

Result in the synthesis of the constituent molecules. Constituent molecules : - Hexosamines - Amino acids - Nucleotides

Question 31

Class 3 reactions

Answer 31

Constituent molecules are then assembled into macromolecules. - Peptidoglycan - Proteins - RNA - DNA

Question 32

Function of pepidoglycans

Answer 32

They make up the cell wall of bacteria and do not occur in eukaryotes. The cell wall is made up from various numbers of strands of pepitdoglycans.

Question 33

Structure of peptidoclycans

Answer 33

The strands of peptidoglycan are made up of multiples of amino-acids; N-acetylglucosamine and N-acetylmuramic acid dimers. The n-acetylmuramic acid has a short peptide side chain (hence peptidoglycan) The peptide side chains are cross linked to form a latticework. Cross linking gives the cell all its strength.

Question 34

Beta-lactam Cephalosporin

Answer 34

TARGET : Penicillin binding proteins MECHANISMS : Preventing peptidoglycan cross-linking EXAMPLES: Penicillin G, Flucloxacillin, Cefoxatin

Question 35

Glycopeptide

Answer 35

TARGET : C-terminal D-Ala-D-Ala MECHANISMS : Prevents transglycolation and transpeptidation EXAMPLES : Vancomycin, Teicoplanin

Question 36

Cyclic peptide

Answer 36

TARGET : C55-isopropyl pyrophosphate MECHANISMS : Prevents carriage of building-blocks of peptidoglycan bacterial cell wall outside of the inner membrane. EXAMPLES : Bacitracin, Polymyxin

Question 37

Phosphonic acids

Answer 37

TARGET : murA protein MECHANISMS : Inhibits first stage of peptidoglycan synthesis EXAMPLES : Fosphomycin

Question 38

Lipopeptides

Answer 38

TARGET : Cell wall stress simulation MECHANISMS : Calcium-dependent membrane depolarisation EXAMPLES : Daptomycin

Question 39

Function of beta lactams

Answer 39

Prevent the cross-linking peptides from binding to the tetra-pepide side chains.

Question 40

Name some penicillins

Answer 40

Penicillins G and V Beta-Lactamase-resistant penicillins Broad spectrum penicillins Extended spectrum penicillins

Question 41

Beta-Lactamase-resistant penicillins

Answer 41

Methicillin Oxacillin Nafcillin Cloxacillin Dicloxacillin

Question 42

Broad spectrum penicillin

Answer 42

Ampicillin Amoxicillin

Question 43

Extended spectrum penicillins

Answer 43

Carbenicillin Ticaracillin Azlocillin Piperacillin

Question 44

Cephalosporins

Answer 44

1. Come from the fungus Cephalosporium Acremonium. 2. Work by the same mechanisms as penicillins 3. Classified by generations in the order in which they were developed. - 1st, 2nd and 3rd generation. 4. Now can be termed by means of administration; Oral is Cephalexin, Parenteral are Cefuroxime & Cefotaxime

Question 45

Bacterial Folate agonists

Answer 45

1. SULPHONAMIDES & TRIMETHOPRIM These are antibiotics which act through an inhibition of the folate pathway in bacteria. 2. Folate system important in cell metabolism 3. Bacteria must make their own supply but we don't as we get it in diet. 4. This makes bacteria susceptible to drugs which interfere with folate metabolism: thus we have our 'selective toxicity' target. 5. Sulphonamides mark the beginning of antimicrobial chemotherapy dating back to the 1930s and preceding the penicillins.

Question 46

Aminoglycosides

Answer 46

1. Form Ionic bonds at the cell surface 2. Penetrate the cell wall by a transport mechanism across the cell membrane. 3. Diffuse into the cytoplasm and then binds to the bacterial ribosomes.

Question 47

Examples of amino glycosides

Answer 47

Streptomycin Kanamycin Neomycin Gentamicin

Question 48

Inhibition of protein synthesis

Answer 48

Genetamcin : 1. Binds to the ribosomes : i. At the interface between the assembled 30s and 50s subunits. ii. Directly to the individual subunits. 2. Inhibits protein synthesis by misreading of mRNA.

Question 49

Tetracyclines

Answer 49

1. They prevent attachment of the tRNA to the acceptor (A) site on the mRNA-ribosomal complex. 2. This prevents the addition of amino acids to the peptide chain. 3. Unlike the aminoglycosides, they are only weakly bound to the ribosomes. 4. Differences in the activity of individual tetracyclines are related to their solubility in the lipid membrane of the bacteria.

Question 50

Chloramphenicol, Erythromycin and Clindamycin

Answer 50

Prevent the addition of new amino acids to the growing peptide chain by binding to the ribosomes. This prevents association of the peptidyl-transferase with the amino acid and no peptide bond is formed i.e. no transpeptidation. May also prevent translocation of the ribosome down the mRNA template (Erythromycin).

Question 51

Fluoroquinolones

Answer 51

Fully synthetic antibiotics

Question 52

Main classes of antibacterial agents - fluoroquinolones

Answer 52

Broad-spectrum agents - Ciprofloxacin - Ofloxacin - Norfloxacin Narrower spectrum agents - Nalidixic acid (first introduced and is not fluorinated)

Question 53

Describe the action of nalidixic acid

Answer 53

1. Act by inhibiting bacterial DNA Topoisomerase II also known as DNA gyrase. 2. This enzyme catalyses the introduction of negative supercoil in DNA permitting transcription and replication.

Question 54

Use of fluoroquinolone

Answer 54

Defined by spectrum of activity Defined by pharmacokinetics Often limited by impact on other flora