Selective imidazoline receptor agonists for metabolic syndrome

Arterial hypertension and metabolic syndrome: specifics of antihypertensive therapy

This review summarizes the literature evidence on humoral disturbances in arterial hypertension (AH), as well as on AH interrelationship with individual components of metabolic syndrome (MS). Based on the results of multi-centre randomised trials, the rationale for the use of antihypertensive agents with favourable metabolic profile is demonstrated, in particular, for antagonists of slow calcium channels, angiotensin-converting enzyme inhibitors, and selective imidazoline receptor agonists.

How can we block sympathetic overactivity? Effects of rilmenidine and atenolol in overweight hypertensive patients

The aim of the present study was to evaluate effects of long-term treatment with rilmenidine compared with atenolol on lipid and glucose metabolism and cardiovascular remodelling in hypertension. In total, 37 patients with hypertension were randomised to rilmenidine 1-2 mg/day or atenolol 50-100 mg/day for 26 weeks. Standard oral glucose tolerance test with a parallel measurement of insulin and glucose levels was performed. The 'areas under the curve' (AUC) for insulin and glucose were calculated. Plasma lipids, left ventricular mass index (LVMI), and intima-media thickness (IMT) were measured. Brachial artery diameter during reactive hyperaemia was used to test endothelium-dependent vasodilatation (EDVD). Blood pressure reduction was equally achieved in both treatment arms. The fasting glucose level increased in the atenolol group from 4.8+/-0.6 to 5.2+/-0.7 mmol/l (P<0.01). The AUC of glucose in rilmenidine group decreased from 860+/-93 to 737+/-66 mmol/min/l (P<0.05), and in the atenolol group it increased from 937+/-86 to 989+/-88 mmol/min/l (P<0.05). Rilmenidine showed a positive effect on lipid levels, whereas in the atenolol group a significant decrease of high-density lipoprotein cholesterol was observed. Left ventricular mass index decreased with rilmenidine by 9.6% and by 6.9% with atenolol (P<0.05). Intima-media thickness significantly decreased in the rilmenidine group. Endothelium-dependent vasodilatation slightly increased on in the rilmenidine group, while on in the atenolol group it remained unchanged. Our data suggest that in hypertensive patients central inhibition of sympathetic drive can produce favourable effects on glucose and lipid metabolism compared with standard beta-blockade with a similar antihypertensive efficacy. Rilmenidine also provides beneficial effects on cardiovascular remodelling and altered endothelial function in hypertension.

Effect of antihypertensive agents on the development of type 2 diabetes mellitus

People with hypertension have a high prevalence of insulin resistance and are at relatively high risk of developing type 2 diabetes mellitus. It is becoming increasingly evident that antihypertensive agents have disparate metabolic effects. For example, recent clinical trials indicate that agents that interrupt the renin-angiotensin axis reduce the risk of developing diabetes compared with other classes of antihypertensive agents. Blockade of the effects of angiotensin II might improve blood flow to insulin-sensitive tissues. Furthermore, interruption of the renin-angiotensin system might provide metabolic benefit through such mechanisms as reduced oxidative stress and restored nitric oxide production, which could lead to improved insulin signaling. Alternatively, collective trials suggest that both diuretics and beta-blockers accelerate the appearance of new-onset type 2 diabetes mellitus in patients with hypertension. Therefore, the risk of new-onset diabetes-associated cardiovascular risks should be factored into future treatment recommendations for patients who require antihypertensive therapy. This will become even more important as the number of insulin-resistant patients with hypertension increases in parallel with the steady growth in the number of sedentary, obese, and aged persons in our population.

Use of moxonidine in elderly patients with resistant hypertension

BACKGROUND

Treatment of hypertension in the elderly people reduces the risk of cardiovascular and cerebrovascular events. Effective treatment often will require the use of two or more antihypertensive agents. Elderly people are at increased risk of adverse events from medication because of physiological changes in pharmacokinetics and pharmacodynamics, polypharmacy and drug interactions. They might not tolerate conventional add-on regimens of antihypertensives as a result.

OBJECTIVE

To investigate the use of the I1-imidazoline receptor agonist moxonidine as an 'add-on' agent in elderly patients with resistant hypertension.

METHODS

We investigated the safety and efficacy of moxonidine (200-400 microg) in a group of elderly patients whose blood pressure (BP) control remained poor despite treatment with two or more antihypertensives. BP was assessed by ambulatory BP monitoring with Spacelabs oscillometric equipment (Model 90207) before and after 6 weeks of treatment with moxonidine used as an 'add-on' agent with the patients normal medication.

RESULTS

Following treatment with moxonidine, the mean daytime systolic BP fell from 169.2 to 153.8 mmHg, a significant reduction of 15.4 +/- 8.9 mmHg (P = 0.003). The mean daytime diastolic BP fell from 91.6 to 84.2 mmHg, a reduction of 7.4 +/- 5.8 mmHg (P = 0.017). For the night-time readings, the systolic BP fell from 151.1 to 141.2 mmHg, a reduction of 9.3 +/- 9.3 mmHg (P = 0.05). The corresponding diastolic fall in BP was not significant (77.9-74.7 mmHg). The 24 h-readings showed a significant reduction in the mean systolic BP from 163.0 to 148.6 mmHg (P = 0.004). The mean diastolic BP also fell significantly from 87.2 to 80.2 mmHg (P = 0.013). Clinical BP readings also showed a significant reduction from 195.9 +/- 19.6 to 174 +/- 17.8 mmHg (P = 0.002) and 103.6 +/- 9.5 to 99.0 +/- 12.4 mmHg (P = 0.013) for systolic and diastolic readings respectively. Moxonidine was well tolerated in 11 of the14 patients.

CONCLUSION

These results suggest that moxonidine might have a place as an 'add-on' treatment in elderly patients whose hypertension is poorly controlled despite treatment with two or more antihypertensive agents.

Imidazoline receptors: new targets for antihyperglycaemic drugs

In both animal models of Type 2 diabetes and in man, it has been evident for many years that certain imidazoline drugs can stimulate insulin secretion and improve glycaemia. This suggests that they may be useful new reagents for use in the management of Type 2 diabetes. However, despite their promise, no imidazoline compound has yet come into clinical use as an effective therapeutic agent in diabetes. This should not be taken as evidence of a flaw in the basic hypothesis, but derives, in part, from continuing ignorance about the molecular characteristics of imidazoline binding proteins, and the precise structure-activity relationships of their ligands. In this review, the mode of action of antihyperglycaemic imidazoline compounds is considered, and the possibility discussed that these agents may interact with a unique subtype of imidazoline binding site associated with ATP-sensitive potassium channels. The functional consequences of this interaction are summarised together with evidence that the binding site may actually lie within the channel complex. Additional data implicating the participation of alpha2-adrenoceptors in some actions of imidazolines are evaluated, and examples of relevant drugs having encouraging therapeutic profiles are highlighted. The possibility that some anti-diabetic imidazoline reagents may exert extra-pancreatic effects is also considered. Overall, the article aims to highlight important developments within the field but also draws attention to those areas where controversy remains.

Antihypertensive drugs interacting with central imidazoline (I1)-receptors

Central imidazoline (I(1))-receptors have been recognised as targets of a new class of centrally acting antihypertensives. The stimulation of these I(1)-receptors induces peripheral sympatho-inhibition and a reduction of (elevated) blood pressure. Moxonidine and rilmenidine are the prototypes of this new class of centrally acting antihypertensives. These imidazoline receptor stimulants are effective antihypertensives with a haemodynamic profile which is attractive from a pathophysiological point of view. Since both moxonidine and rilmenidine have a much weaker affinity for central (2)-adrenoceptors than classic centrally acting drugs, for example, clonidine and alpha-methyl-DOPA, the side-effects profile of the I(1)-receptor stimulants is significantly better. The imidazoline (I(1))-receptor stimulants are the subject of the current survey. They appear to offer the possibility of developing centrally acting antihypertensives with the same attractive haemodynamic characteristics as the classic alpha(2)-adrenoceptor stimulants, but with clearly better tolerability. Their potential use in the treatment of congestive heart failure and the metabolic syndrome is subject to clinical investigation.

Comparison of rilmenidine and lisinopril on ambulatory blood pressure and plasma lipid and glucose levels in hypertensive women with metabolic syndrome

OBJECTIVE

In previous studies, the I1 imidazoline specific agonist rilmenidine effectively lowered office blood pressure (BP) in patients with metabolic syndrome, improved glucose metabolism and did not demonstrate unfavourable effects on plasma lipids. The aim of the present study was to investigate the effects of 12weeks therapy with rilmenidine compared with the ACE inhibitor lisinopril on ambulatory BP, plasma lipid and fasting glucose levels in women with metabolic syndrome.

RESEARCH DESIGN

Prospective randomised open-label, blinded end-points study.

METHODS

Female patients (n = 51) with hypertension and other components of metabolic syndrome were treated with 1 mg rilmenidine (n = 24) or 10 mg lisinopril (n = 27), once- or twice-daily. Anthropometric measurements, office BP and heart rate (HR) measurements, ambulatory BP monitoring, lipid and fasting glucose assessment were performed before and after 12weeks of treatment

MAIN OUTCOME MEASURES

Changes in ambulatory BP and HR, including 24-h, daytime and night-time values, and in lipids and glucose levels. All changes were adjusted for baseline values using the analysis of covariance method.

RESULTS

Ambulatory 24-h systolic BP and diastolic BP were decreased significantly in the rilmenidine group (-11.9 +/- 1.9 and -7.7 +/- 0.8 mm Hg, p < 0.001) respectively and the lisinopril group (-11.0 +/- 1.8 and -6.7 +/- 0.7 mm Hg respectively, p < 0.001). There were no significant differences between the two groups. Rilmenidine reduced 24-h ambulatory HR (-3.6 +/- 0.8 bpm versus 0.3 +/- 0.8 bpm with lisinopril; p = 0.002). The reductions of day-time and night-time BP were also significant for both treatment groups, but the rilmenidine group demonstrated a greater decrease in night-time diastolic BP (p = 0.046). Rilmenidine significantly increased HDL cholesterol and decreased fasting glucose levels (p = 0.009 and p = 0.012, respectively). HDL cholesterol tended to increase and fasting glucose tended to decrease in the lisinopril group. However, differences between groups were not significant.

CONCLUSION

Rilmenidine has similar effects on ambulatory BP patterns in hypertensive women with metabolic syndrome as lisinopril. Rilmenidine compared with lisinopril significantly reduces ambulatory HR. In this study, rilmenidine and lisinopril demonstrate similar effects on plasma lipid and fasting glucose levels.

Hypertension and diabetes

Hypertension is common in people with diabetes with an estimated prevalence using the new criteria (> or = 140/90) at 70%. The recent large randomized controlled studies in diabetic subjects has clearly shown the benefit of blood pressure lowering with reduction in cardiovascular endpoints and microvascular disease. ACE inhibitors, calcium antagonists, thiazide diuretics and beta-blocking agents have been the agents validated by the trials. The challenge now is to implement antihypertensive therapy to achieve tight blood pressure targets (< or = 140/80), usually requiring dual or even triple therapeutic regimens.

The underreporting of results and possible mechanisms of 'negative' drug trials in patients with chronic heart failure

Large drug trials have become very important to determine which drugs should be used in the treatment of patients with chronic heart failure (CHF). When these trials showed "positive" results, publication of the data soon followed, leading to a substantial impact on prescription patterns. In the case of "negative" results, many times they were not published, or were reported as an abstract or as short paper disclosing only the main findings. In this article we will discuss some of these trials that were conducted in the last 10 years, since we believe they may provide insight into the pathophysiology and treatment options in CHF.

Centrally acting imidazoline I1-receptor agonists: do they have a place in the management of hypertension?

Centrally acting imidazoline I(1)-receptor agonists such as moxonidine and rilmenidine induce peripheral sympathoinhibition via the stimulation of hypothetical I(1)-receptors in the rostral ventrolateral medulla. Because of a rather weak affinity for alpha(2)-adrenoceptors, the use of these agents is associated with a lower incidence of adverse reactions, such as sedation and dry mouth, compared with classic centrally acting alpha(2)-adrenoceptor agonists (clonidine, guanfacine, methyldopa). The antihypertensive efficacy of moxonidine and rilmenidine is well documented, and they display a favorable hemodynamic profile. Their tolerability is better than that of the aforementioned centrally acting antihypertensive agents. However, long-term outcome data for moxonidine and rilmenidine are not available, and neither is a quantitative evaluation of their adverse effects. There exists some uncertainty with respect to the identity of the imidazoline I(1)-receptor, which has so far not been cloned. Furthermore, it would be desirable to develop highly selective I(1)-receptor agonists as successor drugs to moxonidine and rilmenidine. Although available data indicate that I(1)-receptor agonists are effective in patients with hypertension, comparative data versus agents such as beta-blockers, diuretics, calcium channel antagonists and ACE inhibitors are required to establish their position in the treatment of hypertension. Finally, I(1)-receptor agonists have potential in the treatment of patients with CHF and those with the metabolic syndrome; syndrome X.

Centrally acting antihypertensive drugs. Present and future

The classic centrally acting antihypertensives such as clonidine, guanfacine and alpha-methyl-DOPA (via its active metabolite alpha-methyl-noradrenaline) induce peripheral sympathoinhibition and a fall in blood pressure as a result of alpha2-adrenoceptor stimulation in the brain stem. These drugs have lost much of their clinical importance because of their unfavourable side-effects (sedation, dry mouth, impotence), which are also mediated by alpha2-adrenoceptors, although in other anatomical regions. Moxonidine and rilmenidine are the examples of a new class of centrally acting antihypertensives, which cause peripheral sympathoinhibition mediated by imidazoline (I1)-receptors in the rostral ventromedulla (RVLM). Their side-effect profile appears to be better than that of clonidine and alpha-methyl-DOPA, probably because of a weaker affinity for alpha2-adrenoceptors. The mode of action, haemodynamic profile, antihypertensive efficacy and adverse reactions of the classic and newer centrally acting antihypertensives are the subject of the present survey. Attention is also paid to other therapeutic applications of centrally acting antihypertensives, such as congestive heart failure and the metabolic syndrome.

Central I1-imidazoline receptors as targets of centrally acting antihypertensive drugs. Clinical pharmacology of moxonidine and rilmenidine

Moxonidine and rilmenidine are moderately selective I1-receptor stimulants. The imidazoline (I1) agonists cause peripheral sympathoinhibition, triggered at the level of central nervous imidazoline receptors. Imidazoline receptor stimulants are effective antihypertensive agents with a hemodynamic profile that is attractive from a pathophysiologic point of view. The antihypertensive activity of these agents is caused by vasodilatation and reduced peripheral vascular resistance. Left ventricular end-diastolic and end-systolic volume is reduced, whereas heart rate, stroke volume, cardiac output, and pulmonary artery pressures are largely unchanged. Long-term left ventricular hypertrophy is reduced. Both drugs, when applied in a once-daily dosage schedule, appear to control hypertension in most patients. Both drugs have been compared with representative agents from the major classes of antihypertensive drugs in controlled trials and found to be equally effective in blood pressure control. The incidence and severity of side effects are lower than those for clonidine, particularly with respect to sedation. A rebound (withdrawal) phenomenon has so far not been reported for moxonidine and rilmenidine. Therefore, I1-receptor stimulants offer the possibility of developing centrally acting agents with a better side-effect profile than do the classic alpha 2-adrenoceptor stimulants.

The effect of moxonidine on feeding and body fat in obese Zucker rats: role of hypothalamic NPY neurones

The antihypertensive agent moxonidine, an imidazoline Ii-receptor agonist, also induces hypophagia and lowers body weight in the obese spontaneously hypertensive rat, but the central mediation of this action and the neuronal pathways that moxonidine may interact with are not known. We studied whether moxonidine has anti-obesity effects in the genetically-obese and insulin-resistant fa/fa Zucker rat, and whether these are mediated through inhibition of the hypothalamic neuropeptide Y (NPY) neurones. Lean and obese Zucker rats were given moxonidine (3 mg kg(-1) day(-1)) or saline by gavage for 21 days. Moxonidine decreased food intake throughout by 20% in obese rats (P<0.001) and by 8% in lean rats (P<0.001), and reduced weight gain that final body weight was 15% lower in obese (P<0.001) and 7% lower in lean (P<0.01) rats than their untreated controls. Plasma insulin and leptin levels were decreased in moxonidine-treated obese rats (P<0.01 and P<0.05), but unchanged in treated lean rats. Uncoupling protein-1 gene expression in brown adipose tissue was stimulated by 40-50% (P< or =0.05) in both obese and lean animals given moxonidine. Obese animals given moxonidine showed a 37% reduction in hypothalamic NPY mRNA levels (P = 0.01), together with significantly increased NPY concentrations in the paraventricular nucleus (P<0.05), but no changes in the arcuate nucleus or other nuclei; this is consistent with reduced NPY synthesis in the arcuate nucleus and blocked release of NPY in the paraventricular nucleus. In lean animals, moxonidine did not affect NPY levels or NPY mRNA. The hypophagic, thermogenic and anti-obesity effects of moxonidine in obese Zucker rats may be partly due to inhibition of the NPY neurones, whose inappropriate overactivity may underlie obesity in this model.

Insulin resistance: site of the primary defect or how the current and the emerging therapies work

Insulin resistance is one of the cardinal pathophysiological components of the metabolic syndrome, type 2 diabetes, and frequently co-exists with essential hypertension. Although insulin resistance is defined as inadequate target organ (muscle, liver and fat) responsiveness and/or sensitivity to insulin, the primary defect may be located in the target organs themselves or at their remote controller--the central nervous system. One of the ways of resolving this dilemma is studying the mechanisms of action of drugs that have insulin-sensitizing properties. In this brief review we discuss how the known and potential insulin sensitizers: metformin, appetite suppressants, thiazolidinediones, and the new class of centrally acting antihypertensive drugs, I1-receptor agonists, may work.