Primary and secondary metabolism of pentamidine by rats

The antiprotozoal drug pentamidine [1,5-bis(4'-amidinophenoxy)pentane] has been previously shown to be metabolized by rat liver microsomes, and five of the seven putative primary metabolites have been identified. With the synthesis and identification of 5-(4'-amidinophenoxy)pentanoic acid and 5-(4'-amidinophenoxy)-1-pentanol as the remaining two metabolites, the primary metabolism of pentamidine in rats appears fully characterized. Use of [14C]pentamidine with rat liver microsomes confirms this conclusion, since no unidentified radioactive peaks were detected by high-performance liquid chromatography (HPLC). Isolated, perfused rat livers were used with [14C]pentamidine to identify secondary metabolites. Only two novel radioactive peaks were detected by HPLC analysis of perfused liver samples. The treatment of liver samples with sulfatase or beta-glucuronidase resulted in the reduction or elimination of these peaks and gave rise to peaks identified as para-hydroxybenzamidine and 5-(4'-amidinophenoxy)pentanoic acid. It was concluded from these results that only these two primary metabolites were conjugated with sulfate or glucuronic acid. After 4 h of incubation in the perfused liver system, approximately 15% of the recovered radiolabel was pentamidine. These results suggest that pentamidine metabolism can be rapid and extensive in rats.