Distribution to the brain and protein binding of 3' and 5-substituted 2',3'-dideoxyuridine derivatives, studied by microdialysis

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 pK(a). 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.