Synthesis and biological activity of cyclolinopeptide A analogues modified with γ3-bis(homophenylalanine)

• Phenylalanine was replaced in Cyclolinopeptide A (CLA) by γ3-bis(homophenylalanine).
• Biological activity of new CLA analogues and their linear precursors was examined.
• Modification of CLA by γ3-hhPhe increased its antiproliferative action.
• Potential immunosuppressive drugs were selected after in vivo tests.

Cyclolinopeptide A, naturally occurring immunomodulatory nonapeptide, was modified with S or R-γ3-bis(homophenylalanine) in positions 3 or 4, or both 3 and 4. The replacement of one or both Phe residues by γ3-hhPhe led to decrease of their conformational flexibility in the analogues in comparison to CLA. All cyclic peptides, except 11, exist as isomers with the cis Pro–Pro peptide bond. Cyclic peptide 11 with single modification S-γ3-hhPhe4 exists as a mixture of two isomers and the major isomer (89%) contains all peptide bonds of the trans geometry.The peptides were subjected to several immunological tests in vitro and in vivo. Linear peptides 1–8, precursors of CLA analogues 9–16, were not toxic against human peripheral blood mononuclear cells (PBMC) but cyclic analogues showed dose-dependent toxicity with exception of peptide 11. Linear peptides did not inhibit mitogen-induced PBMC proliferation whereas cyclic ones inhibited the proliferation in a dose-dependent manner. The actions of linear and cyclic peptides with regard to lipopolysaccharide (LPS) -induced tumour necrosis factor alpha (TNF α) production in whole human blood cultures were differential but particularly suppressive in the case of linear compound 6. Therefore, for in vivo tests compounds 6 and 11 were selected. The compounds showed comparable, suppressive actions in induction and effector phases of delayed type hypersensitivity as well as in the carrageenan-induced foot pad edema in mouse models. In summary, linear peptide 6 and cyclic peptide 11 are attractive as potential immune suppressor drugs.

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