ENANTIOMERS OF THIORPHAN AND ACETORPHAN - CORRELATION BETWEEN ENKEPHALINASE INHIBITION, PROTECTION OF ENDOGENOUS ENKEPHALINS AND BEHAVIORAL-EFFECTS

  • 1 November 1987
    • journal article
    • research article
    • Vol. 243  (2) , 666-673
Abstract
The relationships between various properties of inhibitors of enkephalinase (membrane metalloendopeptidase, EC 3.4.24.11) i.e., enzyme inhibition, protection of endogenous enkephalins, antinociceptive activity and stimulation of locomotor activity was investigated by comparing the relative potencies of the two enantiomers of Thiorphan and acetorphan, its parenterally active prodrug. In vitro (R)- and (S)-Thiorphan were almost equipotent in inhibiting enkephalinase activity (Ki, 1.7 and 2.2 nM, respectively) or thermolysin activity (Ki, 13 and 6 .mu.M, respectively) whereas the (R)-isomer was 44-fold less potent than the (S)-isomer on ACE activity (Ki 4800 and 110 nM, respectively). When tested on slices of rat globus pallidus in the presence of bestatin, to block the aminopeptidase pathway of enkephalin degradation, both Thiorphan enantiomers ensured a complete protection of endogenous (Met5)enkephalin released by depolarization and a suppression of the increase in the extracellular levels of Tyr-Gly-Gly, a characteristic enkephalin metabolite. These two effects occurred at EC50 values of the two enantiomers (10 nM in both cases), consistent with the idea that they were due to enkephalinase inhibition. After i.v. administration of the acetorphan enantiomers to mice, the enkephalinase activity of a rapidly prepared striatal membrane fraction was reduced in a dose-dependent manner with similar "ex vivo" ED50 values (1.0 and 0.3 mg/kg for the (R)- and (S)-isomer, respectively). In contrast the ACE activity of the same preparation was reduced in a significant manner only by (S)-acetorphan (ED50 value of 11 mg/kg). Striatal levels of endogenous Tyr-Gly-Gly in mice treated with (R)- or (S)-acetorphan in increasing dosages were reduced with ED50 values (0.8 and 0.4 mg/kg, respectively) closely similar to those required for "ex vivo" enkephalinase inhibition. Similar ED50 values of acetorphan enantiomers [0.7 and 0.6 mg/kg for the (R)- and (S)-isomer, respectively]also were found in mice for the naloxone-reversible antinociceptive activity of these compounds in the hot-plate jump test, whereas both were inactive in the hot-plate paw licking test. Both compounds also displayed marked antinociceptive activity in the writhing test (1 mg/kg) and enhanced the mouse locomotor activity at similar dosages. It is concluded that the behavioral and enkephalin protective activities of Thiorphan and acetorphan enantiomers are likely to result from enkephalinase inhibition.