Lecture 18: Mechanisms of resistance to chemotherapeutic drugs

Question 1

What is the definition of resistance?

Answer 1

a situation is which disease causing agents are able to survive and/or grow in the presence of concentrations of chemotherapeutic drug that an be safely achieved in patients
This is disease and drug dependent

Question 2

What are the negative consequences of resistance of chemotheraputic drugs?

Answer 2

Increased mortality
Increased morbidity
Increased cost

Question 3

Which drugs develops resistance frequently involving horizontal transfer?

Answer 3

Antibacterial

Question 4

Which drugs do not develops resistance via spreading to a new host?

Answer 4

Anticancer

Question 5

At what % of resistance should a drug not be used as empirical treatment?

Answer 5

5%

Question 6

What is pan resistance?

Answer 6

resistance to all clinically used drugs

Question 7

What are the 2 types of acquired antibiotic resistance?

Answer 7

Spontaneous mutation to resistance - endogenous, usually happens during DNA replication
Horizontal acquisition of antibiotic resistance

Question 8

What are 3 mechanisms of transfer in horizontally-acquired antibiotic resistance?

Answer 8

Conjugation aka bacterial sex
Transduction - DNA from dead cell to host cell
Transformation - New genetic material into non bacterial cells

Question 9

Why does resistance develop?

Answer 9

Natural selection drives the development of resistance
Wide spread abx use = strong evolutionary selection pressure on bacterial pathogens
Survival of the fittest

Question 10

Where do acquired resistance genes originate?

Answer 10

Microorganisms are genetically promiscuous
Horizontally acquired resistance genes may be recruited from: abx producer organisms e.g. soil microorganisms as they usually have a resistance mechanisms as protection or intrinsically resistant bacteria

Question 11

What are the 4 different mechanisms of antibacterial resistance?

Answer 11

1) Altered target site
2) Decreased uptake
3) Enzymatic inactivation
4) Bypass

Question 12

Altering of target site can develop in what 3 ways?

Answer 12

1) Mutation of target
2) Modification of target
3) Increase in target quantity

Question 13

Give 2 examples of alteration of target via mutation of target

Answer 13

1) Alteration of RNA polymerase amino acids mediates rifampicin resistance in S. aureus
2) Beta lactam resistance mediated by mosaic PBPs - addition of genes in naturally transformable bacteria e.g. Neisseria, Streptococcus (interspecies recombination)

Question 14

Give 2 examples of alteration of target via modification of target

Answer 14

1) Methylation of ribosomal RNA, specifically A2503 of 23S rRNA
e. g. Linezolid in S. aureus

2) Vancomycin resistance in enterococci - replacement of D-Ala with D-lactate which means H bond with vancomycin cannot form

Question 15

Explain the mechanism of vancomycin resistance in enterococci

Answer 15

Replacement of D-Ala with D-lactate which means H bond with vancomycin cannot form
van R: switches on other 3 genes (van H, A and X)
van S: detects vancomycin and switches on van R
van H: synthesis of D-lactate from pyruvate
van A: ligase enzyme for coupling of D-Ala to D-Lactate
van X: peptidase that breaks D-Ala-D-Ala linkages

Question 16

Give a example of alteration of target via increase in target quantity

Answer 16

Target overexpression in vancomycin intermediate S.aureus
Numerous genetic/biochemical changes but they all seem to present as a significant thickening of the cell wall
Affinity trapping of vancomycin occurs as well as less cross linking

Question 17

What are the 2 different mechanisms that decreased uptake resistance can occur?

Answer 17

1) Reduced permeability

2) Efflux

Question 18

Give an example of how decreased drug uptake can occur

Answer 18

Loss of porins from the liposaccharide membrane e.g. imipenem-resistant pseudomonas aeruginosa shows reduced levels of OprD porin protein

Question 19

Give an example of the active efflux mechanism of resistance

Answer 19

Big problem in gram -ve
e.g. AcrAB/TolC System in E.coli
Pumps drugs out

Question 20

What are the 2 different mechanisms that enzymatic inactivation can occur?

Answer 20

1) Destruction

2) Modification

Question 21

Give an example of destruction as a mechanism of resistance

Answer 21

B lactamases mediating resistance to B lactams
Cyclic amide bonds of B lactam rings are hydrolyzed
Therefore structure changed at D-Ala and does not bind to transpeptide
Problem in gram -ve

Question 22

Give an example of modification as a mechanism of resistance

Answer 22

Aminoglycosides e.g.

1) Aminoglysocide adenyltransferase (ANT) - catalyses ATP dependent adenylation of hydroxyl group
2) Aminoglycoside acetyltransferases (AAC) - catalyses acetyl CoA dependent acetylation of an amino group
3) Aminoglycoside phosphotransferases (APH) - catalyses ATP-dependent phosphorylation of a hydroxyl group

Question 23

Give an example of target bypass (acquisition of alternative target) as a mechanism of resistance

Answer 23

Methicillin resistance in S aureus
Carrying mecA means that PBP2a can still continue to cross link, though the function is not as good as PBPs 1-3 working together, it still manages to somewhat compensate