Drugs Flashcards
What is absorption?
Process of movement of the unchanged drug from the site of administration to the systemic circulation
What is Cmax?
Drug concentration peak
What is Tmax?
The time at which Cmax us observed
What is bioavailability?
The amount of the drug available for action in the systemic circulation
What factors affect bioavailability?
Formulation
Ability of drug to pass physiological barriers
GI effects
First pass metabolism
What are some physiological barriers drugs encounter?
Lipid solubility
pH
Ionisation
How does ionisation affect drug diffusion?
An ionised drug will not cross the cell membrane
What is the Henderson-Hasselbalch equation used to show?
The relationship between local pH and the degree of ionisation
What is the Henderson-Hasselbalch equation?
pH=pKa+log10([A-]/[HA])
What is the lipid water partition coefficient?
The ratio of the amount of drug which dissolves in the lipid and water phase when they are in contact
How does lipid solubility affect diffusion across a membrane?
A drug must be lipid soluble to diffuse across a membrane
By what methods can drugs pass across the membrane?
Passive diffusion Filtration Bulk flow Active transport Facilitated diffusion Ion pair transport Endocytosis
What drugs cross the membrane via facilitated diffusion?
Monosaccharides, amino acids, vitamins
What is the driving force for filtration, bulk flow and pore transport?
The difference in hydrostatic or osmotic pressure
What gastrointestinal factors affect drug absorption?
Motility
Food
Illness
What is first pass metabolism?
Metabolism of drug prior to it reaching the systemic circulation
What are the 4 primary systems that affect first pass metabolism?
GI lumen
Gut wall enzymes
Bacterial enzymes
Hepatic enzymes
What are the benefits of subcutaneous and IM administration?
Only a small volume required
Avoids first pass metabolism
Water soluble drugs absorbed well
What are the benefits of buccal and sublingual administration?
Avoids first pass metabolism
Enter circulation directly
What are the benefits and disadvantages of suppositories?
Avoids first pass metabolism
Can be used for drugs that irritate stomach
Absorption tends to be slow
What are the benefits of inhalation of drugs?
Can be used for volatile agents
Relatively rapid action
Can be used for topical or systemic effects
What are the advantages and disadvantages of transdermal administration?
Avoids first pass metabolism
Controlled release
Few substances well absorbed
Needs to be nonirritant
What is drug distribution?
Reversible transfer of a drug between the blood and extravascular fluids and tissues of the body
What factor affect tissue distribution?
Plasma protein binding Tissue perfusion Membrane characteristics Transport mechanisms Diseases and other drugs Elimination
What is the effect of plasma protein binding?
Only unbound drugs are biologically active
What factors affect the amount of a drug that binds to plasma proteins?
Renal failure Hypoalbuminaemia Pregnancy Other drugs Saturability of binding
What is the apparent volume of distribution (Vd)?
Theoretical volume that would be necessary to contain the amount of the administered drug at the same concentration as it is in the plasma
What is the correlation between Vd and the ability of a drug to diffuse?
The higher the Vd, the better the drug is at diffusing
What is clearance?
The theoretical volume from which a set amount of a drug is completely removed over time
What is drug clearance dependent on?
Urine flow rate for renal clearance
Metabolism and biliary secretion for hepatic clearance
What is half life (t1/2)?
Time taken for the drug concentration in the blood to half
What is the half life dependent on?
Volume of distribution and rate of clearance
What does prolongation of the half life cause?
Toxicity
When is steady state reached in chronic administration?
Usually after approx 4 half lives
What is elimination?
Removal of the active drug and its metabolites from the body
What are the 3 stages of drug elimination?
Metabolism and excretion
Where does most of drug metabolism take place?
Liver
What is the primary organ of drug excretion?
Kidneys
What are the 3 principle mechanisms of drug excretion?
Glomerular filtration
Passive tubular reabsorption
Active tubular secretion