Tonic versus phasic pain: dose-related effects of ketoprofen

Electroencephalography and analgesics

To assess centrally mediated analgesic mechanisms in clinical trials with pain patients, objective standardized methods such as electroencephalography (EEG) has many advantages. The aim of this review is to provide the reader with an overview of present findings in analgesics assessed with spontaneous EEG and evoked brain potentials (EPs) in humans. Furthermore, EEG methodologies will be discussed with respect to translation from animals to humans and future perspectives in predicting analgesic efficacy. We searched PubMed with MeSH terms 'analgesics', 'electroencephalography' and 'evoked potentials' for relevant articles. Combined with a search in their reference lists 15 articles on spontaneous EEG and 55 papers on EPs were identified. Overall, opioids produced increased activity in the delta band in the spontaneous EEG, but increases in higher frequency bands were also seen. The EP amplitudes decreased in the majority of studies. Anticonvulsants used as analgesics showed inconsistent results. The N-methyl-D-aspartate receptor antagonist ketamine showed an increase in the theta band in spontaneous EEG and decreases in EP amplitudes. Tricyclic antidepressants increased the activity in the delta, theta and beta bands in the spontaneous EEG while EPs were inconsistently affected. Weak analgesics were mainly investigated with EPs and a decrease in amplitudes was generally observed. This review reveals that both spontaneous EEG and EPs are widely used as biomarkers for analgesic drug effects. Methodological differences are common and a more uniform approach will further enhance the value of such biomarkers for drug development and prediction of treatment response in individual patients.

Tonic stimulation of the pharyngeal mucosa causes pain and a reversible increase of inflammatory mediators

Objective and design

To develop a model of the inflammatory component of non-infectious sore throat using tonic stimulation and quantification of inflammatory mediators in pharyngeal lavage fluid.

Material or subjects

Forty-five healthy volunteers.

Treatment

Cold dry air.

Method

Tonic stimulation of the pharynx was achieved using a constant stream of cold dry air to the back of the throat. Following optimization of stimulation conditions (phase 1), pharyngeal pain, irritation, and swallowing discomfort were assessed using visual analog scales, and the concentration of inflammatory markers were measured in pharyngeal lavage fluid (phase 2).

Results

Optimum conditions for tonic pharyngeal stimulation were cold dry air at 12 °C, relative humidity 20 %, at a flow rate of 12 L/min for 15 min. Analysis of pharyngeal lavage fluid collected 5 min after stimulation showed significant increases in prostaglandin E₂ (P = 0.018), thromboxane B₂ (P < 0.001), and substance P (P < 0.001), but no increase in peptidoleukotriene. When the stimulus was removed, the level of inflammatory markers in pharyngeal lavage fluid returned to baseline by 30 min post-stimulation. These objective measures mirrored subjective pain ratings.

Conclusions

Tonic stimulation of the pharyngeal mucosa with cold dry air causes pain and an increase of inflammatory mediators which are reversible.

Electronic supplementary material

The online version of this article (doi:10.1007/s00011-013-0663-7) contains supplementary material, which is available to authorized users.

Dose range finding study for the efficacy of meloxicam administered prior to sodium urate-induced synovitis in cats

OBJECTIVE

To determine the lowest efficacious dose of oral meloxicam for relieving pain in cats with a sodium urate (SU)-induced acute inflammatory synovitis.

STUDY DESIGN

Randomized, blinded, controlled, and four-way crossover study.

ANIMALS

Eight surgically neutered cats (four males, four females) paired according to sex.

METHODS

Each pair of cats was treated with 0 (placebo), 0.025, 0.05, or 0.075 mg kg(-1) oral meloxicam once daily for 4 days prior to injection, into alternating stifles, of 1 mL of 20 mg mL(-1) SU crystals, beginning with the right stifle. Each cat received each of the four treatments, separated by at least 21 days. Analgesic efficacy was evaluated based on objective (e.g., pressure mat data total force, contact pressure, and contact area) and subjective (e.g., scores for Analgesia Scale [AS], Lameness Scale [LS], and Visual Analog Scale [VAS]) outcome measures for pain assessment. All outcome measures were recorded before and during 30 hours after SU injection. The pre-defined primary outcome measure was the area under the response-time curve (AUC(0-30) hours) of the total force of the injected limb. Data were analyzed by analysis of variance. A sequential test procedure was applied and the test sequence stopped in case of a nonsignificant result.

RESULTS

Meloxicam at doses of 0.05 and 0.075 mg kg(-1) day(-1) PO was significantly different from placebo for the pre-defined primary outcome measure (i.e., AUC(0-30) hours of total force). All tested meloxicam doses were lower than placebo for the subjective outcome measures (i.e., AUC(0-30) hours of AS, LS, and VAS).

CONCLUSIONS AND CLINICAL RELEVANCE

The lowest efficacious dose of meloxicam for relieving pain in cats with an SU-induced synovitis was 0.05 mg kg(-1) day(-1) PO according to the pre-defined primary outcome measure. However, lower doses may also be effective as seen in the subjective outcome measures.

Caffeine accelerates absorption and enhances the analgesic effect of acetaminophen

The aim of this study was to determine the analgesic effect of acetaminophen compared to a combination of both caffeine and acetaminophen or caffeine alone using tonic and phasic pain stimulation. Twenty-four subjects were treated orally with 1000 mg acetaminophen, 130 mg caffeine, and a combination of both in a 4-way crossover, double-blind, placebo-controlled study. Pharmacokinetics and analgesic effects were assessed by means of an experimental pain model based on pain-related cortical potentials after phasic stimulation of the nasal mucosa with CO(2) and based on pain ratings after tonic stimulation with dry air. Analgesic effects of acetaminophen and acetaminophen plus caffeine but not caffeine alone caused a significant reduction of pain-related cortical potentials beginning 30 minutes after medication. The combination demonstrated an enhanced effect throughout the observation time up to 3 hours. Caffeine accelerated acetaminophen absorption, indicated by enhanced early AUCs. Significant analgesic effects of the combination on tonic pain ratings were found throughout the observation time as compared to acetaminophen and placebo. In this study, caffeine enhanced and prolonged the analgesic activity of acetaminophen.

Isobolographic analysis of the antinociceptive interactions between ketoprofen and paracetamol

The present study was undertaken to evaluate the antinociceptive interaction between paracetamol and ketoprofen. The antinociceptive effect of oral administration of the drugs alone or in combination was evaluated using the mouse abdominal constriction test. The data were interpreted by isobolographic analysis to establish the nature of the interaction. The effective dose that produced 50% antinociception (ED(50,mix)) was calculated from the log dose-response curve of fixed-ratio combinations of paracetamol with ketoprofen. This ED(50,mix) was compared to the theoretical additive ED(50,add) by isobolographic analysis. The experimental ED(50,mix) was found to be significantly smaller than the theoretically calculated ED(50,add), indicating a synergistic antinociceptive interaction between ketoprofen and paracetamol. Pharmacokinetic studies were carried out with mice treated with combined ketoprofen (12 mg/kg) and paracetamol (36 mg/kg). Plasma levels of ketoprofen were not changed by concurrent paracetamol treatment, and similarly no statistically significant difference was observed between paracetamol alone and the combination with ketoprofen. The pharmacokinetic analysis revealed that the combination of ketoprofen with paracetamol exerted a synergistic (supra-additive) interaction that was not associated with a pharmacokinetic interaction. The results of this study demonstrate significant synergism between ketoprofen and paracetamol.

Biphasic effect of apomorphine on rat nociception and effect of dopamine D2 receptor antagonists

Studies on the effect of dopaminergic agonists in behavioral measures of nociception have gathered numerous but rather conflicting data. We studied the effects of the D(1)/D(2) receptor agonist apomorphine, as well as the modulatory effects of (S)-(-)-sulpiride (selective D(2) receptor antagonist) and domperidone (peripheral D(2) receptor antagonist), on thermal, mechanical and chemical nociception on rats. Apomorphine induced a biphasic dose-response relationship, low doses producing hyperalgesia and high doses inducing antinociception. Tonic (chemical) pain was more sensitive to apomorphine than phasic (thermal and mechanical thresholds) pain. (S)-(-)-sulpiride, but not domperidone, fully antagonized the antinociceptive effect of apomorphine in all three measures of nociception, pointing to a participation of D(2) dopaminergic receptors for the antinociceptive action of apomorphine. Although spinal sites for dopaminergic ligands mechanistically may account for the effects observed, involvement of dopaminergic receptors of the forebrain could probably explain better the antinociceptive effects of apomorphine, especially in chemical tonic pain.

Sex differentiated responses to intranasal trigeminal stimuli

The aim of the study was to address sex-related hemispheric differences in trigeminal event-related potentials while controlling for the subjects' olfactory sensitivity. Event-related potentials to lateralized stimulation using the trigeminal stimulant CO(2) were recorded in 28 healthy young subjects (16 women). There was no sex-related difference in olfactory sensitivity. Results indicated a sex-differentiated response to trigeminally induced pain. Women were found to have generally higher amplitudes and shorter latencies of the late positive component than men. Moreover, men and women exhibited different hemispheric activations in that women expressed shorter latencies over the left hemisphere than men. The pronounced sex-related difference of the late positive component suggests a cognitive/emotional impact on the processing of intranasal pain as indicated by others.

Comparison of the antinociceptive activity of two new NO-releasing derivatives of the NSAID S-ketoprofen in rats

1 Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit cyclooxygenase (COX) enzymes inducing analgesic, anti-inflammatory and antipyretic actions. They are not devoid of severe side effects and so, the search for new compounds with similar or higher effectiveness and a lower incidence of undesired actions is important. Nitric oxide (NO)-releasing NSAIDs resulted from this search. 2 We have compared the antinociceptive effectiveness of cumulative doses of two new NO-releasing derivatives of S-ketoprofen, HCT-2037 and HCT-2040, using the recording of spinal cord nociceptive reflexes in anesthetized and awake rats and after intravenous and oral administration. 3 S-ketoprofen and HCT-2040 were equieffective in reducing responses to noxious mechanical stimulation after i.v. administration in anesthetized animals (ID50s: 1.3+/-0.1 and 1.6+/-0.2 micromol kg(-1) respectively), but did not modify wind-up. HCT-2037 was two-fold more potent (ID50 of 0.75+/-0.1 micromol kg(-1)) in responses to mechanical stimuli and very effective in reducing wind-up (63+/-17% of control; P<0.01; MED: 0.4 micromol kg(-1)), indicating a greater activity than the parent compound. 4 In awake animals with inflammation, HCT-2037 p.o. fully inhibited mechanical allodynia, 91+/-12% reduction, and hyperalgesia, 94+/-8% reduction. Equivalent doses of S-ketoprofen only partially reduced either allodynia (50+/-11%) or hyperalgesia (40+/-4%). The effect on responses to noxious thermal stimulation was similar for the two compounds. 5 We conclude that the molecular changes made in the structure of S-ketoprofen including an NO moiety in its structure, improve the antinociceptive profile of the compound opening new perspectives in a safer use of NSAIDs as analgesic drugs.

Pain in the trigeminal system: irritation of the nasal mucosa using short- and long-lasting stimuli

The paper describes methods which allow intranasal irritation using short- and long-lasting painful stimuli in humans. Short-lasting pain is induced by gaseous CO(2), while long-lasting pain is induced by a stream of dry air. Both models have been explored regarding their major determinants, e.g. stimulus duration, stimulus intensity, or repeated stimulation. Short-lasting, non-inflammatory pain stimuli seem to provide specific indicators of A(delta)-fiber function, while responses to long-lasting, inflammatory pain appear to be indicative of C-fiber function. Responses to both types of painful stimuli are modulated by analgesic drugs. As these well-investigated models allow the detailed and precise analysis of modulatory effects on intranasal nociception, they appear to be suited for the investigation of subtle changes of intranasal irritation, e.g. induced by environmental agents.

Analgesic profile of peroral and topical ketoprofen upon low pH-induced muscle pain

Topical analgesics are widely marketed for treatment of muscle and joint pain. We have recently developed a model of muscle pain and have used this model to evaluate the efficacy of commercially available topical and peroral ketoprofen in order to evaluate the time- and dose-dependence of analgesia. In the present study, we examined the dose- (0, 50, and 100 mg) and time-dependence (hourly to 8 h) of commercially available peroral and topical ketoprofen. In order to achieve infusion times of 8 h (and thus study the time course of analgesic action), we adapted the model of low pH-induced muscle pain in humans to these requirements by applying the infusions continuously for 10 min every hour for 8 h. We found that the 10 min infusion produced reliable and consistent pain levels that were reproducible over the 8 h of the study. The study was performed double-blind, randomized, and with a 1-week interval between each of five different sessions (cross-over). Altogether six volunteers underwent intramuscular infusions of isotonic phosphate-buffered saline solution of pH 5.2; during each 8 h session the infusion was switched on eight times with a duration of 10 min at 50 min intervals (there was no infusion during the 50 min interval). The intramuscular infusion of low pH phosphate buffer induced a localized dull-aching or stinging muscle pain sensation; the flow rate of the pH infusion was individually adjusted to induce pain of a magnitude of 20% on a visual analogue scale (ranging from "no pain" (0%) to "unbearable pain" (100%)). Twenty minutes after starting the infusion the volunteers received a capsule with either a placebo or 50 or 100 mg ketoprofen perorally and, in addition, either placebo gel or 50 or 100 mg of a 2.5% commercial ketoprofen gel was applied topically to the skin. One of the sessions included a placebo gel and an oral placebo. The intensity of the recurrent pain stimulus was significantly reduced by 59% following administration of 100 mg peroral ketoprofen within the first 3 h (P<0.03, Wilcoxon test); this analgesia lasted up to the sixth hour of the experimental protocol. Oral ketoprofen (50 mg) was less effective and reduced the pain intensity by 45% (P<0.05) from only the second to the third hour. In contrast, pain reduction after topical ketoprofen application was not of the same magnitude but appeared to be faster to develop (with a maximum effect within 1 h) on average. The maximum pain suppression with 100 mg topical 2.5% ketoprofen gel was by 51% (significant with P<0.03), while 50 mg topical ketoprofen produced a non-significant reduction of 29%. The apparent analgesia was rapid to develop but transient and pain ratings increased back to baseline values within 3 h for the 100 mg dose and within 2 h for the 50 mg dose. This data suggests that topical application of commercial gel-based systems does not provide long-lasting analgesia in the muscle when compared to perorally-dosed ketoprofen. In addition, the data show that even doses of 100 mg peroral ketoprofen do not provide complete relief of muscle pain.

Effects of azapropazone on pain-related brain activity in human subjects

1. The dose-related effects of azapropazone on (i) event-related and spontaneous EEG-activity and (ii) the subjects' pain ratings were investigated using an experimental human pain model based on both chemo-somatosensory event-related potentials (CSSERP) and subjects' pain ratings. 2. Healthy subjects (n = 20) participated in a placebo-controlled, randomized, double-blind, four-way cross-over study. Single doses of azapropazone (300 mg, 600 mg and 1200 mg) and placebo were administered intravenously. Each experiment consisted of five sessions (before and 1, 2, 4 and 8 h after administration of the medication). Each session lasted for approximately 40 min. In the first 20 min, pain was induced by short CO2-stimuli presented to the right nostril (phasic pain; interstimulus interval 30 s) and EEG was recorded from five positions. CSSERPs were obtained in response to painful CO2-stimuli. In the following 20 min period, tonic pain was induced by a constant stream of dry air introduced in the left nostril. Subjects rated the intensity of both phasic and tonic pain by means of a visual analogue scale. Additionally, a frequency analysis of the spontaneous EEG was performed. 3. Azapropazone reduced the pain-related CSSERP-amplitudes at frontal and parietal recording positions. This topographical pattern was observed in previous studies with opioids, while NSAIDs such as flurbiprofen and ketoprofen exerted effects at frontal and central positions. In contrast to other NSAIDs, administration of azapropazone resulted in a reduction of the frequency bands alpha 1, delta and theta of the spontaneous EEG. At the subjective level, analgesic effects of azapropazone were observed in the ratings of tonic pain. 4. Analgesic properties of azapropazone were demonstrated in man. The topographical pattern of the changes in the CSSERPs and the effects on EEG background activity suggest a central component of the analgesic action of azapropazone.