Pharmacophore-based design and discovery of (-)-meptazinol carbamates as dual modulators of cholinesterase and amyloidogenesis.

Multifunctional carbamate-type acetylcholinesterase (AChE) inhibitors with anti-amyloidogenic properties like phenserine are potential therapeutic agents for Alzheimer's disease (AD). We reported here the design of new carbamates using pharmacophore model strategy to modulate both cholinesterase and amyloidogenesis. A five-feature pharmacophore model was generated based on 25 carbamate-type training set compounds. (-)-Meptazinol carbamates that superimposed well upon the model were designed and synthesized, which exhibited nanomolar AChE inhibitory potency and good anti-amyloidogenic properties in in vitro test. The phenylcarbamate 43 was highly potent (IC50 31.6 nM) and slightly selective for AChE, and showed low acute toxicity. In enzyme kinetics assay, 43 exhibited uncompetitive inhibition and reacted by pseudo-irreversible mechanism. 43 also showed amyloid-ß (Aß) lowering effects (51.9% decrease of Aß42) superior to phenserine (31% decrease of total Aß) in SH-SY5Y-APP695 cells at 50 µM. The dual actions of 43 on cholinergic and amyloidogenic pathways indicated potential uses as symptomatic and disease-modifying agents.

Novel bis-(-)-nor-meptazinol derivatives act as dual binding site AChE inhibitors with metal-complexing property.

The strategy of dual binding site acetylcholinesterase (AChE) inhibition along with metal chelation may represent a promising direction for multi-targeted interventions in the pathophysiological processes of Alzheimer's disease (AD). In the present study, two derivatives (ZLA and ZLB) of a potent dual binding site AChE inhibitor bis-(-)-nor-meptazinol (bis-MEP) were designed and synthesized by introducing metal chelating pharmacophores into the middle chain of bis-MEP. They could inhibit human AChE activity with IC(50) values of 9.63µM (for ZLA) and 8.64µM (for ZLB), and prevent AChE-induced amyloid-ß (Aß) aggregation with IC(50) values of 49.1µM (for ZLA) and 55.3µM (for ZLB). In parallel, molecular docking analysis showed that they are capable of interacting with both the catalytic and peripheral anionic sites of AChE. Furthermore, they exhibited abilities to complex metal ions such as Cu(II) and Zn(II), and inhibit Aß aggregation triggered by these metals. Collectively, these results suggest that ZLA and ZLB may act as dual binding site AChEIs with metal-chelating potency, and may be potential leads of value for further study on disease-modifying treatment of AD.

Bis-(-)-nor-meptazinols as novel nanomolar cholinesterase inhibitors with high inhibitory potency on amyloid-beta aggregation.

Bis-(-)-nor-meptazinols (bis-(-)-nor-MEPs) 5 were designed and synthesized by connecting two (-)-nor-MEP monomers with alkylene linkers of different lengths via the secondary amino groups. Their acetylcholinesterase (AChE) inhibitory activities were more greatly influenced by the length of the alkylene chain than butyrylcholinesterase (BChE) inhibition. The most potent nonamethylene-tethered dimer 5h exhibited low-nanomolar IC 50 values for both ChEs, having a 10 000-fold and 1500-fold increase in inhibition of AChE and BChE compared with (-)-MEP. Molecular docking elucidated that 5h simultaneously bound to the catalytic and peripheral sites in AChE via hydrophobic interactions with Trp86 and Trp286. In comparison, it folded in the large aliphatic cavity of BChE because of the absence of peripheral site and the enlargement of the active site. Furthermore, 5h and 5i markedly prevented the AChE-induced Abeta aggregation with IC 50 values of 16.6 and 5.8 microM, similar to that of propidium (IC 50 = 12.8 microM), which suggests promising disease-modifying agents for the treatment of AD patients.

A novel method to calculate the extent and amount of drug transported into CSF after intranasal administration.

The aim of this paper is to establish a novel method to calculate the extent and amount of drug transported to brain after administration. The cerebrospinal fluid (CSF) was chosen as the target region. The intranasal administration of meptazinol hydrochloride (MEP) was chosen as the model administration and intravenous administration was selected as reference. According to formula transform, the extent was measured by the equation of X(A)CSF, infinity/X0 = Cl(CSF) AUC(0-->infinity)CSF/X0 and the drug amount was calculated by multiplying the dose with the extent. The drug clearance in CSF (Cl(CSF)) was calculated by a method, in which a certain volume of MEP solution was injected directly into rat cistern magna and then clearance was assessed as the reciprocal of the zeroth moment of a CSF level-time curve normalized for dose. In order to testify the accurateness of the method, 14C-sucrose was chosen as reference because of its impermeable characteristic across blood-brain barrier (BBB). It was found out that the MEP concentrations in plasma and CSF after intranasal administration did not show significant difference with those after intravenous administration. However, the extent and amount of MEP transported to CSF was significantly lower compared with those to plasma after these two administrations. In conclusion, the method can be applied to measure the extent and amount of drug transported to CSF, which would be useful to evaluate brain-targeting drug delivery.

Meptazinol: unusual in vivo opioid receptor activity at supraspinal and spinal sites.

Systemic (1-10 mg/kg, s.c.), intracerebroventricular (i.c.v. 20-80 micrograms) and spinal intrathecal (i.t., 5-20 micrograms) administration of meptazinol hydrochloride produced dose-related inhibition of reflex contractions of the urinary bladder, recorded isometrically in urethane-anesthetized rats. The effects of meptazinol were reversed by naloxone administered by the same route. Indeed, this was achieved with intracerebroventricular or intrathecal administration of naloxone (2 micrograms), which also selectively antagonized the mu-receptor ligand [D-Ala2, MePhe4, Gly(ol)5]enkephalin (DAGO). However ICI 174,864 (3 micrograms, i.c.v. or i.t.), a delta-opioid receptor antagonist, did not affect the actions of meptazinol given intracerebroventricularly or intrathecally though it consistently abolished the equieffective actions of a selective delta-receptor ligand (2-D-penicillamine, 5-L-penicillamine) enkephalin (DPLPE). Naloxonazine (5 micrograms, i.c.v. or i.t.), an irreversible mu 1-opioid receptor antagonist, produced prolonged antagonism of the effects of DPLPE and meptazinol. The effects of DPLPE partially or completely recovered by 24 hr, indicating that naloxonazine produced prolonged antagonism of delta-opioid receptors. The effects of maptazinol however only recovered after 72 hr, suggesting that antagonism by naloxonazine of this ligand was irreversible and was mediated through a unique opioid receptor interaction. Subthreshold doses of meptazinol (10 micrograms, i.c.v.; 3 micrograms, i.t.) consistently antagonized the effects of morphine given intracerebroventricularly or intrathecally but not the equieffective doses of DPLPE or DAGO. These observations suggest that meptazinol inhibited reflex contractions of the bladder by supraspinal and spinal mu-opioid receptor activation. Furthermore, its agonistic effect and its antagonistic actions were compatible with interactions at a subpopulation of opioid receptors, possibly mu 1-receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of the effects of meptazinol and morphine on the release of acetylcholine from slices of mouse cerebral cortex.

Morphine and the new centrally-active analgesic agent, meptazinol, both increased the K+-evoked release of tritium from slices of mouse cortex preloaded with [3H]choline. The effect of both compounds was antagonised by naloxone whereas the response to meptazinol, but not that to morphine, was reduced in the presence of either scopolamine or tetrodotoxin. Oxotremorine produced a concentration-related inhibition of tritium release, presumably via an action on presynaptic muscarinic receptors, which was also blocked by scopolamine. These results suggest that there may be an indirect component in the action of meptazinol which may be related to a previous finding in which the antinociceptive response to meptazinol in the mouse was antagonised by both scopolamine and naloxone.

The effects of a new opioid analgesic, meptazinol, on the respiration of the conscious rat.

In the conscious rat arterial PCO2 was measured as an index of respiratory status. The opioid analgesic meptazinol (7.5 - 30 mg kg-1) evoked small but significant increases in arterial PCO2 which were attenuated by naloxone. Meptazinol significantly reduced the increase in arterial PCO2 evoked by morphine. The respiratory depression induced by meptazinol, but not that induced by morphine, was enhanced by pretreatment with atropine. The (+)-enantiomer, but not the (-)-enantiomer of meptazinol increased arterial PCO2. In contrast, only the (-)-enantiomer reduced the respiratory depressant effect of morphine. It is proposed that the degree of respiratory depression induced by meptazinol is limited by its opioid antagonist and cholinomimetic properties.

Prevention of physostigmine-induced lethality by the opioid analgesic meptazinol in the mouse.

The prophylactic action of meptazinol against physostigmine- and neostigmine-induced lethality was evaluated in mice. Meptazinol proved to be effective against physostigmine (1 mg kg-1 i.p.), but not against neostigmine (0.5 mg kg-1 i.p.). The antagonism by meptazinol of physostigmine-induced poisoning was maximal when the drug was administered 15 min before physostigmine. Under these conditions the ED50 (95% confidence limits) of meptazinol was 24 (22.0-26.1) mg kg-1 s.c. A 30 mg kg-1 dose of the drug prevented lethality in 89% of the animals. The action of meptazinol was not antagonized by naloxone hydrochloride (2 mg kg-1 i.p.), injected 10 min before meptazinol. Pretreatment of mice with 30 mg kg-1 meptazinol 15 min before physostigmine (1 mg kg-1) poisoning increased brain acetylcholinesterase (AChE) activity on average, from 8 to 31% of control values. The protection of cholinesterases against physostigmine- and neostigmine-induced inactivation was demonstrated in vitro directly on purified preparations of the enzymes using a dilution method. The ED50 values (95% confidence limits) for the protective effect of meptazinol of electric eel AChE against 1 and 3 microM physostigmine and 1 microM neostigmine were 2.6 (1.4-4.9), 9.5 (5-18) and 3 (1.6-5.7) microM, respectively, while for protection of horse serum butyrylcholinesterase (BuChE) against the same inhibitors, the ED50 values were 12 (5.4-26.4), 42 (27-65.1) and 8 (3.6-17.6) microM, respectively. It is suggested that prevention of physostigmine-induced lethality by meptazinol is a consequence of its protective action on AChE in the central nervous system.

Characterization of the effects of (+/-)-meptazinol, its individual enantiomers and N-methyl meptazinol on food consumption in the rat.

Both (+/-)-meptazinol (2 mg kg-1) and levorphanol (1 mg kg-1) produced hyperphagia over a 4 h period after intraperitoneal injection in free feeding rats during the daylight phase. The individual (+)- and (-)-enantiomers of meptazinol (2 mg kg-1 i.p.) induced comparable increases in cumulative food intake. N-methyl meptazinol (2-10 mg kg-1 i.p.), the quaternary analogue of meptazinol, produced no modification of food intake though it increased food consumption when injected intracerebroventricularly (10-100 micrograms per animal). Meptazinol and levorphanol hyperphagia was abolished by 1 mg kg-1 doses (i.p.) of the opioid antagonists naltrexone, naloxonazine and (-)-Mr 1452 but not by its (+)-enantiomer Mr 1453 which is not effective as an opioid antagonist. Intracerebroventricular administration of the delta-opioid antagonist ICI 154,129 (10 micrograms per animal) suppressed meptazinol but not levorphanol hyperphagia. It was concluded that meptazinol produces centrally mediated stereospecifically reversible hyperphagia through a mu-opioid receptor mechanism common to levorphanol, and also through delta-opioid receptor mechanism(s).

Similarity between mu-opioid receptors in mouse vas deferens and guinea-pig ileum.

The effects of the opioid receptor agonist RX783006 and of the opioid receptor partial agonist (+)-meptazinol have been examined on electrically-induced twitch responses of the guinea-pig isolated ileum and of the mouse isolated vas deferens. Log10 concentration-tissue state curves were determined for (+)-meptazinol and for RX783006, alone, in combination and, when appropriate, in the presence of naloxone (30 nM). Analysis of these log10 concentration-tissue state curves using the null equations derived and verified in the previous paper allows quantitation of the characteristics of the interaction of (+)-meptazinol with the opioid receptors in these tissues. The results indicate that the apparent differences in the actions of (+)-meptazinol on isolated electrically-stimulated guinea-pig ileum and mouse vas deferens can be accounted for without the need to postulate differences between mu-opioid receptors in these two tissues.

Antiarrhythmic actions of meptazinol, a partial agonist at opiate receptors, in acute myocardial ischaemia.

1 The intravenous administration, to anaesthetized rats, of meptazinol (1 and 2 mg kg-1), a partial agonist at opiate receptors, greatly reduced the incidence of ventricular extrasystoles that resulted from acute coronary artery occlusion. The incidence of ventricular fibrillation (VF) was reduced from 50% (in the controls) to 10% and the mortality from 30% to zero. 2 In similar doses, pretreatment with meptazinol also reduced ventricular arrhythmias, including fibrillation, in conscious rats subjected to coronary artery occlusion. In this model, survival at 16 h was increased from 27% in the controls to 50% and 83% respectively in rats pretreated with 1 and 2 mg kg-1 of the drug. 3 In antiarrhythmic doses, meptazinol had little effect on either heart rate or systemic arterial blood pressure. 4 Intracellular action potential recordings from papillary muscle removed from rats given meptazinol (2 mg kg-1) 15 min previously showed an increase in APD50 and APD90 of more than 40%. There was no effect on dV/dtmax. When superfused with meptazinol in vitro normal rat papillary muscle stimulated at 1 or 3 Hz showed an increase in APD90 and a decrease in dV/dtmax. 5 The antiarrhythmic effect of meptazinol in these models can probably be explained by direct actions on the cardiac muscle action potential (increase in APD) although effects on opiate receptors cannot be ruled out. It is suggested that meptazinol might be useful in relieving pain, and in reducing the severity of arrhythmias in the early stages of acute myocardial infarction.

The antinociceptive activity of meptazinol depends on both opiate and cholinergic mechanisms.

1 Antinociceptive responses to meptazinol, morphine and oxotremorine and the effects of pretreatment with naloxone or scopolamine on these responses in mice and rats, were examined. 2 Meptazinol evoked larger increases in nociceptive thresholds in the mouse than in the rat, whereas morphine induced large increases in both species. Oxotremorine was both a more potent and a more effective antinociceptive agent in the mouse than in the rat. 3 Antinociceptive responses to meptazinol were consistently inhibited in animals pretreated with naloxone, whereas scopolamine attenuated the effects of meptazinol in some, particularly the mouse tail immersion test, but not in all of the procedures used. 4 Naloxone inhibited all antinociceptive responses to morphine, and scopolamine inhibited all responses to oxotremorine. However, there was no significant interaction between naloxone and oxotremorine or between scopolamine and various opioid analgesic agents. 5 These results indicate that meptazinol, unlike established opioid drugs, may induce antinociception by a dual action on opiate and cholinergic mechanisms.

Comparison of the cardiovascular effects of meptazinol and naloxone following haemorrhagic shock in rats and cats.

The cardiovascular effects of the opioid mixed agonist-antagonist, meptazinol, and the opioid antagonist, naloxone, have been evaluated in conscious rats, anaesthetized rats and anaesthetized cats following the induction of haemorrhagic shock. The mean arterial pressure of conscious rats decreased by 17-29 mmHg following a haemorrhage of 20% of blood volume. Meptazinol (17 mg kg-1, i.m.) administered after haemorrhage evoked a rapid and sustained increase in mean arterial pressure to pre-haemorrhage levels. Naloxone (10 mg kg-1, i.v.) also increased mean arterial pressure to a level significantly higher than post-haemorrhage values. Neither haemorrhage nor subsequent drug treatments evoked significant changes in the heart rates of conscious rats. In anaesthetized rats, 20% haemorrhage evoked decreases in mean arterial pressure, heart rate and cardiac output. Blood flow to the heart, skin, skeletal muscle, kidneys, spleen and liver (arterial) was decreased. Meptazinol and naloxone increased blood pressure and total peripheral resistance, but did not significantly alter heart rate or cardiac output. Hepatic arterial flow decreased further in both drug and vehicle treated groups. In addition meptazinol slightly reduced skeletal muscle flow. In anaesthetized cats 40% haemorrhage decreased mean arterial pressure by 46 +/- 3 mmHg. An intravenous infusion of either meptazinol or naloxone (cumulative 2 mg kg-1, i.v.) partially restored blood pressure. In experimental animal models of haemorrhagic shock, meptazinol has a similar cardiovascular profile to naloxone. The established analgesic activity of meptazinol may confer an advantage in some shock states.

Meptazinol has a similar agonist action on opioid receptors in field-stimulated mouse vas deferens and guinea-pig ileum.

The effects of the opioid receptor agonist RX783006 and of the opioid receptor partial agonist (+)-meptazinol have been examined on electrically induced twitch responses of the guinea-pig isolated ileum and of the mouse isolated vas deferens. Log10 concentration-tissue state curves were determined for (+)-meptazinol and RX783006, alone, in combination and in the presence of naloxone (30 nM). Analysis of these log10 concentration-tissue state curves using the null equations derived and tested in the preceding paper indicates that the opioid agonist action of (+)-meptazinol on mouse vas deferens is quantitatively similar to that on guinea-pig ileum. The results also suggest that (+)-meptazinol acts as a functional antagonist on the guinea-pig ileum as well as on the mouse vas deferens. The potency of (+)-meptazinol relative to RX783006 has been measured by an indirect method which should eliminate any functional antagonistic action of (+)-meptazinol. This method gives a relative potency of (+)-meptazinol in both tissues which is three to six times greater than that measured directly on guinea-pig ileum. This discrepancy may be due to experimental error but it may also indicate that direct measurements on guinea-pig ileum underestimate the agonist potency of this compound on opioid receptors.

The effects of meptazinol in comparison with pentazocine, morphine and naloxone in a rat model of anaphylactic shock.

The actions of meptazinol, pentazocine, morphine and naloxone on the cardiovascular changes accompanying anaphylactic shock were evaluated in ovalbumin-sensitized anaesthetized rats. Pretreatment with meptazinol and pentazocine prevented the fall in mean arterial pressure associated with antigen challenge, whereas morphine and naloxone attenuated but did not completely prevent, this change. None of the drugs significantly altered the antigen-induced decreases in heart rate. All the drugs partially reversed the fall in mean arterial pressure when given after antigen challenge although the activity of naloxone was less marked. Pretreatment with reserpine prevented the restoration of blood pressure by all drugs. Additional experiments with meptazinol showed that pretreatment with phentolamine prevented its pressor action. In pithed non-sensitized rats the frequency-pressor response curve to splanchnic stimulation was shifted to the left by meptazinol and shifted to the right by pentazocine, but the changes were small Morphine and naloxone had no significant effects. It was concluded that opioid mixed agonist-antagonists reverse the cardiovascular changes associated with anaphylactic shock. These effects appear to be mediated by facilitation of sympathetic neurotransmission.

Meptazinol. A review of its pharmacodynamic and pharmacokinetic properties and therapeutic efficacy.

Meptazinol is a new opioid-type analgesic with mixed agonist/antagonist properties. It may be given orally, intravenously or intramuscularly. In studies in patients with moderate to severe pain of various aetiologies, usually following surgery or in obstetrics, the characteristics of analgesia with meptazinol were comparable to those seen with equianalgesic doses of pentazocine, pethidine or a combination of dextropropoxyphene and paracetamol. Preoperative use and use as a component of anaesthesia require further investigation before conclusions may be drawn on its effectiveness in these areas. Onset of action, recorded in a few studies, was faster than that with the other analgesics but duration was shorter than that of morphine, buprenorphine and pentazocine. Only a small number of patients with chronic pain have received long term therapy with meptazinol; in such patients there was no need for increased doses as treatment progressed. Respiratory depression has only been observed in patients receiving meptazinol as a premedication or while undergoing anaesthesia. Similarly any haemodynamic changes have been limited to preoperative patients or patients undergoing anaesthesia. Like other agonist/antagonist analgesic drugs, the abuse potential of meptazinol seems relatively low, but only wider clinical use for longer periods can establish this with certainty. The most commonly reported side effects have been gastrointestinal in nature, and although the incidence of central nervous system side effects has been relatively low, drowsiness and dizziness have caused occasional problems. Thus, meptazinol is a relatively potent but safe addition to the analgesics available for treatment of the patient with moderate to severe pain.

Restoration of mean arterial pressure in endotoxic shock by meptazinol. Contributions from lysosomal membrane stabilisation, opiate antagonism and noradrenaline release.

Meptazinol elevated mean arterial pressure in rats which had been treated with endotoxin. The drug also reduced the titer of circulating lysosomal enzymes. This effect was secondary to the restoration of mean arterial pressure (MAP). In vitro, meptazinol stabilised lysosomal membranes, increased noradrenaline release and interacted with the opiate receptor (naloxone-binding site) as an antagonist. The relevant contributions of these phenomena to the restoration of MAP are discussed.

Antagonism of morphine analgesia by CCK-8-S does not extend to all assays nor all opiate analgesics.

The conditions under which CCK-8-S may block opiate-induced analgesia were examined in detail. A U-shaped dose-response relationship was observed for the ability of CCK-8-S to attenuate (by approximately 50%, at most) morphine-induced tail flick analgesia. The analgesic effects of morphine in the hot plate or acetic acid-induced stretching tests were not altered by CCK-8-S at doses that antagonized morphine in the tail flick test. Tail flick latency elevations induced by meptazinol, a putative mu-1 receptor agonist, were also attenuated by CCK-8-S according to a U-shaped dose-response relationship, but those induced by U-50,488, a kappa agonist, were not antagonized by CCK-8-S doses that attenuated morphine analgesia. Thus, the ability of CCK-8-S to antagonize opiate analgesia does not follow a conventional dose-response relationship, does not extend to all tests of analgesia and may not extend to all opioid drugs. Analgesia mediated by the mu-1 opioid receptor subtype may be more amenable to antagonism by CCK-8-S than that mediated by the kappa receptor subtype.

Meptazinol cross-tolerance studies between morphine or oxotremorine.

The time course of the development of auto-tolerance to meptazinol as determined by combined heat and pressure nociceptive tests has been examined using two dose levels of meptazinol (10 mg and 30 mgkg-1s.c.) corresponding to the partial agonist and cholinergic components respectively. In mice treated twice daily with meptazinol for eight days, there was cross tolerance to morphine in both tests at each dose level of meptazinol. In chronic morphine treated mice challenged with meptazinol (30 mg kg-1 s.c.), there was no cross tolerance with morphine since meptazinol still retained its antinociceptive effects in both tests. Mice treated chronically with meptazinol (30 mgkg-1 s.c.) did not respond to the cholinomimetic agent oxotremorine, implying that there was cross tolerance between these two analgesics. These data suggest the existence of a one-way tolerance between morphine and meptazinol whilst at higher doses of meptazinol a full tolerance occurs with oxotremorine.

Lack of sensitization during place conditioning in rats is consistent with the low abuse potential of tramadol.

Based on the recent finding that tramadol (TRAM) produces conditioned place preference (CPP) and dopamine release in the nucleus accumbens, it was suggested that the abuse liability of TRAM may be greater than hitherto assumed. We re-evaluated the effects of TRAM in CPP and behavioral sensitization, in comparison with morphine (MOR) and meptazinol (MEPT), an opioid drug with minimal abuse potential. While MOR produced CPP and very strong locomotor sensitization, TRAM and MEPT produced only CPP. It has been suggested that sensitization plays an important role in the development of addiction, hence our results suggest that the abuse potential of TRAM might resemble more that of MEPT than that of MOR, and they are consistent with the clinical picture, in that although TRAM is not completely devoid of positively reinforcing effects, reports on abuse are rare. The low propensity to induce addiction may be related to the lack of changes in the brain circuitry mediating reward and motivation, as evidenced by the lack of sensitization.