Medical Pharmacology Topics
Drug administration triggers a change in homeostasis. Even in a disease state, for example high blood pressure, the disease state is the new homeostatic level (i.e. the elevated blood pressure value). After a drug is administered to correct the disease state, the body may attempt to counter the drug action. The processes the drug will undergo as the body tries to regain homeostasis will determine if the drug will have the desired effect.
The drug must first dissolve in body fluids (pharmaceutical phase), then be distributed to many tissues, and eliminated by either biotransformation and/or excretion (pharmacokinetic phase). Only a fraction of the administered dose will reach the site of action and be available to produce an effect. The fraction at the site of action is involved in drug receptor interactions, usually a specific interaction with another molecule, producing an effect (pharmacodynamic phase).
Drug Administration
Administration routes depend on how the drug can be administered (is the patient conscious, etc.), the site of action and the chemical properties of the drug. Absorption is fast after respiratory administration, making this route advantageous when a fast and controlled effect is needed (anesthesia, smoking). Sites of topical administration include the skin, ear canal and eyes. The effects of topical application are usually local, as high lipophylicity is required for systemic effects.
Enteral administration includes oral ingestion (PO), application to the oral mucosa, and rectal administration. A large amount of oral dose is usually lost before reaching the site of action, especially due to first-pass effect (discussed later).
Parenteral administration is made by subcutaneous (sub Q), intramuscular (IM) or intravenous (IV) injection. Intramuscular injection allows for larger volumes and less irritation than subcutaneous. Sub Q administration is convenient for self-administration and may be used to obtain either rapid onset or prolonged duration of drug action, depending on the chemical characteristics of the drug.Parenteral administration, especially intravenous, may lead to dangerous concentrations if the wrong dose is used, with no avenue to recall it as in oral administration.
Drugs may be administered as topical preparations, by injection or oraly. Injections and oral liquids may be either suspensions or solutions. Suspensions will be absorbed more slowly than solutions. In another form of injection, a drug pellet may be implanted sub Q for sustained release over long periods of time. Oral solids may be enteric-coated or susteined-release formulations. Enteric couated pills do not dissolve in the stomach but in the intestine. Sustained release capsules containe many very small pills with different coatings that late them dissolve at different rates.
Introduction to Pharmacokinetics
After a drug is absorbed from the site of administration into the blood, it reaches other tissues and distributes according to its solubility or binding affinity to different tissues, manly determined by the hydrophilicity vs. lipophilicity of the drug.
Some of the drug may be lost as it passes through the liver right after being absorbed in the intestine. This is known as as the "first-pass" effect: the removal of drug from the portal vein by the liver. Lipid-soluble drugs are more susceptible to first-pass metabolism.
Drug
is eliminated by both biotransformation (metabolism) and excretion. A fraction
of the drug will be metabolized, especially by the liver. Metabolites may be
active or inactive. Drug action is terminated mainly by excretion and biotransformation,
but redistribution is a major factor for some drugs.
In an idealized time-course curve of plasma concentration (for a single dose), the absorptive phase displays a sharp increase, reaching a peak plasma concentration before a rapid decline. The post-absorptive phase is characterized by a slower decline, reaching almost a steady-state concentration if not for excretion.
For a drug that is absorbed faster than it is eliminated, the absorptive phase of the curve will have a sharper slope and a higher peak than for a drug with slower absorption than elimination.
Continue
to "Absorption/Distribution" or take
a quiz: [Q1].
Back to Basics: Homeostasis and Tissue Types (Physiology)
Need more practice? Answer the review questions below (after sponsor).
1- What is the relationship between homeostasis and the effect of a drug?
2- List 6 things that may happen to a drug in the body before it can have an effect.
3- List the 4 main components of pharmacokinetics.
4- What is the first pass effect?
5- List the two components of drug elimination.
6- What is the main site of biotransformation?
7- List 3 ways drug action is terminated.
8- List 3 determinant of the administration route for a given drug.
9- What are the advantages of respiratory administration?
10- List 3 routes of topical drug administration.
11- What are the main characteristics of topical administration?
12- List 3 routes of enteral drug administration.
13- What is the main limitation of oral administration?
14- List 3 routes of parenteral drug administration.
15- What is the main advantage of intramuscular over subcutaneous administration?
16- What is the main disadvantage of parenteral over oral administration?
17- List the 3 main types of drug preparations and their subtypes if any.
18- What preparations forms are common to injections and oral preparations?
19- What is the main difference between suspensions and solutions.
20- What is the main advantage of sub Q drug pellets?
21- What is the purpose of an enteric-coated formulation?
22- What is the purpose of sustained release capsules?
23- Describe an idealized time-course curve of plasma concentration for a single dose.
Continue scrolling to answers below (after sponsor).
Hey! DON'T PEEK!!! Finish the questions fist!
1- What is the relationship
between homeostasis and the effect of a drug?
Drug administration triggers changes in homeostasis. The processes the drug
will undergo as the body tries to regain homeostasis will determine if the drug
will have its desired effect.
2- List 6 things that
may happen to a drug in the body before it can have an effect.
dissolves in body fluids
distribution to many tissues
biotransformation
excretion
reaches a site of action
interacts with a receptor
3- List the 4 main components
of pharmacokinetics.
absorption
distribution
biotransformation
excretion
4- What is the first
pass effect?
Lipid-soluble drugs are more susceptible be lost as they pass through the
liver by secretion into the bile or biotransformation, the removal of drug from
the portal vein.
5- List the two components
of drug elimination.
biotransformation
excretion
6- What is the main site
of biotransformation?
liver
7- List 3 ways drug action
is terminated.
Mainly by excretion or biotransformation, although redistribution is a major
factor for some drugs.
8- List 3 determinant
of the administration route for a given drug.
how can it be administered (is the patient conscious, etc.)
site of action
chemical properties of the drug
9- What are the advantages
of respiratory administration?
Absorption is faster and more controlled than other routes.
10- List 3 routes of
topical drug administration.
skin
ear canal
eyes
11- What are the main
characteristics of topical administration?
Effects are usually local, high lipophilicity is required for systemic effects.
12- List 3 routes of
enteral drug administration.
oral ingestion
application to the oral mucosa
rectal application
13- What is the main
limitation of oral administration?
A large amount of oral dose is usually lost before reaching the site of
action especially due to first-pass effect.
14- List 3 routes of
parenteral drug administration.
subcutaneous
intramuscular
intravenous
15- What is the main
advantage of intramuscular over subcutaneous administration?
Allows for larger volumes and less irritation than subcutaneous.
16- What is the main
disadvantage of parenteral over oral administration?
May lead to dangerous concentrations if the wrong dose is used, with no
avenue for recall as in oral administration (especially intravenous).
17- List the 3 main
types of drug preparations and their subtypes if any.
topical
injection: liquid
and sub Q pellet
oral: liquid and solid
18- What preparations
forms are common to injections and oral preparations?
Injections and oral liquids may be either suspensions or solutions.
19- What is the
main difference between suspensions and solutions.
Suspensions will be absorbed more slowly than solutions.
20- What is the
main advantage of sub Q drug pellets?
May be implanted for sustained release over long periods of time.
21- What is the
purpose of an enteric-coated formulation?
Enteric coated pills do not dissolve in the stomach but in the intestine.
22- What is the
purpose of sustained release capsules?
Sustained release capsules containe many very small pills with different coatings
that late them dissolve at different rates.
23- Describe an idealized
time-course curve of plasma concentration for a single dose.
The early part of the curve displays absorptive phase, reaching a peak plasma
concentration before a rapid decline. The later part of the curve, the post-absorptive
phase, displays a slower decline. Reaching almost steady-state concentration
if not for excretion.