AssessmentsDNA synthesis inhibitors: Fluoroquinolones
DNA synthesis inhibitors: Fluoroquinolones
USMLE® Step 1 style questions USMLE
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A 35-year-old man decides to mow his lawn on a hot Sunday at noon. He forgets his hat inside, but figures he should be fine without it for a little while. It takes him much longer than he expected, but he finally finishes the whole yard. When he walks into the house, his wife gasps when she see his face (see below). Recent use of which of the following medications would most likely cause this to happen?
Content Reviewers:Yifan Xiao, MD
Our bodies consist of eukaryotic cells, while bacterias consist of prokaryotic cells.
So, in order to treat bacterial infections we can develop antimicrobials that only target prokaryotic cells while leaving our cells mostly unharmed.
One of these targets is bacterial DNA, and we call these medications DNA inhibitors or nucleic acid inhibitors.
There are plenty of processes and enzymes involved that we can target and the quinolones and fluoroquinolones inhibit an enzyme called DNA topoisomerase.
So there are many different types of topoisomerases but we’ll be looking at topoisomerase II, also called DNA gyrase, and topoisomerase IV.
Topoisomerase II plays a role in condensing the chromosomes by making a double strand break in the DNA so that it can be more tightly wound, causing a supercoil.
When enough supercoils are induced, the DNA condenses.
Topoisomerase IV plays a role later on, after the chromosome has been replicated, where it causes a double strand break in the DNA so the new DNA strand can be disentangled from the original.
Now, quinolones are created to target bacterial topoisomerases, but it was soon discovered that by adding a fluorine molecule to the quinolones, they become more effective.
So these newer medications, called fluoroquinolones, replaced the older quinolones in most clinical settings.
One way to tell a fluoroquinolone apart from other antimicrobials is that they all have the suffix “-floxacin.”
Now fluoroquinolones are broad spectrum bactericidal antibiotics and ciprofloxacin in particular is widely used.
They are very effective against gram negative bacterias like Enterobacteriaceae, Haemophilus, Legionella, Neisseria, Moraxella, and even Pseudomonas species.
They are also effective against certain mycobacteria and can be used for the treatment of tuberculosis.
However, more and more bacteria are becoming resistant to ciprofloxacin, either through mutations in the genes that code of topoisomerases, or through efflux pumps that pump the medication out.
Another downside is that the older fluoroquinolones are less effective against Gram positive bacteria, but the newer ones, like levofloxacin, gemifloxacin, and moxifloxacin, have improved effectiveness against Gram positive cocci, especially Streptococcus pneumoniae, so they are called respiratory fluoroquinolones since they are often used to treat hospital acquired pneumonia.
The trade off is that they are less effective against pseudomonas than ciprofloxacin.
Most fluoroquinolones are not effective against MRSA.
They’ll bind to each other, a process called chelation, preventing their absorption from the gut.
They are metabolized in the liver by a family of enzymes called cytochrome p450, but they are also inhibitors of this enzyme.
For side effects, fluoroquinolones can cause tendonitis especially in the achilles tendon, where they can cause it to rupture.
Due to this, they should not be used by active athletes and the elderly.