The disposition of pyrimethamine in the isolated perfused rat liver

We have investigated the disposition of pyrimethamine base in the isolated perfused rat liver (IPRL) preparation after the administration of pyrimethamine (0.5 mg, 5 microCi). In the first half hour of the study, pyrimethamine underwent marked hepatic uptake, thereafter perfusate plasma drug levels declined monoexponentially with a half life (t 1/2) of 3.0 +/- 1.0 hr. Area under the perfusate plasma concentration/time curve (AUC)0----infinity was 6.9 +/- 1.9 microgram/hr/ml. Pyrimethamine was found to be a low clearance compound (78.4 +/- 25.3 ml/hr identical to 8.6% of liver perfusate flow) with a large volume of distribution (267.5 +/- 55.3 ml) in the IPRL. The combined AUCS(0----5hr) for pyrimethamine (AUC 4.8 +/- 0.5 microgram/hr/ml) and pyrimethamine 3-N-oxide (AUC0----5hr 0.9 +/- 0.6 microgram/hr/ml) accounted for 57% of the total AUC0----5hr of [14C] radioactivity (10.0 +/- 2.6 micrograms/hr/ml). This indicates the presence of metabolites of pyrimethamine as yet unidentified in the perfusate. Biliary excretion of [14C] during the course of the IPRL preparations was extensive (29.0 +/- 10.3%) though only a small proportion was due to pyrimethamine and the 3-N-oxide metabolite. The majority of radioactivity in the bile was attributable to highly polar, but unidentified metabolites of pyrimethamine. At the conclusion of each experiment (5 hr), a significant proportion of [14C] radioactivity was recovered from the livers (22.9 +/- 5.3%). Subsequent HPLC analysis of the liver tissue indicated this to be unchanged pyrimethamine, with trace levels of the 3-N-oxide metabolite. Sub-cellular fractionation of the homogenized livers revealed the most pronounced localisation of pyrimethamine to be in the lipid rich 10,000 g pellet (13.0 +/- 2.6%), the remainder being distributed equally between the 105,000 g pellet and supernatant. Neither pyrimethamine, [14C] radioactivity, nor pyrimethamine 3-N-oxide were extensively taken up by red cells throughout the study. Therefore, the large volume of distribution (267.5 +/- 55.3 ml) underlines the extent of pyrimethamine localisation in the liver.