Plasma clonidine levels in hypertension

CYP2D6 mediates 4-hydroxylation of clonidine in vitro: implication for pregnancy-induced changes in clonidine clearance

Clonidine is a centrally acting, alpha-2 adrenergic agonist used for the treatment of hypertension during pregnancy. The metabolic pathways of clonidine are poorly understood, and the quantitative contribution of specific human cytochrome P450 (P450) isoforms has not been systematically assessed. In this study, 17 cDNA-expressed P450 enzymes, in addition to pooled human liver microsomes, were evaluated for clonidine 4-hydroxylation activity in vitro. Five P450 enzymes-CYP2D6, 1A2, 3A4, 1A1, and 3A5-catalyzed measurable formation of 4-hydroxyclonidine. Selective inhibition studies in human liver microsomes confirmed that these isoforms are jointly responsible for 4-hydroxylation of clonidine in vitro, and CYP2D6 accounted for approximately two-thirds of the activity. The major role of CYP2D6 in clonidine metabolism might explain the increase in its nonrenal clearance during pregnancy.

Clonidine premedication: a useful adjunct in producing deliberate hypotension

The effect of clonidine on the dose requirements of labetalol and isoflurane for hypotension was studied in 20 adult patients undergoing middle-ear surgery. Group I (10 patients) received as premedication only pethidine 1 mg.kg-1 i.m., and Group II (10 patients) clonidine 4-5 micrograms.kg-1 p.o. in addition to pethidine. Fentanyl was used for analgesia and d-tubocurarine for muscle relaxation. The dose of labetalol required to induce hypotension to the mean arterial pressure (MAP) 50 mmHg (6.7 kPa) was 0.85 +/- 0.08 mg.kg-1 in Group I, and 0.56 +/- 0.08 mg.kg-1 in Group II (P less than 0.05). The mean isoflurane concentration in inspiratory gas for the maintenance of hypotension was 0.8 +/- 0.1 vol% in Group I, and 0.6 +/- 0.1 vol% in Group II (P less than 0.05). Before hypotension, MAP decreased significantly in Group II, and the heart rate (HR) decreased in both groups. There were no significant differences in MAP and HR between the groups during any phase. Urine flow rates (UF) were lower in Group II (0.23 +/- 0.04 ml.min-1) than in Group I (0.68 +/- 0.16 ml.min-1) before hypotension (P less than 0.05) and during hypotension (0.08 +/- 0.02 ml.min-1 vs. 0.68 +/- 0.32 ml.min-1, P less than 0.05). After anaesthesia, there was no difference in UF between the groups. The results indicate that clonidine can be used for deliberate hypotension to decrease the dose requirements of labetalol and isoflurane.

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)

Oral premedication with clonidine: effects on stress responses during general anaesthesia

The effect of clonidine (4.5 micrograms kg-1) on haemodynamics and hormonal stress responses was evaluated in 21 female patients undergoing breast surgery. The standardized general anaesthesia included diazepam as premedicant, thiopentone, enflurane, N2O, fentanyl and vecuronium. Venous plasma concentrations of noradrenaline, adrenaline, growth hormone, vasopressin, and cortisol were assayed at various times before, during and after surgery. Clonidine attenuated the sympathoadrenal response; arterial blood pressure and heart rate increases in association with intubation were lower in clonidine-premedicated patients. Noradrenaline levels were lower throughout and 3 h after surgery in the clonidine group (P less than 0.05). Adrenaline levels were lower in this group 2 min after intubation (P less than 0.05). Growth hormone, vasopressin and cortisol plasma levels were increased at the end of and after surgery, with no differences between the groups. In spite of the effect on sympathoadrenal response, clonidine did not have any significant additive anxiolytic effect. Statistically significant differences were not found as to need for postoperative analgesics.

Transdermal administration of clonidine: a new approach to antihypertensive therapy

This study assessed the antihypertensive efficacy and side effects of clonidine administered transdermally. Twenty-five patients with mild to moderate essential hypertension (seated diastolic blood pressure 95-120 mm Hg with diuretic therapy alone) controlled with oral diuretic plus oral clonidine were enrolled. Transdermal clonidine was substituted for oral clonidine and titrated until adequate blood pressure control (seated diastolic blood pressure less than 90 mm Hg) was attained. At the end of titration, seated morning blood pressure averaged 129/90 +/- 15/5 mm Hg (mean +/- standard deviation) compared to 136/96 +/- 13/7 mm Hg (p less than 0.01/0.001) during oral clonidine administration. Standing morning blood pressure was also lower during transdermal than oral therapy (131/94 +/- 16/5 vs 136/99 +/- 14/7, p less than 0.05/0.001). Afternoon blood pressures (at peak effect of oral dose) were virtually identical during oral and transdermal therapy in both seated and standing positions. Typical side effects of oral clonidine, including dry mouth, drowsiness, and sexual dysfunction, were reduced during transdermal therapy. There was less morning-to-afternoon variability of blood pressure control and plasma clonidine concentrations during transdermal than during oral therapy. One patient left the study because of drowsiness and two because of skin reactions to the transdermal skin patch. Mild transient local skin irritation occurred frequently. Transdermal clonidine plus a diuretic is an effective treatment for mild to moderate essential hypertension, improves compliance and reduces side effects of therapy.