Pharmacokinetics and Absorption

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Questions and Answers

Define pharmacokinetics.

Pharmacokinetics describes the movement of a drug in the body, focusing on how the body acts on the drug.

What are the main components of ADME in pharmacokinetics?

Absorption (correct)

Excretion (correct)

Distribution (correct)

Metabolism (correct)

Match the following drug administration routes with their examples:

Oral = Convenient self-administered route Injectable = Parenteral route Transdermal = Administration through the skin Inhalation = Administration through breathing

Lipid diffusion is a primary mode of movement for drugs in the body.

True Signup and view all the answers

What is bioavailability in pharmacokinetics?

Bioavailability is the quantity of a drug reaching systemic circulation divided by the quantity administered. Signup and view all the answers

Explain the concept of prodrug.

A prodrug is an inactive form of a drug that relies on the body's metabolism to be activated. Signup and view all the answers

What are the advantages of bypassing limitations of oral administration?

Bypass limitations include GI absorption and first pass effect. Signup and view all the answers

Which route of administration provides immediate onset of drug action?

Intravenous Signup and view all the answers

Drugs administered through mucous membranes avoid the first pass effect. (True/False)

True Signup and view all the answers

In biotransformation, Phase II reactions involve the conjugation of drugs to create highly polar ________________.

conjugates Signup and view all the answers

Match the drug metabolism factors with their corresponding influences: Genetics, Age and Sex hormones, Diet and Environment, Disease

Polymorphisms: 2D6, 2C19, 2C9, and Phase II (slow acetylators) = Genetics Newborns lack Glucuronosyltransferase – Chloramphenicol à Gray baby syndrome = Age and Sex hormones Grapefruit, BBQ, lead, supplements = Diet and Environment Liver, cardiac, thyroid, kidney = Disease Signup and view all the answers

What is the primary mechanism that limits reabsorption of drugs in the renal tubules?

pH trapping Signup and view all the answers

What is enterohepatic recycling in drug excretion?

It is the process where drugs reenter the bloodstream from the intestines and are carried back to the liver for reprocessing. Signup and view all the answers

Breast milk being more acidic than plasma may lead to concentration of basic compounds in it.

True Signup and view all the answers

What is the main route for the excretion of volatile materials like nitrous oxide?

Respiration Signup and view all the answers

What does the term 'elimination' refer to in pharmacokinetics?

The process of removal of an administered drug from the body Signup and view all the answers

For a drug administered at steady state, what does [Csteady state] represent in terms of dosing calculation?

Steady-state drug concentration in plasma Signup and view all the answers

Study Notes

Pharmacokinetics

Definition: Describes the movement of a drug in the body (how the body acts on the drug)
ADME: Absorption, Distribution, Metabolism, and Excretion
Importance: The body's actions on the drug affect the drug's efficacy and toxicity

Absorption

Definition: Process of a substance entering the blood circulation
Factors affecting absorption:
- Physical state of the drug
- Water/lipid solubility
- Particle size
- Ionization of the drug
- Disintegration time of the drug
Modes of movement:
- Aqueous diffusion
- Special carriers
- Endocytosis and exocytosis
- Lipid diffusion (most important)
Weak acids and weak bases: only non-ionized forms can diffuse through the lipid membrane

Distribution

Definition: Dispersion or dissemination of substances throughout the fluids and tissues of the body (to the site of action)
Factors affecting distribution:
- Blood flow
- Plasma protein binding
- Lipid solubility
- pH
Volume of distribution (Vd): The apparent volume of fluid that the drug is distributed in
Importance of distribution: To produce a characteristic effect, a drug must present at an appropriate concentration at its intended site of action

Routes of Administration

Enteral (oral, sublingual, rectal)
Parenteral (injectable, transdermal, inhalation)
Advantages and disadvantages of each route

First Pass Metabolism

Definition: Drug passes through the liver where it is metabolized (inactivated) and/or excreted into the bile
Importance: Affects bioavailability and efficacy of the drug

Prodrug

Definition: A drug that is inactive in its administered form and must be converted to an active form in the body
Importance: Takes advantage of first-pass effect to activate the drug

Metabolism

Definition: Biotransformation, may change the substance to a more active form or inactive form
Phase I: Addition of a polar functional group to prepare for conjugation
Phase II: Conjugation or hydrolysis reactions to make the drug more readily excreted
Cytochrome P450 (CYP) enzymes: important for Phase I metabolism
Induction and inhibition: effects of other substances on CYP enzymes

Excretion

Definition: Removal from the body
Renal excretion: major route of excretion
Biliary excretion: minor route of excretion
Other routes: respiration, breast milk, sweat, saliva, and tears

Elimination Kinetics

Definition: The sum of the pharmacokinetics that contribute to the removal of an administered drug
Importance: Determines the duration of the drug's effect### Clinical Pharmacokinetics
Formula to calculate half-life (t½): t½ = (0.693 x Vd)/Clearance
Formula to calculate steady-state concentration [Csteady state]: [Csteady state] = (Bioavailability x Dose)/(Dosing interval x Clearance)
Formula to calculate doseloading: Doseloading = Vd x [Csteady state]
Formula to calculate dose maintenance: Dosemaintenance = Clearance x [Csteady state]
Formula to calculate clearance: Clearance = (0.693 x Vd)/t½
Formula to calculate volume of distribution (Vd): Vd = (t½ x Clearance)/0.693
Alternative formula to calculate Vd: Vd = Dose/[Plasma]

Case Scenario 1: Adverse Drug Reaction

A 58-year-old patient presents with dizziness, nausea, vomiting, and palpitations
The patient is prescribed medication for hypertension
Labs reveal elevated BUN and serum creatinine, indicating Acute Kidney Injury (AKI)
The most likely explanation is a drug-drug interaction affecting kidney function

Case Scenario 2: Drug Interaction with Oral Contraceptives

A 27-year-old female presents with abdominal pain, nausea, and vomiting
The patient is prescribed oral contraceptives and has been taking St. John's Wort supplements
Urine dipstick for human chorionic gonadotropin (HCG) is positive, indicating unintended pregnancy
The best explanation is that St. John's Wort induces CYP3A, which accelerates the metabolism of oral contraceptives, reducing their effectiveness

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Description

Learn about pharmacokinetics, the movement of a drug in the body, and absorption, the process of a substance entering the blood circulation. Understand the factors affecting absorption and its importance in drug efficacy and toxicity.