中国科技论文在线 http://www.paper.edu.cn European Journal of Pharmacology 423Ž. 2001 157–166 www.elsevier.nlrlocaterejphar Tolerance to analgesia and dependence liability by topical application of dihydroetorphine to hairless rats Satoshi Ohmori, Liang Fang, Masami Kawase, Setsuo Saito, Yasunori Morimoto) Department of Pharmaceutics, Faculty of Pharmaceutical Sciences, Josai UniÕersity, 1-1 Keyakidai, Sakado, Saitama 350-0295, Japan Received 19 January 2001; received in revised form 23 May 2001; accepted 29 May 2001 Abstract The tolerance to analgesia and dependence liability of dihydroetorphine following topical application were investigated in hairless rats with and without formalin-induced inflammation. The analgesic effect of dihydroetorphineŽ. s.c. was 4600- to 7200-fold more potent than that of morphine. In non-inflamed rats, the analgesic effect of 24-h topical application of dihydroetorphine tapeŽ. 35 mg and 4-day repeated tape applicationsŽ. 20 mgr5hrday decreased with time after the start of application, even though the plasma dihydroetorphine concentrations did not decrease. In formalin-inflamed rats, however, the tolerance to analgesia diminished. Naloxone-precipitated weight loss was observed after 24-h infusion of dihydroetorphine but not after the tape application in non-inflamed rats. A significant rewarding effect was found in the non-inflamed rats conditioned by s.c. injection and tape application but not in the formalin-inflamed rats. These results indicate that topical application of dihydroetorphine has a tolerance and dependence liability when there is no pain, and therefore, it should be used only for pain relief. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Dihydroetorphine; Hairless, rat; Topical application; Tolerance to analgesia; Physical dependence; Rewarding effect 1. Introduction March 1999, the United Nations decided to include dihy- droetorphine in Schedule I of the Single Convention on DihydroetorphineŽ 7,8-dihydro-7a-1-w Ž.R -hydroxy-1- x Narcotic Drugs of 1961 and that Convention as amended methylbutyl -6,14-endo-ethanotetrahydro-oripavine. pro- by the 1972 ProtocolŽ Commission on Narcotic Drug, duces extraordinarily strong analgesia through the activa- Report on the forty-second session, 1999. Up to now, m tion of -opioid receptorsŽ Katsumata et al., 1995; Wang dihydroetorphine has been clinically used for pain relief et al., 1995; Kamei et al., 1996. , with a potency ratio to under restricted control in ChinaŽ Wang et al., 1999; Zang morphine of 1000 to 12,000Ž Bentley and Hardy, 1967; 1999. Huang and Qin, 1982; Tokuyama et al., 1996. In China, In experiments on the dependence liability of dihydroe- dihydroetorphine began to be clinically used for pain relief torphine, it was shown that dihydroetorphine induced a in 1981. Although dihydroetorphine was registered as an potent reinforcing effect and discriminative stimulus effect analgesic in 1992, it was also used for suppressing opiate in the drug self-administration test in rats and monkeys withdrawal syndrome. However, the Government of China Ž.Martin et al., 1997; Beardsley and Harris, 1997 and a restricted the clinical use of dihydroetorphine in 1993, rewarding effect in the conditioned place preference test in because the number of dihydroetorphine abusers increased ratsŽ. Liu and Zhang, 1999a,b . Furthermore, dihydroetor- quickly as soon as it was commercializedŽ WHO Expert phine produced physical dependence in the case of contin- Committee on Drug Dependence, thirty-first report, 1999. uous exposure by infusionŽ Zhang and Qin, 1994; Aceto et Epidemiological studies showed that the majority of abusers al., 2000.Ž and short-term repeated injections Tokuyama et took dihydroetorphine to avoid the withdrawal syndrome al., 1994.Ž. in rats. Tokuyama et al. 1993 reported the r of heroin or other opiates, whereby potent psychic- and or development of tolerance to analgesia with dihydroetor- mild physical-dependence developedŽ. Liu et al., 1995 . In phine and cross-tolerance with morphine. These studies indicate that dihydroetorphine has significant potential for ) Corresponding author. Tel.: q81-492-71-7685; fax: q81-492-85- abuse like other opioids such as morphine, fentanyl and 5863. heroin. However, it has been reported that the tolerance E-mail address: [email protected]Ž. Y. Morimoto . and dependence liability of morphine are reduced in ani- 0014-2999r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. 转载 PII: S0014-2999Ž. 01 01098-6 中国科技论文在线 http://www.paper.edu.cn 158 S. Ohmori et al.rEuropean Journal of Pharmacology 423() 2001 157–166 mals suffering from formalin-induced painŽ Vaccarino et In all experiments, individual rats received only a single al., 1993; Vaccarino and Couret, 1993; Suzuki et al., 1996. dosing schedule. and in humans with chronic painŽ. Poter and Jick, 1980 . It has not been investigated whether the tolerance to and 2.2. Preparation and topical application of dihydroetor- dependence on dihydroetorphine develops under the influ- phine tape ence of inflammatory nociception. Dihydroetorphine has been clinically administered as The dihydroetorphine tape was prepared according to Ž. sublingual tablets in China Wang et al., 1999 . Because our previous reportŽ. Ohmori et al., 2000b . Briefly, dihy- the sublingual dose exhibits a short duration of analgesic droetorphine, styrene-isoprene-styrene block co-polymer, Ž. effect within 1.5–5.3 h , it has to be administered several rosin ester and isopropyl myristate were dissolved in chlo- Ž. times a day for continuous pain relief Wu and Sun, 1991 . roform. The mixture was cast onto backing film, and then In our previous study, transdermal delivery of dihydroetor- dried. The tape was covered with a release liner, and then phine was shown to produce a prolonged analgesic effect the tape was pulled out circularly to give an area of 0.13, Ž. for more than 24 h in hairless rats Ohmori et al., 2000b . 0.28, 0.38 and 0.50 cm2. The dihydroetorphine content of However, the potential to promote dependence by continu- the tape was 70 mgrcm2. A placebo tape was prepared by ous exposure to dihydroetorphine is of concern and re- the same method excluding the dihydroetorphine. Ž. quires clarification. The objectives of this study are 1 to The 0.13-, 0.28- and 0.38-cm2 tapes were applied to the compare the potency of the analgesic effect and depen- abdominal skin, and the 0.5-cm2 tape was applied to the dence liability of dihydroetorphine and morphine in hair- dorsal skin of rats. The abdominal and dorsal applications Ž. less rats, 2 to compare the tolerance to analgesia and provide steady-state plasma concentrations of the drug for dependence liability of continuous infusion or repeated s.c. 8 and 24 h, which are appropriate for repeated application injection and topical application of dihydroetorphine, and and 24-h continuous exposure, respectivelyŽ Ohmori et al., Ž. 3 to compare the tolerance and dependence liability of 2000b. Because the absorption rate of dihydroetorphine dihydroetorphine in rats with and without formalin-in- through the abdominal and dorsal skin was different, the duced nociception. area of the tape was adjusted to maintain the same plasma concentration. During the repeated application, the tape was re-applied to different sites on the abdominal skin 2. Materials and methods each day, because the skin was slightly damaged when the tape was removedŽ. Ohmori et al., 2000b . 2.1. Animals 2.3. Intra enous infusion of drugs The male hairless ratsŽ. WBNrILA-Ht strain weighing Õ 200–250 gŽ. 10–12 weeks old used in all experiments were supplied by the Ishikawa Experimental Animal Labo- Rats were cannulated with polyethylene tubing in a ratoryŽ. Saitama, Japan . Animals were kept in a room that femoral vein under diethylether anesthesia. The tip of the was maintained at 24"1 8C under a 12-h light–dark cycle cannula was drawn through the skin on the back of the and had free access to a standard rodent chow and clean neck. Rats were placed in a Bollman cage and left for 2 h drinking water. These experiments were performed in ac- after surgery to recover from anesthesia. MorphineŽ 8 r r m r r cordance with the Guide for Laboratory Animal Experi- mg kg h.Ž. , dihydroetorphine 1.8 g kg h or saline r ment adopted by Josai University. Ž.0.5 ml h was infused for 24 h through the venous For the treatment of formalin-inflamed rats, formalin cannula. Ž.2.5%, 50 ml was injected into the plantar surface of the hind paw just once on the initial day. Suzuki et al.Ž. 1996 2.4. Blood sampling for the determination of plasma dihy- reported that, in Sprague–Dawley rats, the formalin-treated droetorphine concentration Ž.2.5%, 50 ml paw continued to swell for 11 days, and a pronounced reduction in the formalin-treated paw pressure To withdraw blood samples for the determination of the threshold was observed for 9 days. In this study, the plasma dihydroetorphine concentration during the 24-h hairless rats showed aversive behavior, and the formalin- infusion, rats were cannulated with polyethylene tubing in treated paw continued to swell for 6 daysŽ. data not shown . a femoral artery, in addition to a femoral vein for the For evaluation of tolerance development, the administra- infusion of dihydroetorphine, under diethylether anesthe- tion of opioids was begun 1 day after the formalin injec- sia. Individual rats were placed in the Bollman cage and tion. For the evaluation of the rewarding effect in the left for 2 h after surgery to recover from anesthesia. conditioned place preference test, animal screening was DihydroetorphineŽ. 1.8 mgrkgrh was infused for 24 h carried out 2 days after and then conditioning following through the venous cannula. BloodŽ. 0.5 ml was with- administration of opioids was begun 3 days after the for- drawn from the arterial cannula at 2, 4, 8, 16 and 24 h after malin injection.