Distribution to the Brain and Protein Binding of 3′and 5-Substituted 2′,3′-Dideoxyuridine Derivatives, Studied by Microdialysis

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The aim of this study was to investigate a series of 3′ and 5-substituted 2′,3′-dideoxyuridine derivatives (ddUD) with respect to plasma protein binding, half-life and distribution across the blood-brain barrier in the rat. The microdialysis technique was used to study protein binding in human plasma ( in vitro), and to sample the extracellular space of rats with microdialysis probes implanted into the striatum of the brain and the gastrocnemic muscle ( in vivo). The compounds were analysed by HPLC with UV-detection. The octanol/water partition coefficients of the ddUD varied from 0.08-0.84. The protein binding of the ddUDs was approximately 80%. After s.c. administration (25 or 50 mg kg−1), the brain and muscle extracellular levels differed; brain levels were 0.18-0.36 of peripheral (muscle) concentrations. A multivariate analysis, which included data on zidovudine, alovudine and thymidine, demonstrated a relationship between the physicochemical and some of the pharmacokinetic properties of uridine analogues. The analysis shows that half-life and protein binding increases with decreasing p Ka. However, penetration to the brain is not correlated with the partition into octanol. It is concluded that the transport to the brain is not primarily dependent upon passive diffusion over a lipophilic barrier but, rather, to other chemical properties of the ddUDs. This is suggestive of a specific transport mechanism, e.g. the thymidine carrier.