CLINICAL ASPECTS OF BLOOD PRESSURE CRISIS DUE TO WITHDRAWAL OF CENTRALLY ACTING ANTIHYPERTENSIVE DRUGS

Hypertensive Crisis in a Pediatric Patient Experiencing Clonidine Withdrawal

Background. Clonidine, a central alpha-adrenoreceptor agonist, was initially developed as an antihypertensive. Though no longer commonly used for its original indication due to rebound hypertension after discontinuation, it is currently widely prescribed as a treatment for many pediatric indications including sleep disorders, behavioral concerns, and attention deficit hyperactivity disorder. Case Report. We describe a girl who developed prolonged symptoms of clonidine withdrawal, including hypertension and elevated serum metanephrines. Discussion. Clonidine withdrawal in pediatric patient can present with hypertensive urgency and other signs of sympathetic stimulation. Withdrawal can also lead to dramatic elevation in serum metanephrines. Treatment with a clonidine taper will reduce development of withdrawal symptoms. Conclusion. Given the rise in clonidine use in pediatric patients, clinicians should be aware of the risk of clonidine withdrawal and how to recognize and avoid its development.

Comparison of the pharmacokinetics, pharmacodynamics, and safety of oral (Catapres) and transdermal (M-5041T) clonidine in healthy subjects

The pharmacokinetic as well as pharmacodynamic properties of a transdermal clonidine, M-5041T (M) and its safety were compared with those of oral clonidine, Catapres (Nippon Boehringer Ingelheim, Hyogo, Japan). One patch of M containing 6 mg of clonidine was applied on the right chest for 3 days or one tablet of Catapres (.075 mg) was given orally every 12 hours for 3 days in eight healthy subjects. The study was conducted by a crossover design with 14 to 16 days' interval between the cross-over. Blood and urine samples for clonidine concentration were obtained, and blood pressure (BP) was measured for a 168-hour period after application of M and for a 96-hour period after initiation of Catapres therapy. Plasma concentration of clonidine increased gradually after application of M and decreased gradually after removal, whereas this parameter increased rapidly during the absorption phase and decreased rapidly in the elimination phase after each dosage of Catapres. Elimination half-life of clonidine after removal of M was significantly greater than that after the final dosage of Catapres. No significant difference was observed in maximum plasma concentration or area under the plasma concentration-time curve between the two trials. The BP lowering effects of M and Catapres did not differ significantly. Adverse symptoms occurred more frequently during Catapres therapy than during treatment with M. Most of these symptoms were observed when plasma clonidine concentration was relatively higher in each trial. These results suggest that M is effective for the treatment of hypertension with a lower incidence of adverse symptoms.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha-adrenoceptors

Major advances have been made in our understanding of the molecular structure and function of the alpha-adrenoceptors. Many new subtypes of the alpha-adrenoceptor have been identified recently through biochemical and pharmacological techniques and several of these receptors have been cloned and expressed in a variety of vector systems. Currently, at least seven subtypes of the alpha-adrenoceptor have been identified and the molecular structure and biochemical functions of these subtypes are beginning to be understood. The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. The mechanisms by which the alpha-adrenoceptor subtypes recognize different G-proteins, as well as the molecular interactions between receptors and G-proteins, are the topics of current research. Furthermore, the physiological and pathophysiological role that alpha-adrenoceptors play in homeostasis and in a variety of disease states is also being elucidated. These major advances made in alpha-adrenoceptor classification, molecular structure, physiologic function, second messenger systems and therapeutic relevance are the subject of this review.

Pharmacokinetics and pharmacodynamics of a new transdermal clonidine, M-5041T, in healthy subjects

The pharmacokinetic as well as the pharmacodynamic properties of a new transdermal clonidine, M-5041T (M), and its safety were evaluated after single and repeated applications. In the single-application study, one patch of M (4 mg-->6 mg-->8 mg) was applied for 3 days in eight healthy subjects. In the repeated-application study, first (0-72 hours), second (72-144 hours), and third (144-216 hours) patches of M 6 mg were applied in seven healthy subjects. In the single-application study, plasma clonidine concentration increased in a dose-dependent manner after application of M. Maximum plasma concentration (Cmax) and area under the plasma concentration-time curve (AUC) increased in a dose-dependent manner, but the difference did not reach significance. Time to maximum concentration, elimination half-life, and total and renal clearance did not differ significantly among three trials. Blood pressure (BP) decreased gradually after application of each dose of M. The BP-lowering effect of M 8 mg was greater than that of M 4 mg and 6 mg. Adverse effects such as erythema and drowsiness were reported in some subjects. No subject had to be withdrawn from the study because of the appearance of adverse effects. In the repeated-application study, plasma concentration of clonidine increased up to 48 hours after application of first patch, and thereafter remained within a relatively narrow range until removal of third patch. The Cmax and AUC did not differ significantly among three trials. Blood pressure during an active period decreased significantly during treatment with M, whereas BP at midnight did not change significantly.(ABSTRACT TRUNCATED AT 250 WORDS)

Abrupt change from a beta-adrenoceptor blocking drug to enalapril in hypertension

The haemodynamic and hormonal effects of an elective change of antihypertensive therapy from a beta-adrenoceptor blocking drug to a converting enzyme inhibitor, enalapril, were monitored in 12 hypertensive in-patients (WHO I). Blood pressure and heart rate were determined every 2-4 h using an automatic sphygmomanometer during an abrupt cessation of the previous beta-adrenoceptor blocking drug and commencement of treatment with enalapril 20 mg o.d. 12 h later. Mean blood pressure values at rest and during the hand grip test were lower when on enalapril, but heart rate was significantly higher, and three patients suffered from palpitations during the change. The change resulted in an improvement in cardiac function both at rest and during isometric work, as shown by echocardiography. A rapid decrease in plasma angiotensin converting enzyme (ACE) activity and an increase in renin activity were also seen after the change, while plasma levels of atrial natriuretic peptide (ANP) decreased towards normal values. The results suggest that an abrupt change from a chronic beta-adrenoceptor blocking drug to enalapril is safe, feasible and is likely to produce favourable haemodynamic and hormonal effects in hypertensive patients.

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)

Clinical consequences of abrupt drug withdrawal

Syndromes due to the abrupt withdrawal of drug treatment occur mainly with adrenal corticosteroids and agents with an action on either the cardiovascular system or central nervous system. The abrupt withdrawal of antihypertensive therapy typically results in symptoms of overactivity in the sympathetic nervous system. Clonidine and beta-adrenoceptor antagonists are clinically the most important of these agents, but numerous other drugs have been implicated. Overall, the problem is small when viewed in the context of the huge scale of prescribing of antihypertensive medicines. A more serious problem is the occurrence of crescendo angina following the abrupt withdrawal of beta-adrenoceptor antagonists. Although other factors may be involved, adaptive up-regulation of beta-adrenoceptor density is the most likely cause of crescendo angina, and renders the patient more susceptible to sympathetic nervous stimulation following withdrawal of treatment. Besides leading to a recrudescence of the disease being treated, the withdrawal of corticosteroids can cause a variety of syndromes. In particular, problems can arise as a result of treatment-induced suppression of the hypothalamic-pituitary-adrenal (HPA) axis. Another steroid withdrawal syndrome of unknown aetiology, without significant abnormalities of the HPA axis occurring, has been described. Benign intracranial hypertension may rarely follow steroid withdrawal in children. The syndromes associated with withdrawal of drugs which have an action on the CNS are poorly understood. Withdrawal of neuroleptic drugs can be followed by symptoms that resemble those described following withdrawal of anticholinergic drugs, and those agents with the greatest muscarinic-receptor-blocking properties are those which are most frequently implicated. However, the less common withdrawal dyskinesias are thought to reflect up-regulation of dopaminergic receptors during long term treatment. Gastrointestinal symptoms predominate following the abrupt withdrawal of antidepressants but hypomania and an 'akathisia-like' syndrome have been reported. Barbiturates are no longer recommended as hypnotics because of severe effects of withdrawal and the existence of safer alternatives. Short acting barbiturates can be withdrawn by replacement with either phenobarbitone (phenobarbitol) or diazepam and subsequent gradual reduction in dose. The recognition of dependency on benzodiazepines has been slow because of the similarity of mild withdrawal symptoms to the original problem which led to treatment being offered.(ABSTRACT TRUNCATED AT 400 WORDS)

The hypotensive activity and side effects of methyldopa, clonidine, and guanfacine

Clonidine (Catapres, Catapresan), guanfacine (Estulic), and methyldopa (Aldomet) are the prototypes of centrally acting antihypertensive drugs. Clonidine and guanfacine are lipophilic drugs that readily penetrate into the brain, where they stimulate alpha-adrenergic receptors in the pontomedullary region. The stimulation of these central alpha-adrenergic receptors has been shown to activate an inhibiting neuron, which causes a reduction of peripheral sympathetic tone and a subsequent fall in arterial blood pressure and heart rate. Both a centrally initiated reduction of vagus reflex activity and the activation of presynaptic alpha 2-adrenergic blocking agents in the heart may contribute to the bradycardia. Studies indicate that methyldopa also penetrates into the brain, where it is converted into alpha-methylnorepinephrine. This amine may stimulate the same central alpha-adrenergic receptors as those activated by clonidine, which will result in a hypotensive effect. Possibly, alpha-methyldopamine might also play a role. Accordingly, the modes of action of clonidine and alpha-methyldopa probably are very similar at a basic level. The central adrenergic receptors probably are located postsynaptically. Their receptor demand corresponds more closely to that of the alpha 2-subtype. Central alpha 1-adrenergic receptors might possibly play a part in the modulation of vagally induced baroreflex bradycardia. A discussion on the pharmacological basis of the side effects of the centrally acting antihypertensives has been limited to those adverse reactions that are somehow related to alpha-adrenergic receptors. Sedation, a common side effect, appears to be mediated by central alpha 2-adrenergic receptors, at least in animal models.(ABSTRACT TRUNCATED AT 250 WORDS)

Withdrawal reactions following cessation of central alpha-adrenergic receptor agonists

Interruption of long-term treatment with alpha 2-adrenergic receptor agonists may be associated with reversal of their hemodynamic effects, clinical and biochemical evidence of increased peripheral sympathetic activity, and behavioral responses similar to those seen after narcotic or alcohol withdrawal. Reactions are most commonly observed after short-acting imidazoline drugs such as clonidine and tiamenidine. Reactions are less common after longer acting agents such as guanfacine. A new management approach to withdrawal has been evaluated, which uses a combination of alpha 1-blockade (prazosin) and cardioselective beta-blockade (atenolol) together with a benzodiazepine (chlordiazepoxide). Withdrawal reactions were not observed in eight patients in whom clonidine was withdrawn under cover of these agents. The mechanism of the withdrawal reaction may involve agonist-induced down regulation of alpha 2-adrenergic receptor affinity, number, or both. Experimental studies with the irreversible alpha-antagonist phenoxybenzamine on the turnover of alpha 2-receptors suggest that recovery of receptor number may be much slower in the brain than in the periphery.