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Clinical Pharmacokinetics & Pharmacodynamics

Humans have devised a variety of techniques to treat and cure illnesses throughout history—from Ayurvedic medicine to chemical-based formulas. In our world, we have access to an abundance of medication to treat many different ailments. Each medicine available works to treat a specific illness; whether for pain relief or fever, these drugs are mainly designed to fulfill their individual purposes. When we feel sick, we take a particular medication depending on our symptoms, and within minutes, we begin to feel better. But have you ever wondered how these drugs work? How do our bodies interact with and process these substances so they perform proficiently? The answers to these questions are more straightforward than you might think, and they are answered by the two significant branches of pharmacology: pharmacokinetics and pharmacodynamics.


Image is Courtesy of UBC Science.


Pharmacokinetics


What is it?


Pharmacokinetics is one of the two major branches of study included in the scientific discipline of pharmacology. Pharmacokinetics can be defined as the science concerned with the movement of drugs. From the site of administration, into and within the body, up to the site of action, and the movement of the drug products up to its ultimate excretion. In simpler terms, pharmacokinetics refers to the movement of a substance into, through, and out of the body of a living organism.


The Four Phases


There are four phases involved in pharmacokinetics; they include absorption, distribution, and the two avenues of drug elimination, metabolism and excretion, also known as ADME.

The Four Phases (ADME) (Technology Networks).


Absorption


The process by which drug molecules achieve access to the bloodstream from drug administration is referred to as drug absorption. The rate and extent to which the drug is absorbed into the body depend on the method by which it is administered. There are three primary means by which a drug may be administered: enterally, topically, or parenterally.

Enteral Drug Administration


The enteral route is the most common as it is the most inexpensive and otherwise the simplest method. This route includes drugs taken orally, nasogastrically or through gastronomy tubes. There are several methods of enteral drug administration:

  • Tablets and capsules: Medicine is administered with tablets and/or capsules though the sublingual route (where the capsule is dissolved under the tongue), and/or the buccal route (where the capsule is placed between the gum and cheek, then absorbed slowly).

  • Nasogastric administration: Medicine is administered through nasal tubes which are inserted in the nasopharynx, or the oropharynx, and emptied out into the stomach (short-term administration).

  • Gastronomy administration: Tubes are surgically placed in the stomach, and used for administration of medicine in liquid form (long-term administration).


Topical Drug Administration


The topical route is one of several means of administration in which medications are applied to the outer surface of the body, or to the membranes that cover the body’s internal surfaces, such the respiratory tract. There are several means of topical administration:

  • Transdermal delivery system: Drugs are applied to the skin through transdermal patches.

  • Ophthalmic administration: Drugs are administered through the means of eye drops, eye irrigation, medicated disks, or ointment.

  • Otic administration: Medication is administered through ear drops or irrigation, nasal administration, and vaginal/rectal administration.

Topical drugs are sometimes preferred as they provide a local effect, and are better for absorption, and general circulation.


Parenteral Drug Administration


The parenteral route delivers medication directly into the body, via injection, into layers of the patient’s skin, tissue, muscles or veins. There are four different methods of parenteral drug administration:

  • Intradermal administration: This involves the injection of a substance into the dermis, which is located between the epidermis and hypodermis.

  • Subcutaneous administration: This method is the injection of a substance into subcutaneous tissue, located underneath the dermis.

  • Intramuscular administration: This method involves the injection of medication into a specific muscle.

  • Intravenous administration (IV): The medication is injected into a vein.


Distribution


The process by which drugs are transported through the bloodstream after absorption. There are several factors that influence the distribution of a drug through an organism:

  • Their ability to bond with receptors (binding).

  • Bioavailability: The amount of a drug available after an injection to produce a biological effect

  • Crossing the blood-brain barrier.

  • Crossing the placental barrier.

  • Passing through the blood-testicular barrier.


Two Methods of Elimination


Metabolism

The process or rate at which chemical reactions in the body take place. A variety of factors such as age, body size, gender, physical activity, health issues, genetics and hormonal factors can affect metabolism.


Excretion

Most substances that enter the body are eliminated by urination, exhalation, defecation or perspiration. Drugs are expelled from the body by the kidneys, respiratory tract, bile or by glandular activity.

  • Kidney Excretion: The kidney purifies the blood by removing harmful substances and then completes the task of excreting the unusable parts with the help of the urinary bladder.

  • Respiratory Excretion: Drugs are excreted by the lungs in the form of gaseous molecules. The rate of respiratory excretion is dependent on diffusion, gas composition, and blood flow.

  • Bile Excretion: Components of bile are circulated back to the liver; this process is referred to as enterohepatic recirculation. Recirculated drugs are then metabolized by the liver and excreted by the kidneys.

  • Gland Excretion: Drugs are excreted in the form of body fluids such as saliva, perspiration, and urine.


Image is courtesy of the Orlando Clinical Research Center (OCRC)


Pharmacodynamics


What is it?


The study of pharmacodynamics is the second of the two branches included in the scientific discipline of pharmacology. It is the quantitative investigation of the relationship between drug exposure and its pharmacologic and toxic effects. Pharmacodynamics encompasses the biochemical, physiologic, and molecular effects of drugs on the body and includes such concepts as receptor binding, postreceptor effects, and chemical interactions. Simply stated, pharmacodynamics is concerned with the impact of drugs on living organisms and the methods by which these effects are achieved.


The Difference between Pharmacokinetics and Pharmacodynamics


According to Elizabeth Dodds-Ashley, PharmD, MHS, from the Department of Medicine at Duke University, School of Medicine, “Pharmacokinetics is how the body processes a drug. So how the drug is absorbed, how it is distributed throughout the body, where it is distributed, how you metabolize the drug and then eliminate it. It is mostly focused on the drug and the body. Pharmacodynamics takes into account the complex interactions between the drug, the human body, and then the pathogen that might be causing an infection in the patient.” To summarize, pharmacokinetics is concerned with what the body does to a drug, whereas pharmacodynamics is concerned with what the drug does to the body.


References


Briscoe, C. (2020, October 05). The Difference between Pharmacokinetics and Pharmacodynamics. Retrieved November 06, 2020, from https://www.bioagilytix.com/blog/2020/10/05/the-difference-between-pharmacokinetics-and-pharmacodynamics/

Contagion Editorial Staff. (2016, June 30). What is the Difference between Pharmacokinetics and Pharmacodynamics? Retrieved November 06, 2020, from https://www.contagionlive.com/view/what-is-the-difference-between-pharmacokinetics-and-pharmacodynamics

Daymar College. (n.d.). Pharmaceutical Drug Administration & Pharmacokinetics Defined. Retrieved November 06, 2020, from https://www.daymarcollege.edu/blog/pharmaceutical-drug-administration-pharmacokinetics-defined

Diaz, D., Terézhalmy, G. (n.d.). General Principles of Pharmacology. Retrieved November 07, 2020, from https://www.dentalcare.com/en-us/professional-education/ce-courses/ce580/pharmacodynamic-mechanisms

Le, J. (October, 2020), Overview of Pharmacokinetics. Retrieved November 06, 2020, from https://www.merckmanuals.com/professional/clinical-pharmacology/pharmacokinetics/overview-of-pharmacokinetics

Orlando Clinical Research Center (OCRC). 2019, May 22). How Does The Body Metabolize Medication? Retrieved November 07, 2020, from https://ocrc.net/how-does-the-body-metabolize-medication/

Pharmacology Education Project. (n.d.). Clinical Pharmacokinetics. Retrieved November 06, 2020, from https://www.pharmacologyeducation.org/clinical-pharmacology/clinical-pharmacokinetics

Pharmacology Education Project. (n.d.). Pharmacodynamics. Retrieved November 07, 2020, from https://www.pharmacologyeducation.org/pharmacology/pharmacodynamics



Article Author: Risheena Banerji

Article Editors: Edie Whittington, Olivia Ye