In Vivo Transport of a Dynorphin-like Analgesic Peptide, E-2078, Through the Blood–Brain Barrier: An Application of Brain Microdialysis

In vivo transport through the blood–brain barrier (BBB) has been demonstrated for a dynorphin-like analgesic peptide, CH₃-[¹²⁵I]Tyr-Gly-Gly-Phe-Leu-Arg-CH₃Arg-D-Leu-NHC₂H₅ ([¹²⁵I]E-2078). A remarkable time-dependent increase in the distribution volume of [¹²⁵I]E-2078 in the brain parenchyma separated from blood vessels and capillaries was observed during a brain perfusion. The distribution volume of [¹²⁵I]E-2078 in the brain parenchyma after 20 min of perfusion was 2.18 ± 0.09 µl/g brain (mean ± SE) and was significantly greater than the distribution volume of [³H]inulin (0.994 ± 0.138 (µl/g brain), providing in vivo evidence for the penetration of [¹²⁵I]E-2078 into the brain parenchyma. Brain microdialysis was carried out to collect directly the brain interstitial fluid (ISF) during the brain perfusion of [¹²⁵I]E-2078. No metabolite of [¹²⁵I]E-2078 in the brain ISF was found by high-performance liquid chromatographic analysis of the brain dialysate. The concentrations of [¹²⁵I]E-2078 and [¹⁴C]sucrose in the brain ISF were estimated based on an in vitro evaluation of dialysis clearance. The concentration ratio of [¹²⁵I]E-2078 between the brain ISF and the brain perfusate was determined to be 2.92 × 10^−l ± 0.50 × 10^−l and was approximately 100 times higher than that of [¹⁴C]sucrose (2.71 × 10⁻³ ± 1.43 × 10⁻³), demonstrating transport of [¹²⁵I]E-2078 through the BBB in vivo. On the other hand, no remarkable difference in the cerebrospinal fluid (CSF)-to-perfusate concentration ratios of [¹²⁵I]E-2078 and [¹⁴C]sucrose was observed, indicating little contribution of the blood–CSF barrier (BCSF barrier) transport to the penetration of [¹²⁵I]E-2078 into the brain.