Drugs Used in the Treatment of Gastrointestinal Disease
Many of the drug groups discussed elsewhere in this book have important applications in the treatment of diseases of the gastrointestinal tract and other organs. Other groups are used almost exclusively for their effects on the gut; these are discussed in the following text according to their therapeutic uses.
Drugs Used in Acid-Peptic Diseases
Acid-peptic diseases include gastroesophageal reflux, peptic ulcer (gastric and duodenal), and stress-related mucosal injury. In all these conditions, mucosal erosions or ulceration arise when the caustic effects of aggressive factors (acid, pepsin, bile) overwhelm the defensive factors of the gastrointestinal mucosa (mucus and bicarbonate secretion, prostaglandins, blood flow, and the processes of restitution and regeneration after cellular injury). Over 90% of peptic ulcers are caused by infection with the bacterium Helicobacter pylori or by use of nonsteroidal anti-inflammatory drugs (NSAIDs). Drugs used in the treatment of acid-peptic disorders may be divided into two classes: agents that reduce intragastric acidity and agents that promote mucosal defense.
Agents that Reduce Intragastric Acidity
Physiology of Acid Secretion
The parietal cell contains receptors for gastrin (CCK-B), histamine (H2), and acetylcholine (muscarinic, M3) (Figure 62-1). When acetylcholine (from vagal postganglionic nerves) or gastrin (released from antral G cells into the blood) bind to the parietal cell receptors, they cause an increase in cytosolic calcium, which in turn stimulates protein kinases that stimulate acid secretion from a H+,K+ ATPase (the proton pump) on the canalicular surface.
In close proximity to the parietal cells are gut endocrine cells called enterochromaffin-like (ECL) cells. ECL cells also have receptors for gastrin and acetylcholine, which stimulate histamine release. Histamine binds to the H2 receptor on the parietal cell, resulting in activation of adenylyl cyclase, which increases intracellular cyclic adenosine monophosphate (cAMP) and activates protein kinases that stimulate acid secretion by the H+,K+ ATPase. In humans, it is believed that the major effect of gastrin upon acid secretion is mediated indirectly through the release of histamine from ECL cells rather than through direct parietal cell stimulation. In contrast, acetylcholine provides potent direct parietal cell stimulation.
Antacids have been used for centuries in the treatment of patients with dyspepsia and acid-peptic disorders. They were the mainstay of treatment for acid-peptic disorders until the advent of H2-receptor antagonists and proton pump inhibitors. They continue to be used commonly by patients as nonprescription remedies for the treatment of intermittent heartburn and dyspepsia.
Antacids are weak bases that react with gastric hydrochloric acid to form a salt and water. Their principal mechanism of action is reduction of intragastric acidity. After a meal, approximately 45 mEq/h of hydrochloric acid is secreted. A single dose of 156 mEq of antacid given 1 hour after a meal effectively neutralizes gastric acid for up to 2 hours. However, the acid-neutralization capacity among different proprietary formulations of antacids is highly variable, depending on their rate of dissolution (tablet versus liquid), water solubility, rate of reaction with acid, and rate of gastric emptying.
Sodium bicarbonate (eg, baking soda, Alka Seltzer) reacts rapidly with hydrochloric acid (HCL) to produce carbon dioxide and sodium chloride. Formation of carbon dioxide results in gastric distention and belching. Unreacted alkali is readily absorbed, potentially causing metabolic alkalosis when given in high doses or to patients with renal insufficiency. Sodium chloride absorption may exacerbate fluid retention in patients with heart failure, hypertension, and renal insufficiency. Calcium carbonate (eg, Tums, Os-Cal) is less soluble and reacts more slowly than sodium bicarbonate with HCl to form carbon dioxide and calcium chloride (CaCl2). Like sodium bicarbonate, calcium carbonate may cause belching or metabolic alkalosis. Calcium carbonate is used for a number of other indications apart from its antacid properties (see Chapter 42). Excessive doses of either sodium bicarbonate or calcium carbonate with calcium-containing dairy products can lead to hypercalcemia, renal insufficiency, and metabolic alkalosis (milk-alkali syndrome).
Formulations containing magnesium hydroxide or aluminum hydroxide react slowly with HCl to form magnesium chloride or aluminum chloride and water. Because no gas is generated, belching does not occur. Metabolic alkalosis is also uncommon because of the efficiency of the neutralization reaction. Because unabsorbed magnesium salts may cause an osmotic diarrhea and aluminum salts may cause constipation, these agents are commonly administered together in proprietary formulations (eg, Gelusil, Maalox, Mylanta) to minimize the impact on bowel function. Both magnesium and aluminum are absorbed and excreted by the kidneys. Hence, patients with renal insufficiency should not take these agents long-term.
All antacids may affect the absorption of other medications by binding the drug (reducing its absorption) or by increasing intragastric pH so that the drug's dissolution or solubility (especially weakly basic or acidic drugs) is altered. Therefore, antacids should not be given within 2 hours of doses of tetracyclines, fluoroquinolones, itraconazole, and iron.
From their introduction in the 1970s until the early 1990s, H2-receptor antagonists (commonly referred to as H2 blockers) were the most commonly prescribed drugs in the world (see Clinical Uses). With the recognition of the role of H pylori in ulcer disease (which may be treated with appropriate antibacterial therapy) and the advent of proton pump inhibitors, the use of prescription H2 blockers has declined markedly.
Chemistry & Pharmacokinetics
Four H2 antagonists are in clinical use: cimetidine, ranitidine, famotidine, and nizatidine. All four agents are rapidly absorbed from the intestine. Cimetidine, ranitidine, and famotidine undergo first-pass hepatic metabolism resulting in a bioavailability of approximately 50%. Nizatidine has little first-pass metabolism. The serum half-lives of the four agents range from 1.1 to 4 hours; however, duration of action depends on the dose given (Table 62-1). H2 antagonists are cleared by a combination of hepatic metabolism, glomerular filtration, and renal tubular secretion. Dose reduction is required in patients with moderate to severe renal (and possibly severe hepatic) insufficiency. In the elderly, there is a decline of up to 50% in drug clearance as well as a significant reduction in volume of distribution.
Table 62-1 Clinical Comparisons of H2-Receptor Blockers.
BID, twice daily; HS, bedtime.
H2-receptor antagonists continue to be prescribed but proton pump inhibitors (see below) are steadily replacing H2 antagonists for most clinical indications. However, the over-the-counter preparations are heavily used by the public.
Gastroesophageal Reflux Disease (GERD)
Patients with infrequent heartburn or dyspepsia (fewer than 3 times per week) may take either antacids or intermittent H2 antagonists. Because antacids provide rapid acid neutralization, they afford faster symptom relief than H2 antagonists. However, the effect of antacids is short-lived (1-2 hours) compared with H2 antagonists (6-10 hours). H2 antagonists may be taken prophylactically before meals in an effort to reduce the likelihood of heartburn. Frequent heartburn is better treated with twice-daily H2 antagonists (Table 62-1) or proton pump inhibitors. In patients with erosive esophagitis (approximately 50% of patients with GERD), H2 antagonists afford healing in less than 50% of patients; hence proton pump inhibitors are preferred because of their superior acid inhibition.
Peptic Ulcer Disease
Proton pump inhibitors have largely replaced H2 antagonists in the treatment of acute peptic ulcer disease. Nevertheless, H2 antagonists are still sometimes used. Nocturnal acid suppression by H2 antagonists affords effective ulcer healing in most patients with uncomplicated gastric and duodenal ulcers. Hence, all the agents may be administered once daily at bedtime, resulting in ulcer healing rates of more than 80-90% after 6-8 weeks of therapy. For patients with ulcers caused by aspirin or other NSAIDs, the NSAID should be discontinued. If the NSAID must be continued for clinical reasons despite active ulceration, a proton pump inhibitor should be given instead of an H2 antagonist to more reliably promote ulcer healing. For patients with acute peptic ulcers caused by H pylori, H2 antagonists no longer play a significant therapeutic role. H pylori should be treated with a 10- to 14-day course of therapy including a proton pump inhibitor and two antibiotics (see below). This regimen achieves ulcer healing and eradication of the infection in more than 90% of patients. For the minority of patients in whom H pylori cannot be successfully eradicated, H2 antagonists may be given daily at bedtime in half of the usual ulcer therapeutic dose to prevent ulcer recurrence (eg, ranitidine, 150 mg; famotidine, 20 mg).
H2 antagonists are commonly used as over-the-counter agents and prescription agents for treatment of intermittent dyspepsia not caused by peptic ulcer. However, benefit compared with placebo has never been convincingly demonstrated.
Prevention of Bleeding from Stress-Related Gastritis
Clinically important bleeding from upper gastrointestinal erosions or ulcers occurs in 1-5% of critically ill patients as a result of impaired mucosal defense mechanisms caused by poor perfusion. Although most critically ill patients have normal or decreased acid secretion, numerous studies have shown that agents that increase intragastric pH (H2 antagonists or proton pump inhibitors) reduce the incidence of clinically significant bleeding. However, the optimal agent is uncertain at this time. For patients without a nasoenteric tube or with significant ileus, intravenous H2 antagonists are preferable over intravenous proton pump inhibitors because of their proven efficacy and lower cost. Continuous infusions of H2 antagonists are generally preferred to bolus infusions because they achieve more consistent, sustained elevation of intragastric pH.
H2 antagonists are extremely safe drugs. Adverse effects occur in less than 3% of patients and include diarrhea, headache, fatigue, myalgias, and constipation. Some studies suggest that intravenous H2 antagonists (or proton pump inhibitors) may increase the risk of nosocomial pneumonia in critically ill patients.
Mental status changes (confusion, hallucinations, agitation) may occur with administration of intravenous H2 antagonists, especially in patients in the intensive care unit who are elderly or who have renal or hepatic dysfunction. These events may be more common with cimetidine. Mental status changes rarely occur in ambulatory patients.
Cimetidine inhibits binding of dihydrotestosterone to androgen receptors, inhibits metabolism of estradiol, and increases serum prolactin levels. When used long-term or in high doses, it may cause gynecomastia or impotence in men and galactorrhea in women. These effects are specific to cimetidine and do not occur with the other H2 antagonists.
Although there are no known harmful effects on the fetus, H2 antagonists cross the placenta. Therefore, they should not be administered to pregnant women unless absolutely necessary. The H2 antagonists are secreted into breast milk and may therefore affect nursing infants.
H2 antagonists may rarely cause blood dyscrasias. Blockade of cardiac H2 receptors may cause bradycardia, but this is rarely of clinical significance. Rapid intravenous infusion may cause bradycardia and hypotension through blockade of cardiac H2 receptors; therefore, intravenous injections should be given over 30 minutes. H2 antagonists rarely cause reversible abnormalities in liver chemistry.
Cimetidine interferes with several important hepatic cytochrome P450 drug metabolism pathways, including those catalyzed by CYP1A2, CYP2C9, CYP2D6, and CYP3A4 (see Chapter 4). Hence, the half-lives of drugs metabolized by these pathways may be prolonged. Ranitidine binds 4-10 times less avidly than cimetidine to cytochrome P450. Negligible interaction occurs with nizatidine and famotidine.
H2 antagonists compete with creatinine and certain drugs (eg, procainamide) for renal tubular secretion. All of these agents except famotidine inhibit gastric first-pass metabolism of ethanol, especially in women. Although the importance of this is debated, increased bioavailability of ethanol could lead to increased blood ethanol levels.
Proton Pump Inhibitors
Since their introduction in the late 1980s, these efficacious acid inhibitory agents have assumed the major role for the treatment of acid-peptic disorders. Proton pump inhibitors (PPIs) are now among the most widely prescribed drugs worldwide due to their outstanding efficacy and safety.
Chemistry & Pharmacokinetics
Five proton pump inhibitors are available for clinical use: omeprazole, lansoprazole, rabeprazole, pantoprazole, and esomeprazole. All are substituted benzimidazoles that resemble H2 antagonists in structure (Figure 62-3) but have a completely different mechanism of action. Omeprazole is a racemic mixture of R- and S-isomers. Esomeprazole is the S-isomer of omeprazole. All are available in oral formulations. Esomeprazole and pantoprazole are also available in intravenous formulations
Table 62-2 Pharmacokinetics of Proton Pump InhibItors.
Proton pump inhibitors are administered as inactive prodrugs. To protect the acid-labile prodrug from rapid destruction within the gastric lumen, oral products are formulated for delayed release as acid-resistant, enteric-coated capsules or tablets. After passing through the stomach into the alkaline intestinal lumen, the enteric coatings dissolve and the prodrug is absorbed. For children or patients with dysphagia or enteral feeding tubes, capsules may be opened and the microgranules mixed with apple or orange juice or mixed with soft foods (eg, applesauce). Lansoprazole is also available as a tablet formulation that disintegrates in the mouth, or it may be mixed with water and administered via oral syringe or enteral tube. Omeprazole is also available as a powder formulation (capsule or packet) that contains sodium bicarbonate (1100-1680 mg NaHCO3 ; 304-460 mg of sodium) to protect the naked (non-enteric-coated) drug from acid degradation. When administered on an empty stomach by mouth or enteral tube, this "immediate-release" suspension results in rapid omeprazole absorption (Tmax < 30 minutes) and onset of acid inhibition.
The proton pump inhibitors are lipophilic weak bases (pKa 4-5) and after intestinal absorption diffuse readily across lipid membranes into acidified compartments (eg, the parietal cell canaliculus). The prodrug rapidly becomes protonated within the canaliculus and is concentrated more than 1000-fold by Henderson-Hasselbalch trapping (see Chapter 1). There, it rapidly undergoes a molecular conversion to the active form, a reactive thiophilic sulfenamide cation, which forms a covalent disulfide bond with the H+,K+ ATPase, irreversibly inactivating the enzyme.
From a pharmacokinetic perspective, proton pump inhibitors are ideal drugs: they have a short serum half-life, they are concentrated and activated near their site of action, and they have a long duration of action.
In contrast to H2 antagonists, proton pump inhibitors inhibit both fasting and meal-stimulated secretion because they block the final common pathway of acid secretion, the proton pump. In standard doses, proton pump inhibitors inhibit 90-98% of 24-hour acid secretion (Figure 62-2). When administered at equivalent doses, the different agents show little difference in clinical efficacy. In a crossover study of patients receiving long-term therapy with all five proton pump inhibitors, the mean 24-hour intragastric pH varied from 3.3 (pantoprazole, 40 mg) to 4.0 (esomeprazole, 40 mg) and the mean number of hours the pH was higher than 4 varied from 10.1 (pantoprazole, 40 mg) to 14.0 (esomeprazole, 40 mg).
Gastroesophageal Reflux Disease (GERD)
Proton pump inhibitors are the most effective agents for the treatment of nonerosive and erosive reflux disease, esophageal complications of reflux disease (peptic stricture or Barrett's esophagus), and extraesophageal manifestations of reflux disease. Once-daily dosing provides effective symptom relief and tissue healing in 85-90% of patients; up to 15% of patients require twice-daily dosing.
GERD symptoms recur in over 80% of patients within 6 months after discontinuation of a proton pump inhibitor. For patients with erosive esophagitis or esophageal complications, long-term daily maintenance therapy with a full-dose or half-dose proton pump inhibitor is usually needed. Many patients with nonerosive GERD may be treated successfully with intermittent courses of proton pump inhibitors or H2 antagonists taken as needed ("on demand") for recurrent symptoms.
In current clinical practice, many patients with symptomatic GERD are treated empirically with medications without prior endoscopy, ie, without knowledge of whether the patient has erosive or nonerosive reflux disease. Empiric treatment with proton pump inhibitors provides sustained symptomatic relief in 70-80% of patients, compared with 50-60% with H2 antagonists. Because of recent cost reductions, proton pump inhibitors are being used increasingly as first-line therapy for patients with symptomatic GERD.
Sustained acid suppression with twice-daily proton pump inhibitors for at least 3 months is used to treat extraesophageal complications of reflux disease (asthma, chronic cough, laryngitis, and noncardiac chest pain).
Peptic Ulcer Disease
Compared with H2 antagonists, proton pump inhibitors afford more rapid symptom relief and faster ulcer healing for duodenal ulcers and, to a lesser extent, gastric ulcers. All the pump inhibitors heal more than 90% of duodenal ulcers within 4 weeks and a similar percentage of gastric ulcers within 6-8 weeks.
H pylori-Associated Ulcers
For H pylori-associated ulcers, there are two therapeutic goals: to heal the ulcer and to eradicate the organism. The most effective regimens for H pylori eradication are combinations of two antibiotics and a proton pump inhibitor. Proton pump inhibitors promote eradication of H pylori through several mechanisms: direct antimicrobial properties (minor) and—by raising intragastric pH—lowering the minimal inhibitory concentrations of antibiotics against H pylori. The best treatment regimen consists of a 14-day regimen of "triple therapy": a proton pump inhibitor twice daily; clarithromycin, 500 mg twice daily; and either amoxicillin, 1 g twice daily, or metronidazole, 500 mg twice daily. After completion of triple therapy, the proton pump inhibitor should be continued once daily for a total of 4-6 weeks to ensure complete ulcer healing. Recently, 10 days of "sequential treatment" consisting on days 1-5 of a proton pump inhibitor twice daily plus amoxicillin, 1 g twice daily, and followed on days 6-10 by five additional days of a proton pump inhibitor twice daily, plus clarithromycin, 500 mg twice daily, and tinidazole, 500 mg twice daily, has been shown to be a highly effective treatment regimen.
For patients with ulcers caused by aspirin or other NSAIDs, either H2 antagonists or proton pump inhibitors provide rapid ulcer healing so long as the NSAID is discontinued; however continued use of the NSAID impairs ulcer healing. In patients with NSAID-induced ulcers who require continued NSAID therapy, treatment with a once- or twice-daily proton pump inhibitor more reliably promotes ulcer healing.
Asymptomatic peptic ulceration develops in 10-20% of people taking frequent NSAIDs, and ulcer-related complications (bleeding, perforation) develop in 1-2% of persons per year. Proton pump inhibitors taken once daily are effective in reducing the incidence of ulcers and ulcer complications in patients taking aspirin or other NSAIDs.
Prevention of Rebleeding from Peptic Ulcers
In patients with acute gastrointestinal bleeding due to peptic ulcers, the risk of rebleeding from ulcers that have a visible vessel or adherent clot is increased. Rebleeding of this subset of high-risk ulcers is reduced significantly with proton pump inhibitors administered for 3-5 days either as high-dose oral therapy (eg, omeprazole, 40 mg orally twice daily) or as a continuous intravenous infusion. It is believed that an intragastric pH higher than 6 may enhance coagulation and platelet aggregation. The optimal dose of intravenous proton pump inhibitor needed to achieve and maintain this level of near-complete acid inhibition is unknown; however, initial bolus administration (80 mg) followed by constant infusion (8 mg/h) is commonly recommended.
Proton pump inhibitors have modest efficacy for treatment of nonulcer dyspepsia, benefiting 10-20% more patients than placebo. Despite their use for this indication, superiority to H2 antagonists (or even placebo) has not been conclusively demonstrated.
Prevention of Stress-Related Mucosal Bleeding
As discussed previously (see H2-Receptor Antagonists) proton pump inhibitors (given orally, by nasogastric tube, or by intravenous infusions) may be administered to reduce the risk of clinically significant stress-related mucosal bleeding in critically ill patients. The only proton pump inhibitor approved by the Food and Drug Administration (FDA) for this indication is an oral immediate-release omeprazole formulation, which is administered by nasogastric tube twice daily on the first day, then once daily. For patients with nasoenteric tubes, immediate-release omeprazole may be preferred to intravenous H2 antagonists or proton pump inhibitors because of comparable efficacy, lower cost, and ease of administration.
For patients without a nasoenteric tube or with significant ileus, intravenous H2 antagonists are preferred to intravenous proton pump inhibitors because of their proven efficacy and lower cost. Although proton pump inhibitors are increasingly used, there are no controlled trials demonstrating efficacy or optimal dosing.
Gastrinoma and Other Hypersecretory Conditions
Patients with isolated gastrinomas are best treated with surgical resection. In patients with metastatic or unresectable gastrinomas, massive acid hypersecretion results in peptic ulceration, erosive esophagitis, and malabsorption. Previously, these patients required vagotomy and extraordinarily high doses of H2 antagonists, which still resulted in suboptimal acid suppression. With proton pump inhibitors, excellent acid suppression can be achieved in all patients. Dosage is titrated to reduce basal acid output to less than 5-10 mEq/h. Typical doses of omeprazole are 60-120 mg/d.
Proton pump inhibitors are extremely safe. Diarrhea, headache, and abdominal pain are reported in 1-5% of patients, although the frequency of these events is only slightly increased compared with placebo. Proton pump inhibitors do not have teratogenicity in animal models; however, safety during pregnancy has not been established.
Acid is important in releasing vitamin B12 from food. A minor reduction in oral cyanocobalamin absorption occurs during proton pump inhibition, potentially leading to subnormal B12 levels with prolonged therapy. Acid also promotes absorption of food-bound minerals (iron, calcium, zinc); however, no mineral deficiencies have been reported with proton pump inhibitor therapy. Recent case-control studies have suggested a modest increase in the risk of hip fracture in patients taking proton pump inhibitors over a long term compared with matched controls. Although a causal relationship is unproven, proton pump inhibitors may reduce calcium absorption or inhibit osteoclast function. Pending further studies, patients who require long-term proton pump inhibitors—especially those with risk factors for osteoporosis—should have monitoring of bone density and should be provided calcium supplements.
Respiratory and Enteric Infections
Gastric acid is an important barrier to colonization and infection of the stomach and intestine from ingested bacteria. Increases in gastric bacterial concentrations are detected in patients taking proton pump inhibitors, which is of unknown clinical significance. Some studies have reported an increased risk of both community-acquired respiratory infections and nosocomial pneumonia among patients taking proton pump inhibitors.
A small increased risk of enteric infections may exist in patients taking proton pump inhibitors, especially when traveling in underdeveloped countries. Hospitalized patients may have an increased risk for Clostridium difficile infection.
Potential Problems Due to Increased Serum Gastrin
Gastrin levels are regulated by intragastric acidity. Acid suppression alters normal feedback inhibition so that median serum gastrin levels rise 1.5- to 2-fold in patients taking proton pump inhibitors. Although gastrin levels remain within normal limits in most patients, they exceed 500 pg/mL (normal, < 100 pg/mL) in 3%. Upon stopping the drug, the levels normalize within 4 weeks.
The rise in serum gastrin levels in patients receiving long-term therapy with proton pump inhibitors raises a theoretical concern because gastrin may stimulate hyperplasia of ECL cells. In female rats given proton pump inhibitors for prolonged periods, gastric carcinoid tumors developed in areas of ECL hyperplasia. Although humans who take proton pump inhibitors for a long time may exhibit ECL hyperplasia in response to hypergastrinemia, carcinoid tumor formation has not been documented. At present, routine monitoring of serum gastrin levels is not recommended in patients receiving prolonged proton pump inhibitor therapy.
Other Potential Problems Due to Decreased Gastric Acidity
Among patients infected with H pylori, long-term acid suppression leads to increased chronic inflammation in the gastric body and decreased inflammation in the antrum. Concerns have been raised that increased gastric inflammation may accelerate gastric gland atrophy (atrophic gastritis) and intestinal metaplasia—known risk factors for gastric adenocarcinoma. A special FDA Gastrointestinal Advisory Committee concluded that there is no evidence that prolonged proton pump inhibitor therapy produces the kind of atrophic gastritis (multifocal atrophic gastritis) or intestinal metaplasia that is associated with increased risk of adenocarcinoma. Routine testing for H pylori is not recommended in patients who require long-term proton pump inhibitor therapy. Long-term proton pump inhibitor therapy is associated with the development of small benign gastric fundic-gland polyps in a small number of patients, which may disappear after stopping the drug and are of uncertain clinical significance.
Decreased gastric acidity may alter absorption of drugs for which intragastric acidity affects drug bioavailability, eg, ketoconazole, itraconazole, digoxin, and atazanavir. All proton pump inhibitors are metabolized by hepatic P450 cytochromes, including CYP2C19 and CYP3A4. Because of the short half-lives of proton pump inhibitors, clinically significant drug interactions are rare. Omeprazole may inhibit the metabolism of warfarin, diazepam, and phenytoin. Esomeprazole also may decrease metabolism of diazepam. Lansoprazole may enhance clearance of theophylline. Rabeprazole and pantoprazole have no significant drug interactions.
Mucosal Protective Agents
The gastroduodenal mucosa has evolved a number of defense mechanisms to protect itself against the noxious effects of acid and pepsin. Both mucus and epithelial cell-cell tight junctions restrict back diffusion of acid and pepsin. Epithelial bicarbonate secretion establishes a pH gradient within the mucous layer in which the pH ranges from 7 at the mucosal surface to 1-2 in the gastric lumen. Blood flow carries bicarbonate and vital nutrients to surface cells. Areas of injured epithelium are quickly repaired by restitution, a process in which migration of cells from gland neck cells seals small erosions to reestablish intact epithelium. Mucosal prostaglandins appear to be important in stimulating mucus and bicarbonate secretion and mucosal blood flow. A number of agents that potentiate these mucosal defense mechanisms are available for the prevention and treatment of acid-peptic disorders.
Chemistry & Pharmacokinetics
Sucralfate is a salt of sucrose complexed to sulfated aluminum hydroxide. In water or acidic solutions it forms a viscous, tenacious paste that binds selectively to ulcers or erosions for up to 6 hours. Sucralfate has limited solubility, breaking down into sucrose sulfate (strongly negatively charged) and an aluminum salt. Less than 3% of intact drug and aluminum is absorbed from the intestinal tract; the remainder is excreted in the feces.
A variety of beneficial effects have been attributed to sucralfate, but the precise mechanism of action is unclear. It is believed that the negatively charged sucrose sulfate binds to positively charged proteins in the base of ulcers or erosion, forming a physical barrier that restricts further caustic damage and stimulates mucosal prostaglandin and bicarbonate secretion.
Sucralfate is administered in a dosage of 1 g four times daily on an empty stomach (at least 1 hour before meals). At present, its clinical uses are limited. Sucralfate (administered as a slurry through a nasogastric tube) reduces the incidence of clinically significant upper gastrointestinal bleeding in critically ill patients hospitalized in the intensive care unit, although it is slightly less effective than intravenous H2 antagonists. Sucralfate is still used by many clinicians for prevention of stress-related bleeding because of concerns that acid inhibitory therapies (antacids, H2 antagonists, and proton pump inhibitors) may increase the risk of nosocomial pneumonia.
Because it is not absorbed, sucralfate is virtually devoid of systemic adverse effects. Constipation occurs in 2% of patients due to the aluminum salt. Because a small amount of aluminum is absorbed, it should not be used for prolonged periods in patients with renal insufficiency.
Sucralfate may bind to other medications, impairing their absorption.
Chemistry & Pharmacokinetics
The human gastrointestinal mucosa synthesizes a number of prostaglandins (see Chapter 18); the primary ones are prostaglandins E and F. Misoprostol, a methyl analog of PGE1 , has been approved for gastrointestinal conditions. After oral administration, it is rapidly absorbed and metabolized to a metabolically active free acid. The serum half-life is less than 30 minutes; hence, it must be administered 3-4 times daily. It is excreted in the urine; however, dose reduction is not needed in patients with renal insufficiency.
Misoprostol has both acid inhibitory and mucosal protective properties. It is believed to stimulate mucus and bicarbonate secretion and enhance mucosal blood flow. In addition, it binds to a prostaglandin receptor on parietal cells, reducing histamine-stimulated cAMP production and causing modest acid inhibition. Prostaglandins have a variety of other actions, including stimulation of intestinal electrolyte and fluid secretion, intestinal motility, and uterine contractions.
Peptic ulcers develop in approximately 10-20% of patients who receive long-term NSAID therapy (see Proton Pump Inhibitors, above). Misoprostol reduces the incidence of NSAID-induced ulcers to less than 3% and the incidence of ulcer complications by 50%. It is approved for prevention of NSAID-induced ulcers in high-risk patients; however, misoprostol has never achieved widespread use owing to its high adverse-effect profile and need for multiple daily dosing. As discussed, proton pump inhibitors may be as effective as and better tolerated than misoprostol for this indication. Cyclooxygenase-2-selective NSAIDs, which may have less gastrointestinal toxicity (see Chapter 36), offer another option for patients at high-risk for NSAID-induced complications.
Adverse Effects & Drug Interactions
Diarrhea and cramping abdominal pain occur in 10-20% of patients. Because misoprostol stimulates uterine contractions (see Chapter 18), it should not be used during pregnancy or in women of childbearing potential unless they have a negative serum pregnancy test and are compliant with effective contraceptive measures. No significant drug interactions are reported.
Chemistry & Pharmacokinetics
Two bismuth compounds are available: bismuth subsalicylate, a nonprescription formulation containing bismuth and salicylate, and bismuth subcitrate potassium. In the USA, bismuth subcitrate is available only as a combination prescription product that also contains metronidazole and tetracycline for the treatment of H pylori. Bismuth subsalicylate undergoes rapid dissociation within the stomach, allowing absorption of salicylate. Over 99% of the bismuth appears in the stool. Although minimal (< 1%), bismuth is absorbed; it is stored in many tissues and has slow renal excretion. Salicylate (like aspirin) is readily absorbed and excreted in the urine.
The precise mechanisms of action of bismuth are unknown. Bismuth coats ulcers and erosions, creating a protective layer against acid and pepsin. It may also stimulate prostaglandin, mucus, and bicarbonate secretion. Bismuth subsalicylate reduces stool frequency and liquidity in acute infectious diarrhea, due to salicylate inhibition of intestinal prostaglandin and chloride secretion. Bismuth has direct antimicrobial effects and binds enterotoxins, accounting for its benefit in preventing and treating traveler's diarrhea. Bismuth compounds have direct antimicrobial activity against H pylori.
In spite of the lack of comparative trials, nonprescription bismuth compounds are widely used by patients for the nonspecific treatment of dyspepsia and acute diarrhea. Bismuth subsalicylate also is used for the prevention of traveler's diarrhea (30 mL or 2 tablets four times daily).
Bismuth compounds are used in 4 drug regimens for the eradication of H pylori infection. One regimen consists of a proton pump inhibitor twice daily combined with bismuth subsalicylate (2 tablets; 262 mg each), tetracycline (250-500 mg), and metronidazole (500 mg) four times daily for 10-14 days. Another regimen consists of a proton pump inhibitor twice daily combined with three capsules of a combination prescription formulation (each capsule containing bismuth subcitrate 140 mg, metronidazole 125 mg, and tetracycline 125 mg) taken four times daily for 10 days. Although these are effective, standard "triple therapy" regimens (ie, proton pump inhibitor, clarithromycin, and amoxicillin or metronidazole twice daily for 14 days) generally are preferred for first-line therapy because of twice-daily dosing and superior compliance. Bismuth-based quadruple therapies commonly are used as second-line therapies.
All bismuth formulations have excellent safety profiles. Bismuth causes harmless blackening of the stool, which may be confused with gastrointestinal bleeding. Liquid formulations may cause harmless darkening of the tongue. Bismuth agents should be used for short periods only and should be avoided in patients with renal insufficiency. Prolonged usage of some bismuth compounds may rarely lead to bismuth toxicity, resulting in encephalopathy (ataxia, headaches, confusion, seizures). However, such toxicity is not reported with bismuth subsalicylate or bismuth citrate. High dosages of bismuth subsalicylate may lead to salicylate toxicity.
Drugs Stimulating Gastrointestinal Motility
Drugs that can selectively stimulate gut motor function (prokinetic agents) have significant potential clinical usefulness. Agents that increase lower esophageal sphincter pressures may be useful for GERD. Drugs that improve gastric emptying may be helpful for gastroparesis and postsurgical gastric emptying delay. Agents that stimulate the small intestine may be beneficial for postoperative ileus or chronic intestinal pseudo-obstruction. Finally, agents that enhance colonic transit may be useful in the treatment of constipation. Unfortunately, only a limited number of agents in this group are available for clinical use at this time.
Gastroesophageal Reflux Disease
Metoclopramide is available for clinical use in the USA; domperidone is available in many other countries. These agents are sometimes used in the treatment of symptomatic GERD but are not effective in patients with erosive esophagitis. Because of the superior efficacy and safety of antisecretory agents in the treatment of heartburn, prokinetic agents are used mainly in combination with antisecretory agents in patients with regurgitation or refractory heartburn.
Impaired Gastric Emptying
These agents are widely used in the treatment of patients with delayed gastric emptying due to postsurgical disorders (vagotomy, antrectomy) and diabetic gastroparesis. Metoclopramide is sometimes administered in hospitalized patients to promote advancement of nasoenteric feeding tubes from the stomach into the duodenum.
These agents lead to symptomatic improvement in a small number of patients with chronic dyspepsia.
Prevention of Vomiting
Because of their potent antiemetic action, metoclopramide and domperidone are used for the prevention and treatment of emesis.
Postpartum Lactation Stimulation
Domperidone is sometimes recommended to promote postpartum lactation (see also Adverse Effects).
The most common adverse effects of metoclopramide involve the central nervous system. Restlessness, drowsiness, insomnia, anxiety, and agitation occur in 10-20% of patients, especially the elderly. Extrapyramidal effects (dystonias, akathisia, parkinsonian features) due to central dopamine receptor blockade occur acutely in 25% of patients given high doses and in 5% of patients receiving long-term therapy. Tardivedyskinesia, sometimes irreversible, has developed in patients treated for a prolonged period with metoclopramide. For this reason, long-term use should be avoided unless absolutely necessary, especially in the elderly. Elevated prolactin levels (caused by both metoclopramide and domperidone) can cause galactorrhea, gynecomastia, impotence, and menstrual disorders.
Domperidone is extremely well tolerated. Because it does not cross the blood-brain barrier to a significant degree, neuropsychiatric and extrapyramidal effects are rare.
Macrolide antibiotics such as erythromycin directly stimulate motilin receptors on gastrointestinal smooth muscle and promote the onset of a migrating motor complex. Intravenous erythromycin (3 mg/kg) is beneficial in some patients with gastroparesis; however, tolerance rapidly develops. It may be used in patients with acute upper gastrointestinal hemorrhage to promote gastric emptying of blood before endoscopy.
The overwhelming majority of people do not need laxatives; yet they are self-prescribed by a large portion of the population. For most people, intermittent constipation is best prevented with a high-fiber diet, adequate fluid intake, regular exercise, and the heeding of nature's call. Patients not responding to dietary changes or fiber supplements should undergo medical evaluation before initiating long-term laxative treatment. Laxatives may be classified by their major mechanism of action, but many work through more than one mechanism.
Bulk-forming laxatives are indigestible, hydrophilic colloids that absorb water, forming a bulky, emollient gel that distends the colon and promotes peristalsis. Common preparations include natural plant products (psyllium, methylcellulose) and synthetic fibers (polycarbophil). Bacterial digestion of plant fibers within the colon may lead to increased bloating and flatus.
Stool Surfactant Agents (Softeners)
These agents soften stool material, permitting water and lipids to penetrate. They may be administered orally or rectally. Common agents include docusate (oral or enema) and glycerin suppository. In hospitalized patients, docusate is commonly prescribed to prevent constipation and minimize straining. Mineral oil is a clear, viscous oil that lubricates fecal material, retarding water absorption from the stool. It is used to prevent and treat fecal impaction in young children and debilitated adults. It is not palatable but may be mixed with juices. Aspiration can result in a severe lipid pneumonitis. Long-term use can impair absorption of fat-soluble vitamins (A, D, E, K).
The colon can neither concentrate nor dilute fecal fluid: fecal water is isotonic throughout the colon. Osmotic laxatives are soluble but nonabsorbable compounds that result in increased stool liquidity due to an obligate increase in fecal fluid.
Nonabsorbable Sugars or Salts
These agents may be used for the treatment of acute constipation or the prevention of chronic constipation. Magnesium hydroxide (milk of magnesia) is a commonly used osmotic laxative. It should not be used for prolonged periods in patients with renal insufficiency due to the risk of hypermagnesemia. Sorbitol and lactulose are nonabsorbable sugars that can be used to prevent or treat chronic constipation. These sugars are metabolized by colonic bacteria, producing severe flatus and cramps.
High doses of osmotically active agents produce prompt bowel evacuation (purgation) within 1-3 hours. The rapid movement of water into the distal small bowel and colon leads to a high volume of liquid stool followed by rapid relief of constipation. The most commonly used purgatives are magnesium citrate and sodium phosphate. Sodium phosphate is available as a nonprescription liquid formulation and by prescription as a tablet formulation. When taking these agents, it is very important that patients maintain adequate hydration by taking increased oral liquids to compensate for fecal fluid loss. Sodium phosphate frequently causes hyperphosphatemia, hypocalcemia, hypernatremia, and hypokalemia. Although these electrolyte abnormalities are clinically insignificant in most patients, they may lead to cardiac arrhythmias or acute renal failure due to tubular deposition of calcium phosphate (nephrocalcinosis). Sodium phosphate preparations should not be used in patients who are frail or elderly, have renal insufficiency, have significant cardiac disease, or are unable to maintain adequate hydration during bowel preparation.
Balanced Polyethylene Glycol
Lavage solutions containing polyethylene glycol (PEG) are used for complete colonic cleansing before gastrointestinal endoscopic procedures. These balanced, isotonic solutions contain an inert, nonabsorbable, osmotically active sugar (PEG) with sodium sulfate, sodium chloride, sodium bicarbonate, and potassium chloride. The solution is designed so that no significant intravascular fluid or electrolyte shifts occur. Therefore, they are safe for all patients. The solution should be ingested rapidly (2-4 L over 2-4 hours) to promote bowel cleansing. For treatment or prevention of chronic constipation, smaller doses of PEG powder may be mixed with water or juices (17 g/8 oz) and ingested daily. In contrast to sorbitol or lactulose, PEG does not produce significant cramps or flatus.
Stimulant laxatives (cathartics) induce bowel movements through a number of poorly understood mechanisms. These include direct stimulation of the enteric nervous system and colonic electrolyte and fluid secretion. There has been concern that long-term use of cathartics could lead to dependence and destruction of the myenteric plexus, resulting in colonic atony and dilation. More recent research suggests that long-term use of these agents probably is safe in most patients. Cathartics may be required on a long-term basis, especially in patients who are neurologically impaired and in bed-bound patients in long-term care facilities.
Aloe, senna, and cascara occur naturally in plants. These laxatives are poorly absorbed and after hydrolysis in the colon, produce a bowel movement in 6-12 hours when given orally and within 2 hours when given rectally. Chronic use leads to a characteristic brown pigmentation of the colon known as "melanosis coli." There has been some concern that these agents may be carcinogenic, but epidemiologic studies do not suggest a relation to colorectal cancer.
Bisacodyl is available in tablet and suppository formulations for the treatment of acute and chronic constipation. It also is used in conjunction with PEG solutions for colonic cleansing prior to colonoscopy. It induces a bowel movement within 6-10 hours when given orally and 30-60 minutes when taken rectally. It has minimal systemic absorption and appears to be safe for acute and long-term use. Phenolphthalein, another agent in this class, was removed from the market owing to concerns about possible cardiac toxicity.
Chloride Channel Activator
Lubiprostone is a prostanoic acid derivative labeled for use in chronic constipation and irritable bowel syndrome (IBS) with predominant constipation. It acts by stimulating the type 2 chloride channel (ClC-2) in the small intestine. This increases chloride-rich fluid secretion into the intestine, which stimulates intestinal motility and shortens intestinal transit time. Over 50% of patients experience a bowel movement within 24 hours of taking one dose. There appears to be no loss of efficacy with long-term therapy. After discontinuation of the drug, constipation may return to its pretreatment severity. Lubiprostone has minimal systemic absorption but is designated category C for pregnancy because of increased fetal loss in guinea pigs. Lubiprostone may cause nausea in up to 30% of patients due to delayed gastric emptying.
Opioid Receptor Antagonists
Acute and chronic therapy with opioids may cause constipation by decreasing intestinal motility, which results in prolonged transit time and increased absorption of fecal water (see Chapter 31). Use of opioids after surgery for treatment of pain as well as endogenous opioids also may prolong the duration of postoperative ileus. These effects are mainly mediated through intestinal mu ()-opioid receptors. Two selective antagonists of the -opioid receptor are commercially available: methylnaltrexone bromide and alvimopan. Because these agents do not readily cross the blood-brain barrier, they inhibit peripheral -opioid receptors without impacting analgesic effects within the central nervous system. Methylnaltrexone is approved for the treatment of opioid-induced constipation in patients receiving palliative care for advanced illness who have had inadequate response to other agents. It is administered as a subcutaneous injection (0.15 mg/kg) every 2 days. Alvimopan is approved for short-term use to shorten the period of postoperative ileus in hospitalized patients who have undergone small or large bowel resection. Alvimopan (12 mg capsule) is administered orally within 5 hours before surgery and twice daily after surgery until bowel function has recovered, but for no more than 7 days. Because of possible cardiovascular toxicity, alvimopan currently is restricted to short-term use in hospitalized patients only.
Serotonin 5-HT4-Receptor Agonists
Stimulation of 5-HT4 receptors on the presynaptic terminal of submucosal intrinsic primary afferent nerves enhances the release of their neurotransmitters, including calcitoningene-related peptide, which stimulate second-order enteric neurons to promote the peristaltic reflex (Figure 62-4). These enteric neurons stimulate proximal bowel contraction (via acetylcholine and substance P) and distal bowel relaxation (via nitric oxide and vasoactive intestinal peptide).
Tegaserod is a serotonin5-HT4 partial agonist that has high affinity for 5-HT4 receptors but no appreciable binding to 5-HT3 or dopamine receptors. Tegaserod was approved for the treatment of patients with chronic constipation and IBS with predominant constipation. Although tegaserod initially appeared to be extremely safe, it was voluntarily removed from the general market in 2007 because of an increased incidence of serious cardiovascular events. These adverse events have been attributed to inhibition of the 5-HT1B receptor. Another partial 5-HT4 agonist, cisapride, was also associated with an increased incidence of cardiovascular events that was attributed to inhibition of cardiac hERG (human ether-a-go-go-related gene) K+ channels, which resulted in QTc prolongation in some patients.
Prucalopride is a high-affinity 5-HT4 agonist that is in clinical development. In contrast to cisapride and tegaserod, it does not appear to have significant affinities for either hERG channels or 5-HT1B. In a recent 12-week clinical trial of patients with severe chronic constipation, it resulted in a significant increase in bowel movements compared with placebo. The long-term efficacy and safety of this agent require further study.
Antidiarrheal agents may be used safely in patients with mild to moderate acute diarrhea. However, these agents should not be used in patients with bloody diarrhea, high fever, or systemic toxicity because of the risk of worsening the underlying condition. They should be discontinued in patients whose diarrhea is worsening despite therapy. Antidiarrheals are also used to control chronic diarrhea caused by such conditions as irritable bowel syndrome (IBS) or inflammatory bowel disease (IBD).
As previously noted, opioids have significant constipating effects (see Chapter 31). They increase colonic phasic segmenting activity through inhibition of presynaptic cholinergic nerves in the submucosal and myenteric plexuses and lead to increased colonic transit time and fecal water absorption. They also decrease mass colonic movements and the gastrocolic reflex. Although all opioids have antidiarrheal effects, central nervous system effects and potential for addiction limit the usefulness of most.
Loperamide is a nonprescription opioid agonist that does not cross the blood-brain barrier and has no analgesic properties or potential for addiction. Tolerance to long-term use has not been reported. It is typically administered in doses of 2 mg taken one to four times daily. Diphenoxylate is a prescription opioid agonist that has no analgesic properties in standard doses; however, higher doses have central nervous system effects, and prolonged use can lead to opioid dependence. Commercial preparations commonly contain small amounts of atropine to discourage overdosage (2.5 mg diphenoxylate with 0.025 mg atropine). The anticholinergic properties of atropine may contribute to the antidiarrheal action.
Colloidal Bismuth Compounds
See the section under Mucosal Protective Agents in earlier text.
Kaolin & Pectin
Kaolin is a naturally occurring hydrated magnesium aluminum silicate (attapulgite), and pectin is an indigestible carbohydrate derived from apples. Both appear to act as absorbents of bacterial toxins and fluid, thereby decreasing stool liquidity and number. They may be useful in acute diarrhea but are seldom used on a chronic basis. A common nonprescription preparation is Kaopectate. The usual dosage is 1.2-1.5 g after each loose bowel movement (maximum: 9 g/d). Kaolin-pectin formulations are not absorbed and have no significant adverse effects except constipation. They should not be taken within 2 hours of other medications (which they may bind).
Bile Salt-Binding Resins
Conjugated bile salts are normally absorbed in the terminal ileum. Disease of the terminal ileum (eg, Crohn's disease) or surgical resection leads to malabsorption of bile salts, which may cause colonic secretory diarrhea. The bile salt-binding resins cholestyramine, colestipol, or colesevelam, may decrease diarrhea caused by excess fecal bile acids (see Chapter 35). These products come in a variety of powder and pill formulations that may be taken one to three times daily before meals. Adverse effects include bloating, flatulence, constipation, and fecal impaction. In patients with diminished circulating bile acid pools, further removal of bile acids may lead to an exacerbation of fat malabsorption. Cholestyramine and colestipol bind a number of drugs and reduce their absorption; hence, they should not be given within 2 hours of other drugs. Colesevelam does not appear to have significant effects on absorption of other drugs.
Somatostatin is a 14-amino-acid peptide that is released in the gastrointestinal tract and pancreas from paracrine cells, D cells, and enteric nerves as well as from the hypothalamus (see Chapter 37). Somatostatin is a key regulatory peptide that has many physiologic effects:
1. It inhibits the secretion of numerous hormones and transmitters, including gastrin, cholecystokinin, glucagon, growth hormone, insulin, secretin, pancreatic polypeptide, vasoactive intestinal peptide, and 5-HT.
2. It reduces intestinal fluid secretion and pancreatic secretion.
3. It slows gastrointestinal motility and inhibits gallbladder contraction.
4. It induces direct contraction of vascular smooth muscle, leading to a reduction of portal and splanchnic blood flow.
5. It inhibits secretion of some anterior pituitary hormones.
The clinical usefulness of somatostatin is limited by its short half-life in the circulation (3 minutes) when it is administered by intravenous injection. Octreotide is a synthetic octapeptide with actions similar to somatostatin. When administered intravenously, it has a serum half-life of 1.5 hours. It also may be administered by subcutaneous injection, resulting in a 6- to 12-hour duration of action. A longer-acting formulation is available for once-monthly depot intramuscular injection.
Inhibition of Endocrine Tumor Effects
Two gastrointestinal neuroendocrine tumors (carcinoid, VIPoma) cause secretory diarrhea and systemic symptoms such as flushing and wheezing. For patients with advanced symptomatic tumors that cannot be completely removed by surgery, octreotide decreases secretory diarrhea and systemic symptoms through inhibition of hormonal secretion and may slow tumor progression.
Other Causes of Diarrhea
Octreotide inhibits intestinal secretion and has dose-related effects on bowel motility. In low doses (50 mcg subcutaneously), it stimulates motility, whereas at higher doses (eg, 100-250 mcg subcutaneously), it inhibits motility. Octreotide is effective in higher doses for the treatment of diarrhea due to vagotomy or dumping syndrome as well as for diarrhea caused by short bowel syndrome or AIDS. Octreotide has been used in low doses (50 mcg subcutaneously) to stimulate small bowel motility in patients with small bowel bacterial overgrowth or intestinal pseudo-obstruction secondary to scleroderma.
Because it inhibits pancreatic secretion, octreotide may be of value in patients with pancreatic fistula. The role of octreotide in the treatment of pituitary tumors (eg, acromegaly) is discussed in Chapter 37. Octreotide is sometimes used in gastrointestinal bleeding (see below).
Impaired pancreatic secretion may cause steatorrhea, which can lead to fat-soluble vitamin deficiency. Alterations in gastrointestinal motility cause nausea, abdominal pain, flatulence, and diarrhea. Because of inhibition of gallbladder contractility and alterations in fat absorption, long-term use of octreotide can cause formation of sludge or gallstones in over 50% of patients, which rarely results in the development of acute cholecystitis. Because octreotide alters the balance among insulin, glucagon, and growth hormone, hyperglycemia or, less frequently, hypoglycemia (usually mild) can occur. Prolonged treatment with octreotide may result in hypothyroidism. Octreotide also can cause bradycardia.
Drugs Used in the Treatment of Irritable Bowel Syndrome
IBS is an idiopathic chronic, relapsing disorder characterized by abdominal discomfort (pain, bloating, distention, or cramps) in association with alterations in bowel habits (diarrhea, constipation, or both). With episodes of abdominal pain or discomfort, patients note a change in the frequency or consistency of their bowel movements.
Pharmacologic therapies for IBS are directed at relieving abdominal pain and discomfort and improving bowel function. For patients with predominant diarrhea, antidiarrheal agents, especially loperamide, are helpful in reducing stool frequency and fecal urgency. For patients with predominant constipation, fiber supplements may lead to softening of stools and reduced straining; however, increased gas production may exacerbate bloating and abdominal discomfort. Consequently, osmotic laxatives, especially milk of magnesia, are commonly used to soften stools and promote increased stool frequency.
For chronic abdominal pain, low doses of tricyclic antidepressants (eg, amitriptyline or desipramine, 10-50 mg/d) appear to be helpful (see Chapter 30). At these doses, these agents have no effect on mood but may alter central processing of visceral afferent information. The anticholinergic properties of these agents also may have effects on gastrointestinal motility and secretion, reducing stool frequency and liquidity. Finally, tricyclic antidepressants may alter receptors for enteric neurotransmitters such as serotonin, affecting visceral afferent sensation.
Several other agents are available that are specifically intended for the treatment of IBS.
Some agents are promoted as providing relief of abdominal pain or discomfort through antispasmodic actions. However, small or large bowel spasm has not been found to be an important cause of symptoms in patients with IBS. Antispasmodics work primarily through anticholinergic activities. Commonly used medications in this class include dicyclomine and hyoscyamine (see Chapter 8). These drugs inhibit muscarinic cholinergic receptors in the enteric plexus and on smooth muscle. The efficacy of antispasmodics for relief of abdominal symptoms has never been convincingly demonstrated. At low doses, they have minimal autonomic effects. However, at higher doses they exhibit significant additional anticholinergic effects, including dry mouth, visual disturbances, urinary retention, and constipation. For these reasons, antispasmodics are infrequently used.
Serotonin 5-HT3-Receptor Antagonists
5-HT3 receptors in the gastrointestinal tract activate visceral afferent pain sensation via extrinsic sensory neurons from the gut to the spinal cord and central nervous system. Inhibition of afferent gastrointestinal 5-HT3 receptors may inhibit unpleasant visceral afferent sensation, including nausea, bloating, and pain. Blockade of central 5-HT3 receptors also reduces the central response to visceral afferent stimulation. In addition, 5-HT3-receptor blockade on the terminals of enteric cholinergic neurons inhibits colonic motility, especially in the left colon, increasing total colonic transit time.
Alosetron is a 5-HT3 antagonist that has been approved for the treatment of patients with severe IBS with diarrhea (Figure 62-5). Four other 5-HT3 antagonists (ondansetron, granisetron, dolasetron, and palonosetron) have been approved for the prevention and treatment of nausea and vomiting (see Antiemetics); however, their efficacy in the treatment of IBS has not been determined. The differences between these 5-HT3 antagonists that determine their pharmacodynamic effects have not been well studied.
Pharmacokinetics & Pharmacodynamics
Alosetron is a highly potent and selective antagonist of the 5-HT3 receptor. It is rapidly absorbed from the gastrointestinal tract with a bioavailability of 50-60% and has a plasma half-life of 1.5 hours but a much longer duration of effect. It undergoes extensive hepatic cytochrome P450 metabolism with renal excretion of most metabolites. Alosetron binds with higher affinity and dissociates more slowly from 5-HT3 receptors than other 5-HT3 antagonists, which may account for its long duration of action.
Alosetron is approved for the treatment of women with severe IBS in whom diarrhea is the predominant symptom ("diarrhea-predominant IBS"). Its efficacy in men has not been established. In a dosage of 1 mg once or twice daily, it reduces IBS-related lower abdominal pain, cramps, urgency, and diarrhea. Approximately 50-60% of patients report adequate relief of pain and discomfort with alosetron compared with 30-40% of patients treated with placebo. It also leads to a reduction in the mean number of bowel movements per day and improvement in stool consistency. Alosetron has not been evaluated for the treatment of other causes of diarrhea.
In contrast to the excellent safety profile of other 5-HT3-receptor antagonists, alosetron is associated with rare but serious gastrointestinal toxicity. Constipation occurs in up to 30% of patients with diarrhea-predominant IBS, requiring discontinuation of the drug in 10%. Serious complications of constipation requiring hospitalization or surgery have occurred in 1 of every 1000 patients. Episodes of ischemic colitis—some fatal—have been reported in up to 3 per 1000 patients. Given the seriousness of these adverse events, alosetron is restricted to women with severe diarrhea-predominant IBS who have not responded to conventional therapies and who have been educated about the relative risks and benefits.
Despite being metabolized by a number of CYP enzymes, alosetron does not appear to have clinically significant interactions with other drugs.
Serotonin 5-HT4-Receptor Agonists
The pharmacology of tegaserod was discussed previously under Laxatives. This agent was approved for the short-term treatment of women with IBS who had predominant constipation. Controlled studies demonstrated a modest improvement (approximately 15%) in patient global satisfaction and a reduction in severity of pain and bloating in patients treated with tegaserod, 6 mg twice daily, compared with placebo. Owing to an increased number of cardiovascular deaths observed in post-marketing studies in patients taking tegaserod, it was voluntarily removed from the market and is no longer clinically available.
Chloride Channel Activator
As discussed previously, lubiprostone is a prostanoic acid derivative that stimulates the type 2 chloride channel (ClC-2) in the small intestine and is used in the treatment of chronic constipation. Lubiprostone recently was approved for the treatment of women with IBS with predominant constipation. Its efficacy for men with IBS is unproven. The approved dose for IBS is 8 mcg twice daily (compared with 24 mcg twice daily for chronic constipation). Lubiprostone has not been compared with other less expensive laxatives (eg, milk of magnesia). Lubiprostone is listed as category C for pregnancy and should be avoided in women of childbearing age.
Nausea and vomiting may be manifestations of a wide variety of conditions, including adverse effects from medications; systemic disorders or infections; pregnancy; vestibular dysfunction; central nervous system infection or increased pressure; peritonitis; hepatobiliary disorders; radiation or chemotherapy; and gastrointestinal obstruction, dysmotility, or infections.
The brain stem "vomiting center" is a loosely organized neuronal region within the lateral medullary reticular formation and coordinates the complex act of vomiting through interactions with cranial nerves VIII and X and neural networks in the nucleus tractus solitarius that control respiratory, salivatory, and vasomotor centers. High concentrations of muscarinic M1, histamine H1, neurokinin 1 (NK1), and serotonin5-HT3 receptors have been identified in the vomiting center (Figure 62-6).
There are four important sources of afferent input to the vomiting center:
1. The "chemoreceptor trigger zone" or area postrema is located at the caudal end of the fourth ventricle. This is outside the blood-brain barrier but is accessible to emetogenic stimuli in the blood or cerebrospinal fluid. The chemoreceptor trigger zone is rich in dopamine D2 receptors and opioid receptors, and possibly serotonin5-HT3 receptors and NK1 receptors.
2. The vestibular system is important in motion sickness via cranial nerve VIII. It is rich in muscarinic M1 and histamine H1 receptors.
3. Vagal and spinal afferent nerves from the gastrointestinal tract are rich in 5-HT3 receptors. Irritation of the gastrointestinal mucosa by chemotherapy, radiation therapy, distention, or acute infectious gastroenteritis leads to release of mucosal serotonin and activation of these receptors, which stimulate vagal afferent input to the vomiting center and chemoreceptor trigger zone.
4. The central nervous system plays a role in vomiting due to psychiatric disorders, stress, and anticipatory vomiting prior to cancer chemotherapy.
Identification of the different neurotransmitters involved with emesis has allowed development of a diverse group of antiemetic agents that have affinity for various receptors. Combinations of antiemetic agents with different mechanisms of action are often used, especially in patients with vomiting due to chemotherapeutic agents.
Serotonin 5-HT3 Antagonists
Pharmacokinetics & Pharmacodynamics
Selective 5-HT3-receptor antagonists have potent antiemetic properties that are mediated in part through central 5-HT3-receptor blockade in the vomiting center and chemoreceptor trigger zone but mainly through blockade of peripheral 5-HT3 receptors on extrinsic intestinal vagal and spinal afferent nerves. The antiemetic action of these agents is restricted to emesis attributable to vagal stimulation (eg, postoperative) and chemotherapy; other emetic stimuli such as motion sickness are poorly controlled.
5-HT3-receptor antagonists do not inhibit dopamine or muscarinic receptors. They do not have effects on esophageal or gastric motility but may slow colonic transit.
Chemotherapy-Induced Nausea and Vomiting
5-HT3-receptor antagonists are the primary agents for the prevention of acute chemotherapy-induced nausea and emesis. When used alone, these drugs have little or no efficacy for the prevention of delayed nausea and vomiting (ie, occurring > 24 hours after chemotherapy). The drugs are most effective when given as a single dose by intravenous injection 30 minutes prior to administration of chemotherapy in the following doses: ondansetron, 8 mg or 0.15 mg/kg; granisetron, 1 mg; dolasetron, 100 mg; or palonosetron, 0.25 mg. A single oral dose given 1 hour before chemotherapy may be equally effective in the following regimens: ondansetron 8 mg twice daily or 24 mg once; granisetron, 2 mg; dolasetron, 100 mg. Although 5-HT3-receptor antagonists are effective as single agents for the prevention of chemotherapy-induced nausea and vomiting, their efficacy is enhanced by combination therapy with a corticosteroid (dexamethasone) and NK1-receptor antagonist (see below).
Postoperative and Postradiation Nausea and Vomiting
5-HT3-receptor antagonists are used to prevent or treat postoperative nausea and vomiting. Because of adverse effects and increased restrictions on the use of other antiemetic agents, 5-HT3-receptor antagonists are increasingly used for this indication. They are also effective in the prevention and treatment of nausea and vomiting in patients undergoing radiation therapy to the whole body or abdomen.
The 5-HT3-receptor antagonists are well-tolerated agents with excellent safety profiles. The most commonly reported adverse effects are headache, dizziness, and constipation. All four agents cause a small but statistically significant prolongation of the QT interval, but this is most pronounced with dolasetron. Although cardiac arrhythmias have not been linked to dolasetron, it should not be administered to patients with prolonged QT or in conjunction with other medications that may prolong the QT interval (see Chapter 14).
No significant drug interactions have been reported with 5-HT3-receptor antagonists. All four agents undergo some metabolism by the hepatic cytochrome P450 system but they do not appear to affect the metabolism of other drugs. However, other drugs may reduce hepatic clearance of the 5-HT3-receptor antagonists, altering their half-life.
Corticosteroids (dexamethasone, methylprednisolone) have antiemetic properties, but the basis for these effects is unknown. The pharmacology of this class of drugs is discussed in Chapter 39. These agents appear to enhance the efficacy of 5-HT3-receptor antagonists for prevention of acute and delayed nausea and vomiting in patients receiving moderately to highly emetogenic chemotherapy regimens. Although a number of corticosteroids have been used, dexamethasone, 8-20 mg intravenously before chemotherapy, followed by 8 mg/d orally for 2-4 days, is commonly administered.
Neurokinin Receptor Antagonists
Neurokinin 1 (NK1)-receptor antagonists have antiemetic properties that are mediated through central blockade in the area postrema. Aprepitant (an oral formulation) is a highly selective NK1-receptor antagonist that crosses the blood-brain barrier and occupies brain NK1 receptors. It has no affinity for serotonin, dopamine, or corticosteroid receptors. Fosaprepitant is an intravenous formulation that is converted within 30 minutes after infusion to aprepitant.
The oral bioavailability of aprepitant is 65%, and the serum half-life is 12 hours. Aprepitant is metabolized by the liver, primarily by the CYP3A4 pathway.
Aprepitant is used in combination with 5-HT3-receptor antagonists and corticosteroids for the prevention of acute and delayed nausea and vomiting from highly emetogenic chemotherapeutic regimens. Combined therapy with aprepitant, a 5-HT3-receptor antagonist, and dexamethasone prevents acute emesis in 80-90% of patients compared with less than 70% treated without aprepitant. Prevention of delayed emesis occurs in more than 70% of patients receiving combined therapy versus 30-50% treated without aprepitant. NK1-receptor antagonists may be administered for 3 days as follows: oral aprepitant 125 mg or intravenous fosaprepitant 115 mg given 1 hour before chemotherapy, followed by oral aprepitant 80 mg/d for 2 days after chemotherapy.
Adverse Effects & Drug Interactions
Aprepitant may be associated with fatigue, dizziness, and diarrhea. The drug is metabolized by CYP3A4 and may inhibit the metabolism of other drugs metabolized by the CYP3A4 pathway. Several chemotherapeutic agents are metabolized by CYP3A4, including docetaxel, paclitaxel, etoposide, irinotecan, imatinib, vinblastine, and vincristine. Drugs that inhibit CYP3A4 metabolism may significantly increase aprepitant plasma levels (eg, ketoconazole, ciprofloxacin, clarithromycin, nefazodone, ritonavir, nelfinavir, verapamil, and quinidine). Aprepitant decreases the international normalized ratio (INR) in patients taking warfarin.
Phenothiazines & Butyrophenones
Phenothiazines are antipsychotic agents that can be used for their potent antiemetic and sedative properties (see Chapter 29). The antiemetic properties of phenothiazines are mediated through inhibition of dopamine and muscarinic receptors. Sedative properties are due to their antihistamine activity. The agents most commonly used as antiemetics are prochlorperazine, promethazine, and thiethylperazine.
Antipsychotic butyrophenones also possess antiemetic properties due to their central dopaminergic blockade (see Chapter 29). The main agent used is droperidol, which can be given by intramuscular or intravenous injection. In antiemetic doses, droperidol is extremely sedating. Until recently, it was used extensively for postoperative nausea and vomiting, in conjunction with opiates and benzodiazepines for sedation for surgical and endoscopic procedures, for neuroleptanalgesia, and for induction and maintenance of general anesthesia. Extrapyramidal effects and hypotension may occur. Droperidol may prolong the QT interval, rarely resulting in fatal episodes of ventricular tachycardia including torsade de pointes. Therefore, droperidol should not be used in patients with QT prolongation and should be used only in patients who have not responded adequately to alternative agents.
Substituted benzamides include metoclopramide (discussed previously) and trimethobenzamide. Their primary mechanism of antiemetic action is believed to be dopamine-receptor blockade. Trimethobenzamide also has weak antihistaminic activity. For prevention and treatment of nausea and vomiting, metoclopramide may be given in the relatively high dosage of 10-20 mg orally or intravenously every 6 hours. The usual dose of trimethobenzamide is 250 mg orally, 200 mg rectally, or 200 mg by intramuscular injection. The principal adverse effects of these central dopamine antagonists are extrapyramidal: restlessness, dystonias, and parkinsonian symptoms.
H1 Antihistamines & Anticholinergic Drugs
The pharmacology of anticholinergic agents is discussed in Chapter 8 and that of H1 antihistaminic agents in Chapter 16. As single agents, these drugs have weak antiemetic activity, although they are particularly useful for the prevention or treatment of motion sickness. Their use may be limited by dizziness, sedation, confusion, dry mouth, cycloplegia, and urinary retention. Diphenhydramine and one of its salts, dimenhydrinate, are first-generation histamine H1 antagonists that also have significant anticholinergic properties. Because of its sedating properties, diphenhydramine is commonly used in conjunction with other antiemetics for treatment of emesis due to chemotherapy. Meclizine is an H1 antihistaminic agent with minimal anticholinergic properties that also causes less sedation. It is used for the prevention of motion sickness and the treatment of vertigo due to labyrinth dysfunction.
Hyoscine (scopolamine), a prototypic muscarinic receptor antagonist, is one of the best agents for the prevention of motion sickness. However, it has a very high incidence of anticholinergic effects when given orally or parenterally. It is better tolerated as a transdermal patch. Superiority to dimenhydrinate has not been proved.
Dronabinol is 9-tetrahydrocannabinol (THC), the major psychoactive chemical in marijuana (see Chapter 32). After oral ingestion, the drug is almost completely absorbed but undergoes significant first-pass hepatic metabolism. Its metabolites are excreted slowly over days to weeks in the feces and urine. Like crude marijuana, dronabinol is a psychoactive agent that is used medically as an appetite stimulant and as an antiemetic, but the mechanisms for these effects are not understood. Because of the availability of more effective agents, dronabinol now is uncommonly used for the prevention of chemotherapy-induced nausea and vomiting. Combination therapy with phenothiazines provides synergistic antiemetic action and appears to attenuate the adverse effects of both agents. Dronabinol is usually administered in a dosage of 5 mg/m2 just prior to chemotherapy and every 2-4 hours as needed. Adverse effects include euphoria, dysphoria, sedation, hallucinations, dry mouth, and increased appetite. It has some autonomic effects that may result in tachycardia, conjunctival injection, and orthostatic hypotension. Dronabinol has no significant drug-drug interactions but may potentiate the clinical effects of other psychoactive agents.
Nabilone is a closely related THC analog that has been available in other countries and is now approved for use in the USA.