Pharmacokinetics Flashcards
What is Pharmacokinetics
What the body does to the drug
Concepts of Pharmacokinetics a drug has to be
Name 5
Available in a suitable form; Administered by an appropriate route; Absorbed and distributed – effect; Metabolised; and then Excreted
Name 4 stages of a drug in the body
Absorption
Distribution
Metabolism
Excretion
Name the two factors of absorption
Physiological
Physico-chemical
Describe the Physiological factors of absorption
Time of arrival and contact time at the absorption site.
Total surface area for absorption
Blood flow to the absorbing site
Describe the Physio-chemical factors of absorption
Solubility- how soluble the drug is in body fluids
Chemical stability- will it break down readily
Lipid to water partition coefficient- is it more fat-soluble than water-soluble
Degree of ionisation- chemical charge
Name the factors affecting the distribution
blood flow,
tissue mass
partition coefficient between blood and tissues
Describe protein binding
Plasma proteins such as albumin can bind drug molecules. Drugs bound to plasma proteins are pharmacologically inert, only free drugs are active.
Name a drug that can be displaced from plasma binding by another drug
Warfarin can be displaced by aspirin
What must a drug be to cross the blood-brain barrier
Highly lipid-soluble (lipophilic)
How many phases of metabolism are there?
There are 2
First pass (Oxidation)
Second pass (Conjugation)
Describe First Pass Metabolism
The drug is absorbed from the GI tract and passes via the portal vein into the liver where some drugs are metabolised. Sometimes the result of first pass metabolism means that only a proportion of the drug reaches the circulation.
First pass metabolism can occur in the gut and the liver. For example, first pass metabolism occurs in the gut for benzylpenicillin and insulin and in the liver for propranolol, lignocane, chloromethiasole and GTN.
Describe Second phase Metabolism
The reactive metabolite is conjugated with another substance which is more water-soluble to allow it to be excreted
Describe renal excretion
Glomerular filtration;
Active secretion of ionised drugs into the proximal tubule;
In the distal tubule there is passive absorption and excretion of unionised drugs according to a concentration gradient
What is time to onset?
The time it takes for a drug to reach its site of action at a concentration necessary for it to work
What is duration of effect?
The time interval where the concentration of drug at its site of action is above its therapeutic concentration – may be prolonged in the elderly or people with poor liver or kidney function
What is First Pass metabolism?
when a drug is completely destroyed in the stomach before being absorbed, or when it is metabolised by the liver before reaching the systemic circulation
What is half-life?
the time it takes for the peak concentration of a drug in the plasma to reduce by half
What is steady-state?
when drug going into the body = drug going out, achieved after 5 half-lives
What is clearance?
a measurement of a person’s ability to metabolise and excrete a drug, or how quickly a drug is ‘cleared’ from the blood, affected by renal function, blood pressure and urine pH
What is bioavailabily?
the fraction or %age of an administered drug that reaches the systemic circulation after absorption unless given IV where it is 100% of the administered dose
Define polypharmacy
A patient on FOUR or more medications
What is volume of distribution?
that volume of plasma in which the total amount of drug in the body would need to be dissolved in order to reflect the drug concentration in the plasma
Describe pharmacokinetic drug interactions
occurs when one drug alters the absorption, protein binding, metabolism or excretion of another leading to altered bioavailability of the drug to have its pharmacological effect.
