Prospects for a Novel K-Opioid Receptor Agonist, TRK-820, in Uremic Pruritus

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naltrexone for uremic pruritus (7). Recently, we have found that a nopioid agonist suppresses scratching in the mouse pruritus model (8). These

reports suggest that A- and n-opioid systems are involved in the itching

mechanism.

We now describe our studies on the relationship between endogenous

opioid peptides and itching, and evaluate n-opioid agonist TRK-820 as a

novel antipruritic drug for the treatment of uremic pruritus.



II.



MATERIALS AND METHODS



A.



Subjects



Forty healthy volunteers (26 males and 14 females, mean age 55.4 F 13.3 yr)

and 37 hemodialysis patients (20 males and 17 females, mean age: 60.7 F

13.1 years) were screened for this study. Hemodialysis patients were divided

into three groups comprised of those with no itching, moderate itching, or

severe itching because of uremic pruritus. Six patients (4 males and 2 females, mean age: 56.2 F 6.2 yr) complaining of severe uremic pruritus were

enrolled for the preliminary open-labeled clinical pharmacological study of

TRK-820. Itching intensity was determined by each individual patient.

B.



Determination of Serum Factors and Opioid Peptides



Blood samples were collected from each subject and separated into serum

and cell precipitate. Sera were subjected to the determination of serum factors and endogenous opioid peptides. Serum factors measured included

histamine, serotonin, intact parathyroid hormone (PTH), and eosinophil

cationic protein (ECP), which have been previously suggested to be involved in the pathogenesis of itching. Endogenous opioid peptides h-endorphin and dynorphin A were also measured. All serum factors and opioid

peptides were tested at BML Inc. (Tokyo, Japan), a professional clinical

laboratory.

C.



Drug



A novel n-opioid agonist TRK-820 ((À)-17-(cyclopropylmethyl)-3,14 h-dihydroxy-4,5 a-epoxy-6-[N-methyl-trans-3-(3-furyl) acrylamidol morphinan

hydrochloride, Toray Industries, Inc., Tokyo, Japan) (see also Chapter 11)

was given orally to all six patients at a single dose of 10 Ag in the form of

a soft capsule. The patients were prohibited from eating for 3 hr after

administration.



Prospects for a Novel K-Opioid Receptor



D.



281



Clinical Evaluation of the Drug



Pharmacokinetics, safety, and antipruritic efficacy of TRK-820 were evaluated as follows:

For pharmacokinetics, blood samples were collected and the plasma

concentration of TRK-820 was determined. Pharmacokinetic parameters,

Tmax (the time it took to reach Cmax of plasma) and t1/2 (half-life), were

calculated.

For assessing safety and toxicity, the physical findings, vital signs, and

laboratory parameters (hematology, blood biochemistry, endocrine test, and

urinalysis) were determined for 48 hr after administration of the drug.

For evaluation of antipruritic efficacy, the visual analog scale (VAS)

and itching intensity, categorized according to five grades (none, slight, mild,

moderate, and severe) were used. VAS is a linear scale of 100 mm that ranged

from 0, or ‘‘No itching,’’ to 100, or ‘‘Intolerable itching.’’ VAS score was

assessed by the patient. The physician assessed patient itching and categorized the intensity according to five grades of intensity.



III.



RESULTS



A.



Serum Factors and Opioid Peptides



Figure 1 shows that serum levels of known mediators involved in itching,

such as histamine, serotonin, intact PTH, and ECP, did not differ between

healthy volunteers and hemodialysis patients. However, the serum level of

intact PTH was higher in patients than in 40 healthy volunteers, although

there was no significant correlation between intact PTH and itching intensity. As shown in Figure 1, none of the measured serum factors

correlated with the itching intensity in hemodialysis patients.

Serum levels of h-endorphin and dynorphin A in healthy volunteers and

hemodialysis patients are illustrated in Figure 2. There was an apparent

increase of h-endorphin level proportionate to the increase in itching intensity in hemodialysis patients (Fig. 2a). We assumed that the balance of the

A- and n-opioid systems may be a key factor controlling itching sensation. The

ratio of concentrations of h-endorphin to dynorphin A in individuals was

calculated. An arbitrary index between h-endorphin and dynorphin A, the [hendorphin/dynorphin A] ratio (E/D ratio), is shown in Fig. 2b. The E/D ratios

were 2.17 F 0.38, 2.48 F 1.08, 2.83 F 1.74, and 3.59 F 1.36 for healthy

volunteers, patients with no itching, moderately itchy patients, and severely

itchy patients, respectively. These results imply that serum levels of opioid

peptides differ among individuals, and the balance of endogenous A- and nopioid systems is relevant to understanding the itch status.



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Figure 1 Serum factors, histamine, serotonin, intact PTH, and ECP. The number

in parentheses represents the number of patients. Bar denoted as the mean F SD.



Figure 2 Serum concentration of endogenous opioid peptides in each group of

healthy volunteers and hemodialysis patients with pruritus; (a) concentration of hendorphin and dynorphin A in the sera; (b) balance of endogenous A- and n-opioid

peptides. [h-endorphin/dynorphin A] ratio calculated from the data of serum concentrations. Bar denoted as the mean F SD.



Prospects for a Novel K-Opioid Receptor



B.



283



Clinical Evaluation of a Novel K-Opioid Agonist TRK-820



The pharmacokinetics of TRK-820 were analyzed in only five patients. As

shown in Figure 3, Tmax was 4.00 F 4.47 hr, Cmax was 14.3 F 1.3 pg/mL,

and t1/2 was 16.8 F 10.2 hr, suggesting a long-acting drug.

Concerning assessment of safety, the adverse drug reactions of somnolence (one case) and asthenia (one case) were mild, and both cases recovered without any additional treatment. Mild laboratory abnormalities

were observed in two patients—a plasma testosterone decrease and a leukocytosis, respectively. No serious clinical adverse drug reactions were observed. Thus, all adverse drug reactions observed in this study were mild in

intensity and transient.

A remarkable antipruritic effect of TRK-820 was observed. Mean

VAS score before administration was 54.2 F 3.8 mm, and the scores decreased in all patients after administration of TRK-820, as shown in Figure

4. The mean VAS scores 4 and 12 hr after administration were 12.2 F 2.4

and 1.8 F 0.6 mm, respectively, and this score showed an increase, with the

exclusion of one patient, 24 hr after administration.

The categorical assessment of itching intensity also significantly

changed as shown in Figure 5. The initial itching intensity was considered

as ‘‘moderate itching’’ in 3 patients, ‘‘mild itching’’ in 2 patients, and ‘‘slight

itching’’ in 1 patient. The severity ameliorated to ‘‘mild itching’’ or less in all



Figure 3 Pharmacokinetics profile of TRK-820 after oral administration in hemodialysis patients (n = 5).



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Figure 4 VAS score profiles after oral administration of TRK-820 in uremic

pruritus patients (n = 6).



Figure 5 Itching intensity profiles after oral administration of TRK-820 in uremic

pruritus patients (n = 6).



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285



patients from 2 hr after administration. Twelve hours after administration, 3

patients described themselves as ‘‘slightly itching’’ and another 3 patients as

‘‘not itching.’’

These results suggest that oral administration of TRK-820 in hemodialysis patients is a promising treatment for uremic pruritus.

IV.



DISCUSSION



The underlying mechanism of uremic pruritus is still not clear. Opioids have

been proposed as a possible pathogenic factor for pruritus, but to date only

A-opioid agonists are known to be pruritogenic. It is well known to clinicians that A-opioids induce itching in postoperative pain management and

A opioid agonists induce itching through central A-opioid receptors in mice

(9). In earlier studies, h-endorphin, an endogenous A-opioid peptide, was

elevated in severe atopic dermatitis patients (10,11), but no relationship to

uremic pruritus could be found (12). Concerning met-enkephalin, another Aopioid peptide, conflicting findings have been reported (13,14).

We initially focused on the endogenous n-opioid peptide dynorphin

and found that the balance of A- and n-opioid peptides could potentially

be related to the pathogenesis of uremic pruritus. Thus, our data suggests

that the major contributors to uremic pruritus are not the serum factors

previously suggested, such as histamine and serotonin, but rather the A- and

n-opioid ratio. Our findings also support the idea that the A-opioid system

is itch-inducible, while the n-opioid system is itch-suppressive (Fig. 6). In

support of this hypothesis, a recently published review article showed that

activation of the n-receptor antagonized various A-receptor-mediated

actions (15). This implies that the n-opioid system may perform actions

opposing A-opioid system.

We have demonstrated a remarkable antipruritic efficacy of a novel

n-opioid agonist TRK-820 for uremic patients in a small uncontrolled study.



Figure 6 Our hypothesis of pruritus/itching control by the opioid system. This

hypothesis implies that the A-opioid system induces itching while the n-opioid system

suppresses itching.



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Although the present clinical pharmacological study was open-labeled and

small in scale, rendering it necessary to further investigate and confirm the

efficacy of TRK-820, the results suggest that oral administration of TRK820 in hemodialysis patients may be of value in the treatment of uremic

pruritus.



REFERENCES

1.

2.

3.



4.

5.

6.



7.



8.

9.



10.



11.



12.



13.

14.



15.



Szepietowski JC, Schwartz RA. Uremic pruritus. Int J Dermatol 1998; 37:247–

253.

Schwartz IF, Iaina A. Uraemic pruritus. Nephrol Dial Transplant 1999; 14:34–

839.

Omori K, Aioke I, Aoyanagi H, et al. Risk factors for uremic pruritus in long

term hemodialysis patients. J Jpn Soc Dial Ther 2001; 34:1469–1477 (in Japanese).

Duffy BL. Itching as a side-effect of epidural morphine. Anesthesia 1981; 36:67.

Ballantyne JC, Loach AB, Carr DB. Itching after epidural and spinal opiates.

Pain 1988; 33:149–160.

Peer G, Kivity S, Aqami O, Fireman E, Silverberg D, Blum M, Iaina A. Randomised crossover trial of naltrexone in uraemic pruritus. Lancet 1996; 348:

1552–1554.

Pauli-Magnus C, Mikus G, Alscher DM, et al. Naltrexone does not relieve uremic pruritus: results of a randomized, double-blind, placebo-controlled crossover study. J Am Soc Nephrol 2000; 11:514–519.

Togashi Y, Umeuchi H, Okano K, et al. Antipruritic activity of the kappaopioid receptor agonist, TRK-820. Eur J Pharmacol 2002; 435:259–264.

Kuraishi Y, Yamaguchi T, Miyamoto T. Itch–scratch responses induced by

opioids through central mu opioid receptors in mice. J Biomed Sci 2000; 7:248–

252.

Glinski W, Brodecka H, Glinska-Ferenz M, Kowalski D. Increased concentration of beta endorphin in the sera of patients with severe atopic dermatitis.

Acta Derm Venereol 1995; 75:9–11.

Georgala S, Schulpis KH, Papaconstantinou ED, Stratigos J. Raised betaendorphin serum levels in children with atopic dermatitis and pruritus. J Dermatol Sci 1994; 8:125–128.

Mettang T, Fischer FP, Dollenbacher U, Kuhlman U. Uraemic pruritus is not

related to beta-endorphin serum levels in haemodialysis patients. Nephrol Dial

Transplant 1998; 13:231–232.

Danno K, Nishiura K, Tanaka M. Increased met-enkephalin plasma levels in

hemodialysis patients with or without pruritus. J Dermatol Sci 1995; 10:238–240.

Odou P, Azar R, Luyckx M, Brunet C, Dine T. A hypothesis for endogenous

opioid peptides in uraemic pruritus: role of enkephalin. Nephrol Dial Transplant 2001; 16:1953–1954.

Pan ZZ. mu-Opposing actions of the kappa-opioid receptor. Trends Pharmacol

Sci 1998; 19:94–99.



28

Treatment of Pruritic Skin Diseases

with Topical Capsaicin

¨

Sonja Stander and Dieter Metze

¨

¨

University of Munster, Munster, Germany



Abbreviations

CGRP

DRG

NKA

PGP 9.5

SIC

SP

VIP

VR1

VRL-1

VRL-2

VR.5Vsv



I.



calcitonin gene-related peptide

dorsal root ganglion

neurokinin A

protein gene product 9.5

mechanosensitive stretch-inhibitable cation channel

substance P

vasoactive intestinal polypeptide

vanilloid (capsaicin) receptor 1

vanilloid receptor-like protein 1

vanilloid receptor-like protein 2

vanilloid receptor 5V splice variant



CAPSAICIN AND THE VANILLOID RECEPTOR



Capsaicin (trans-8-methyl-N-vanillyl-6-nonenamide)(Fig. 1) is a naturally

occurring lipophile alkaloid derived from plants of the nightshade family

and is the major pungent of hot chili peppers (1). Capsaicin is an exogenous,

but not an endogenous expressed, ligand at a capsaicin-specific receptor, i.e.,

287



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288



Figure 1



Structural formula of capsaicin. (From Ref. 1.)



the vanilloid receptor 1 (VR1) (2,3). Vanilloid receptor 1 is also activated by

the endogenous cannabinoid anandamide (4–6), increase in temperature

(above 42jC), and protons (pH below 5.9) (2–8). Recently, other members

of the vanilloid receptor family were identified (9–14). These include vanilloid

receptor-like protein 1 (VRL-1), vanilloid receptor-like protein 2 (VRL-2),

vanilloid receptor 5V splice variant (VR.5Vsv), and a mechanosensitive stretchinhibitable cation channel (SIC) with different distributions in the tissues

together with corresponding ligands and biological functions. However,

capsaicin has been, up to now, known only to exert its functions via the VR1.

In animal studies, VR1 could be demonstrated in small myelinated Aytype and C-type sensory nerve fibers of the spinal cord and dorsal root ganglion (DRG) (3,15) as well as in central nervous system, sciatic nerve, and small

nerve fibers in the skin and cornea of rat (2,8,12,16–18). Interestingly, VRL-2

was found in animal cutaneous sympathetic and parasympathetic nerve fibers,

kidney, trachea, and salivary gland (9). In humans, VR1 is expressed in the

dorsal horn of the human spinal cord (12), dorsal root ganglia (19), and central

nervous system (17). In human skin, VR1 could recently be shown on cultured

(20) and in vivo epidermal keratinocytes (21).

Receptor binding of capsaicin opens nonselective cation channels with

high permeability to calcium (13). Receptor activation on nerve fibers and

subsequent calcium currents into the axon result in depolarization of the nerve

fibers and release of secretory granules containing neuropeptides such as

substance P (SP), calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide (VIP), and neurokinin A (NKA) (1,10,12,14,22,23). In animal

studies it could be demonstrated that systemic application of capsaicin at high

concentrations is associated with neurotoxic effects. Moreover, systemic application of capsaicin in neonatal mice induces permanent degeneration of

primary sensory neurons (11,22,24–26). Likewise, in sensory neurons of cultured rat dorsal root ganglion, capsaicin irreversibly damages C-type nerves

via activation of intracytoplasmic calcium-sensitive proteases (11,27–29). In

morphological studies in man, it was demonstrated that epidermal protein



Treatment of Pruritic Skin Diseases



289



gene product (PGP) 9.5-positive nerve fiber density is decreased upon intradermal injection and topical application of capsaicin but nearly normalized

within 4–6 weeks after capsaicin therapy (30,31), while dermal nerve fibers

remain unaffected (32,33). However, repeated topical application of capsaicin

in low concentrations (0.025–0.3%) does not lead to any degeneration or

inflammation of epidermal and dermal nerve fibers (33). Furthermore,

capsaicin influences other skin cells upon external application. Human cultured keratinocytes and fibroblasts showed a reduced cell growth under capsaicin concentrations of 0.025–0.2% (34).

Repeated application of capsaicin results in suppression of pain and itch

sensations which are mediated by unmyelinated C-fibers, while tactile sensations remain unaffected (28,29,35,36). After 3–5 days of continuous treatment,

capsaicin-sensitive nerve fibers are desensitized and reaccumulation of neuropeptides is inhibited (37). Furthermore, the axoplasmic transport of neuropeptides to the periphery is suppressed (28,38). In morphological studies the

specific effect of capsaicin could be confirmed in skin biopsies obtained before

and during capsaicin treatment by means of confocal laser scanning microscopy (32,33) and immunofluorescence microscopy (39). Therefore, capsaicin

treatment could be shown to be associated with a complete attenuation of SP

expression in dermal papillary nerve fibers. The antipruritic potency of

capsaicin has been confirmed in experimental studies suppressing histamineinduced itch (40). Interestingly, capsaicin was not able to reduce serotonininduced pruritus (41).



II.



TOPICAL APPLICATION OF CAPSAICIN

FOR THERAPEUTIC PURPOSES



A.



Capsaicin Preparations and Concentrations



For the topical application, a liquid capsaicin extract consisting of different

capsaicinoids with 80–90% capsaicin and dihydrocapsaicin has proved to be

efficacious and safe (32,33). However, the extraction of pure capsaicin, a white

powder, from chili plants is very difficult and expensive, and only a few capsaicin ointments are commercially available; these include DolenonR (0.05%

capsaicinoids) and ZostrixR (0.025% and 0.075% capsaicin). However, the

oily 1% capsaicin extract (Extractum Capsici aetherea 1%; Caesar and Loretz

GmbH, Hilden, Germany) could be added easily to various emollients or

ointments and selectively applied to the skin lesions (32,33,42). As the

capsaicin extract is dark red, the capsaicin ointments of 0.025–0.5% concentration are orange-red, occasionally staining the clothing. Capsaicin should be

utilized regularly four to six times daily to prevent reaccumulation of neuropeptides and, thus, recurrence of itch and pain. In order to enhance the



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penetration rate through the markedly hyperkeratotic lesions of, e.g., prurigo

nodularis, capsaicin can be administrated twice daily under occlusion bandages for the initial three days. Before starting capsaicin therapy, erosive

lesions should be pretreated with a topical antiseptic or corticosteroidal

ointment to avoid burning sensations on erosive skin (33). Because of poor

compliance due to the initial side effects of neurogenic inflammation, the

necessity of frequent daily application and occasional staining of the orange

ointment on clothes, and in order to improve the evaluation of the therapeutic

effect, it is recommended to hospitalize the patients for a short time. Generally,

the practicability of capsaicin therapy is largely limited by the high application

frequency as long as capsaicin analogues with prolonged tissue persistence are

not available. When starting, capsaicin cream should be used in low concentrations of 0.025% (e.g., Extractum capsici 1% 2.5 g in ointment ad 100.0 g) or

0.05%. After the cessation of the primary symptoms of neurogenic inflammation, capsaicin concentration can be individually raised in steps of 0.025%

every 3–5 days until a total relief of pruritus is achieved. The concentration

applied regularly should not exceed 0.1%; in rare cases, capsaicin concentration can be raised carefully to 0.3% or 0.5% (32,33,42). After the discontinuation of the capsaicin therapy, pruritus and pain recur immediately

within 18 days due to restoration of neuropeptide deposits in sensory nerve

fibers.



B.



Side Effects



The side effects of the capsaicin treatment can be attributed to the primary

release of neuropeptides and as such are limited. The transient symptoms are

those of neurogenic inflammation, i.e., stinging, pricking, and burning sensations, erythema, as well as increase of pain or itch caused by the action of

the neuropeptides on mast cells and blood vessels (33,42–44). The symptoms

of neurogenic inflammation start within 20–30 min after application and

last for no longer than 30–60 min (33,45). Interestingly, pretreatment with

a topical anesthetic (EMLAR) significantly reduced the burning sensations

from capsaicin (46).

Topical application of capsaicin has never been reported to lead to systemic effects. The latter fact is of crucial importance because upon systemic

administration and in cultured sensory neurons, capsaicin potentially damages C-type nerves in an irreversible fashion due to activation of intracytoplasmic calcium-sensitive proteases (see above). However, ultra-structural

investigations showed that cutaneous nerve fibers appeared to be regular and

revealed no degenerative changes during or after capsaicin treatment (33).

Despite long-term therapy and application of high concentrations of capsaicin, loss of skin sensibility has never been reported. Furthermore, contact