Controlled release of phenolic opioids

A compound of structural Formula (I):

• or a pharmaceutically acceptable salt thereof wherein:
• X is a phenolic opioid, wherein the hydrogen atom of the phenolic hydroxyl group is replaced by a covalent bond to —C(O)—Y—(C(R1)(R2))n—N—(R3)(R4);
• Y is —NR5—and R5 is (1-4C)alkyl;
• n is 2 or 3;
• R1 and R2 are each hydrogen;
• R3 is hydrogen or (1-4C)alkyl;
• and
• R4 is a residue of an L-amino acid selected from alanine, arginine, asparagine, aspartic acid, cysteine, glycine, glutamine, glutamic acid, histidine, isoleucine, leucine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, lysine and valine;
• a residue of a dipeptide or tripeptide composed of two or three L-amino acid residues selected independently from alanine, arginine, asparagine, aspartic acid, cysteine, glycine, glutamine, glutamic acid, histidine, isoleucine, leucine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, lysine and valine;
• or a residue of an N-acyl derivative thereof.