Pharmacokinetics and pharmacodynamics of nitroglycerin and its dinitrate metabolites in conscious dogs: Intravenous infusion studies

Abstract

Intravenous infusions of nitroglycerin (GTN), 1,2-glyceryl dinitrate (1,2-GDN), and 1,3-glyceryl dinitrate (1,3-GDN) were given to four conscious dogs at 10 μg/min, 30 μg/min, 50 μg/min, and 70 μg/min of GTN and 20 μg/min and 100 μg/min of GDNs. The steady state plasma concentrations (Css)of GTN were reached after about 60 min whereas for 1,2-GDN and 1,3-GDN the Csswere reached at about 150 min after the infusion began. Except for one dog, the Cssof GTN were not proportional to infusion rate, however, all dogs together showed a good linear relationship between Cssof GTN and infusion rates with an average correlation coefficient of 0.917±0.102. Large variability in GTN clearance after various infusion rates was observed in all dogs. The Cssratios of 1,2-GDN/GTN and 1,3-GDN/GTN yield overall averages of 31.5 ±17.2 and 5.47 ±3.19,respectively. Average Cssratios of metabolites 1,2-GDN/1,3-GDN were 5.78±1.23. This ratio is different from those obtained after iv bolus and oral dosing indicating that the biotransformation of GTN to 1,2-GDN and 1,3-GDN differs for each dosing route. The clearances for 1,2-GDN and 1,3-GDN were not changed over the dose range of 20 μg/min to 100 μg/min. Terminal half-lives of 1,2-GDN and 1,3-GDN postinfusion were similar to those values obtained after a single bolus dose (45 min). It appears that all the GTN dose at steady state can be accounted for by the formation of measurable 1,2-GDN and 1,3-GDN. Large intra- and interdog variations in systolic blood pressure decrease (SPD) following infusions of GTN were observed, however, all dogs showed a clear systolic blood pressure decrease when the highest infusion rate (70 μg/min) was given. No significant systolic blood pressure drop was detected following 20 μg/min infusions of 1,2-GDN or 1,3-GDN. It was clear that systolic blood pressure in all dogs decreased following 100 μg/min infusions of 1,2-GDN or 1,3-GDN. When SPD values were plotted vs. log GTN concentrations following the infusion of 70 μg/min of GTN in all four dogs, a counterclockwise hysteresis was observed indicating the significant contribution of the active dinitrate metabolites to GTN pharmacodynamics.