Antihypertensive efficacy and acceptability of rilmenidine in elderly hypertensive patients

Elderly hypertensive patients (older than 70 years, with a diastolic blood pressure [BP] between 95 and 114 mm Hg) were entered into a study after a 2-week wash-out period and randomized to 2 parallel groups: rilmenidine (n = 28) and methyldopa (n = 30). The initial dose (rilmenidine, 1 mg once daily A.M. or methyldopa, 250 mg twice daily) was doubled (1 mg twice daily or 500 mg twice daily, respectively) on day 21 if supine diastolic BP remained greater than 90 mm Hg. After a 6-week treatment period (days 0 to 42, with weekly examinations), the effects of treatment withdrawal (day 42) were evaluated twice daily (days 43 to 45), with a final examination on day 49. Most of the 58 patients (70%) (aged 81.5 +/- 0.8 years) with a mean diastolic BP of 100.2 +/- 0.7 mm Hg remained treated with the initial dose in both groups. Efficacy in both groups was identical on day 42: decrease in systolic and diastolic BP of approximately 18 mm Hg, with 85% of patients having BP levels normalized (supine diastolic BP less than or equal to 90 mm Hg). Compared with the reference period, no increase in adverse effects was noted apart from a moderate dryness of mouth in 15% of patients in both groups; no orthostatic hypotension was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of rilmenidine on bronchomotor responses in the guinea pig

This study compares the effects of 2 alpha 2-adrenergic agonists, rilmenidine (S 3341) and clonidine, on histamine-induced bronchoconstriction (HIB) in guinea pigs. Clonidine has previously been shown to potentiate HIB in guinea pigs. The study was conducted in anesthetized, paralyzed and ventilated guinea pigs. Six groups of at least 6 guinea pigs were pretreated with saline, rilmenidine (0.1, 0.3 and 1 mg/kg intravenously) or clonidine (0.01 and 0.03 mg/kg intravenously), respectively. Conductance, dynamic compliance and static compliance of the respiratory system were measured before and during histamine infusion (80 ng/kg/s) and expressed as percent variation. Clonidine potentiated HIB as previously reported. In contrast, rilmenidine potentiated HIB only at the dose of 1 mg/kg. As it is well known that HIB is influenced by adrenergic outflow, these results suggest that rilmenidine has less inhibitory effect than clonidine on adrenal secretion.

A multicenter double-blind comparative study of rilmenidine and clonidine in 333 hypertensive patients

The efficacy and acceptability of rilmenidine were studied in a double-blind clonidine-controlled multicenter trial; after a 4-week placebo run-in period, patients with supine diastolic blood pressure (BP) between 95 and 115 mm Hg received as monotherapy either rilmenidine or clonidine over 6 weeks. The initial dose (rilmenidine 1 mg/day or clonidine 0.15 mg/day) was doubled (1 mg or 0.15 mg twice a day, respectively) after 2 weeks if diastolic BP remained greater than or equal to 90 mm Hg. Three hundred and thirty-three patients (mean age 57.8 +/- 0.7 years) with a systolic BP of 170.53 +/- 0.92 mm Hg and a diastolic BP of 101.57 +/- 0.30 mm Hg were randomly divided into 2 homogenous groups (rilmenidine, n = 162 and clonidine, n = 171). All patients taking rilmenidine completed the trial. Seventeen patients taking clonidine (10%, p less than 0.01 vs rilmenidine) were withdrawn because of severe side effects. Systolic and diastolic BP were significantly reduced in both groups at every examination (at 2, 4 and 6 weeks). The mean decreases in supine and erect BP were identical in both groups: systolic BP 19 mm Hg and diastolic BP 12 mm Hg after 6 weeks. BP was normalized (systolic BP less than 160 and diastolic BP less than or equal to 90 mm Hg) in 57% of patients taking rilmenidine and 56% of patients taking clonidine (60% of normalized patients had been taking the single dose in both groups).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics of rilmenidine

Rilmenidine, an alpha 2-adrenoceptor agonist, was studied (1 mg single dose) in order to determine the effects of pathology on its basic pharmacokinetic parameters. Because of the mainly renal elimination of rilmenidine, studies involved hypertensive, elderly hypertensive, renal insufficient and hepatic insufficient patients. Hypertension was found to influence neither the absorption, the distribution nor the elimination processes; the linearity in the range of 1 to 2 mg and the absence of accumulation in long-term treatment were confirmed. In contrast, in the elderly, the absorption phase was delayed. The slight decrease in the apparent volume of distribution (-12%), with a notable decrease in the apparent total clearance (-50%) led to a prolonged elimination half-life (+50%). In renal failure, linear relations between the degree of renal impairment and the elimination parameters were shown. These relations allow the evaluation of the predicted steady-state level of rilmenidine for a given degree of renal failure. In hepatic insufficiency, the modification of rilmenidine disposition concerned exclusively the elimination phase in which apparent clearance was decreased approximately 20%. In conclusion, these results lead to a decreased dosage regimen in patients with severe renal failure.

Hemodynamic and electrophysiologic effects of a new alpha 2-adrenoceptor agonist, rilmenidine, for systemic hypertension

The hemodynamic and electrophysiologic effects of rilmenidine were examined after single oral administration to hypertensive patients. In 8 untreated hypertensive patients, cardiac output, pulmonary pressure and blood pressure were measured before and for 10 hours after the administration of 25 micrograms/kg of rilmenidine (1.3 to 2.4 mg, mean 1.88). In addition, electrophysiologic investigations were performed before and 2 hours after administration. Hemodynamics were repeated in 8 other hypertensive patients receiving 50 micrograms/kg rilmenidine (3.0 to 4.8 mg, mean 3.85 mg). The electrophysiologic study was repeated in 8 other hypertensive patients receiving 50 micrograms/kg of rilmenidine (3.2 to 4.4 mg, mean 3.90). In contrast to the results obtained at the dose of 50 micrograms/kg, there was no significant variation in pulmonary arterial pressure, cardiac index or stroke index after administration of 25 micrograms/kg. No significant variation was observed in heart rate, sinus function, conduction parameters or atrial, nodal and ventricular refractory periods after administration of 25 and 50 micrograms/kg. Rilmenidine, after single oral administration at the 25 micrograms/kg dose, led to a significant reduction in blood pressure and peripheral resistance without any significant change in cardiac output; the 25- and 50-micrograms/kg doses led to no alteration in heart rate and cardiac electrophysiology.

Effects of continuous infusions (10 days) and cessation of infusions of clonidine and rilmenidine (S 3341) on cardiovascular and behavioral parameters of spontaneously hypertensive rats

Clonidine is a centrally acting antihypertensive drug that acts in vivo at both alpha 1- and alpha 2-adrenoceptor sites, whereas rilmenidine (S 3341) is more selective for alpha 2 adrenoceptors. The present study compared the effects of continuous 10-day infusions of clonidine (5 micrograms/kg/hour) with those of rilmenidine (100 micrograms/kg/hour) on various cardiovascular and behavioral parameters in the spontaneously hypertensive rat. The changes in these parameters after cessation of the infusions were also compared. At these rates of infusion, clonidine and rilmenidine produced quantitatively similar reductions in mean arterial pressure (MAP), lability of MAP and cardiovascular responsiveness during normal behaviors such as eating and grooming. Neither drug infusion affected heart rate. The cessation of the clonidine infusion resulted in a "withdrawal" syndrome characterized by prominent rapid eye movement-sleep rebound, and cardiovascular and behavioral disturbances including an increased lability of MAP, exaggerated cardiovascular responses during normal behaviors, tachycardia, and an "opiate abstinence-like" syndrome including head and body shakes. Cessation of rilmenidine infusion resulted in somewhat similar cardiovascular and behavioral disturbances, but unlike clonidine there was a return to normal rapid eye movement sleep without rebound.

Prejunctional alpha 2-adrenergic properties of S-3341 (dicyclopropylmethyl)amino-2-delta-2-oxazoline: a comparison with clonidine

The prejunctional activities of S-3341 and clonidine have been studied in the transmural field-stimulated epididymal part of the rat vas deferens. Both S-3341 and clonidine inhibited these neuronally induced contractions. At high concentrations, these agonists induced spontaneous contractions in the preparation, which were abolished by 10(-7) M prazosin. The inhibitory effects of S-3341 and clonidine were antagonized in a competitive manner by rauwolscine and were found not to be modified in a statistically significant manner by 10(-7) M prazosin. The pA2 values of rauwolscine against S-3341 and clonidine were the same, indicating the receptors influenced by these drugs were the same also. However, the efficacy of S-3341 is lower than that of clonidine (by a factor of approximately 100). This lower efficacy of S-3341 at the prejunctional level could be linked to the lack of a sedative effect of this compound at therapeutic concentrations.

Rapid eye movement sleep and the associated cardiovascular changes during and after continuous 10-day infusion of clonidine in normotensive rats: comparison with lofexidine and S3341

The centrally acting alpha-adrenoceptor agonists clonidine, lofexidine, and S3341 vary in their selectivity for the alpha 2-adrenoceptor subtype (S3341 greater than clonidine greater than lofexidine). The aim of this study was to examine the frequency and duration of rapid eye movement (REM) sleep episodes, and the cardiovascular changes associated with these episodes, during and after the continuous (10-day) infusion of saline (0.9% wt/vol), clonidine (10 micrograms/kg/h), lofexidine (10 micrograms/kg/h), or S3341 (50 or 100 micrograms/kg/h) in unrestrained, chronically instrumented rats. During a typical 24-h period, the saline infusion animals (n = 4) had 46 +/- 3 REM sleep episodes of 2.4 +/- 0.1 min duration. The majority of these episodes were associated with relatively minor phasic and tonic changes in arterial blood pressure and heart rate. During infusion, the three drugs markedly reduced the cardiovascular and behavioural signs of REM sleep. On cessation of the clonidine and lofexidine infusions, rebound increases (i.e., to values greater than controls) in the frequency and duration of REM sleep episodes were observed. These episodes were associated with markedly exaggerated fluctuations in arterial blood pressure and heart rate. The REM sleep rebound and associated cardiovascular responses were greater during the post-lofexidine infusion phase. A relatively minor rebound increase in total REM sleep time was observed after cessation of the 100-micrograms/kg/h, but not the 50-micrograms/kg/h, infusion rate of S3341. The cardiovascular responses during the REM sleep episodes were mildly exaggerated after cessation of the higher infusion rate of S3341.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative analysis of S3341 in human plasma and urine by combined gas chromatography-negative ion chemical ionization mass spectrometry: 15 month inter-day precision and accuracy validation

A new quantitative assay for the determination of S3341, an alpha-2 agonist antihypertensive drug, has been developed using combined gas chromatography-negative ion chemical ionization mass spectrometry. The [M]-. ions from TFA derivatives of S3341 (m/z 276) and the internal standard (2H4)S3341 (m/z 280) are monitored simultaneously by selected ion monitoring. For S3341 concentrations ranging from the limit of detection (0.2 ng ml-1 using 1 ml of plasma) to 5 ng ml-1, the average assay precision (CV) is approximately 7% while the average assay accuracy (percentage of error) is 4%. Validation of the day-to-day precision and accuracy was realized after analysing control plasma samples (n = 295) concurrently with the biological samples collected during the pharmacokinetic studies conducted over 15 months. The average day-to-day precision (CV) and accuracy (percentage of error) are 10% and 6% respectively, thus indicating that this assay procedure routinely provides reliable analytical data.

Alpha 2-adrenoceptor agonists stimulate growth hormone secretion but have no acute effects on plasma cortisol under basal conditions

Ten normal male subjects were administered clonidine (0.1 and 0.2 mg) or a highly-selective alpha 2-adrenoceptor agonist S 3341 (1 and 2 mg) on separate occasions in a double-blind randomised placebo-controlled study; blood samples were obtained for measurement of serum GH and plasma cortisol via an indwelling venous cannula for 4 h after each drug administration. Both clonidine and S 3341 were equally effective at lowering supine, and particularly standing BP; both also caused mild sedation, although this was slightly less marked for S 3341. High doses of both clonidine and S 3341 significantly increased serum GH (peak increment after clonidine: 18.8 +/- 5.5 mU/l (mean +/- SEM); peak increment after S 3341: 21.1 +/- 4.8 mU/l). Neither drug affected plasma cortisol. It is concluded that GH release may be stimulated by alpha 2-adrenoceptor agonism. As S 3341 has slightly less central activity than clonidine at the doses employed, but is at least equally potent at stimulating GH release, it is probable that the alpha 2-adrenoceptor stimulation of GH release occurs outside the blood-brain barrier. The pituitary-adrenal axis appears resistant to manipulation of alpha 2-adrenoceptors under basal conditions.

Effect of the amino-oxazoline derivative S-3341 on pre- and postjunctional alpha-adrenoceptors in isolated blood vessels

In segments of isolated dog saphenous veins, S-3341 (10(-8) to 10(-4) M) induced concentration-dependent contractions which, like those to clonidine, were progressively inhibited by increasing concentrations of yohimbine (10(-8) to 10(-6) M) but not by prazosin (10(-8) to 10(-6) M). In strips of the rabbit main pulmonary artery, previously labelled with [3H]noradrenaline and mounted for superfusion, S-3341 and clonidine both decreased the overflow of [3H] caused by electrical stimulation; these effects of S-3341 and clonidine were inhibited by 10(-6) M of yohimbine but not by 10(-6) M of prazosin. Our results illustrate that both S-3341 and clonidine cause contraction by activation of postjunctional alpha 2-adrenoceptors and decrease noradrenaline-release by stimulating prejunctional alpha 2-adrenoceptors. At the postjunctional alpha 2-adrenoceptors clonidine is 27 times more potent than S-3341, while at the prejunctional alpha 2-adrenoceptors, clonidine is only 15 times more potent than S-3341.

[Effects of acute and chronic treatment with an adrenergic alpha receptor agonist, LE S3341, on the rate of catecholamine turnover in various peripheral organs and brain structures in the rat]

The turnover of catecholamines (CA) in some peripheral tissues and various areas of the rat brain was estimated by measuring the amine depletion after inhibition of their biosynthesis by alpha-methyl-p-tyrosine (alpha-MPT). Acute or chronic treatment with S3341 (3mg/kg i.p.) reduced NA turnover in submaxillary glands, brain stem and rest of the brain but had no effect on NA turnover in the hypothalamus. The dopamine (DA) levels were unaltered following chronic S3341 treatment but the alpha-MPT induced disappearance of DA was significantly retarded in the striatum and rest of the brain. The results of the present study demonstrate that no tolerance to the effect of S3341 on brain CA turnover develops during chronic drug administration. The central biochemical effects of S3341 appear different from those of other agonists of central alpha 2-adrenoceptors such as clonidine.

Influence of the alpha-2 agonist oxaminozoline (S3341) on firing rate of central noradrenergic and serotonergic neurons in the rat. Comparison with clonidine

The firing rate of central locus coeruleus (LC) noradrenergic neurons and dorsal raphe (DR) serotonergic neurons was recorded in rats anaesthetized with chloral hydrate. The iontophoretic application or the i.v. perfusion of S3341, a new antihypertensive drug or clonidine decreased the frequency of discharge of LC neurons. Depending on the mode of administration clonidine was 54-63 times more potent than S3341. The selectivity of action of both drugs on alpha-2 vs. alpha-1 adrenoceptors was confirmed using yohimbine and prazosin: yohimbine completely blocked the inhibitory effect of S3341 or clonidine while prazosin did not prevent this effect. S3341 and clonidine regularly reduced the firing rate of DR neurons during i.v. perfusion but not during iontophoretic application. From these experiments is it concluded that S3341 and clonidine have a direct inhibitory effect on LC neurons via stimulation of alpha-2 autoreceptors and that both drugs have an indirect inhibitory effect on DR neurons, probably via impairment of noradrenergic transmission. Clinical studies show that S3341 induces much less sedative side effects than clonidine. In view of the great difference in the potency of these drugs to inhibit the firing rate of monoaminergic neurons which are known to be involved in sleep mechanisms, it is possible that the electrophysiological effects reported here relate to the sedative effects of these drugs.

Central and peripheral effects of S 3341 [(N-dicyclopropylmethyl)-amino-2-oxazoline] in animal models

The effects on blood pressure and heart rate of S 3341 [(N-dicyclopropylmethyl)-amino-2-oxazoline] were investigated in rats and cats. Upon i.v. injection in pithed rats, S 3341 produced pressor responses which were antagonized by prazosin as well as by yohimbine. In pentobarbitone-anesthetized rats S 3341 produced a short-lasting increase in mean arterial pressure, followed by a long-lasting reduction. During a 12-day, continuous subcutaneous infusion in conscious spontaneously hypertensive rats, S 3341 reduced blood pressure and heart rate. Upon cessation of treatment, an overshoot of heart rate and blood pressure lability occurred. The hypotensive effect was abolished by reserpinization of the rats. S 3341 was more potent in reducing mean arterial pressure upon infusion via the vertebral artery than after infusion via the femoral artery of chloralose-anesthetized cats. S 3341 did not prolong the hexobarbitone-induced loss of righting reflex in mice. S 3341 was more potent in displacing [3H]clonidine than [3H]prazosin from their specific binding sites in rat brain membranes. These data characterize S 3341 as a clonidine-like, centrally acting antihypertensive drug. The lack of sedative effect, as assessed by the prolongation of hexobarbitone-induced sleeping in mice, remains to be clarified.

Gas chromatographic negative ion mass spectrometric assay of 2-dicyclopropylmethylamino-2-oxazoline (S-3341), a new antihypertensive drug

The quantification in plasma and urine of 2-dicyclopropylmethylamino-2-oxazoline (S-3341), a new antihypertensive drug is described using a sensitive gas chromatographic negative ion mass spectrometric method with ammonia as moderating gas. After a two-step extraction, derivatization is carried out with 3,5-bis(trifluoromethyl)benzoyl chloride and the abundance of the molecular ion (m/z 420) obtained is compared with that of the tetradeuterated standard (m/z 424). The low background due to the high mass and negative ion detection provides a detection limit of about 1 pg per injection. Oral administration of 1 or 2 mg S-3341 to patients gives a maximum concentration of 3.3 +/- 0.7 ng ml-1 and 7.6 +/- 2.0 ng ml-1 at 1.8 +/- 0.6 h and 1.4 +/- 0.7 h and an average elimination half-life of 6.7 h.

Pharmacological properties of (N-dicyclopropylmethyl) amino-2-oxazoline (S 3341), an alpha-2 adrenoceptor agonist

Pharmacological actions of (N-dicyclopropylmethyl)-amino-2-oxazoline (S 3341), an agonist of alpha-2 adrenoceptors, were examined in acute animal studies. In the normotensive anaesthetized dog S 3341 (0.3 mg/kg, i.v.) produced an initial transient increase followed by a marked, prolonged fall in mean arterial pressure (MAP) of 20 mmHg. Central actions of S 3341 were demonstrated by administration of low doses into the vertebral artery of the anaesthetized dog. A rapid and marked fall in MAP resulted which was antagonised by piperoxan. Splanchnic discharges were strongly decreased following S 3341 i.v. administration, suggesting a centrally mediated diminution of sympathetic tone. Peripheral actions of S 3341 were observed in the pithed rat where a dose-dependent increase in MAP was noted which was somewhat antagonised by prazosin and largely by prazosin plus yohimbine. S 3341 reduced hypertension and tachycardia due to stimulation of the sympathetic outflow in the pithed rat, an effect also antagonised by piperoxan. These effects were more marked and prolonged than those of clonidine. S 3341 reduced the tachycardia resulting from stimulation of the cardioaccelerator nerve in the anaesthetized, spinalised and bilateraly vagotomised dog, this effect was reversed by piperoxan. S 3341 did no change the tachycardia induced by noradrenaline or tyramine. Plasma renin activity was significantly decreased after S 3341 treatment in dogs on low normal or high sodium diets. In rats S 3341 decreased the rate of discharge of noradrenergic cells located in the locus coeruleus which are believed to be involved in wake/sleep mechanisms. This depression was 63 times less than that of clonidine. At effective hypotensive doses S 3341 produced no sedation (i.e. loss of righting reflex) in 2 day old chicks. In addition the sedative action of clonidine was inhibited by S 3341 pretreatment. In the mouse tail flick model, the antinociceptive effects of S 3341 were 45 times less than those of clonidine. S 3341, an oxazoline derivative, appears to have haemodynamic effects similar to those of other agonists of central alpha-2 adrenoceptors but with fewer side-effects, and therefore could be of interest an as antihypertensive agent.

Bistrifluoromethylaryl derivatives for drug analysis by gas chromatography electron capture negative ion chemical ionization mass spectrometry. Application to the measurement of (N-dicyclopropylmethyl)amino-2-oxazoline in plasma

A gas chromatographic mass spectrometric assay for (N-dicyclopropylmethyl) amino-2-oxazoline in plasma with a detection limit of 0.1 ng ml-1 was required. Various fluoroaryl derivatives of this compound (code name S3341) were synthesized and their positive ion chemical ionization and electron capture negative ion chemical ionization mass spectra recorded. While fluorobenzyl derivatives of S3341 could be made by heating with the requisite benzyl bromide and diisopropylethylamine in acetonitrile, initial efforts to synthesize corresponding fluorobenzoyl derivatives using a benzoyl chloride in dry ethyl acetate at 60 degrees C were unsuccessful. Mass spectral data indicated that only a fragment of the oxazoline ring was retained in the reaction product and that an N-(2-chloroethyl)benzamide was formed. However, when diisopropylethylamine was included in the reaction mixture, a benzoyl derivative of the complete molecule was obtained. The mechanisms of these reactions are discussed. The negative ion mass spectrum of the 3,5-bistrifluoromethylbenzoyl derivative of S3341 has a base peak at m/z 420 (the molecular ion) and, when this ion is specifically monitored, an amount of derivative equivalent to 1 pg of S3341 can be detected. This allowed the development of an assay for S3341 in plasma with a precision of 9% (SD) at 0.2 ng ml-1 and a lower limit for quantitative determination of 0.1 ng ml-1.

Rapid eye movement (REM)-sleep during and after continuous infusion of the oxazoline derivate S3341: a comparison with clonidine

The frequency and duration of rapid eye movement (REM)-sleep episodes were examined in normotensive rats during and after cessation of continuous infusion of either clonidine (10 micrograms/kg/h) or the oxazoline derivative S3341 (50 micrograms/kg/h) for 10 days. Both compounds abolished REM-sleep during the infusion. On cessation of the clonidine infusion there was a rebound increase in the frequency and duration of REM-sleep episodes. These episodes were associated with markedly exaggerated blood pressure fluctuations. On cessation of the S3341 infusion, the frequency and duration of REM-sleep episodes returned to control levels. Blood pressure responses were not different from controls. These post-infusion differences may be related to the greater degree of selectivity of S3341 for the alpha 2-adrenoceptor rather than the alpha 1-adrenoceptor subtype.

Preliminary clinical pharmacological studies of S3341, a new hypotensive agent, and comparison with clonidine in normal males

S3341, a new hypotensive agent which binds to alpha2-receptors in animal brain preparations, was studied in normal healthy male volunteers. A dose ranging study with 15 and 25 micrograms/kg of S3341 was performed in a double blind, placebo controlled randomised and balanced manner with 3 subjects. A decrease or BP without noticeable sedation (assessed by visual analogue scales) was seen. One, 2 mg of S3341, 0.1 and 0.2 mg of clonidine were then compared in a double blind, placebo controlled, randomised and balanced manner in 10 subjects. BP, heart rate, systolic time intervals (STI), critical flicker frequency, choice reaction time, pursuit rotor, stimulated salivary volume, and dryness of mouth and sedation with visual analogue scales, were measured at 0, 1.5, 3.0, 4.5 and 6.0 h after drug administration. The relationship between decreases in BP and sedation was assessed by linear regression analysis, with the former as the independent (predictor) response and the latter as the dependent (response) variate. Both drugs produced a similar decrease of BP which was significantly different from placebo. Changes in psychomotor function tests were not significant. Both drugs produced dryness of mouth and sedation which were significantly different from placebo but changes were less with S3341. Clonidine showed a significantly steeper slope than S3341 in the relationship between decreases in BP and sedation. This must be interpreted with caution as there was wide variation in the correlation between decreases in BP and sedation, but it may be possible to achieve lesser sedation with S3341.

[Pharmacokinetics of S 3341. Analysis by inhibition of the specific binding of 3H-clonidine]

1. The radioreceptor assay for S 3341, an alpha 2-adrenergic agonist, is based on the ability of the S 3341 to compete with the specific binding of [3H] clonidine to alpha-receptors of rat cerebral cortex membranes. The technique described was used for S 3341 plasma determination. 2. S 3341 levels in plasma and blood pressure were measured in hypertensive subjects 1, 3, 5, 8, 12, 24 and 48 hours after ingestion of the drug. 3. The s 3341 plasma level rose to a maximum between the first and the third hours after ingestion and then declined, with a mean half-life of 14 hours. There were substantial inter-individual variations, ranging from 3 to 24 hours. 4. The fall in blood pressure mirrored the increase in the s 3341 plasma level during the three hours following the ingestion of the drug.

[Relative affinities of S 3341 and clonidine for the alpha 2- and alpha 2-adrenoceptors of the rat brain (author's transl)]

1. 3H-prazosin binds to brain alpha 1-adrenoceptors with high affinity (K(D) 25 degrees C approximately or equal to 1 nM). The number of binding sites in different regions of the brain varied from 44-114 fmoles/mg of protein: striatum = medulla oblongata less than hypothalamus less than cortex. 2. 3H-p-aminoclonidine labels brain alpha 1-adrenoceptors. Its binding capacity (66-209 fmoles/mg prot). is higher in the hypothalamus greater than cortex greater than striatum greater than or equal to medulla oblongata. Its affinity varies from one region to another (0.85 mM in cortex, 3.0 nM in hypothalamus). 3. S 3341 in a potent inhibitor of 3H-p-aminoclonidine like clonidine and a weak inhibitor of 3H-prazosin. 4. Comparison of the inhibitory effect of S 3341 and clonidine on the binding of 3H-prazosin and 3H-aminoclonidine indicates that S 3341 is 2-3 times more specific for alpha 2-adrenoceptors than clonidine. 5. Inhibition constants of S 3341 and clonidine (K1) are not identical in various brain regions but clonidine and S 3341 constants vary in parallel.