The organization of these drugs is based on the organization of the gastrointestinal (GI) tract. There are drugs that are used in treating ulcers in the stomach and duodenum. There are also drugs that affect motility in the upper GI tract. Then, moving down to the large intestine, we can divide the agents into those that enhance motility and those that reduce motility. Finally, there are agents that specifically target diseases of the lower GI tract.
DRUGS THAT ACT IN THE UPPER GI TRACT
Orlistat is a lipase inhibitor being used to treat obesity.
Orlistat binds to pancreatic and gastric lipase and inactivates the enzyme. This reduces the absorption of dietary fat by about 30%. Adverse effects include flatulence, oily spotting, and fecal urgency.
Duodenal and gastric ulcers are often caused by the bacterium Helicobacter pylori. The treatment objective is eradication of H. pylori with a combination of antibiotics and H2 blockers. Bismuth (PEPTO-BISMOL) appears to be bactericidal to H. pylori.
The “proton pump inhibitors” (“-prazoles”) inhibit the H+-K+-ATPase enzyme of the parietal cell. This reduces acid secretion.
Proton pump inhibitors are used principally to promote healing of duodenal and gastric ulcers and in the treatment of GERD (gastric esophageal reflux disease).
H2 receptor antagonists prevent histamine-induced acid release. H2 antagonists include:
These drugs are easily recognizable by the “-tidine” ending. These drugs are used to promote healing of ulcers. Cimetidine binds to cytochrome P450. Therefore, adverse drug interactions with drugs transformed via the cytochrome P450 system are common with cimetidine.
The aluminum salts and calcium carbonate antacids cause constipation. The magnesium salts cause diarrhea. Therefore, they are often mixed.
Antacids can decrease the absorption of other drugs because they alter the stomach and duodenal pH. They can also bind to drugs and block their absorption. This is particularly true for the aluminum salts. Antacids also have systemic effects. Magnesium salts can cause hypermagnesemia and aluminum salts can cause hypophosphatemia.
SUCRALFATE forms a protective coating on the mucosa, particularly ulcerated areas.
Sucralfate is only minimally absorbed. Constipation is the main side effect.
Metoclopramide and cisapride increase the rate of gastric emptying.
A diverse group of agents enhance coordination and increase transit of material in the GI tract. Cholinergic agonists (bethanechol) and acetylcholinesterase inhibitors, such as neostigmine, increase motility. Serotonin (5-HT) affects both secretion and motility. Metoclopramide is considered to act predominantly as an agonist at 5-HT4 receptors. Cisapride has been removed from the market in the United States due to cardiac arrhythmias.