No abstract

A series of 4-amino-2,2-diarylbutyronitriles (3) prepared for testing as inhibitors of gastrointestinal propulsive activity did not show any enhancement over such existing agents as diphenoxylate and loperamide. However, conversion of the nitrile group to a 2-methyl-1,3,4-oxadiazol-5-yl function led to compounds 5g and 5j, statistically equipotent to diphenoxylate and loperamide in the mouse and showing a very low order of analgesic activity. Structural modifications determined that the best separation of antipropulsive and analgesic effects was obtained when the amino group was bicyclic and the oxadiazole ring had a 2-methyl substituent. The most potent compounds were and analogues of diphenoxylate and loperamide where the oxadiazole ring was present, but these compounds had marked analgesic activity.

One of the most serious side effects of nonsteroidal anti-inflammatory drugs (NSAIDs) is upper gastrointestinal mucosal damage that may result in erosions, ulcerations and other serious complications. NSAIDs reduce endogenous prostaglandins, and this reduction is relevant to their pharmacology and toxicity. The stomach and to some extent the duodenum are the major organs involved in the mucosal toxicity of NSAIDs. With the availability of the synthetic prostaglandin misoprostol, it has become possible to prevent NSAID-induced gastroduodenal ulcers without compromising the beneficial antirheumatic and analgesic effects of NSAID therapy. In fact, misoprostol is the only drug with established long-term efficacy in preventing NSAID-induced gastroduodenal ulcers in rheumatic patients. The purpose of this communication is to critically review the efficacy of gastric antisecretory drugs, mucosal protective drugs and misoprostol when used for the prevention of NSAID-induced ulcers, considering only data from well-controlled, randomized, double-blind clinical studies. The histamine H₂-receptor antagonist ranitidine has been shown to be effective in preventing NSAID-induced duoenal ulcers, but has no efficacy in preventing NSAID-induced gastric ulcers. In a direct comparative trial with ranitidine, misoprostol (200 µg qid) was significantly more effective than ranitidine (150 mg bid) in preventing gastric ulcers in chronic NSAID users. The inactivity of ranitidine in preventing gastric ulcers indicates that the pathogenesis of NSAID-induced gastric ulcers is not related to gastric acid. Limited but conflicting data exist with omeprazole. The mucosal-coating drug sucralfate has not been found effective in preventing NSAID ulcers. In fact, in a direct comparative trial, misoprostol (200 µg qid) was significantly more effective than sucralfate (1 g qid) in preventing gastric ulcers in patients receiving chronic NSAID therapy. No meaningful data exist with organic bismuth salts, a group of drugs which has mucosal coating and protective properties. From this brief overview, we conclude: (1) mucosal-coating compounds have no therapeutic role in preventing NSAID-induced ulceration; (2) gastric antisecretory drugs are not effective in preventing NSAID-induced gastric ulcers, and (3) misoprostol is the only antiulcer drug proven to be effective for preventing NSAID-induced gastric and duodenal ulcers in patients receiving chronic NSAID. Misoprostol represents a major therapeutic advance for the management of NSAID-induced mucosal injury.

Misoprostol is a synthetic prostaglandin which is related structurally to naturally occurring prostaglandin E1 (PGE1). PGE1 has long been recognized as an effective inhibitor of gastric acid secretion when administered intravenously. However, three major problems have prevented the use of natural PGE1 as a therapeutic treatment for peptic ulcer disease. Each of these problems, lack of oral activity, side-effects, and short duration of action, has been overcome by the chemical development of misoprostol from PGE1. The major structural alteration of PGE1 was the relocation of the 15-hydroxy group to the adjacent 16-position. This important modification significantly reduced typical prostaglandin side-effects such as diarrhea, yet retained full antisecretory potency and also provided a degree of oral activity. The second modification was the addition of a methyl group to carbon-16 to prevent metabolic oxidation of the hydroxy group. This structural change greatly increased both oral potency and duration of action and completed the synthetic development of misoprostol. Misoprostol is chemically unstable at room temperature, as is PGE1. This problem has been solved by pharmaceutical formulation studies which led to a stable and solid dosage form.

Over the last 25 years, a remarkable revolution in the pathophysiology and treatment of gastric and duodenal ulcers has occurred. Effective therapies were developed not only to heal ulcers, but also to cure most patients. The two principal causes for gastric and duodenal ulcers are either infection with Helicobacter pylori or the use of non-steroidal anti-inflammatory drugs (NSAIDs). With H. pylori eradication, gastric and duodenal ulcers are rapidly becoming historical diseases. This communication reviews the salient pharmacology of the novel anti-ulcer drugs currently in development, with particular emphasis on the treatment of gastric and duodenal ulcers. Intense research is currently focused on the development of proton pump inhibitors primarily for the treatment and prevention of gastroesophageal reflux disease. The older proton pump inhibitors, omeprazole and lansoprazole, are effective in healing gastric and duodenal ulcers. Furthermore, both drugs are effective in eradicating H. pylori when given with various antibiotics. Pantoprazole, rabeprazole and esomeprazole are new proton pump inhibitors, which appear to have comparable therapeutic profiles with omeprazole and lansoprazole. Rebamipide is a new mucosal protective drug, which is effective in healing gastric ulcers. Polaprezinc and nocloprost are also mucosal protective drugs, which are in clinical development. However, none of these three cytoprotective drugs have been evaluated for their efficacy in eradicating H. pylori when given in combination with antibiotics. Likewise, no published literature exists on the use of these drugs for preventing NSAID-induced ulcers. With the rapid eradication of H. pylori currently happening in the developed world, the therapeutic challenge is now directed toward preventing NSAID-associated ulcer. Significant reduction of NSAID-induced ulcers is achieved by using continuous prophylactic anti-ulcer therapy (misoprostol or omeprazole) or by using NSAIDs possessing selective COX-2 inhibitory activity. However, outcome clinical studies are needed to compare the adjuvant anti-ulcer therapies given with COX-1 inhibitors versus the selective COX-2 inhibitors given alone.