Tumor-cell-targeted methionine-enkephalin analogues containing unnatural amino acids: Design, synthesis, and in vitro antitumor activity

A series of new peptides (8-25) containing different unnatural amino acids of the adamantane type (1-6), was synthesized. Possible cytotoxic activity on human cervical adenocarcinoma ( HeLa), larynx carcinoma (HEp-2), colon carcinomas (HT-29, Caco-2), poorly differentiated cells from lymph node metastasis of colon carcinoma (SW-620), mammary gland adenocarcinoma (MCF-7), and melanoma (HBL) cells were tested by the MTT assay. The results were compared with the effect of methionine-enkephalin (Tyr-GlyGly-Phe-Met, or opioid growth factor, OGF), and its shorter N-terminal fragments. Peptide analogues containing CRR-dialkylated glycine (Aaa1, 1) or CR-alkylated glycine (Aaa2, 2) amino acid residues showed antitumor activity against melanoma, larynx carcinoma, colon carcinomas, and colon metastasis cell lines in vitro. The pentapeptide Tyr-(R,S)-Aaa2-Gly-Phe-Met (18) was the most effective analogue especially against the most antitumor drug-resistant cell lines HEp-2 and SW-620. Apoptosis as a mode of cell death was confirmed in these tumor cells after exposure to pentapeptide 18.