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GI Drugs Flashcards

1
Q

Regulation of gastric acid secretion

A

Histamine (endocrine)

- H2 receptor coupled to Gs 
- Increased cAMP in parietal cell 
- Protein kinases upregulate activity of proton pump --> pumps acid into gastric lumen

Ach (neural via vagus nerve)

- M3 coupled to Gq 
- Increased intracellular Ca2+ 
- Increased activity of proton pump

Gastrin (paracrine)

- Does not use a gastrin receptor 
- Uses CCKb receptor, results in increase in intracellular Ca2+

Prostaglandin E2

- Coupled to Gi 
- Inhibitory activity --> decreased AC and cAMP and downregulates the proton pump 
- Has a protective activity
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2
Q

Antacids MOA

A

Weak bases

Act as chemical antagonist –> neutralisation reaction with acid to produce salt and water –> increases gastric pH –> protects esophageal mucosa from reflex corrosion

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3
Q

3 antacids and their AE

A

Aluminium hydroxide = constipation
Magnesium hydroxide = osmotic diarrhoea
—-> both combined to produce no change in bowel movements

Calcium carbonate = belching –> metabolic acidosis –> milk alkali syndrome –> renal failure

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4
Q

What drugs cannot be given along with antacids?

A

Alter dissolution of weakly charged acids due to increased pH

Decreased absorption of co-administered tetracyclines, fluorquinolones, itraconazole and iron.

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5
Q

H2 receptor antagonists MOA and drugs

A

Selective competitive inhibition at parietal cell H2 Gs coupled receptor

Suppress histamine inducing acid secretion –> Upstream blockade. Only partial suppression of acid production as the Ach and Gastrin pathways are still active.

Suppress basal gastric acid secretion but modest effect on meal stimulated secretion

1st gen = cimetidine
2nd gen = ranitidine, famotidine and nizatidine

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6
Q

H2RA AE

A

Tachyphlaxis develops in 2 to 6 weeks = repeated exposure to drug will cause down regulation of expression of H2 receptors and will lead to non-responsiveness

Cimetidine

  • Gynecomastia, galactorrhea and male impotence (Block androgen receptor and stimulates prolactin)
  • Crosses BBB: confusion, dizziness and headaches
  • B12 deficiency and myelosuppression in long-term use due to increased gastric pH (meat and animal products cannot be digested properly)
  • Inhibits CYP450: increased conc of drugs (warfarin, diazepam, phenytoin)
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7
Q

H2RA uses

A

GERD
PUD
Nonulcer dyspepsia
Prophylaxis against stress-related gastritis (burn and head-trauma patients)

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8
Q

PPIs drugs and MOA

A 
Omeprazole 
Esomeprazole 
Lansoprazole 
Rabeprazole 
Pantoprazole

Irreversibly inhibit H+/K+ ATPase proton pump in parental cells (final common pathway)–> inhibit 90-98% of 24 hours acid secretion

Suppress both basal and meal stimulated gastric acid production

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9
Q

PPIs uses

A

Patients who fail twice daily H2RA therapy

Severe GERD

PUD (H. pylori, NSAID associated) and prevention of peptic ulcer rebreeding

Nonulcer dyspepsia

Prophylaxis against stress-related gastritis (burn and head-trauma patients

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10
Q

PPIs AE with long term use (4)

A

Vitamin B12 deficiency due to reduced pepsin function (due to increased pH)

Risk for community acquired pneumonias and C. difficile colitis

Hypomagnesemia

Osteopenia: reduced Ca2+ absorption or osteoclast inhibition –> increased risk of hip, spine, wrist fractures

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11
Q

PPIs drug interactions

A

Omeprazole = inhibit CYP450 enzymes (increased conc of drugs)

Omeprazole, Esomeprazole, Lansoprazole = inhibit CYP2C19 –> If clopidogrel is given with these drugs, it will not be activated as it is a prodrug

Pantoprazole or rabeprazol is preferred in people on clopidogrel

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12
Q

Why use PPIs for h. pylori eradication

A

1) direct antimicrobial properties
2) increased ph –> microbe will be actively dividing so antibiotics will be more effective –> lowers MIC of antibiotics needed to clear the organism

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13
Q

Triple therapy for h. pylori eradication

A

Clarithromycin + amoxicillin + PPI

In penicillin allergic patients:
Clarithromycin + metronidazole + PPI

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14
Q

Quadruple therapy for h. pylori eradication

A

Bismuth subsalicylate + metronidazole + tetracycline + PPI

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15
Q

Misoprostol MOA

A

Analog of PGE1

Binds EP3 receptor –> stimulates Gi pathways to decrease gastric acid secretion

Stimulates mucus and bicarb secretion

Enhances mucosal blood flow

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16
Q

Misoprostol uses

A

Prevention of NSAID-induced ulcers in high risk patients

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17
Q

Misoprostol AE and contraindications

A

Diarrhea, abdominal pain, cramps

Contraindicated in pregnancy –> causes uterine contractions –> abortion

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18
Q

Sucralfate MOA and use

A

Forms a viscous paste that binds selectively to ulcers –> neg charged sucrose sulphate binds pos charged proteins forming a physical barriers which stops further damage

Stimulated mucosal prostaglandin and bicarb secretion

Used for initial management of GERD in pregnancy

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19
Q

Bismuth Subsalicylate MOA and uses

A

Suppresses H. pylori but no neutralisation action of acid

Used in quadruple therapy

Used for dyspepsia and acute diarrhea

20
Q

Bismuth Subsalicylate AE and contraindications

A

Bismuth toxicity rare

Metabolite bismuth sulfide causes harmless blackening of the stool

Can cause salicylate toxicity in combination with other salicylate products –> contraindicated in renal failure

21
Q

Prokinetic agents general MOA and drugs

A

Should enhance coordinated GI motility

Activating M1 receptors does not enhance coordinated contractions so drugs should act upstream of Ach at receptor sites on enteric neuron itself or higher

Erythromycin
Cisapride
Metoclopramide

22
Q

How is GI peristalsis regulated

A

1) Serotonin. Binds first and stimulates enteric neurons via 5-HT4 receptor–> which stimulate Ach via M1 to stimulate muscles –> More coordinated response of muscle contraction
2) Motilin binds directly to receptor on muscularis to stimulate contraction

23
Q

Erythromycin MOA and use

A

Macrolide antibiotic

Agonist effects the motion receptor

Rapid down-regulation of motion receptor –> early tolerance so only used for short courses

Used for diabetic gastroparesis

24
Q

Cisapride MOA and uses

A

5-HT4 receptor agonist, 5-HT3 antagonist and direct smooth muscle stimulant

No longer used as it is a pro-arrhythmic drug –> serious and fatal cardiac ventricular arrhythmias

25
Metoclopramide MOA and uses
5-HT4 receptor agonist, vagal and central 5-HT3 antagonist and D2 receptor antagonist Effects upper GI tract only: Increased LES tone Stimulates astral and small intestinal contractions Uses: - Gastroparesis - Anti-emetic
26
Metoclopramide AE
Extrapyramidal effects - due to dopamine antagonism (effects children and young adults) Galactorrhea - block inhibitory effect of dopamine on prolactin --> increased prolactin release
27
Chemotherapy induced N/V phases
Associated with some chemotherapeutic agents like cisplatin Acute phase = universally experienced (within 24 hours) Delayed phase = only affects some patients (on day 2-5)
28
Prevention and treatment of motion sickness
Scopolamine (anticholinergic) as motion sickness is mediated by H1 and M receptors from inner ear to cerebellum Not first line for CINV
29
H1 antagonists used for motion sickness and post-operative emesis
Diphenhydramine, meclizine and cyclizine Act on vestibular afferents and brainstem
30
Ondansetron and granisetron MOA and uses
5-HT3 antagonists --> present in vagal afferents, the solitary tract nucleus, chemoreceptor trigger zone and area postrema DOC for proxylaxis against immediate CINV Also used for hyperemesis gravidarum in pregnancy
31
NK1 antagonists and uses
Aprepitant - oral Fosaprepitant - parenteral Antagonist for NK1 receptor for substance P Used for prophylaxis against delayed CINV
32
Aprepitant metabolism
NK1 antagonist Extensive CYP3A4 metabolism --> may affect metabolism of warfarin and OCP
33
D2 antagonists drugs, use and AE
Promethazine Droperidol Antagonism at chemoreceptor trigger zone Also has anti-histamine and anti-cholinergic effects = good for treating motion sickness AE: extrapyramidal effects
34
Dexamethasone Methylprednisolone MOA and uses
Corticosteriods. Effective in treatment of nausea in patients with metastatic cancer => suppression of peritumoral inflammation and prostaglandin production
35
Lorazepam Alprazolam Diazepam Use as anti-emetics MOA and AE
Benzos: No intrinsic anti-emetic effects Useful adjuncts to do sedative, amnesic and anti-anxiety effects which reduce the anticipatory component of N/V MOA: facilitate GABAa action in CNS by increasing Cl- channel opening AE: CNS depression and dependence
36
Dronabinol MOA and use as anti-emetics
Naturally occurring cannabinoid Stimulates CB1 receptor in brainstem Prophylaxis in patients receiving chemotherapy when other anti-emetic medications are not effective
37
Dronabinol AE
Marijuana-like highs CNS activity = palpitations, tachycardia, vasodilation, hypotension and bloodshot eyes Paranoid reactions and thinking Abrupt withdrawal => abstinence syndrome (irritability, insomnia, restlessness)
38
Bulk-forming laxatives (3) MOA
Methylcellulose Psyllium Bran Non-digestable colloids --> absorb water to form bulky, soft jelly that distends the colon to promote peristalsis
39
Contraindications of bulk-forming laxatives
Immobile patients Patients on long-term opioid therapy Can result in intestinal obstruction
40
Cathartics (stimulant laxatives) drugs, MOA and contraindications
Directly stimulate enteric nervous system to increase intestinal motility Caster oil = broken down into ricinoleic acid in SI. Contraindicated in pregnancy as it causes uterine contractions. Bisacodyl = acts at level of colon. Minimal systemic absorption --> can be used in acute and long-term. Senna = chronic use can cause melanosis coli --> harmless brown pigmentation of colonic mucosa (unrelated to colon cancer risk)
41
Stool softeners (2)
Docusate Glycerin Surfactants --> allow water to penetrate and soften formed stool in the bowel
42
Lubricant laxative
Mineral oil = coats decal material and prevents water reabsorption Cannot be given at the same times as stool softeners
43
Osmotic laxatives (3)
MOA: non-absorbable sugars or salts --> exert osmotic pull to retain water in intestinal lumen Lactulose - metabolised by colonic bacteria and can cause severe flatus with cramping Magnesium salts - should not be used for long time in patients with renal insufficiency as thy can cause hypermagnesemia Polyethylene glycol - water soluble - low toxicity and systemic absorption - creates very high osmotic pressures in gut lumen - used for total bowel prep before GI endoscopy - less AE, better for long term use
44
Lubiprostone
Stimulates type 2 Cl- channels in SI --> increases Cl- secretion --> increased intestinal motility Uses: - chronic constipation - IBS with predominant constipation AE: diarrhea Contraindicated in children
45
Alvimopan | Methynaltrexone
Selective Mu-Opioid receptor antagonists Use of opioid analgesics causes constipation due to stimulation of Mu receptors in bowel Do not cross BBB --> do not negate analgesic effect of opioids
46
Loperamide | Diphenoxylate
Low potential for addiction

Pharmacology (20 decks)

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