L3: Drug absorption Flashcards
(25 cards)
What are the different routes of drug administration?
Oral
Sublingual
Rectal,
Application to epithelial surfaces (e.g. skin, nasal mucosa)
Inhalation,
Injection (subcutaneous, intramuscular, intravenous,
intrathecal)
What determines drug absorption from the intestine?
Passive transfer rate governed by ionisation and lipid solubility
What are the main factors affecting GI absorption?
GI motility
Splanchnic blood flow
Particle size and formulation Physicochemical properties (ionisation, pKa, LogKow)
How does GI motility affect absorption?
Decreased motility (e.g. gastric stasis) reduces absorption
GI diseases and coeliac disease also reduce absorption
Some drugs increase motility (e.g. metoclopramide)
Food generally decreases drug absorption.
Name 3 drug-drug or food interactions that reduce absorption.
Tetracycline + Ca²⁺ (milk), Cholestyramine + warfarin/thyroxine, Milk + antibiotics
Name drugs that are designed to remain in the GIT.
Vancomycin (C. difficile), Mesalazine, Olsalazine
What is first-pass metabolism?
Metabolism of a drug before it reaches systemic circulation
What are the sites of presystemic metabolism?
Gut wall (e.g. MAO), Liver (Phase I & Phase II metabolism)
Which vein is key for liver perfusion?
Portal vein (provides ~80% of perfusion)
What happens in liver metabolism?
Phase I (oxidation, reduction, hydrolysis)
Phase II (conjugation with water-soluble groups like glucuronide, sulfate)
Name drugs with high hepatic extraction.
Morphine, Glyceryl trinitrate, Propranolol, Ergotamine
Name drugs with intermediate hepatic extraction.
Aspirin, Codeine, Nortriptyline
Name drugs with low hepatic extraction.
Diazepam, Paracetamol, Theophylline, Warfarin
What is systemic availability (bioavailability)?
The amount of drug that reaches systemic circulation intact
What determines rate and extent of bioavailability?
Rate: pharmaceutical formulation and GI factors
Extent: absorption and presystemic metabolism
Define Cmax, Tmax, and AUC.
Cmax = peak plasma concentration; Tmax = time to reach peak; AUC = total systemic exposure over time
What are the benefits of transdermal delivery?
Avoids GI tract and first-pass metabolism,
steady plasma levels
reduced dosing frequency
improved compliance
fewer side effects
What are the limitations of transdermal delivery?
Variable absorption
incorrect patch placement
slow onset
limited to lipophilic drugs
Describe the scopolamine transdermal system.
Drug diffuses from a patch along a concentration gradient; constant rate is maintained as long as reservoir has excess drug
Name four drugs delivered via transdermal patch.
Nitroglycerin (Transderm-Nitro), Clonidine (Catapres TTS), Estradiol (Estraderm), Nicotine (Nicorette)
A lipophilic weak base with high first-pass metabolism is administered orally. What is the most effective way to increase its systemic bioavailability?
A) Increase dose frequency
B) Deliver via IV
C) Use a coated oral tablet
D) Co-administer with bile acid-binding resin
B) Deliver via IV
Which of the following would most likely increase the plasma levels of a drug like warfarin?
A) Co-administration with cholestyramine
B) Switching to a transdermal patch
C) Increasing food intake before dosing
D) Accelerated GI motility
B) Switching to a transdermal patch
Why is digoxin particularly sensitive to changes in formulation and patient variability?
A) It is ionised at all physiological pHs
B) It has low lipid solubility
C) It has a narrow therapeutic window
D) It bypasses hepatic metabolism
C) It has a narrow therapeutic window
A patient using a transdermal scopolamine patch shows prolonged symptoms even after removal. What explains this?
A) Rapid diffusion into systemic tissues
B) Incomplete skin absorption
C) Drug remaining in skin reservoir
D) Enzyme inhibition in skin
C) Drug remaining in skin reservoir
