Prazosin

Chemical Name 1 -(4-Amino-6,7-dimethoxy-2-quinazollnyl)-4-(2-furanylcarbonyl)piperazine Common Name Furazosin Structural Formula  

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OC-Adrenoceptor Blockers. Nonselective a-adrenoceptor blockers (Table 6), such as phentolamine, which block both a - and a2 adrenoceptors, produce vasodilation by antagonizing the effects of endogenous norepinephrine. They also produce severe tachycardia and have been replaced by selective a -adrenoceptor blockers, such as prazosin, terazosin, and doxazosin, which do not usually cause severe tachycardia. 

Prazosin, a selective a -adrenoceptor antagonist, exerts its antihypertensive effect by blocking the vasoconstrictor action of adrenergic neurotransmitter, norepinephrine, at a -adrenoceptors in the vasculature (200,227,228). Prazosin lowers blood pressure without producing a marked reflex tachycardia. It causes arteriolar and venular vasodilation, but a significant side effect is fluid retention. Prazosin increases HDL cholesterol, decreases LDL cholesterol, and does not cause glucose intolerance. 

Terazosin is selective a -adrenoceptor blocker having hypotensive efficacy equal to that of prazosin. Terazosin has a longer duration of action and better gastrointestinal absorption profile than prazosin. 

In general, activation of alpha-1 adrenergic receptors causes a contraction of smooth muscle and of blood vessels, pilomotor muscles, dilator pupillae, vas deferens, nictitating membrane, splenic capsule, and sphincters of the intestine and urinary bladder and of the bile duct. An exception is the relaxation of the smooth muscle of the intestine. Prazosin [19216-56-9] indoramin [26844-12-2] and WB-4101 are relatively selective antagonists of these receptors. 

Another quinazoline, prazosin (1040), is now a widely used antihypertensive drug, acting by vasodilation (77MI21300, B-81MI21301) its preparation is outlined in the patent literature (72NEP7206067). 

Alfuzosin (91) is a prazosin-like hypotensive adrenergic a-1 receptor blocker with the special structural feature that two carbons have been excised conceptually from the piperazine ring normally present in this series. Following the usual sequence for this series, reaction of 4-amino-2-chloro-7-dimethoxyquinazoline (89) with the tetrahydro-2-furyl amide of 3-methylaminopropyla-mine (90) gives alfuzosin (91) [25], Alfuzosin is claimed to cause less orthostatic hypotention (dizziness or fainting upon sudden rising) than prazosin. 

A large number of diugs interfere with the smooth muscle contraction. These compounds lower blood pressure and are referred to as antihypertensive. In this section, only those coumpounds will be mentioned that have a direct effect on smooth muscle tone. Phenylephrine is an agonist on most smooth muscles and activates ax adrenoceptors. Carbachol is an agonist on some smooth muscles and activates contraction through muscarinic receptors. Blockers of the ax-adrenoceptors such as prazosin and urapidil are competitive inhibitors of the ax-receptor in vascular and bladder smooth muscle. Phenoxybenzamine is an ineversible blocker of ax receptors and phentol-amine blocks ax and a2 receptors. Ca2+ channel blockers such as the dihydropyiidines, phenylalkyla-mines and benzothiazepines lower smooth muscle tone by blocking the L-type calcium channel. 

Alpha (a)-adrenergic blocking dragp—for example, doxazosin (Cardura) and prazosin (Minipress)... 

Metatensin Tablets—trichlormethiazide, reserpine Minizide—polythiazide, prazosin Prinzide—hydrochlorotliiazide, lisinopril Rauzide Tablets—rauwolfia, bendroflumethiazide Regroton—chlorthalidone, reserpine Salutensin Tablets—hydroflumethiazide, reserpine Salutensin-Demi—hydrochlorothiazide, reseqiine Ser-Ap-Es—hydrochlorotliiazide, reseqiine, hydralazine HC1... 

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