Medical School · Year 2 · Gastrointestinal · includes a discussion video

Lecture 16: Gastrointestinal Pharmacology

Unit 2.2: Gastrointestinal System

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the mechanisms and uses of acid-suppressing medications
2. Explain antiemetic drug classes and their applications
3. Describe laxative and antidiarrheal agents
4. Explain medications for inflammatory bowel disease
5. Describe drugs used in liver disease
6. Explain prokinetic agents and their indications

Lecture Outline

I. Acid-Suppressing Medications

Gastric acid secretion from parietal cells is the final common pathway for peptic ulcer disease, gastroesophageal reflux, and stress-related mucosal injury. Pharmacologic acid suppression targets the cellular mechanisms of hydrochloric acid production.

Proton Pump Inhibitors (PPIs) represent the most potent acid-suppressing agents available. First-generation agents include omeprazole and lansoprazole; second-generation agents include esomeprazole (the S-enantiomer of omeprazole), pantoprazole, rabeprazole, and dexlansoprazole. PPIs are prodrugs that require acidic activation; absorbed from the small intestine, they concentrate in the acidic environment of parietal cell canaliculi, where they undergo protonation and conversion to active sulfenamide compounds. This active form then binds irreversibly to the hydrogen-potassium ATPase (H⁺/K⁺-ATPase, the proton pump) on the luminal surface of parietal cells, permanently inactivating the enzyme. Because inhibition is irreversible, acid secretion remains suppressed until new proton pumps are synthesized—approximately 24-48 hours—explaining the prolonged duration despite short plasma half-lives.

Pharmacokinetic considerations are clinically important. PPIs should be administered 30-60 minutes before meals because they only inhibit actively secreting pumps; eating stimulates pump activity through gastrin release, maximizing the number of active targets. Full acid suppression requires 2-3 days of dosing as successive cohorts of pumps are inactivated. Dexlansoprazole uses dual delayed-release technology allowing flexible timing.

Clinical uses span the spectrum of acid-related diseases. PPIs are first-line therapy for GERD, providing symptom relief and healing erosive esophagitis. In peptic ulcer disease, they accelerate healing and are essential components of H. pylori eradication regimens. High-dose continuous infusion (80 mg bolus followed by 8 mg/hour) reduces rebleeding after endoscopic treatment of bleeding peptic ulcers. Zollinger-Ellison syndrome requires high-dose therapy (often 60-120 mg daily in divided doses) to control massive acid hypersecretion. PPIs provide gastroprotection in high-risk patients requiring chronic NSAID therapy.

Adverse effects, while PPIs are generally well-tolerated, have raised concerns with long-term use. The reduced gastric acid barrier may increase Clostridioides difficile infection risk and possibly community-acquired pneumonia. Fracture risk may increase modestly with long-term use, possibly through impaired calcium absorption. Hypomagnesemia is an idiosyncratic effect, potentially severe, occurring through unclear mechanisms. Vitamin B12 deficiency results from impaired acid-pepsin release of B12 from food proteins. Acute interstitial nephritis is a rare hypersensitivity reaction. Possible associations with dementia and increased cardiovascular events remain controversial. The benefits typically outweigh risks when PPIs are appropriately indicated, but deprescribing when no longer needed is prudent.

H2 Receptor Antagonists (H2RAs) including famotidine, nizatidine, and cimetidine competitively inhibit histamine at H2 receptors on parietal cells. Histamine, released from enterochromaffin-like (ECL) cells in response to gastrin, is a major stimulant of acid secretion. H2RAs produce less profound acid suppression than PPIs and are subject to tolerance (tachyphylaxis) with continuous use, limiting their utility for chronic therapy. Ranitidine was withdrawn due to NDMA contamination concerns. Clinical uses include mild GERD, stress ulcer prophylaxis in ICU patients, and nocturnal acid breakthrough when added to PPI therapy. Cimetidine uniquely inhibits cytochrome P450 enzymes, causing drug interactions.

<image>Panel A: Parietal cell cross-section showing apical H+/K+-ATPase proton pump and basolateral receptors (histamine H2, gastrin CCK-B, acetylcholine M3) with intracellular signaling pathways (cAMP, calcium/IP3) converging on pump activation. Panel B: PPI mechanism showing prodrug activation in the acidic canaliculus to sulfenamide form, irreversible binding to proton pump, and requirement for 30-60 minute pre-meal dosing with 2-3 days for full effect. Panel C: H2RA mechanism showing competitive reversible blockade of histamine H2 receptors with development of tolerance (tachyphylaxis) limiting chronic use. Panel D: Clinical comparison of PPIs (potent, irreversible, no tolerance, meal timing required) versus H2RAs (moderate, competitive, tolerance develops) with indications and adverse effects.</image>

II. Other Acid-Related Medications

Beyond PPIs and H2RAs, several agents protect the gastroduodenal mucosa or neutralize secreted acid through distinct mechanisms.

Antacids directly neutralize gastric acid through chemical reaction, providing rapid but brief symptom relief. Calcium carbonate (Tums) provides potent neutralization with rapid onset but may cause rebound acid secretion and constipation; chronic use risks hypercalcemia and milk-alkite syndrome. Magnesium hydroxide (Milk of Magnesia) is effective but causes diarrhea through osmotic effects. Aluminum hydroxide causes constipation and binds dietary phosphate, potentially useful in hyperphosphatemia but problematic long-term. Combination products (Maalox, Mylanta) balance the bowel effects of aluminum and magnesium. All antacids have limited duration (20-60 minutes) and provide no mucosal healing—their role is primarily symptomatic relief of occasional heartburn.

Sucralfate is an aluminum hydroxide-sucrose sulfate complex that polymerizes in the acidic gastric environment, forming a viscous, sticky paste that selectively binds to ulcer craters and erosions. This physical barrier protects damaged mucosa from acid, pepsin, and bile, allowing healing. Sucralfate stimulates prostaglandin synthesis and may have additional cytoprotective effects. Clinical uses include peptic ulcer disease (adjunctive therapy) and stress ulcer prophylaxis (advantageous because it does not alter gastric pH, theoretically reducing pneumonia risk compared to acid suppressants). Administration requires an empty stomach, and sucralfate binds numerous medications (fluoroquinolones, levothyroxine, phenytoin), requiring 2-hour separation.

Misoprostol, a synthetic prostaglandin E1 (PGE1) analog, replaces the cytoprotective prostaglandins depleted by NSAID-induced COX-1 inhibition. Prostaglandins promote mucosal defense by increasing mucus and bicarbonate secretion, enhancing mucosal blood flow, and modestly reducing acid secretion. Misoprostol is FDA-approved for preventing NSAID-induced gastric ulcers in high-risk patients. However, use is limited by side effects—dose-dependent diarrhea and abdominal cramping are common. Misoprostol is absolutely contraindicated in pregnancy due to abortifacient properties (uterotonic effects causing cervical ripening and uterine contractions—it is actually used off-label for cervical ripening and labor induction). Women of childbearing potential require pregnancy testing and contraception counseling.

Bismuth compounds (bismuth subsalicylate—Pepto-Bismol; bismuth subcitrate) have multiple mechanisms: direct antibacterial activity against H. pylori, stimulation of prostaglandin and bicarbonate secretion, and coating of ulcer bases. Bismuth is a component of quadruple therapy for H. pylori (bismuth, metronidazole, tetracycline, PPI). Bismuth subsalicylate is also effective for traveler's diarrhea and dyspepsia. Side effects include harmless black discoloration of stool and tongue. High doses or prolonged use may cause bismuth neurotoxicity (encephalopathy).

<image>Panel A: Antacids showing neutralization reactions, agent comparison (calcium carbonate with constipation risk, magnesium hydroxide with diarrhea, aluminum hydroxide with phosphate binding), and 20-60 minute duration. Panel B: Sucralfate mechanism showing polymerization in acidic environment and selective adherence to ulcer craters creating a physical barrier against acid and pepsin, with empty stomach administration requirement. Panel C: Misoprostol as PGE1 analog showing prostaglandin effects (increased mucus, bicarbonate, and mucosal blood flow) with pregnancy contraindication due to abortifacient properties. Panel D: Bismuth compounds showing direct antibacterial activity against H. pylori, ulcer surface coating, and role in quadruple therapy (bismuth, metronidazole, tetracycline, PPI).</image>

III. Antiemetics

Nausea and vomiting result from complex neural pathways involving the chemoreceptor trigger zone (CTZ) in the area postrema, the vomiting center in the medulla, vestibular afferents, and vagal afferents from the GI tract. Multiple neurotransmitter systems participate, explaining why different antiemetic classes target distinct receptors and work best for specific causes of nausea.

Dopamine (D2) Antagonists block dopamine receptors in the CTZ, which lies outside the blood-brain barrier and senses circulating emetogenic substances. Metoclopramide combines D2 antagonism with 5-HT4 agonism, providing both antiemetic and prokinetic effects; it crosses the blood-brain barrier, causing CNS side effects. Prochlorperazine, a phenothiazine, is a potent antiemetic for various causes including postoperative nausea and migraines. Domperidone is a peripheral D2 antagonist that does not significantly cross the blood-brain barrier, reducing extrapyramidal risks, but is not available in the United States due to cardiac concerns. Side effects of D2 antagonists include extrapyramidal symptoms (acute dystonia, akathisia, parkinsonism) and, with prolonged use, tardive dyskinesia—metoclopramide carries a black box warning limiting use to 12 weeks maximum. Hyperprolactinemia (galactorrhea, gynecomastia) occurs through dopamine blockade in the tuberoinfundibular pathway.

Serotonin (5-HT3) Antagonists including ondansetron, granisetron, and palonosetron block serotonin receptors both in the CTZ and on vagal afferents from the GI tract. Chemotherapy and radiation cause release of serotonin from enterochromaffin cells in the intestinal mucosa, activating vagal afferents—explaining why 5-HT3 antagonists are particularly effective for chemotherapy-induced nausea and vomiting (CINV) and radiation-induced emesis. They are also first-line for postoperative nausea and vomiting (PONV). Palonosetron has a longer half-life (40 hours vs 3-5 hours) and higher receptor affinity, potentially offering superior efficacy for delayed CINV. Side effects are generally mild: headache, constipation, and dose-dependent QT prolongation (requiring caution in patients at risk for arrhythmias).

Neurokinin-1 (NK1) Antagonists including aprepitant (oral), fosaprepitant (IV prodrug), and rolapitant block substance P at NK1 receptors in the vomiting center and CTZ. Substance P mediates the delayed phase of CINV (occurring 24-120 hours after chemotherapy), which is poorly controlled by 5-HT3 antagonists alone. NK1 antagonists are used for highly emetogenic chemotherapy (cisplatin, anthracycline/cyclophosphamide) as part of three-drug regimens with a 5-HT3 antagonist and dexamethasone. Aprepitant is a moderate CYP3A4 inhibitor and inducer, causing drug interactions (notably with warfarin and dexamethasone dosing adjustments).

Other antiemetics target different pathways for specific indications. Scopolamine, a muscarinic antagonist available as a transdermal patch, is highly effective for motion sickness and vestibular-mediated nausea by blocking acetylcholine in the vomiting center and vestibular nuclei; side effects include dry mouth, blurred vision, and urinary retention. Meclizine and dimenhydrinate, H1 antihistamines with anticholinergic properties, also treat motion sickness and vertigo with fewer anticholinergic effects. Dexamethasone, through unclear mechanisms, enhances antiemetic efficacy and is included in most CINV regimens. Cannabinoids (dronabinol, nabilone) activate CB1 receptors centrally, useful for refractory chemotherapy-induced nausea.

<image>Panel A: Vomiting pathway diagram showing chemoreceptor trigger zone (D2, 5-HT3, NK1 receptors), vomiting center in medulla (H1, M1, NK1), vestibular nuclei (H1, M1), and vagal afferents from GI tract (5-HT3). Panel B: Drug classes at sites of action showing D2 antagonists (metoclopramide, prochlorperazine) at CTZ, 5-HT3 antagonists (ondansetron) at CTZ and vagal afferents, NK1 antagonists (aprepitant) at vomiting center, and scopolamine at vestibular nuclei. Panel C: Indication-based drug selection for CINV (acute phase 5-HT3 antagonist, delayed phase NK1 antagonist, highly emetogenic all three agents), PONV, and motion sickness (scopolamine, meclizine). Panel D: Side effects highlighting extrapyramidal symptoms and tardive dyskinesia for D2 antagonists with black box warning, QT prolongation for 5-HT3 antagonists, and CYP3A4 interactions for NK1 antagonists.</image>

IV. Laxatives

Laxatives treat constipation through various mechanisms: increasing stool bulk and water content, stimulating intestinal secretion, or enhancing motility. Selection depends on the cause and severity of constipation, patient preferences, and specific clinical situations.

Bulk-Forming Agents including psyllium (Metamucil), methylcellulose (Citrucel), and polycarbophil are soluble or insoluble fibers that absorb water, increase stool mass, and distend the colon, triggering peristalsis. They are considered first-line therapy for chronic constipation and are also useful for IBS with mixed bowel habits. Onset is gradual (12-72 hours), and adequate water intake is essential to prevent impaction. Side effects include bloating and flatulence, particularly initially.

Osmotic Laxatives draw water into the intestinal lumen through osmotic gradients. Polyethylene glycol (PEG/Miralax) is a non-absorbable, non-metabolized polymer producing a dose-dependent osmotic effect without electrolyte disturbances; it is effective, well-tolerated, and available over-the-counter. Higher doses (PEG-electrolyte solutions—GoLYTELY, NuLYTELY) are used for colonoscopy bowel preparation. Lactulose, a synthetic disaccharide, is not absorbed and reaches the colon, where bacterial fermentation produces organic acids, lowering colonic pH and creating osmotic draw; beyond constipation, lactulose is first-line treatment for hepatic encephalopathy (discussed below). Magnesium-based laxatives (magnesium citrate, magnesium hydroxide) provide both osmotic and secretory effects for rapid action but require caution in renal insufficiency due to magnesium accumulation. Sorbitol is an inexpensive osmotic alternative to lactulose.

Stimulant Laxatives including bisacodyl (Dulcolax) and senna directly stimulate colonic motility and secretion through effects on enteric neurons and enterocytes. They produce rapid, often predictable bowel movements within 6-12 hours (or faster with suppository formulations). While effective for rescue therapy and bowel preparation, chronic use has traditionally been discouraged due to concerns about dependence and "cathartic colon," though these fears may be overstated. Melanosis coli, a benign brown pigmentation of the colonic mucosa, occurs with chronic anthraquinone (senna) use.

Secretory Agents represent a newer class targeting intestinal ion channels to increase fluid secretion. Linaclotide and plecanatide are guanylate cyclase-C (GC-C) agonists that bind to receptors on the luminal surface of intestinal epithelial cells, increasing intracellular cyclic GMP, which opens chloride channels and increases fluid secretion. Additionally, GC-C activation reduces visceral pain signaling—providing benefit for IBS with constipation (IBS-C) and chronic idiopathic constipation (CIC). Lubiprostone activates type 2 chloride channels (ClC-2) on the apical membrane, increasing chloride and fluid secretion; it is approved for IBS-C, CIC, and opioid-induced constipation.

Opioid-Induced Constipation (OIC) is particularly challenging because opioids directly inhibit intestinal motility through mu-opioid receptors throughout the GI tract. Tolerance to constipation does not develop as it does for other opioid effects. Peripherally Acting Mu-Opioid Receptor Antagonists (PAMORAs) selectively block peripheral mu-opioid receptors without crossing the blood-brain barrier, reversing GI effects without precipitating withdrawal or reducing analgesia. Methylnaltrexone (subcutaneous) is used for OIC in advanced illness when other laxatives fail. Naldemedine and naloxegol (oral) are approved for OIC in chronic non-cancer pain.

<image>Panel A: Laxative mechanisms with colon cross-sections showing bulk-forming agents (fiber absorbing water, increasing stool mass), osmotic agents (PEG/lactulose drawing water into lumen), stimulant agents (bisacodyl/senna on enteric neurons), and secretory agents (linaclotide at GC-C receptor). Panel B: Specific agents with onset times including fiber (12-72 hours), PEG (1-3 days), lactulose (24-48 hours), magnesium (30 minutes to 6 hours), bisacodyl and senna (6-12 hours), and linaclotide and lubiprostone (24 hours). Panel C: Opioid-induced constipation treatment showing mu-opioid receptor in GI tract causing decreased motility, with PAMORAs (methylnaltrexone, naloxegol, naldemedine) blocking peripheral receptors while preserving CNS analgesia. Panel D: Clinical indication summary for IBS-C (linaclotide, lubiprostone), chronic idiopathic constipation (PEG, linaclotide, plecanatide), and opioid-induced constipation (PAMORAs, lubiprostone).</image>

V. Antidiarrheal Agents

Antidiarrheal therapy aims to reduce stool frequency and improve consistency, but the underlying cause determines appropriateness—antidiarrheals are contraindicated in bloody diarrhea and suspected C. difficile infection where slowing transit may worsen toxin accumulation.

Opioid Agonists reduce intestinal motility by activating mu-opioid receptors on enteric neurons, decreasing peristalsis and increasing intestinal transit time, while also reducing intestinal secretion and increasing water and electrolyte absorption. Loperamide (Imodium) is a synthetic opioid that does not significantly cross the blood-brain barrier at standard doses due to P-glycoprotein efflux, providing peripheral antidiarrheal effects without CNS effects or abuse potential—hence available over-the-counter. It is effective for acute infectious diarrhea (in combination with rehydration), chronic diarrhea (IBS-D, short bowel syndrome), and traveler's diarrhea. Diphenoxylate with atropine (Lomotil) is prescription only (Schedule V); atropine is added in subtherapeutic doses to deter abuse (causes unpleasant anticholinergic effects at high doses). Codeine and other opioids have antidiarrheal effects but CNS effects limit routine use.

Antisecretory Agents reduce intestinal fluid secretion. Bismuth subsalicylate has antisecretory properties through prostaglandin and chloride secretion inhibition, plus antimicrobial effects; it is useful for traveler's diarrhea (treatment and prevention) and nonspecific diarrhea. Crofelemer, a botanical compound from Croton lechleri, inhibits both CFTR chloride channels and calcium-activated chloride channels, reducing secretory diarrhea; it is specifically approved for HIV-associated diarrhea in patients on antiretroviral therapy.

Bile Acid Sequestrants including cholestyramine, colesevelam, and colestipol are anion-exchange resins that bind bile acids in the intestinal lumen, preventing their colonic irritation which causes secretory diarrhea. They are first-line treatment for bile acid diarrhea (BAD), which occurs after ileal resection, in Crohn's disease affecting the ileum, after cholecystectomy (in some patients), and as primary/idiopathic bile acid malabsorption. Response to empiric bile acid sequestrant trial can be both diagnostic and therapeutic. Side effects include bloating, constipation, and binding of other medications (separate administration by hours).

Other Antidiarrheal Agents target specific mechanisms. Octreotide, a somatostatin analog, reduces secretion and slows transit; it is used for secretory diarrheas including carcinoid syndrome, VIPoma, dumping syndrome, and refractory diarrhea. Rifaximin, a non-absorbed antibiotic, is approved for IBS-D (reducing bacterial metabolites and subtle SIBO) and also for hepatic encephalopathy and traveler's diarrhea. Eluxadoline combines mu-opioid receptor agonism, delta-opioid receptor antagonism, and kappa-opioid receptor agonism for IBS-D, but is contraindicated in patients without a gallbladder or with a history of pancreatitis due to sphincter of Oddi spasm risk. Alosetron, a 5-HT3 antagonist, is restricted to severe IBS-D in women due to ischemic colitis risk.

<image>Panel A: Intestinal cross-section with drug targets showing loperamide at mu-opioid receptors (decreasing motility), bile acid sequestrants binding bile acids in the lumen, and crofelemer blocking CFTR and CaCC chloride channels. Panel B: Agent comparison of loperamide (OTC, does not cross BBB), diphenoxylate/atropine (Schedule V), bismuth (antimicrobial plus antisecretory), and bile acid sequestrants (BAD-specific). Panel C: Specific indications showing bile acid diarrhea pathway (ileal disease to colonic bile acids to sequestrant binding), IBS-D agents (rifaximin, eluxadoline, alosetron), and secretory diarrhea (octreotide for carcinoid/VIPoma). Panel D: Contraindications and special considerations including avoidance in bloody diarrhea and C. difficile infection, HIV-associated diarrhea (crofelemer), and medication binding interactions with sequestrants.</image>

VI. Inflammatory Bowel Disease Drugs

IBD pharmacotherapy aims to induce and maintain remission, with drug selection guided by disease location, severity, phenotype, and prior treatment response. The therapeutic ladder has largely been replaced by more individualized, often "top-down" approaches using biologics earlier.

Aminosalicylates (5-ASA) exert topical anti-inflammatory effects on the intestinal mucosa through mechanisms including inhibition of prostaglandin and leukotriene synthesis, scavenging of reactive oxygen species, and interference with NF-κB signaling. Sulfasalazine combines 5-ASA linked to sulfapyridine (the carrier); colonic bacteria cleave the azo bond, releasing 5-ASA in the colon. Sulfapyridine causes most side effects (headache, nausea, rash, hemolysis, oligospermia), limiting tolerability. Mesalamine (5-ASA without carrier) is available in multiple delivery systems: pH-dependent release (Asacol—releases at pH 7 in terminal ileum and colon; Lialda/MMX—releases throughout colon), time-dependent release (Pentasa—releases throughout small bowel and colon), and topical preparations (suppositories for proctitis, enemas for left-sided colitis). Olsalazine and balsalazide are azo-bonded 5-ASA prodrugs releasing 5-ASA in the colon. Aminosalicylates are effective for mild-to-moderate ulcerative colitis induction and maintenance but have limited efficacy in Crohn's disease. Nephrotoxicity (interstitial nephritis) is rare but requires periodic creatinine monitoring.

Corticosteroids remain the most effective agents for inducing remission in moderate-to-severe IBD flares but are not appropriate for maintenance due to toxicity. Systemic steroids (prednisone, IV methylprednisolone) are used for acute flares. Budesonide undergoes extensive first-pass hepatic metabolism, producing high local concentrations with minimal systemic effects; formulations target the ileum and right colon (Entocort for Crohn's) or entire colon (Uceris for UC). Steroid side effects include hyperglycemia, hypertension, osteoporosis, adrenal suppression, cataracts, and opportunistic infections.

Immunomodulators provide steroid-sparing maintenance therapy. Azathioprine and its metabolite 6-mercaptopurine (6-MP) are purine antimetabolites inhibiting lymphocyte proliferation. TPMT (thiopurine methyltransferase) enzyme testing is required before initiation—deficient patients accumulate toxic metabolites causing severe myelosuppression. Onset is slow (3-6 months), requiring bridging with steroids. Side effects include myelosuppression, hepatotoxicity, pancreatitis, and increased lymphoma risk. Methotrexate, a dihydrofolate reductase inhibitor, is effective for Crohn's disease maintenance (administered weekly intramuscularly or subcutaneously) with required folic acid supplementation; side effects include hepatic fibrosis, pneumonitis, and myelosuppression.

Biologics have transformed IBD treatment, particularly for moderate-to-severe disease. Anti-TNF agents (infliximab, adalimumab, certolizumab pegol, golimumab) bind and neutralize TNF-α, a key pro-inflammatory cytokine. Infliximab (chimeric antibody) is administered intravenously; adalimumab and golimumab (fully human antibodies) and certolizumab (pegylated Fab fragment) are subcutaneous. All require TB screening before initiation. Vedolizumab, an anti-integrin antibody targeting α4β7, blocks lymphocyte trafficking to the gut selectively ("gut-selective"), reducing infection risk compared to non-selective immunosuppression. Ustekinumab targets the p40 subunit shared by IL-12 and IL-23. Newer anti-IL-23 agents (risankizumab, mirikizumab) target the p19 subunit specific to IL-23, with emerging efficacy data.

Small Molecules offer oral alternatives to biologics. JAK inhibitors (tofacitinib—JAK1/3; upadacitinib—JAK1 selective) block Janus kinase signaling downstream of multiple cytokine receptors, are approved for ulcerative colitis with rapid onset. Risks include herpes zoster, thromboembolism, and cardiovascular events (requiring monitoring). S1P modulators (ozanimod) sequester lymphocytes in lymph nodes, preventing gut trafficking; approved for UC.

<image>Panel A: Inflammatory cascade with drug targets showing anti-TNF agents blocking macrophage TNF, ustekinumab/IL-23 inhibitors blocking dendritic cell cytokines, JAK inhibitors blocking T cell signaling, vedolizumab blocking alpha4-beta7 lymphocyte trafficking, and ozanimod modulating S1P receptors. Panel B: Aminosalicylate delivery systems including sulfasalazine (azo bond cleavage in colon), mesalamine pH-release, mesalamine MMX, and rectal formulations with coverage zones. Panel C: Biologic and small molecule drug cards for anti-TNF agents (IV versus SC, TB screening), vedolizumab (gut-selective), ustekinumab (IL-12/23), JAK inhibitors (oral, rapid onset, VTE/CV monitoring), and S1P modulators. Panel D: Treatment algorithm pyramid from mild UC (5-ASA) through moderate (steroids to induce, immunomodulators) to severe/refractory disease (biologics or small molecules).</image>

VII. Liver Disease Medications

Pharmacotherapy for liver disease addresses specific complications of cirrhosis and portal hypertension, as well as treatment of the underlying liver disease.

Hepatic Encephalopathy (HE) results from accumulation of neurotoxins, particularly ammonia, due to impaired hepatic clearance and portosystemic shunting. Treatment targets ammonia reduction. Lactulose, a non-absorbable disaccharide, is first-line therapy working through multiple mechanisms: colonic bacteria metabolize lactulose to organic acids (lactic, acetic), lowering colonic pH; acidification converts ammonia (NH₃) to ammonium (NH₄⁺), which cannot cross the colonic mucosa; cathartic effect reduces ammonia absorption and contact time; and altered bacterial metabolism. Dosing is titrated to achieve 2-3 soft bowel movements daily. Lactulose enemas are used for patients unable to take oral medications. Rifaximin, a non-absorbed antibiotic, reduces ammonia-producing intestinal bacteria and is used as add-on therapy to lactulose for preventing HE recurrence—studies show significant reduction in breakthrough HE episodes.

Portal Hypertension management focuses on preventing variceal bleeding. Non-selective beta-blockers (NSBBs) reduce portal pressure by decreasing cardiac output (β1 blockade) and causing splanchnic vasoconstriction (β2 blockade, allowing unopposed alpha-adrenergic tone). Propranolol and nadolol are traditional agents, dosed to achieve target heart rate of 55-60 bpm or 25% reduction from baseline. Carvedilol adds alpha-1 blockade, providing greater portal pressure reduction; however, it may cause more hypotension, limiting use in patients with refractory ascites. NSBBs are used for primary prophylaxis (preventing first variceal bleed in patients with medium/large varices) and secondary prophylaxis (preventing rebleeding after variceal hemorrhage, combined with endoscopic band ligation). In acute variceal bleeding, octreotide or terlipressin cause splanchnic vasoconstriction, reducing portal flow; octreotide is preferred in the US, while terlipressin (vasopressin analog) is used elsewhere.

Ascites is managed with sodium restriction and diuretics. Spironolactone, an aldosterone antagonist, is first-line because hyperaldosteronism drives sodium retention in cirrhosis. Furosemide is added when spironolactone alone is insufficient. The traditional starting ratio is 100 mg spironolactone to 40 mg furosemide, maintaining this ratio when escalating. Weight loss goals are 0.5 kg/day without edema or 1 kg/day with peripheral edema; faster diuresis risks hepatorenal syndrome. Spironolactone causes gynecomastia and hyperkalemia; amiloride is an alternative potassium-sparing diuretic without antiandrogenic effects.

Other Liver Disease Medications include ursodeoxycholic acid (UDCA), a hydrophilic bile acid that reduces biliary toxicity, approved for primary biliary cholangitis (PBC) and used for cholestasis of pregnancy and gallstone dissolution. Obeticholic acid, a farnesoid X receptor (FXR) agonist, is second-line for PBC when UDCA response is inadequate. N-acetylcysteine (NAC) is the antidote for acetaminophen hepatotoxicity, replenishing glutathione stores, and is also used in acute liver failure of other etiologies.

<image>Panel A: Hepatic encephalopathy showing gut-liver-brain axis with ammonia production, portosystemic shunting, and lactulose mechanism (colonic acidification trapping NH4+, cathartic effect) plus rifaximin reducing bacterial load. Panel B: Portal hypertension management with NSBB mechanism (decreased cardiac output via beta-1, splanchnic vasoconstriction via beta-2 blockade), propranolol/nadolol versus carvedilol comparison, and octreotide for acute variceal bleeding. Panel C: Ascites pathophysiology (portal hypertension, splanchnic vasodilation, RAAS activation, sodium retention) and treatment with spironolactone (100 mg) and furosemide (40 mg) at 100:40 ratio with weight loss targets. Panel D: Other liver medications including UDCA for primary biliary cholangitis, obeticholic acid as second-line FXR agonist, and N-acetylcysteine for acetaminophen toxicity and acute liver failure.</image>

VIII. Prokinetic Agents

Prokinetics enhance coordinated gastrointestinal motility, used primarily for gastroparesis and other motility disorders. Options are limited by efficacy and safety concerns.

Dopamine Antagonists enhance gastric motility by blocking inhibitory dopamine D2 receptors on gastric smooth muscle and the myenteric plexus. Metoclopramide is the only FDA-approved medication for gastroparesis, combining D2 antagonism with 5-HT4 agonism (which stimulates acetylcholine release from myenteric neurons). It accelerates gastric emptying and has antiemetic effects. However, metoclopramide crosses the blood-brain barrier, causing significant CNS effects: drowsiness, restlessness, and most importantly, extrapyramidal symptoms (acute dystonic reactions, akathisia, drug-induced parkinsonism) and tardive dyskinesia. A black box warning limits recommended use to 12 weeks maximum, though many patients with chronic gastroparesis require longer therapy. Domperidone, which does not cross the BBB, would be preferable but is unavailable in the US due to concerns about QT prolongation and cardiac arrhythmias; it is available in Canada and Europe.

Motilin Agonists stimulate motilin receptors, which normally mediate the migrating motor complex (phase III contractions that sweep debris from the stomach during fasting). Erythromycin, a macrolide antibiotic, is also a potent motilin receptor agonist at lower-than-antimicrobial doses. Intravenous erythromycin (250 mg) produces rapid gastric emptying and is useful for acute gastroparesis or to prepare the stomach for endoscopy in upper GI bleeding. However, tachyphylaxis develops rapidly (within days to weeks), making erythromycin unsuitable for long-term treatment. Oral erythromycin has variable efficacy. QT prolongation is a concern, particularly with IV administration.

Serotonin (5-HT4) Agonists stimulate 5-HT4 receptors on myenteric neurons, increasing acetylcholine release and enhancing motility throughout the GI tract. Prucalopride is FDA-approved for chronic idiopathic constipation and shows promise in gastroparesis, though not FDA-approved for this indication. It has a favorable safety profile without significant cardiac concerns. Tegaserod, an older 5-HT4 agonist, was withdrawn due to cardiovascular events but has been reintroduced with restricted indications (IBS-C in women under 65 without cardiovascular risk factors). Cisapride, a potent 5-HT4 agonist with prokinetic effects throughout the GI tract, was withdrawn due to fatal arrhythmias from QT prolongation.

Investigational Agents include relamorelin, a ghrelin receptor agonist that stimulates gastric motility; it has shown efficacy in diabetic gastroparesis in clinical trials but is not yet approved.

<image>Panel A: Enteric nervous system receptor targets showing inhibitory D2 receptors on gastric muscle (blocked by metoclopramide/domperidone), stimulatory 5-HT4 receptors on myenteric neurons (activated by metoclopramide, prucalopride), and motilin receptors (activated by erythromycin). Panel B: Agent comparison of metoclopramide (FDA-approved, EPS risk, 12-week black box warning), domperidone (not available in US, QT risk), erythromycin (acute IV use, tachyphylaxis), and prucalopride (CIC-approved, favorable safety). Panel C: Clinical applications showing gastroparesis algorithm (metoclopramide first-line, erythromycin for acute bridge, dietary modification) and chronic constipation (prucalopride), with investigational ghrelin receptor agonist relamorelin. Panel D: Safety concerns including extrapyramidal symptoms and tardive dyskinesia with metoclopramide, QT prolongation with multiple agents shown on EKG strip, and tachyphylaxis limiting chronic erythromycin use.</image>

IX. Pancreatic Enzyme Replacement

Pancreatic enzyme replacement therapy (PERT) is essential for managing pancreatic exocrine insufficiency (PEI), providing exogenous enzymes to compensate for inadequate pancreatic secretion.

Components of PERT include three enzyme classes: lipase for fat digestion (the most critical component, as fat malabsorption causes most symptoms), amylase for carbohydrate digestion, and protease for protein digestion. Products contain porcine-derived pancreatic enzymes in delayed-release capsules with enteric-coated microspheres or minimicrospheres that protect enzymes from gastric acid degradation and release in the proximal small intestine at pH above 5.5. Available products include Creon, Pancreaze, Zenpep, and Pertzye; Viokace is an immediate-release formulation requiring concurrent acid suppression.

Dosing is based on lipase units, as lipase is the most critical enzyme and has no alternative secretory source (unlike amylase from saliva and protease from gastric chief cells). The starting dose for adults is 25,000-50,000 lipase units per meal and half that dose with snacks. Enzymes should be taken at the start of or during meals—not before, as enzymes empty with food for optimal mixing. Doses are adjusted based on response (stool character, weight, fat-soluble vitamin levels). The maximum recommended dose is 10,000 lipase units/kg/day or 2,500 units/kg/meal due to concern for fibrosing colonopathy at very high doses, primarily reported in cystic fibrosis patients.

Optimization Strategies address inadequate response. Adding a proton pump inhibitor improves enzyme activity by raising duodenal pH, as acid inactivates enzymes and may prevent enteric coating dissolution. High-fat meals may require increased dosing. Strict meal timing should be encouraged. If symptoms persist despite optimized PERT, consider alternative diagnoses (SIBO, which commonly accompanies chronic pancreatitis).

Monitoring includes nutritional assessment (weight, anthropometrics), stool character (resolution of steatorrhea), and fat-soluble vitamin levels (A, D, E, K) with supplementation as needed. Bone density monitoring is appropriate given vitamin D malabsorption risk.

Complications are uncommon. Fibrosing colonopathy, a rare condition with colonic strictures, has been reported almost exclusively in cystic fibrosis patients receiving very high doses. Hyperuricemia may occur due to purine content of enzymes. Allergic reactions to porcine proteins are rare; there is no commercially available non-porcine PERT.

<image>Panel A: Enteric-coated microsphere mechanism showing enzyme core (lipase, amylase, protease) with acid-resistant coating intact at stomach pH 2, dissolving at duodenal pH 6+, and enzyme actions on triglycerides, starch, and proteins. Panel B: Dosing guidance showing 25,000-50,000 lipase units per meal and half dose with snacks, timing with meals rather than before, and capsule or sprinkle administration options. Panel C: Optimization strategies including PPI addition to raise duodenal pH, monitoring checklist (weight, stool quality, fat-soluble vitamin levels), and maximum dose warning of 10,000 units/kg/day for fibrosing colonopathy risk. Panel D: Fat-soluble vitamin supplementation (A, D, E, K), clinical pearl to consider SIBO if inadequate response despite optimized PERT, and available product formulations (Creon, Pancreaze, Zenpep, Viokace).</image>

X. Miscellaneous GI Drugs

Several additional medications address specific gastrointestinal conditions not covered in preceding sections.

Ursodeoxycholic Acid (UDCA) is a naturally occurring hydrophilic bile acid that, when administered exogenously, enriches the bile acid pool with less toxic bile acids. Mechanisms include displacing hydrophobic bile acids from hepatocytes (reducing their toxicity), immunomodulatory effects, stimulating bile flow (choleresis), and reducing biliary cholesterol secretion. Primary biliary cholangitis (PBC) is the main indication, where UDCA improves liver biochemistry and may slow progression when started early. UDCA is also used for gallstone dissolution (small cholesterol stones in patients who are not surgical candidates; efficacy is limited and stones typically recur), intrahepatic cholestasis of pregnancy (symptomatic relief of pruritus), and primary sclerosing cholangitis (controversial, may improve biochemistry but unclear effect on outcomes). Dose is typically 13-15 mg/kg/day in divided doses. Side effects are minimal (diarrhea, weight gain).

Chenodeoxycholic Acid (CDCA), a primary hydrophobic bile acid, was historically used for gallstone dissolution but abandoned due to hepatotoxicity and diarrhea. Its current indication is cerebrotendinous xanthomatosis (CTX), a rare bile acid synthesis disorder where CDCA replacement corrects the metabolic defect.

Teduglutide, a glucagon-like peptide-2 (GLP-2) analog, stimulates intestinal adaptation by enhancing crypt cell proliferation, increasing villus height, and improving intestinal absorptive capacity. It is approved for short bowel syndrome (SBS) in patients dependent on parenteral nutrition, reducing PN requirements by improving intestinal absorption. Administration is by daily subcutaneous injection. Colonoscopy is required before and during treatment to monitor for colorectal polyps.

Somatostatin Analogs (octreotide, lanreotide) have multiple GI applications. They reduce splanchnic blood flow (used in acute variceal bleeding and hepatorenal syndrome with terlipressin). They suppress hormone secretion from neuroendocrine tumors: carcinoid syndrome (diarrhea, flushing from serotonin excess), VIPoma (watery diarrhea), and glucagonoma. They reduce pancreatic secretion, useful for pancreatic fistula management. Dumping syndrome after gastric surgery responds to octreotide when dietary measures fail. Telotristat, a tryptophan hydroxylase inhibitor, reduces serotonin synthesis and is used for carcinoid syndrome diarrhea inadequately controlled by somatostatin analogs.

<image>Panel A: Ursodeoxycholic acid showing hydrophilic bile acid structure, displacement of toxic bile acids from hepatocytes, gallstone dissolution, and indications (PBC, gallstones, cholestasis of pregnancy). Panel B: Teduglutide as GLP-2 analog showing small bowel cross-section with increased villus height and crypt depth, reduced parenteral nutrition dependence, and colonoscopy surveillance requirement. Panel C: Somatostatin analogs (octreotide, lanreotide) showing splanchnic vasoconstriction for variceal bleeding, hormone suppression in carcinoid and VIPoma, and reduced pancreatic secretion for fistula management. Panel D: Telotristat blocking tryptophan hydroxylase in the tryptophan-to-serotonin pathway for carcinoid syndrome refractory to somatostatin analogs, with combination therapy approach.</image>

Summary

Proton pump inhibitors irreversibly inactivate H⁺/K⁺-ATPase, requiring administration before meals for optimal efficacy; long-term risks include C. difficile, fractures, hypomagnesemia, and B12 deficiency. H2 receptor antagonists provide weaker acid suppression with tolerance development limiting chronic use.

Antiemetics target specific receptors: 5-HT3 antagonists are first-line for chemotherapy and postoperative nausea; D2 antagonists risk extrapyramidal symptoms and tardive dyskinesia; NK1 antagonists address delayed chemotherapy-induced nausea.

Laxatives span bulk-forming agents (first-line for chronic constipation), osmotic agents (PEG, lactulose), stimulant laxatives (rescue therapy), and secretory agents (linaclotide for IBS-C). PAMORAs specifically treat opioid-induced constipation without compromising analgesia.

Antidiarrheals include loperamide (peripheral mu-opioid agonist), bile acid sequestrants for bile acid diarrhea, and specific agents for IBS-D (rifaximin, eluxadoline).

IBD therapy includes aminosalicylates for mild ulcerative colitis, corticosteroids for induction only, immunomodulators (thiopurines, methotrexate) for steroid-sparing maintenance, biologics (anti-TNF, vedolizumab, ustekinumab, anti-IL-23), and JAK inhibitors for moderate-to-severe disease.

Liver disease medications include lactulose and rifaximin for hepatic encephalopathy, non-selective beta-blockers for variceal prophylaxis, and spironolactone-based diuresis for ascites.

Prokinetics are limited: metoclopramide has EPS risk with 12-week recommended maximum; erythromycin works acutely but develops tachyphylaxis; prucalopride offers promise for gastroparesis.

PERT dosing is lipase-based, typically 25,000-50,000 units per meal, with PPI optimization if inadequate response.

Key Terms

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