Reactivity of flavone acetic acid and its acyl glucuronide

Glycerolysis of acyl glucuronides as an artifact of in vitro drug metabolism incubations

During an investigation of the in vitro glucuronidation of benoxaprofen by human liver S-9 fraction, an unusual drug-related entity possessing a protonated molecular ion that was 74 mass units greater than the parent drug was observed. It was identified as the glycerol ester of benoxaprofen. Formation of this entity required inclusion of uridine diphosphoglucuronic acid (UDPGA) in the incubation, suggesting the formation of benoxaprofen acyl glucuronide followed by transesterification with the glycerol present in the incubation due to its presence as a stabilizer for liver subcellular fractions. Formation occurred during the sample work-up procedure while the samples were subjected to evaporation in vacuo, which does not remove glycerol. Conversion of purified benoxaprofen acyl glucuronide to the glycerol ester was demonstrated in glycerol at 37 degrees C. Other drugs that are converted to acyl glucuronides in vitro (diclofenac, mefenamic acid, tolmetin, and naproxen) were also shown to form corresponding glycerol esters when incubated with human liver S-9 fraction and UDPGA. The potential formation of glycerol esters of carboxylic acid drugs undergoing acyl glucuronidation in vitro represents an experimental artifact to which drug metabolism scientists should be aware.

Identification of New Flavone-8-Acetic Acid Metabolites Using Mouse Microsomes and Comparison with Human Microsomes

Flavone-8-acetic acid (FAA) is a potent anticancer agent in mouse but has not shown activity in humans. Because FAA metabolism could play a role in this interspecies difference, our aim was to identify the metabolites formed in vitro using mouse microsomes compared with those in human microsomes. Mouse microsomes produced six metabolites as detected by reversed-phase high-performance liquid chromatography-mass spectrometry (MS). Three metabolites were identified as the 3'-, 4'-, or 6-hydroxy-FAA, by comparison with retention times and UV and MS spectra of standards. Two metabolites presented a molecular weight of 296 (FAA = 280) indicating the presence of one oxygen but did not correspond to any monohydroxylated FAA derivative. These two metabolites were identified as epoxides because they were sensitive to epoxide hydrolase. The position of the oxygen was determined by the formation of the corresponding phenols under soft acidic conditions: one epoxide yielded the 3'- and 4'-hydroxy-FAA, thus corresponding to the 3',4'-epoxy-FAA, whereas the other epoxide yielded 5- and 6-hydroxy-FAA, thus identifying the 5,6-epoxy-FAA. The last metabolite was assigned to the 3',4'-dihydrodiol-FAA because of its molecular weight (314) and sulfuric acid dehydration that indicated that the 3'- and 4'-positions were involved. Compared with mouse microsomes, human microsomes (2 pools and 15 individual microsomes) were unable to metabolize FAA to a significant extent. In conclusion, we have identified six new FAA metabolites formed by mouse microsomes, whereas human microsomes could not metabolize this flavonoid to a significant extent. The biological importance of the new metabolites identified herein remains to be evaluated.

Predicting pharmacokinetics and drug interactions in patients from in vitro and in vivo models: the experience with 5,6-dimethylxanthenone-4-acetic acid (DMXAA), an anti-cancer drug eliminated mainly by conjugation

The novel anti-tumor agent 5,6-dimethylxanthenone-4-acetic acid (DMXAA) was developed in the Auckland Cancer Society Research Center. Its pharmacokinetic properties have been investigated using both in vitro and in vivo models, and the resulting data extrapolated to patients. The metabolism of DMXAA has been extensively studied mainly using hepatic microsomes, which indicated that UGT1A9 and UGT2B7-catalyzed glucuronidation on its acetic acid side chain and to a lesser extent CYP1A2-catalyzed hydroxylation of the 6-methyl group are the major metabolic pathways, resulting in DMXAA acyl glucuronide (DMXAA-G) and 6-hydroxymethyl-5-methylxanthenone-4-acetic acid. The predominant metabolite in human urine (up to 60% of total dose) was identified as DMXAA-G, which was chemically reactive, undergoing hydrolysis, intramolecular rearrangement, and covalent binding to plasma proteins. In vivo formation of DMXAA-protein adducts were also observed in cancer patients receiving DMXAA treatment. The comparison of the in vitro human hepatic microsomal metabolism and inhibition of DMXA by UGT and/or CYP substrates with animal species indicated species differences. Renal microsomes from all animal species examined had glucuronidation activity for DMXAA, but lower than the liver. In vitro-in vivo extrapolations based on human microsomal data indicated a 7-fold underestimation of plasma clearance in patients. In contrast, allometric scaling using in vivo data from the mouse, rat, and rabbit predicted a plasma clearance of 3.5 mL/min/kg, similar to that observed in patients (3.7 mL/min/kg). Based on in vitro metabolic inhibition studies, it appears possible to predict the effects on the plasma kinetic profile of DMXAA of drugs such as diclofenac, which are mainly metabolized by UGT2B7. However, it did not appear possible to predict the effect of thalidomide on the pharmacokinetics of DMXAA in patients based on in vitro inhibition and animal studies. These data indicate that preclincial pharmacokinetic studies using both in vitro and in vivo models play an important but different role in predicting pharmacokinetics and drug interactions in patients.

Disposition of the novel antitumour agent xanthenone-4-acetic acid in the mouse: identification of metabolites and routes of elimination

1. Xanthenone-4-acetic acid (XAA) is an experimental antitumour agent which resembles flavone-8-acetic acid in its induction of cytokine synthesis, nitric oxide production and tumour haemorrhagic necrosis. We have investigated the excretion and metabolic fate of XAA in the BDF1 mouse. 2. XAA was administered intravenously at the maximal tolerated dose (1090 mumol/kg). Urine, plasma and bile were collected and subjected to analysis by hplc. Urine samples demonstrated labile metabolites which released XAA following incubation with beta-glucuronidase/sulphatase or at pH 9.0. The structures of isolated XAA metabolites were characterized by ms or 1H-NMR spectra at 400 MHz. 3. The major metabolite pathway of XAA involves conjugation with glucuronic acid, since the resulting metabolite, XAA acyl glucuronide, accounts for 25% of the dose excreted in the urine. Other metabolite pathways include alpha-oxidation of the acetic acid side chain and aromatic hydroxylation of the xanthenone ring.

Flavone acetic acid (FAA) with recombinant interleukin-2 (rIL-2) in advanced malignant melanoma. IV: Pharmacokinetics and toxicity of flavone acetic acid and its metabolites

Flavone acetic acid (FAA) was administered at a dose of 4.8 g m-2 over 1 h to patients with advanced malignant disease in combination with Interleukin II. A new high performance liquid chromatography method is described to determine both the parent compound and eight drug-related products, and the conditions required to determine these components in plasma are discussed. The half-life over the first 8 h was 2.3 h, but the terminal clearance of the drug was extremely slow. Severe (WHO Grade 4) hypotension was observed in some patients. However, incidence of this did not appear to be associated with any differences in FAA plasma concentrations, nor were there differences in FAA clearance between those patients whose tumour responded to the drug combination and those who did not.