The physical stability of one type of stabilized molded and three types of compressed nitroglycerin tablets was studied. The evaporation rate of nitroglycerin was controlled by its vapour pressure and by the matrix effect of the dosage forms. The four products showed different vapour pressures and matrix effects. In time nitroglycerin escapes from the outmost layers of the tablets; the dosage form in which the drug showed the lowest vapour pressure (the stabilized molded tablet) was found to be the most stable one. However, from the time when the drug had escaped from the outmost layers of the dosage form, the matrix effect became dominant. When nitroglycerin tablets were stored in tightly closed containers at room temperature potency loss was minimal. Even when the bottles were opened regularly this did not result in a significant loss of the drug. Measures are suggested to minimize drug evaporation.
This study tested the efficacy of enprostil given both at potent antisecretory (35 micrograms twice daily) and weak antisecretory (7 micrograms twice daily) doses in preventing aspirin-induced gastric blood loss measured chemically in gastric washings in 10 volunteers. Aspirin (500 mg four times a day) increased gastric blood loss compared with placebo (p less than 0.001). When enprostil was given in addition to aspirin, gastric blood loss was not significantly different from basal value. The pH of gastric washings was significantly increased by the large but not by the low dose of enprostil compared with placebo. The two doses of enprostil were equally efficacious, suggesting a mucosal protective effect of enprostil independent of acid inhibition.
The in vitro and in vivo availability of nitroglycerin from three compressed and one stabilized molded tablet have been studied. Compressed nitroglycerin tablets have to disintegrate before the drug is released. The disintegration and release of nitroglycerin from these tablets appeared to be strongly dependent upon conditions in the in vitro dissolution tests, such as the degree of agitation, and in vivo factors, such as tongue pressure and/or movement. The stabilized molded nitroglycerin tablet did not disintegrate but dissolved completely. Because the dissolution of the molded tablet was fast, the release rate of nitroglycerin was far less influenced by the in vitro and in vivo factors described above. The potency and in vitro and in vivo availability of fresh tablets and tablets which had been stored in tightly closed glass containers for over one year did not differ significantly. It is conjectured that the supposed potency loss of properly stored compressed nitroglycerin tablets is rather a release problem than a stability problem.
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