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Principles of Drug Action (2)
Terms in this set (63)
Time course of drug action
Relationship between the efficacy of an individual drug dose(s) and the time that passes by after administration of the drug.
Stages of Time Course of Drug Action
1- Onset of Drug Action
2- Time of Peak Effect
3- Duration of Action
4- Drug Half-Life (Elimination Half Life)
Onset of Drug Action
Time it takes after the drug is administered to reach an effective plasma or tissue concentration that is capable of a minimal clinical response.
Time to Peak Effect
Time it takes after a drug is administered to reach a plasma or tissue concentration capable of producing it's maximal clinical response.
Peak Plasma Level
Highest plasma level achieved by the administration of a single dose of the drug.
Trough Plasma Level
Drug concentration in the body is the lowest.
Duration of Action
Total time that a drug is present in the plasma or tissues for it to produce any clinical effects (minimal or maximal).
Drug Half-Life (or elimination half-life)
Time required for the amount of drug in the body to decrease by 50%.
Drug Elimination Half-Life Example
Amount of drug in the body initially = 100%.
Amount remaining after 4 hours = 50%.
Amount remaining after 8 hours = 25%.
Amount remaining after 12 hours = 12.5%.
Amount remaining after 16 hours = 6.25%.
Drug Dosage Depends On:
1- Dose response relationship.
2- Drug Therapeutic Index.
3- Plasma Level Profile.
4- Drug Half-Life or elimination half life.
Dose Response Curve
Determines the minimum amount of drug that we can use, the maximum response that a drug can produce, and how much we need to increase the dosage to produce the desired increase in response.
Therapeutic Plasma Level Range
Associated with drug efficacy and minimal toxicity.
- Doses below the established dose range do not produce meaningful pharmacologic responses.
- Doses above this range often do not produce additional pharmacologic change and/or may be associated with other unwanted effects.
Provides a quantitative measure of the relative safety of a drug. Represents a ratio between the lethal dose and the effective dose.
- The closer the ratio is to 1, the greater is the danger involved in administering that drug to human beings. The higher the Therapeutic Index, the safter the drug is likely to be.
Therapeutic Index Calculation
Median Lethal Dose LD50
Median Effective Dose ED 50
Plasma Level Profile
Demonstrates the relationship between the plasma drug concentration and the level of therapeutic effectiveness over time.
Plasma level profile terms.
A drug with short half life (2-3 hours) needs to be administered more often than one with a long half life (12 hours).
The drug half life does not change with the size of the drug dose.
Some drug's half life is only minutes, others can be more than a week.
Lessining of syptoms.
Reduction of morbidity.
Prevention of mortality.
Altering the course of a disease process.
Improving a sense of wellbeing.
Improving a patient's functional status.
Usually predictable and unavoidable secondary effects produced by the drug at the usual therapeutic doses. The intensity is usually dose-dependent.
Unintended, undesirable, and often unpredictable drug effects. Every medication has a potential for causing harm.
Predictable factors that can cause Adverse Drug Effects
Time of administration.
Pathologic state, psychological factors.
Types of Adverse Drug Reactions
1- Toxic Reaction.
2- Allergic Reaction.
5- Cumulative Effect.
6- Idiosyncrasy or paradoxic response.]
7- Drug Dependence.
8- Teratogenic Reaction.
9- Mutagenic Reaction.
10- Carcinogenic Effect.
The effects of one drug are modified by prior or concurrent administration of another drug, this increasing the pharmacologic action of each other.
This can be beneficial (Probenecid and PCN) or detrimental (Sulfas and anticoagulants = harmful bleeding).
You can use the two to help or assist.
Is a measureable modification of the magnitude or duration of one drug caused by the prior administration, current administration, or the withdrawl of another drug.
Overall effects can be:
Increase in the repsonse or clinical effects of one or both drugs. This can be beneficial or harmful. There is a summation effect which can be beneficial (aspirin and codeine better pain relief) or harmful (diazepam and morphine have additive CNS depression).
Decrease in the response or clinical effects of one or both drugs. This can be beneficial or harmful.
Can be beneficial in that naloxone reverses respiratory overdose effects of meperidine.
The combination of effects of two drugs is less than the sum of the drugs acting separately.
Types of Drug Interactions:
1- Direct physical or chemical interaction: Outside the patient.
2- Pharmacodynamic drug-drug interaction: Potentiative or Inhibitory.
3- Pharmacokinetic drug-drug interaction: Absorption, distribution, metabolism, or excretion.
In most cases, the time course of drug action is directly related to what?
The concentration of a drug in the blood.
If the half-life of Morphine is 3 hours, this means that the body's storage of Morphine decreases by how much?
Decreases by 50% every 3 hours.
The half life of a drug determines what?
The dosing intervals.
After one dose is administered, the time course of the amount of drug in the body depends on its rate of absorption, distribution, metabolism, and excretion.
Before a dose.
Amount of drug going into the body equals the amount leaving the body.
Young children have incomplete metabolic and excretory mechanisms (immature hepatic and renals systems, as well as decreased liver metabolism), and older adults have different responses to drug therapy (decline in hepatic and renal function). Older adults usually have low nutritional health also.
The average adult dosage is calculated on the basis of the drug quantity that produces a particular effect in 50% of persons who are between the ages of 18 and 65 years and weigh approximately 150 pounds (70 kg). With children, drug dosage is determined on the basis of amount of drug per kilogram of body weight or body surface area.
Women are usually smaller than men, which may lead to higher drug concentrations in women. There are also differences in relative proportions of fat and water.
Sensory deprivation and sensory overload may also affect repsonses to drugs. Temperature can also affect drug activity (vasodilation, or vasoconstriction). Oxygen deprivation (altitude) may increase sensitivity to drugs.
Time of Administration
Absorbed more rapidly if GI is free of food. Circadian rhythm, sleep-wake rhythm may contribute to the effective, ineffective, adverse, or toxic response to particular drugs.
Hepatic, renal dysfunction, or disease at the cellular level.
Can cause abnormal response to a drug.
Some lack the enzyme needed to break down a drug.
Individual's symbolic investment in the drug and faith in its effects strongly influence and usually potentiate drug effects. "Placebo Effect".
Often considered a drug poisoning caused by either a quantitative or a qualitative (excessively sensitive) overdose of the drug.
The drug activates an immune response because of a prior recognition by the patient's immune system of the drug as a foreign substance.
Decreases physiologic response that occurs after repeated administration of a drug or chemically related substance. Requires an increase in dosage to maintain a given therapeutic effect. A cross tolerance to pharmacologically similar drugs may also develop.
Fast developing tolerance that occurs after repeated administration of a drug. Rapid onset, patient's initial response to the drug cannot be reproduced even with larger doses of the drug. Example: CNS stimulants.
The body cannot metabolize one dose of a drug before another dose is administered. Each new dose adds more to the total quantity in the blood and organs than is lost in the same amount of time by excretion: toixic effects.
Idiosyncrasy or Paradoxic Response
Abnormal or peculiar response to a drug which can be overresponse or abnormal susceptibility to a drug, underresponse with abnomral tolerance, excitation after the administration of a sedative, unexplainable symptoms. The etiology is probably genetic enzymatic deficiencies, which lead to abnormal drug metabolism. This is the drug doing the opposite of what you think that it will do. Example: Benadryl. Sometimes it can be from unexplained physiologic makeup.
Instead of "habituation" or "addiction". Can be physiological or psychological.
A state of physiologic drug adaptation that manifests itself by intense physical disturbance when the drug is withdrawn. This is like taking alcohol away from an alcoholic. They will have withdrawls.
A state of emotional reliance on a drug to maintain an effect. The patient would take it for the feeling or the state of well being. If you take it away, you will not have the same effect.
Causes malformations in the developing organ system of a fetus when given to a pregnancy woman during the first trimester of pregnancy. This takes place after conception. The women can get this through the medications.
This happens in the organ or system usually in the first trimester (12 weeks).
You can have dependence as both what?
Therapeutic or Non-Therapeutic.
Induces alteration in the chromosomes of sperm or egg cells of an adult who can then pass these defects along to his/her offspring. Development of eggs or "Spermatogenesis".
Induces transformation of benign cells into malignant.
Addition of additive effect of the combined effect of two drugs produces a result that equals the sum of the individual effect of each agent. 1+1=2. The combination allows the administration of a lower dose of each drug with a resultant decrease in adverse effects (aspirin and codeine).
The combied effect of drugs is greater than the sum of each individual agent acting independently. 1+1=3 or mroe (e.g. use of combination of drugs to treat hypertension).
Concurrent administration of 2 drugs in which one drug increases the effect of the other drug.
Direct Physical or Chemical Interaction
Outside the body.
Occurs outside the patient, for example syringe, IV bottle or IV tubing. This is when looking at IV, or IM.
Pharmacodynamic Drug-Drug Interactions
The pharmacodynamics of one drug influences the action of a second concurrently administered drug. Can be potentiative or inhibitory.
Beneficial (potentiative) long acting Beta 2 agonist bronchodilator for asthma as prophylaxis together with an additional short-acting Beta 2 agonist used for acute intervening bronchospasm. The combination provides better long-term control of bronchospasm in asthmatics.
Harmful (Inhibitory) concurrent administration of flurazepam a CNS depressant for sleep together with theophylline, a CNS stimulant and bronchodilator. They cancel out each others effects.
Pharmacokinetic Drug-Drug Interactions
The pharmacokinetics (absorption, distribution, metabolism, or excretion) of one drug alters the effects of another drug or drugs by altering its pharmacokinetics. They occur between drugs as they are being processed by the body. These interactions can be potentiative or inhibitory and beneficial or harmful.
- Alteration of gastric pH.
- Alteration of gastric emptying time.
- Alteration of GI motility.
- Presence of "interfering" substances in the GI tract.
(Diarrhea will eliminate drug faster; If vomiting, then do not give anythign orally).
Competition for protein binding sites: They can "knock" each other off the albumin thus increasing their free drug plasma levels and increasing the pharmacologic activity.
Alteration of extracellular pH: They alter the degree of ionization (and the water/fat solubility) of other drugs and affect their movements across cell membranes and between body compartments. Example: Sodium bicarb.
Hepatic enzyme induction or reduction:
Drugs which can increase the rate of synthesis of hepatic drug-metabolizing enzymes will enhance their own metabolism and reduce the clinical effects. Example: Cigarettes, barbiturates, phenytoin, rifampin, chronic etanol ingestion.
Drugs which can decrease the rate of synthesis of hepatic drug-metabolizing enzymes will inhibit their own metabolism and increase the clinical effects. Example: Cimetidine, oral contraceptives, sulfonamides, erythromycin, and some antifungals.
Increased renal blood flow.
Competition for renal tubule excretion ports.
Alteration of the pH in the urine; less ionized drugs (and more fat soluble) at a given urinary pH will be more likely to be reabsorbed from the urine back into their blood in the distal renale tubule, decreasing their excretion of the formation and flow of urine: high urinary flow rates will increase the excretion of drugs, decreasing clinical effects.
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