Inter-species comparison of microsomal reductive transformation of biologically active benfluron N-oxide

Use of chiral liquid chromatography for the evaluation of stereospecificity in the carbonyl reduction of potential benzo[c]fluorene antineoplastics benfluron and dimefluron in various species

Benfluron (B) [5-(2-dimethylaminoethoxy)-7H-benzo[c]fluorene-7-one hydrochloride] is a potential antineoplastic agent. In the organism, B undergoes a rapid phase I biotransformation through oxidative and reductive metabolic pathways. The carbonyl reduction of B leads to reduced benfluron, red-B, this is one of the principal pathways for the deactivation of this compound. The structure of B was modified to suppress its rapid deactivation via the carbonyl reduction on C7. Dimefluron, D (3,9-dimethoxy-benfluron) is one of the derivatives of B, in which an alternative metabolic pathway (O-desmethylation) prevails over the carbonyl reduction. The goal of this study was to develop HPLC methods enabling chiral separations of the red-B and -D enantiomers. The separation of red-B enantiomers was successful done on a Chiralcel OD-R column (250 mm x 4.6 mm ID, 5 microm) using a mobile phase acetonitrile-1 M NaClO4 (40:60, v/v). Another mobile phase, methanol-1 M NaClO4 (75:25, v/v), had to be employed for the sufficient resolution of red-D enantiomers. Flow rate was 0.5 ml min(-1) in both cases. Red-B was detected at 340 nm, red-D at 370 nm. The above chiral HPLC methods were used for the study of the biotransformation of B and D in the microsomal fractions of liver homogenates prepared from various species (rat, rabbit, pig, guinea pig, goat and human). The enantiospecificity of the respective carbonyl reductases was evaluated and discussed for both prochiral compounds, B and D.

Disposition study of a new potential antineoplastic agent dimefluron in rats using high-performance liquid chromatography with ultraviolet and mass spectrometric detection

The disposition of a new potential antineoplastic drug dimefluron after an oral administration to rats was investigated. Dimefluron, 3,9-dimethoxy-5-(2-dimethylaminoethoxy)-7H-benzo[c]fluoren-7-one hydrochloride, was administered in a single oral dose (250 mg kg(-1) of body weight) in the form of an aqueous solution via a gastric probe. Dimefluron metabolites were being searched for in rat faeces. Synthetic standards of the expected phase I metabolites (the products of O- and N-desmethylation, N-oxidation and carbonyl reduction of dimefluron) were prepared and used together with dimefluron and internal standard in the development of two HPLC bioanalytical methods based on different separation principles. The first separation of dimefluron and the phase I metabolites was tested on a 250 mm x 4 mm chromatographic column with LiChrospher 60 RP-selectB 5 microm (Merck) using an isocratic mobile phase containing 0.01 M nonylamine buffer (pH 7.4) and acetonitrile in the 1:2 ratio (v/v). The second separation was performed on a 250 mm x 4 mm chromatographic column Discovery HS F5, 5 microm (Supelco) using a linear gradient mode with the mobile phase containing acetonitrile and phosphate buffer (0.05 M KH2PO4, pH 3). The flow rate was 1 ml min(-1) in both cases. UV detection was performed in the dual wavelength mode, with 317 nm having been used for dimefluron and all 7H-benzo[c]fluoren-7-one metabolites, 367 nm for 7H-benzo[c]fluoren-7-ol metabolites. A higher homologue of dimefluron served as an internal standard. The identity of the dimefluron metabolites in biological samples was confirmed using HPLC-MS experiments. The elimination study showed that the concentration maximum for dimefluron and its metabolites in rat faeces was reached 48 h after the administration of the parent drug. O-Desmethylated derivatives of dimefluron prevailed among the phase I metabolites.

Human liver microsomal reduction of pyrrolizidine alkaloid N-oxides to form the corresponding carcinogenic parent alkaloid

Retronecine-based pyrrolizidine alkaloids, such as riddelliine, retrorsine, and monocrotaline, are toxic to domestic livestock and carcinogenic to laboratory rodents. Previous in vitro metabolism studies showed that (+/-)6,7-dihydro-7-hydroxy-1-(hydroxymethyl)-5H-pyrrolizine (DHP) and pyrrolizidine alkaloid N-oxides were the major metabolites of these compounds. DHP is the reactive metabolite of pyrrolizidine alkaloids and pyrrolizidine alkaloid N-oxides are generally regarded as detoxification products. However, a previous study of rat liver microsomal metabolism of riddelliine N-oxide demonstrated that DHP and its parent compound, riddelliine, were generated as the major metabolites of riddelliine N-oxide. In this study the metabolic activation of the three retronecine-based pyrrolizidine alkaloid N-oxides by human liver microsomes is investigated under oxidative and hypoxic conditions. Results shows that both the DHP and the corresponding parent pyrrolizidine alkaloids are the major metabolites of the human liver microsomal metabolism of pyrrolizidine alkaloid N-oxides. Under oxidative conditions, reduction of the N-oxide to pyrrolizidine alkaloid is inhibited and while under hypoxic conditions, DHP formation is dramatically decreased. The oxidative and reductive products generated from the metabolism of pyrrolizidine alkaloid N-oxides are substrate-, enzyme- and time-dependent. In the presence of troleandomycin, a microsomal CYP3A inhibitor, DHP formation is inhibited by more than 70%, while the N-oxide reduction was not affected. The level of microsomal enzyme activity in human liver is comparable with rats. The rate of in vitro metabolism by either human and rat liver microsomes follows the order of riddelliine > or = retrorsine > monocrotaline, and DHP-derived DNA adducts are detected and quantified by 32P-postlabeling/HPLC analysis. Similar DHP-derived DNA adducts are found in liver DNA of F344 rats gavaged with the pyrrolizidine alkaloid N-oxides (1.0 mg/kg). The levels of in vivo DHP-DNA adduct formation is correlated with the level of in vitro DHP formation. Our results indicate that pyrrolizidine alkaloid N-oxides may be hepatocarcinogenic to rats through a genotoxic mechanism via the conversion of the N-oxides to their corresponding parent pyrrolizidine alkaloids, and these results may be relevant to humans.