Labetalol: bioavailability, drug plasma levels, plasma renin and catecholamines in acute and chronic treatment of resistant hypertension

Labetalol (Trandate, Glaxo Inc.; Normodyne, Schering Corp.)

Labetalol is a competitive antagonist of α1-, β1-, and β2-adrenergic receptors. The hemodynamic effects of the drug include reduced blood pressure, heart rate, and peripheral resistance, with little change in resting cardiac output or stroke volume. In open trials and controlled studies, labetalol was an effective antihypertensive. Labetalol compared favorably with β-blockers alone or in combination with vasodilators, for the treatment of hypertension. Reductions in heart rate are less pronounced with labetalol as compared with propranolol. Labetalol produces rapid reductions in blood pressure when administered intravenously for severe hypertension. The most frequent adverse reactions to the drug include fatigue, postural symptoms, headache, and gastrointestinal complaints. Labetalol may prove advantageous when vasodilation in addition to β-blockade is desired, or for selected patients experiencing adverse effects attributable to β-blockade. Until the clinical profile of labetalol is better defined, the use of the drug should be limited.

Single-dose pharmacokinetics of labetalol in healthy young men

Single doses of labetalol (200 mg i.v. over 5 min, 200 and 400 mg orally) were given to five healthy men on three different occasions. Plasma levels were followed for up to 25 h and blood pressure for 5 h. The elimination half-life was 1.6 to 8.5 h for the first 8 h. The oral bioavailability ranged from 4 to 23%. All doses induced a significant fall in systolic blood pressure at 2 h, the peak effect occurring at 30--120 (mean 63-72) min. After intravenous administration the peak supine blood-pressure fall was significant for both systolic and diastolic blood pressure and occurred 16 min after administration.

Case report: Profound hypotension associated with labetalol therapy in a patient with cerebral aneurysms and subarachnoid hemorrhage

PURPOSE

Labetalol is an effective antihypertensive medication frequently used to treat systemic hypertension in acute care settings, including the management of hypertension associated with a subarachnoid hemorrhage. We present a case of profound hypotension, refractory to inotropic and vasopressor therapy following an iv infusion of labetalol.

CLINICAL FEATURES

Initiation of an iv labetalol infusion resulted in good blood pressure control in a patient suffering from a Fisher grade 3 subarachnoid hemorrhage with an initial Glascow coma scale of 14/15 and mild hydrocephalus. Progressive deterioration of neurological symptoms and evidence of worsening hydrocephalus preceded the sudden development of profound hypotension (60/35 mmHg) and bradycardia with a minimum heart rate of 40 beats.min(-1). Initial resuscitative efforts included administration of intravascular fluid, hypertonic saline, atropine, adrenalin (more than 10 mg in divided doses) and noradrenalin. These measures restored the blood pressure to 80/45 with a HR of 98 beats.min(-1). Intraoperative placement of an intraventricular drain released cerebrospinal fluid under pressure with an initial intracranial pressure of 15 cm H(2)O. A combination of adrenalin, noradrenalin, dopamine and vasopressin infusions were required to restore the blood pressure to 130/65 mmHg after an additional two hours. All inotropic and vasopressor support was weaned off after the 14th hr (about two drug half-lives). The patient was awake and responsive the following day, with no obvious neurological consequences. No evidence of neurological injury, drug administration error or myocardial dysfunction was documented.

CONCLUSION

The episode of profound hypotension which occurred after initiating a labetolol infusion required maximal combined vasopressor therapy to restore the blood pressure suggesting that this patient demonstrated an extreme sensitivity to labetalol. Combination therapy with adrenergic and nonadrenergic agonists may be required for optimal treatment of profound hypotension associated with labetalol-induced vasoplegia.

The effects of a single dose of dilevalol on [3H]-noradrenaline plasma kinetics and plasma lipoprotein cholesterol concentrations

1. A single oral dose of dilevalol (200 mg or 400 mg) or placebo was administered to 15 normal male volunteers in a double-blind, random order crossover study. 2. Dilevalol had no significant effect on supine blood pressures or heart rates, but caused a significant fall in systolic blood pressure 1 and 30 min following standing, and attenuated the rise in diastolic blood pressure and heart rate that accompanies standing. 3. Dilevalol caused a dose dependent increase in plasma noradrenaline levels from arterialized blood which was due to an increase in noradrenaline spillover with no change in clearance. 4. Dilevalol increased plasma levels of the noradrenaline metabolite 3,4-dihydroxyphenylethylene glycol (DHPG) (which is formed in sympathetic nerves following neuronal uptake of noradrenaline), indicating that the increase in noradrenaline spillover was not due to the blockade of neuronal uptake. 5. Acute dilevalol administration had no effect on total plasma cholesterol, HDL-cholesterol or LDL-cholesterol levels.

Labetalol. A reappraisal of its pharmacology, pharmacokinetics and therapeutic use in hypertension and ischaemic heart disease

Since labetalol was first reviewed in the Journal (1978), its scope of therapeutic use has expanded and become better defined. Labetalol is an adrenoceptor blocking drug with combined alpha- and beta-blocking properties. These result in a more favourable haemodynamic profile for labetalol compared with 'pure' beta-blockers or pure alpha-blockers, but also contribute to a wider range, but not an overall increased incidence, of adverse effects. The drug is effective and well-tolerated in patients with all grades of hypertension, but is of particular value in special subgroups such as Black patients, the elderly and patients with renal hypertension. While comparative studies are not extensive, available data show that the drug reduces blood pressure to a similar extent, and in a similar proportion of patients, as 'pure' beta-blockers such as propranolol, pure alpha-blockers such as prazosin, calcium antagonists (nifedipine, verapamil), and centrally acting drugs (clonidine and methyldopa). Labetalol is very effective in hypertensive pregnant women and in hypertensive crises, where it provides good control of blood pressure without serious adverse effects, and where few therapeutic options exist. Few controlled studies have investigated the use of labetalol in deliberate induction of hypotension or prevention of hypertension during anaesthesia, and also in patients with ischaemic heart disease. However, available evidence suggests a role for labetalol in these indications and further studies should aid in clarification of its efficacy in these areas. Thus, with its broad scope of therapeutic use in hypertension labetalol remains an important therapeutic option, and the drug may well find an additional place in the treatment of myocardial ischaemia if further evidence confirms encouraging preliminary findings.

Interindividual and intraindividual variability in labetalol pharmacokinetics

The between-subject and within-subject variability in the pharmacokinetics of labetalol at steady state were determined. Sixteen nonobese normal volunteers (mean age, 27 years) received five different formulations of labetalol orally on five different occasions every 12 hours for five doses. A 7-day washout separated each administration phase. Plasma concentration-time data for labetalol were obtained over the 24-hour period after the fifth dose in each phase. Labetalol concentrations in plasma were measured using high-performance liquid chromatography (HPLC). Pharmacokinetic parameters for each subject after each study phase were estimated. The mean V beta/F, Vdss/F, TBC/F, t1/2 beta, and AUC tau 0 for each subject ranged between 18.1 and 161.9 L/kg, 7.1 and 53.9 L/kg, 1.3 and 5.72 L/hr/kg, 6.9 and 11.0 hours, and 154 and 520 micrograms.hr/L, respectively, indicating large interindividual variability. Considerable intraindividual variability in each of the pharmacokinetic parameters was also observed. These data indicate that a larger number of subjects will be required to detect "significant" differences in the disposition of labetalol.

Pharmacokinetics and systemic availability of the antihypertensive agent indoramin and its metabolite 6-hydroxyindoramin in healthy subjects

We have studied the pharmacokinetics and absolute systemic availability of indoramin (50 mg) given orally in solution or as a tablet with reference to intravenously administered drug (0.15 mg/kg) in 9 healthy volunteers. After intravenous administration the median apparent volume of distribution was 6.3 l X kg-1, plasma clearance was 20.0 ml X min-1 X kg-1, and terminal half-time was 4.1 h. When given by tablet indoramin was absorbed with moderate rapidity, with a median tmax of 1.5 h. The median systemic availability was 24%. After oral administration in solution the drug was more rapidly absorbed, with a median tmax of 1.0 h (p less than 0.01). The median systemic availability was 43% (15-85%). Plasma concentrations of an active metabolite, 6-hydroxyindoramin, after single oral doses in either dosage form, were of a similar order to those of unchanged drug and fell with similar rapidity. After intravenous administration, however, concentrations of the metabolite were negligible.

Falsely elevated urinary excretion of catecholamines and metanephrines in patients receiving labetalol therapy

Five patients with essential hypertension who were receiving treatment with the beta- and alpha-adrenoreceptor-blocking drug labetalol had elevated urinary excretion of catecholamines. When measured with a radioenzymatic assay, all five subjects had elevated urinary excretion of norepinephrine and normal urinary excretion of epinephrine; when measured with a fluorometric assay, four subjects had elevated urinary excretion of epinephrine with normal urinary excretion of norepinephrine, and one subject had elevated urinary norepinephrine excretion with normal urinary excretion of epinephrine. In three subjects who could be reevaluated three days to two months after labetalol therapy was discontinued, the urinary catecholamine levels had returned toward normal. Our studies indicate that labetalol produced a false elevation of urinary catecholamine levels. Although labetalol also interfered with the measurement of urinary excretion of metanephrine, it did not interfere with the measurement of urinary excretion of vanillylmandelic acid, homovanillic acid, 5-hydroxyindoleacetic acid, and serotonin, and it probably did not interfere with the measurement of plasma concentrations of dopamine, norepinephrine, and epinephrine. We conclude that labetalol therapy should be discontinued, and another beta-blocking drug, such as propranolol, that does not interfere with these tests should be temporarily substituted at least three days before a patient collects a 24-hour urine for measurement of the urinary excretion of catecholamines and metanephrine.

Pharmacology of combined alpha-beta-blockade. I

Several compounds of the chemical class arylethanolamines have been shown to possess combined alpha- or vasodilator and beta-adrenoceptor blocking properties. The first drug was labetalol (AH5158)[5-(1-hydroxy-2)1-methyl-3-phenylpropyl(amino)-ethyl (salicylamide)]. Others include medroxalol, bucindolol and YM-09538, which differ from labetalol either by the nature of the substitution on the primary benzene ring and/or on the terminal nitrogen. All of these drugs are non-selective beta-blockers, except for bucindolol whose selectivity has not been carefully defined. The rationale for the development of this group of drugs was the knowledge that blockade of one adrenoceptor subtype causes reflex stimulation of the other, i.e. vasoconstriction after nonspecific beta-blockade and tachycardia after alpha-blockade. Since both of these compensatory responses act to prevent a fall in blood pressure, a relatively weak blockade of both receptor types should act synergistically to produce a lowering of blood pressure with minimal physiological disturbance. Haemodynamic studies have confirmed that the additional alpha-blocking properties of labetalol produce a pattern of haemodynamic changes unlike that of propranolol and other simple beta-adrenoceptor blocking agents. Peripheral vascular resistance, which falls acutely during the initial administration of the drug, tends to fall further during prolonged administration and the pulse rate tends to remain only slightly lower than pretreatment levels. In addition, at normal dose levels cardiac output is maintained by a compensatory increase in stroke volume. Thus, blood pressure is lowered largely by a reduction in vascular resistance, and although the heart rate falls significantly during exercise, the cardiac output is maintained by an increase in stroke volume. This pattern of events is different to that seen with beta-blocking agents which consistently reduce cardiac output during exercise. Currently labetalol is the only member of this group of drugs which is in established clinical use. Its antihypertensive efficacy has been confirmed in many studies and it has been shown to be effective in the management of both hypertensive emergencies and in the long term management of severe hypertension. It is particularly valuable in allowing a reduction in the number of drugs required for adequate blood pressure control. The early theoretical prediction that postural hypotension would occur with high doses is now acknowledged to be labetalol's major dose-limiting side effect. Most of the available pharmacokinetic data on labetalol were derived from studies which utilised a fluorimetric assay.(ABSTRACT TRUNCATED AT 400 WORDS)

Labetalol: a review of its pharmacology, pharmacokinetics, clinical uses and adverse effects

Labetalol is a combined alpha- and beta-adrenoceptor blocking agent for oral and intravenous use in the treatment of hypertension. It is a nonselective antagonist at beta-adrenoceptors and a competitive antagonist of postsynaptic alpha 1-adrenoceptors. Labetalol is more potent at beta that at alpha 1 adrenoceptors in man; the ratio of beta-alpha antagonism is 3:1 after oral and 6.9:1 after intravenous administration. Labetalol is readily absorbed in man after oral administration, but the drug, which is lipid soluble, undergoes considerable hepatic first-pass metabolism and has an absolute bioavailability of approximately 25%. There are no active metabolites, and the elimination half-life of the drug is approximately 6 hours. Unlike conventional beta-adrenoceptor blocking drugs without intrinsic sympathomimetic activity, labetalol, when given acutely, produces a decrease in peripheral vascular resistance and blood pressure with little alteration in heart rate or cardiac output. However, like conventional beta-blockers, labetalol may influence the renin-angiotensin-aldosterone system and respiratory function. Clinical studies have shown that the antihypertensive efficacy of labetalol is superior to placebo and to diuretic therapy and is at least comparable to that of conventional beta-blockers, methyldopa, clonidine and various adrenergic neuronal blockers. Labetalol administered alone or with a diuretic is often effective when other antihypertensive regimens have failed. Studies have shown that labetalol is effective in the treatment of essential hypertension, renal hypertension, pheochromocytoma, pregnancy hypertension and hypertensive emergencies. In addition, preliminary studies indicate that labetalol may be of value in the management of ischemic heart disease. The most troublesome side effect of labetalol therapy is posture-related dizziness. Other reported side effects of the drug include gastrointestinal disturbances, tiredness, headache, scalp tingling, skin rashes, urinary retention and impotence. Side effects related to the beta-adrenoceptor blocking effect of labetalol, including asthma, heart failure and Raynaud's phenomenon, have been reported in rare instances.

The treatment of hypertension by labetalol--a new alpha- and beta-adrenoreceptor blocking agent

A new alpha- and beta-blocking agent, labetalol, was used to treat 50 hypertensive patients, 34 of them with refractory hypertension. A significant reduction in hypertension was achieved with relatively few adverse reactions. The combination of alpha and beta blockers in labetalol with their mutually opposing characteristics offers an interesting and promising approach to the control of hypertension.

Labetalol steady-state pharmacokinetics in hypertensive patients

1 During chronic treatment of hypertensive patients observed mean steady-state plasma labetalol levels in ten patients were consistently and significantly higher than predicted from initial acute pharmacokinetic analysis. 2 Mean steady-state levels varied from 36 to 183 ng/ml on a chronic dose of 200 mg twice daily. 3 Patients with lower steady-state levels on a fixed dose of labetalol usually required higher maintenance doses to control blood pressure.

Elimination kinetics of labetalol in severe renal failure

1 The pharmacokinetics of intravenously administered labetalol were studied in four patients with severe renal failure and in three normal subjects. 2 A new and sensitive method for the assay of plasma labetalol concentrations using high performance liquid chromatography is described. 3 Labetalol has a relatively large apparent volume of distribution (3.3-7.9 l/kg, two-compartment open model) and a relatively high plasma clearance (0.3-1.6 1 h-1 kg-1). 4 There were no significant differences between the patients with severe renal failure and the controls for any of the pharmacokinetic parameters measured. 5 In the presence of renal functional impairment, no modification of labetalol dosage is required. 6 In another study of 31 patients, glomerular filtration rate was measured at 3-month intervals for 3-18 months. In ten patients there was no change, in ten there was an improvement and in 11 there was deterioration, but in only three was this attributable to treatment. 7 In our experience with over 300 patients, we conclude that labetalol plus diuretic treatment is efficacious and safe in all grades of hypertension including those with all degrees of renal insufficiency.

Effects of chronic alpha and beta adrenoceptor blockade with labetalol on plasma catecholamines and renal function in hypertension

Plasma catecholamines and renal function were evaluated in 18 patients with essential hypertension treated with the alpha and beta adrenoceptor blocking agent, labetalol. Following 6 weeks of labetalol therapy, blood levels of epinephrine and norepinephrine remained unaltered. Glomerular filtration rate and renal plasma flow were decreased similarly by about 20% (P less than 0.025). Tubular rejection fraction of sodium was increased by 36% (P less than 0.001) while sodium excretion was comparable to control conditions. Labetalol's potential to cause a mild reduction in kidney function should be considered, but may have no clinical consequences in most hypertensive patients receiving such treatment. The lack of increased plasma catecholamine levels during therapy supports the concept that labetalol's alpha-blocking potential is limited to post-junctional receptors, leaving the prejunctional feedback control of catecholamine release intact. Moreover, labetalol's blood pressure-lowering mechanism may be largely independent of changes in sympathetic nervous activity.