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An overview of the biomimetic catalysts used in the oxidative activation of drugs is given, with an emphasis on the use of synthetic metalloporphyrins as models of cytochrome P450 to mimic the in vivo metabolism of pharmaceuticals. In addition, a special focus is directed towards the recent results from the authors' group on the oxidative activation of isoniazid, an antitubercular drug.

Isoniazid (INH) is easily oxidized with manganese(III) pyrophosphate, a chemical model of the KatG protein involved in activation of INH inside the bacteria Mycobacterium tuberculosis. Performed in the presence of NAD(+), this oxidation generates a family of isomeric INH-NAD(H) adducts, which have been shown to be effective inhibitors of InhA, an enzyme essential in mycolic acid biosynthesis. In this work, we fully characterized by (1)H and (13)C NMR spectroscopy four main species of INH-NAD(H) adducts that coexist in solution. Two of them are open diastereoisomers consisting of the covalent attachment of the isonicotinoyl radical at position four of the nicotinamide coenzyme. The other two result from a cyclization involving the amide group from the nicotinamide and the carbonyl group from the isonicotinoyl radical to give diastereoisomeric hemiamidals. Although an INH-NAD(H) adduct with a 4S configuration has been characterized within the active site of InhA from Xray crystallography and this bound adduct interpreted as an open form (Rozwarski et al., Science 1998, 279, 98-102), it is legitimate to raise the question about the effective active form(s), open or cyclic, of INH-NAD(H) adduct(s). Is there a single active form or are several forms able to inhibit the InhA activity with different levels of inhibitory potency?

The antituberculosis drug isoniazid (INH) is quickly oxidised by stoichiometric amounts of manganese(III)-pyrophosphate and the following products were identified: isonicotinic acid 1 , isonicotinamide 2 and isonicotinaldehyde 3 , the acid being the major product. The oxidation of INH with Mn III -pyrophosphate was carried out in either H 2 16 O, or H 2 18 O or D 2 O and under varied atmosphere composition (argon, air, O 2 or 18 O 2 ). LC–MS analyses of isotope incorporation suggest the simultaneous presence of two competitive pathways leading to the formation of acid 1 , with the isonicotinoyl radical as a common intermediate. One route is oxygen-dependent and the other is water-dependent. Analyses of isotope incorporation in amide 2 and aldehyde 3 also support this mechanism.

Oxygenation reactions (hydroxylation, epoxidation, N- or S-oxide formation, etc.) catalyzed by cytochrome P450 enzymes and related biomimetic models involve an electrophilic oxidative species as the active species, namely a high-valent metal-oxo intermediate. Among the different methods to study the oxygenation reactions mediated by high-valent metal-oxo porphyrin complexes, the recent discovery of oxo-hydroxo tautomerism provides a useful tool to investigate the mechanism of O-atom transfer reactions in aqueous media.

In contrast to the abundant literature available on the oxidation of primary and secondary alcohols, examples of oxidation of tertiary alcohols are rather rare and a general review on this topic does not exist. Therefore, this microreview will briefly describe the different existing methods, mainly metal-mediated, which consist of one or two-electron oxidations and usually lead to rearrangement, cyclization or fragmentation products. Some related data is also given for fragmentation reactions observed in basic or acid medium, or catalyzed by metal complexes without a real oxidation step.

Oxidation of 4-(1-hydroxy-1-phenylethyl)benzoic acid HPEBA with a water-soluble metalloporphyrin as catalyst and KHSO5 as oxygen atom donor gives the major products, acetophenone AC and acetylbenzoic acid ABA, by a Caliph–CAr bond cleavage, but a minor product, benzoyloxybenzoic acid BOBA, requires the insertion of an oxygen atom to form the ester. This compound becomes the main oxidation product on increasing the amount of acetonitrile in the reaction medium, and its formation is oxygen-dependent. The conversion is drastically lowered by using D2O instead of H2O, suggesting that an alkoxyl radical is formed in the rate-determining step. Labeling experiments using 18O2 or H218O under different reaction conditions show that the carbonyl oxygen atoms of AC and ABA originate either from substrate, water or dioxygen. However, the carbonyl oxygen atom in the ester group of BOBA originates from dioxygen while the other oxygen atom of the ester remains unlabeled. These results can be explained by an O-neophyl rearrangement of the initial alkoxyl radical to afford a carbon-based radical which then reacts with dioxygen or MnIV-OH/water. In a competitive reaction pathway, direct β-scission of the alkoxyl radical leads to unlabeled products. The oxidation of other tertiary diaryl alcohols is also discussed.

Autoxidation of sulfite is catalyzed by the water-soluble complex (meso-tetrakis(4-N-methylpyridiniumyl)porphyrinato)manganese(III) (Mn-TMPyP), leading to an intermediate species capable of hydroxylation of p-isopropylbenzoic acid, epoxidation of carbamazepine, and generation of single-strand breaks both in plasmid ΦX174 DNA and in a 167-base-pair fragment of pBR322. 18O-Labeling studies confirm that the high-valent manganese−oxo intermediate undergoes oxo−hydroxo tautomerism in competition with oxidation of substrate or reduction by excess sulfite. In contrast to the classical reductive activation of dioxygen by metalloporphyrins (dithiothreitol or ascorbate in the presence of O2), a mechanism is proposed in which SO3•-, formed by oxidation with MnIII, is trapped by O2, leading to a species (SO5•-) capable of generating a MnVO complex. Importantly, these studies point to the use of an alternative, biocompatible oxidation system compared to the preformed oxidant KHSO5.

The iron complexes of a new series of weakly acidic acetamidosulfonylated tetraarylporphyrins were tested in aqueous medium as biomimetic oxidation catalysts of the usual peroxidase substrates; tetramethylbenzidine, guaiacol and ABTS. One of them ( meso -tetrakis[4-(acetamidosulfonyl)phenyl]porphyrinato-iron(III), activated by potassium monopersulfate, was shown to be much more efficient than the iron complexes of sulfonated tetraarylporphyrins and at least as efficient as horseradish peroxidase/H 2 O 2 used as reference system. The biomimetic oxidation of acetaminophen is presented as a convenient application of such chemical catalysts in oxidative drug metabolism studies.

High valent metal–oxo species have been evoked as active intermediates in many different oxidation reactions using manganese or iron porphyrin complexes as catalysts and oxygen atom donors (H2O2, PhIO, NaOCl, KHSO5, ... etc.) or dioxygen associated to a reductant as oxygen atom source. When these metalloporphyrin-catalysed oxidations are performed in water, such metal–oxo species are able to transfer an oxygen atom coming from either the oxygen source or from bulk water. This fact has been explained by the so-called oxo–hydroxo tautomerism, a mechanism involving a rapid shift of two electrons and one proton from a hydroxo ligand (electron-rich ligand formed by deprotonation of an aqua ligand) to the trans oxo species (electron-poor ligand) leading to the transformation of the hydroxo ligand into an electrophilic oxo entity on the opposite side of the initial oxo. This ‘oxo–hydroxo tautomerism’, evidenced by using 18O-labelled water, has been used as mechanistic tool to unambiguously characterize oxygen atom transfer mechanisms mediated by metal–oxo species in opposition to mechanisms related to free radical oxidation reactions.

International audience; The metalloporphyrin-catalyzed oxidation of SSR180575, a ligand of the peripheral benzodiazepine receptor, that contains both N-methylindole and N,N-dimethylamide functions, produces a high yield of a stable carbinolamide derivative.; Le composé SSR180575 est un ligand du récepteur périphérique des benzodiazépines, qui contient à la fois un cycle N-méthylindole et une chaîne N,N-diméthylamide. L’oxydation de ce composé catalysée par des métalloporphyrines produit avec un bon rendement un carbinolamide stable.

This review article focuses on the molecular aspects of DNA cleavage by synthetic chemical nucleases (transition metal complexes endowed with redox properties and DNA affinity) and natural drugs (cytotoxic agents such as bleomycins or enediynes). Unlike deoxyribonucleases, which catalyze the nucleophilic attack of water on the phosphorus atom of a particular phosphodiester entity, these nonhydrolytic DNA-cleavers are able to oxidize the sugar units, generally by hydrogen atom abstraction. Examples of oxidative attack on each of the five different CH bonds of deoxyribose are known, depending on the nature, structure, type of activation, or mode of DNA interaction of the DNA-cleaver. Further evolution at the site of the initial lesion leads to the release of bases, oxidized deoxyribose units, or oxidized sugar fragments appended to the base or the terminal phosphate. In most cases the loss of a part (at least) of a nucleoside, with the concomitant loss of one base information, primarily induces the cleavage of the DNA strand. For both types of DNA cleavage reagents studied within the two last decades, the modes of activation and DNA binding are presented, as well as the details on the mechanism of deoxyribose oxidative degration. Because of the need for highly efficient and highly specific reagents, the development of new artificial and selective DNA cleavers, supported by an improved knowledge of these different mechanisms of DNA cleavage, is to-day a challenging area in the rational design of antitumoral or antiviral agents, as well as in the field of molecular biology.

The chemical nuclease Mn-TMPyP/KHSO 5 is able to hydroxylate 5' carbon-hydrogen bonds of deoxyribose units at three contiguous A-T base pairs sites and initiate DNA rupture. Double-strand cleavage sites consist of trinucleotide sequences with a four nucleotide 3' stagger of the cleaved residues. Both strand nicks are identical and consist of 3'-phosphate termini facing a 5'-aldehyde residue which is easily reduced to a 5'-alcohol by NaBH 4 . These two successive treatments (oxidation plus reduction) are equivalent to a hydrolysis of the phosphodiester bond. Such mechanism is reminiscent of DNA restriction enzymes but with 5'-OH and 3'-phosphate ends at the site of cleavage

In contrast to the abundant literature available on the oxidation of primary and secondary alcohols, examples of oxidation of tertiary alcohols are rather rare and a general review on this topic does not exist. Therefore, this microreview will briefly describe the different existing methods, mainly metal-mediated, which consist of one or two-electron oxidations and usually lead to rearrangement, cyclization or fragmentation products. Some related data is also given for fragmentation reactions observed in basic or acid medium, or catalyzed by metal complexes without a real oxidation step.

The monopersulfate oxidation of 4-isopropylbenzoic acid performed in H(2)(18)O and catalyzed by a water-soluble metalloporphyrin indicated that half of the oxygen atoms incorporated in 4-(1-hydroxy-1-methylethyl)benzoic acid, the primary hydroxylation product, came from water. A redox tautomerism of the active high-valent hydroxo-metal-oxo porphyrin intermediate coupled with an oxygen rebound mechanism explained this result. Under similar conditions, ketoprofen was directly oxidized to 3-benzoylacetophenone, via at least two different reaction pathways. Trapping of radical intermediates by molecular oxygen competed with the oxygen rebound mechanism.

The metalloporphyrin-catalyzed oxidation of 2-methynaphthalene (1) by potassium monopersulfate produced mainly two naphthoquinones: 2-methyl-1,4-naphthoquinone (2) (menadione or vitamin K(3)) and 6-methyl-1,4-naphthoquinone (3). In aqueous solution and at room temperature in the presence of 5 mol % of the water-soluble metalloporphyrins MnTPPS or FeTMPS, 2-methylnaphthalene was quantitatively oxidized to quinones 2 and 3. Based on experiments performed in(18)O-labeled water and according to the "redox tautomerism" mechanism previously described for such catalysts, the oxidation to quinones is proposed to be mainly due to a cytochrome P-450-type oxygenation reaction (oxygen atom transfer), rather than a peroxidase-type oxidation (electron transfer).

Reported studies indicate that the association of potassium monopersulfate with [Mn(TMPyP)](OAc)5, a water-soluble manganese porphyrin complex, leads to an efficient reagent for the oxidative cleavage of DNA. Single-strand breaks (SSBs) are observed on double-stranded DNA at manganese porphyrin concentrations as low as 0.5 nM with a short incubation time of 1 min. The number of SSBs linearly varies with the concentration of the manganese complex, and potassium monopersulfate is at least 3 orders of magnitude more efficient as oxygen source than hydrogen peroxide. Cleavage efficiency is optimal in the pH range 7.5-9.0 for a NaCl concentration between 80 and 150 mM or for a MgCl2 concentration of 10 mM. At very low manganese porphyrin concentration and by increasing the incubation time a catalytic cleavage activity of the complex is evidenced: up to 5 SSBs per manganese porphyrin are observed. The high cleavage activity of the monopersulfate-manganese porphyrin system makes it a good candidate for DNA-footprinting experiments.

The association of potassium monopersulfate, KHSO5, with a water-soluble manganese porphyrin complex has been recently reported as being a very efficient system for the oxidative cleavage of DNA (Fouquet et al., J. Chem. Soc., Chem. Commun., 1987, 1169); similar results are observed with RNA. As puzzle piece of the mechanism of this oxidative degradation of nucleic acids, we report in the present communication the identification of 8-hydroxyadenosine-5′-monophosphate as oxidation product of adenosine-5′-monophosphate (AMP) by KHSO5/[Mn(Mepy)4P](OAc)5. In the absence of manganese catalyst, the only detectable compound is the adenosine-N1-oxide- 5′-monophosphate.