Pharmacokinetics and concentration-effect relationships of intervenous and oral clonidine

A comparison of the haemodynamic and behavioural effects of moxonidine and clonidine in normotensive subjects

1. This randomised double-blind placebo controlled crossover study in healthy normotensive males compared the haemodynamic and behavioural responses following single oral doses of moxonidine (200 micrograms), clonidine (200 micrograms) and placebo. 2. Both active drugs significantly reduced blood pressure as compared with placebo: on average (over the study day) by -5.6/-0.8 with moxonidine and by -13.3/-5.3 mm Hg with clonidine. The hypotensive effect of clonidine was significantly greater (95% CI 3.2-12.2). Heart rate was unchanged by either drug. 3. Psychomotor testing, salivary flow and side effect reporting showed a consistent treatment rank order similar to that of the hypotensive response: clonidine greater than moxonidine greater than placebo. 4. Although moxonidine produced less adverse effects than clonidine, an equivalent hypotensive response was not demonstrated in normal subjects. Further study at comparable antihypertensive doses is required to clarify the relative side effect profile of these agents.

Do centrally-acting antihypertensive drugs act at non-adrenergic as well as alpha-2 adrenoceptor sites?

Rabbits were treated with guanabenz, clonidine and rilmenidine for 6 days via osmotic minipumps. Blood pressure, heart rate and responses to intracisternal clonidine were measured after 1 and 6 days treatment. Radioligand binding to forebrain and hindbrain membranes after 6 days treatment was examined using [3H]yohimbine to measure the number of adrenergic binding sites and [3H]clonidine and [3H]idazoxan to assess nonadrenergic imidazoline sites. No change in nonadrenergic imidazoline binding was observed but adrenergic binding was decreased in forebrain and hindbrain by guanabenz and in hindbrain by clonidine treatment. Resting heart rate was decreased after 1 day's treatment with partial recovery by day 6. At this time heart rate significantly reduced in the clonidine and rilmenidine treated groups but not the guanabenz group. No significant change in baseline blood pressure was observed in normotensive rabbits. Both depressor and bradycardia responses to intracisternal clonidine were attenuated after 1 day's dosing but only depressor responses were influenced after 6 days. Blood pressure and heart rate thus appeared to be regulated independently. It is possible that imidazoline receptors predominate in the central control of blood pressure while central alpha-2 adrenoceptors play a greater part in heart rate regulation.

Dose-response relationships with antihypertensive drugs

A variety of antihypertensive drugs have been introduced into clinical practice at excessively high dose. Examples include most thiazide diuretics, propranolol, oxprenolol, atenolol, methyldopa, hydralazine and captopril. These very high doses have usually resulted from studies in which doses have been increased at regular intervals until the desired antihypertensive effect has been achieved or until unacceptable adverse effects have resulted. Frequently the starting doses were too high and the intervals between dose adjustment too short. In many cases these large doses resulted in unnecessary adverse effects--the adverse biochemical effects of thiazide diuretics, nephrotic syndrome, taste disturbances and neutropenia with captopril, the lupus syndrome with hydralazine and the central nervous system effects of methyldopa. Parallel group design with single doses and sufficient statistical power to distinguish between the upper and lower ends of the antihypertensive dose-response relationship should replace the dose-escalating design.

The effect of intravenous clonidine on the forearm circulation

The effect of two doses of clonidine on forearm blood flow was compared with an inert treatment using mercury strain gauge venous occlusive plethysmography. In the clonidine treated groups, forearm blood flow was unaffected in the resting state, but decreased sharply with tracheal intubation. In the saline group, blood flow increased with intubation. Forearm vascular resistance increased in the clonidine treated groups, but decreased in the saline group. These results suggest that clonidine has a peripheral action in anaesthetised normotensive individuals, and is not a purely centrally acting drug.

Transdermal and oral clonidine

The antihypertensive efficacy and side effects of transdermal clonidine (Catapres-TTS) and oral clonidine in equivalent doses on a weight basis were compared under double blind (double dummy) and cross over conditions in 16 outpatients with mild to moderate hypertension. After four weeks of placebo TTS and placebo tablet treatment, the patients were randomly placed into groups for six weeks of active treatment and, after an intervening week of placebo treatment, a second six week treatment period. Transdermal clonidine reduced supine and standing blood pressures (P less than 0.01) and heart rates (P less than 0.05) compared with the values at the end of the placebo periods, while oral clonidine did so to the extent of supine systolic blood pressure (P less than 0.01) and standing heart rate (P less than 0.05), respectively. There were, however, no differences in the values between transdermal and oral clonidine at the end of these six week periods. The plasma clonidine concentration was lower 12 hours after a dose of oral clonidine than after transdermal clonidine (P less than 0.05). The side effects did not differ. Seven patients said afterwards that they preferred the transdermal treatment, two preferred the oral treatment and four could not state any preference. It is concluded that transdermal clonidine is similar in its effect to oral clonidine in mild to moderate hypertension. Transdermal clonidine once a week may increase patients' compliance with antihypertensive treatment.

Cardiovascular effects of intravenous clonidine. Partial attenuation of the pressor response to intubation by clonidine

The effect of clonidine on the pressor and heart rate response to tracheal intubation was studied in a placebo-controlled, randomised, double-blind trial. Thirty patients were pretreated with either clonidine 1.25 micrograms/kg, or clonidine 0.625 microgram/kg or an equivalent volume of normal saline, given intravenously 15 minutes before induction of anaesthesia. The attenuation of the pressor response to intubation of both clonidine groups was statistically significant compared to the saline group. Neither dose of clonidine completely abolished the increase in either heart rate or blood pressure. There was no difference in attenuation between the clonidine treatments; this indicated that the lower dose may be the more appropriate.

Influence of some alpha 2-receptor agonists and antagonists on the excitatory non-adrenergic, non-cholinergic neurotransmission in the airways of guinea-pigs in vivo

Neural control of smooth muscle tension in the airways of guinea-pigs can be subdivided into at least four components: an excitatory cholinergic, an inhibitory noradrenergic, an excitatory non-adrenergic, non-cholinergic and an inhibitory non-adrenergic, non-cholinergic component. The existence of alpha 2-adrenoceptors that modulate the excitatory non-adrenergic, non-cholinergic nerve activity has also been demonstrated. The aim of the present study was to investigate the dose-dependence of the selective alpha 2-receptor agonists UK 14,304, dexmedetomidine and clonidine on these alpha 2-adrenoceptors in the airway of guinea-pigs in vivo. Electrical stimulation of the cervical vagus nerves in anaesthetized guinea-pigs resulted in a biphasic response in insufflation pressure: an immediate increase due to excitation of cholinergic nerve fibres and a slower, longer-lasting increase due to activation of the excitatory non-adrenergic, non-cholinergic fibres to be studied. The cholinergic compound was abolished by atropine, leaving the excitatory non-adrenergic, non-cholinergic component as the sole response recorded during vagal stimulation. UK 14,304 and dexmedetomidine attenuated the response in a dose-dependent manner, whereas the effect of the partial agonist clonidine was less dose-related. UK 14,304 and dexmedetomidine reduced the insufflation pressure about 60 and 53% respectively at the highest dose. In conclusion, the neurotransmission in the excitatory non-adrenergic, non-cholinergic nerves is attenuated by selective alpha 2-receptors in a dose-dependent manner. The present study adds support to earlier observations that the excitatory non-adrenergic, non-cholinergic neural system in the airways of guinea-pigs is modulated by alpha 2-adrenoceptors.

[Effects of intravenous clonidine on postoperative shivering]

A study was designed to assess the possible effects of intravenous clonidine on postoperative shivering. Fifty patients undergoing spinal fusion under isoflurane anaesthesia were randomly assigned to two groups (n = 25). Patients in one group were given intravenous clonidine (5 micrograms.kg-1 during the first hour, and 0.3 microgram.kg-1.h-1 thereafter) or placebo. Shivering intensity (rated from 0 to 2) and pulmonary artery blood temperature (theta) was recorded every ten minutes for two hours. Haemodynamic and metabolic data were obtained by way of a Swan-Ganz catheter prior to administering the test drug, and then after 1 and 2 hours. On arrival at the recovery room, theta was 36.2 +/- 0.3 degrees C in the clonidine group, and 36.5 +/- 0.2 degrees C in the placebo group. There were no significant differences in shivering between the two groups. Clonidine significantly decreased blood pressure, heart rate, cardiac output and whole body oxygen consumption. The patients in these series were free from any cardiac disease. Further studies are required to assess the possible beneficial effects of clonidine on postoperative oxygen consumption in patients suffering from heart failure.

Relationship of plasma clonidine to growth hormone concentrations in children and adolescents

The relationships of plasma clonidine to increases in growth hormone, blood pressure response and degree of sedation were studied in nine children with short stature after a 0.15 mg/m2 oral clonidine tolerance test. A direct correlation was found between the peak plasma clonidine concentration and the increase in plasma growth hormone (P = 0.055). The patients experienced varying degrees of blood pressure reduction and sedation that were unrelated to age. No correlation was found between the plasma clonidine concentration and degree of blood pressure reduction or degree of sedation. The half-life of oral clonidine ranged from 1.75 to 2.38 hours, which is significantly less than that for adults.

Pharmacokinetics and antihypertensive effects of low dose clonidine during chronic therapy

Using a sensitive and specific radioimmunoassay the pharmacokinetic disposition of clonidine was determined in hypertensive patients after a single dose and then after 5, 28 and 56 days of chronic dosing with 75 micrograms bd. Following a single dose of clonidine maximal plasma concentrations of 0.34 +/- 0.06 ng/ml were achieved after 3.6 +/- 1.2 hours. After 5 days of repetitive dosing the maximal concentration was significantly higher, 0.66 +/- 0.06 ng/ml and remained so throughout chronic therapy (P = 0.018). The AUC, Tmax and T1/2 did not differ significantly between the acute dose and the chronic dosing pharmacokinetic studies. Clonidine also produced a significant fall in blood pressure. Supine diastolic blood pressure fell from 106 +/- 5 mmHg predose to 99 +/- 6 mmHg 2 hours after the first dose (P less than 0.05). The corresponding values after cyclopenthiazide alone were 108 +/- 8 and 105 +/- 8 mmHg (P = 0.13). Similar falls in blood pressure were produced during chronic therapy.

Catecholamines and the covert orientation of attention in humans

The role of brain catecholamines in covert orienting was tested in normal subjects using a cued reaction time paradigm which measures the directional engagement, disengagement and movement of attention. Droperidol and clonidine were administered intravenously to suppress central dopamine and noradrenaline transmission. Both drugs produced reductions in the cost of invalid cueing without change in the benefit of valid cueing suggesting that both noradrenaline and dopamine are involved in facilitating the disengagement of attention. These results are relevant to the slowed disengagement observed with parietal lesions in monkeys and humans since this region of the cortex associated with visuospatial analysis receives a dense innervation by both dopamine and noradrenaline projections in primates.

Abolition of clonidine's effects on ventricular refractoriness by naloxone in the conscious dog

The interaction between opiate and adrenergic receptors on cardiac electrophysiologic function in the conscious dog was addressed in our study. We examined the effects of opiate receptor blockade with naloxone on clonidine-induced changes in refractoriness of the cardiac ventricle. Nine dogs were chronically instrumented for recording mean arterial blood pressure, administration of drugs and for measurement of effective refractory period of the ventricle. Clonidine (10 micrograms/kg, i.v.) significantly (p less than 0.05) decreased heart rate to 72 +/- 5 beats/minute from 108 +/- 8 beats/minute; mean arterial pressure decreased significantly (p less than 0.05) to 83 +/- 3 mmHg from 91 +/- 4 mmHg. Ventricular refractoriness was increased significantly (p less than 0.05) at current levels of 7 and 10 mA and pacing rates 180 and 200 beats/minute. Naloxone (3-10 mg/kg, i.v.) abolished clonidine's effects on heart rate, mean arterial pressure and ventricular refractoriness. We conclude that ventricular refractoriness may be regulated in part by interactions between central adrenergic and opioidergic systems.

Medetomidine--a novel alpha 2-adrenoceptor agonist: a review of its pharmacodynamic effects

1. The pharmacodynamic effects of medetomidine, a novel alpha 2-adrenoceptor agonist, are reviewed. 2. In receptor binding experiments, and in isolated organ preparations medetomidine shows high specificity and selectivity to alpha 2-adrenoceptors. Its alpha 2/alpha 1 selectivity ratio is 1620 compared to 220 of clonidine. It is a highly potent full agonist at alpha 2-adrenoceptors, a fact that also distinguishes it from clonidine. 3. Medetomidine induces a dose-dependent decrease in the central release and turnover of norepinephrine (NE) measured as changes in metabolite concentrations or using pharmacological intervention techniques. 4. The selectivity, specificity and potency of medetomidine is further supported by various in vivo experiments showing dose-dependent hypotensive, bradycardic, sedative, anxiolytic mydriatic, hypothermic and analgesic effects. 5. The pharmacological, neurochemical and behavioral effects of medetomidine can be inhibited by prior, simultaneous or subsequent administration of selective and specific alpha 2-antagonists. 6. In humans medetomidine is well-tolerated and pharmacodynamic effects including e.g. dose-dependent decrease of vigilance, blood pressure, heart rate, salivary secretion and plasma NE are compatible with an agonistic action at alpha 2-adrenoceptors.

Quantitative measurement of clonidine in human plasma by combined gas chromatography/electron capture negative ion chemical ionization mass spectrometry

A new quantitative and selective assay was developed for the measurement of low levels of clonidine in human plasma. The drug and the deuterated internal standard (2H4)clonidine are quantified by gas chromatography/negative ion chemical ionization mass spectrometry with methane as the reagent gas. After a three-step extraction procedure, the two compounds of interest are converted to their fluorinated derivatives using 3,5-bis(trifluoromethyl)benzoyl chloride. The mass spectrometer is focused to monitor abundant and stable characteristic high-mass ions (m/z 673 and 709) which are generated in the ion source by a resonance electron capture process. This assay requires only 1 ml of plasma sample and the limit of detection of the method is 5 pg ml-1 with a relative standard deviation of less than 7%.

Clinical pharmacokinetics of clonidine

Clonidine is a centrally active antihypertensive agent effective in the treatment of mild, moderate and severe hypertension, alone or in combination with other drugs. Use of oral clonidine has often been limited by side effects which include dry mouth and drowsiness. Transdermal clonidine was therefore developed as an alternative to oral therapy. Ideally, a drug administered at a constant rate into the systemic circulation should attain steady-state concentrations with less peak-to-trough fluctuation than that associated with intermittent oral dosing. In theory, transdermal administration should thus minimise the adverse effects associated with peak plasma drug concentration, while avoiding the potential for decreased efficacy associated with trough levels. Clonidine has been incorporated into a small, pliable adhesive cutaneous delivery device designed to provide therapeutically effective doses of drug at a constant rate for at least 7 days. The transdermal therapeutic system is a laminate consisting of an external film impermeable to moisture and to the drug, a thin layer of active drug dispersed within a highly drug-permeable matrix, a membrane with a controlled intrinsic permeability regulating the rate of delivery of drug to the skin, and an adhesive coating that attaches the system to the skin surface. The permeation of drug through the skin occurs primarily by diffusion. Application of the clonidine transdermal system to both normotensive and hypertensive subjects has consistently reduced systolic and diastolic blood pressures. Maximum reduction in blood pressure occurs 2 to 3 days after initial application, and is maintained for at least 7 days or until the system is removed. The rate at which clonidine is presented to the skin surface is controlled by the microporous membrane: this rate is the same for all strengths of transdermal clonidine, the amount of clonidine released being proportional to its surface area. Thus, the daily dose is regulated by the area of skin covered. Typically, steady-state plasma concentrations are reached on the fourth day after initial transdermal system application. The lack of dose dependency in half-life and renal clearance estimates emphasise that the transdermal absorption of clonidine is linear. The plasma clonidine concentration produced by a particular transdermal dose varies considerably between individuals as a result of interindividual variation in renal clearance. For this reason, it is recommended that dosages be titrated up from the smallest system (3.5 cm2) until the desired pharmacological effect has been obtained.(ABSTRACT TRUNCATED AT 400 WORDS)

Advantages and disadvantages of alpha 2-adrenoceptor agonists for systemic hypertension

Alpha 2-receptor agonists are effective antihypertensive drugs that reduce sympathetic activity by both central and peripheral mechanisms. Plasma noradrenaline and renin concentrations are reduced. Hemodynamic disturbances are minor with a near-normal cardiovascular response to standing, exercise and volume depletion. The main problems are due to central adverse effects, particularly sedation, and a syndrome of sympathetic overactivity on sudden withdrawal. These problems can be minimized by using lower doses, while therapy with a useful antihypertensive drug is maintained. Clonidine and its analogs have a low toxicity and very few contraindications. They remain a useful part of the therapeutic armamentarium in hypertension.

Catecholamines and attention. II: Pharmacological studies in normal humans

Part I of this review [17] found evidence from animal and clinical studies of a role for catecholamines in the control of attention. This part of the review examines the results of a systematic study of the acute effects of a number of catecholamine active drugs on measures of attention in normal adults. The results are interpreted within the context of a general capacity model of information processing in which level of arousal determines the capacity to process stimulus information and level of activation the capacity for processing response demands. Whilst the dopamine antagonist, droperidol, and the alpha agonist, clonidine, had similar effects on behavioural indices of processing capacity and reports of the degree of effort required to carry out processing, they differentially affected subjective state and measures of activation and arousal. These differences provide some support for the hypothesis that central noradrenaline and dopamine modulate the capacity for early and late processing of information, respectively, in humans.

Sedative and cardiovascular effects of medetomidine, a novel selective alpha 2-adrenoceptor agonist, in healthy volunteers

1. Single intravenous doses (25, 50 and 100 micrograms) of medetomidine (MPV-785, an imidazole derivative), a selective alpha 2-adrenoceptor agonist, were administered to eight healthy male volunteers in a double-blind, placebo-controlled study. 2. The following dose-related effects, all of which were compatible with an agonistic action of the drug at alpha 2-adrenoceptors, were noted: reductions of systolic and diastolic blood pressure (maximum 18/11 mm Hg), heart rate (maximum 10 beats min-1), saliva secretion (maximum 84%) and noradrenaline levels in plasma (maximum 70%). 3. Dose-dependent sedation or impairment of vigilance was also observed, both by subjective and objective (critical flicker fusion threshold) assessments, with the highest dose actually inducing sleep in five of the subjects. 4. The observed effects were in general agreement with those previously seen after intravenous administration of the centrally acting antihypertensive alpha 2-adrenoceptor activating drug, clonidine, but of a shorter duration. 5. The relative importance of alpha 2-adrenoceptors located in peripheral tissues and in the central nervous system for the drug's cardiovascular effects could not be determined, but the high lipid solubility of the compound and the rapid onset of sedation are in favour of a major central component. 6. Medetomidine may be a useful tool for the investigation of the physiology and pharmacology of alpha 2-adrenoceptors in man. In addition, the therapeutic and diagnostic uses of the compound should be investigated in pathological conditions related to increased sympathetic neuronal activity.

Efficacy of clonidine as transdermal therapeutic system: the international clinical trial experience

Worldwide clinical trial data concerning the pharmacokinetic and pharmacodynamic characteristics of the clonidine transdermal therapeutic system (TTS) are reviewed with reference to its antihypertensive efficacy. The amount of clonidine delivered to the systemic circulation is a direct function of TTS size. After initial patch application, there is a delay of 2 to 3 days before the onset of action, but after removal of the patch, plasma clonidine levels decline slowly, at an elimination half-life of about 20 hours. Evaluation in approximately 2000 patients with mild to moderate hypertension has shown that the bioavailability of transdermal clonidine is comparable to that of oral clonidine and that equivalent blood pressure reductions are achieved. The rate at which dose increases were found necessary to maintain adequate blood pressure control over extended periods reflects a low incidence of tolerance to this new once-a-week dosage form of clonidine, and there has been little evidence of rebound hypertension after discontinuation of TTS treatment.

Clonidine and the sympatico-adrenal response to coronary artery by-pass surgery

Clonidine was administered intravenously in an attempt to limit sympatico-adrenal activity and thereby reduce the incidence of arterial hypertension associated with coronary artery by-pass graft surgery (CABG). Forty patients scheduled for CABG were assigned to two groups. Twenty patients received clonidine 4 micrograms kg-1 before surgery, 2 micrograms kg-1 after cardiopulmonary by-pass and 1 microgram kg-1 when the skin was sutured. The other 20 patients served as controls. All patients were anesthetized with fentanyl, droperidol, nitrous oxide and alcuronium. During surgery 5 min after sternotomy, mean arterial pressure was 13 mmHg lower (P less than 0.01) in the clonidine group, while after operation the difference between the groups was negligible. Both during and after surgery the plasma catecholamine concentrations were significantly lower in the clonidine group (P less than 0.01). The greatest difference between the groups was seen 90 min after operation, when plasma noradrenaline and plasma adrenaline concentrations in the clonidine group were less than 1/3 of those in the control group (P less than 0.01). As judged by catecholamine concentrations clonidine was effective in attenuating sympatico-adrenal hyperactivity during and after surgery. Postoperative arterial hypertension was not reduced, however, and it is concluded that other factors besides sympatico-adrenal hyperactivity must be important.

Growth hormone and cortisol secretion after oral clonidine in healthy adults

The purpose of this study was to evaluate oral clonidine for testing growth hormone (GH) responsiveness in healthy adults. Oral clonidine (0.15 mg) produced a satisfactory GH response (greater than 4 ng/ml from basal) in eight out of 10 subjects, which is similar to rates reported after an equivalent intravenous dose. Elevated GH levels at baseline occurred in four out of five female subjects; this did not affect the clonidine-induced GH release. There were no significant differences at any time point in plasma prolactin or cortisol levels following clonidine, compared to placebo controls. Adequate plasma clonidine levels (greater than 0.4 ng/ml) were achieved in all subjects, with corresponding reductions in mean arterial blood pressure, but with only minimal adverse effects. Results from this study indicate that oral clonidine is a reliable method for testing GH responsiveness in adult subjects.

Role of monoamine pathways in attention and effort: effects of clonidine and methylphenidate in normal adult humans

Methylphenidate (0.65 mg/kg), clonidine (200 micrograms) or placebo were administered to normal adult males undertaking a dichotic monitoring task in which they were required to detect nominated target words and discriminate them from phonemic distractors. Following placebo, performance was better when attention was focused than when divided. Following clonidine, subjects were poorer and slower at discriminating targets during both divided and focused attention and subjectively were withdrawn and reported difficulties with concentration. Methylphenidate had no effect on target discrimination or response time but raised the rate of response and had marked effects on spontaneous behaviour in which an increased attention capacity was generally reported. The effects on attention of the pharmacological agents employed in this study are attributed to their effects on central monoamines. The disparity noted between objective and subjective assessments of attention is discussed in terms of the voluntary allocation of effort.

Selective impairment of paired associate learning after administration of a centrally-acting adrenergic agonist (clonidine)

Eight normal volunteers had IV infusions of 200 micrograms clonidine (a centrally-acting adrenergic agonist which reduces noradrenaline release), and saline in a "double-blind" cross-over design. Clonidine reduced subjective estimates of arousal but did not affect performance on the Digit Symbol Substitution Test. Clonidine impaired paired-associate learning, but it did not affect performance on a number of measures of short and long term memory. The findings suggest either 1) that there is a specific (adrenergic) mechanism involved in the acquisition of novel associations, but not in other types of learning, or 2) that paired associate learning is more vulnerable than other learning tasks to disruption of adrenergic transmission.

Bis(trifluoromethyl)aryl derivatives for drug analysis by gas chromatography electron capture negative ion chemical ionization mass spectrometry. Application to the measurement of low levels of clonidine in plasma

A gas chromatographic mass spectrometric assay for clonidine in plasma with a detection limit of a few picograms per ml was required. The p-trifluoromethylbenzyl, pentafluorobenzyl and pentafluorobenzoyl derivatives of clonidine were synthesized and the electron capture negative ion chemical ionization mass spectra of these compounds show extensive fragmentation with prominent ions at m/z 35 and 37 due to the two chlorine atoms in the clonidine molecule. Selected ion monitoring of specific high mass ions in these mass spectra indicated that the required sensitivity could not be obtained with these derivatives. Several bis(trifluoromethyl)pyrimidines were synthesized and these compounds were found to give an intense negative ion current under conditions of resonance electron capture. Consequently, a derivative of clonidine containing a bis(trifluoromethyl)aryl group was synthesized by reacting the drug with 3,5-bis(trifluoromethyl)benzoyl chloride. The negative ion mass spectrum of the reaction product has a base peak at m/z 673 and, when this ion is specifically monitored, an amount of derivative equivalent to 1 picogram of clonidine can be detected. This allowed the development of an assay for clonidine in plasma with a precision of 8% (SD) at 50 pg ml-1, 22% (SD) at 20 pg ml-1 and a lower limit for quantitative determination of 10 pg ml-1. Plasma concentrations of clonidine in 10 subjects given a single 50 micrograms oral dose are reported.

Effect of indoramin, labetalol and alinidine on sympathetic function in normal man

The effects of single oral doses of indoramin (mean dose 58 mg), abetalol (mean dose 150 mg), alinidine 80 mg and placebo on arterial pressure and heart rate in the supine and standing positions were studied in six normal volunteers. Doses were chosen to give equivalent reductions of arterial pressure in the standing position. Observations were made before and at 2 and 4 h after drug administration. Plasma noradrenaline (NA) was measured at each time interval in the supine position, and after 4 min of standing. Plasma renin activity (PRA) was measured at each time interval after 30 min in the standing position. In the supine position, alinidine produced a significant reduction of systolic arterial pressure from 124.0 +/- 3.0 mm Hg to 104.3 +/- 4.1 mm Hg at 2 h (P less than 0.01) and to 101.7 +/- 2.2 mm Hg at 4 h (P less than 0.01). Diastolic pressure was reduced from 74.7 +/- 2.6 mm Hg to 57.0 +/- 4.6 mm Hg at 4 h (P less than 0.01). Arterial pressure was unchanged after indoramin or labetalol administration. In the supine position, heart rate was unchanged after indoramin, and small reductions were observed after labetalol and alinidine. Indoramin produced a significant increase in plasma NA. A small increase of plasma NA was observed after labetalol, and a small decrease after alinidine. In the standing position, the three active drugs reduced systolic arterial pressure to a similar extent (indoramin, -26.7 mm Hg at 4 h after drug administration; labetalol, -21.3 mm Hg at 2 h; alinidine, -21.7 mm Hg at 4 h).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics of nitroglycerin and clonidine delivered by the transdermal route

This article describes in vitro and in vivo performance of two transdermal drug delivery systems. Transderm -Nitro delivers nitroglycerin for the treatment and prevention of angina for 24 hours following a single application. In a three-way crossover study comparing Transderm -Nitro with two other transdermal nitroglycerin products, mean plasma levels of drug were similar 0.5 and 6 hours after application; however, with Transderm -Nitro the area under the plasma concentration curve was highest and the coefficient of variation was least. A transdermal therapeutic system for delivering clonidine ( Catapres -TTS) is used for the treatment of hypertension. In a two-way crossover study comparing Catapres -TTS and oral Catapres , plasma levels of clonidine with use of the transdermal system reached a steady-state value in 2 to 3 days and remained steady for the duration of the wearing period; plasma levels with oral Catapres , however, fluctuated markedly. The ratio of maximum to minimum plasma levels during a dosing interval was 2 for oral Catapres and approximately 1 for Catapres -TTS. The potential use of intact skin as a route of entry for controlled delivery of drugs to the systemic circulation is promising.

The involvement of central alpha adrenoceptors in the antihypertensive actions of methyldopa and clonidine in the rat

The antihypertensive effects of methyldopa and clonidine result from agonist activity at alpha adrenoceptor sites within the brain. Methyldopa is converted enzymatically to alpha-methylnoradrenaline in noradrenergic neurones in rat brain and replaces the natural transmitter, noradrenaline. Radioligand receptor studies show that alpha-methylnoradrenaline differs from noradrenaline in being much more selective (70 times) for the alpha 2 subclass of adrenoceptors than noradrenaline and it is likely that the antihypertensive action of methyldopa results from selective activation of alpha 2 adrenoceptors by alpha-methylnoradrenaline in the nucleus tractus solitarius and the anterior hypothalamus. Radioligand studies also show that clonidine is a selective alpha 2 adrenoceptor agonist although it probably interacts with alpha 1 adrenoceptors at higher concentrations. With regard to a withdrawal syndrome after cessation of clonidine treatment, the cardiovascular and behavioural components can now be characterised in a rat model. The components include increases in basal blood pressure and heart rate, as well as increases in cardiovascular reactivity and also increases in rapid eye movement (REM) sleep, body shakes and tremor which is reminiscent of an opiate withdrawal syndrome. Increased central noradrenergic activity is involved in this syndrome and alpha 1 and alpha 2 adrenoceptors mediate opposing effects on the REM sleep rebound component.

Pharmacokinetics and pharmacodynamics of transdermally administered clonidine

Clonidine was applied to the skin of healthy volunteers once weekly by means of a Transdermal Therapeutic System (TTS). The plasma concentration and renal excretion of clonidine, and its effects on mean arterial blood pressure (MAP) and heart rate (HR) were recorded for 7 days, followed by a three-day observation period when a second TTS was applied. Subjective side effects were semiquantitatively recorded. Four different TTS formulations were tested; of which TTS-RP 600679 was the most effective. Following application of this formulation, the plasma level of the drug built-up up during the first 2 days and then remained stable for 120 h at therapeutic concentrations between 0.5 and 0.7 ng/ml; MAP was consistently reduced. During the steady state period the daily urinary clonidine excretion was in the same range as during chronic administration of Catapres tablets 0.15 mg every 12 h, or Catapres Perlongets 0.25 mg every 24 h. Transdermal clonidine applications renewed weekly provide the following therapeutic advantages: 1. patients are protected continuously throughout the entire steady state period; 2. daily fluctuations in plasma clonidine concentration are minimized, which may result in a marked reduction in side effects; and, 3. drug compliance should be improved.

Crossover comparison of moxonidine and clonidine in mild to moderate hypertension

The antihypertensive effect of moxonidine X HCl X H2O (MOX) was compared with that of clonidine X HCl (CLON) in a randomized double-blind crossover study in 20 hypertensive outpatients (BP range 154-178/96-108 mmHg). After 2 weeks without antihypertensive medication, either MOX 0.2 mg daily or CLON 0.2 mg daily was given and the dose was titrated until the diastolic blood pressure fell below 90 mmHg. The first treatment period was continued for 2 weeks and, after crossover without a wash-out period, it was followed by the second treatment for a further 2 weeks. Within the first 4 days of administration 0.2-0.4 mg of either agent caused a significant decrease in BP (p less than 0.001) from a mean of 166/100 mmHg to 149/86 mmHg after CLON (approx. -10/-14%), and 163/99 mmHg to 146/84 mmHg after MOX (approx. -10/-15%). No significant difference in the fall in BP or pulse rate was detected between the two drugs. In the mean daily dose of 0.3 mg both drugs showed the same antihypertensive activity, but on CLON a higher incidence of side effects (p = 0.003) was noted, and after discontinuation of therapy a more rapid rise in BP (systolic BP p less than 0.01, diastolic BP p less than 0.02) was found. 17 patients on CLON complained of side effects, especially tiredness and dry mouth, whilst only 6 patients on MOX were affected (p = 0.003).

An assessment of the contribution of clonidine metabolised from alinidine to the cardiovascular effects of alinidine

Five healthy volunteers (mean age 20.6 years, mean weight 71 kg) received in random order on day 1 and day 8 a single dose of alinidine 40 mg, clonidine 0.1 mg or placebo and on days 2-7 alinidine 40 mg, clonidine 0.1 mg or placebo given three times a day with 1 week between treatment periods. Blood samples were taken for measurement of concentrations of alinidine and clonidine during alinidine administration and of clonidine during clonidine dosing. Heart rate and blood pressure were recorded in supine and standing positions and heart rate after 3 min exercise. Plasma concentrations of alinidine reached a maximum of 163.6 +/- 10.0 ng/ml 2 h after alinidine administration on day 1 and during chronic administration similar concentrations were achieved. Clonidine plasma concentrations reached 0.3 +/- 0.11 ng/ml 6 h after alinidine 40 mg on day 1, and during chronic administration of alinidine, increased to a steady state on day 5 with trough and 2 h values of 0.73 +/- 0.15 and 0.86 +/- 0.14 ng/ml respectively. After the first dose of clonidine on day 1, the maximum plasma concentration of clonidine was 0.32 +/- 0.1 ng/ml at 4 h, during chronic administration clonidine plasma concentration rose to 1.04 +/- 0.14 ng/ml 2 h after a dose on day 5. Alinidine produced a greater reduction in the exercise tachycardia than clonidine.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute hypotensive action of clonidine after intravenous infusion in hypertensive emergencies

Clonidine was administered by intravenous infusion to 12 patients classified as having exaggerated arterial hypertension, their systolic (SAP), diastolic (DAP) and mean (MAP) arterial pressures were significantly reduced from the third min. The maximal percentage reduction (Mean +/- SEM) reached 30.1 +/- 3.1% (SAP) and 24.7 +/- 2.9% (DAP) after 30 to 110 min of infusion. Initially there were transitory initial increases in SAP (3 patients) and DAP (1 patient). The increases in blood pressure were related to low body surface area (BSA). The dose of clonidine per m2BSA able to reduce by 10% either SAP or DAP (active dose-10), and the dose able to reduce SAP or DAP by 10 mmHg in one minute (systolic or diastolic clonidine unit) were calculated, providing indices for detecting clonidine responsiveness in patients with exaggerated hypertension. This method is advantageous when using clonidine intravenously because it diminishes the risk of overdosage.

Pharmacokinetics and pharmacodynamics of clonidine in varying states of renal function

Previous studies have reported a therapeutic window of 0.8 to 2.0 ng/ml within which clonidine exerts its antihypertensive effect. This study was designed to investigate whether there was any loss of blood pressure control when plasma clonidine concentrations exceeded this window owing to its accumulation in renal insufficiency. The results indicate that clonidine concentrations of up to 30 ng/ml in patients with end-stage renal disease (ESRD) can be associated with a maintenance of blood pressure control. It remains to be learned whether this phenomenon of high concentrations and control of blood pressure in our ESRD patient population may be related to altered (decreased) peripheral alpha-receptor sensitivity and inability to develop vasoconstriction. There is precedence for autonomic imbalance and altered tissue sensitivity in ESRD.

New aspects of the pharmacokinetics and pharmacodynamics of clonidine in man

Using considerably improved analytical methods, the kinetics and effects of clonidine were observed in healthy volunteers over periods of time more than 3 times longer than those previously reported. The high sensitivity and small work load of the newly developed method permitted the performance of low-dose and multipledose trials. 1. The complete bioavailability of clonidine and its elimination half-life (20 to 25.5 h) remained constant after single and multiple doses. 2. Approximately 62% of a given dose was excreted unchanged in the urine, independent of the quantity administered (0.075, 0.15, 0.2, 0.25 or 0.3 mg), the drug formulation (solution, tablet, Perlonget) or of the mode of administration (i.v., p.o.; single or multiple doses). 3. As the pharmacokinetics of the drug were affected by entero-hepatic circulation, it cannot be described by a conventional, open one or two compartment model. 4. The time courses of the plasma clonidine concentration and its drug effects ran asynchronously. 5. On cessation of chronic clonidine administration, blood pressure and plasma catecholamine levels increased to pretreatment levels without exhibiting any "overshoot" reaction.