13 Sulfonamides Duncan

Question 1

What are the general characteristics of Sulfonamides?

Answer 1

Active against both Gram (-) and Gram (+) bacteria. Metabolic antagonists

Question 2

What is Prontosil?

Answer 2

A prodrug. Converted to sulfanilamide, the active metabolite. Bacteriostatic

Question 3

What are the SARs for Sulfonamides?

Answer 3

1) COO- mimicking group. 2) H replaced with 1-2 EWD groups. 3) Intact benzene ring. 4) NH2 must be para. The NH2 at #2 is considered the “R” group

Question 4

What are the characteristics of Sulfonamides R group?

Answer 4

In Sulfanilamide, the pKa of the N-attached hydrogens is 10.5. In PABA the pKa of the hydrogen is ~6.5. Modifications in the R group that reduce the pKa of the remaining hydrogen to 6.5 or less - as occurs with several heterocyclic R groups - greatly increases the sulfonamides potency

Question 5

What are the characteristics of Sulfonamides N4 amino group?

Answer 5

An amino group is an absolute requirement for enzyme inhibition. However, sulfonamides containing a modified N4 to NHR are active. They are prodrugs; the compound is metabolically activated to NH2 form. Modified N4 forms that have reduced solubility are nephrotoxic (e.g. acetylated form)

Question 6

What are the different activity classes of Sulfonamides?

Answer 6

1) Rapid absorption, rapid action, rapid excretion (Sulfisoxazole, Sulfadiazine). 2) No absorption - for intestinal infections (Sulfasalazine). 3) Topically applied, topically active (Sulfacetamide, Silver Sulfadiazine)

Question 7

What are the Rules of the “Game” when it comes to competition in metabolic inhibition?

Answer 7

The inhibitor (antibiotic) competes with the natural substrate for the enzyme. The competitor with the highest concentration wins (most of the time). Binding affinity will influence competition. If the antagonist binds less avidly, likely toxic concentrations will be required for antibiotic activity. Structural flexibility can decrease affinity

Question 8

What is Transient inhibition?

Answer 8

The UFC story he told. An antibiotic can inhibit a receptor, but as more and more of the natural substrate build up to a point where they over power the antibiotic, the antibiotic will start losing its effect

Question 9

What can make inhibition not transient?

Answer 9

High concentration of antibiotic at target site. Feedback inhibition. Alternate metabolic pathways. Suicide substrate

Question 10

What is the Folate Biosynthesis Pathway?

Answer 10

Precursors for nucleic acid synthesis

Question 11

What metabolic processes are affected by sulfonamides?

Answer 11

Block the folate biosynthesis pathway. Sulfonamides have a shape similar to PABA (basis for their antimicrobial activity). Sulfonamides block the activity of dihydropteroate synthetase (competitive inhibitor, involves formation of covalently-linked dead-end derivatives)

Question 12

What do Pyrimidines such as Trimethoprim do?

Answer 12

Inhibit another step in the Folate Pathway. Trimethoprim inhibits Dihydrofolate Reductase (metabolic antagonist (like sulfonamides), competitive inhibitor). Trimethoprim has a similar shape as dihydrofolate, confuses DHFR enzyme. Sulfonamides plus pyrimidines give synergism

Question 13

How does sensitivity and resistance of Sulfonamides occur?

Answer 13

High PABA in bacteria (or human body) reduces effectiveness (diminishes competition). Adequate supply of purines, pyrimidines in environment (or diet) bypasses requirement for the biosynthetic pathway (some organisms rely primarily on biosynthesis, others mainly on scavenging)

Question 14

Why are humans not harmed by sulfonamides?

Answer 14

They primarily get NA precursors from their diet

Question 15

What are the genetic pathways to sulfonamide resistance?

Answer 15

DHPS or DHFR enzyme mutations (most common basis for sulfonamide resistance). Alteration of the PABA biosynthetic pathway to hyperactivity. Transport IN pathways down-regulated

Question 16

What are the general characteristics of Quinolones?

Answer 16

The prototype quinolone is Nalidixic Acid. High activity against Gram (+) and Gram (-)

Question 17

What are Fluoroquinolones?

Answer 17

The addition of C6 fluorine significantly increases activity

Question 18

What is the SAR summary for Fluoroquinolones?

Answer 18

Question 19

For Fluoroquinolones, what can be done to increase activity against topoisomerase, and against resistant Gram (-) bacteria?

Answer 19

Addition of C8 methoxy

Question 20

How do Quinolones work?

Answer 20

Inhibit the replication of DNA (DNA synthesis). A variety of natural antibiotics target the same step and enzyme (Coumarins: natural inhibitors of DNA Gyrase (aka Topoisomerase II)

Question 21

What is the specific mechanism of Quinolones?

Answer 21

Interact with and inhibit DNA Gyrase (aka Topoisomerase II). Topoisomerases change DNA shape, involvies: cutting, twisting, and resealing supercoiled double helix. C8 methoxy fluoroquinolones also inhibit Topoisomerase IV

Question 22

Which Topoisomerases do Fluoroquinolones inhibit?

Answer 22

Topoisomerase II and IV

Question 23

What are the genes involved in Topoisomerase II and IV?

Answer 23

II (gyrA, gyrB). IV (parC, parE)

Question 24

How does DNA Gyrase catalyze the supercoiling step?

Answer 24

DNA Gyrase is a heterotetramer: 2A, 2B. DNA wraps around the tetramer; this creates the substrate for cutting, resealing. The reaction involves large conformational changes. One conformational state of DNA gyrase creates an excellent binding pocket for FQs. Binding site created in open conformation

Question 25

What is sensitivity and resistance like for Quinolones?

Answer 25

Humans have topoisomerase II too (binding is 300-1000 fold weaker). In tissue culture, topo II inhibition can create cancerous cells (hasn’t been proven). Resistance occurs due to restricted transport in, or altered DNA gyrase (transport mutants often cross-resistant to penicillins)

Question 26

What is Methenamine?

Answer 26

Other UTI Antibiotic. Generates formaldehyde. Highly reactive. Alkylates (inactivates) proteins. Humans are better at detoxifying. “Inhactivated” in stomach, unless protected

Question 27

What is Nitrofurantoin?

Answer 27

A nitrofuran. Prodrug. Requires reduction for activation. Generates radicals which can damage DNA, proteins, etc. Bacteria reduce subsequentially more rapidly (basis for selective action)

Question 28

What does Metronidazole look like?

Answer 28

Question 29

What is Metronidazole?

Answer 29

Prodrug (activated by pyruvate-ferrodoxin oxidoreductase). Readily enters cells. Nitro group accepts electrons, becomes free radical (highly reactive in this form). Intracellular “land mine” (destroys anything that gets too close: DNA, RNA, proteins, membranes)

Question 30

What are some general characteristics of Metronidazole?

Answer 30

Radicals are inactivated by reduction: higher activity of “detoxifying” enzymes in humans. Metronidazole is specifically useful against anaerobes and micraerophiles; these organisms have low-redox-potential electron transport systems that activate Metronidazole. Metronidazole is concentrated in the liver

Question 31

When does resistance to Metronidazole occur?

Answer 31

When activating enzymes mutate to decreased activity