Physical dependence potential of an enkephalin analog, EK-399, in rats.
The physical dependence potential of Tyr-D-Met(O)-Gly-EtPhe-NHNHCOCH3.AcOH (EK-399), a novel enkephalin analog with a potent analgesic effect, was assessed in rats. The animals were given EK-399 (0.008, 0.032, 0.125, or 0.5 mg/kg), morphine (0.125, 0.5, or 2 mg/kg), pethidine (2 or 8 mg/kg), or pentazocine (2 or 8 mg/kg) every hour through an implanted intravenous cannula. After 3 days of treatment, precipitated withdrawal tests were conducted: naloxone (5 mg/kg) was administered subcutaneously. Rats treated with morphine showed withdrawal signs such as hyperirritability, salivation, diarrhea, and weight loss. Rats treated with pethidine, pentazocine, or EK-399 showed similar signs, but they were less evident than those in morphine-treated rats. In abrupt withdrawal tests after 7 days of treatment, rats treated with morphine, pethidine, or pentazocine showed weight loss, whereas rats treated with EK-399 showed little or no weight loss. In substitution tests, EK-399 suppressed the withdrawal signs in morphine-dependent rats, and vice versa. These results show that EK-399 has a morphine-like physical dependence potential that is weaker than that of morphine, pethidine, or pentazocine in rats.
Nishida
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Research and Development Division, Takeda Chemical Industries, Ltd., Takatsuki, Osaka, Japan.
D000818:Animals; D001835:Body Weight / Q000187:drug effects; D004745:Enkephalins / Q000008:administration & dosage / Q000493:pharmacokinetics / Q000494:pharmacology*; D007262:Infusions, Intravenous; D008297:Male; D008614:Meperidine / Q000494:pharmacology; D009020:Morphine / Q000494:pharmacology; D009270:Naloxone; D009293:Opioid-Related Disorders / Q000523:psychology*; D010423:Pentazocine / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013375:Substance Withdrawal Syndrome / Q000523:psychology
D004745:Enkephalins; D009270:Naloxone; D009020:Morphine; C057683:tyrosyl-methionyl(O)-glycyl-ethylphenylalanine-2-acetylhydrazide; D008614:Meperidine; D010423:Pentazocine