New analogues of leucine-methionine-enkephalin.

Nine analogues of the opioid pentapeptides leucine-/ methionine-enkephalinamide, involving replacement of amino acid at position 5 or amino acids at positions 2 and 5, have been synthesized by the solid phase method using mainly 9-fluorenylmethyloxycarbonyl amino acid trichlorophenyl esters in the presence of 1-hydroxybenzotriazole, the solid support being the Merrifield resin. All the analogues were effective in inhibiting the electri cally stimulated contractions of the guinea pig ileum (in vitro) and one of them, tyrosyl-Dnorvalyl- glycyl-phenylalanyl-methioninamide was found to be about 82 times more active than morphine. They also exhibited analgesic activity as well as antidiarrhoeal activity in mice (in vivo).