Synthesis and biological activity of endomorphin-2 analogs incorporating piperidine-2-, 3- or 4-carboxylic acids instead of proline in position 24391, Staniszewska, R., Fichna J., Gach K., Toth G., Poels J, J. Broeck Vanden, and Janecka A.E. , Chem.Biol.Drug Des, 0/7/2008, Volume 72, Issue 1, Number 94, p.91 - 94, (2008)