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. This eventually leads to dephosphorylation of myosin light
chains, which regulates the contractile state in smooth muscle and results 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 the 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 (after load), 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 a reflex 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 pressure 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.
Pharmacokinetics
Nitroglycerin is rapidly
absorbed following lingual spray administration. In a pharmacokinetic study
when a single 1200 mcg dose (three activations of a 400 mcg dose) of NitroMist
was administered to healthy volunteers (n=12), all subjects had detectable
trinitroglycerin plasma levels (mean Cmax 0.8 ng/mL ± 0.7 ng/mL and tmax of 8
minutes, range 4 minutes to 15 minutes) beginning at 2 minutes post-dose and
higher levels of the 1,2-(mean Cmax 3.7 ng/mL ± 1 ng/mL and tmax 34 minutes ±
21 minutes, range 15 minutes to 90 minutes) and 1,3-dinitroglycerin metabolites
(mean Cmax 1 ng/mL ± 0.3 ng/mL and mean tmax 41 minutes ± 20 minutes, range 20
minutes to 90 minutes).
The volume of distribution of
nitroglycerin following intravenous administration is 3.3 L/kg.
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, 1,2-and 1,3dinitroglycerin are
found in plasma. The mean elimination half-life of both 1,2-and
1,3-dinitroglycerin is about 40 minutes. The 1,2-and 1,3-dinitroglycerin metabolites
have been reported to possess some pharmacological activity, whereas the
glycerol mononitrate metabolites of nitroglycerin are essentially inactive.
Higher plasma concentrations of the dinitro metabolites, with their nearly
8-fold longer elimination half-lives, may contribute significantly to the
duration of pharmacologic effect.
In the above referenced
pharmacokinetic study the average initial half-lives (T½α) of
nitroglycerin, and its 1,2-and 1,3-dinitroglycerin metabolites were estimated
to be 3 minutes, 10 minutes, and 11 minutes, respectively. The half-life of
disappearance of the nitroglycerin (T½β) (5 minutes) was significantly
less than the half-life of appearance (T½α) of the 1,2-and
1,3-dinitroglycerin metabolites suggesting the possibility of an additional
compartment into which the nitroglycerin disappears from plasma prior to being
metabolized into the dinitroglycerin metabolites. A second indication of this
other compartment is that the appearance of nitroglycerin metabolites in plasma
was delayed in some subjects, with zero plasma levels seen for 4 minutes to 6
minutes after dosing. In some subjects, nitroglycerin metabolites appeared only
after nitroglycerin Cmax had been observed.
Clinical Studies
In a randomized, double-blind,
single-center, single-administration, placebo-controlled, 4-period cross-over
study in 30 subjects with stable angina pectoris, statistically significant
dose-related increases in exercise tolerance were seen following doses of 200
mcg, 400 mcg, and 800 mcg of nitroglycerin delivered by NitroMist compared to
placebo.