Adriana Miriam Domínguez-Ramírez

Anti-nociceptive synergism of morphine and gabapentin in neuropathic pain induced by chronic constriction injury.

In order to detect an anti-nociceptive interaction between morphine and gabapentin, the anti-allodynic and anti-hyperalgesic effects of these drugs, administered either separately or in combination, were determined using the von Frey and acetone tests in a rat model of neuropathic pain (Bennett model). Morphine and gabapentin individually induced moderate attenuation of mechanical hyperalgesia, whereas the morphine and gabapentin combination completely decreased hyperalgesia. Morphine showed its maximal effect at 30 min post-injection in the acetone test; however, this effect gradually returned to the baseline value. Gabapentin did not produce an anti-allodynic effect, whereas the morphine and gabapentin combination completely decreased allodynia behavior at 30 min post-injection, an effect that persisted until 120 min. The area under the curve (AUC) of the anti-allodynic or anti-hyperalgesic effects produced by the combinations were significantly greater than the theoretical sum of effects produced by each drug alone or similar to the theoretical sum. The analysis of the effect, expressed as the AUC of the time course, supports the hypothesis that the combination of these drugs is useful in neuropathic pain therapy.

Effect of metamizol on morphine pharmacokinetics and pharmacodynamics after acute and subchronic administration in arthritic rats

The aim of the present study was to investigate whether metamizol alters the pharmacokinetics of morphine and to determine the relationship between morphine plasma levels and antinociceptive effect produced after co-administration of drugs under acute and subchronic treatments using the pain-induced functional impairment model in rat (PIFIR model). Administration of morphine+metamizol under acute treatment produced a significantly higher antinociceptive effect than that obtained with morphine alone (P<0.05). This effect remained unaltered after subchronic treatments for 6 and 12 days. In addition, after the simultaneous administration of the drugs in a single dose, a pharmacokinetic interaction was found, which significantly (P<0.001) increased maximum plasma concentration (C(max)), concentration at 4h (C(4h)), partial areas under the plasma concentration-time curve from zero to 4h (AUC(0-4)) and from zero to 24h (AUC(0-24)). Moreover, whereas plasma concentration of morphine markedly decreased up to 4h (C(4h)) after subchronic administration of the opioid, multiple dosing of the morphine+metamizol combination produced an accumulation of the drug in plasma (P<0.001). The increase observed in morphine plasma levels after co-administration of metamizol may be explained by a possible enzymatic inhibition of the glucuronosyl-transferase system involved in the metabolism of morphine. This study reveals both a pharmacodynamic and a pharmacokinetic interaction between morphine and metamizol, leading to an increased antinociceptive effect and a delay in tolerance development.

High-performance liquid chromatographic assay for metamizol metabolites in rat plasma: application to pharmacokinetic studies.

In order to evaluate the pharmacokinetics of metamizol in the presence of morphine in arthritic rats, after subcutaneous administration of the drugs, an easy, rapid, sensitive and selective analytical method was proposed and validated. The four main metamizol metabolites (4-methylaminoantipyrine, 4-aminoantipyrine, 4-acetylaminoantipyrine and 4-formylaminoantipyrine) were extracted from plasma samples (50-100μl) by a single solid-phase extraction method prior to reverse-phase high performance liquid chromatography with diode-array detection. Standard calibration graphs for all metabolites were linear within a range of 1-100μg/ml (r(2)≥0.99). The intra-day coefficients of variation (CV) were in the range of 1.3-8.4% and the inter-day CV ranged from 1.5 to 8.4%. The intra-day assay accuracy was in the range of 0.6-9.6% and the inter-day assay accuracy ranged from 0.9 to 7.5% of relative error. The lower limit of quantification was 1μg/ml for all metabolites using a plasma sample of 100μl. Plasma samples were stable at least for 4 weeks at -20°C. This method was found to be suitable for studying metamizol metabolites pharmacokinetics in arthritic rats, after simultaneous administration of metamizol and morphine, in single dose.

Antinociceptive effects of tramadol in co-administration with metamizol after single and repeated administrations in rats

Combinations of two analgesic drugs of the same or different class are widely used in clinical therapy to enhance its antinociceptive effects and reduce the side effects. In order to evaluate a possible antinociceptive synergistic interaction of metamizol s.c., a nonsteroidal antiinflammatory drug (NSAID), and tramadol s.c., an atypical opioid (opioid receptor agonist), were administered alone or in combination. In the present study, the antinociceptive efficacy and the possible development of pharmacological tolerance produced by the combination tramadol plus metamizol during a 4-day treatment in rats using the plantar test was evaluated. Male Wistar rats were s.c. injected with tramadol (17.8 mg/kg), metamizol (177.8 mg/kg) or the combination tramadol plus metamizol three times a day for 4 days. Both metamizol and tramadol produced antinociceptive effects with a low rate trend towards tolerance development at the end of the treatment. The antinociceptive efficacy of tramadol and metamizol co-administration gradually decreased after the second injection. These data suggest that when the combination is given in a unique administration it results in an important potentiation of their individual antinociceptive effects. But, the repeated coadministration of tramadol plus metamizol results in a development of tolerance.

Antinociceptive effects of a new sigma-1 receptor antagonist (N-(2-morpholin-4-yl-ethyl)-2-(1-naphthyloxy)acetamide) in two types of nociception

Pain has become an active clinical challenge due its etiological heterogeneity, symptoms and mechanisms of action. In the search for new pharmacological therapeutic alternatives, sigma receptors have been proposed as drug targets. This family consists of sigma-1 and sigma-2 receptors. The sigma-1 system is involved in nociception through its chaperone activity. Additionally, it has been shown that agonist to these receptors promote related sensitisation and pain hypersensitisation, suggesting the possible use of antagonists for sigma-1 receptors as an alternative therapy. The aim of this study was to evaluate the antinociceptive effect of a new sigma-1 receptor antagonist N-(2-morpholin-4-yl-ethyl)-2-(1-naphthyloxy)acetamida (NMIN) in two types of pain (arthritic and neuropathic) and to compare its efficacy and potency with reference drugs. The antinociceptive effects of NMIN were quantitatively evaluated using the pain-induced functional impairment model in the rat and the acetone test in a rat model of neuropathic pain. NMIN (sigma-1 receptor affinity of 324nM) did not show any antinociceptive activity in the arthritic pain model but showed a dose-dependent anti-allodynic effect in neuropathic pain. NMIN showed a similar efficacy compared to the effects obtained with morphine and the sigma-1 antagonist BD-1063. However, these reference drugs showed increased potency compared with NMIN. Our results suggest that sigma-1 receptors may play an important direct role in neuropathic pain but not in arthritic pain, supporting the hypothesis that NMIN may be useful for the treatment of neuropathic pain.

The Antinociceptive Effects of Tramadol and/or Gabapentin on Rat Neuropathic Pain Induced by a Chronic Constriction Injury

Preclinical Research

HPLC Method with Solid-Phase Extraction for Determination of (R)- and (S)-Ketoprofen in Plasma without Caffeine Interference: Application to Pharmacokinetic Studies in Rats

A fast and reproducible high-performance liquid chromatography method has been developed for the determination of (R)- and (S)-ketoprofen. Ketoprofen enantiomers were determined in plasma samples (50 µL), after solid-phase extraction, using diclofenac as internal standard. Analyses were performed on a (S, S)-Whelk-O 1 stainless steel column (5 µm, 250 × 4.6 mm) using hexane-ethanol-acetic acid (93:7:0.5, v/v/v) as the mobile phase and detection at 254 nm. The method was selective for ketoprofen enantiomers in the presence of caffeine and endogenous plasma compounds. Standard curves were linear (R(2) > 0.999) over the concentration range of 0.25-12.50 and 0.25 µg/mL was taken as the limit of quantification. The intra- and interday precision (relative standard deviation) values were <15.0% and the accuracy (relative error) was within ±12.0% at 1.0, 5.0 and 10.0 µg/mL. Enantiomer recoveries yielded 100.0 ± 15%. No significant differences were determined in plasma samples stored at room temperature for 24.0 h, after two freeze-thaw cycles, and between 0 and 4 weeks at -20°C (P > 0.05). The validated method was successfully applied in determination of (S)-ketoprofen in Wistar rats after oral administration of 3.2 mg/kg of (S)-ketoprofen alone or 3.2 mg/kg of (S)-ketoprofen + 17.8 mg/kg of caffeine.

Effect of tramadol on metamizol pharmacokinetics and pharmacodynamics after single and repeated administrations in arthritic rats

Combined administration of certain doses of opioid compounds with a non-steroidal anti-inflammatory drug can produce additive or supra-additive effects while reducing unwanted effects. We have recently reported that co-administration of metamizol with tramadol produces antinociceptive effect potentiation, after acute treatment. However, none information about the effect produced by the combination after chronic or repeated dose administration exists. The aims of this study were to investigate whether the antinociceptive synergism produced by the combination of metamizol and tramadol (177.8 + 17.8 mg/kg, s.c. respectively) is maintained after repeated treatment and whether the effects observed are primarily due to pharmacodynamic interactions or may be related to pharmacokinetics changes. Administration of metamizol plus tramadol acute treatment significantly enhanced the antinociceptive effect of the drugs given alone (P < 0.05). Nevertheless, this effect decreased about 53% after the chronic treatment (3 doses per day, for 4 days). No pharmacokinetic interaction between metamizol and tramadol was found under acute treatment (P > 0.05). The mechanism involved in the synergism of the antinociceptive effect observed with the combination of metamizol and tramadol in single dose cannot be attributed to a pharmacokinetic interaction, and other pharmacodynamic interactions have to be considered. On the other hand, when metamizol and tramadol were co-administered under repeated administrations, a pharmacokinetic interaction and tolerance development occurred. Differences found in metamizol active metabolites’ pharmacokinetics (P < 0.05) were related to the development of tolerance produced by the combination after repeated doses. This work shows an additional preclinical support for the combination therapy. The clinical utility of this combination in a suitable dose range should be evaluated in future studies.

Antinociceptive Activity of Metamizol Metabolites in a Rat Model of Arthritic Pain

Preclinical Research

Pharmacokinetics and pharmacodynamics of metamizol in co‐administration with morphine under acute and chronic treatments in arthritic rats

To investigate the relationship between metamizol pharmacokinetics and the antinociceptive effect produced after subcutaneous administration of metamizol (177.8 mg/kg) alone or in combination with morphine (3.2 mg/kg), under acute and chronic treatments.