Codeine Epoxide, a New Metabolite of Codeine, and Epoxides of Morphine and Heroin on the Electrically Stimulated Guinea Pig Ileum Were Studied

EFFECTS OF EPOXIDES OF HEROIN, MORPHINE AND CODEINE ON THE ELECTRICALLY STIMULATED GUINEA PIG ILEUM

Issei TAKAYANAGI, Naoki MIYATA*, Kiyoko UBA*, Keizo WATANABE* and Masaaki HIROBE*

Department of Chemical Pharmacology, Toho University School of Pharmaceutical Sciences , Miyama, Funabashi, Chiba 274, Japan and *Department of Bioorganic and Medicinal chemistry , Faculty of Pharmaceutical Sciences, University of Tokyo, Bunkyo-ku , Tokyo 113, Japan

Accepted March 6, 1981

Abstract-Modes of action of codeine epoxide, a new metabolite of codeine, and epoxides of morphine and heroin on the electrically stimulated guinea pig ileum were studied. The opiates and their epoxides inhibited the twitch response of the ileum to electrical stimulation. The inhibitory actions of the test drugs were antagonized by naloxone. The activities of epoxides were not strikingly different from those of the parent opiates. Dependence liabilities of the epoxides were also tested on the guinea pig ileum which was treated with a high concentration of the test drugs. Epoxydation of 7,8-double bond in the parent opiates showed a trend toward a decrease in dependence liability.

Codeine-7,8-oxide (codeine epoxide) has, suspended in a 20 ml organ bath filled with recently, been identified as a new metabolite Locke Ringer solution kept at 32°C and of codeine in the rat (1). Derivatives of gassed with a mixture of 95% oxygen and 5% 7,8-oxides of morphine and heroin are carbon dioxide. The twitch responses of the assumed to be metabolites of morphine and ileum to electrical stimulation were recorded heroin. As opiate receptors in the chol isometrically under an initial tension of 1.0 g. inergic nerves of the guinea pig ileum are The electrodes were made of platinum and similar to those in the central nervous the intraluminal electrode was the anode. system (2). The isolated guinea pig ileum is Rectangular pulses of 0.1 msec duration used as a model to investigate the mode of were used at a frequency of 0.1 Hz and at a action of narcotic analgesics in the central voltage sufficent to give a maximal responses nervous system. We studied the modes of (3). Locke Ringer solution used had the action of codeine, morphine, heroin and following composition (mM); NaCI 154, their epoxides on the electrically stimulated KCI 5.6, CaC12 2.2, MgC12 2.1, NaHC03 5.9 guinea pig ileum and our findings are and glucose 2.8. In order to obtain a dose reported herein. inhibitory response curve of drug, the drug was applied cumulatively. MATERIALSAND METHODS The guinea pig ileum was also used to Male guinea pigs weighing 300 to 350 g assess the dependence liability of the test were killed, the ileum isolated and a section compounds. Two pieces of ileum were (3 to 5 cm) taken from the middle ileum was isolated from an adjacent area of the middle Table 1. Effects of naloxone (10-8 M) on the test compound-treated guinea pig ileum stimulated electrically in the presence of different concentrations (see Materials and Methods).

ileum : one was used for the test of a parent in the Department of Bioorganic and compound and the other for evaluating the Medicinal Chemistry, Faculty of Phar corresponding epoxide. The piece of ileum maceutical Sciences, University of Tokyo (4, was incubated for 24 hr in Locke Ringer 5). The test compounds were dissolved in solution containing of the test compounds Locke Ringer solution and the concentrations in the concentration listed in Table 1, was used refered to their bases. kept at 20°C and gassed with a mixture of 95% oxygen and 5% carbon dioxide and RESULTS then washed every 10 min for 60 min with All the test compounds dose-dependently Locke Ringer solution containing one tenth inhibited the twitch responses to electrical of the concentration required to produce 50% stimulation as shown in Fig. 1. A parallel of inhibition (IC50). As listed in Table 1, line assay was employed for the inhibitory the concentrations of morphine, heroin and their epoxides in an incubation medium were 100 times higher than their IC50's, however, those of codeine and its epoxide were approximately the same as their IC50's because of the large IC50's. The ileum treated above was adjusted a length of 4 cm under a tension of 1.0 g and electrically stimulated as mentioned in the experiments with the normal untreated ileum. Naloxone (10-6 M) was applied to this ileum in the presence of a tenth of IC50 of the test compound and changes in the base line Fig. 1. Effects of the test compounds on the twitch tension and twitch response were observed. response of guinea pig ileum to electrical Drug used: Morphine hydrochloride stimulation. Values are the means of 9 experi ments. (Sankyo, Japan), codeine sulfate (Tanabe, -s-: morphine , --S : morphine epoxide, Japan) and naloxone (Sankyo, Japan). -A-: heroin , ---A- : heroin epoxide, Heroin and all the epoxides were synthesized -0-: codeine , - 0 : codeine epoxide. potency ratios using morphine as a standard. Table 2. The inhibitory activity of each The potentcy ratios of the test compounds epoxide was not strikingly different from that relative to morphine are summarized in of the corresponding parent opiate. The

Table 2. Potency ratios of the test compounds on the electrically stimulated guinea pig ileum.

Fig. 2. Effects of naloxone (10-6 M) on the inhibitory actions of the test compounds on the electrically stimulated guinea pig ileum. Similar results were observed in 6 strips from 6 guinea pigs. inhibitory actions of the epoxides were Ringer solution which did not contain the antagonized by a narcotic antagonist, opiate was challenged by naloxone (10-6 M) naloxone (10-6 M), thereby indicating that in the presence of morphine (a tenth of IC50; they were narcotics (Fig. 2). 1.5x10-8 M), no contracture of ileum was In the following experiments the ileum observed and the twitch responses to treated with a high concentration for 24 hr electrical stimulation were not influenced by (see Materials and Methods) was used to morphine (1.5x10-8 M). compare the dependence liability of a epoxide with that of the corresponding parent opiate. DISCUSSION After the ileum had been incubated for 24 hr The potency ratios of codeine and heroin with the concentration listed in Table 1, the relative to morphine were similar to the values electrically stimulated ileum was challenged reported by Audigier et al. (6) who estimated by naloxone (10-6 M) in the presence of one the analgesia induced by intracerebro tenth of IC50. Addition of naloxone produced ventricular injection of the both drugs to a large contracture of the ileum and increased mice and also in binding studies with 3H the twitch response (Fig. 3). An area (S etrophine performed on a rat brain homoge in Fig. 3) surrounded with a base line (dotted nate. The inhibitory actions of all the test line in Fig. 3) and contracture curve was compounds on the electrically stimulated measured and a ratio (y'/y in Fig. 3) of the ileum were antagonized by naloxone. twitch responses before and after naloxone Analgesic of the parent opiates, given s.c. to (10-6 M) were also calculated. The change(s) rats (5), through the inhibitory actions on the in the base line tension of the epoxide twitch response were not strikingly influ treated ileum after naloxone challenge was enced by epoxydation in this study. The smaller than that in the base line tension of analgesic activities in the rats (5) were the ileum treated by the corresponding dependent on affinities and intrinsic activities parent opiate. The ratios (y'/y) obtained in and also on distribution and metabolism of the ileum treated by the epoxides were not the test compounds. The potency ratios of significantly different from those in the the test compounds in this study are con ileum treated by the corresponding parent sidered to reflect their relative affinities to opiates. All the ratios (y'/y) were larger the opiate receptors. than one and are summarized in Table 1. Ehrenpreis et al. (7, 8) found that the When the ileum incubated for 24 hr in Locke small intestine of chronically morphinized guinea pig characteristically contracted upon naloxone exposure. Furthermore, myenteric plexus-longitudinal muscle strips prepared from tolerant/dependent guinea pig and continuously exposed to normorphine, dis played a contracture upon naloxone chal lenge (9-11). These phenomena are con sidered to represent a sign of abstinence from Fig. 3. Recording of the naloxone-induced con narcotics (9-11), since naloxone does not tracture of the ileum treated with morphine cause any change in the tension of prepara (2 x 10_6 M) for 24 hr. The ileum were electrically tions obtained from naive rats. It was demon stimulated in the presence of 1 x10-8 M (a tenth of IC50) of morphine. See Materials and strated that the contracture of the preparation Methods. after naloxone challenge was more intense in the presence of opiate than in the absence of were not strikingly different from those of it (9, 11). Therefore, Locke Ringer solution the parent opiates. Although epoxydation of containing one tenth of IC50 of one of the 7,8-double bond in the opiates tends toward test compounds was used as the bath fluid a decrease in dependence liability, epoxides in this study, when naloxone was being may still have a high degree of dependence administered. The change in the base liability. line tension of epoxide-treated ileum after naloxone challenge was smaller than that in REFERENCES

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