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Sulfonamides, Trimethoprim and Fluoroquinolones – Antimetabolites

by Pravin Shukle, MD
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    Let's talk about the sulfonamides. Now sulfonamides are weak acids. And they're congeners or structurally similar to a chemical called paraminobenzoic acid which is actually shown in an illustration right here. Now these are bacteriostatic inhibitors of folic acid synthesis. And they may also be involved in the production of faulty folic acid. So mammals obtain their folic acid from the diet, whereas bacteria have to manufacture it. Now most of this particular drug is going to be excreted in the urine. So trimethoprim is often added to the sulfonamides. These are selective inhibitors of dihydrofolate acid reductase. It inhibits the bacterial form and there isn't really a mammalian form to inhibit to speak of. It prevents formation of tetrahydrofolate. Now bacterial dihydrofolate reductase is five times more sensitive to trimethoprim then the human version of the same enzyme. So from a trimethoprim point of view, it's really a non-issue. We combine sulfomethoxazole with trimethoprim to create an agent called septra which is sold commercially are also called SMP/TMP. Now bacterial synergy is obtained because we're acting on two different levels of the folic acid production pathway. Now the sulfonamides work on these particular enzyme at the top of this structure. And trimethoprim works on dihydrofolate acid reductase at the middle of this structural pathway. These drugs are concentrated in the bladder so they're particularly effective in bladder infections. Now we also use this agent in pneumocystis carini infections and toxoplasmosis infections in HIV positive patients. So it's not just used in urinary tract infections alone. Now septra toxicity includes hypersensitivity reactions which are actually surprisingly common. A lot of patients are allergic to sulpha and they will have a cross allergy to this agent. There are also cross allergies with other agents as well. So an example for this...

    About the Lecture

    The lecture Sulfonamides, Trimethoprim and Fluoroquinolones – Antimetabolites by Pravin Shukle, MD is from the course Antimicrobial Pharmacology. It contains the following chapters:

    • Sulfonamides
    • Trimethoprim
    • Sulfamethoxazole/Trimethoprim
    • Flouroquinolones

    Included Quiz Questions

    1. It concentrates in the bladder before being excreted in the urine.
    2. It requires a low pH environment to become activated.
    3. They are also effective against certain fungal infections which are commonly diagnosed in bladder infections.
    4. They are weak acids that must be deprotonated to exert their effect.
    5. Cells of the bladder do not require folate, so there is less organ damage compared to other drugs.
    1. ACE inhibitors
    2. Fluoroquinolones
    3. Tetracyclines
    4. Probenicid
    5. Warfarin
    1. Atypical bacteria
    2. Gram positive cocci
    3. VRSA
    4. MRSA
    5. Anaerobes
    1. Moxifloxacin
    2. Ciprofloxacin
    3. Levofloxacin
    4. Norfloxacin
    5. Oflaxacin
    1. These agents are bacteriostatic.
    2. They have good oral bioavailability.
    3. They work by interfering with DNA topoisomerase II in gram negative organisms.
    4. They are usually administered twice daily.
    5. Kidney excretion of fluroquinolones is blocked by probenecid which may lead to toxic accumulation.

    Author of lecture Sulfonamides, Trimethoprim and Fluoroquinolones – Antimetabolites

     Pravin Shukle, MD

    Pravin Shukle, MD


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