40 Questions
What is the main purpose of Phase II metabolism reaction?
To add an endogenous water soluble group to the drug
What is the factor that determines the bioavailability of a drug?
Particle size of the drug
What is the primary reason why water soluble drugs cannot filter into the CSF or brain?
Due to the blood-brain barrier
What is the term for the process by which a drug is converted into a more water soluble product?
Biotransformation
What is the equation for calculating the volume of distribution of a drug?
Vd = Cp / t
What type of reaction occurs in Phase I metabolism?
Non-synthetic reaction
What is the purpose of glucuronidation in Phase II metabolism?
To add a water soluble group to the drug
What is the term for the process by which a drug is absorbed into the systemic circulation?
Absorption
What is the process that occurs in the intestinal lumen and intestinal wall during first pass effect?
Intestinal first pass effect
Which of the following is an example of a drug that is affected by enzyme induction by rifampine?
All of the above
What happens to the first pass effect in the presence of hepatic diseases?
It decreases
Which of the following is an example of a drug that is affected by enzyme induction by barbiturates?
All of the above
What is the result of saturation of metabolizing enzymes on the first pass effect?
It decreases
Which of the following is an example of a conjugation reaction in biotransformation?
All of the above
What is the result of enzyme induction on drug efficacy and plasma half-life?
It significantly reduces drug efficacy and plasma half-life
Which of the following is an example of an enzyme inducer that affects the metabolism of benzodiazepines and paracetamol?
Cigarettes
Which of the following enzymes is induced by cimetidine?
Hepatic microsomal enzyme
Which of the following drugs is a prodrug that is converted to its active metabolite?
Prednisone
What is the main mechanism of drug elimination by the kidney?
Tubular secretion
Which of the following enzymes is responsible for the metabolism of sulphonamides?
Acetylase enzyme
What happens to the biological activity of a drug during metabolism?
Biologically active to biologically inactive
Which of the following factors affects drug metabolism?
Age
What is the route of excretion of lithium?
Renal
Which of the following enzymes is inhibited by erythromycin?
Hepatic microsomal enzyme
Which of the following drugs does not follow a single compartment distribution model?
Phenytoin
What is the characteristic of 'hit and run' drugs in terms of their half-life?
Their half-life is not indicative of their duration of action and dosage schedule
What is the time required to reach the steady-state level for most drugs that follow first-order kinetics?
5 half-lives
What is the factor that determines the steady-state level of a drug?
Dose and dose interval of the drug
What is the primary consideration in designing a dosage regimen for a drug with a short plasma half-life?
Giving smaller doses at shorter intervals
Which of the following is a characteristic of a multi-compartment distribution model?
Drugs undergo an early distribution phase followed by a slower elimination phase
What is the primary reason why the half-life of a drug is not always indicative of its duration of action and dosage schedule?
All of the above
What is the term for the level of a drug reached when its intake is equal to its elimination?
Steady-state level
What is the effect of plasma protein-binding on the rate of renal excretion of drugs?
Decreases excretion
Which of the following routes of excretion is responsible for the elimination of volatile gases and gaseous anesthetics?
Lungs
What is the total clearance of a drug equivalent to?
Cl renal + Cl liver + Cl others
Which of the following drugs has a clearance less than the glomerular filtration rate?
Aminoglycosides
What is the characteristic of first-order kinetics?
Non-saturable; log plasma concentration-time curve is linear
Which of the following factors determines the duration of the half-life of a drug?
Rate of clearance and distribution
What is the effect of highly distributed and sequestrated drugs in tissue on the half-life?
Increases the half-life
Which of the following drugs follows zero-order kinetics?
Alcohol, phenytoin, salicylates
Study Notes
Pharmacokinetics
- Bioavailability: The fraction of the administered dose that reaches the systemic circulation, defined as unity (or 100%) in the case of intravenous administration.
-
Factors determining bioavailability:
- Biological factors: GIT degradation, food and other drugs, absorption, first pass metabolism, entero-hepatic circulation.
- Pharmaceutical factors: particle size, disintegration agents, type of excipient.
Distribution
- Volume of Distribution (Vd): The volume of distribution is calculated by dividing the dose of the drug by the estimated plasma concentration per liter at the time of plasma-tissue equilibrium after i.v. injection.
- Blood-brain and CSF barriers: Water-soluble drugs cannot filter into CSF or brain, crossing occurs either by diffusion or carrier-mediated transport.
- Placental barrier: Important for drug distribution in pregnant women.
- Plasma protein binding: Important for drug distribution and elimination.
Metabolism (Biotransformation)
- Metabolic processes: Convert lipid-soluble drugs into water-soluble products, which are easily eliminated either in the urine or through the bile.
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Types of chemical reactions:
- Phase I reaction: Non-synthetic, occur by microsomal enzymes, including oxidation, reduction, and hydrolysis.
- Phase II reaction: Synthetic, involving the addition of an endogenous water-soluble group (conjugation) by transferases enzymes.
- First pass effect: Pre-systemic elimination, occurs only with oral administration, includes intestinal and hepatic first pass effects.
- Enzyme induction: Increases the metabolism of drugs, leading to decreased efficacy and plasma half-life.
- Enzyme inhibition: Decreases the metabolism of drugs, leading to increased efficacy and plasma half-life.
Elimination
- Rout of excretion: Renal, liver, lungs, gastro-intestinal tract, breast milk, and miscellaneous.
- Clearance: The volume of plasma cleared of the drug in a unit of time, total clearance is the sum of renal, liver, and other clearances.
- Kinetics of clearance: First-order kinetics (non-saturable) and zero-order kinetics (saturable).
- Half-life (t ½): The time taken for the circulatory plasma concentration to fall by 50%, determined by the rate of clearance, distribution, and storage, and plasma protein binding.
Pharmacokinetic Models
- Multi-compartment distribution: Many drugs undergo an early distribution phase followed by a slower elimination phase, modeled by a "two compartment model".
- Single compartment distribution: A few drugs may behave as if they are distributed to only one compartment.
Steady State (SS) Level
- Steady state level: When the intake of a drug is equal to its elimination, reached after 5 half-lives (t ½) in most drugs that follow first-order kinetics.
- Significance of steady state level: The time to reach SS level is a function of t ½, and the SS level reached is a function of the dose and dose interval.
Dosage Regimen
- Variables to consider: Amount of single dose, route of administration, dose interval, and total duration of therapy.
- Types of treatment: Single dose treatment (e.g., hypnotics, analgesics, purgatives) and multiple dosing (better to give smaller doses at shorter intervals).
- Variants in dosage schedule: Drugs with short plasma t ½ (e.g., dopamine) require different dosage schedules.
This quiz covers the essential concepts of pharmacokinetics, including bioavailability, oral plasma concentration-time curve, and factors that determine bioavailability of a drug.
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