Transport Characteristics of Endomorphin-2 Analogues in Brain Capillary Endothelial Cells

Because of their poor metabolic stability and limited blood–brain barrier permeability, endomorphins have a low analgesic efficacy when administered systemically. Therefore, it is of great importance to design analogues with improved peptidase resistance and better delivery to the central nervous system. Recently, novel endomorphin‐2 analogues have been synthesized, which proved to bind with high affinity and selectivity to the μ‐opioid receptors and showed proteolytic resistance. In this study, we have analysed the transport characteristics of endomorphin‐2 and three of its analogues [Dmt‐Pro‐Phe‐Phe‐NH2, Tyr‐(1S,2R)Acpc‐Phe‐Phe‐NH2 and Tyr‐(1S,2R)Achc‐Phe‐Phe‐NH2] using an in vitro blood–brain barrier model. The lipophilicity of the analogues, as assessed by their octanol/water partition coefficients, was higher than that of endomorphin‐2. The flux of all four peptides from the apical (blood) side to the basolateral (brain) side was not saturable in the 10 nm–1 mm concentration range, suggesting that a passive mechanism plays a major role in their transport. The permeability coefficient of the analogues was significantly higher than that of endomorphin‐2, suggesting increased blood–brain barrier penetration properties. We conclude that because of their good peptidase resistance and improved transport through brain endothelial cells, these endomorphin‐2 analogues will have better analgesic properties in vivo. In this study we have analyzed the transport characteristics of EM‐2 and three of its analogues (Dmt‐Pro‐Phe‐Phe‐NH2, Tyr‐(1S,2R)Acpc‐Phe‐Phe‐NH2 and Tyr‐(1S,2R)Achc‐Phe‐Phe‐NH2) using an in vitro BBB model. The permeability coefficient of the analogues was significantly higher than that of EM‐2, suggesting increased BBB penetration properties. We conclude that due to their good peptidase resistance and improved transport through brain endothelial cells these EM‐2 analogues may have better analgesic properties in vivo.