[Biotransformation of benzamidine and benzamidoxime by microsomal enzymes of the rabbit]

The History of mARC

The mitochondrial amidoxime-reducing component (mARC) is the most recently discovered molybdoenzyme in humans after sulfite oxidase, xanthine oxidase and aldehyde oxidase. Here, the timeline of mARC's discovery is briefly described. The story begins with investigations into N-oxidation of pharmaceutical drugs and model compounds. Many compounds are N-oxidized extensively in vitro, but it turned out that a previously unknown enzyme catalyzes the retroreduction of the N-oxygenated products in vivo. After many years, the molybdoenzyme mARC could finally be isolated and identified in 2006. mARC is an important drug-metabolizing enzyme and N-reduction by mARC has been exploited very successfully for prodrug strategies, that allow oral administration of otherwise poorly bioavailable therapeutic drugs. Recently, it was demonstrated that mARC is a key factor in lipid metabolism and likely involved in the pathogenesis of non-alcoholic fatty liver disease (NAFLD). The exact link between mARC and lipid metabolism is not yet fully understood. Regardless, many now consider mARC a potential drug target for the prevention or treatment of liver diseases. This article focusses on discoveries related to mammalian mARC enzymes. mARC homologues have been studied in algae, plants and bacteria. These will not be discussed extensively here.

Reduction of N-hydroxylated compounds: amidoximes (N-hydroxyamidines) as pro-drugs of amidines

In order to examine the importance of metabolic cycles and in particular of reductions of N-hydroxylated compounds, the reversible metabolism at the amidine, guanidine, and amidinohydrazone nitrogen atoms of various drugs and model compounds was investigated. Many of these N-oxygenated metabolites are very easily reduced back into the starting materials. A comparison of the kinetic data for the N-hydroxylation and reduction suggests that the reduction should predominate in vivo. This could be verified by in vivo studies. Thus, N-hydroxylated amidines (amidoximes) can be used as pro-drugs of amidines. Because of their strong basicity, amidines, guanidines, and amidinohydrazones are protonated under physiological conditions, are very hydrophilic, and are usually not absorbed from the gastrointestinal tract. The N-hydroxylated derivatives of amidines (amidoximes), guanidines (N-hydroxyamidines), and amidinohydrazones (N-hydroxyamidinohydrazones) are less basic because of the introduction of the oxygen atom. They are absorbed from the gastrointestinal tract and then reduced to the active amidines, guanidines, and amidinohydrazones. The pro-drug principle was originally developed in our laboratory for pentamidine and then applied to other amidines such as sibrafiban and melagatran (ximelagatran). The enzymatic basis of N-oxidative processes is very well understood, whereas reductions have been less extensively investigated. We purified an enzyme system from pig and human liver consisting of cytochrome b5, its reductase, and a P450 enzyme, which is involved in the reduction of the N-hydroxylated compounds. Similar activities were found in all species studied so far. Furthermore, comparable reductive reactions could also be demonstrated with microsomal fractions from organs other than liver. In addition, mitochondria are highly capable of performing the reductions of these N-hydroxylated compounds. Thus, several organs and cell organelles are involved in the reduction explaining the extensive reduction of the pro-drugs in vivo underlying the suitability of the concept for drug development.

N-hydroxylation and N-dealkylation by P4502C3 of N-methylbenzamidine: N-oxygenation and N-oxidative dealkylation of one functional group

1. The first detection of a microsomal N-hydroxylation of an N-alkylated benzamidine possessing alpha-H atoms by P450 is now reported in the present in vitro biotransformation studies. 2. The newly found metabolites, N-hydroxy-N-methylbenzamidine and N-methylbenzamidoxime, were identified after hplc separation by comparison of their retention times with those of synthetic reference compounds and by comixing methods. N-hydroxy-N-methylbenzamidine exists predominantly in the aminonitrone form and constitutes a novel type of metabolite. 3. By means of reconstitution experiments with purified P4502C3 from rabbit liver and with purified variants of 2C3 expressed in Escherichia coli, it has been shown that the N-hydroxylation and the N-dealkylation of N-methylbenzamidine are catalysed by the same P450 isoenzyme. 4. A reaction mechanism is proposed in which the P450-dependent N-oxygenations and N-dealkylation of N-methylbenzamidine are derived from a common intermediate. It is obvious that if alpha-H atoms are present N-dealkylation is observed; however, in contrast with previous concepts, N-oxygenation is also possible.

Biotransformation of benzamidine and benzamidoxime in vivo

After administration of benzamidine (1) or benzamidoxime (2), respectively, to rats and rabbits, plasma from rats and rabbits as well as urine from rats were examined for the presence of benzamidoxime (2) or benzamidine (1). Some of the samples were worked-up directly and the others after enzymatic pretreatment with beta-glucuronidase or arylsulfatase, respectively. HPLC analysis was employed for the detection of the metabolites. After administration of 1, an in vivo N-hydroxylation of an amidine to an amidoxime was demonstrated for the first time. The metabolite 2 could only be detected after enzymatic cleavage of the glucuronide or sulfate, respectively, and only in plasma at a low concentration. After administration of benzamidoxime (2), on the other hand, benzamidine (1) was detected in very high concentrations in all biological samples. Benzamidine was present in the free state but indications for glucuronidization and sulfatation were also detectable. These investigations suggest that the benzamidoxime (2) formed by an in vivo N-hydroxylation undergoes ready retro-reduction but that further transformations of the metabolite 2, such as conjugation to a glucuronide or a sulfate, respectively, prevent complete back reaction. Furthermore, benzamide (3) could be detected as a transformation product in urine after administration of either 1 or 2.