What are the general characteristics of Sulfonamides?
Active against both Gram (-) and Gram (+) bacteria. Metabolic antagonists
What is Prontosil?
A prodrug. Converted to sulfanilamide, the active metabolite. Bacteriostatic
What are the SARs for Sulfonamides?
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
What are the characteristics of Sulfonamides R group?
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
What are the characteristics of Sulfonamides N4 amino group?
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)
What are the different activity classes of Sulfonamides?
1) Rapid absorption, rapid action, rapid excretion (Sulfisoxazole, Sulfadiazine). 2) No absorption - for intestinal infections (Sulfasalazine). 3) Topically applied, topically active (Sulfacetamide, Silver Sulfadiazine)
What are the Rules of the "Game" when it comes to competition in metabolic inhibition?
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
What is Transient inhibition?
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
What can make inhibition not transient?
High concentration of antibiotic at target site. Feedback inhibition. Alternate metabolic pathways. Suicide substrate
What is the Folate Biosynthesis Pathway?
Precursors for nucleic acid synthesis
What metabolic processes are affected by sulfonamides?
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)
What do Pyrimidines such as Trimethoprim do?
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
How does sensitivity and resistance of Sulfonamides occur?
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)
Why are humans not harmed by sulfonamides?
They primarily get NA precursors from their diet
What are the genetic pathways to sulfonamide resistance?
DHPS or DHFR enzyme mutations (most common basis for sulfonamide resistance). Alteration of the PABA biosynthetic pathway to hyperactivity. Transport IN pathways down-regulated
What are the general characteristics of Quinolones?
The prototype quinolone is Nalidixic Acid. High activity against Gram (+) and Gram (-)
What are Fluoroquinolones?
The addition of C6 fluorine significantly increases activity
What is the SAR summary for Fluoroquinolones?
For Fluoroquinolones, what can be done to increase activity against topoisomerase, and against resistant Gram (-) bacteria?
Addition of C8 methoxy
How do Quinolones work?
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)
What is the specific mechanism of Quinolones?
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
Which Topoisomerases do Fluoroquinolones inhibit?
Topoisomerase II and IV
What are the genes involved in Topoisomerase II and IV?
II (gyrA, gyrB). IV (parC, parE)
How does DNA Gyrase catalyze the supercoiling step?
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
What is sensitivity and resistance like for Quinolones?
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)
What is Methenamine?
Other UTI Antibiotic. Generates formaldehyde. Highly reactive. Alkylates (inactivates) proteins. Humans are better at detoxifying. "Inhactivated" in stomach, unless protected
What is Nitrofurantoin?
A nitrofuran. Prodrug. Requires reduction for activation. Generates radicals which can damage DNA, proteins, etc. Bacteria reduce subsequentially more rapidly (basis for selective action)
What does Metronidazole look like?
What is Metronidazole?
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)
What are some general characteristics of Metronidazole?
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
When does resistance to Metronidazole occur?
When activating enzymes mutate to decreased activity