In‐vivo Blood‐brain Barrier Transport of a Novel Adrenocorticotropic Hormone Analogue, Ebiratide, Demonstrated by Brain Microdialysis and Capillary Depletion Methods

Abstract— The transport of ebiratide, a novel adrenocorticotropic hormone (ACTH) analogue, [H‐Met‐(O₂)‐Glu‐His‐Phe‐d‐Lys‐Phe‐NH(CH₂)₈‐NH₂], through the blood‐brain barrier was directly demonstrated in‐vivo. [¹²⁵I]Ebiratide (16·9 MBq mL⁻¹) or [¹⁴C]sucrose (29·2 MBq mL⁻¹) known to be restrictively transported through the blood‐brain barrier was infused into the rat internal carotid artery at a flow rate of 50 μL min⁻¹ for 10 min, and after 15 min infusion the distribution volume of each compound in the brain parenchyma was determined by the capillary depletion method. The distribution volume of [¹²⁵I]ebiratide was 167·8 ± 62·2 μL (g brain)⁻¹, which was about seven times higher than that of [¹⁴C]sucrose (24·9 ± 4·0 μL g brain)⁻¹, indicating the uptake of ebiratide into brain parenchymal cells. During the infusion into the internal carotid artery, brain microdialysis was simultaneously performed to directly collect the brain interstitial fluid as the dialysate. Radioactivity was detected in the dialysate during the [¹²⁵I]ebiratide infusion and HPLC analysis of the dialysate revealed that the intact ebiratide accounted for ≥ 80% total radioactivity. The concentrations of [¹²⁵I]ebiratide and [¹⁴C]sucrose in the brain interstitial fluid were estimated based on the relative recovery obtained in the in‐vitro recovery study. The brain interstitial fluid/internal carotid arterial blood concentration ratio for [¹²⁵I]ebiratide was determined to be 1·47 × 10⁻² ± 0·17 × 10⁻² and was about eight times higher than that for [¹⁴C]sucrose (1·92 × 10⁻³ ± 0·36 × 10⁻³), indicating significant transport of ebiratide to the brain interstitial fluid. Accordingly, it was demonstrated that ebiratide is taken up into the brain in the intact form possibly via an absorptive‐mediated transport through the blood‐brain barrier. 1992 Royal Pharmaceutical Society of Great Britain