Respiratory & Gastrointestinal Pharmacology

Although both asthma and COPD cause obstructive airflow limitation (FEV₁/FVC <0.70), their underlying immunopathology dictates fundamentally different stepwise pharmacotherapy rules. Asthma is driven by eosinophilic, Th2/IgE-mediated airway hyperresponsiveness and reversible bronchoconstriction; therefore, contemporary GINA guidelines mandate that every adolescent and adult with asthma must receive an Inhaled Corticosteroid (ICS: budesonide, fluticasone, beclomethasone, mometasone)—increasing annexin-1 (lipocortin-1) to inhibit phospholipase A₂ and suppressing NF-κB inflammatory gene transcription—either as needed combined with formoterol in mild asthma or as daily maintenance controller therapy. Crucially, long-acting β₂-agonists (LABAs: salmeterol, formoterol, vilanterol) are STRICTLY CONTRAINDICATED as monotherapy in asthma without a concomitant ICS because unopposed β₂ receptor downregulation masks worsening eosinophilic airway inflammation and increases asthma-related mortality (whereas formoterol + budesonide can be used as single maintenance and reliever therapy [SMART] because formoterol has a rapid 3-minute onset similar to albuterol). Always instruct patients on ICS inhalers to rinse the mouth and spit after every use to prevent oropharyngeal candidiasis (thrush) and dysphonia (vocal cord myopathy). Conversely, in COPD (neutrophilic/macrophage protease-antiprotease destruction), first-line maintenance therapy is long-acting bronchodilation with a LAMA (tiotropium, umeclidinium) and/or LABA, reserving ICS addition only for patients with frequent exacerbations and blood eosinophils ≥300 cells/μL (since ICS increases pneumonia risk in COPD).

Arachidonic acid cleaved from membrane phospholipids by phospholipase A₂ enters either the cyclooxygenase (COX) pathway or the 5-lipoxygenase (5-LOX) pathway in mast cells, eosinophils, and basophils. When aspirin or NSAIDs block COX-1, arachidonic acid is shunted exclusively down the 5-LOX pathway, generating massive quantities of cysteinyl leukotrienes (LTC₄, LTD₄, LTE₄)—which are 1,000 times more potent bronchoconstrictors than histamine and increase microvascular permeability/mucus secretion—alongside neutrophil chemotactic LTB₄. This shunting produces Aspirin-Exacerbated Respiratory Disease (AERD / Samter Triad: asthma, chronic rhinosinusitis with nasal polyps, and acute bronchospasm after aspirin/NSAIDs). Two classes target the leukotriene pathway: montelukast and zafirlukast competitively block the CysLT₁ receptor (first-line oral adjuncts for AERD, exercise-induced bronchospasm, and pediatric allergic rhinitis; montelukast carries an FDA boxed warning for neuropsychiatric events including agitation, depression, sleep disturbances, and suicidal ideation), whereas zileuton directly inhibits 5-lipoxygenase (blocking both LTB₄ and cysteinyl leukotrienes, but requiring serial liver function monitoring due to hepatotoxicity). Theophylline inhibits phosphodiesterase (PDE3/PDE4, raising cAMP) and antagonizes adenosine A₁/A_2B receptors, but has a narrow therapeutic window ( metabolized by CYP1A2/3A4) where toxicity causes tachyarrhythmias and refractory generalized seizures.

Proton pump inhibitors (PPIs: omeprazole, pantoprazole, esomeprazole, lansoprazole) are acid-activated prodrugs that concentrate inside the acidic secretory canaliculi of parietal cells and form an irreversible covalent disulfide bond with the luminal H⁺/K⁺-ATPase, suppressing >90% of basal and meal-stimulated acid secretion for 24–48 hours until new pumps are synthesized (administered 30 minutes before breakfast). Chronic PPI therapy causes hypergastrinemia and impairs pH-dependent intestinal absorption of insoluble calcium carbonate, iron (Fe³⁺→Fe²⁺ reduction), magnesium, and food-bound vitamin B₁₂, increasing risks of hip/vertebral fractures, Clostridioides difficile colitis, community-acquired pneumonia, and acute interstitial nephritis; additionally, omeprazole and esomeprazole inhibit CYP2C19, blocking bioactivation of the prodrug clopidogrel (use pantoprazole if a PPI is required with clopidogrel). Among reversible H₂ receptor antagonists (famotidine, ranitidine, nizatidine, cimetidine), cimetidine is notorious on board exams because it potently inhibits hepatic CYP450 enzymes, blocks androgen receptors (causing gynecomastia, impotence, and hyperprolactinemia/galactorrhea), crosses the blood-brain barrier (delirium in the elderly), and blocks renal tubular creatinine secretion. Sucralfate (aluminum sucrose octasulfate) requires an acidic pH (<4) to polymerize into a viscous cytoprotective gel over ulcer craters (so it must NEVER be given simultaneously with PPIs, H₂ blockers, or antacids!). Misoprostol (a PGE₁ analog) increases gastric mucus/HCO₃⁻ to prevent NSAID ulcers but is strictly contraindicated in pregnancy (Category X abortifacient).

Diabetic and postsurgical gastroparesis requires prokinetic agents that accelerate antral contractions and gastroduodenal coordination. Metoclopramide is a central and peripheral D₂ receptor antagonist (and 5-HT₄ agonist) that relieves nausea and releases the dopaminergic brake on enteric acetylcholine release; however, because it crosses the blood-brain barrier, chronic use (>12 weeks) carries an FDA boxed warning for irreversible tardive dyskinesia, acute dystonia, parkinsonism, and hyperprolactinemia (and metoclopramide is strictly contraindicated in complete mechanical bowel obstruction, GI perforation, pheochromocytoma, and Parkinson disease). Intravenous erythromycin acts directly as a motilin receptor agonist on enteric smooth muscle to trigger migrating motor complexes in acute gastroparesis and prior to upper endoscopy for GI bleeding. For chemotherapy-induced nausea and vomiting (CINV), highly emetogenic regimens (cisplatin) combine a 5-HT₃ antagonist (ondansetron, palonosetron—side effects: QT prolongation, constipation, headache) with a central neurokinin-1 (NK₁ / Substance P) receptor antagonist (aprepitant, fosaprepitant) and dexamethasone. In chronic constipation and irritable bowel syndrome with constipation (IBS-C), lubiprostone activates apical type-2 chloride channels (ClC-2), linaclotide and plecanatide activate brush-border guanylyl cyclase-C (GC-C) to increase intracellular cGMP and stimulate CFTR chloride/water secretion, and lactulose is fermented by colonic bacteria into lactic/acetic acids that lower stool pH to trap ammonia (NH₃ + H⁺ → NH₄⁺) in hepatic encephalopathy.

World Health Organization Group 1 Pulmonary Arterial Hypertension (PAH—mean pulmonary artery pressure >20 mmHg, PCWP ≤15 mmHg, and PVR >2 Wood units) is driven by an imbalance of excessive vasoconstrictive/proliferative endothelin-1 and deficient vasodilatory nitric oxide and prostacyclin (PGI₂). Targeted PAH pharmacotherapy acts on three distinct endothelial-smooth muscle pathways: (1) Endothelin Receptor Antagonists (-sentans: bosentan, ambrisentan, macitentan) competitively block endothelin ET_A (and ET_B) G_q-coupled receptors on pulmonary arterial smooth muscle to prevent vasoconstriction and vascular remodeling; all endothelin antagonists are potent teratogens (Category X—requiring mandatory monthly pregnancy tests under REMS), cause peripheral edema/flushing, and bosentan causes dose-dependent hepatotoxicity (via bile salt export pump inhibition, requiring monthly LFTs); (2) Nitric Oxide–cGMP Enhancers include PDE-5 inhibitors (sildenafil, tadalafil), which prevent cGMP degradation, and riociguat, which directly stimulates soluble guanylyl cyclase independent of NO (both are strictly contraindicated with nitrates or with each other due to severe hypotension); and (3) Prostacyclin (PGI₂) Pathway Agonists include parenteral/inhaled epoprostenol, treprostinil, and iloprost and the oral selective IP receptor agonist selexipag, which activate G_s → cAMP vasodilation and inhibit platelet aggregation (epoprostenol has a 3- to 5-minute half-life requiring continuous central venous infusion; abrupt pump interruption precipitates fatal rebound pulmonary hypertension).

Cystic fibrosis is an autosomal recessive channelopathy caused by mutations in the CFTR gene on chromosome 7q31.2, encoding an ATP-gated cAMP-regulated chloride and bicarbonate channel on the apical membrane of respiratory, pancreatic, biliary, intestinal, and sweat duct epithelia. Precision CFTR modulator therapy depends on the specific mutation class. In Class III "gating" mutations—classically G551D—the CFTR protein is trafficked normally to the apical cell surface, but the ATP-binding channel gate fails to open; ivacaftor is a CFTR potentiator that increases the open probability (channel gating time) of cell-surface CFTR channels. In contrast, the most common mutation worldwide (~85–90% of alleles)—Class II ΔF508 (deletion of phenylalanine at position 508)—causes misfolding of the CFTR protein in the rough endoplasmic reticulum, resulting in ubiquitin-proteasome degradation before the channel can ever reach the apical membrane. Therefore, ΔF508 requires CFTR chaperone "correctors" (elexacaftor, tezacaftor, lumacaftor) to rescue protein folding and trafficking to the cell surface, combined with the potentiator ivacaftor to open the rescued channels at the membrane. Triple combination therapy (elexacaftor + tezacaftor + ivacaftor) dramatically improves FEV₁, reduces sweat chloride, and lowers Pseudomonas exacerbations in patients with at least one ΔF508 allele, alongside maintenance inhaled dornase alfa (recombinant human DNase that cleaves extracellular neutrophil DNA in purulent sputum), hypertonic 7% saline, and pancreatic enzyme replacement.

Pharmacotherapy for Inflammatory Bowel Disease (IBD) stratifies agents into acute induction of remission (systemic or topical glucocorticoids such as oral budesonide—which undergoes 90% first-pass hepatic CYP3A4 metabolism to minimize systemic Cushingoid toxicity in ileocecal Crohn disease—and biologics) versus steroid-sparing maintenance of remission. For mild-to-moderate Ulcerative Colitis (UC), first-line induction and maintenance utilizes 5-aminosalicylic acid (5-ASA / mesalamine) derivatives, which act topically on colonic epithelium to inhibit NF-κB, leukotriene, and prostaglandin synthesis. Because free oral 5-ASA is rapidly absorbed in the jejunum, prodrugs and targeted release formulations are used to deliver 5-ASA to the colon: sulfasalazine links 5-ASA to sulfapyridine via an azo bond cleaved by colonic bacterial azoreductase (the sulfapyridine moiety causes sulfa allergy, reversible oligospermia, hemolysis in G6PD deficiency, and folate malabsorption—requiring folic acid supplementation), whereas mesalamine uses pH-dependent Eudragit coatings or rectal suppositories/enemas for ulcerative proctitis. For moderate-to-severe UC and Crohn disease, maintenance biologics include anti-TNF-α monoclonal antibodies (infliximab, adalimumab, certolizumab pegol, golimumab—requiring mandatory pre-treatment IGRA/PPD tuberculosis and Hepatitis B screening), the gut-selective anti-α₄β₇ integrin antibody vedolizumab (which blocks lymphocyte binding to mucosal addressin cell adhesion molecule-1 [MAdCAM-1] exclusively in GI vessels, completely avoiding natalizumab's brain PML risk!), ustekinumab/risankizumab (anti-IL-12/23 or IL-23), and oral JAK inhibitors (tofacitinib, upadacitinib).

Irritable Bowel Syndrome (IBS—diagnosed via Rome IV criteria of recurrent abdominal pain ≥1 day/week for 3 months associated with defecation and changes in stool frequency or form) is managed with subtype-specific enteric receptor modulators. In constipation-predominant IBS (IBS-C), when soluble fiber (psyllium) and osmotic polyethylene glycol are insufficient, intestinal secretagogues restore luminal fluid: linaclotide and plecanatide agonize luminal guanylyl cyclase-C (GC-C) to raise intracellular cGMP (activating CFTR Cl⁻/HCO₃⁻ secretion while extracellular cGMP inhibits submucosal visceral nociceptive pain fibers), lubiprostone activates apical ClC-2 chloride channels, and tenapanor inhibits the sodium-hydrogen exchanger-3 (NHE3). Conversely, in diarrhea-predominant IBS (IBS-D), targeted options include loperamide (a peripheral μ-opioid agonist on myenteric plexus neurons that slows peristalsis and increases anal sphincter tone without crossing the blood-brain barrier due to P-glycoprotein efflux), eluxadoline (mixed μ-opioid agonist / δ-opioid antagonist; contraindicated in patients without a gallbladder due to sphincter of Oddi spasm and acute pancreatitis), rifaximin (non-absorbable oral rifamycin that treats small intestinal bacterial overgrowth [SIBO] and hepatic encephalopathy), and alosetron (a selective 5-HT₃ antagonist restricted to women with severe refractory IBS-D due to risk of ischemic colitis). Post-cholecystectomy or ileal-resection bile acid diarrhea responds dramatically to bile acid resins (cholestyramine, colestipol, colesevelam), while octreotide reduces splanchnic blood flow in acute esophageal variceal hemorrhage and suppresses secretory diarrhea in VIPoma and carcinoid syndrome.

(1) Endothelin Receptor Antagonists (ERAs: bosentan [non-selective ET_A/ET_B], ambrisentan [selective ET_A], macitentan) block Endothelin-1 (the most potent endogenous vasoconstrictor, which acts via G_q-coupled vascular smooth muscle ET_A receptors to increase IP₃/Ca²⁺ and drive intimal plexiform proliferation). Bosentan causes dose-dependent hepatotoxicity (requires monthly LFTs), peripheral edema, flushing, and severe teratogenicity.

(2) Nitric Oxide–cGMP Pathway Enhancers include PDE-5 Inhibitors (sildenafil, tadalafil), which inhibit phosphodiesterase-5 degradation of cGMP in pulmonary arterial smooth muscle, and Soluble Guanylate Cyclase (sGC) Stimulators (riociguat), which directly stimulate sGC independently of NO and synergize with endogenous NO to synthesize cGMP (used in both Group 1 PAH and Group 4 Chronic Thromboembolic Pulmonary Hypertension [CTEPH]). Crucially, never combine riociguat with PDE-5 inhibitors or nitrates due to fatal systemic hypotension!

(3) Prostacyclin (PGI₂) Pathway Agonists include parenteral/inhaled prostacyclin analogs (epoprostenol [continuous IV infusion with a 3–5 minute half-life; abrupt pump interruption causes fatal rebound pulmonary hypertension!], treprostinil, iloprost) and the oral selective non-prostanoid IP prostacyclin receptor agonist selexipag. These agents stimulate G_s-coupled IP receptors to increase intracellular cAMP, causing potent pulmonary vasodilation and inhibiting platelet aggregation (classic side effects: jaw pain, flushing, headache, diarrhea, and hypotension). Only the small subset (~10%) of idiopathic PAH patients who demonstrate a positive acute vasoreactivity test (inhaled NO or IV epoprostenol during right heart catheterization) are candidates for high-dose dihydropyridine calcium channel blockers (nifedipine, amlodipine, diltiazem).

Cystic Fibrosis (autosomal recessive mutation in the CFTR gene on chromosome 7q31.2 encoding an ABC-transporter Cl⁻ channel) is classified by mutation mechanism: Class II mutations (most notably F508del, accounting for ~85–90% of patients) cause protein misfolding in the rough endoplasmic reticulum and premature ubiquitin-proteasome degradation before reaching the apical membrane; Class III mutations (most notably G551D) reach the apical cell surface normally but exhibit defective ATP-dependent channel gating (opening). Pharmacologic CFTR modulators are divided into Potentiators (ivacaftor), which bind cell-surface CFTR to increase channel open probability (effective as monotherapy ONLY in Class III G551D gating mutations!), and Correctors (elexacaftor, tezacaftor, lumacaftor), which act as molecular chaperones to rescue misfolded F508del CFTR folding and trafficking to the apical membrane. Triple-combination therapy (elexacaftor + tezacaftor + ivacaftor) dramatically improves FEV₁, reduces sweat chloride, and prevents pulmonary exacerbations in patients with at least one F508del allele. Supportive airway clearance combines inhaled dornase alfa (recombinant human deoxyribonuclease I [DNase]), which cleaves extracellular leukocyte DNA released by degenerating neutrophils to reduce sputum viscosity, with inhaled hypertonic (7%) saline (hydrates periciliary mucus layer) and cycled inhaled tobramycin or aztreonam for chronic Pseudomonas aeruginosa colonization.

Selecting the optimal antiemetic on board exams requires matching the clinical etiology to its dominant receptor: For chemotherapy-induced nausea and vomiting (CINV), 5-HT₃ receptor antagonists (ondansetron, granisetron, palonosetron) block vagal afferent and central CTZ serotonin receptors (adverse effects: headache, constipation, QTc prolongation, and serotonin syndrome). Highly emetogenic chemotherapy (cisplatin) requires adding a Neurokinin-1 (NK₁) receptor antagonist (aprepitant, fosaprepitant, rolapitant)—which blocks Substance P in the area postrema during delayed emesis—plus dexamethasone and olanzapine.

For motion sickness and vestibular vertigo, M₁ muscarinic antagonists (transdermal scopolamine patch) and first-generation H₁ antihistamines (meclizine, dimenhydrinate, diphenhydramine, promethazine) block labyrinthine-vestibular signaling (causing anticholinergic dry mouth, blurred vision, urinary retention, and sedation). For hyperemesis gravidarum / Nausea and Vomiting of Pregnancy, first-line therapy is Pyridoxine (Vitamin B₆) + Doxylamine (H₁ antihistamine). For diabetic gastroparesis, the only FDA-approved prokinetic agents are metoclopramide (D₂ antagonist + 5-HT₄ agonist + M₃ sensitizer) and erythromycin (macrolide antibiotic that acts as a direct motilin receptor agonist on gastroduodenal smooth muscle to trigger migrating motor complexes; rapid tachyphylaxis limits chronic use).

Constipation and Irritable Bowel Syndrome (IBS) therapeutics target luminal osmolarity, enteric peristalsis, and apical enterocyte chloride/sodium channels: (1) Bulk-forming laxatives (psyllium, methylcellulose) absorb water to distend the colon; (2) Osmotic laxatives (polyethylene glycol [PEG], lactulose, magnesium hydroxide/citrate, sorbitol) retain intraluminal water (note: non-absorbable lactulose is fermented by colonic bacteria into lactic/acetic acid, lowering luminal pH to trap diffusable NH₃ as non-absorbable NH₄⁺ for fecal excretion in hepatic encephalopathy!); (3) Stimulant/enteric laxatives (senna, bisacodyl) irritate myenteric plexus neurons (chronic senna use causes benign brown lipofuscin-laden macrophages in the colonic lamina propria: melanosis coli); and (4) Emollient stool softeners (docusate) lower surface tension.

In IBS with Constipation (IBS-C) or chronic idiopathic constipation, specialized intestinal secretogogues include: Lubiprostone (activates apical type-2 chloride channels [ClC-2] to secrete Cl⁻ and water into the lumen), Linaclotide and Plecanatide (guanylate cyclase-C [GC-C] agonists that increase intracellular cGMP to activate CFTR chloride/bicarbonate secretion while decreasing visceral pain fiber firing), Tenapanor (inhibits the intestinal sodium-hydrogen exchanger 3 [NHE3]), and Prucalopride (selective high-affinity 5-HT₄ agonist prokinetic). Conversely, in IBS with Diarrhea (IBS-D) or acute non-infectious diarrhea, loperamide and diphenoxylate (+ atropine to deter abuse) agonize peripheral enteric mu-opioid receptors on the myenteric plexus to slow propulsive peristalsis (loperamide is extruded from the CNS by P-glycoprotein; strictly contraindicated in bloody/inflammatory dysentery like Shigella, C. difficile, or EHEC due to toxic megacolon and HUS!), while alosetron (5-HT₃ antagonist; restricted due to ischemic colitis), eluxadoline (mixed mu-agonist/delta-antagonist; contraindicated if prior cholecystectomy due to Sphincter of Oddi spasm/pancreatitis), and rifaximin (non-absorbable rifamycin) treat refractory IBS-D.

Proton Pump Inhibitors (PPIs: omeprazole, pantoprazole, esomeprazole, lansoprazole, rabeprazole) are weak-base prodrugs that accumulate in the acidic secretory canaliculi of stimulated parietal cells, where protonation converts them into reactive sulfenamides that irreversibly inhibit the H⁺/K⁺-ATPase via covalent disulfide bonds (suppressing >90% of 24-hour acid output until new pumps are synthesized; must be taken 30–60 minutes BEFORE meals so pumps are actively translocated to the canalicular membrane!). Chronic PPI therapy carries high-yield board risks: (1) Clostridioides difficile colitis and community-acquired pneumonia (loss of gastric acid barrier), (2) Malabsorption of calcium, magnesium, iron, and Vitamin B₁₂ (acid is required to cleave B₁₂ from dietary animal proteins and solubilize calcium carbonate/ferric iron → osteoporotic hip fractures and hypomagnesemia), (3) Acute Interstitial Nephritis (AIN), and (4) Omeprazole/Esomeprazole inhibition of CYP2C19, which blocks conversion of the antiplatelet prodrug clopidogrel to its active thiol metabolite (use pantoprazole if a PPI is needed alongside clopidogrel!).

H₂ Receptor Antagonists (famotidine, nizatidine, cimetidine) reversibly block histamine H₂ receptors on parietal cells (suppressing ~60–70% of acid secretion, predominantly nocturnal basal acid, but exhibiting rapid tachyphylaxis). Mucosal cytoprotective agents include Sucralfate, an aluminum sucrose octasulfate polymer that requires an acidic environment (pH <4) to cross-link into a viscous negatively charged gel that binds positively charged proteins in ulcer craters (never co-administer simultaneously with PPIs, H₂ blockers, or antacids, as raising gastric pH prevents sucralfate polymerization!), and Bismuth subsalicylate / subcitrate, which coats ulcers, stimulates bicarbonate/prostaglandin secretion, exerts direct bactericidal activity against Helicobacter pylori (part of first-line Bismuth Quadruple Therapy: PPI + Bismuth + Tetracycline + Metronidazole for 10–14 days), and binds enterotoxins in traveler's diarrhea (causes harmless black tongue and black stools from bismuth sulfide formation; avoid in children with viral illness due to salicylate Reye syndrome risk).

In Ulcerative Colitis and Crohn Disease, mild-to-moderate colonic inflammation is treated with 5-Aminosalicylic Acid (5-ASA / mesalamine) derivatives, which topically inhibit mucosal NF-κB, leukotriene, and prostaglandin synthesis. Because oral 5-ASA is rapidly absorbed in the proximal jejunum, delivery to the distal ileum and colon uses three prodrug/formulation strategies: Sulfasalazine (5-ASA linked via an azo bond to sulfapyridine; colonic bacterial azoreductase cleaves the bond to release active 5-ASA in the colon; sulfapyridine absorption causes malaise, sulfa allergy, G6PD hemolysis, folate deficiency [megaloblastic anemia], and reversible oligospermia/male infertility), Balsalazide and Olsalazine (azo-linked prodrugs lacking the toxic sulfapyridine moiety), and pH/time-dependent coated mesalamine (Pentasa releases across the small/large bowel for Crohn; Lialda/Asacol release in the terminal ileum/colon for UC; rectal mesalamine suppositories are first-line for isolated ulcerative proctitis). Moderate-to-severe IBD requires corticosteroid induction (prednisone or oral budesonide—which has 90% first-pass hepatic metabolism to minimize systemic steroid toxicity in ileocecal Crohn disease) bridged to steroid-sparing maintenance with thiopurines (azathioprine, 6-MP) or biologics: Anti-TNF-α (infliximab, adalimumab, certolizumab, golimumab), the gut-selective anti-α₄β₇-integrin antibody vedolizumab (blocks MAdCAM-1 gut homing with ZERO risk of JC virus PML, unlike natalizumab!), the anti-IL-12/23p40 antibody ustekinumab (or anti-IL-23p19 risankizumab), oral JAK inhibitors (upadacitinib, tofacitinib), or S1P modulators (ozanimod).