Distribution of antiviral nucleoside analogues into brain and skin

Abstract: The aim of this thesis was to study physiological and physico-chemical factors influencing the pharmacokinetics and distribution over the blood-brain barrier of antiviral nucleoside analogues in the rat, and to study the distribution to the human skin in vivo of an anti-herpetic nucleoside analogue. Microdialysis was used to sample the free (unbound) extracellular concentration of the nucleoside analogues from the brain tissue, muscle and blood in the rat and from the dermis of healthy human volunteers. Microdialysis was also used in vitro to determine the plasma protein binding of the nucleoside analogues. HPLC was used for analysis and quantification of the nucleoside analogues. The HPLC assays were optimised by using PLS (Partial Least Square analysis), which allows a rapid development of assays for new nucleoside analogues. The effect of varying perfusion medium osmolality on the in vivo microdialysis recovery of caffeine and 5-chloro-2', 3'-dideoxy-3'-fluorouridine was investigated in rat brain. A linear correlation between the recovery over microdialysis membrane and osmolality of perfusion medium was demonstrated for both substances within a certain interval. At higher osmolality (>627 mosmol/l) of the perfusion medium the increase of the recovery of caffeine was non-linear, indicating that there is an upper limit of the change in extracellular volume in the brain. The distribution of alovudine (3'-fluorothymidine) to the brain and to the cerebrospinal fluid was studied after i. v. infusion and after s.c. injection. The concentration gradient between brain extracellular fluid and cerebrospinal fluid indicates that there is an active efflux of alovudine from the brain. Acetazolamide, a carbonic anhydrase inhibitor, inhibiting the cerebrospinal fluid production in the choroid plexus, had no influence on the AUC (Area Under the plasma concentration-time Curve) ratio brain/blood. This suggests that alovudine transport to the brain does not occur via the cerebrospinal fluid, but via cerebrovascular endothelium. Thymidine, administered locally or systemically, had no influence on the AUC ratio brain/blood, indicating that thymidine transport is not involved. Two inhibitors of transport proteins, probenecid and quinidine, were also used to study the alovudine transport system. Quinidine, but not probenecid, significantly decreased the alovudine concentration in the brain and increased the concentration gradient. Both perfusion through the microdialysis probe with alovudine solution at increasing concentration and quirridine administration significantly increased the microdialysis recovery of alovudine. The distribution into the rat brain of 5-substituted alovudine analogues and physico-chemical properties of these substances was studied and a multivariate quantitative structure-kinetic relationship was calculated. No correlation between the transport over the blood-brain barrier and the lipophilicity (octanol/water partition coefficient) could be demonstrated. Instead the pKa and the electronic distribution properties of the 5-substituent were found to influence the concentration gradient across the blood-brain barrier. This result indicates that alovudine analogues are subject to active transport (carrier-mediated) rather than passive diffusion across the blood-brain barrier. The distribution to the brain of the cytidine analogues: 2', 3'-dideoxycytidine and 3'-hydroxymethyl-2', 3'- dideoxycytidine over the blood-brain barrier were studied after s.c. administration. The pharmacokinetic properties were found to be similar. There was no significant difference with respect to blood-brain barrier transport between 2', 3'-dideoxycytidine and 3'-hydroxymethyl-2', 3'-dideoxycytidine. Thus, the sugar structure did not influence their transport into the brain. The distribution to the brain of penciclovir after i.v. injection or p.o. administration of its prodrug famciclovir and during i.v. infusion of penciclovir or famciclovir was studied in rats. Penciclovir was found to cross the blood-brain barrier and achieved significant concentrations in the brain. The distribution of penciclovir into the dermis of healthy volunteers was studied after a single dose of its prodrug, famciclovir, by microdialysis and by the suction blister technique. The results obtained with the suction blister technique and by microdialysis were similar and both showed that pencilovir reaches the skin in concentrations expected to inhibit herpes virus replication. However, microdialysis allows continuous sampling of the drug over a prolonged time after administration. The microdialysis concentration was decreased by cold and by adrenaline- mediated vasoconstriction.