Chemistry, pharmacology, and structure-activity relationships with a new type of imidazolines exerting a specific bradycardic action at a cardiac site

Imidazoline Receptor System: The Past, the Present, and the Future

Imidazoline receptors historically referred to a family of nonadrenergic binding sites that recognize compounds with an imidazoline moiety, although this has proven to be an oversimplification. For example, none of the proposed endogenous ligands for imidazoline receptors contain an imidazoline moiety but they are diverse in their chemical structure. Three receptor subtypes (I₁, I₂, and I₃) have been proposed and the understanding of each has seen differing progress over the decades. I₁ receptors partially mediate the central hypotensive effects of clonidine-like drugs. Moxonidine and rilmenidine have better therapeutic profiles (fewer side effects) than clonidine as antihypertensive drugs, thought to be due to their higher I₁/α ₂-adrenoceptor selectivity. Newer I₁ receptor agonists such as LNP599 [3-chloro-2-methyl-phenyl)-(4-methyl-4,5-dihydro-3H-pyrrol-2-yl)-amine hydrochloride] have little to no activity on α ₂-adrenoceptors and demonstrate promising therapeutic potential for hypertension and metabolic syndrome. I₂ receptors associate with several distinct proteins, but the identities of these proteins remain elusive. I₂ receptor agonists have demonstrated various centrally mediated effects including antinociception and neuroprotection. A new I₂ receptor agonist, CR4056 [2-phenyl-6-(1H-imidazol-1yl) quinazoline], demonstrated clear analgesic activity in a recently completed phase II clinical trial and holds great promise as a novel I₂ receptor-based first-in-class nonopioid analgesic. The understanding of I₃ receptors is relatively limited. Existing data suggest that I₃ receptors may represent a binding site at the Kir6.2-subtype ATP-sensitive potassium channels in pancreatic β-cells and may be involved in insulin secretion. Despite the elusive nature of their molecular identities, recent progress on drug discovery targeting imidazoline receptors (I₁ and I₂) demonstrates the exciting potential of these compounds to elicit neuroprotection and to treat various disorders such as hypertension, metabolic syndrome, and chronic pain.

HCN Channels Modulators: The Need for Selectivity

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels, the molecular correlate of the hyperpolarization-activated current (If/Ih), are membrane proteins which play an important role in several physiological processes and various pathological conditions. In the Sino Atrial Node (SAN) HCN4 is the target of ivabradine, a bradycardic agent that is, at the moment, the only drug which specifically blocks If. Nevertheless, several other pharmacological agents have been shown to modulate HCN channels, a property that may contribute to their therapeutic activity and/or to their side effects. HCN channels are considered potential targets for developing drugs to treat several important pathologies, but a major issue in this field is the discovery of isoform-selective compounds, owing to the wide distribution of these proteins into the central and peripheral nervous systems, heart and other peripheral tissues. This survey is focused on the compounds that have been shown, or have been designed, to interact with HCN channels and on their binding sites, with the aim to summarize current knowledge and possibly to unveil useful information to design new potent and selective modulators.

Antiischemic and antiarrhythmic activities of some novel alinidine analogs in the rat heart

The antiischemic and antiarrhythmic effects of alinidine and a number of novel alinidine analogs were examined by using perfused rat-heart models. In the isolated working rat heart, the alinidine analog TH91:21 (10 microM; a butyl derivative) significantly increased the postischemic recovery of the heart in terms of both power and efficiency when compared with the control group. In the in situ perfused heart model, this same compound, along with TH91:22 (10 microM; a pentyl derivative) also significantly reduced the severity of both ischemia- and reperfusion-induced arrhythmias in both paced and unpaced hearts. Thus this study is the first to demonstrate the potent antiarrhythmic efficacy of two novel alinidine analogs TH91:21 and TH91:22, with TH91:21 also demonstrated to be a potent antiischemic agent in the isolated working rat heart. Although the mode of action of these compounds remains unclear, results from this study suggest that it is not simply a result of bradycardia or blockade of KATP channels, two actions these compounds possess. These compounds thus possess a novel and beneficial pharmacologic profile worthy of further study.

Comparison of the cromakalim antagonism and bradycardic actions of a series of novel alinidine analogues in the rat

Alinidine, and eight derivatives, were synthesized and tested for their ability to antagonise the actions of the K+ channel opener cromakalim in rat thoracic aorta, and for their ability to induce bradycardia in rat isolated spontaneously beating right atria. Ring segments of rat thoracic aorta were suspended in organ baths to record isometric tension. Tissues were precontracted with K+ (20 mM), and full concentration-relaxation curves constructed to cromakalim (0.01-30 microM) in the absence and presence of increasing concentrations of alinidine/derivative. The majority of the compounds tested caused rightward shifts in the cromakalim concentration-effect curves. Rat spontaneously beating right atria were suspended in organ baths to record rate of contraction. Addition of alinidine/derivative caused a concentration-dependent negative chronotropic response. In terms of structure-activity relationships, increasing the length of the N-allyl side-chain on the alinidine molecule (from 3 carbon (3C), to 5C) resulted in a significant increase in the activity of the compounds as both bradycardic agents and cromakalim antagonists. The most potent compounds in both cases (bradycardic agent and cromakalim antagonist) had no double bond in the side chain. The results suggest that the carbon side-chain influences the activity of alinidine-related compounds both as cromakalim antagonists and as bradycardic agents. However, while similar structure-activity relationships appear to apply for both effects in some instances, there was no significant correlation between the two actions of the alinidine analogues. The results suggest that the ability of alinidine-derivatives to induce bradycardia or to block K+ channels opened by cromakalim can be differentiated on the basis of structure.

Intraoperative heart rate reduction--alinidine versus metoprolol

In a double-blind randomized protocol the effectiveness of the specific bradycardic agent alinidine (0.6 mg.kg-1 i.v.) was compared to that of the betablocker metoprolol (0.035 mg.kg-1 i.v.). Twenty-four coronary artery disease patients undergoing a bypass procedure with an intraoperative heart rate increase of more than 20% were included. Patients with a concomitant intraoperative mean arterial pressure increase of more than 30% or with an intraoperative wedge pressure higher than 15 mmHg (2.0 kPa) were excluded. After application of alinidine and metoprolol, heart rate decreased significantly (P less than 0.01) in the alinidine group from 88 +/- 19 beats per min to 72 +/- 13 and in the metoprolol group from 82 +/- 16 to 72 +/- 12. Baseline values were not obtained. Compared to the hemodynamic changes in the metoprolol group, the alterations of pulmonary capillary wedge pressure (PCWP) (P less than 0.05), stroke volume index (SVI) (P less than 0.05), left ventricular stroke work index (LVSWI) (P less than 0.01) and right ventricular stroke work index (RVSWI) (P less than 0.05) in the alinidine group were statistically significantly different. PCWP remained unchanged after alinidine and increased in the metoprolol group (1.4 +/- 0.4 to 1.6 +/- 0.4 kPa). In the alinidine group LVSWI (43.1 +/- 15 to 49.2 +/- 18 g-m.m-2), RVSWI (5.1 +/- 4 to 6.6 +/- 3 g-m.m-2) and SVI (37.2 +/- 12.2 to 42.5 +/- 12.8 ml.m-2) increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood level, distribution, metabolite pattern and excretion of [14C]alinidine in mice and rats

Following oral and intravenous administration the absorption, distribution, metabolite pattern and excretion of [14C]alinidine, a drug with specific bradycardic efficacy, was studied in mice and rats. [14C]alinidine was rapidly and extensively absorbed. The distribution of radio-labelled drug over the entire animal body was rapid as indicated by blood level curves as well as by whole body autoradiography. In both species radioactive compounds were eliminated from blood with half-lives ranging from 5.6 h to 7.4 h. More than 50% of the renally excreted radioactivity was a uniform substance behaving in in TLC and HPLC experiments like the drug administered. From rat urine this compound could be identified as [14C]alinidine using mass spectrometry. In mice and rats no definite substance with clonidine-like chromatographic properties was found. Biliary excretion was demonstrated in both species. The renal portion of the total radioactivity elimination was 67.2-70.1% of the dose administered in mice and 68.1-85.1% in rats. Total excretion was 85.1-101.3% of radioactivity given and was complete 3-4 days after [14C]alinidine administration. No significant differences in pharmacokinetic behavior in mice and rats could be found.

The effect of alinidine (St 567) on emotionally induced tachycardia in man

Experiments were performed in a randomized double-blind cross-over study in 6 healthy volunteers to answer the question if alinidine, a new analogue of clonidine with a bradycardic effect directly on the sinus node, would have an effect on an emotionally-induced tachycardia. Alinidine 40 mg orally significantly (1%) reduced the tachycardia and the concomitant rise in diastolic blood pressure during the stress of performing a mental task. Forearm blood flow and the efficiency (total score in the mental task) remained unchanged by alinidine.