Drug absorption Flashcards
Discuss the physico-chemical factors that affect the transfer of drugs across cell membranes.
Lipid solubility
Charge of the drug
Size of the drug
Discuss the factors that affect absorption of a drug from the GI tract
Motility - Speed of gastric absorption will affect speed at which drug reaches site of absorption (most drugs are absorbed in small intestine). Can be affected by other drugs, food/drink and illnesses (esp pain).
Food - Can enhance or impair rate of absorption.
Illness - Malabsorption (eg Coeliac disease) can increase or decrease rate of absorption. Migraine reduces rate of stomach emptying and therefore rate of absorption of analgesic drugs.
Discuss the medical importance of first pass metabolism.
If dose is too low, drug reaching systemic circulation may not reach therapeutic concentration in the blood.
Discuss the benefits of intravenous medication.
Bypasses first pass metabolism
100% bioavailability
Fast response
Discuss the benefits of transdermal medication.
- Avoids first pass metabolism.
* Can provide controlled release
Discuss the benefits of inhaled medication.
- Relatively rapid action.
- Better for volatile agents.
- Usually used for topical effect or to avoid problems of oral absorption (eg nausea).
What is pharmacokinetics?
Pharmacokinetics sometimes abbreviated as PK, is a branch of pharmacology dedicated to determining the fate of substances administered to a living organism
What does the study of pharmacokinetics allow us to do?
- Enables understanding of:
- Dosage
- Drug administration
- Drug handling
- Patient variability
- Potential for harm (toxicology)
Define absorption
Absorption is defined as the process of movement of unchanged drug from the site of administration to the systemic circulation.
Describe the different means of drug absorption
- Oral
- Intravenous
- Subcutaneous
- Intramuscular
- Other GI - Sublingual, rectal
- Inhalation
- Nasal
- Transdermal
What does the AUC represent?
The area under the drug concentration-time curve (AUC) represents total drug exposure over time and the drugs bioavailability
Define the therapeutic range of a drug
A drug is active and therapeutically effective over a range of concentrations
Below this there will be insufficient or no pharmacological action.
Above this toxicity occurs
Define bioavailability of a drug
Bioavailability – the fraction of an administered dose of unchanged drug that reaches the systemic circulation (AUC)
What is the bioavailibity of intravenous drugs?
100%
What factors affect bioavailability of oral drugs?
- Formulation
- Ability of drug to pass physiological barriers
- Gastrointestinal effects e.g. motility, food, illness
- First pass metabolism
What is ion trapping?
An acidic drug will be more concentrated in the compartment with high pH (‘ion trapping’)
What does the Henderson-Hasselbalch equation describe?
In chemistry, the Henderson–Hasselbalch equation describes the derivation of pH as a measure of acidity (using pKa, the negative log of the acid dissociation constant) in biological and chemical systems.
What factors can affect diffusion of a drug?
Ionization/change in pH
Lipid solubility
Name a drug that is actively transported
Levadopa by brain for Parkinson’s.
What is first pass metabolism?
Metabolism of drug prior to reaching systemic circulation involving the intestinal and hepatic degradation or alteration of a drug or substance taken by mouth, after absorption, removing some of the active substance from the blood before it enters the general circulation.
What parts of the body contribute to first pass metabolism?
- Gut lumen (acid, enzymes)
- Gut wall (metabolic enzymes)
- Liver (hepatic enzymes)
What factors affect the viability of intramuscular and subcutaneous administration?
Depends on blood flow to site.
What are the benefits of intramuscular and subcutaneous administration?
Bypasses first pass metabolism
What are the benefits of sublingual/buccal administration?
Bypasses first pass metabolism
