Chapter 1

Introduction to pharmacology

I. Definition of terms

Absorption the movement of a drug from its site of administration (oral, topical, or injection) into the systemic circulation (bloodstream).

Adverse drug event (ADE) injury resulting from the use of a drug. It is an unfavorable and unintentional response resulting from an administered medication. Includes medication errors such as miscalculation of dosage or ­misreading a prescription.

Adverse drug reaction (ADR) harm to the body due to a medication that was properly prescribed (e.g., drug taken at normal doses and the correct route of administration). Examples of an ADR are allergy to penicillin or adverse (side) effect of a drug.

Affinity the ability of a drug to bind to the receptor to cause a therapeutic response.

Antibiotic prophylaxis antibiotic given to prevent an infection.

Bioavailability the amount of a drug (expressed as a percentage) that reaches the systemic circulation. For example, any drug administered intravenously has a 100% bioavailability

Biologics agents that are naturally produced in an animal or human body.

Clearance quantitative measure of the rate of drug elimination from the body divided by the concentration.

Creatinine a waste product in skeletal muscle by the breakdown of creatinine phosphate.

Creatinine clearance (CrCl) a test that compares the level of creatinine in the urine with that of creatinine in the blood and it determines normal functioning of the kidneys.

Cytochrome P450 (CYP) enzymes found primarily in the liver that are responsible for the metabolism of many drugs. Many drug interactions occur because some drugs are inhibitors or inducers of the substrate (drug being metabolized) resulting in high or low blood levels of one or the other drug.

Distribution movement of a drug through the body to the various target tissues/organs (site of action) after it enters the bloodstream.

Dose the amount of drug taken at any one time.

Drug any substance which changes a physiological function or modifies a disease process.

Drug action the response of living matter to administered chemicals. Levels of drug action include cellular or ­molecular. Cellular site of drug action is defined as all foreign parts that enter the body, will react with at least one portion of the cell. The initial reaction occurs here. At the molecular level the molecules of the drug will react with the molecules of the body.

Efficacy the ability of a drug to stimulate the receptor and produce the maximum response achievable by the drug. Two drugs can have the same efficacy but different potencies where one drug is more potent (stronger strength) than the first drug but both will have the same effect.

Elimination half-life (t½ ): The time required to reduce the amount of drug in the body or concentration of drug in blood by 50%. However, once the first 50% is gone, it will take the body more time to clear 50% of the remaining medication. Usually it takes about 5 half-lives to clear 99% of the medication. To determine the time it takes for a drug to be 99% eliminated from the body multiply the half-life of the drug by 5.

First-pass effect (or first-pass metabolism) before an orally administered drug enters the systemic circulation it goes to the liver to be metabolized or biotransformed. Some oral drugs can undergo extensive first-pass effect that they are ineffective by the time of entering the bloodstream while other drugs undergo little first-pass effect and ­maintain the original efficacy. Drugs that undergo extensive first-pass effect cannot be given orally because it becomes pharmacologically ineffective by the time it enters the general circulation. Lidocaine is an example of a drug that cannot be given orally because it undergoes extensive first-pass effect.

First-order kinetics the rate of drug elimination decreases with time. That is, the rate if drug elimination falls as the concentration falls. Most drugs are removed from the body by first-order kinetics.

Loading dose (LD) an initial higher dose of a drug that may be given at the beginning of a course of treatment (to get initial quick plasma levels) before dropping down to a lower maintenance dose afterward. A loading dose is given on the first day of drug treatment.

Maintenance dose (MD) a lower drug dose allowing the dose that keeps the plasma drug concentration continuously within the therapeutic range. The maintenance dose is given starting after the loading dose on day 1 of drug therapy.

Metabolism (biotransformation) the primary mechanism of drug elimination from the body. Biotransformation will usually end the pharmacologic action of the drug.

Pharmacodynamics describes how the drug actually works; mechanism of action. How drug interact with receptors and what happens once the drug binds to the receptor.

Pharmacokinetics study of the action of drug once it is in the patient. It describes the absorption, distribution, ­metabolism and elimination of the drug from the body

Pharmacology is a Greek word defined as the science dealing with drugs and their interaction with the body’s components

Pharmacogenetics the convergence of pharmacology and genetics that deals with genetic factors that influence an organism’s response to a drug.

Pharmacognosy study of drugs derived from herbal and other natural sources.

Pharmacotherapeutics the medical use of drugs in the prevention, diagnosis, treatment of diseases.

Polypharmacy many different medications including over-the-counter (OTC) and prescription drugs are taken by the patient.

Potency strength of the drug.

Prodrug a drug that becomes active only after it is ingested and metabolized in the liver. Codeine is converted from an inactive form to the pharmacologically active form, morphine, by first pass metabolism.

Protein binding attachment of a drug to proteins in the plasma. Drugs that are protein bound are inactive and become active in the free unbound form.

Steady state The point at which the rate of input of drug into the body is equal to the rate of elimination. As such, the amount or concentration in the body reaches a plateau.

Therapeutics branch of medicine that deals with the treatment of disease.

Therapeutic index (TI) (therapeutic ratio) a measure of the relative safety of a drug. Therapeutic index is expressed as the ratio of the lethal or toxic dose (LD) to the therapeutic dose (TD). For example, lithium has a narrow therapeutic index so if the dose is just slightly more than the therapeutic range toxicity can occur. Patients must be on chronic lithium maintenance treatment to avoid toxicity. On the other hand, penicillin has a wide therapeutic index so that slightly more than the usually dose will not cause toxicity.

Therapeutic range the dosage range of a drug that achieves the desired pharmacologic response.

Toxicology study of poisons and poisonings.

Zero-order kinetics the drug is removed at a constant rate regardless of the drug concentration; it is linear with time. The elimination from the body of a large concentration of alcohol is an example of a drug that follows zero-order kinetics. (Weinberg, 2002; Gossel, 1998a, b).

II. Pharmacokinetics

Q. What is the definition of pharmacokinetics and why is it important to know?

A. Pharmacokinetics describes the actions of the drug as it moves through the body and how the body influences drug concentrations. It is easiest to remember pharmacokinetics by the acronym: ADME (A = absorption into the systemic circulation; D = distribution to the target tissues and organs; M = metabolism or biotransformation; E = elimination from the body). It is important to know the basics of pharmacokinetics in order to understand the basic principles of ­prescribing medications. Pharmacokinetics (e.g., absorption of the drug into the blood) may be altered when certain antibiotics prescribed in dentistry are taken with food. Instructions must be verbally expressed to the patient and ­documented in the patient’s chart on how to take medications that are prescribed by dentists (e.g., antibiotics, ­antimicrobial agents, analgesics, antifungal agents, antiviral agents, fluorides).

Q. What factors affect the rate of drug absorption?

A. In the gastrointestinal tract, many factors can influence the rate of drug absorption into the systemic circulation including acidity of the stomach and food in the stomach.

    Some medications used in dentistry should be taken with food to reduce gastrointestinal irritation, some medications should be taken on an empty stomach because the food could delay the absorption of the drug and some medications can be taken with or without food because food would not interfere with absorption. Usually the absorption of the total amount of drug is not reduced but rather it will just take longer to get absorbed. Usually antibiotics have the most restrictions regarding taking with meals. Nonsteroidal anti-inflammatory drugs such as ibuprofen must be taken with food to avoid gastric irritation. Specific drugs will be discussed within the chapters.

Q. What does “take on an empty stomach” mean?

A. Take on an empty stomach means to take the drug within 1 hour before eating or 2 hours after eating. Take on an empty stomach is not interpreted as not eating.

Q. What is the pharmacokinetics of an orally administered drug?

A. The pharmacokinetics of a drug administered orally such as penicillin VK is as follows (Weinberg, 2002; Gossel 1998a, b):

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