The invention provides a compound of formula (I), wherein R1 is H, C(NH)NH2, an amino acid, or a peptide; X is OH, NH2, NHR2, NR2R3, an amino acid, or a peptide; R2 and R3 are selected from alkyl, alkylenearyl, or alkyleneheteroaryl; each R4 and R5 is independently H or CH3; Z is 2-amino-2,3-dihydro-1H-indene-2-carboxylic acid; 2-amino-1,2,3,4-tetrahydronaphthalene-2-carboxylic acid; 6-amino-6,7,8,9-tetrahydro-5H-benzo[7]annulene 6-carboxylic acid; cyclohexylalanine; cyclohexylglycine; homophenylalanine; 1-naphthylalanine; 2-naphthylalanine; 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid; or octahydro-1H-indole-2-carboxylic acid; n is 0, 1, 2, 3, or 4; with the proviso that X is not NH2 when R1 is H, R4 is H, R5 is CH3, Z is Aci, and n is 2; or a pharmaceutically acceptable salt, ester or solvate thereof. A method of treating pain and a method for treating a mu-opioid receptor mediated disorder and/or a delta-opioid receptor mediated disorder also are provided.