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Prazosin: A Review of its Pharmacological Properties and Therapeutic Efficacy in Hypertension

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Summary

Synopsis: Prazosin1 is a peripheral vasodilator antihypertensive agent derived from quinazoline. Animal studies suggest that its hypotensive effect results from relaxation of peripheral arterioles as a consequence of blockade of post-synaptic α-adrenoceptors. In man, the most important haemodynamic effect is a reduction in total peripheral resistance, but in contrast to hydrallazine, tachycardia is seldom produced.

Therapeutic trials have shown prazosin to be effective in lowering blood pressure in all grades of hypertension: in mild and some cases of moderate hypertension when used alone and in moderate and severe hypertension when used in combination with other agents. The antihypertensive effect of prazosin is enhanced by the concomitant administration of a thiazide diuretic or a β-adrenoceptor blocking drug. Clinical experience with combination antihypertensive therapy in severe hypertension indicates that a combination of prazosin, a β-adrenoceptor blocking drug and a thiazide diuretic is effective in many such patients. In patients with mild or moderate hypertension, the reduction in blood pressure induced by prazosin is comparable with that of α-methyldopa when the dose of both drugs is adjusted to obtain optimum effect and the overall incidence of side-effects and their severity has been similar with both drugs under the conditions of controlled therapeutic trials, although the nature of the side-effects differs (drowsiness or tiredness being more common and dizziness more persistent with methyldopa).

The most important side-effect is marked postural hypotension after the initial dose and sometimes after a rapid dose increment. This effect can be minimised by beginning treatment with a low dose (0.5mg tablets 2 or 3 times daily; 1mg capsules 3 times daily) and increasing dosage gradually. Overall, syncope has occurred in fewer than 0.2% of patients given low initial doses. Postural hypotension is not usually a problem during continued treatment.

Animal studies: Studies in normal and hypertensive rats and dogs have demonstrated the antihypertensive effects of oral and intravenous prazosin. Unlike hydrallazine, prazosin does not consistently increase heart rate in animals. The effect of prazosin on plasma renin activity has varied according to the hypertensive animal model used and possibly may be a function of dosage.

Prazosin appears to produce its antihypertensive effect by relaxation of peripheral arterioles as a consequence of post-synaptic α-adrenoceptor blockade, rather than by direct relaxation of arteriolar vascular muscle. It differs from phentolamine, which acts at both pre-and post-synaptic α-adrenoceptors, and from diazoxide and hydrallazine which directly affect vascular smooth muscle. Like diazoxide and hydrallazine, however, prazosin acts primarily on arterioles. Prazosin appears to have no central action on blood pressure and does not affect neuronal adrenergic function.

Studies with 14C-labelled prazosin in dogs indicate that the drug is rapidly taken up by the tissues. High concentrations of the drug are present in lung, coronary arteries, aorta, paw arteries and heart. The plasma half-life is reported to be about 1 to 2 hours. Prazosin is extensively metabolised with only about 6% being excreted unchanged in the rat and dog.

Human pharmacology: The haemodynamic effects of prazosin have been studied in hypertensive patients after single intravenous or oral doses, and multiple oral doses sufficient to lower blood pressure given over a short period or for several weeks or months. In male hypertensive patients studied before and after 11 to 12 months of treatment with prazosin, significant lowering of arterial pressure and total peripheral resistance occurred at rest and during exercise, whereas changes in cardiac index, heart rate or stroke index were not significant.

Single oral or intravenous doses of prazosin and therapeutic doses administered for several weeks to small groups of hypertensive patients have produced no demonstrable deterioration in renal function. The lack of adverse effect of prazosin on renal function in clinical experimental studies has been confirmed in therapeutic trials involving hypertensive patients with impaired renal function.

Plasma renin activity has consistently been reduced in hypertensive patients treated with therapeutic doses of prazosin alone for at least 4 weeks.

Studies of the pharmacokinetics of prazosin in man are few. Available data indicate that prazosin is readily absorbed after oral administration, but that there is considerable variation between individuals in the peak serum concentration and the time required to attain it, although the pattern of absorption is consistent within individuals. The effect of food on serum concentrations also varies between individuals; concentrations being increased in some subjects and reduced in others. The plasma half-life is about 2.5 to 4 hours in man. There appears to be no correlation between plasma levels and antihypertensive effect and the duration of effect is longer than expected from the plasma half-life. Prazosin is metabolised in the liver and is possibly subject to significant first pass metabolism. Higher and more prolonged plasma levels are attained in hypertensive patients with renal impairment and smaller than usual doses of prazosin are indicated in patients with chronic renal failure, especially initially.

Therapeutic trials: Studies conducted in hypertensive patients throughout the world have demonstrated the efficacy of prazosin in reducing all grades of hypertension; when used alone in mild and some moderate cases and in combination with a thiazide diuretic and/or other antihypertensive drugs in moderate to severe cases. When given in combination with a thiazide diuretic it is generally effective in controlling blood pressure in a high proportion of patients with severe as well as moderate hypertension.

Prazosin may be particularly useful as a substitute for hydrallazine or as an adjunct to existing antihypertensive drugs with a different mode of action in patients who are unable to tolerate further increments of these drugs or in whom blood pressure is not adequately controlled by them.

In newly diagnosed hypertensive patients, prazosin is particularly effective when used in combination with a thiazide diuretic (polythiazide 2 to 4mg daily has been used in studies to date). The thiazide can be given initially, and then prazosin added, or alternatively, introduced to an initial prazosin regimen. Although in many studies the thiazide has not been added to the regimen until the chosen maximum dosage of prazosin was reached, there is some evidence that the earlier introduction of a thiazide may produce equally satisfactory blood pressure control with a lower dosage of prazosin in some patients.

Studies comparing prazosin and α-methyldopa in patients with mild or moderate hypertension and in which the dosage of each drug was titrated to achieve optimum control of blood pressure, have shown little difference in their antihypertensive efficacy. In the studies reported to date, the overall incidence of side-effects and the proportion necessitating withdrawal of therapy have been similar with both drugs when studied under the controlled conditions of therapeutic trials, but the nature of side-effects differ; tiredness is more common with methyldopa and although dizziness is more common with prazosin initially, it tends to be more persistent with methyldopa. In comparison with hydrallazine, limited therapeuticstudies indicate that prazosin is comparable in antihypertensive effect but it appears to cause fewer troublesome side-effects.

High blood pressure is often difficult to treat in patients with impaired renal function and it is usually necessary to combine several antihypertensive agents to control it. Prazosin, with its different mode of action, has been added to the treatment regimen of several patients with impaired renal function with a consequent improvement in blood pressure. At present there is no evidence that the addition of prazosin to existing combined antihypertensive therapy results in further deterioration in residual renal function. Renal function impairment was not associated with a reduced response rate or an increased incidence of side-effects. Some patients with chronic renal failure respond with a fall in blood pressure after low dosages of prazosin. It would thus seem wise to use low dosages initially in such patients.

Side-effects: At dosages usually given for the control of hypertension, prazosin has been relatively well tolerated by most patients and only occasionally has it been necessary to discontinue the drug because of adverse effects. Dizziness and faintness, possibly associated with postural hypotension, have been the most commonly reported effects. In some patients severe postural reactions occur with the initial or first few doses of prazosin, but this effect may be minimised by beginning treatment with a low dose, and usually disappears with continued treatment. In a few patients, postural effects may persist beyond the first few doses, or have occurred after a sharp dose increase. Unlike hydrallazine, prazosin usually does not cause a rise in heart rate. Palpitations and tachycardia have been reported only occasionally. Other more commonly reported side-effects include headache, lassitude, dry mouth, nausea, and urinary frequency with urgency.

Dosage: As some patients experience postural symptoms with the initial or first few doses of prazosin it is essential to begin treatment with a low dose; e.g. 0.5mg (tablets) 2 to 3 times daily or 1mg (as capsules*) 3 times daily, with the first dose preferably at bedtime. Subsequently, the dose should be gradually increased to 3 to 20mg daily in 2 or 3 divided doses.

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‘Minipress’, ‘Hypovase’, ‘Hypovasol’, ‘Peripress’ (Pfizer)

In the USA, prazosin is available as capsules. The initial dosage recommendation with this dose form is different from the tablets because the capsule formulation results in lower peak plasma levels of prazosin.

Various sections of the manuscript reviewed by: A. Amery, Department of Medicine, Academisch Ziekenhuis Sint-Rafael. Belgium; J.R. Curtis, Charing Cross Hospital, London, England; R.W. Gifford, The Cleveland Clinic Foundation, Cleveland, Ohio, USA; L.J. Goldberg, Department of Pharmacological and Physiological Sciences, The University of Chicago, Illinois, USA; F. Gross, Department of Pharmacology, University of Heidelberg, Federal Republic of Germany; Priscilla Kincaid-Smith, Department of Medicine, Royal Melbourne Hospital, Victoria, Australia; P. Lund-Johansen, Department of Medicine, University of Bergen School of Medicine, Norway; M. Moser, Davis Avenue Medical Center, White Plains, New York, U.S.A.; G. Onesti, Division of Nephrology and Hypertension, Hahnemann, Medical College, Philadelphia, USA; J.S. Peel, Hawke’s Bay Medical Research Foundation, Napier, New Zealand; J.J.S. Robertson, Medical Research Council Blood Pressure Unit. Western Infirmary, Glasgow, Scotland; F.O. Simpson, Department of Medicine, University of Otago, Dunedin, New Zealand; P. Sleight, Department of Cardiovascular Medicine, Radcliffe Infirmary, Oxford, England. G.S. Stokes, Medical Research Department, Sydney Hospital, Australia; A.S. Turner, Cardiology Department, Napier Hospital, New Zealand.

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Brogden, R.N., Heel, R.C., Speight, T.M. et al. Prazosin: A Review of its Pharmacological Properties and Therapeutic Efficacy in Hypertension. Drugs 14, 163–197 (1977). https://doi.org/10.2165/00003495-197714030-00002

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