FP - Inhibitors of Nucleic Acid Synthesis

Question 1

What are inhibitors of DNA topoisomerases (2) and inhibitors of mRNA synthesis (1)?

Answer 1

Inhibitors of DNA topoisomerases

• Fluoroquinolones
• Quinolone

Inhibitors of mRNA synthesis

• Rifampicin

Question 2

What are 5 features of Rifampicin?

Answer 2

• Member of the ansamycin group
• Bactericidal
• Mainly active against Gram-positive bacteria
• Especially useful against tuberculosis, leprosy and Legionnaires’ disease
• Inducer of P450

Question 3

What is the mechanism of action of Rifampicin? (2)

Answer 3

1) Inhibitor of RNA polymerase

• Binds to the β-subunit (rpoB gene)

2) Inhibits initiation, not elongation of mRNA synthesis:

• It does not block the formation of the first phosphodiester bond, but prevents further additions

Question 4

How did resistance for Rifampicin develop?

Answer 4

Resistance rapidly develops from a single point mutation (Ser531Leu) in the β-subunit

• Should not be used in monotherapy!!!

Question 5

What does the selectivity of rifampicin depend on?

Answer 5

High affinity of rifampicin for the prokaryotic RNA polymerase.

• The selectivity of rifampicin depends on the fact that they have a very poor affinity for the analogous mammalian enzyme

Question 6

What are 3 features of Quinolones?

Answer 6

• Active against Gram-positive and few Gram negative
• Bacteriostatic at low concentrations, bactericidal at high concentrations
• Used almost exclusively for urinary tract infections

Question 7

What is the mechanism of action of quinolones? (2)

Answer 7

1) Interfere with DNA gyrase

2) Stabilises the DNA-gyrase complex at the point of cleavage

• Gyrase-Quinolone-DNA is a stable complex

Question 8

What are 3 features of DNA gyrase:

Answer 8

• A2B2 structure
• Unwinds double-stranded DNA and so introduces negative supercoils into circular DNA
• ATP-dependent

Question 9

How does resistance develop for Quinolones?

Answer 9

Resistance develops from a single point mutation in gyrA

Question 10

What do Fluoroquionolones inhibit and what are the stable complexes formed? (2)

Answer 10

Inhibit DNA gyrase and topoisomerase IV

• Prevents formation of supercoiled DNA
• Prevents decantenation

Gyrase-Fluoroquinolone-DNA Complex

Topoisomerase IV-Fluoroquinolone-DNA Complex

Question 11

What are the mechanisms contributing to resistance against fluoroquinolones? (3)

Answer 11

• 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

Question 12

What do Fluoroquinolones inhibit?

Answer 12

Inhibitor of cytochrome P450 – CYP2C9 and CYP3A4

Question 13

What are 3 serious side effects caused by fluoroquinolones?

Answer 13

1. Inflamed or torn tendon
2. Muscle pain or weakness
3. Joint pain or swelling

Question 14

What are 2 other antibiotics that affect nucleic acids?

Answer 14

1. Nitrofurantoin
2. Metronidazole

Reduction in vivo —> reactive compounds —> mutagenesis and nucleus acid breaks