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Hydralazine and Dihydralazine

Hydralazinum, Dihydralazinum

For medical students2 min readUpdated 2026-10-10

Hydralazine and dihydralazine are direct-acting myotropic vasodilators that lower blood pressure by directly relaxing resistance vessels. In clinical pharmacology, these drugs serve as a classic example of how a patient's genetic profile (acetylation rate) critically affects drug bioavailability and metabolism.

GeneticsBioavailability strictly depends on genetically determined acetylation rate.

TargetDrugs act predominantly on resistance vessels (arterioles and small arteries).

DurationPeak effect occurs in 1–2 hours, with a total duration of 6–8 hours.

ToxicityOverdose carries the risk of developing a specific drug-induced lupus-like syndrome.

Mechanism of Action and Pharmacodynamics

The precise molecular mechanism by which hydralazine (Hydralazinum) affects vascular smooth muscle cells remains unclear. However, the pharmacodynamics and hemodynamic effects of the drug have been studied in detail.

The primary site of action for this direct vasodilator is the resistance vessels, which include arterioles and small arteries. By acting on these vessels, the drug causes pronounced vasodilation (widening of the lumen). The physiological consequence of this smooth muscle relaxation is a significant decrease in total peripheral vascular resistance (TPVR). This drop in TPVR drives the ultimate therapeutic effect: a marked reduction in blood pressure.

Pharmacokinetics and Genetic Polymorphism

The pharmacokinetic profile of hydralazine is of particular interest due to pronounced genetic polymorphism. The drug can be administered both intravenously and orally (per os). When taken orally, absorption from the gastrointestinal tract is very high, with approximately 90% of the active substance absorbed.

However, high absorption does not guarantee high systemic concentrations. Bioavailability is low and highly variable due to a potent first-pass hepatic metabolism effect, where the drug undergoes extensive biotransformation into active metabolites. Acetylation is the key pathway here.

The activity of the metabolizing enzyme is genetically determined, dividing patients into two main groups:

"Slow acetylators": Metabolism proceeds more slowly, resulting in a systemic bioavailability of approximately 30%.
"Fast acetylators": The enzyme is highly active, breaking down most of the drug before it reaches the systemic circulation. In these patients, bioavailability drops to 10%.

Regarding temporal parameters, oral administration yields a peak hypotensive effect in 1–2 hours, with a total duration of action ranging from 6 to 8 hours.

Side Effects and Safety Profile

Despite its antihypertensive efficacy, the clinical use of hydralazine is limited by its adverse effect profile, which impacts multiple body systems:

Cardiovascular system: Reflex tachycardia occurs in response to vasodilation and falling blood pressure. Additionally, sodium and water retention are common, clinically manifesting as peripheral edema.
Gastrointestinal tract: Patients frequently experience dyspeptic symptoms, including nausea, vomiting, diarrhea, and anorexia (loss of appetite).
Central nervous system: The most frequent central nervous system adverse event is headache.

A major safety concern is the drug's specific toxicity. Overdose with hydralazine can trigger a severe complication known as drug-induced lupus-like syndrome, which clinically mimics systemic lupus erythematosus.

Dihydralazine

Dihydralazine (Dihydralazinum) is a related compound that is virtually identical to hydralazine in its chemical structure and pharmacological profile.

In modern medical practice, dihydralazine is rarely used as a standalone monotherapy. As a rule, it is included in fixed-dose combination drugs. A classic example is Adelphane-Esidrex, where dihydralazine is combined with other active agents.

Mnemonic

To remember the specific adverse effect of hydralazine, use the association: "HYDRA bites like a WOLF" — HYDRAlezine overdose causes a WOLF-like (lupus-like) syndrome.

Frequently asked questions

What is the molecular mechanism of vascular smooth muscle relaxation by hydralazine?

The exact molecular mechanism by which hydralazine relaxes vascular smooth muscle remains unclear. It is known that the primary targets are resistance vessels (arterioles and small arteries), and action on these structures decreases total peripheral vascular resistance (TPVR) and blood pressure.

What are the primary clinical indications for hydralazine?

The main indications for hydralazine include chronic congestive heart failure and hypertensive emergencies.

Chronic congestive heart failure — prescribed as an alternative to ACE inhibitors when they cannot be used (e.g., marked renal failure or hypotension).
Eclampsia — used as a direct vasodilator for rapid blood pressure reduction.

Hydralazine is not used as a first-line monotherapy agent.

What active components are included in the combination drug Adelphane-Esidrex besides dihydralazine?

Besides dihydralazine, the fixed-dose combination antihypertensive drug Adelphane-Esidrex includes three other active components:

Reserpine — a sympatholytic agent (0.0001 g).
Hydrochlorothiazide — a diuretic (0.01 g).
Potassium chloride — an electrolyte supplement (0.6 g) added to prevent hypokalemia induced by the thiazide diuretic.

This combination provides synergistic action and patient convenience.

Why does hydralazine bioavailability vary so widely among patients?

This is due to genetic polymorphism in hepatic enzymes responsible for acetylation. In "fast acetylators," the drug is metabolized more aggressively, dropping bioavailability to 10%, whereas in "slow acetylators," it reaches about 30%.

Which blood vessels are primarily targeted by hydralazine?

The drug selectively relaxes resistance vessels—arterioles and small arteries. This leads to a decrease in total peripheral vascular resistance (TPVR) and a drop in blood pressure.

What is the clinical role of dihydralazine?

It is structurally and pharmacologically similar to hydralazine, but in clinical practice, it is typically used in fixed-dose combination medications rather than monotherapy (e.g., Adelphane-Esidrex).

Go deeper

Molecular hypotheses regarding the mechanism of action of hydralazine on smooth muscle.
Pathogenesis of drug-induced lupus-like syndrome in hydralazine overdose.
Mechanisms of reflex tachycardia in response to direct vasodilation.
Clinical significance of sodium and water retention during myotropic vasodilator therapy.
Principles of combination therapy utilizing dihydralazine.