Oxygen-insensitive enzymatic reduction of oximes to imines

The mitochondrial amidoxime reducing component-from prodrug-activation mechanism to drug-metabolizing enzyme and onward to drug target

The mitochondrial amidoxime-reducing component (mARC) is one of five known molybdenum enzymes in eukaryotes. mARC belongs to the MOSC domain superfamily, a large group of so far poorly studied molybdoenzymes. mARC was initially discovered as the enzyme activating N-hydroxylated prodrugs of basic amidines but has since been shown to also reduce a variety of other N-oxygenated compounds, for example, toxic nucleobase analogs. Under certain circumstances, mARC might also be involved in reductive nitric oxide synthesis through reduction of nitrite. Recently, mARC enzymes have received a lot of attention due to their apparent involvement in lipid metabolism and, in particular, because many genome-wide association studies have shown a common variant of human mARC1 to have a protective effect against liver disease. The mechanism linking mARC enzymes with lipid metabolism remains unknown. Here, we give a comprehensive overview of what is currently known about mARC enzymes, their substrates, structure, and apparent involvement in human disease.

Pharmacokinetics of three novel pyridinium aldoxime acetylcholinesterase reactivators in female rats

A platform of novel lipophilic substituted phenoxyalkyl pyridinium oximes was invented to reactivate organophosphate-inhibited acetylcholinesterase. This platform has provided superior efficacy in rats to the current standard of care, 2-PAM, for survival of lethal doses of nerve agent surrogates as well as evidence of brain penetration and neuroprotection. The pharmacokinetics of three of these novel oximes in female rats was studied for comparison to previous data in male rats. Compared to the published half-life of 2-PAM (less than 2 h), the lead novel oxime, Oxime 20, displayed a plasma half-life of about 5 h in both sexes of rats following intramuscular administration. Very few sex differences in pharmacokinetic parameters were apparent. Oxime 20 displayed an increase in brain concentration to plasma concentration over the initial 2 h following intramuscular administration in male rats, with a plateau at 1 h; there were no differences in brain concentrations between the sexes at 2 h. Hepatic metabolism of Oxime 20 was higher in rat microsomes than in human microsomes. The relatively long plasma half-life is likely an important factor in both the enhanced survival and the neuroprotection previously observed for Oxime 20. The metabolism data suggest that the clearance of Oxime 20 could be slower in humans than was observed in rats, which might allow less frequent administration than 2-PAM for therapy of organophosphate acute toxicity. Therefore, the pharmacokinetic data combined with our earlier efficacy data suggest that Oxime 20 has potential as a superior therapeutic for nerve agent poisoning.

Evaluations of In Vitro Metabolism, Drug–Drug Interactions Mediated by Reversible and Time-Dependent Inhibition of CYPs, and Plasma Protein Binding of MMB4 DMS

1,1′-Methylenebis[4-[(hydroxyimino)methyl]-pyridinium] (MMB4) dimethanesulfonate (DMS) is a bisquaternary pyridinium aldoxime that reactivates acetylcholinesterase inhibited by organophosphorus nerve agent. Drug metabolism and plasma protein binding for MMB4 DMS were examined using various techniques and a wide range of species. When 14C-MMB4 DMS was incubated in liver microsomes, 4-pyridine aldoxime (4-PA) and an additional metabolite were detected in all species tested. Identity of the additional metabolite was postulated to be isonicotinic acid (INA) based on liquid chromatography with a tandem mass spectrometry analysis, which was confirmed by comparison with authentic INA. Formation of INA was dependent on species, with the highest level found in monkey liver microsomes. The MMB4 DMS exhibited reversible inhibition in a concentration-dependent manner toward cytochrome P450 1A2 (CYP1A2), CYP2C9, CYP2C19, CYP2D6, and CYP3A4 in human liver microsomes showing the highest inhibition for CYP2D6. Human recombinant CYPs were used to evaluate inhibitory curves more adequately and determine detailed kinetic constants for reversible inhibition and potential time-dependent inhibition (TDI). The MMB4 DMS exhibited reversible inhibition toward human-recombinant CYP2D6 with an inhibition constant ( Ki) value of 66.6 µmol/L. Based on the kinact/ KI values, MMB4 DMS was found to exhibit the most potent TDI toward CYP2D6. The MMB4 DMS at 5 different concentrations was incubated in plasma for 5 hours using an equilibrium dialysis device. For all species tested, there were no concentration-dependent changes in plasma protein binding, ranging from 10% to 17%. These results suggest that MMB4 was not extensively bound to plasma protein, and there were no overt species-related differences in the extent of MMB4 bound to plasma protein.

Absorption, Distribution, Metabolism, and Excretion of 14C-MMB4 DMS Administered Intramuscularly to Sprague-Dawley Rats and New Zealand White Rabbits

1,1′-Methylenebis[4-[(hydroxyimino)methyl]-pyridinium] dimethanesulfonate (MMB4 DMS) is currently under development for the treatment of chemical warfare organophosphorus nerve agent poisoning. The present study evaluates the absorption, distribution, metabolism, and excretion of 14C-MMB4 DMS administered intramuscularly to rats and rabbits. The formulated mixture of radiolabeled and nonradiolabeled MMB4 DMS was administered as a single or 7-day repeated dose. Rat doses were 55 or 220 mg/kg (100 µCi/kg), and rabbit doses were 25 or 100 mg/kg (31.25 and 62.5 µCi/kg, respectively). Urine, bile (rats only), feces, blood, and tissues were collected for up to 72 hours. Metabolic profiling using high-performance liquid chromatography with radiodetection was performed on selected urine samples. For both animal species, the majority of the total radioactivity was excreted in the urine (74%-94%) by 72 hours after dosing with greater than 90% of the radioactivity measured in the urine within 8 to 12 hours after dosing. There were no apparent species or dose differences in the urine excretion pattern. The distribution of 14C-MMB4 DMS-derived radioactivity was rapid and generally reached the highest concentration by the first collection time point (0.25 hours). The tissue–blood concentration ratios were highest at the injection sites and in the kidneys and gastrointestinal tract contents for both the species. Two metabolites of MMB4 DMS were detected in rat and rabbit urine; their structure was confirmed by liquid chromatography with tandem mass spectrometry as 4-pyridine aldoxime and isonicotinic acid (pyridine-4-carboxylic acid).

Non-oxime pyrazole based inhibitors of B-Raf kinase

The synthesis and biological evaluation of non-oxime pyrazole based B-Raf inhibitors is reported. Several oxime replacements have been prepared and have shown excellent enzyme activity. Further optimization of fused pyrazole 2a led to compound 38, a selective and potent B-Raf inhibitor.

CYP201A2, a cytochrome P450 from Rhodopseudomonas palustris, plays a key role in the biodegradation of tributyl phosphate

Tributyl phosphate (TBP) is a toxic organophosphorous compound widely used in nuclear fuel processing and chemical industries. Rhodopseudomonas palustris, one of the most metabolically versatile photosynthetic bacteria, is shown here to degrade TBP efficiently under photosynthetic conditions. This study shows that this O(2)- and NADPH/FMNH(2)-dependent process was also catalyzed when TBP was incubated with membrane-associated proteins extracted from this strain. The effects of several regulators of cytochrome P450 activity on the TBP consumption suggest a key role for a cytochrome P450 in this process. Disruption of the rpa0241 gene encoding a putative cytochrome P450 led to a 60% decrease of the TBP catabolism, whereas reintroducing the gene in the mutant restored the wild-type phenotype. The rpa0241 gene was expressed and purified in Escherichia coli. Characterization by UV-visible spectroscopy of the purified recombinant membrane-bound protein (CYP201A2) encoded by the rpa0241 gene revealed typical spectral characteristics of cytochrome P450 with a large spin state change of the heme iron associated with binding of TBP (K (d) approximately 65 microM). It is proposed that CYP201A2 catalyzes the initial step of the biodegradation process of TBP.

The biochemistry of drug metabolism--an introduction: Part 2. Redox reactions and their enzymes

This review continues a general presentation of the metabolism of drugs and other xenobiotics started in a recent issue of Chemistry & Biodiversity. This Part 2 presents the numerous oxidoreductases involved, their nomenclature, relevant biochemical properties, catalytic mechanisms, and the very diverse reactions they catalyze. Many medicinally, environmentally, and toxicologically relevant examples are presented and discussed. Cytochromes P450 occupy a majority of the pages of Part 2, but a large number of relevant oxidoreductases are also considered, e.g., flavin-containing monooxygenases, amine oxidases, molybdenum hydroxylases, peroxidases, and the innumerable dehydrogenases/reductases.

Mimicking damaged DNA with a small molecule inhibitor of human UNG2

Human nuclear uracil DNA glycosylase (UNG2) is a cellular DNA repair enzyme that is essential for a number of diverse biological phenomena ranging from antibody diversification to B-cell lymphomas and type-1 human immunodeficiency virus infectivity. During each of these processes, UNG2 recognizes uracilated DNA and excises the uracil base by flipping it into the enzyme active site. We have taken advantage of the extrahelical uracil recognition mechanism to build large small-molecule libraries in which uracil is tethered via flexible alkane linkers to a collection of secondary binding elements. This high-throughput synthesis and screening approach produced two novel uracil-tethered inhibitors of UNG2, the best of which was crystallized with the enzyme. Remarkably, this inhibitor mimics the crucial hydrogen bonding and electrostatic interactions previously observed in UNG2 complexes with damaged uracilated DNA. Thus, the environment of the binding site selects for library ligands that share these DNA features. This is a general approach to rapid discovery of inhibitors of enzymes that recognize extrahelical damaged bases.

Synthesis and high-throughput evaluation of triskelion uracil libraries for inhibition of human dUTPase and UNG2

Human nuclear uracil DNA glycosylase (UNG2) and deoxyuridine triphosphate nucleotidohydrolase (dUTPase) are the primary enzymes that prevent the incorporation and accumulation of deoxyuridine in genomic DNA. These enzymes are desirable targets for small molecule inhibitors given their roles in a wide range of biological processes ranging from chromosomal rearrangements that lead to cancer, viral DNA replication, and the formation of toxic DNA strand breaks during anticancer drug therapy. To accelerate the discovery of such inhibitors, we have developed a high-throughput approach for directed library synthesis and screening. In this efficient technology, a uracil-aldehyde ligand is covalently tethered to one position of a trivalent alkyloxyamine linker via an oxime linkage, and then the vacant linker positions are derivatized with a library of aldehydes. The resulting triskelion oximes were directly screened for inhibitory activity and the most potent of these showed micromolar binding affinities to UNG2 and dUTPase.