This is post number three of our six part post on Pharmacology. Our posts will focus on the following topics:
Part 1. Introduction
Part 2. Pharmacokinetics
Part 3. Pharmacodynamics
Part 4. Bioavailabilty
Part 5. “Free versus Bound Drug”
Part 6. Elucidating Pharmacodynamic Effect from an Analytical Chemistry Result

Pharmacodynamics is the study of the time course of drug effects. In other words, what the drug does to the body, good (therapeutic) or bad (toxic). Pharmacodynamic equations describe the relationships between the drug concentration-time profile and therapeutic, impairment and toxic effects.

Oversimplified, pharmacodynamics is the study of how a drug interacts with a receptors. Simply put, a receptor functions as a keyhole and the drug is like the key. If the right key finds the right keyhole, then it unlocks what is on the other side. In biochemistry and pharmacology, if the right drug finds the right receptor then it opens a neural path towards possible affect on the human being. There are different types of receptors, often referred to by the type of interaction they have with a drug or drug class, for example: opioid receptor, benzodiazepine receptor, nicotine receptor, etc.

In the central nervous system (CNS) which comprises the brain and all of the spinal nerves radiating from the spinal column, propagation of nerve transmission are facilitated at junctions called synapse (similar to a circuit breaker or fuse in an electrical system). The chemical that facilitates the nerve transmission at the synapse is termed a neurotransmitter. Some common neurotransmitters are acetylcholine (Ach), epinephrine (Adrenalin®), norepinephrine, serotonin, and dopamine.

• A neurotransmitter released from the pre-synaptic cleft has a specific shape to fit into a receptor site situated in the post-synaptic cleft and cause a pharmacological response such as a nerve impulse being sent. The neurotransmitter is similar to a substrate in an enzyme interaction. After attachment to a receptor site, a drug may either initiate a response or prevent a response from occurring. A drug must be a close “mimic” of the neurotransmitter.
• An agonist is a drug which produces a stimulation type response. The agonist is a very close mimic and “fits” with the receptor site and is thus able to initiate a response.
• An antagonist drug interacts with the receptor site and blocks or depresses the normal response for that receptor because it only partially fits the receptor site and can not produce an effect. However, it does block the site preventing any other agonist or the normal neurotransmitter from interacting with the receptor site.
• Source: http://www.elmhurst.edu/~chm/vchembook/660drugreceptor.html

One of the most important aspects of pharmacodynamics is to remember that everyone’s response to a drug is different. If I take 1 mg of Alprazolam (Xanax), I will have a different reaction than you will.

The causes of this variability in drug response is really multifaceted and perhaps can be best categorized into two broad, yet interconnected, categories: (1) sources of variability that are related to the biological system of the particular human being, and (2) sources of variability due to the administration of the drug itself to that particular human being.

Sources of variability that are related to the biological system of the particular human being:

1. Body size and body mass (volume of distribution)
2. Gender
3. Chronological age
4. Genetics-pharmacogenetics
5. General health of the person
6. Psychological aspects (e.g., placebo effect)

Sources of variability due to the administration of the drug itself to that particular human being:

1. Dosage
2. Potency of the drug.
3. How the drug is formulated (e.g., liquid, capsule, tablet, sustained release)
4. Route of ingestion (IV, oral, smoked, etc.)
5. Single dose (acute) versus maintenance (chronic) versus steady state
6. Physiological tolerance of the drug (i.e., drug allergy, drug resistance)
7. Interaction with other drugs and other substances that may be present in the body