Melatonin synergizes with the antinociceptive effect of N-palmitoylethanolamide and paracetamol.

Melatonin has been shown to have an antinociceptive effect and its administration could enhance the antinociceptive effect of other drugs. This study assessed the antinociceptive effects of melatonin in combination with paracetamol and N-palmitoylethanolamide (PEA) using the formalin test in mice. Melatonin, paracetamol, and PEA were administered intraplantarly (paw) alone or combined to mice. A concentration-response curve was generated to determine the concentration needed to reach 30% of the maximal antinociceptive effect (EC30). Melatonin, paracetamol and PEA induced a concentration-dependent antinociceptive effect in both phases of the formalin test, being PEA more potent (EC30 = 7.4±0.2 mg/paw) than melatonin (EC30 = 20.5±3.1 mg/paw) or paracetamol (EC30 = 41.8±2.6 mg/paw). Combinations of melatonin with paracetamol or PEA also induced a concentration-dependent antinociceptive effect in the formalin test. Isobolographic analysis showed that melatonin interacts synergistically with either paracetamol or PEA to reduce formalin-induced inflammatory pain. However, the experimental values of EC30 were significantly smaller than those calculated theoretically.

The peripheral antinociceptive effect of resveratrol is associated with activation of potassium channels.

The possible participation of K(+) channels in the antinociceptive action induced by resveratrol was assessed in the 1% formalin test. Local administration of resveratrol produced a dose-dependent antinociception in the second phase of the test. The antinociception produced by resveratrol was due to a local action as its administration in the contralateral paw was not active. Local pretreatment of the injured paw with glibenclamide, tolbutamide or glipizide (ATP-sensitive K(+) channel inhibitors) did not modify resveratrol-induced antinociception. In contrast, charybdotoxin and apamin (large and small conductance Ca(2+) activated-K(+) channel blockers, respectively), 4-aminopyridine or tetraethylammonium (voltage-dependent K(+) channel inhibitors) dose-dependently prevented resveratrol-induced antinociception. Local peripheral administration of glibenclamide, but not charybdotoxin or apamin, significantly reduced the antinociceptive effect produced by peripheral morphine (positive control). At the highest effective doses, none of the drugs used induced behavioral side effects as revealed by the evaluation of stepping, righting, corneal and pinna reflexes. In addition, when given alone, none of the inhibitors modified the nociceptive behavior induced by 1% formalin. The results suggest that resveratrol opens large and small conductance Ca(2+)-activated K(+) channels, but not ATP-sensitive K(+) channels, in order to produce its peripheral antinociceptive effect in the formalin test. The participation of voltage-dependent K(+) channels was also suggested, but since non-selective inhibitors were used the data awaits further confirmation.

Evidence for the participation of the nitric oxide-cyclic GMP pathway in the antinociceptive action of meloxicam in the formalin test.

The involvement of the nitric oxide-cyclic GMP pathway in the antinociceptive action of the cyclooxygenase-2 preferential inhibitor meloxicam was assessed in the rat formalin test. Rats received local pretreatment with saline or meloxicam and then 50 microl of dilute formalin (1%). Local administration of meloxicam produced a dose-dependent antinociception in the second phase of the formalin test. The antinociception produced by meloxicam was due to a local action as its administration in the contralateral paw was ineffective. Local pretreatment of the paws with saline or N(G)-D-nitro-arginine methyl ester (D-NAME) did not affect the antinociception produced by meloxicam. However, N(G)-L-nitro-arginine methyl ester (L-NAME, a NO synthesis inhibitor) or 1H-(1,2,4)-oxadiazolo(4,2-a)quinoxalin-1-one (ODQ, a soluble guanylyl cyclase inhibitor) blocked in a dose-dependent way the effect of meloxicam. It is concluded that the peripheral antinociceptive effect of meloxicam involves a local NO-cyclic GMP pathway.

Participation of the nitric oxide-cyclic GMP-ATP-sensitive K(+) channel pathway in the antinociceptive action of ketorolac.

The involvement of nitric oxide (NO), cyclic GMP and ATP-sensitive K(+) channels in the antinociceptive effect of ketorolac was assessed using the formalin test in the rat. Local administration of ketorolac in a formalin-injured paw produced a dose-dependent antinociceptive effect due to a local action, as drug administration in the contralateral paw was ineffective. Pretreatment of the injured paw with N(G)-L-nitro-arginine methyl ester (L-NAME, an NO synthesis inhibitor), 1H-(1,2,4)-oxadiazolo(4,2-a)quinoxalin-1-one (ODQ, a soluble guanylyl cyclase inhibitor) or glibenclamide (an ATP-sensitive K(+) channel blocker) prevented ketorolac-induced antinociception. However, pretreatment with saline or N(G)-D-nitro-arginine methyl ester (D-NAME) did not block antinociception. Local administration of S-nitroso-N-acetylpenicillamine (SNAP, an NO donor) was inactive by itself, but increased the effect of ketorolac. The present results suggest that the antinociceptive effect of ketorolac involves activation of the NO-cyclic GMP pathway, followed by an opening of ATP-sensitive K(+) channels at the peripheral level.

Sildenafil produces antinociception and increases morphine antinociception in the formalin test.

The antinociceptive activity of an inhibitor of phosphodiesterase 5 alone or combined with morphine was assessed in the formalin test. Local administration of 1-[4-ethoxy-3-(6, 7-dihydro-1-methyl-7-oxo-3-propyl-1H-pyrazolo [3, 4-d]pyrimidin-5-yl)phenylsulfonyl]-4-methyl piperazine (sildenafil, inhibitor of phosphodiesterase 5) produced a dose-dependent antinociceptive effect in the second phase of the formalin test in female Wistar rats. In contrast, morphine produced antinociception in both phases. Sildenafil significantly increased the morphine-induced antinociception. The antinociception produced by the drugs alone or combined was due to a local action, as its administration in the contralateral paw was ineffective. Pretreatment of the paws with N(G)-L-nitro-arginine methyl ester (L-NAME, nitric oxide (NO) synthesis inhibitor), 1H-[1,2, 4]-oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, guanylyl cyclase inhibitor) or naloxone blocked the effect of the combination. Results suggest that opioid receptors, NO and cyclic GMP are relevant in the combination-induced antinociception. In conclusion, sildenafil produced antinociception and increased that produced by morphine, probably through the inhibition of cyclic GMP degradation.

Evidence for the involvement of nitric oxide in the antinociceptive effect of ketorolac.

The involvement of nitric oxide in the antinociception produced by ketorolac was assessed using the pain-induced functional impairment model in the rat: 800 micrograms of NG-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthesis, or saline was injected intra-articularly in a hind limb joint previously injured with uric acid. Animals then received ketorolac, dipyrone or no drug. Ketorolac and dipyrone produced a significant antinociceptive effect which was reduced by pretreatment with NG-nitro-L-arginine methyl ester, but not with saline. It is concluded that the antinociceptive effect of both drugs involves the local participation of nitric oxide.

Effect of coadministration of caffeine and either adenosine agonists or cyclic nucleotides on ketorolac analgesia.

Caffeine potentiation of ketorolac-induced antinociception in the pain-induced functional impairment model in rats was assessed. Caffeine alone was ineffective, but increased the effect of ketorolac without affecting its pharmacokinetics. Intra-articular administration of adenosine and N6-cyclohexyladenosine (CHA, an adenosine A1 receptor agonist), but not 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS-21680, an adenosine A2A receptor agonist), significantly increased ketorolac antinociception. This effect was not local, as contralateral administration was also effective. Ipsilateral and contralateral administration of adenosine and CHA also increased antinociception by ketorolac-caffeine. Intra-articular 8-Bromo-adenosine cyclic 3',5'-hydrogen phosphate sodium or 8-Bromo-guanosine-3',5'-cyclophosphate sodium (cGMP) given ipsilaterally or contralaterally did not affect ketorolac-induced antinociception. Nevertheless, ipsilateral, but not contralateral, administration of 8-Br-cGMP significantly increased antinociception by ketorolac-caffeine, suggesting a local effect. The results suggest that caffeine potentiation of ketorolac antinociception is mediated, at least partially, by a local increase in cGMP and rule out the participation of adenosine receptor blockade.

Evidence for the involvement of the nitric oxide-cGMP pathway in the antinociception of morphine in the formalin test.

The effect of inhibition of nitric oxide synthesis and guanylate cyclase on the peripheral antinociceptive effect of morphine was assessed by using the formalin test in the rat. Saline, N(G)-monomethyl-L-arginine, a nitric oxide synthesis inhibitor (50 microg) and methylene blue, a guanylate cyclase inhibitor (500 microg), did not exhibit any antinociceptive activity. However, morphine (10 microg) produced a significant antinociceptive effect in phases 2a and 2b, which was reduced by pretreatment with either N(G)-monomethyl-L-arginine or methylene blue. These results suggest that the local administration of morphine induces antinociception by the activation of the L-arginine-nitric oxide-cGMP pathway.

Effect of caffeine coadministration and of nitric oxide synthesis inhibition on the antinociceptive action of ketorolac.

The effects of caffeine and nitric oxide synthesis inhibition on the antinociceptive action of ketorolac were assessed using the pain-induced functional impairment model in the rat. Nociception was induced by the intra-articular injection of uric acid. Ketorolac, but not caffeine, produced an antinociceptive effect which was reduced by NG nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthesis. Caffeine coadministration potentiated the ketorolac effect. L-NAME induced a dose-dependent reduction of this potentiation. The results suggest the participation of the L-arginine-nitric oxide-cyclic GMP pathway in the caffeine potentiation of ketorolac-induced antinociception.

Sildenafil increases diclofenac antinociception in the formalin test.

The antinociceptive activity of an inhibitor of phosphodiesterase 5, alone or combined with diclofenac, was assessed in the formalin test. Local administration of diclofenac produced a significant antinociception in both phases of the formalin test in female Wistar rats. In contrast, 1-[4-ethoxy-3-(6,7-dihydro-1-methyl-7-oxo-3-propyl-1H-pyrazolo [3,4-d]pyrimidin-5-yl)phenylsulfonyl]-4-methyl piperazine (sildenafil, an inhibitor of phosphodiesterase 5) produced significant antinociception, only during the second phase of the formalin test. Non-effective doses of sildenafil (25-100 microg/paw) significantly increased the antinociceptive effect of an inactive dose of diclofenac (25 microg) in both phases of the test. The antinociception produced by the drugs alone or the combination was due to a local action, as its administration in the contralateral paw was ineffective. Since sildenafil is a potent and selective inhibitor of phosphodiesterase 5, our results suggest that this drug produced its antinociceptive activity, and increased that of diclofenac, probably through the inhibition of cyclic GMP degradation.

A review of the pharmacokinetic and pharmacodynamic factors in the potentiation of the antinociceptive effect of nonsteroidal anti-inflammatory drugs by caffeine.

Caffeine is an effective analgesic adjuvant because it increases the antinociceptive effect of NSAIDs while reducing the probability of side effects. The mechanism by which caffeine increases the antinociceptive action of NSAIDs does not appear to include a pharmacokinetic interaction. The potentiation appears to be due to a pharmacokinetic mechanism including actions at the central and the peripheral levels. Because caffeine shifts the effect-compartment concentration-effect relation of NSAIDs to the left and this relationship is sigmoidal, there is no potentiation if the NSAID concentrations are too high or too low with respect to EC(50). The best potentiation can be observed if the NSAID doses used yield effect-compartment concentrations in the vicinity of EC(50). Therefore further investigation of the PK/PD relations of caffeine-NSAID combinations for different pain states and intensities is needed to optimize the therapeutic use of these mixtures.

Evidence for a peripheral mechanism of action for the potentiation of the antinociceptive effect of morphine by dipyrone.

The potentiation of the antinociceptive effect of morphine by dipyrone (metamizol) and the possible participation of a peripheral mechanism on such synergism were studied with the use of the formalin test in the rat. Nociception was induced by the intraplantar injection of diluted formalin (1%) in the right hind paw. Local administration of either dipyrone or morphine in the site of injury produced a dose-dependent antinociceptive effect. When combined, noneffective doses of morphine (1.25 microg/paw) and dipyrone (100 microg/paw) produced a significantly greater antinociceptive effect compared with either drug alone or saline. The opioid antagonist naloxone partly reversed the effect of the dipyrone-morphine combination. On the other hand, the inhibitor of nitric oxide (NO) synthesis, N(G)-L-nitro-arginine methylester (L-NAME), but not its inactive isomer, D-NAME, completely antagonized the effect of the dipyrone-morphine combination. These results suggest that the potentiation of morphine-induced antinociception by dipyrone in the formalin test requires an important participation of local release of NO, activating the NO-cyclic GMP pathway at the peripheral level.

Evidence for the participation of the nitric oxide-cyclic GMP pathway in the antinociceptive effect of nimesulide.

The involvement of the nitric oxide-cyclic GMP pathway in the peripheral antinociception induced by the COX-2 preferential inhibitor nimesulide was assessed by using the formalin test in the rat. Intraplantar administration of nimesulide in the formalin-injured paw produced a significant antinociceptive effect that was due to a local action, because nimesulide administration in the contralateral paw was ineffective. Local pretreatment of the paws with saline or N(G)-D-nitro-arginine methyl ester (D-NAME, the inactive isomer of L-NAME) did not affect the antinociception produced by nimesulide. However, local administration of L-NAME (a nitric oxide synthesis inhibitor) or 1H-(1,2,4)-oxadiazolo(4, 2-a)quinoxalin-1-one (ODQ, a soluble guanylyl cyclase inhibitor) blocked the effect of nimesulide. Moreover, the antinociceptive effect of local nimesulide was potentiated by the coadministration of 3-morpholino-sydnonimine-HCl (SIN-1, a nitric oxide donor). It is concluded that nimesulide produces antinociception by a peripheral mechanism of action requiring activation of the nitric oxide-cyclic GMP pathway at the local level.

Pharmacokinetic-pharmacodynamic modeling: why?

At present, pharmacokinetic-pharmacodynamic (PK-PD) modeling has emerged as a major tool in clinical pharmacology to optimize drug use by designing rational dosage forms and dosage regimes. Quantitative representation of the dose-concentration-response relationship should provide information for prediction of the level of response to a certain level of drug dose. Several mathematical approaches can be used to describe such relationships, depending on the single dose or the steady-state measurements carried out. With concentration and response data on-phase, basic models such as fixed-effect, linear, log-linear, E(MAX), and sigmoid E(MAX) can be sufficient. However, time-variant pharmacodynamic models (effect compartment, acute tolerance, sensitization, and indirect responses) can be required when kinetics and response are out-of-phase. To date, methodologies available for PK-PD analysis barely suppose the use of powerful computing resources. Some of these algorithms are able to generate individual estimates of parameters based on population analysis and Bayesian forecasting. Notwithstanding, attention must be paid to avoid overinterpreted data from mathematical models, so that reliability and clinical significance of estimated parameters will be valuable when underlying physiologic processes (disease, age, gender, etc.) are considered.

Analgesic efficacy and bioavailability of ketorolac in postoperative pain: a probability analysis.

BACKGROUND: The analgesic efficacy and bioavailability of 30 mg intramuscular ketorolac was studied in 24 patients with severe or very severe postoperative pain. METHODS: Pain and pain relief were determined by a five-point verbal rating scale and data were submitted to a probability analysis. Ketorolac plasma levels were determined by high-performance liquid chromatography. RESULTS: Two patients chose not to finish the study; 22 patients completed the study achieving at least good pain relief. Of these 22 patients, 13 reached complete pain relief. Ketorolac was rapidly absorbed. Notwithstanding, pain relief increased gradually, showing considerable delay with regard to plasma concentrations. Analysis of the probability-time curves revealed that 25% of the patients obtained moderate pain relief at 7 min after ketorolac administration, 50% at 11 min, 75% at 29 min, and 95% at 60 min. Good pain relief was achieved in 25, 50, and 75% of the patients at 1.1, 1.8, and 2.7 h, respectively. Complete pain relief was achieved in 25% and 50% of the patients at 2.6 h and 3.7 h, respectively. The probability of exhibiting an acceptable pain relief in responsive patients for more than 5 h was 0.97. No serious side effects were detected. CONCLUSIONS: Results show that 30 mg intramuscular ketorolac is an adequate treatment for postoperative pain in the Mexican population. Therefore, the use of higher doses is not justified. Due to gradual installation of analgesia, administration of additional analgesic medication before 1 h is not recommended.

Pharmacokinetics of oral ranitidine in Mexicans.

The pharmacokinetics of oral ranitidine were studied in 24 Mexican male healthy volunteers. Subjects received a tablet containing 150 mg of ranitidine (Azantac, Glaxo de México, Mexico City) after an overnight fast and blood samples were drawn at several times for a period of 24 h. Ranitidine concentration in plasma was measured by high performance liquid chromatography and pharmacokinetic parameters were determined by non-compartmental analysis. Ranitidine plasma concentration increased with time, reaching a maximum of (mean +/- SEM) 484 +/- 34 ng/ml in 2.7 +/- 0.2 h. Plasma levels then decayed with a terminal half-life of 4.8 +/- 0.3 h. The area under the plasma concentration against time curve was 2440 +/- 126 ngh/ml. Oral ranitidine pharmacokinetic parameters in Mexicans appeared to be similar to those previously reported for Caucasians.

The use of the plasma concentration-effect relationship as a tool for the study of the mechanism of action of naloxone effects on mood and endocrine function.

The relationship between naloxone-plasma concentrations and their effects on mood and endocrine function was studied. Ten healthy volunteers received 1.0 mg/kg i.v. naloxone or placebo following a randomized double-blind design. Effects on mood, determined by a visual analogue scale and luteinizing hormone (LH) and naloxone-plasma concentrations were measured at selected times. Naloxone induced significant effects on confusion, bewilderment and indifference, and an increment in LH levels. The timecourse of the responses on confusion and bewilderment was similar to that of naloxone-plasma concentration, suggesting that these effects are directly related to the action of naloxone on its receptors. Responses for indifference and LH, however, exhibited a delayed onset. This delay could be due to an indirect action, i.e. to the participation of additional physiological mechanisms in a cascade-like manner. The results show that analysis of the concentration-effect relationship can be a useful tool for understanding naloxone effects on mood and endocrine function.

Evidence against the participation of mu- and kappa-opioid receptors in the analgesic activity of ketorolac in rats.

The possibility that activation of opioid receptors is involved in the analgesic activity of ketorolac was explored. The analgesic effects of ketorolac, of ketocyclazocine, the prototype kappa-agonist, and of morphine, the prototype mu-agonist, were assayed in the pain-induced functional impairment model in the rat. All three drugs induced a significant analgesic effect in this model. Naloxone was able to antagonize the effects of ketocyclazocine and morphine. However, the effect of ketorolac was not blocked by naloxone, although a high dose, 3.2 mg kg-1, capable of blocking kappa-receptors was used. It is concluded that activation of mu- or kappa-opioid receptors, by either a direct or an indirect mechanism, does not play a role in the analgesic activity of ketorolac.

Relationship between paracetamol plasma levels and its analgesic effect in the rat.

The relationship between plasma levels of paracetamol and its analgesic effect was studied in the rat using a model of pain-induced functional impairment (PIFI). Female Wistar rats received an intraarticular injection of 30% uric acid in the knee of the right hind limb, inducing its dysfunction. Animals then received oral paracetamol at doses of 178, 316 or 562 mg kg-1 and the recovery of functionality over time was considered as an expression of analgesia. Paracetamol plasma levels were determined by HPLC. Results showed that there is a direct relationship between paracetamol plasma levels and its analgesic effect that follows a sigmoidal model according to the Hill equation. The PIFI model appears to be a useful tool to establish pharmacokinetic/pharmacodynamic relationships for non-narcotic analgesics.

Characterization of the analgesic effect of paracetamol and caffeine combinations in the pain-induced functional impairment model in the rat.

The analgesic activities of paracetamol (100, 178, 316 and 562 mg kg-1), caffeine (10, 18, 32 and 56 mg kg-1) and combinations of these doses were tested on a pain-induced functional impairment model in the rat. Dysfunction of the right hind limb was induced by an intra-articular injection of 30% uric acid in the knee. Drugs were given orally and the recovery of functionality over time was considered as an expression of analgesia. Paracetamol alone induced a dose-dependent analgesic effect whereas caffeine alone did not show any activity at the assayed doses. Combinations of 316 mg kg-1 paracetamol with either 10, 18, 32 or 56 mg kg-1 caffeine yielded analgesic effects significantly greater than that of paracetamol alone. The highest potentiation was observed with a paracetamol-caffeine mixture of 316-32 mg kg-1. Caffeine coadministration, however, did not significantly change paracetamol plasma levels. No potentiation was obtained with other combinations. Paracetamol plasma levels and analgesic effect observed with administration of 316 mg kg-1 paracetamol alone or 316-32 mg kg-1 of paracetamol-caffeine were fitted to the sigmoidal Emax model according to the Hill equation. The curves obtained were parallel, but that of the combination was shifted to the left. It is concluded that caffeine is able to potentiate the analgesic effect of paracetamol by a pharmacodynamic mechanism, but this only occurs at certain dose combinations.