Cardiovascular Actions of U50,488H and Related Kappa Agonists

Discovery of attenuation effect of orexin 1 receptor to aversion of nalfurafine: Synthesis and evaluation of D-nor-nalfurafine derivatives and analyses of the three active conformations of nalfurafine

The D-nor-nalfurafine derivatives, which were synthesized by contraction of the six-membered D-ring in nalfurafine (1), had no affinity for orexin 1 receptors (OX₁Rs). The 17N-lone electron pair in 1 oriented toward the axial direction, while that of D-nor-derivatives was directed in the equatorial configuration. The axial lone electron pair can form a hydrogen bond with the 14-hydroxy group, which could push the 6-amide side chain toward the downward direction with respect to the C-ring. The resulting conformation would be an active conformation for binding with OX₁R. The dual affinities of 1 for OX₁R and κ opioid receptor (KOR) led us to elucidate the mechanism by which only 1 showed no aversion but U-50488H. Actually, 1 selectively induced severe aversion in OX₁R knockout mice, but not in wild-type mice. These results well support that OX₁R suppresses the aversion of 1. This is the elucidation of long period puzzle which 1 showed no aversion in KOR.

Pharmacological and toxicological activity of RSD921, a novel sodium channel blocker

BACKGROUND

RSD921, the R,R enantiomer of the kappa (k) agonist PD117,302, lacks significant activity on opioid receptors.

METHODS

The pharmacological and toxicological actions were studied with reference to cardiovascular, cardiac, antiarrhythmic, toxic and local anaesthetic activity.

RESULTS

In rats, dogs and baboons, RSD921 dose-dependently reduced blood pressure and heart rate. In a manner consistent with sodium channel blockade it prolonged the PR and QRS intervals of the ECG. Furthermore, in rats and NHP, RSD921 increased the threshold currents for induction of extra-systoles and ventricular fibrillation (VFt), and prolonged effective refractory period (ERP). In rats, RSD921 was protective against arrhythmias induced by electrical stimulation and coronary artery occlusion. Application of RSD921 to voltage-clamped rat cardiac myocytes blocked sodium currents. RSD921 also blocked transient (ito) and sustained (IKsus) outward potassium currents, albeit with reduced potency relative to sodium current blockade. Sodium channel blockade due to RSD921 in myocytes and isolated hearts was enhanced under ischaemic conditions (low pH and high extracellular potassium concentration). When tested on the cardiac, neuronal and skeletal muscle forms of sodium channels expressed in Xenopus laevis oocytes, RSD921 produced equipotent tonic block of sodium currents, enhanced channel block at reduced pH (6.4) and marked use-dependent block of the cardiac isoform. RSD921 had limited but quantifiable effects in subacute toxicology studies in rats and dogs. Pharmacokinetic analyses were performed in baboons. Plasma concentrations producing cardiac actions in vivo after intravenous administration of RSD921 were similar to the concentrations effective in the in vitro assays utilized.

CONCLUSIONS

RSD921 primarily blocks sodium currents, and possesses antiarrhythmic and local anaesthetic activity.

A review of the properties of spiradoline: a potent and selective kappa-opioid receptor agonist

The selective kappa-opioid receptor agonist spiradoline mesylate (U62,066E), an arylacetamide, was synthesized with the intention of creating an analgesic that, while still retaining its analgesic properties, would be devoid of the, mainly mu receptor mediated, side effects such as physical dependence and respiratory depression associated with morphine. Spiradoline is highly selective for the kappa receptor with K(i) of 8.6 nM in guinea pig. Examination of the enantiomers of spiradoline, showed the (-)enantiomer to be responsible for the kappa agonist properties. Spiradoline easily penetrates the blood brain barrier, and does not seem to have any significant active metabolites. In preclinical studies, spiradoline has a short duration of action with a peak at around 30 min after administration. The analgesic properties of spiradoline are well documented in mice and rats. Antitussive properties have also been reported in rats. Furthermore, spiradoline was reported to display effects suggestive of neuroprotective properties in animal models of ischemia. In humans, spiradoline is a potent diuretic. It also produces significant sedation presumably due to its antihistamine properties. Preclinical studies have shown that spiradoline reduces blood pressure and heart rate, and has possible antiarrhythmic properties. Clinical studies did not confirm these findings. kappa Receptors inhibit dopaminergic neurotransmission. Spiradoline, given systematically to rats, produces a significant and long lasting decrease in dopamine release, and in locomotor activity. It has also antipsychotic-like effect in animal behavioral tests. At low doses spiradoline was reported to decrease tics in patients with Tourette's syndrome. Although spiradoline had promising effects in animal tests of analgesia, and a reasonably good safety profile in preliminary studies, it did not replace morphine as an analgesic. The available clinical data suggest that spiradoline produces disturbing adverse effects such as diuresis, sedation, and dysphoria at doses lower than those needed for analgesic effects. Thus, future development of spiradoline-like analgesic compounds should preferably focus on reduction of unwanted effects on the central nervous system. Spiradoline, which currently is commercially available for preclinical research, might prove useful in some psychiatric conditions and possibly as a neuroprotective agent.

The diverse molecular mechanisms responsible for the actions of opioids on the cardiovascular system

The actions of opioid agonist and antagonist drugs have not been well characterized in the heart and cardiovascular system. This stems from the limited role opioid receptors have been perceived to have in the regulation of the cardiovascular system. Instead, the focus of opioid receptor research, for many years, relates to the characterization of the actions of opioid drugs in analgesia associated with receptor activation in the CNS. However, recent studies suggest that opioid receptors have a role in the heart and cardiovascular system. While some of these actions may be mediated by activation of peripheral opioid receptors, others are not, and may result from direct or receptor-independent actions on cardiac tissue and the peripheral vascular system. This review will outline some of the diverse molecular mechanisms that may be responsible for the cardiovascular actions of opioids, and will characterize the role opioid receptors have in several cardiovascular pathophysiological disease states, including hypertension, heart failure, and ischaemic arrhythmogenesis. In many instances, it would appear that the effects of opioid agonists (and antagonists) in cardiovascular disease models may be mediated by opioid receptor-independent actions of these drugs.

Spiradoline, a kappa opioid receptor agonist, produces tonic- and use-dependent block of sodium channels expressed in Xenopus oocytes

Spiradoline, an arylacetamide kappa (kappa) opioid receptor agonist, produced a potent tonic block of rat neuronal (EC(50)= 34+/-5 microM) and heart (EC(50)= 183+/-13 microM) sodium channels and also blocked IFMQ3 mutant neuronal sodium channels (EC(50)= 130+/-34 microM) that lack fast inactivation when expressed in Xenopus oocytes. Spiradoline produced a hyperpolarizing shift in the voltage-dependence of sodium channel inactivation and exhibited a marked frequency-dependent component to blockade of sodium channels. The onset of open channel block of the IFMQ3 channel by spiradoline was best fit with a first-order blocking scheme, yielding an affinity constant of 116 +/- 33 microM. Thus, spiradoline blocks sodium channels by interacting with the major states of the channel which could result in local anesthetic action in nerves and antiarrhythmic action in the heart.

Molecular analysis of the Na+ channel blocking actions of the novel class I anti-arrhythmic agent RSD 921

RSD 921 is a novel, structurally unique, class I Na+ channel blocking drug under development as a local anaesthetic agent and possibly for the treatment of cardiac arrhythmias. The effects of RSD 921 on wild-type heart, skeletal muscle, neuronal and non-inactivating IFMQ3 mutant neuronal Na+ channels expressed in Xenopus laevis oocytes were examined using a two-electrode voltage clamp. RSD 921 produced similarly potent tonic block of all three wild-type channel isoforms, with EC50 values between 35 and 47 microM, whereas the EC50 for block of the IFMQ3 mutant channel was 110+5.5 microM. Block of Na+ channels by RSD 921 was concentration and use-dependent, with marked frequency-dependent block of heart channels and mild frequency-dependent block of skeletal muscle, wild-type neuronal and IFMQ3 mutant channels. RSD 921 produced a minimal hyperpolarizing shift in the steady-state voltage-dependence of inactivation of all three wild-type channel isoforms. Open channel block of the IFMQ3 mutant channel was best fit with a first order blocking scheme with k(on) equal to 0.11+/-0.012x10(6) M(-1) s(-1) and k(off) equal to 12.5+/-2.5 s(-1), resulting in KD of 117+/-31 microM. Recovery from open channel block occurred with a time constant of 14+/-2.7 s(-1). These results suggest that RSD 921 preferentially interacts with the open state of the Na+ channel, and that the drug may produce potent local anaesthetic or anti-arrhythmic action under conditions of shortened action potentials, such as during anoxia or ischaemia.

Antinociceptive effects of morphine and U-50,488H on vaginal distension in the anesthetized rat

The antinociceptive activity of the kappa- and mu-opioid receptor agonists, (+/-)-U-50,488H and morphine, was examined in a vaginal distension model in anaesthetized female rats. Vaginal distension induced a reproducible cardiovascular response (CVR) which was inhibited in a dose related manner by morphine (0.03-1.0 mg/kg i.v., ED50 = 0.16 mg/kg) and (+/-)-U-50,488H (0.08-1.6 mg/kg i.v., ED50 = 0.49 mg/kg). Morphine (0.3 microg/rat) administered i.c.v. inhibited the CVR by 81.6 +/- 7.9% whereas (+/-)-U-50,488H (30-300 microg/rat) was inactive by this route. A low dose of naloxone (30 microg/kg i.v.) blocked the effect of morphine but not that of (+/-)-U-50,488H. The kappa-opioid antagonist, nor-binaltorphimine (10 mg/kg s.c.) abolished the response to (+/-)-U-50,488H but not that of morphine. This demonstrates that both central and peripheral mu-opioid receptors may be involved in morphine-induced antinociception whereas the kappa-opioid agonist, (+/-)-U-50,488H, blocks vaginal nociception by acting on peripheral kappa-opioid receptors.

Comparison of the effects of a series of kappa-opioid receptor agonists upon sodium channel function in rat brain miniprisms

The blockade of veratrine-stimulated phosphoinositide breakdown in rat cerebral cortical miniprisms has been used as a model of drug action on voltage-dependent sodium channels. The kappa-opioid agonists bremazocine, (+/-)- and (+)-trans-U-50488, U-62066 (spiradoline) and U-69593 inhibited the response to veratrine with IC50 values of 35, 13, 15, 9, and > 100 microM, respectively. Bremazocine, at concentrations inhibiting the response to veratrine, did not inhibit the phosphoinositide breakdown response to the sodium ionophore monensin, indicating the specificity of the assay for sodium channels. The inhibitory actions of bremazocine upon veratrine-stimulated phosphoinositide breakdown were not antagonised by naloxone. This study thus confirms previous data suggesting that the kappa-opioid receptor agonists can affect Na(+)-channel function in a manner unrelated to their actions at kappa-opioid receptors. However, for the compounds tested, such effects are only found at rather high concentrations of the compounds.

Peripheral kappa-opioid receptors mediate the antinociceptive effect of fedotozine (correction of fetodozine) on the duodenal pain reflex inrat

Fedotozine has been shown to act on gastrointestinal sensitivity through peripheral kappa-opioid receptors. The present study investigated the action of fedotozine and reference compounds, morphine and (+/-)-U-50,488H, on duodenal pain in anesthetized rats. The noxious stimulus was produced by duodenal distension (100 mm Hg; 30 s). Fedotozine (1-5 mg/kg i.v.) produced a dose-dependent inhibition of the cardiovascular reflex induced by duodenal distension (ED50 = 1.87 mg/kg) but had no effect at doses up to 300 micrograms/rat by either intracerebroventricular (i.c.v.) or intrathecal routes (i.t.). The mu-opioid receptor agonist, morphine, was active by both i.v. (ED50 = 0.62 mg/kg) and i.c.v. routes (ED50 = 2.17 micrograms/rat) as was the kappa-opioid receptor agonist, (+/-)-U-50,488H (trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1- pyrrolidinyl]cyclohexyl)benzeneacetamide) (ED50 = 0.25 mg/kg and 149 micrograms/rat for i.v. and i.c.v. routes, respectively). The selective kappa-opioid receptor antagonist, nor-binaltorphimine (10 mg/kg s.c.), abolished the response to fedotozine (5 mg/kg i.v.) and (+/-)-U-50,488H (2 mg/kg i.v.) but not that to morphine (1 mg/kg i.v.). In contrast, naloxone (30 micrograms/kg i.v.) blocked the response to morphine (1 mg/kg i.v.) but not that to fedotozine (5 mg/kg i.v.) or (+/-)-U-50,488H (2 mg/kg i.v.). It is concluded that the antinociceptive effects of fedotozine on duodenal pain are mediated by peripheral kappa-opioid receptors.

An electrophysiological basis for the antiarrhythmic actions of the kappa-opioid receptor agonist U-50,488H

This study examined the actions of the selective kappa-opioid receptor agonist, U-50,488H, on voltage activated Na+ and K+ currents in isolated rat cardiac myocytes. U-50,488H produced a concentration-dependent block of the transient Na+ current with an ED50 of about 15 microM, and, at higher concentrations (40-50 microM), a block of the plateau K+ current and an increase in the rate of decay of the transient K+ current. In addition U-50,488H produced a hyperpolarising shift in the inactivation curve for the transient Na+ current without altering the voltage dependence for activation and without an effect on the voltage dependence of inactivation or activation of K+ currents. The block of Na+ currents by U-50,488H showed pronounced use dependence. The kappa-opioid receptor antagonist MR2266 did not itself produce any change in the Na+ or K+ currents and did not change the channel blocking properties of U-50,488H. Thus, since the antiarrhythmic actions of U-50,488H are not blocked by MR2266 or naloxone, the effects of U-50,488H to block Na+ and K+ currents are the most likely reasons for its antiarrhythmic actions, rather than an action at kappa-opioid receptors.