BLOOD-TO-CENTRAL NERVOUS-SYSTEM ENTRY AND STABILITY OF BIPHALIN, A UNIQUE DOUBLE-ENKEPHALIN ANALOG, AND ITS HALOGENATED DERIVATIVES

Biphalin (Tyr-D-Ala-Gly-Phe-NH)(2) is a unique opioid peptide analog t hat contains two active enkephalin pharmacophores and is more potent t han morphine and etorphine in eliciting analgesia after intrathecal ad ministration. After systemic administration, only a small amount was d etected in the brain, but analgesia was observed. Because halogenation of enkephalin analogs has been shown to increase the brain uptake aft er systemic administration, our research group synthesized both p-[Cl- Phe(4,4')]biphalin and p-[F-Phe(4,4')]biphalin. The aim of the present study was to characterize and compare the blood-to-central nervous sy stem (CNS) pharmacokinetics and biological stability of biphalin and r elated halogenated analogs. The initial screening used an in vitro blo od-brain barrier model and identified p-[Cl-Phe(4,4')]biphalin as the enkephalin analog with the best potential for greater CNS entry. The C NS uptake and stability of biphalin and p-[Cl-Phe(4,4')]biphalin was e xamined further using an in situ brain perfusion technique coupled to high-performance liquid chromatography analysis. Both biphalin and its chlorohalogenated analog, were found to significantly enter the CNS t hrough both the blood-brain and blood-cerebrospinal fluid barriers. Ch lorohalogenation of biphalin was shown to both improve CNS entry, most likely through an enhancement in lipophilicity, and increase biologic al stability. This study suggests that incorporation of chlorohalogens at the p-Phe(4,4') position is a promising structural modification in the development of biphalin as a successful opioid drug for the clini c.