Pharmacology of Organic Nitrates

Introduction and Physiological Mechanism of Action

• Organic nitrates are a specific class of pharmaceuticals primarily utilized for both the acute treatment and the long-term prevention of ischemic heart disease.

• Comparison with Other Anti-anginals:

  • Beta blockers and calcium channel blockers prevent angina exclusively by reducing myocardial oxygen demand.

  • Organic nitrates are unique in their clinical utility because they can be used both to treat an active, acute angina attack and to prevent future occurrences.

• Endogenous Role of Nitric Oxide: In a healthy cardiovascular system, the endothelium naturally produces nitric oxide ($NO$), which serves as a potent signaling molecule to promote vasodilation. However, in patients with stable angina, the presence of thick atherosclerotic plaques causes arterial walls to become rigid, thereby preventing natural dilation.

• Metabolic Activation Pathway:

  • Nitrates are rapidly absorbed into the systemic circulation following administration.

  • The drugs travel to the veins, where they are broken down by the enzyme aldehyde dehydrogenase 2 ($ALDH2$).

  • This enzymatic breakdown results in the spontaneous release of nitric oxide ($NO$).

  • The localized increase in $NO$ levels triggers an increase in cyclic GMP ($cGMP$) within the vasculature.

  • Elevated $cGMP$ serves to relax vascular smooth muscles, resulting in systemic vasodilation.

Hemodynamic Effects and Therapeutic Goals

• Venodilation (Preload Reduction):

  • At standard therapeutic doses, nitrates act primarily as venodilators because the enzyme $ALDH2$ is found in higher concentrations within the veins.

  • By dilating the veins, the drug reduces the volume of blood returning to the heart, which effectively reduces preload.

• Arteriolar Dilation (Afterload Reduction):

  • If the dosage of the nitrate is increased, the drug can also affect the arteries (arteriolar dilation).

  • This systemic arterial dilation subsequently reduces afterload.

• Ultimate Clinical Goal: Despite their primary classification as venodilators, the overarching therapeutic effect of all nitrates is the reduction of myocardial oxygen demand, thereby alleviating the symptoms of ischemia.

Classification and Clinical Comparison of Nitrates

• There are three primary types of organic nitrates utilized in clinical practice:

  1. Glycerol Trinitrate (also known as Nitroglycerin or GTN).

  2. Isosorbide Dinitrate (ISDN).

  3. Isosorbide Mononitrate (ISMN).

• Available Routes of Administration:

  • Sublingual ($SL$) tablets.

  • Sublingual ($SL$) sprays.

  • Transdermal patches.

  • Intravenous ($IV$) infusions.

  • Conventional oral tablets.

  • Controlled-release ($CR$) tablets.

• Selection Based on Therapeutic Goal:

  • To Treat Acute Angina: Requires formulations with a rapid onset of action. Preferred routes include sublingual tablets, sublingual sprays, or $IV$ infusions (typically reserved for emergency or hospital settings).

  • To Prevent Angina (Prophylaxis): Requires formulations with a long duration of action. Preferred routes include transdermal patches, conventional oral tablets, or controlled-release tablets.

  • Contraindication for Acute Use: Any nitrate with a long duration but delayed onset (e.g., patches or $CR$ tablets) is strictly prophylactic and cannot be used to treat an active angina episode.

Glycerol Trinitrate (GTN / Nitroglycerin)

• Pharmacokinetics and Metabolism:

  • GTN is characterized by a very rapid onset of action but suffers from extremely low oral bioavailability.

  • If administered orally (swallowed), it is extensively metabolized by the reductase enzyme in the liver via first-pass metabolism.

  • The hepatic reductase enzyme converts the trinitrate form into dinitrate forms ($Glyceryl 1,2-dinitrate$ and $Glyceryl 1,3-dinitrate$). While these metabolites are active and possess vasodilatory effects, their onset of action is too slow for acute relief.

• Acute Formulations:

  • Sublingual Tablet vs. Spray: These are used for immediate symptom relief. Administering the drug in the buccal region allows it to enter the systemic circulation nearly immediately through the sublingual capillary network, successfully bypassing hepatic first-pass metabolism.

  • Stability of Sublingual Tablets: These tablets are highly volatile and have a limited shelf life of approximately $3\,\text{months}$. They are sensitive to heat, moisture, and light.

  • Advantages of the Sublingual Spray: Modern practice often prefers the spray because it is less volatile. It has a shelf life of up to $2\,\text{years}$ after opening. The device must be primed before initial use, but its longevity makes it more practical for patients.

• Maintenance Formulations:

  • Intravenous ($IV$) Infusion: The duration of action is coupled entirely to the duration of administration; the veins remain dilated for the duration of the infusion.

  • Transdermal Patch: Used strictly for prevention. It has a slow onset of action (approximately $40\,\text{minutes}$) because it relies on the slow release and metabolism of the drug into active dinitrate metabolites.

Isosorbide Nitrates: Dinitrate and Mononitrate

• Isosorbide Dinitrate (ISDN):

  • Like GTN, ISDN has poor oral bioavailability due to the hepatic reductase enzyme.

  • Its metabolism in the liver produces active metabolites: Isosorbide 2-mononitrate and Isosorbide 5-mononitrate ($ISMN$).

  • ISDN is available as an oral tablet (prevention) or a unique sublingual formulation that can be used for acute attacks.

• Isosorbide Mononitrate (ISMN):

  • ISMN is the active metabolite of the dinitrate form.

  • It is $100\%$ bioavailable when administered orally because it is not susceptible to further degradation by the hepatic reductase enzyme.

  • It has a significantly long half-life ($4\,\text{hours}$).

  • Controlled Release ($CR$) ISMN: Used strictly for prevention. The formulation is designed to create a profile of alternating high and low nitrate levels throughout the day to maximize efficacy while minimizing the risk of tolerance.

Comparison of Pharmacokinetic Properties

• Glycerol Trinitrate (GTN):

  • Route: Sublingual ($SL$) tablet/spray, Transdermal patch, $IV$ infusion.

  • Half-life ($t_{1/2}$): $1-3\,\text{minutes}$.

  • First-pass Metabolism: Yes (if taken $PO$).

  • Primary Metabolism: Nitrate reductase enzyme; Denitration to dinitrate ($t_{1/2} = 40\,\text{minutes}$).

  • Elimination: Glucuronidation and renal excretion.

• Isosorbide Dinitrate (ISDN):

  • Route: Sublingual ($SL$) tablet, Oral tablet.

  • Half-life ($t_{1/2}$): $45\,\text{minutes}$.

  • First-pass Metabolism: Yes.

  • Primary Metabolism: Nitrate reductase enzyme; Denitration to mononitrate ($t_{1/2} = 2-4\,\text{hours}$).

  • Elimination: Glucuronidation and renal excretion.

• Isosorbide Mononitrate (ISMN):

  • Route: Controlled Release ($CR$) tablet.

  • Half-life ($t_{1/2}$): $4\,\text{hours}$.

  • First-pass Metabolism: Minimal.

  • Primary Metabolism: Denitration to isosorbide.

  • Elimination: Glucuronidation and renal excretion.

Nitrate Tolerance and Patient Management

• Mechanism of Tolerance: Continuous exposure to nitrates leads to a rapid loss of efficacy, known as nitrate tolerance.

• Prevention Strategy: To maintain therapeutic effectiveness, patients must have a nitrate-free period of at least $8\,\text{hours}$ every $24\,\text{hours}$. This allows systemic levels to return to zero and reverses the tolerance effect.

• Clinical Application for Patches:

  • Patients should apply the patch during their most vulnerable periods.

  • For standard use, a patch might be applied at $8\,\text{AM}$ and removed before bed.

  • If a patient suffers from nocturnal angina, the patch should be applied in the evening and removed before noon.

Adverse Effects and Local Reactions

• Vasodilatory Side Effects:

  • Dizziness and lightheadedness (specifically when standing up quickly).

  • Headache (this is considered very common).

  • Flushing.

  • Orthostatic Hypotension: The venodilatory effect reduces preload and interrupts the natural baroreceptor reflex (involving the carotid sinus and aortic baroreceptors), leading to a significant drop in blood pressure upon standing.

  • Reflex Tachycardia: Occurs more frequently at higher doses. As peripheral resistance and blood pressure drop, the body compensates by increasing heart rate and myocardial contractility.

• Local Effects: Transdermal patches may cause contact dermatitis at the site of application.

Major Drug Interaction: Sildenafil (Viagra)

• Interaction with Phosphodiesterase 5 (PDE-5) Inhibitors:

  • Sildenafil and similar drugs are used for erectile dysfunction or pulmonary hypertension.

• Mechanism of Interaction:

  • Nitrates increase the production of $cGMP$.

  • Under normal conditions, the enzyme phosphodiesterase 5 ($PDE5$) breaks down $cGMP$.

  • Sildenafil blocks $PDE5$, leading to a massive, uncontrolled buildup of $cGMP$ and extreme relaxation of the vascular smooth muscles.

• Clinical Risks:

  • Severe Hypotension: A lethal drop in blood pressure can occur even if the drugs are taken $24\,\text{hours}$ apart.

  • Priapism: Excessive dilation of blood vessels in the corpus cavernosum can cause priapism (a painful erection lasting more than $2\,\text{hours}$). This is a medical emergency; if it persists beyond $4\,\text{hours}$, it can cause permanent tissue damage and impotence.

• Emergency Reversal: Life-threatening vasodilation or priapism caused by this interaction is treated with pseudoephedrine. Pseudoephedrine acts as a vasoconstrictor by binding to alpha receptors in the periphery, effectively reversing the effects of excessive $cGMP$.