GI Pharmacology Flashcards
What gastric ph is desired prior to anaesthetic
3.5
Examples of H2 receptor antagonists
How do they work
Cimetidine, ranitidine
Reversible competative inhibitor at H2 receptor which usually stimulates acid secretion
Mimic the imidazole ring end of histamine to bond. Inhibits both basal level and stimulated release of acid.
Where are h2 receptors present
Effect of antagonists at these locations
Uterus, heart, blood vessels, los
Very little
Side effects of cimetidine
Gynaecomastia and impotence (antiandrogen effects)
Cyp450 inhibition
Reduces hepatic blood flow
Inhibition of suppressor T cells worsening autoimmune conditions
Bradyarrhythmias
Time to peak effect of cimetidine
80mins
Time to peak effect of ranitidine
Pharmacokinetics of oral administration
100mins
T1/2 2.5hrs
Substantial first pass metabolism
Metabolised in liver and causes some inhibition of cyp450
How do proton pump inhibitors work?
Effect acid secreting pumps of gastric parietal cells
Structure and pharmacokinetics of ppi
Imidazole and benzene ring together
Weak base with a high PKA
Administered in buffered capsules and slowly released
Absorbed whilst non ionised then activated by protination on luminal side (prodrug)
Accumulates and binds strongly to hkatpase pumps preventing h passage
What is the active form of omeprazole
Sulphenamide
What is the half life of omeprazole
Why does it last longer
3 minutes in plasma
Accumulates around receptors and binds strongly
How is omeprazole metabolised
Hepatically
Adverse effects of omeprazole
Hypergastrinaemia
Cyp450 inhibition
Effect of prostaglandins on stomach acid secretion
Example
Eg misoprostol
Reduce stomach acid secretion and stimulate mucus production thus useful in nsaid induced ulceration
Examples of antacids used in gastric acidic control
Mechanism
Aluminium hydroxide, sodium citrate
Simple chemical reaction of acid +base to salt + water
Side effects of antacid use
Co2 production with beltching and gastric distension
Very alkaline thus aspiration of the antacid could also cause damage
Alkalosis
salt and water retention (as a lot has to be given for significant effect)
Which antacid is non-particulate
Consequence?
Sodium citrate
Less lung damage if aspirated
Examples of antispasmodics
Class of drug
Anticholinergic
Atropine, hyoscine
Effects of anticholinergics on gi tract
Reduce acid secretion (higher dose, side effects prohibitory for primary use)
Reduce gi tone (antispasmodic) (lower dose, side effects can be managed usually)
Reduce los tone
What improves side effect profile of antispasmodic anticholinergics
Not crossing bbb
Specificity for m1 receptor
Other than anticholinergics what other drugs can be used as antispasmodics, how do they work
Meberverine, Peppermint oil
Direct relaxation of smooth muscle
Examples of prokinetic agents
How do they work
Domperidone, erythromycin, metoclopramide
Domperidone and metoclopramide acts on D2 receptors peripherally
Erythromycin is a motility receptor agonist
Final common pathway via acetylcholine
Catagories mechanism and examples of laxatives
Bulking agents - increase stool bulk and retention of water, eg isphaghula husk
Faecal softeners - oily compounds that soften stool, eg liquid parrifin
Osmotic laxatives - draw water into large bowel, eg magnesium salts, lactulose, phosphates
Stimulants - stimulate bowel movement and permeability, eg senna, bisacodyl
Risks of stimulant laxatives
Hepatotoxicity
Which group of laxatives are used in bowel prep
Osmotic
Issue with faecal softeners
Decreased absorption of fat soluble vitamins
Protein binding of lansoprazole and omeprazole
97 and 95% respectively
Bioavailability of lansoprazole and omeprazole
85 vs 35%
Side effects of lansoprazole and omeprazole
rash, puritis, eosinophilia, gynaecomastia, liver dysfunction, low na and Mg
