Halogen-Substituted Triazolethioacetamides as a Potent Skeleton for the Development of Metallo-ß-Lactamase Inhibitors.

Metallo-ß-lactamases (MßLs) are the target enzymes of ß-lactam antibiotic resistance, and there are no effective inhibitors against MßLs available for clinic so far. In this study, thirteen halogen-substituted triazolethioacetamides were designed and synthesized as a potent skeleton of MßLs inhibitors. All the compounds displayed inhibitory activity against ImiS with an IC50 value range of 0.032⁻15.64 µM except 7. The chlorine substituted compounds (1, 2 and 3) inhibited NDM-1 with an IC50 value of less than 0.96 µM, and the fluorine substituted 12 and 13 inhibited VIM-2 with IC50 values of 38.9 and 2.8 µM, respectively. However, none of the triazolethioacetamides exhibited activity against L1 at inhibitor concentrations of up to 1 mM. Enzyme inhibition kinetics revealed that 9 and 13 are mixed inhibitors for ImiS with Ki values of 0.074 and 0.27µM using imipenem as the substrate. Docking studies showed that 1 and 9, which have the highest inhibitory activity against ImiS, fit the binding site of CphA as a replacement of ImiS via stable interactions between the triazole group bridging ASP120 and hydroxyl group bridging ASN233.

A Mercaptoacetamide-Based Class II Histone Deacetylase Inhibitor Suppresses Cell Migration and Invasion in Monomorphic Malignant Human Glioma Cells by Inhibiting FAK/STAT3 Signaling.

Histone deacetylase inhibitors (HDACIs) have emerged as potential anticancer agents for the treatment of solid and hematopoietic cancers. Several HDACIs delay cell growth, induce differentiation, or activate apoptosis in multiple types of tumors, including glioblastomas. In the present study, we showed that the mercaptoacetamide-based HDACI W2 inhibits cell migration and invasion in monomorphic malignant human glioma cells. W2 treatment significantly decreased the activity and expression levels of matrix metalloprotease-2 in malignant A172 cells but not in U373MG cells. Key signaling pathways involved in cell migration and invasion, including PI3K-AKT, ERK-JNK-P38, and FAK/STAT3, were examined to identify the mechanism of action of W2. W2 increased the phosphorylation of AKT and altered cell migration and invasion in an AKT-independent manner. W2 inhibited the phosphorylation of FAK/STAT3, and treatment with a FAK/STAT3 inhibitor significantly suppressed cancer cell migration and MMP-2 activity in the presence of W2. In addition, W2 significantly inhibited the nuclear translocation of phospho-STAT3. Taken together, our results suggest that W2 suppresses cancer cell migration and invasion by inhibiting FAK/STAT3 signaling and STAT3 translocation to the nucleus in monomorphic malignant human glioma cells. J. Cell. Biochem. 118: 4672-4685, 2017. © 2017 Wiley Periodicals, Inc.

Azolylthioacetamides as a potent scaffold for the development of metallo-ß-lactamase inhibitors.

In an effort to develop new inhibitors of metallo-ß-lactamases (MßLs), twenty-eight azolylthioacetamides were synthesized and assayed against MßLs. The obtained benzimidazolyl and benzioxazolyl substituted 1-19 specifically inhibited the enzyme ImiS, and 10 was found to be the most potent inhibitor of ImiS with an IC50 value of 15 nM. The nitrobenzimidazolyl substituted 20-28 specifically inhibited NDM-1, with 27 being the most potent inhibitor with an IC50 value of 170 nM. Further studies with 10, 11, and 27 revealed a mixed inhibition mode with competitive and uncompetitive inhibition constants in a similar range as the IC50 values. These inhibitors resulted in a 2-4-fold decrease in imipenem MIC values using E. coli cells producing ImiS or NDM-1. While the source of uncompetitive (possibly allosteric) inhibition remains unclear, docking studies indicate that 10 and 11 may interact orthosterically with Zn2 in the active site of CphA, while 27 could bridge the two Zn(II) ions in the active site of NDM-1 via its nitro group.

Discovery and validation of small-molecule heat-shock protein 90 inhibitors through multimodality molecular imaging in living subjects.

Up-regulation of the folding machinery of the heat-shock protein 90 (Hsp90) chaperone protein is crucial for cancer progression. The two Hsp90 isoforms (α and ß) play different roles in response to chemotherapy. To identify isoform-selective inhibitors of Hsp90(α/ß)/cochaperone p23 interactions, we developed a dual-luciferase (Renilla and Firefly) reporter system for high-throughput screening (HTS) and monitoring the efficacy of Hsp90 inhibitors in cell culture and live mice. HTS of a 30,176 small-molecule chemical library in cell culture identified a compound, N-(5-methylisoxazol-3-yl)-2-[4-(thiophen-2-yl)-6-(trifluoromethyl)pyrimidin-2-ylthio]acetamide (CP9), that binds to Hsp90(α/ß) and displays characteristics of Hsp90 inhibitors, i.e., degradation of Hsp90 client proteins and inhibition of cell proliferation, glucose metabolism, and thymidine kinase activity, in multiple cancer cell lines. The efficacy of CP9 in disrupting Hsp90(α/ß)/p23 interactions and cell proliferation in tumor xenografts was evaluated by non-invasive, repetitive Renilla luciferase and Firefly luciferase imaging, respectively. At 38 h posttreatment (80 mg/kg × 3, i.p.), CP9 led to selective disruption of Hsp90α/p23 as compared with Hsp90ß/p23 interactions. Small-animal PET/CT in the same cohort of mice showed that CP9 treatment (43 h) led to a 40% decrease in (18)F-fluorodeoxyglucose uptake in tumors relative to carrier control-treated mice. However, CP9 did not lead to significant degradation of Hsp90 client proteins in tumors. We performed a structural activity relationship study with 62 analogs of CP9 and identified A17 as the lead compound that outperformed CP9 in inhibiting Hsp90(α/ß)/p23 interactions in cell culture. Our efforts demonstrated the power of coupling of HTS with multimodality molecular imaging and led to identification of Hsp90 inhibitors.

Protein targets of thioacetamide metabolites in rat hepatocytes.

Thioacetamide (TA) has long been known as a hepatotoxicant whose bioactivation requires S-oxidation to thioacetamide S-oxide (TASO) and then to the very reactive S,S-dioxide (TASO2). The latter can tautomerize to form acylating species capable of covalently modifying cellular nucleophiles including phosphatidylethanolamine (PE) lipids and protein lysine side chains. Isolated hepatocytes efficiently oxidize TA to TASO but experience little covalent binding or cytotoxicity because TA is a very potent inhibitor of the oxidation of TASO to TASO2. However, hepatocytes treated with TASO show extensive covalent binding to both lipids and proteins accompanied by extensive cytotoxicity. In this work, we treated rat hepatocytes with [(14)C]-TASO and submitted the mitochondrial, microsomal, and cytosolic fractions to 2DGE, which revealed a total of 321 radioactive protein spots. To facilitate the identification of target proteins and adducted peptides, we also treated cells with a mixture of TASO/[(13)C2D3]-TASO. Using a combination of 1DGE- and 2DGE-based proteomic approaches, we identified 187 modified peptides (174 acetylated, 50 acetimidoylated, and 37 in both forms) from a total of 88 nonredundant target proteins. Among the latter, 57 are also known targets of at least one other hepatotoxin. The formation of both amide- and amidine-type adducts to protein lysine side chains is in contrast to the exclusive formation of amidine-type adducts with PE phospholipids. Thiobenzamide (TB) undergoes the same two-step oxidative bioactivation as TA, and it also gives rise to both amide and amidine adducts on protein lysine side chains but only amidine adducts to PE lipids. Despite their similarity in functional group chemical reactivity, only 38 of 62 known TB target proteins are found among the 88 known targets of TASO. The potential roles of protein modification by TASO in triggering cytotoxicity are discussed in terms of enzyme inhibition, protein folding, and chaperone function, and the emerging role of protein acetylation in intracellular signaling and the regulation of biochemical pathways.

Metabolism and toxicity of thioacetamide and thioacetamide S-oxide in rat hepatocytes.

The hepatotoxicity of thioacetamide (TA) has been known since 1948. In rats, single doses cause centrolobular necrosis accompanied by increases in plasma transaminases and bilirubin. To elicit these effects, TA requires oxidative bioactivation, leading first to its S-oxide (TASO) and then to its chemically reactive S,S-dioxide (TASO(2)), which ultimately modifies amine-lipids and proteins. To generate a suite of liver proteins adducted by TA metabolites for proteomic analysis and to reduce the need for both animals and labeled compounds, we treated isolated hepatocytes directly with TA. Surprisingly, TA was not toxic at concentrations up to 50 mM for 40 h. On the other hand, TASO was highly toxic to isolated hepatocytes as indicated by LDH release, cellular morphology, and vital staining with Hoechst 33342/propidium iodide. TASO toxicity was partially blocked by the CYP2E1 inhibitors diallyl sulfide and 4-methylpyrazole and was strongly inhibited by TA. Significantly, we found that hepatocytes produce TA from TASO relatively efficiently by back-reduction. The covalent binding of [(14)C]-TASO is inhibited by unlabeled TA, which acts as a "cold-trap" for [(14)C]-TA and prevents its reoxidation to [(14)C]-TASO. This in turn increases the net consumption of [(14)C]-TASO despite the fact that its oxidation to TASO(2) is inhibited. The potent inhibition of TASO oxidation by TA, coupled with the back-reduction of TASO and its futile redox cycling with TA, may help explain phenomena previously interpreted as "saturation toxicokinetics" in the in vivo metabolism and toxicity of TA and TASO. The improved understanding of the metabolism and covalent binding of TA and TASO facilitates the use of hepatocytes to prepare protein adducts for target protein identification.

Covalent modification of lipids and proteins in rat hepatocytes and in vitro by thioacetamide metabolites.

Thioacetamide (TA) is a well-known hepatotoxin in rats. Acute doses cause centrilobular necrosis and hyperbilirubinemia while chronic administration leads to biliary hyperplasia and cholangiocarcinoma. Its acute toxicity requires its oxidation to a stable S-oxide (TASO) that is oxidized further to a highly reactive S,S-dioxide (TASO(2)). To explore possible parallels among the metabolism, covalent binding, and toxicity of TA and thiobenzamide (TB), we exposed freshly isolated rat hepatocytes to [(14)C]-TASO or [(13)C(2)D(3)]-TASO. TLC analysis of the cellular lipids showed a single major spot of radioactivity that mass spectral analysis showed to consist of N-acetimidoyl PE lipids having the same side chain composition as the PE fraction from untreated cells; no carbons or hydrogens from TASO were incorporated into the fatty acyl chains. Many cellular proteins contained N-acetyl- or N-acetimidoyl lysine residues in a 3:1 ratio (details to be reported separately). We also oxidized TASO with hydrogen peroxide in the presence of dipalmitoyl phosphatidylenthanolamine (DPPE) or lysozyme. Lysozyme was covalently modified at five of its six lysine side chains; only acetamide-type adducts were formed. DPPE in liposomes also gave only amide-type adducts, even when the reaction was carried out in tetrahydrofuran with only 10% water added. The exclusive formation of N-acetimidoyl PE in hepatocytes means that the concentration or activity of water must be extremely low in the region where TASO(2) is formed, whereas at least some of the TASO(2) can hydrolyze to acetylsulfinic acid before it reacts with cellular proteins. The requirement for two sequential oxidations to produce a reactive metabolite is unusual, but it is even more unusual that a reactive metabolite would react with water to form a new compound that retains a high degree of chemical reactivity toward biological nucleophiles. The possible contribution of lipid modification to the hepatotoxicity of TA/TASO remains to be determined.

Synthesis and preliminary antimicrobial activities of new arylideneamino-1,3,4-thiadiazole-(thio/dithio)-acetamido cephalosporanic acids.

New derivatives of 7-aminocephalosporanic acid 1-8 were synthesized by acylation of the 7-amino group of the cephem nucleus with various arylidinimino-1,3,4-thiadiazole-thio(or dithio)-acetic acid intermediates 3a-d and 5a-d, respectively, so the acyl side chains of these new cephalosporins contained a sulfide or disulfide bond. This unique combination of a Schiff base with the sulfide or disulfide bonds in the acyl side chain afforded new cephalosporins of reasonable potencies, some of which were found to possess moderate activities against the tested microorganisms. Their chemical structures were characterized by ¹H-NMR, IR spectroscopy and elemental microanalysis. Preliminary in vitro antimicrobial activities of the prepared cephalosporins were investigated using a panel of selected microorganisms. Results indicated that the newly synthesized cephalosporins containing disulfide bonds (compounds 5-8) exhibited better activities against Staphylococcus aureus and Escherichia coli. The cephalosporins cross-linked by a sulfide bond (compounds 1-4) showed a slight change in antimicrobial activities when compared with that of the reference cephalosporin (cephalexin).

Fluoroalkene modification of mercaptoacetamide-based histone deacetylase inhibitors.

Inhibitors of histone deacetylases (HDAC) are emerging as a promising class of anti-cancer agents. The mercaptoacetoamide-based inhibitors are reported to be less toxic than hydroxamate and are worthy of further consideration. Therefore, we have designed a series of analogs as potential inhibitors of HDACs, in which the mercaptoacetamide group was replaced by (mercaptomethyl)fluoroalkene, and their HDAC inhibitory activity was evaluated. Subnanomolar inhibition was observed for all synthetic compounds.

Botulinum neurotoxin serotype A inhibitors: small-molecule mercaptoacetamide analogs.

Botulinum neurotoxin elicits its paralytic activity through a zinc-dependant metalloprotease that cleaves proteins involved in neurotransmitter release. Currently, no drugs are available to reverse the effects of botulinum intoxication. Herein we report the design of a novel series of mercaptoacetamide small-molecule inhibitors active against botulinum neurotoxin serotype A. These analogs show low micromolar inhibitory activity against the isolated enzyme. Structure-activity relationship studies for a series of mercaptoacetamide analogs of 5-amino-3-phenylpyrazole reveal components essential for potent inhibitory activity.

Balancing reactivity and antitumor activity: heteroarylthioacetamide derivatives as potent and time-dependent inhibitors of EGFR.

Second- and third-generation inhibitors of EGFR possess an acrylamide group which alkylates Cys797, allowing to overcome resistance due to insurgence of T790M mutation. Less reactive warheads, yet capable to bind the target cysteine, may be useful to design newer and safer inhibitors. In the present work, we synthesized a 2-chloro-N-(4-(phenylamino)quinazolin-6-yl)acetamide (8) derivative as a prototype of EGFR inhibitor potentially able to react with Cys797 by nucleophilic substitution. We then tuned the reactivity of the acetamide fragment by replacing the chlorine leaving group with (hetero)-aromatic thiols or carboxylate esters. Among the synthesized derivatives, the 2-((1H-imidazol-2-yl)thio)acetamide 16, while showing negligible reactivity with cysteine in solution, caused long-lasting inhibition of wild-type EGFR autophosphorylation in A549 cells, resulted able to bind recombinant EGFR L858R/T790M in a time-dependent manner, and inhibited both EGFR autophosphorylation and proliferation in gefitinib-resistant H1975 lung cancer cells (expressing EGFR L858R/T790M mutant) at low micromolar concentration.

Methionine sulfoxides on PrPSc: a prion-specific covalent signature.

Prion diseases are fatal neurodegenerative disorders believed to be transmitted by PrP (Sc), an aberrant form of the membrane protein PrP (C). In the absence of an established form-specific covalent difference, the infectious properties of PrP (Sc) were uniquely ascribed to the self-perpetuation properties of its aberrant fold. Previous sequencing of the PrP chain isolated from PrP(27-30) showed the oxidation of some methionine residues; however, at that time, these findings were ascribed to experimental limitations. Using the unique recognition properties of alphaPrP mAb IPC2, protein chemistry, and state of the art mass spectrometry, we now show that while a large fraction of the methionine residues in brain PrP (Sc) are present as methionine sulfoxides this modification could not be found on brain PrP (C) as well as on its recombinant models. In particular, the pattern of oxidation of M213 with respect to the glycosylation at N181 of PrP (Sc) differs both within and between species, adding another diversity factor to the structure of PrP (Sc) molecules. Our results pave the way for the production of prion-specific reagents in the form of antibodies against oxidized PrP chains which can serve in the development of both diagnostic and therapeutic strategies. In addition, we hypothesize that the accumulation of PrP (Sc) and thereafter the pathogenesis of prion disease may result from the poor degradation of oxidized aberrantly folded PrP.

3'-Methylacetanilide and N-benzylthioacetamide at low temperature.

Molecules of the title compounds, 3'-methylacetanilide [or N-(m-tolyl)acetamide], C(9)H(11)NO, (I), and N-benzylthioacetamide, C(9)H(11)NS, (II), are connected by a framework of intermolecular N-H...O and N-H...S hydrogen bonds, respectively, forming chains with the graph-set description C(4), which run along the b axis. Analyses of the crystal structures of (I) and (II) are helpful in the elucidation of a generation mechanism of the IR spectra of hydrogen-bonded molecular crystals. The correlation between the IR spectra of studied compounds and structural data is also discussed.

Toxicokinetics and toxicity of thioacetamide sulfoxide: a metabolite of thioacetamide.

Thioacetamide (TA) is bioactivated by CYP2E1 to TA sulfoxide (TASO), and to the highly reactive sulfdioxide (TASO(2)), which initiates hepatic necrosis by covalent binding. Previously, we have established that TA exhibits saturation toxicokinetics over a 12-fold dose range, which explains the lack of dose-response for bioactivation-based liver injury. In vivo and in vitro studies indicated that the second step (TASO-->TASO(2)) of TA bioactivation is less efficient than the first one (TA-->TASO). The objective of the present study was to specifically test the saturation of the second step of TA bioactivation by directly administering TASO, which obviates the contribution from first step, i.e. TA-->TASO. Male SD rats were injected with low (50mg/kg, ip), medium (100mg/kg) and high (LD(70), 200mg/kg) doses of TASO. Bioactivation-mediated liver injury that occurs in the initial time points (6 and 12h), estimated by plasma ALT, AST and liver histopathology over a time course, was not dose-proportional. Escalation of liver injury thereafter was dose dependent: low dose injury subsided; medium dose injury escalated upto 36h before declining; high dose injury escalated from 24h leading to 70% mortality. TASO was quantified in plasma by HPLC at various time points after administration of the three doses. With increasing dose (i.e., from 50 to 200mg/kg), area under the curve (AUC) and C(max) increased more than dose proportionately, indicating that TASO bioactivation exhibits saturable kinetics. Toxicokinetics and initiation of liver injury of TASO are similar to that of TA, although TASO-initiated injury occurs at lower doses. These findings indicate that bioactivation of TASO to its reactive metabolite is saturable in the rat as suggested by previous studies with TA.

Photoswitchable elements within a peptide backbone-ultrafast spectroscopy of thioxylated amides.

A series of thioxo compounds, thioacetamide, N-methylthioacetamide, a cyclic thioxoamide [(S)-5-thioxopyrrolidine-2-carboxylic acid ethyl ester], two thioxylated dipeptides (Ala-Psi[CS-NH]-Ala and Phe-Psi[CS-NH]-Ala) and a thioxylated dodecapeptide (Lys-Glu-Thr-Ala-Ala-Ala-Lys-Phe-Glu-Arg-Gln-His-Psi[CS-NH]-Nle-Asp-Ser-Ser-Thr-Ser-Ala-Ala, or [thioxo-His(12)]-S-peptide; Nle = norleucine) are investigated by ultrafast spectroscopy in the visible and near UV. The different molecules show very similar absorption dynamics featuring a rise of a strong visible absorption band on the subpicosecond and picosecond time scale. The decay of the visible absorption occurs within 150-600 ps. The observations are interpreted by the ultrafast formation of triplet states and their decay on the subnanosecond time scale. Comparison with published IR experiments on N-methylthioacetamide indicates that the cis-trans isomerization around the thioxopeptide bond is terminated within less than 1 ns.

Clofibrate, pirinixil (BR 931) and WY-14,643 do not affect body cholesterol in Sprague-Dawley rats.

Cholesterol levels in plasma and different tissues were determined in Sprague-Dawley male rats, on standard and cholesterol-cholic acid enriched diets, after short term treatment with the absorbable hypolipidemic agents, clofibrate, WY-14,643 and Pirinixil (BR 931). The objective of the study was to evaluate the mode of action of these drugs in decreasing plasma cholesterol, be it by increased tissue mobilization, or by redistribution from plasma to tissues. After one or two weeks on a standard diet, none of the three agents significantly affected total body cholesterol stores. In spite of the liver enlargement induced by all three, in no case was total liver cholesterol significantly raised. Only clofibrate significantly increased colonic cholesterol concentrations. On a cholesterol-cholic acid regimen, some cholesterol mobilization was noted with all three drugs. However, only Pirinixil significantly reduced total liver cholesterol as well as the estimated total body cholesterol. A parallel effect of diet and drugs on plasma and body cholesterol pools is not constantly observed. In the examined rat model, clofibrate and two chemically unrelated compounds with a probably similar mechanism of action, markedly reduce plasma cholesterol levels while not affecting or decreasing total body cholesterol stores.

N-[2-[N'-pentyl-(6,6-dimethyl-2,4-heptadiynyl)amino]ethyl]- (2-methyl-1-naphthylthio)acetamide (FY-087). A new acyl coenzyme a:cholesterol acyltransferase (ACAT) inhibitor of diet-induced atherosclerosis formation in mice.

FY-087 (N-[2-[N'-pentyl-(6,6-dimethyl-2,4-heptadiynyl)amino]ethyl]- (2-methyl-1-naphthylthio)acetamide) was found to be a competitive inhibitor of human microsomal acyl coenzyme A:cholesterol acyltransferase (ACAT) with an IC50 value of 0.11 microM. Under our assay conditions, other ACAT inhibitors tested, specifically YM-750, E-5324, and melinamide, all of which are now in phase I clinical trials or in clinical use in Japan, inhibited this enzyme with IC50 values of 0.18, 0.14, and 3.2 microM, respectively. FY-087 also inhibited ACAT in acetyl-low density lipoprotein loaded human macrophages (THP-1 cells) with an IC50 of 0.17 microM. Following the oral administration of FY-087 (30 mg/kg) to rats, the plasma concentration of FY-087 reached 0.42 microgram/mL after 2 hr. This concentration of FY-087 was enough to inhibit blood vessel ACAT activity. Cholesterol-lowering and anti-atherogenic effects of FY-087 were examined using C57BL/6J mice fed an atherogenic diet. In this mouse model, treatment with FY-087 (28 mg/kg) inhibited the increase in plasma cholesterol levels by about 20% and decreased the hepatic accumulation of free and esterified cholesterol by 61 and 67%, respectively. FY-087 also significantly inhibited the atherogenic diet-induced increase in the fatty-streak lesion area of the proximal aorta by 57% in C57BL/6J mice. These results indicate that FY-087 is not only a therapeutically bioavailable ACAT inhibitor that lowers plasma cholesterol levels, but also an effective anti-atherogenic agent in mice fed an atherogenic diet.

Anodic cyclization reactions: reversing the polarity of ketene dithioacetal groups.

Intramolecular coupling reactions of ketene dithioacetal groups with enol ether and alcohol nucleophiles have been studied. The reactions were initiated by an anodic oxidation of the ketene dithioacetal and proved to be compatible with the formation of five- or six-member rings, as well as the stereoselective generation of quaternary carbons.

Additional observations on cholecystokinin and the vasoactive intestinal polypeptide.

During the isolation of cholecystokinin from natural sources, as well as during its bioassay, inactivation by oxidation can cause problems. We have attempted to reactivate oxidized CCK by reduction at room temperature with N-methylmercaptoacetamide, recently stated to be the reducing agent of choice for the reduction of methionine sulfoxide to methionine [22]. We have not yet been unequivocally successful in these attempts, but the results seem promising. In the case of oxidized VIP and of oxidized tetragastrin, reduction with N-methylmercaptoacetamide does seem to result in reconversion of the peptides to their preoxidation states, as evidenced by thin layer chromatography on silica gel. We have, together with A. Holmgren and A. Ehrnberg, made observations suggesting the presence in rate liver cytosol of an enzyme which catalyzes the reductive reactivation of oxidized CCK with reduced thioredoxin as the immediate hydrogen donor. In collaboration with A. Light, Purdue University, we have found that enterokinase cleaves 39-CCK and 33-CCk with release of 8-CCK and the tetrapeptide immediately preceding it in the peptide chain. The conversion of 39-CCK to 33-CCK by the action of dipeptidyl amino-peptidase I has been confirmed.

Role of the microsomal FAD-containing monooxygenase in the liver toxicity of thioacetamide S-oxide.

To evaluate the different contributions of either microsomal FAD-containing ( FADM ) or cytochrome P-450 dependent monooxygenases in the bioactivation and liver toxicity of thioacetamide-S-oxide ( TASO ) (a proximate metabolite of the liver toxin and carcinogen thioacetamide), this compound: (i) was given to rats pretreated with methimazole (a substrate and inhibitor of FADM ), SKF 525-A (an inhibitor of cytochrome P-450) and cobalt protoporphyrin IX (a synthetic porphyrin which induces a long-lasting depletion of the hepatic cytochrome P-450); and (ii) was added to liver microsomes performing oxidation of model FADM or cytochrome P-450 substrates. Whereas the prior administration of methimazole alleviated the TASO induced liver necrosis, SKF 525-A was almost ineffective. Also pretreatment with cobalt protoporphyrin IX prevented liver necrosis. However, this porphyrin derivative was found to depress both cytochrome P-450 dependent and the FADM dependent biotransformations. On the other hand, addition of TASO to liver microsomes in vitro induced changes in the kinetics of S-oxidation of thiobenzamide and of N-oxidation of dimethylaniline, whereas the O-deethylation of ethoxycoumarin was unchanged. The overall results show the necessity of TASO bioactivation by mixed-function monooxygenases for the toxic action to be apparent; at the same time, the findings suggest FADM as the system mainly involved in TASO metabolism.