Drug Terms + Equations Flashcards
Ligand
A substance that is bound to a protein
Affinity
The tendency (attraction) of a ligand to bind to its receptor
Drug
A chemical that affects physiological function in a specific way
Agonist
A ligand that binds to a receptor and alters receptor state, resulting in biological response
Mimics action of NT
Antagonist
A drug that reduces the action of another drug by binding to (and therefore blocking) its receptor
Partial agonist
An agonist that cannot elicit a large effect (hence partial)
Efficacy
The ability of an agonist to activate a receptor and evoke a response
Drug potency
An expression of the activity of a drug in terms of concentration/amount needed to have an effect
Drug A requires 0.2 conc to evoke a response compared to Drug B at 1.5 conc
Drug A more potent
Drug selectivity
The affinity a drug has for its intended target to the exclusion of other (‘off’) targets
Greater selectivity of target = greater likelihood drug will evoke intended action
Greater selectivity of ‘off’ targets = adverse effects
Dose
A specified quantity of drug administered once/at stated intervals
Can be relevant to body weight (mg/kg)
k+1
A + R ⇌ AR
k-1
A - ligand
R - receptor
k+1 - association (forward) rate constant
k-1 - disassociation (backward) rate constant
PAR = [A]/ KA + [A]
Lower KA = greater affinity of ligand for receptor
KA = disassociation equilibrium constant = conc of ligand required to occupy 50% of receptors
Competitive antagonism
Antagonist competes with agonist for the SAME binding site on receptor
Reversible, concentration-response curve shifted to right
Non-competitive antagonism
Antagonist binds to separate site on the receptor
Receptor only active when agonist is bound alone
Action is reversible or irreversible
Chemical antagonism
Antagonist combines in solution directly with the chemical being antagonised
(eg. Chelating agents, used to treat lead poisoning, bind to heavy metals and form a less toxic chelate)
Physiological antagonism
Two agonists that produce opposite physiological actions cancel each other out
Pharmacokinetic antagonism
The ‘antagonist’ reduces conc of active drug at its site of action
Pharmacodynamics
What a drug does to the body
Affinity, potency, efficacy
Pharmacokinetics
What the body does to a drug
Apparent volume of distribution, elimination half-life, clearance, bioavailability
What can a drug be eliminated through
Urine, faeces, milk, sweat, expired air
What method of drug administration avoids first pass metabolism
IV - straight into blood circulation
Emax
The concentration of a drug that gives 100% maximal response
EC50
The concentration of a drug that gives 50% maximal response
Absorption
Drug is absorbed from site of administration - entry into plasma
Distribution
Drug leaves bloodstream and is distributed into interstitial + intracellular fluids
Metabolism
Drug transformation by metabolism by liver
Excretion
Drug and/or drug metabolites excreted in urine, faeces or bile
Ways in which drugs can cross cell membranes
Passive diffusion
Facilitated diffusion
Active transport
Endocytosis
Passive diffusion
Drug moves directly through lipids in the cell membrane
Facilitated diffusion
Drug moves across cell membrane via carrier proteins - conformational change occurs
Doesn’t need ATP
Requires concentration gradient
Active transport
Drug moves across cell membrane via carrier proteins
Requires ATP
Moves against concentration gradient
Endocytosis
Invagination of part of the cell membrane. Drug is encased in a small vesicle then released inside the cell
Apparent volume of distribution (Vd)
The extent to which a drug partitions between the plasma and tissue compartments
Vd = dose/[drug]plasma
As Vd increases, drug plasma concentration …
Decreases
And vice versa
Metabolism in the liver …. the bioavailability of drugs when administered orally
Reduces
Bioavailability (F)
The amount of drug that eventually reaches systemic circulation
F = quantity of drug reaching systemic circulation/dose
100% for IV
Cmax and Tmax
The peak or maximum drug plasma concentration (Cmax), and the time when it occurs (Tmax)
Clearance (CL)
The volume of blood removed (or cleared) of drugs when administered per unit of time
CL= rate of drug elimination/[D]plasma
First order kinetics
Rate of drug elimination increases as plasma drug concentration increases
Vmax
Maximum rate of drug elimination
Km
Drug concentration at which rate of elimination is 1/2Vmax
[Drug]plasma
The drug concentration in plasma
Steady state
Rate of drug administration = rate of drug elimination
Input = output
Dosage rate =
[Drug]plasma X CL
t1/2 =
(0.693 X Vd)/CL
How many half lives are required to reach steady state
5
What is elimination half life (t1/2)
Determines how much time is required for drug to be eliminated from body
MLCK
Responsible for smooth muscle contraction
