INHIBITOR OF PROTEIN DEPHOSPHORYLASE

PROBLEM TO BE SOLVED: To obtain an inhibitor of a protein dephosphorylase, comprising a pyrrolidone derivative as an active ingredient and useful as an antitumor agent, an immunosuppressant, a therapeutic agent for diabetes, etc. SOLUTION: This inhibitor of a protein dephosphorylase comprises a compound of formula I (R is methyl, or together with R denotes =CH2 ; R is OH, a 1-10C alkoxy, a halogen or nitroxy; R is cyano, a 2-10C alkanoyl or formula II) or its salt as an active ingredient. The compound of formula I is preferably obtained by condensing 2,3-dioxobutane with a compound of the formula R CH2 CONH2 and treating the resultant compound with an acid (e.g. 10-camphorsulfonic acid). The compound of formula I is directly used or formulated into a preparation according to a conventional method and used as the inhibitor of the protein dephosphorylase. 3-Cyano-4-methyl-5-vinylidene-2- pyrrolidone is exemplified as the compound of formula I.