Pharmacokinetic analysis of factors determining elimination pathways for sulfate and glucuronide metabolites of drugs. I: Studies by in vivo constant infusion

1. The hepatic and renal handling of glucuronides and sulphates of three phenolic compounds, 4-methylumbelliferone (4-MU), p-nitrophenol (pNP) and acetaminophen (APAP), were evaluated pharmacokinetically by in vivo constant infusion experiments in rat. It was shown that the urinary excretion rate at steady-state was larger than the biliary excretion rate for both glucuronides and sulfates, and sulfates, in particular, were extensively excreted into the urine. 2. For each glucuronide, however, biliary excretion clearances (CL_b) calculated based on the total concentration and unbound concentration in the liver were much larger than the corresponding renal excretion clearances (CL_r). Even in the case of sulfates, there was not any large difference between CL_r and CL_b based on the total and unbound concentration in tissues, which could not explain their extensive urinary excretion. From these results, these excretion clearances were recognized not to reflect necessarily the actual excretion rate obtained. 3. On the other hand, the tissue-to-plasma concentration ratio (K_p) of both glucuronides and sulfates for every phenolic compound was much higher in the kidney than that in the liver. The results suggested that one of the most important determinants for the preferential excretion of these conjugates into the bile or urine is the extent of disposition of each compound to the liver or kidney. 4. In addition, K_p of both glucuronides and sulfates in the liver, where these conjugates are mainly formed, was small. The K_p of sulfates was quite low, suggesting that sulfates generated in the liver were subject to extensive sinusoidal efflux.