**6. Comparison of pharmacological properties between TRK-820 and conventional agonists**

We described in the previous sections some pharmacological properties of TRK-820 that are different from conventional agonists, arylacetamides such as U-50,488H and U-69,593: binding properties (section 2) and exhibition of no preferential and no aversive effect in the CPP paradigm (section 5). As described below, drug discrimination procedures indicate conclusive difference between TRK-820 and arylacetamides.

#### **6.1 Discriminative tests**

90 Pharmacology

a rewarding effect (Dykstra et al., 1997). However in the CPP test, animals avoid an environment associated with the administration of the agonists, indicating that these drugs have aversive effects (Barr et al., 1994; Funada et al., 1993). In contrast to conventional agonists, such as U-50,488H, TRK-820 (3.0-30 g/kg, s.c.) did not induce significant place aversion in mice at doses producing significant antinociception (Fig. 6) (Nagase, 2010). Notably, TRK-820 exhibited neither preferential nor aversive properties. Recently, the peroral administration of TRK-820 (5.0 g/day) was reported to show no signs of psychological or physical dependence in an open-labeled clinical trial for one year (Nagase & Fujii, 2011).

Fig. 6. The effect of TRK-820 in the CPP test. Reprinted with permission from Nagase, 2010.

The mechanism of agonist-induced rewarding effects is outlined below. The activation of the receptor on -aminobutyric acid-containing interneurons is likely to disinhibit ventrotegmental area dopaminergic neurons, thereby increasing dopamine release in their terminal areas, including the nucleus accumbens (N.Acc). On the other hand, the activation of the receptor decreases dopamine release in the N.Acc (Di Chiara & Imperato, 1988; Spanagel et al., 1992). Therefore, agonists may be useful for treating morphine dependence. Indeed, the pretreatment with U-50,488H attenuated the morphine-induced place preference in mice (Funada et al., 1993). TRK-820 also significantly suppressed the place preference produced by morphine, and the effect of TRK-820 was antagonized by pre-treatment with nor-BNI (3.0 mg/kg, s.c.) in mice (Tsuji et al., 2001). In addition, TRK-820 was effective in reducing the rewarding effect produced by cocaine. TRK-820 (20 and 40 g/kg, i.p.), at doses producing no aversive or sedative effects, suppressed the rewarding effect of cocaine (4.0 mg/kg, i.p.) in rats (Mori et al., 2002). U-50,488H and U-69,593 exhibited similar effects as TRK-820 (Shippenberg et al., 1996; Suzuki et al., 1992). Drug discrimination procedures provide relevant information about neuropharmacological mechanisms underlying the subjective effects of abused drugs, including cocaine, methamphetamine, and opioids, in animals. Therefore, the procedures are potentially useful for identifying candidate therapeutics for the management of drug abuse (Schuster & Johanson, 1988). Pre-treatment with TRK-820 (10 and 20 g/kg, s.c.) significantly

**5.2 The effects of TRK-820 on morphine and cocaine-induced rewarding effects** 

Drug discrimination procedures have shown that the properties of TRK-820 differ from those of conventional agonists, such as U-50,488H. In the cross-substitution tests using rats, U-50,488H (1.0-3.0 mg/kg) substituted for the discriminative stimulus effects of TRK-820 (40 g/kg, i.p.), whereas TRK-820 (10-76 g/kg) did not completely substitute for those of U-50,488H (3.0 mg/kg, i.p.). E-2078 (0.3-3.0 mg/kg), but not R-84760 (0.01-0.3 mg/kg), substituted for the discriminative stimulus effects of both TRK-820 and U-50,488H. KT-90 (0.03-3.0 mg/kg), CI-977 (1-30 mg/kg), or ICI-199441 (3.0-56 mg/kg) substituted for the discriminative stimulus effects of U-50,488H, but not for those of TRK-820 (Mori et al., 2004).

Opioid Kappa Receptor Selective Agonist TRK-820 (Nalfurafine Hydrochloride) 93

TRK-820 was a selective agonist. However, its pharmacological properties were different from those of conventional arylacetamide agonists, including U-50,488H. A noteworthy feature of TRK-820 was that it showed no preferential or aversive properties, whereas U-50,488H produced aversion. This disparity of properties between TRK-820 and arylacetamide agonists was reported to stem from the difference in receptor subtypes each compound interacted with: arylacetamide agonists would interact with 1 receptor subtype, whereas TRK-820 may interact with another receptor subtype (perhaps 3) (Endoh et al., 1999; 2000; 2001; Tsuji et al., 2000a; 2000b). Although opioid receptors have been classified historically into three types (, , and types) and further divided into several subtypes from the pharmacological viewpoint (Dhawan et al., 1996), only the three major types have been cloned (Satoh & Minami, 1995). Much evidence has been compiled indicating that various receptors, including opioid receptors, exist as homo- or hetero dimers of the receptors (George et al., 2000, 2002; Gomes et al., 2000, 2004; Devi, 2001; Levac et al., 2002; Wang, D et al., 2005), and receptor dimerization has been invoked to explain the discrepancy between widely varied pharmacologies and the identification of only three opioid receptor types. Therefore, the disparity of properties between TRK-820 and arylacetamide agonists may stem from the difference in receptor dimers each compound interacts with. Both TRK-820 and arylacetamide agonists are expected to be useful tools for the investigation of receptor dimerization and/or receptor subtype. As mentioned in section 2, a binding assay using [3H]TRK-820 and [3H]U-69,593 is thought to be a facile and useful method for achieving that purpose. However, [3H]TRK-820 is not currently available. In addition to antipruritic and antinociceptive effects, TRK-820 exhibited various pharmacological effects, such as the treatment of the symptoms of schizophrenia or dyskinesia symptoms of parkisonian patients, or remedy for drug addiction. Moreover, TRK-820 has been already launched in Japan. TRK-820 is expected not only to be developed with the other indication, such as symptoms of schizophrenia or parkinson's disease, but

also to be utilized to investigate pharmacology *via* the receptor.

Barr, G. A.; Wang, S.; Carden, S. (1994). Aversive properties of the

*Anesthesiology*, 61, 3, 276-310, ISSN 0003-3022.

*Pain*, 33, 2, 149-160, ISSN 0304-3959.

Ballantyne, J.C.; Loach, A. B. & Carr, D. B. (1988). Itching after epidural and spinal opiates,

in the week-old rat pup, *Psychopharmacology,* 113, 3-4, 422-428, ISSN 0033-3158. Cousins, M.J. & Mather, L.E. (1984). Intrathecal and Epidural Administration of Opioids,

Hamon, M. (1996). International Union of Pharmacology. XII. Classification of

dopamine concentrations in the mesolimbic system of freely moving rats, *Proc.* 

Devi, L. A. (2001). Heterodimerization of G-protein-coupled receptors: pharmacology, signaling and trafficking, *Trends Pharmacol. Sci.*, 22, 10, 532-537, ISSN 0165-6147. Dhawan, B. N.; Cesselin, F.; Raghubir, R.; Reisine, T.; Bradley, P. B.; Portoghese, P. S. &

Di Chiara, G.; Imperato, A. (1988). Drugs abused by humans preferentially increase synaptic

Opioid Receptors, *Pharmacol. Rev.*, 48, 4, 567-592, ISSN 0031-6997.

*Natl. Acad. Sci. U. S. A.,* 85, 14, 5274-5278, ISSN 0027-8424.

opioid agonist U50,488

**8. Conclusion** 

**9. References** 

In this study, cross-substitution between the discriminative effects of U-50,488H and TRK-820 was not observed. The agonists tested in this study, except E-2078, tended to substitute for the discriminative stimulus effects of U-50,488H rather than those of TRK-820. These results suggest that U-50,488H and TRK-820 have differential properties. Furthermore, noncompetitive NMDA antagonists phencyclidine (PCP, 0.5-2.0 mg/kg) and MK-801 (10-80 g/kg) dose-dependently generalized to the discriminative stimulus effects of U-50,488H (3.0 mg/kg, i.p.) in the cross-substitution tests. On the other hand, PCP and MK-801 at doses that generalized to the discriminative stimulus effects of U-50,488H did not generalize to those of TRK-820 (40 g/kg, i.p.) (Mori et al., 2006). The outcomes clearly indicate different properties between TRK-820 and U-50,488H.
