Effect of Systemic Adenosine on Pain andSecondary Hyperalgesia Associated With theHeat/Capsaicin Sensitization Modelin Healthy Volunteers

Jesper Dirks, M.D., Karin L. Petersen, M.D., Michael C. Rowbotham, M.D.,and Jørgen B. Dahl, M.D., Ph.D.

Background and Objectives: Adenosine is an endogenous compound that may have analgesic effects. Resultsfrom clinical trials are not consistent, however, and there is a need for large-scale, randomized, placebo-controlled studies to clarify the role of adenosine in the treatment of pain states, including acute nociceptive painand pain involving central sensitization.

Methods: The analgesic and antihyperalgesic effect of systemic adenosine on the heat/capsaicin sensitizationmodel of experimental pain was investigated in 25 healthy human volunteers. Sensitization was produced byheating the skin to 45°C for 5 minutes, followed by a 30-minute application of 0.075% capsaicin cream, andmaintained by periodically reheating the sensitized skin to 40°C for 5 minutes at 40-minute intervals. Subjectsreceived intravenous adenosine 60 �g/kg/min or saline for 85 minutes. Areas of secondary hyperalgesia to vonFrey hair and brush stimulation, heat-pain detection thresholds (HPDTs) in normal and sensitized skin, andpainfulness of stimulation with 45°C for 1 minute (LTS) in normal skin were quantified before, during, and afterstudy drug infusion.

Results: Systemic adenosine had no effect on the area of secondary hyperalgesia to von Frey hair or brushstimulation, HPDT in normal or sensitized skin, or painfulness of LTS in normal skin.

Conclusion: We conclude that adenosine has no effect on acute nociceptive pain induced by heat stimulationor on secondary hyperalgesia induced by heat/capsaicin sensitization in healthy volunteers. Reg Anesth Pain Med2001;26:414-419.

Key Words: Adenosine, Allodynia, Central sensitization, Hyperalgesia, Pain.

Adenosine is an endogenous compound withmodulatory effects in both the peripheral and

central nervous system, mediated through specificcell-surface–associated receptors.1 Four P1-puriner-gic receptors subtypes designated A1, A2a, A2b, andA3 have now been cloned.2 In the periphery, A1

receptor activation is believed to result in antinoci-ception, whereas A2 and A3 receptor activation isbelieved to be pronociceptive. In the spinal cord, A1receptors have been demonstrated both pre- andpostsynaptically, and activation is believed to resultin reduction in neurotransmitter release as well asreduction of sensitization of central pain-transmit-ting neurons.1 Adenosine has been suggested to beinvolved in the analgesic action of both opioid-typeanalgesics as well as amitriptyline. Caffeine andtheophylline have been shown to be adenosine an-tagonists.3

Both peripheral, systemic, and intrathecal admin-istration of adenosine and its agonists have beenshown to have antinociceptive and antihyperalgesicproperties in a number of different animal models.1

In healthy human volunteers, systemic bolus ad-ministration (ranging from 10.6 mg to 37.1 mgadenosine) results in angina-like pain.4 In contrast,systemic administration of doses under 80 �g/kg/min have been shown to have antinociceptive and

From the Laboratory of Pain Physiology (J.D., J.B.D.), Depart-ment of Anaesthesiology, Herlev Hospital, Denmark; UCSF PainClinical Research Center, Departments of Neurology (K.L.P.,M.C.R.) and Anesthesia (M.C.R.), University of California, SanFrancisco.

Accepted for publication May 7, 2001.Supported by Danish Medical Research Council (Reg. No.

28809) and Novo Nordisk Foundation. K.L.P. is supported bythe VZV Foundation, Inc. M.C.R. is supported by NIH Grant K24NS02164.

Reprint requests: Jesper Dirks, M.D., Department of Anaes-thesiology, Herlev University Hospital, DK-2730 Herlev, Den-mark. E-mail: jedi@herlevhosp.kbhamt.dk

© 2001 by the American Society of Regional Anesthesia andPain Medicine.

1098-7339/01/2605-0017$35.00/0doi:10.1053/rapm.2001.22256

414 Regional Anesthesia and Pain Medicine, Vol 26, No 5 (September–October), 2001: pp 414–419

antihyperalgesic effect in healthy volunteers,5-9 surgi-cal patients,10-12 and in patients with chronic pain.13,14

However, as most of these studies have included onlya few volunteers or patients, and because the resultsare not consistent, there is a need for large-scale,randomized, placebo-controlled studies to clarify therole of adenosine in the treatment of various painstates, including acute nociceptive pain and pain in-volving central sensitization.

Human experimental pain models can simulateclinical pain conditions. An intense thermal and/orchemical stimulation of the skin will produce revers-ible cutaneous hyperalgesia, characterized by an areaof primary hyperalgesia corresponding to the stimu-lated skin surrounded by an area of secondary hyper-algesia in unstimulated skin.15,16 Through such mod-els temporary inflammatory changes and sensitizationof both the peripheral and the central nervous systemcan be studied.17 The heat/capsaicin sensitizationmodel is a recently developed, noninvasive humanexperimental pain model that combines noninvasivethermal and chemical methods of nociceptor stimula-tion to produce stable and long-lasting secondary hy-peralgesia. Because 2 stimuli are applied in sequenceat noninjurious intensity levels, the model has a lowpotential for skin injury, and thus only a minor risk oflasting skin injury for the volunteers.16 Secondaryhyperalgesia in this model was reliably and potentlysuppressed by administration of intravenous (IV) opi-oid18 and partly suppressed by IV lidocaine.19

The aim of the present study was to investigatethe effect of IV adenosine on acute nociceptive paininduced by heat stimuli and heat/capsaicin-inducedcutaneous hyperalgesia in healthy volunteers.

Methods

We studied 25 healthy unmedicated male volun-teers, ages 20 to 30 years. The volunteers were freeof caffeine and nicotine for 24 hours before eachstudy day. Informed consent was obtained from allvolunteers, and the study was approved by theregional Ethics Committee and the Danish NationalHealth Board.

The study was performed in a quiet room withsubjects in a semi-supine position. Each volunteerhad been familiarized with the measurement pro-cedures on a separate day. All thermal stimulationsand measurements were performed with a comput-er-controlled thermode (12.5 cm2) (Thermotest;Somedic A/B, Horby, Sweden).

Induction and Maintenance of Heat/CapsaicinSensitization

Sensitization was produced by heating the skin ofthe dominant forearm to 45°C for 5 minutes with

the thermode. Immediately thereafter, the skin wascovered with capsaicin cream (0.075% Capsaicin;Clay-Park Labs Inc, Bronx, NY) for 30 minutes (seeFig 1A). The sensitization was rekindled twice dur-ing each infusion session by applying the thermodeat 40°C for 5 minutes on the stimulation location.The first rekindle (RK-1) was performed 40 minutesafter the capsaicin was removed (time [t] � 80minutes) and the second rekindle (RK-2) 40 min-utes later (t � 120). The timing of the procedures issummarized in Fig 1B.

Measurement of Secondary Hyperalgesia

Immediately after removal of the capsaicin cream(t � 40 minutes) and after each rekindle (t � 80and 120 minutes), the area of secondary hyperal-gesia was quantified with a foam paint brush andwith a 21.5-g von Frey hair. The borders of hyper-algesia were determined by stimulating along 4 lin-ear paths arranged radially around the stimulationsite in steps of 5 mm at intervals of 1 second.Stimulation started in normal skin and continuedtoward the stimulation site until subjects reported aclear change in sensations (“burning,” “tender-ness,” “more intense pricking”). The borders weremarked with a felt pen and the distances measuredfor later surface-area calculations.

Heat Stimulation

Heat-pain detection threshold (HPDT), defined asthe lowest temperature perceived as painful, wasdetermined in normal skin on the nondominantforearm, and in sensitized skin on the dominantforearm. The baseline temperature of the thermodewas 32°C and the rate of increase was 1°C/s. Bypressing a button, subjects indicated when thethreshold was reached. If the cutoff limit (52°C)was reached before the pertinent threshold, thethermode returned automatically to the baselinevalue and 52°C was registered. Each threshold wascalculated as an average of 4 stimulations; stimula-tions were 6 to 10 seconds apart.

Subjects were further asked to rate the painful-ness of a 1-minute long 45°C heat stimulus (LTS)applied on nonsensitized skin on the electronic vi-sual analog scale (VAS) consisting of a vertical baron a computer screen (sample rate, 2 Hz) anchoredwith the descriptors “no pain” (numeric value � 0)and “worst possible pain” (numeric value � 100).The volunteers were able to see the VAS ratingscreen during each assessment and each VAS mea-surement was independent of the previous mea-surement. All LTS measurements were performedin normal skin on the nondominant forearm andrepeated 3 times during each session.

Adenosine in Cutaneous Hyperalgesia • Dirks et al. 415

Study Drug Infusion and Side Effects

The study design was double-blind, randomized,and with a cross-over. On 2 separate days subjectsreceived IV adenosine (60 �g/kg/min) or placebofor a total of 85 minutes. The infusion was initiatedimmediately after the heat/capsaicin sensitizationwas established and baseline measurements hadbeen performed (Fig 1, t � 45 minutes) and con-tinued until RK-2 and accompanying measure-ments were completed (Fig 1, t � 130 minutes).Heart rhythm was monitored through continuouselectrocardiogram during the infusion. Side effects(light-headedness, drowsiness, chest pressure, gen-eral discomfort, flushing, nausea, and dizziness)were assessed after 10 minutes of infusion (t � 55minutes) and after each measurement cycle (80minutes and 120 minutes, Fig 1B). Subjects wereasked to rate side effects on a 4-point verbal scale(0 � none; 1 � mild; 2 � moderate; 3 � severe).

Statistical Analysis

The sample size was based on a power calculationwhich showed that 25 volunteers were necessary toachieve 80% power to detect a 20% treatmentdifference in area of secondary hyperalgesia, with� � 0.05 (2-tailed). The painfulness of LTS was

Fig 1. (A) Heat/capsaicin sensiti-zation and rekindling procedure(see Methods section for details).(B) Timing of heat/capsaicin sen-sitization (H/C), rekindling (RK),and measurements (M1, M2, M3).

416 Regional Anesthesia and Pain Medicine Vol. 26 No. 5 September–October 2001

calculated as area under the curve and then con-verted to mean VAS values (0 to 100). Data arepresented as medians (lower and upper quartiles).Variables were evaluated with Wilcoxon’s test forpaired data. All significant P values were correctedwith Bonferroni’s test for repeated measurements.Before entering statistical analyses, data were nor-malized in relation to values obtained before ad-ministration of study drugs to achieve the samepoint of reference. P � .05 was considered statisti-cally significant. Calculations were performed usingStatgraphics Plus 7.0. (Manugistics, Inc, Rockville,MD).

Results

All of the 25 subjects completed both sessions.The data are presented in Table 1.

Combined sensitization with heat and capsaicinproduced areas of secondary hyperalgesia to vonFrey hair and brush stimulation that could bemapped easily in all subjects (Table 1). On the IVadenosine infusion day, the baseline areas of sec-ondary hyperalgesia to von Frey hair and brushstimulation were comparable to the placebo infu-sion day. During infusions, no differences were ob-served in areas of secondary hyperalgesia to vonFrey hair and brush stimulation between the pla-cebo and the adenosine infusion day (Table 1).

HPDTs were reduced after sensitization with heatand capsaicin compared with baseline measure-ments. HPDTs in sensitized and normal skin andpainfulness of LTS were not significantly differentbetween the placebo and the adenosine infusiondays (Table 1).

Side Effects

Chest pressure was observed more frequentlyduring infusion of adenosine compared with pla-cebo (9 v 2 volunteers, P � .05), and was rated as“mild” to “moderate” by all who experienced it. Theincidence of other side effects (light-headedness,drowsiness, general discomfort, flushing, nausea,and dizziness) was not different between studydays. All volunteers were able to cooperate fullyduring the various assessments without problems.Blistering, skin color, or any other changes werenot observed in any of the volunteers.

Discussion

Systemic adenosine had no effect on the size ofthe skin area displaying secondary hyperalgesia (al-lodynia) to brush or to mildly noxious von Frey hairstimulation. HPDTs in sensitized and normal skinand the painfulness of prolonged noxious heatstimuli (LTS) in normal skin were also unaffectedby adenosine. A post hoc power analysis revealedan intra-individual standard deviation of 30 of ourmain outcome parameter, area of secondary hyper-algesia to von Frey hair stimulation. This meansthat this study had a power of 80% to detect a 17%treatment difference in area of secondary hyperal-gesia, with � � 0.05 (2-tailed).

Our findings are in conflict with some of theresults from previous studies in human volun-teers.5-9

Two placebo-controlled studies have addressedthe effect of systemic adenosine on secondary hy-peralgesia induced by experimental pain models. In

Table 1. Area of Secondary Hyperalgesia, HPDTs, and VAS Rating During LTS (Real Data), in VolunteersReceiving Adenosine 60 �g/kg/min and Placebo

Time PlaceboAdenosine 60

�g/kg/min P

Area of secondary hyperalgesia, Baseline 94 (70-122) 88 (69-116)von Frey hair (cm2) 80 min 85 (53-111) 71 (53-102) .15

Median (quartiles) 120 min 95 (51-116) 70 (53-111) .55

Area of secondary hyperalgesia, Baseline 60 (53-94) 60 (35-75)Brush (cm2) 80 min 48 (20-73) 46 (16-74) .70

Median (quartiles) 120 min 38 (9-67) 41 (13-64) .35

HPDT (°C) Baseline 42.7 (41.1-44.0) 43.3 (41.4-44.7)Normal skin 80 min 42.9 (41.4-44.3) 42.7 (41.0-44.4) .46Median (quartiles) 120 min 42.3 (41.1-44.2) 42.3 (40.4-43.7) .28

HPDT (°C) Baseline 43.6 (41.4-45.0) 43.8 (42.2-45.4)Sensitized skin 80 min 41.7 (39.4-43.8) 41.1 (38.6-43.5) .95Median (quartiles) 120 min 43.4 (41.7-44.5) 42.5 (40.2-44.4) .06

VAS rating during LTS (45°C, 1 min) Baseline 23 (14-42) 22 (14-44)Median (quartiles) 80 min 27 (17-44) 27 (14-39) .33

120 min 27 (17-40) 22 (14-31) .17

NOTE. P values based on normalized data (for explanation, see Methods, Statistical Analysis).

Adenosine in Cutaneous Hyperalgesia • Dirks et al. 417

the first study, IV adenosine 50 �g/kg/min reducedmustard oil–induced von Frey and brush allodyniain 6 volunteers.7 In the second study, the findingswere reproduced using a slightly higher dose ofadenosine (60 �g/kg/min) in 10 volunteers and,moreover, it was shown that secondary hyperalge-sia to von Frey hair stimulation induced by a cuta-neous burn injury (47°C, 7 minutes) was also re-duced by adenosine.8 In the latter study, adenosinehad no effect on HPDTs or pain scores to supra-threshold thermal stimulation (45°C for 30 seconds)in areas of primary or secondary hyperalgesia. Incontrast, a previous uncontrolled study reportedincreased HPDTs in normal skin with IV adenosinedoses of 50 to 80 �g/kg/min.9 Finally, in a recentunblinded study intrathecal adenosine, in dosesranging from 500 to 2,000 �g in 12 healthy volun-teers, reduced von Frey but not brush allodyniainduced by mustard oil in a non–dose-dependentmanner.20 HPDTs in normal and sensitized skinwere unaffected.20

In 2 studies of ischemic experimental pain, ad-ministration of IV adenosine 70 �g/kg/min had ananalgesic effect.5,6

Three placebo-controlled studies have investi-gated the analgesic effects of intraoperative IVadenosine infusion (80 �g/kg/min) during breastsurgery,10 shoulder joint surgery,21 and abdominalhysterectomy,11 respectively. Intraoperative isoflu-rane concentrations were significantly reducedwith adenosine in all studies. In addition, the post-operative 24-hour opioid requirements were re-duced by 18% and 27% in 2 of these studies,10,11

whereas no effect on postoperative pain or analge-sic requirements was observed in the third study.21

WAG 994, a novel adenosine agonist, did not showefficacy in the management of postoperative painafter third-molar surgery.12 In a recent study, intra-operative use of IV adenosine during major gyne-cologic surgery was associated with a decreased re-quirement for opioid analgesics during the first 24hours after operation compared with remifen-tanil.22 This observation, however, may be ex-plained by acute opioid tolerance induced by intra-operative remifentanil, as observed in anotherstudy.23 Finally, intrathecal adenosine (500 �g) didnot influence the requirement of anesthetic drug orpostoperative analgesics after hysterectomy, com-pared with placebo.24

In 2 studies, the effect of IV adenosine has beeninvestigated in patients with chronic neuropathicpain. Sollevi et al.13 administered IV adenosine (50to 70 �g/kg/min) to 2 patients with postsurgicalneuropathic pain in an unblinded study and re-ported reduction of tactile allodynia, thermal, andspontaneous pain. In a study by Belfrage et al.,25 7

patients with neuropathic pain (surgical, entrap-ment, rhizopathy) received IV adenosine (60 �g/kg/min) or placebo in a double-blind design. Back-ground pain and pinprick hyperalgesia wasreduced, but as in the present study of healthyvolunteers, there was no effect on experimentallyinduced thermal pain thresholds in either normalor painful skin. In a recent unblinded study of in-trathecal administration of adenosine in 14 patientswith chronic neuropathic pain, the same investiga-tors found reduction in spontaneous and evokedpain and increased tactile pain thresholds in areaswith allodynia.14

In summary, at doses that produced definite sideeffects, there was no antinociceptive or antihyper-algesic effect of IV adenosine in 25 healthy volun-teers in the present placebo-controlled study. Pre-vious studies investigating the effect of adenosine inhealthy volunteers using mustard oil or burn-injurymodels have shown some antihyperalgesic effect,and IV adenosine may reduce ischemic pain. It ap-pears consistent across all the studies that adeno-sine does not affect HPDTs and the response tosupra-threshold thermal stimulation in normal andsensitized skin.5-8 The results from studies investi-gating the effect of adenosine on perioperative painare discordant.10,11,21,22 Three smaller studies in pa-tients with chronic pain,13,14,25 2 of which wereopen, suggested analgesic effects of systemic andintrathecal adenosine on chronic pain and allo-dynia. With opioids, analgesic effects are consistentacross healthy volunteer pain model studies, peri-operative pain studies, and chronic neuropathicpain studies. In contrast, IV sodium channel block-ers such as lidocaine have little effect on experi-mental pain, a very modest effect on perioperativepain, and sometimes dramatic effects on chronicneuropathic pain. For adenosine, although furthercontrolled studies of acute and chronic pain areneeded, antinociceptive and antihyperalgesic ef-fects on healthy volunteers appear minor or absent.

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