Clinical pharmacokinetics of organic nitrates

Plasma concentrations of glyceryl trinitrate (nitroglycerin), isosorbide dinitrate and isosorbide 2- and 5-mononitrates in man have been measured after administration via different routes. Appropriate precautions have to be taken in the administration of these agents (to avoid loss during intravenous infusion), and in their sampling and assay. Pharmacokinetic calculations based on plasma concentrations should be viewed with caution, as the data on which these calculations are based are often very limited, and the very rapid disappearance of for example glyceryl trinitrate from plasma makes the choice of an appropriate kinetic model and exact calculations difficult. Glyceryl trinitrate disappears from plasma within a few minutes, and a high apparent volume of distribution and a very high systemic clearance are found. After oral administration, plasma concentrations are very low; with sublingual or cutaneous administration, higher plasma concentrations can be obtained, suggesting a high first-pass extraction after oral administration, but quantitative data on bioavailability are lacking. For isosorbide dinitrate the systemic clearance, although high, is lower than for glyceryl trinitrate; disappearance from the plasma is slower and plasma concentrations after different routes of administration are much higher. Here too, quantitative data on bioavailability are lacking. High plasma concentrations of isosorbide 2-mononitrate and isosorbide 5-mononitrate are found in plasma after administration of isosorbide dinitrate. These metabolites have a good bioavailability, and half-lives of around 2.5 hours for isosorbide 2-mononitrate and 5 hours for isosorbide 5-mononitrate. Only very limited data are available about the influence of disease states and interactions with food and other drugs on the kinetics of the organic nitrates. It is very difficult to correlate the effects of the nitrates to their plasma concentrations; counter-regulation, development of tolerance, and the presence of metabolites could disturb the interpretation of such a relationship. It is at present impossible to predict the pharmacological effects or the efficacy of organic nitrates on the basis of their plasma concentrations.