Centrally acting antihypertensive agents: a comparison of lofexidine with clonidine

Evaluation of the effects of lofexidine and clonidine on naloxone-precipitated withdrawal in opioid-dependent humans

AIMS

To examine the efficacy of lofexidine, an alpha2 adrenergic agonist, to suppress opioid withdrawal symptoms in opioid-dependent humans under in-patient laboratory conditions by using a naloxone challenge procedure.

DESIGN

Randomized, within-subject, cross-over design with drug administration taking place under double-blind and triple-dummy conditions.

SETTING

A 14-bed in-patient hospital research unit dedicated to the conduct of behavioral pharmacology studies.

PARTICIPANTS

Eight healthy adult volunteers (two female/six male) with histories of polysubstance abuse and current physical dependence on opioids.

INTERVENTION

Participants were stabilized onto methadone and maintained on 30 mg/day, p.o. throughout the study. Oral placebo, lofexidine (0.4, 0.8 and 1.6 mg, p.o.) and clonidine (0.1 and 0.2 mg, p.o.) were each tested as pre-treatments once in combination with each of three intramuscular naloxone doses (0, 0.1 and 0.3 mg, i.m.) during 18 separate experimental sessions.

MEASUREMENTS

An array of physiological indices (e.g. heart rate, blood pressure, pupil diameter) as well as a number of subjective and observer-rating scales sensitive to opioid withdrawal effects.

FINDINGS

As expected, lofexidine and clonidine both produced dose-related decreases in blood pressure and heart rate but few subjective effects; naloxone increased opioid withdrawal signs and symptoms in a dose- and time-dependent fashion. Although both lofexidine and clonidine reduced the cardiovascular response to naloxone challenge, close inspection of the data reveal that this occurred only to the extent that baseline physiological parameters were reduced, while neither drug significantly modified the overall magnitude of the response to naloxone. Moreover, neither lofexidine nor clonidine suppressed the subjective discomfort of opioid withdrawal or significantly reduced other autonomic signs of opioid withdrawal, such as lacrimation or rhinorrhea.

CONCLUSIONS

These data suggest that lofexidine is well tolerated even at supratherapeutic acute doses. However, its failure to modify most signs and symptoms of opioid withdrawal suggest that its effective use in spontaneous withdrawal will require concomitant medications for improved therapeutic response.

Pharmacology of moxonidine: an I1-imidazoline receptor agonist

The I1-imidazoline receptor is a novel neurotransmitter receptor found mainly in the brainstem, adrenal medulla and kidney. The actions of moxonidine are described at the level of individual biomolecules, cells, tissues, organs and finally with integrative functions. The receptor functions at the cellular level works through arachidonic acid and phospholipid signaling cascades in neuronal cells with the net result of inhibiting sympathetic premotor neurons.

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.

Long-term experience with lofexidine in the treatment of mild-to-moderate essential hypertension

This study describes sustained efficacy and safety of lofexidine in hypertensive patients. Twenty-one male patients (mean age 55 +/- 9 yr) who had previously completed a short-term trial of lofexidine entered this open-label trial. The daily dose of lofexidine was titrated over three months to a maximum of 1.6 mg or an erect diastolic blood pressure less than 90 mm Hg. Hydrochlorothiazide was added when necessary. Thereafter, each patient was evaluated for blood pressure (BP) response, compliance, side effects, hepatic, renal, or hematological abnormalities every three months for two years. Sustained BP reduction with lofexidine was achieved but not without concomitant diuretic therapy. There were no clinically important changes in heart rate or hepatic, renal, or hematological profiles. Side effects were frequent but severe enough to warrant discontinuation in only four patients. The side-effect profile was similar to that of clonidine. We recommend concurrent diuretic use to maximize effectiveness of lofexidine.

Central and peripheral blockade of the sympathetic nervous system

Agents that work primarily by inhibiting effects of the sympathetic nervous system continue to be widely used for the therapy of hypertension. There are two principal types of drugs: those that work in the central nervous system, and those that act primarily at peripheral neuromuscular sites. Both types of drugs are often effective as single-agent therapy although they also work well in combination with diuretics and other antihypertensive agents. Recently, there has been interest in a new transdermal method for administering the centrally-acting drug clonidine; the consistency of the plasma drug concentrations achieved by this form of treatment appears to minimize adverse effects while providing continuous therapeutic efficacy. Interest is also focusing on the ability of sympathoinhibitory agents such as alpha-methyldopa to produce regression of left ventricular hypertrophy, possibly by mechanisms independent of their antihypertensive action.

Comparative efficacy and safety of lofexidine and clonidine in mild to moderately severe systemic hypertension

The comparative efficacy and safety of lofexidine and clonidine in patients with mild to moderate systemic hypertension were evaluated. Patients who met established criteria were administered lofexidine or clonidine in a double-blind manner. The dose of either drug, along with hydrochlorothiazide if necessary, was titrated to blood pressure response or to the occurrence of side effects. Treatment with the dose established during titration was continued for 12 weeks. Blood pressure and the occurrence of side effects were evaluated during weekly clinic visits. Twenty-six patients completed all portions of the study. The decrease in blood pressure compared with that with the placebo was significant for both drugs (p less than 0.05). The daily dose of clonidine required for blood pressure control was less than that of lofexidine (p less than 0.05). Concomitant diuretic therapy was required equally as often for both drugs. Clonidine caused adverse effects more frequently than did lofexidine. The effectiveness of lofexidine is comparable to that of clonidine, but lofexidine has a lower incidence of toxicity.