CLINICAL PHARMACOLOGY
Mechanism Of Action
Nitroglycerin forms free
radical nitric oxide (NO) which activates guanylate cyclase, resulting in an
increase of guanosine 3'5' monophosphate (cyclic GMP) in smooth muscle and other
tissues. These events lead to dephosphorylation of myosin light chains, which
regulate the contractile state in smooth muscle, and result in vasodilatation.
Pharmacodynamics
The principal pharmacological
action of nitroglycerin is relaxation of vascular smooth muscle. Although
venous effects predominate, nitroglycerin produces, in a dose-related manner,
dilation of both arterial and venous beds. Dilation of postcapillary vessels,
including large veins, promotes peripheral pooling of blood, decreases venous
return to the heart, and reduces left ventricular end-diastolic pressure
(preload).
Nitroglycerin also produces arteriolar relaxation,
thereby reducing peripheral vascular resistance and arterial pressure
(afterload), and dilates large epicardial coronary arteries; however, the
extent to which this latter effect contributes to the relief of exertional
angina is unclear.
Therapeutic doses of nitroglycerin may reduce systolic,
diastolic, and mean arterial blood pressure. Effective coronary perfusion
pressure is usually maintained, but can be compromised if blood pressure falls
excessively, or increased heart rate decreases diastolic filling time.
Elevated central venous and pulmonary capillary wedge
pressures, and pulmonary and systemic vascular resistance are also reduced by
nitroglycerin therapy. Heart rate is usually slightly increased, presumably due
to a compensatory response to the fall in blood pressure. Cardiac index may be
increased, decreased, or unchanged. Myocardial oxygen consumption or demand (as
measured by the pressure-rate product, tension-time index, and stroke-work
index) is decreased and a more favorable supply-demand ratio can be achieved. Patients
with elevated left ventricular filling pressures and increased systemic
vascular resistance in association with a depressed cardiac index are likely to
experience an improvement in cardiac index. In contrast, when filling pressures
and cardiac index are normal, cardiac index may be slightly reduced following
nitroglycerin administration.
Consistent with the symptomatic relief of angina, digital
plethysmography indicates that onset of the vasodilatory effect occurs
approximately 1 to 3 minutes after sublingual nitroglycerin administration and
reaches a maximum by 5 minutes postdose. Effects persist for at least 25
minutes following NITROSTAT administration.
Pharmacokinetics
Absorption
Nitroglycerin is rapidly absorbed following sublingual
administration of NITROSTAT tablets. Mean peak nitroglycerin plasma
concentrations occur at a mean time of approximately 6 to 7 minutes postdose
(Table 1). Maximum plasma nitroglycerin concentrations (Cmax) and area under
the plasma concentration-time curves (AUC) increase dose-proportionally
following 0.3 to 0.6 mg NITROSTAT. The absolute bioavailability of
nitroglycerin from NITROSTAT tablets is approximately 40% but tends to be
variable due to factors influencing drug absorption, such as sublingual hydration
and mucosal metabolism.
Table 1
Parameter |
Mean Nitroglycerin (SD) Values |
2 x 0.3 mg NITROSTAT Tablets |
1 x 0.6 mg NITROSTAT Tablets |
Cmax, ng/mL |
2.3 (1.7) |
2.1 (1.5) |
Tmax, min |
6.4 (2.5) |
7.2 (3.2) |
AUC(0-∞), min |
14.9 (8.2) |
14.9 (11.4) |
t½, min |
2.8 (1.1) |
2.6 (0.6) |
Distribution
The volume of distribution (VArea)
of nitroglycerin following intravenous administration is 3.3 L/kg. At plasma
concentrations between 50 and 500 ng/mL, the binding of nitroglycerin to plasma
proteins is approximately 60%, while that of 1,2-and 1,3-dinitroglycerin is 60%
and 30%, respectively.
Metabolism
A liver reductase enzyme is of
primary importance in the metabolism of nitroglycerin to glycerol di-and
mononitrate metabolites and ultimately to glycerol and organic nitrate. Known
sites of extrahepatic metabolism include red blood cells and vascular walls. In
addition to nitroglycerin, 2 major metabolites 1,2and 1,3-dinitroglycerin, are
found in plasma. Mean peak 1,2-and 1,3-dinitroglycerin plasma concentrations
occur at approximately 15 minutes postdose. The elimination half-life of
1,2-and 1,3-dinitroglycerin is 36 and 32 minutes, respectively. The 1,2-and
1,3-dinitroglycerin metabolites have been reported to possess approximately 2%
and 10%, respectively, of the pharmacological activity of nitroglycerin. Higher
plasma concentrations of the dinitro metabolites, along with their nearly
10-fold longer elimination half-lives, may contribute significantly to the
duration of pharmacologic effect. Glycerol mononitrate metabolites of
nitroglycerin are biologically inactive.
Elimination
Nitroglycerin plasma
concentrations decrease rapidly, with a mean elimination half-life of 2 to 3
minutes. Half-life values range from 1.5 to 7.5 minutes. Clearance (13.6 L/min)
greatly exceeds hepatic blood flow. Metabolism is the primary route of drug
elimination.
Drug Interactions
Aspirin
Coadministration of
nitroglycerin with high dose aspirin (1000 mg) results in increased exposure to
nitroglycerin. The vasodilatory and hemodynamic effects of nitroglycerin may be
enhanced by concomitant administration of nitroglycerin with high dose aspirin.
Tissue-Type Plasminogen
Activator (t-PA)
Concomitant administration of
t-PA and intravenous nitroglycerin has been shown to reduce plasma levels of
t-PA and its thrombolytic effect.