Drugs Affecting the Gastrointestinal System. Antacids, Constipation, Increasing gastrointestinal motility

Transcription

1 Drugs Affecting the Gastrointestinal System Antacids, Constipation, Increasing gastrointestinal motility

2 Cells of the Gastric Gland parietal cells: produce & secrete HCl primary site of action for many drugs chief cells: secrete pepsinogen [= a proenzyme] pepsinogen acid pepsin[ breaks down proteins] mucoid cells: mucus-secreting cells purpose = protective mucous coat [HCl]

3 Hydrochloric Acid secreted by parietal cells maintains stomach ph at 1 to 4 stimulants to secretion: large, fatty meals excessive amounts of ETOH emotional stress

4 Parietal Cell Stimulation & Secretion Gastric Gland Histamine H-2 Blocker Illustrations from LifeART, MediClip, Williams & Wilkins, a Waverly Company.

5 Vagus Nerve mast cell Histamine from circulation H-2 Blocker Ca++ Ca++ K+ Energy H+/K+ ATPase Pump H+

6 Management Therapy is aimed at Healing the ulcer Preventing recurrence Drug therapy has several major thrusts Raising gastric ph antacids Inhibitors of acid secretion H2 antagonists Proton pump inhibitors Mucosal protection H Pylori eradication The predominant causes of peptic ulcer disease are H pylori infection and use of NSAIDs accounting for 48% and 24% of cases, respectively.

7 Antacids Neutralize gastric ph Largely replaced in recent years by other drugs Used for fast relief of gastric discomfort, often in combination with other therapies Onset: 5-15 minutes Duration: 2 hours must be used repeatedly to reduce acidity

8 Antacids Formulations: chewable tablets, liquids liquids are more effective Formulations of aluminum, magnesium and calcium, often combinations are made

9 MOA Antacids Do nothing to stop overproduction of acid Promote gastric mucosal defenses by: stimulating mucus, prostaglandins, and bicarbonate secretion from the gastric glands

10 Effects of Antacids raising ph 1.3 to 1.6 acid in gastric juices 50% neutralized raising ph 1.3 to 2.3 reduction of acid by 90% reduces pain assoc. with acid-release

11 Therapeutic Uses Heart burn DU GU Doses: 1-2 tablespoon 1-3 hours after meals ( meq)

12 Side Effects Minimal and depend upon compound May be unpleasant or chalky tasting flavors are better tolerated magnesium: Diarrhea, in RF causes CNS toxicity aluminum & calcium constipation

13 Drug Interactions Four basic mechanisms chelation: binding or inactivation of another drug chemical inactivation: produces insoluble complexes increased stomach ph: increases absorption of basic drugs decreases absorption of acidic drugs increased urinary ph increases excretion of acidic drugs decreases excretion of basic drugs

14 Cimetidine Ranitidine Famotidine Nizatidine

15 H 2 Antagonists Histamine type 2 receptor [H 2 ] competitive antagonist prototypical acid secretory antagonists Reduce, but not abolish acid secretion one of most frequently Rx d drugs

16 MOA 2 mechanism: Competitively block the histamine (H 2 ) receptor of acid-producing parietal cells Also up to 90% inhibition of vagal stimulated and gastrin stimulated acid secretion

17 Therapeutic Effects Proven effective: PUD = gastric & duodenal ulcer Gastroesophageal reflux disease [GERD] Upper GI bleed [GIB] May be effective: Prevention of stress ulcers Peptic esophagitis Less potent than PPIs but still suppress acid by 60-70% over 24 hrs Complete inhibition has not been shown.

18 70-85% of patients with DU respond in 4-6 weeks and 87% to 94% after 8 weeks. Suppression of acid secretion during night % of patients with GU respond in 6-8 weeks. It is used in half of usual dose to prevent recurrence of ulcer.

19 Side Effects Cimetidine: Overall low < 3% Headache, dizziness, confusion, rash, liver & kidney impairment, muscle cramp, fever Impotence, increased prolactin, & gynecomastia Dosage: qid or at hs Ranitidine: Headache, arrhythmia, confusion Dosage: bid or at hs

20 Drug Interactions Cimetidine: Decrease the activity of cytochrome P 450 Ranitidine Inhibits CYP 450, 4-10 times less than cimetidine. Concurrent use with omeprazole decreases its effect Cigarette decreases their effects

21 Anticholenergics Clidinium, Dicyclomine, Glycopyrrolate, Propanthelene, Pirenzepine Decreases the effect of Ach on the parietal cells Therapeutic Uses: Not a mainstay of therapy Should not be used in GERD Side effects: Blurred vision, dry mouth, tachycardia, urinary retention, constipation

22 Omeprazole Rabeprazole Pantoprazole Lansoprazole Esomeprazole

23 H + /K + ATPase = site of action Final common step in acid production Vagus Nerve Acetylcholine atropine mast cell Ca++ ATP Histamine H-2 Blocker camp from circulation Ca++ Gastrin K+ Energy H+/K+ ATPase Pump H+

24 MOA Proton Pump Inhibitors convert in to their active form in the acidic environment (Activated only when ph decreases below 4) irreversibly bind to H + /K + ATPase prevent H + ion production & secretion block all acid secretion = achlorhydria to return to normal must synthesize new H + /K + ATPase Delayed effect

25 should be administered approximately 1 hour before a meal have a short serum half-life concentrated and activated near their site of action have a long duration of action.

26 Therapeutic Effects GERD Drug of choice PUD recurrent [Helicobacter pylori] Better than H 2 antagonists Erosive esophagitis Zollinger-Ellison syndrome [ZE] NSAID associated ulcers Prevention of Rebleeding from Peptic Ulcers Prevention of stress related mucosal bleeding

27 Dosage: 20 mg qd for 4-8 weeks Most effective after a prolonged fast when large amounts of active proton pumps are present (i.e. breakfast) PPIs provide superior acid suppression, healing rates, and symptom relief vs H2 blockers and heal 95% of duodenal ulcers and 80% to 90% of gastric ulcers by 4 weeks

28 Side Effects & Interactions Side effects: (mild) nausea, headache, diarrhea, constipation, flatulence, hypergastrinemia ECL hypeplasia May affect absorption of other drug May decrease calcium absorption or osteoclast, which may lead to hip fracture. nosocomial pneumonia

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30 Sucralfate cytoprotective agent once comes in contact with acid in stomach dissociate into aluminum hydroxide & sulfate anions attracted to and bind to the base of ulcers & erosions forms a protective barrier over the base of this area binds to exposed proteins of ulcers limits pepsin s proteolytic action here

31 Sucralfate Uses: gastric, duodenal and oral ulcers, GI bleed, stress ulcer prevention Minimal absorption Dosing: 1 g qid, 1 hr before meals & hs on empty stomach

32 Side effects: constipation, sleepiness, hypophosphatemia Drug interaction: decreases absorption of many drugs and fat soluble vitamins

33 Bismuth subcitrate Coats ulcer at low ph Assists H Pylori eradication Causes black tongue and stool Must be taken at least half an hour before meals Bismuth based preparations have more adverse effects, and also ~85% success. However cheapest. Bismuth, metronidazole and tetracycline, and a proton pump inhibitor for 2 weeks

34 H Pylori eradication regimens Clarithromycin based quadruple therapy has highest eradication rate (~95%) Omeprazole (any PPI), clarithromycin, amoxicillin and metronidazole, all twice daily for a week Non-clarithromycin based regimens: cheaper, but lower eradication rate (~85%) Omeprazole (or high dose ranitidine), amoxicillin and metronidazole for 2 weeks

35 Eradication of H pylori reduces the incidence of peptic ulcer disease recurrence, from 67% to 6% for DU and from 59% to 4% for GU. Continue PPIs after 6 weeks of triple therapy, especially if ulcer is >1 cm.

36 Laxatives Bulk-formers Osmotic laxatives Stool softeners Stimulant Laxatives

37 Bulk-forming Laxatives Psyllium, bran, etc. Swells in water to form gel that increases bulk and softens stool, increase peristalsis Similar to the action of dietary fiber Useful in chronic constipation, and in irritable bowel syndrome Onset: hrs Powder needs reconstitution in at least 8 oz (1 glass) water or juice Drink 6-8 glasses of water

38 Osmotic laxatives Increase colonic fluid retention Nonabsorbable Stimulate peristalsis Include two types: Nonabsorbable Sugars Nonabsorbable salts

39 Nonabsorbable Sugars Lactulose In the colon hydrolyzes to lactic acid, formic acid, and acetic acid Onset: hr Sorbital Side effects: flatulence, abdominal bloating, diarrhea Polyethylene Glycol - does not produce significant cramps or flatus.

40 Nonabsorbable salts Milk of Magnesia, Magnesium oxide Used in simple constipation & postop Onset: hours Use cautiously in renal impairment Potential for hypermagnesemia due to reduced excretion

41 Stool softeners Docusate, Paraffin (Mineral oil), Glycerin Reduce surface tension in the bowel increasing water absorption into stool Onset: 6-8 hrs Side effects: (rare) GI pain, cramping, rash, decrease absorption of fat soluble vitamins Administer with adequate fluids

42 Stimulant Laxatives May stimulate myenteric plexus Increase intestinal peristaltic motility and secretion Onset: 2-6 hours Potent, may cause watery stools, cramping Often used for preprocedure bowel preps, postop Side effects: abdominal discomfort, nausea, cramps

43 Castor oil Besides irritation, it has surfactant activity It may acts through inhibition of Na + /k + ATPase, camp, PGs

44 Bisacodyl (Dulcolax) inhibition of Na + /k + ATPase, camp, PGs Tablets Onset: 6-12 hrs Suppository Onset: minutes Used in conjunction with PEG

45 Senokot (senna) Natural, obtained from dried cassia leaflets Used for simple constipation, postop, other Onset: 6-12 hours (up to 24)

46 Drugs promoting gastrointestinal motility Time to empty half of liquid is half an hour and to empty solids is 2 hours Factors that delay the gastric emptying: Neuropathy, myopathy, electrolyte disturbances, gastroenteritis, anticholenergics, TCA, levodopa, β agonists Factors that enhance the gastric emptying: Metoclopramide, Cisapride, Domperidone, cholinomimetics

47 Metoclopramide Mechanism: CNS: D 2 antagonist Peripheral: cholinergic agonist, dopamine antagonist, 5-HT 4 agonist, 5-HT 3 blocker, sensitize intestinal smooth muscles to the Ach Increases gastric contractions, gastric emptying and peristaltic movement, raises pressure of lower esophageal sphincter block dopamine D 2 receptors in the CTZ of the medulla, resulting in potent anti N/V

48 Indications: 1. Decreases gastric emptying time 2. GERD 3. Nausea 4. Before induction of anesthesia 5. Facilitates small bowel intubation 6. Increases lactation

49 Side effects Insomnia, fatigue, restlessness, unordinary weakness Galactorrhea, menstrual disorders Extraperimedial side effects: parkinson, dystonic reactions, akatesia, tardive dyskenesia Especially in elders and youngsters

50 Drug Interactions Increases the effect of CNS depressant Increases the EP side effects of haloperidol, phenothiazines, thioxanthins Antimuscarinics neutralize the effects of metoclopramide

51 Other drugs Domperidone Like metoclopramide except it does not enter the CNS Cisapride No effect on DA, activation of 5-HT 4 and Ach Causes fatal ventricular arrhythmia, diarrhea Cholinergic agonists Increase the effect of metoclopramide Erythromycin Prokinetic activity