Prazosin

Absorption

After administration of an oral dose, peak plasma concentrations are attained at approximately 3 hours . There is a linear association between the prazosin dose given and plasma concentration at steady state .

Toxicity

TDLO, LD50: Oral TDLO (human): 285 μg/kg; Oral TDLO (woman): 10 μg/kg .
Oral LD50 (rat): 1950 mg/kg; Intraperitoneal LD50 (rat): 102 mg/kg .
Overdose information
Accidental ingestion of at least 50 mg of prazosin by a two-year-old child led to severe drowsiness with depressed reflexes. There was no fall in blood pressure, and the child recovered without complication .
Use in pregnancy
There are no adequate and well-controlled studies determining the safety of prazosin use during pregnancy. It is considered a pregnancy category C drug. Prazosin should be used during pregnancy only in cases where the benefit outweighs the possible risk to the mother and fetus . In specific cases where blood pressure control was emergent during pregnancy, prazosin has been used and no effects on the fetus or neonate were reported .
Use in nursing
This drug is found excreted in small concentrations in human milk. This drug should be used with caution when used during nursing .

Originator

Hypovase, Pfizer, UK,1974

The Uses of Prazosin

Prazosin is used for treating mid-to-moderate hypertension. When using this drug, blood pressure is reduced without any significant change in indicators of cardiac function such as frequency, coronary flow, or cardiac output.

The Uses of Prazosin

Antihypertensive.

The Uses of Prazosin

Prazosin is used to treat average or moderate hypertension. Upon taking this drug, blood pressure drops without substantial changes in indicators of heart work, such as rate, coronary flow, and cardiac output. Prazosin is used for weak to moderate hypertension.

Background

Prazosin is a drug used to treat hypertension. Prazosin is marketed by Pfizer and was initially approved by the FDA in 1988. It belongs to the class of drugs known as alpha-1 antagonists.
Recently, many studies have evaluated the benefits of this drug in controlling the symptoms of post-traumatic stress disorder (PTSD) and associated nightmares.

Indications

This drug is indicated for the treatment of hypertension (high blood pressure). Prazosin can be given alone or given with other blood pressure-lowering drugs, including diuretics or beta-adrenergic blocking agents .
Prazosin does not negatively impact lung function, and therefore may be used to manage hypertension in patients who are asthmatic or patients with chronic obstructive lung disease (COPD).

What are the applications of Application

Prazosin is a useful α1 and α2B-adrenoceptor antagonist

Definition

ChEBI: A member of the class of piperazines that is piperazine substituted by a furan-2-ylcarbonyl group and a 4-amino-6,7-dimethoxyquinazolin-2-yl group at positions 1 and 4 respectively.

Manufacturing Process

Preparation of 2-Chloro-4-Amino-6,7-Dimethoxyquinazoline: To 800 ml of a solution of anhydrous ammonia in tetrahydrofuran at room temperature is added 30 g of 2,4-dichloro-6,7-dimethoxyquinazoline [F.H.S. Curd et al., J. Chem. Soc., p 1759 (1948)]. The mixture is stirred for 44 hours. The precipitate (29 g, MP 267° to 268°C) is filtered and recrystallized from methanol to yield 19 g of 2-chloro-4-amino-6,7-dimethoxyquinazoline, MP 302°C (dec.).
Preparation of 2-(1-Piperazinyl)-4-Amino-6,7-Dimethoxyquinazoline: To 5 g of 2-chloro-4-amino-6,7-dimethoxyquinazoline, is added 20 g of a 25% solution of piperazine in ethanol. The mixture is heated at 160°C for 16 hours in a pressure bottle. The solvent is then evaporated and the residue is recrystallized from methanol/water.
Preparation of 2[4-(2-Furoyl)-Piperazinyl]-4-Amino-6,7-Dimethoxyquinazoline: To 0.10 mol 2-(1-piperazinyl)-4-amino-6,7-dimethoxyquinazoline in 300 ml methanol is added with vigorous stirring, 0.10 mol 2-furoyl chloride. After addition is complete, the mixture is stirred for 3 hours at room temperature. The solids are filtered to give the desired product, MP 278° to 280°C.

brand name

Minipress (Pfizer).

Therapeutic Function

Antihypertensive

Pharmacokinetics

Effects on blood pressure
The pharmacodynamic and therapeutic effect of this drug includes is a decrease in blood pressure as well as clinically significant decreases in cardiac output, heart rate, blood flow to the kidney, and glomerular filtration rate. The decrease in blood pressure may occur in both standing and supine positions .
Many of the above effects are due to vasodilation of blood vessels caused by prazosin, resulting in decreased peripheral resistance , . Peripheral resistance refers to the level resistance of the blood vessels to blood that flows through them. As the blood vessels constrict (narrow), the resistance increases and as they dilate (widen), and peripheral resistance decreases, lowering blood pressure .
Effects on sleep disturbance related to post-traumatic stress disorder (PTSD)
Some studies have suggested that this drug improves sleep in patients suffering from insomnia related to nightmares and post-traumatic stress disorder, caused by hyperarousal . This effect likely occurs through the inhibition of adrenergic stimulation found in states of hyperarousal .

Clinical Use

Prazosin is effective in reducing all grades of hypertension. The drug can be administered alone in mild and (in some instances) moderate hypertension.When the hypertension is moderate or severe, prazosin generally is given in combination with a thiazide diuretic and a -blocker. The antihypertensive actions of prazosin are considerably potentiated by coadministration of thiazides or other types of antihypertensive drugs.
Prazosin may be particularly useful when patients cannot tolerate other classes of antihypertensive drugs or when blood pressure is not well controlled by other drugs. Since prazosin does not significantly influence blood uric acid or glucose levels, it can be used in hypertensive patients whose condition is complicated by diabetes mellitus or gout. Prazosin and other -antagonists find use in the management of benign prostatic obstruction, especially in patients who are not candidates for surgery. Blockade of -adrenoceptors in the base of the bladder and in the prostate apparently reduces the symptoms of obstruction and the urinary urgency that occurs at night.

Side Effects

Although less of a problem than with phenoxybenzamine or phentolamine, symptoms of postural hypotension, such as dizziness and light-headedness, are the most commonly reported side effects associated with prazosin therapy. These effects occur most frequently during initial treatment and when the dosage is sharply increased. Postural hypotension seems to be more pronounced during Na deficiency, as may occur in patients on a low-salt diet or being treated with diuretics, - blockers, or both.

Synthesis

Prazosin, 1-(4-amino-6,7-dimethoxy-2-quinazolinyl)-4-(2-furoyl)-piperazine (12.2.12), is synthesized from 2-amino-4,5-dimethoxybenzoic acid, which upon reaction with sodium cyanate undergoes heterocyclation into 2,4-dihydroxy-6,7-dimethoxyquinazoline (12.2.9). Substituting hydroxyl groups of this compound with chlorine atoms by reaction with thionyl chloride, or a mixture of phosphorous oxychloride with phosphorous pentachloride gives 2,4-dichloro-6,7-dimethoxyquinazoline (12.2.10). Upon subsequent reaction with ammonia, the chlorine atom at C4 of the pyrimidine ring is replaced with an amino group, which leads to the formation of 4-amino-2-chloro-6,7-dimethoxyquinazoline (12.2.11). Introducing this into a reaction with 1-(2-furoyl)piperazine gives prazosin (12.2.12) [38¨C47].

Veterinary Drugs and Treatments

Prazosin is less well studied in dogs than hydralazine, and its capsule dosage form makes it less convenient for dosing. Prazosin, however, appears to have fewer problems with causing tachycardia, and its venous dilation effects may be an advantage over hydralazine when preload reduction is desired. It could be considered for therapy for the adjunctive treatment of CHF, particularly when secondary to mitral or aortic valve insufficiency when hydralazine is ineffective or not tolerated. Prazosin may also be used for the treatment of systemic hypertension or pulmonary hypertension in dogs.

Drug interactions

Potentially hazardous interactions with other drugs
Anaesthetics: enhanced hypotensive effect.
Antidepressants: enhanced hypotensive effect with MAOIs.
Avanafil, vardenafil, sildenafil and tadalafil: enhanced hypotensive effect - avoid.
Beta-blockers: enhanced hypotensive effect, increased risk of first dose hypotensive effect.
Calcium-channel blockers: enhanced hypotensive effect, increased risk of first dose hypotensive effect.
Diuretics: enhanced hypotensive effect, increased risk of first dose hypotensive effect.
Moxisylyte: possibly severe postural hypotension when used in combination.

Metabolism

In animals, prazosin hydrochloride is heavily metabolized. This occurs through liver demethylation and conjugation . Some studies in humans or human cells in vitro show similar prazosin metabolism .

Metabolism

Prazosin is extensively metabolised in the liver, mainly by demethylation and conjugation; some of the metabolites have antihypertensive activity.
It is excreted as metabolites and 5-11% as unchanged prazosin mainly in the faeces via the bile.