Doxycycline: Pharmacokinetics & Pharmacodynamics

Instructor: Charity Hacker

I am a nursing instructor with over 20 years of nursing experience and a Masters Degree in Nursing Education.

Healthcare professionals should learn as much as possible about any drug before administering it, and doxycycline is a popular medication. In this lesson, you'll learn about the pharmacokinetics and pharmacodynamics of doxycycline.

Getting to Know Doxycycline

So, you're a new nurse and you have an order to give a patient doxycycline. Part of your job as a nurse is to be knowledgeable about the medications you are administering and provide patient education. The patient is in the hospital for pneumonia and has a history of chronic kidney disease. He also has a lot of family members at the bedside and you want to be prepared. You decide to do some research prior to taking the medication to the patient.

The first thing you learn is that doxycycline is not a new anti-infective medication. You learn that it also falls into the anti-bacterial, antimalarial, antiparasitic, antiprotozoal, cytrochrome P-450, and tetracycline drug classes. It has been around for quite a while and is used to treat a long list of medical problems, including but not limited to cholera, E. coli, the plague, respiratory infections, STDs, and tick-borne disease. It is generally used to treat gram-negative bacteria but (with a susceptible culture) can also be used to treat gram-positive, anaerobic, and other bacteria as well as some parasites.

Pharmacodynamics of Doxycycline

What is pharmacodynamics? Pharmacodynamics is what a drug does to your body. Doxycycline stops the protein synthesis of susceptible organisms. How does that help? Protein synthesis is the necessary step in the replication of genetic material needed for organisms to grow and reproduce. It does this by binding, thus blocking reproduction, to the microorganism's 30S ribosomal subunit. This process is known as the drug's mechanism of action or how it exerts its desired effect on the body.

What else does this drug do? It may enhance the effects of anticoagulants, such as warfarin, while it may reduce the effectiveness of birth control. Some heartburn medicines, vitamins, and anti-seizure medications can keep it from working as well. Also, it should not be given at the same time as penicillin: they just don't get along.

Pharmacokinetics of Doxycycline

Now let's see what your body does with the drug once administered: this is called pharmacokinetics.

Absorption

The human body does a good job of absorbing almost all of the doxycycline. It does this job quickly and reaches its peak concentration in about 2 hours. The clinical significance of taking doxycycline with or without meals has not been established. A high fat meal has been shown to slightly reduce absorption, but some people tolerate the medication better when taking it with food. This is good information because your patient is very busy with tests; sometimes he can eat and sometimes he can't. Your patient will not need to miss any doses based on whether he has eaten or not.

Distribution

Once the medication has been absorbed into the blood system, it is then further dispersed throughout the body by binding to proteins. Greater than 90% of doxycycline administered will bind to protein for distribution. Proteins in your blood include albumin, as well as immunity and clotting factors. These are the things that carry the drug to the location where the doxycycline will exert its mechanism of action.

Metabolization

When your body is done with the medication, meaning its potential effectiveness has ran out, it is sent to the liver. It is metabolized there and concentrated there in the bile. Bile is used by the body for digestion. Your patient does not have any liver issues, so you are not worried about his ability to metabolize the drug.

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