Thiorphan
Thiorphan is the active metabolite of the antidiarrheal racecadotril (acetorphan). It prevents the degradation of endogenous enkephalins by acting as an enkephalinase inhibitor. References 

Enkephalinase inhibitors
Antidiarrhoeals
Alpha-Amino acids
Amino acid derivatives
Thiols
Carboxamides
Propionamides