Distribution of 6-mercaptopurine ribonucleoside and other purine analogs to brain.

ase is also described. Radioactivity in plasma, brain, and muscle was determined after administration of 6-mercap topurine, 6-mercaptopurine ribonucleoside, 6-thioguanine, or 6-thioguanine ribonucleoside in mice or rats. At the times chosen for tissue sampling, there were no major differences in brain uptake between the base and ribonu cleoside pairs. Muscle radioactivity ranged from 3 to 7 times that in brain, indicating a barrier to the penetration of these agents into brain. Brain to plasma concentration ratio after 6-mercaptopurine ribonucleoside administration was independent of dose (0.02 to 50 ¿¿moles/mouse; 0.03 to 20 /¿moles/rat), route of administration (i.p., p.o., i.V.), and experimental duration (0.5 to 2 hr). Analyses of cerebrospinal fluid radioactivity in rats indicate that 6-mer captopurine ribonucleoside penetrates this compartment only to a limited extent.