Antinociceptive effects of meptazinol and its isomers on carrageenan-induced thermal hyperalgesia in rats.
Using the latency of paw withdrawal (PWL) from a noxious thermal stimulus as a measure of hyperalgesia, the effects of i.p. injection of meptazinol and its isomers, 112824 and 112825, on carrageenan-induced thermal hyperalgesia were studied in awaked carrageenan-inflamed rats. Peripheral inflammation was induced by intraplantar (i.pl.) injection of carrageenan (2 mg/100 microl) into one hindpaw in rats. Carrageenan produced marked inflammation (edema and erythema) and thermal hyperalgesia in the injected paws, which peaked at 3 h after injection and showed little change in magnitude for another 3 h. Injection of 0.1 mg/kg meptazinol (i.p.) at 3 h after carrageenan had no effect on the PWLs of either inflamed or non-inflamed hindpaw during the next 100 min (P>0.05, n=8). At the dosage of 1 and 10 mg/kg, meptazinol produced marked anti-nociception and anti-hyperalgesia in non-inflamed and inflamed hindpaw, respectively (P<0.05, n=8-11). The prolonging effect of meptazinol on PWL in inflamed hindpaw was more potent than that in non-inflamed hindpaw. Pre-administration of 1.5 mg/kg naloxone significantly antagonized meptazinol-induced anti-nociception and anti-hyperalgesia. Intraperitoneal injection of an isomer of meptazinol, 112825 (1.5 mg/kg), but not 112824 (1 mg/kg), markedly increased the PWL of the non-inflamed hindpaw. Nevertheless, both the isomers produced similar anti-hyperalgesic effect to that of meptazinol (P<0.05, n=8), which was completely reversed by naloxone (1.5 mg/mg). The results suggest that meptazinol and its isomers have anti-nociceptive and anti-hyperalgesic properties with the former more potent. The effects are mainly mediated by mu opioid receptors. This study provides an important clue for extending clinical utilization of meptazinol and its isomers. (+info
Structural comparisons of meptazinol with opioid analgesics.
AIM: To investigate the mechanism of action of a potent analgesic, (+/-)-meptazinol. METHODS: The structures of meptazinol enantiomers were compared with opioid pharmacophore and tramadol. RESULTS: Neither enantiomer of meptazinol fitted any patterns among the opioid pharmacophore and tramadol, although they did share some structural and pharmacological similarities. However, the structure superpositions implied that both enantiomers of meptazinol might share some similar analgesic mechanisms with typical opiate analgesics. CONCLUSION: Meptazinol should have a different mechanism of action to known analgesics, which would be helpful in further investigations of meptazinol in the search for non-addictive analgesics. (+info
Conformational re-analysis of (+)-meptazinol: an opioid with mixed analgesic pharmacophores.
AIM: To further investigate the analgesic pharmacophore of (+)-meptazinol. METHODS: Two different opioid pharmacophores, Pharm-I and Pharm-II, were established from structures of nine typical opiates and meperidine by using molecular modeling approaches according to their different structure activity relationship properties. They were further validated by a set of conformationally constrained arylpiperidines. Two conformers of (+)-meptazinol (Conformer-I and Conformer-II) detected in solution were then fitted into the pharmacophores, respectively, by Fit Atoms facilities available in SYBYL, a computational modeling tool kit for molecular design and analysis. RESULTS: Conformer-I fit Pharm-I from typical opiates well. However, Conformer-II fit none of these pharmacophores. Instead, it was found to be similar to another potent analgesic, benzofuro[2,3-c]pyridin-6-ol, whose pharmacophore was suggested to hold the transitional state between the two established pharmacophores. Unlike typical analgesics derived from 4-aryl piperidine (eg, meperidine) with one conformer absolutely overwhelming, the (+)-meptazinol exists in two conformers with similar amounts in solution. Furthermore, both conformers can not transform to each other freely in ordinary conditions based on our NMR results. CONCLUSION: (+)-meptazinol was suggested to be an opioid with mixed analgesic pharmacophores, which may account for the complicated pharmacological properties of meptazinol. (+info
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. (+info
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. (+info
Traditional Chinese acupuncture: a potentially useful antiemetic?
Two consecutive studies were undertaken to evaluate the effectiveness of acupuncture as an antiemetic used in addition to premedication with opioids in patients undergoing minor gynaecological operations. In the first study 25 of the 50 patients underwent acupuncture immediately after premedication with 100 mg meptazinol, the rest receiving the drug alone, and in the second 75 patients were allocated randomly to one of three groups: a group receiving 10 mg nalbuphine and acupuncture, a group receiving premedication and dummy acupuncture, and a group receiving premedication alone. Manual needling for five minutes at the P6 acupuncture point (Neiguan) resulted in a significant reduction in perioperative nausea and vomiting in the 50 patients who underwent acupuncture compared with the 75 patients who received no acupuncture. These findings cannot be explained, but it is recommended that the use of acupuncture as an antiemetic should be explored further. (+info
Meptazinol and pentazocine: plasma catecholamines and other effects in healthy volunteers.
1. This double-blind, random-order study was designed to compare the clinical effects and the plasma catecholamine responses after i.v. administration of meptazinol at doses 0.7 and 1.4 mg kg-1, pentazocine at doses 0.3 and 0.6 mg kg-1 and saline placebo to six healthy volunteers. 2. Mean arterial pressure was not affected by either drug. Heart rate showed slight drug-related changes. Respiratory rate fell slightly with both drugs, but independently of dose. 3. The critical flicker fusion threshold-test and Maddox wing readings could both clearly differentiate active drugs from placebo. Meptazinol caused more nausea and dysphoria as expressed with visual analogue scales. Both analgesics caused short-lived feelings of euphoria. 4. After pentazocine plasma noradrenaline increased almost two-fold in 10-20 min. The effect of meptazinol was slightly smaller, whereas meptazinol caused a pronounced increase in plasma adrenaline concentrations in two of six subjects. Pentazocine had a smaller, but significant effect on plasma adrenaline. 5. We conclude that the effects of meptazinol in healthy volunteers do not differ markedly from those of pentazocine, although it may cause more nausea and dysphoria. The pronounced increase in plasma adrenaline concentrations in two of six subjects calls for caution in its use in patients with cardiac diseases. (+info
Enhanced oral bioavailability of meptazinol in cirrhosis.
Kinetic analysis was carried out after single intravenous (25 mg) and oral (200 mg) doses of the novel partial opioid agonist meptazinol (Meptid) in patients with non-cirrhotic liver disease (NCLD) and biopsy proven cirrhosis. Comparison was made with a group of patients with normal hepatic function. Elimination half-lives after the intravenous dose were slightly prolonged in the cirrhotics (n = 10; 4.2 +/- 0.6 h) compared with the control (n = 8; 2.7 +/- 0.2 h: p less than 0.05) and NCLD (n = 8; 3.2 +/- 0.5 h) groups. There was no significant difference in meptazinol plasma clearance between the groups (cirrhotics = 72 +/- 8 l/h; NCLD = 89 +/- 9 l/h; control = 83 +/- 10 l/h). After the oral dose, seven of 15 cirrhotic patients vomited but only one patient in each of the other groups was unable to tolerate the drug (p = 0.06). This may be explained by very much higher peak meptazinol concentrations in the cirrhotic (n = 8; 184 +/- 37 ng/ml, p less than 0.01) and NCLD (n = 8; 131 +/- 38 ng/ml, p less than 0.05) patients than those of the controls (n = 7; 53 +/- 12 ng/ml) reflecting a mean four-fold and two-fold increase in oral bioavailability respectively (cirrhotics: n = 8; 27.9 +/- 5.3%: p less than 0.001; NCLD: n = 7; 13.7 +/- 3.9% p less than 0.05; controls: n = 7; 6.5 +/- 1.3%). There was no evidence of accumulation after chronic dosing with 200 mg meptazinol four times daily for 13 doses in seven control, seven NCLD and six cirrhotic patients. There were no detectable differences in psychomotor function measured objectively using the Leeds Psychomotor Tester of subjectively by linear analogue scoring between the groups in all three parts of the study. The oral use of meptazinol in patients with chronic liver disease is associated more with the development of nausea and vomiting rather than excessive sedation. These data suggest that dosage reduction in cirrhotic patients is advisable particularly if the drug is taken by mouth. (+info