Effect of sulfaphenazole on tolbutamide distribution in rabbits: analysis of interspecies differences in tissue distribution of tolbutamide.

The effect of sulfaphenazole on the distribution of tolbutamide was examined by comparing the change in the steady-state volume of distribution (Vdss) determined from in vivo plasma elimination with the tissue-to-plasma concentration ratio of various tissues (Kp) in rabbits; this effect was compared with that previously reported in rats. In rabbits, the Kp values of six tissues studied (i.e., brain, heart, spleen, small intestine, muscle, and skin) increased in the presence of sulfaphenazole ; except for brain, lung, and adipose tissue, the tissue-to-plasma unbound concentration ratio (Kp,f) of other tissues did show a significant decrease. This suggested that both the tissue and plasma protein binding of tolbutamide were affected by sulfaphenazole and that the increase in Kp was due mainly to the displacement of plasma protein binding of tolbutamide by sulfaphenazole , which was greater than that of tissue binding, while no change in Kp was due to a parallel change in both the plasma protein binding and tissue binding of tolbutamide. In both rabbits and rats, the Vdss calculated from plasma concentration versus time curve was very close to that calculated from the Kp values and volumes of various tissues in the presence and absence of sulfaphenazole , respectively. The interspecies difference of the effect of sulfaphenazole on the tissue distribution of tolbutamide between rabbits and rats was elucidated from both in vivo tissue distribution and in vitro plasma protein binding studies.