Section 4.13: Clinical Pharmacology: Gastrointestinal & Hepatic Disorders

Clinical Pharmacology: Gastrointestinal & Hepatic Disorders

Gastrointestinal and hepatic disorders require an understanding of gastric acid suppression, infectious disease eradication, inflammatory bowel disease control, and managing liver cirrhosis complications.

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1. Gastroesophageal Reflux Disease (GERD)

GERD is caused by the retrograde flow of gastric contents into the esophagus. Pharmacological management scales with symptom severity:

• Antacids: Provide rapid but transient relief by neutralizing gastric acid.
• H2 Receptor Antagonists (H2RAs - e.g., Famotidine): Reversibly inhibit H2 receptors on parietal cells, reducing acid secretion. Tachyphylaxis (tolerance) can develop within 1–2 weeks of daily use.
• Proton Pump Inhibitors (PPIs - e.g., Omeprazole, Esomeprazole, Pantoprazole): Irreversibly inhibit the active H+/K+ ATPase pump in parietal cells. They are the most effective agents for healing esophagitis.
  • PPI Safety Risks (Chronic Use): Hypomagnesemia, osteoporosis and bone fractures (due to decreased calcium absorption), Clostridioides difficile-associated diarrhea (CDAD), Vitamin B12 deficiency, and acute interstitial nephritis (AIN).

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2. Peptic Ulcer Disease (PUD)

PUD refers to mucosal defects in the stomach or duodenum. The two primary etiologies are NSAID use and Helicobacter pylori infection.

NSAID-Induced Ulcers

• Prevention: Co-administer a PPI or Misoprostol (synthetic PGE1 analog). Misoprostol replaces mucosal prostaglandins depleted by NSAIDs. Contraindicated in pregnancy due to its abortifacient properties (uterine contractions).

H. pylori Eradication Regimens (10–14 Days)

Due to increasing clarithromycin resistance, regimen selection must be precise:

• Bismuth Quadruple Therapy (First-line): PPI (BID) + Bismuth subsalicylate (300mg QID) + Metronidazole (250-500mg QID) + Tetracycline (500mg QID).
• Concomitant (Non-Bismuth) Quadruple Therapy: PPI (BID) + Amoxicillin (1g BID) + Clarithromycin (500mg BID) + Metronidazole (500mg BID).
• Clarithromycin Triple Therapy: PPI (BID) + Amoxicillin (1g BID) + Clarithromycin (500mg BID). Reserved only for areas where local clarithromycin resistance is documented to be < 15% and the patient has no prior macrolide exposure.

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3. Inflammatory Bowel Disease (IBD)

IBD includes Crohn's Disease (CD - transmural, affects any part of GI tract) and Ulcerative Colitis (UC - mucosal inflammation restricted to colon/rectum).

• Aminosalicylates (5-ASA): e.g., Mesalamine, Sulfasalazine. Used for induction and maintenance of remission in mild-to-moderate UC. Mesalamine is available in site-specific formulations (e.g., Rowasa enema for distal colitis, Canasa suppository for proctitis, Pentasa/Asacol release tablets for small bowel/colonic delivery).
• Corticosteroids: e.g., Prednisone, Budesonide. Used for induction of remission during active flares only. Budesonide has high first-pass metabolism, reducing systemic side effects.
• Immunomodulators: e.g., Azathioprine, 6-Mercaptopurine, Methotrexate (for CD).
• Biologics: Anti-TNF agents (Infliximab, Adalimumab) and anti-integrins (Vedolizumab) are used for moderate-to-severe disease.

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4. Cirrhosis & Complications

Cirrhosis is characterized by diffuse fibrosis and hepatic architecture distortion, leading to portal hypertension.

Cirrhosis Portal Hypertension Complications:
- Variceal Hemorrhage: Prophylaxis with Non-selective Beta-Blockers (Propranolol, Nadolol)
- Ascites: Dual Diuretics (Spironolactone + Furosemide in 100:40 ratio)
- Hepatic Encephalopathy: Lactulose + Rifaximin
- Spontaneous Bacterial Peritonitis (SBP): Treatment (Ceftriaxone) and Prophylaxis (Ciprofloxacin/TMP-SMX)

Portal Hypertension & Variceal Bleeding

• Primary/Secondary Prophylaxis: Non-selective Beta-Blockers (NSBBs - Propranolol, Nadolol, Carvedilol). They reduce portal pressure by blocking beta-2 receptors (causing splanchnic vasoconstriction) and beta-1 receptors (decreasing cardiac output). Target heart rate is 55–60 bpm.
• Acute Variceal Bleeding Management:
  • Vasoactive drug: Octreotide (somatostatin analog; causes selective splanchnic vasoconstriction).
  • Infection prophylaxis: Ceftriaxone (1g IV daily) for 7 days (reduces bacterial translocation and mortality in patients with cirrhosis and active bleeding).

Ascites

Accumulation of fluid in the peritoneal cavity.

• Pharmacotherapy: Dual-diuretic therapy with Spironolactone and Furosemide in a 100:40 ratio (typically Spironolactone 100 mg and Furosemide 40 mg daily, titrated up to 400:160 mg). This ratio maintains potassium homeostasis.
• Paracentesis: For large volume paracentesis (> 5 L removed), Albumin (6–8 g per liter of fluid removed) must be administered to prevent post-paracentesis circulatory dysfunction.

Hepatic Encephalopathy (HE)

Reversible neuropsychiatric syndrome caused by accumulation of neurotoxins (mainly ammonia, $NH_3$) due to hepatic dysfunction and shunting.

• Lactulose (First-line): Osmotic laxative metabolized by colonic bacteria to organic acids. This acidifies the colon, converting diffusible ammonia ($NH_3$) into non-absorbable ammonium ($NH_4^+$) (ion trapping), driving excretion. Dose is titrated to achieve 2–3 soft bowel movements per day.
• Rifaximin (Second-line/Add-on): Non-absorbable antibiotic that eliminates ammonia-producing gut bacteria.

Spontaneous Bacterial Peritonitis (SBP)

Infection of ascitic fluid without an intra-abdominal source.

• Diagnosis: Ascitic fluid absolute polymorphonuclear (PMN) count >= 250 cells/mm³.
• Treatment: Ceftriaxone or Cefotaxime for 5–7 days.
• Prophylaxis (Secondary): Daily oral Ciprofloxacin or TMP-SMX to prevent recurrence.

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5. Viral Hepatitis

• Hepatitis B (HBV): Controlled, not cured. First-line oral therapies are nucleoside/nucleotide reverse transcriptase inhibitors: Tenofovir (TDF or TAF) or Entecavir.
• Hepatitis C (HCV): Curable with Direct-Acting Antivirals (DAAs) for 8–12 weeks. Regimens include Sofosbuvir/Velpatasvir (Epclusa) or Glecaprevir/Pibrentasvir (Mavyret).
  • Black Box Warning: All DAAs carry a warning for the risk of Hepatitis B reactivation. All patients must be screened for active/prior HBV infection before starting HCV therapy.