Enzymatic S-methylation of 6-n-propyl-2-thiouracil and other antithyroid drugs.

Mouse kidney thiol transmethylase and S-adenosylmethionine were incubated with the radioactive antithyroid drugs, 2-thiouracil (TU), 6-propyl-2-thiouracil (PTU), methimazole (MMI). 6-methyl-2-thiouracil (6-methyl TU) or thiourea. Radioactive metabolites were produced with TU, PTU and 6-methyl TU and, in each case. were identified as the corresponding S-methyl derivatives. No measurable metabolism of MMI or thiourea was observed. Kinetic studies with the partially purified enzyme demonstrated K_m values for TU, PTU and 6-methyl TU of 1 × 10^?3 M, 2.5 × 10^?3 M and 1.54 × 10^?3 M respectively. Extensive investigation with PTU demonstrated that methylation was to the sulfur rather than the nitrogen of the thiopyrimidine and that the pH optimum for PTU was 8.0. Methylation of PTU was proportional to enzyme concentration, with little spontaneous methylation occurring, and was not reversible. TU and 6-methyl TU inhibited PTU metabolism and were apparently competitive substrates. Thiouracil nucleoside and thiouracil nucleotide were not substrates for the enzyme. Studies with porcine thyroid peroxidase demonstrated that S-methylation of PTU. TU and MMI abolished the antiperoxidase activity observed with the parent compound. The results obtained demonstrate that S-methylation is a general pathway of metabolism for thiopyrimidine antithyroid drugs, but not for thiourea or MMI, which markedly decreases the antiperoxidase activity of the parent compound.     

Site of glucuronide conjugation to the antithyroid drug 6-n-propyl-2-thiouracil.

Chromatographyically pure, essentially salt-free radioactive 6-n-propyl-2-thiouracil (PTU) glucuronides were isolated from rat bile and urine and synthesized with guinea pig liver microsomes to determine if more than one PTU glucuronide was formed and to determine which group or groups in the PTU molecule was glucuronidated. Analyses on Bio-Gel P-2 and DEAE-Sephadex A-25 columns and on TLC sheets in five solvent systems demonstrated that the three glucuronide preparations were chromatographically identical. Furthermore, the reactivities of the three glucuronides with 1 N HCl, methyl iodide, sodium azide-iodine reagent, 2,6-dichloroquinone-chloroimide and H₂O₂ were also identical strongly indicating that a single PTU glucuronide was formed. The PTU glucuronide was partially hydrolyzed by 1 N HCl to 6-n-propyl-uracil (PU), a reaction typical of S-conjugated PTU; demonstrated greatly reduced reactivity with methyl iodide whereas the S of PTU was readily methylated; exhibited a negative reaction in the azide-iodine test which was a certain indication that the C—SH or CS group was not present; failed to react with 2,6-dichloroquinone-chloroimide which reacts with the C=S of PTU and provides the basis for a colorimetric assay for PTU; and was not oxidized by H₂O₂ to form sulfate as are all PTU derivatives except S conjugates of PTU. Furthermore PU, which possesses identical potential conjugation sites with the exception of the S, was not glucuronidated under conditions in which PTU was readily conjugated. The results obtained strongly indicate that the glucuronide is conjugated to the S of PTU.     

N-Hydroxylation of 2-aminofluorene in the guinea pig and by guinea pig liver microsomes in vitro

Summary N-hydroxy-2-aminofluorene was found in the urine of guinea pigs intraperitoneally injected with 2-aminofluorene. The hydroxylamine was oxidized to the nitroso analogue and this was identified and determined in the carbon tetrachloride extract by its characteristic UV absorption, by thin-layer chromatography, and by the formation of a diazo compound in the reaction with nitrous acid. Only a small fraction of the 2-aminofluorene injected appeared in the urine as N-hydroxy derivative.Guinea pig liver microsomes were observed to N-hydroxylate 2-aminofluorene rather rapidly, the reaction proceeding at least as rapidly as the N-hydroxylation of aniline.The results of this paper were presented at meetings of the Deutsche Pharmakologische Gesellschaft in Mainz, April 26 to 28, 1965 (Kampffmeyer and Kiese) and Göttingen, September 27 to 30, 1965 (von Jagow, Kiese, Renner, and Wiedemann).     

Glucuronide and glucoside conjugation of mycophenolic acid by human liver, kidney and intestinal microsomes

Mycophenolic acid (MPA) is primarily metabolized to a phenolic glucuronide (MPAG) as well as to two further minor metabolites: an acyl glucuronide (AcMPAG) and a phenolic glucoside (MPAG1s). This study presents investigations of the formation of these metabolites by human liver (HLM), kidney (HKM), and intestinal (HIM) microsomes, as well as by recombinant UDP-glucuronosyltransferases. HLM (n=5), HKM (n=6), HIM (n=5) and recombinant UGTs were incubated in the presence of either UDP-glucuronic acid or UDP-glucose and various concentrations of MPA. Metabolite formation was followed by h.p.l.c. All microsomes investigated formed both MPAG and AcMPAG. Whereas the efficiency of MPAG formation was greater with HKM compared to HLM, AcMPAG formation was greater with HLM than HKM. HIM showed the lowest glucuronidation efficiency and the greatest interindividual variation. The capacity for MPAGls formation was highest in HKM, while no glucoside was detected with HIM. HKM produced a second metabolite when incubated with MPA and UDP-glucose, which was labile to alkaline treatment. Mass spectrometry of this metabolite in the negative ion mode revealed a molecular ion of m/z 481 compatible with an acyl glucoside conjugate of MPA. All recombinant UGTs investigated were able to glucuronidate MPA with K:(M:) values ranging from 115.3 to 275.7 microM l(-1) and V(max) values between 29 and 106 pM min(-1) mg protein(-1). Even though the liver is the most important site of MPA glucuronidation, extrahepatic tissues particularly the kidney may play a significant role in the overall biotransformation of MPA in man. Only kidney microsomes formed a putative acyl glucoside of MPA.     

Preliminary evaluation of an in utero-lactation assay using 6-n-propyl-2-thiouracil

In this preliminary study, the potential of an in utero-lactation assay to detect thyroid effectors was evaluated by treating three dams/group with 6-n-propyl-2-thiouracil (PTU), a known thyroid antagonist, by oral gavage at doses of 0, 0.0032, 0.016, 0.08 and 0.4 mg/kg/day during fetal organogenesis and lactation. Hearing disturbances and an elevated relative thyroid weight were observed in offspring of both sexes in the 0.4 mg/kg/day group. The Biel-type water T-maze test showed an increase in the number of errors made by females in the 0.4 mg/kg/day group. Histopathologically, flattening of follicular epithelium, a decrease in resorptive colloid droplets, degeneration of follicular epithelium, and hyperplasia of follicular epithelium were observed in males belonging to the 0.4 mg/kg/day group. Histopathological abnormalities were also observed in some offspring belonging to the 0.08 mg/kg/day group. In the dams, hypertrophy of the follicular epithelium of the thyroid was observed in the 0.4 mg/kg/day group. Although we could not explain the mechanism for the difference in the effects seen in the offspring and the dams, the effect of PTU in utero through lactational exposure is apparently different from that resulting from exposure in homeostatically mature rats. Most reports studying PTU have involved administration in water or in food, and reports on the oral gavage of PTU during the fetal organogenesis and lactation period are very rare. This assumes that dosages >0.4 mg/kg/day would also produce clear anti-thyroid effects by oral gavage and, possibly, emphasizes that dosages <0.4 mg/kg/day did not have a noticeable effect. Based on the present results, a study to determine the reproducibility of the data in a much larger number of dams will be performed to confirm the findings in the present study, and to evaluate other endpoints, such as hormonal evaluation of dams and their offspring, sexual developmental landmarks, and fertility of the offspring.     

Metoprolol oxidation by rat liver microsomes. Inhibition by debrisoquine and other drugs

The oxidative metabolism of metoprolol has been shown to display genetic polymorphism of the debrisoquine-type. The use of in vitro inhibition studies has been proposed as a means of defining whether one or more forms of cytochrome P-450 are involved in the monogenically-controlled metabolism of two substrates. We have, therefore, tested the ability of debrisoquine and other substrates to inhibit the oxidation of metoprolol by rat liver microsomes. Debrisoquine and guanoxan were potent competitive inhibitors of the alpha-hydroxylation and O-desmethylation of metoprolol as well as its metabolism by all routes (measured by substrate disappearance). Cimetidine and ranitidine, drugs which are known to impair the clearance of metoprolol in man, showed an inhibitory action comparable to that of debrisoquine in rat liver microsomes. Antipyrine, a compound whose metabolism is not impaired in poor metabolisers of debrisoquine, was found to be only a weak inhibitor of the metabolism of metoprolol. These findings suggest that the oxidation of metoprolol is linked closely to that of debrisoquine, cimetidine and ranitidine but not to that of antipyrine in the rat.     

Mefloquine metabolism by human liver microsomes. Effect of other antimalarial drugs.

A number of drugs have been studied for their effect on the metabolism of the antimalarial drug mefloquine by human liver microsomes (N = 6) in vitro. The only metabolite generated was identified as carboxymefloquine by co-chromatography with the authentic standard. Ketoconazole caused marked inhibition of carboxymefloquine formation with IC50 and Ki values of 7.5 and 11.2 microM, respectively. The inhibition of ketoconazole, a known inhibitor of cytochrome P450 isozymes, and the dependency of metabolite formation on the presence of NADPH indicated that cytochrome P450 isozyme(s) catalysed metabolite production. Of compounds actually or likely to be coadministered with mefloquine to malaria patients only primaquine and quinine produced marked inhibition (IC50, 17.5 and 122 microM; Ki, 8.6 and 28.5 microM, respectively). However, despite these in vitro data with primaquine, clinical studies have failed to show any significant effect of single dose primaquine on the pharmacokinetics of mefloquine. With quinine, because peak plasma concentrations are very close to the Ki value, there is likely to be inhibition of mefloquine metabolism in patients receiving both drugs. Sulfadoxine, artemether, artesunate and tetracycline did not significantly inhibit carboxymefloquine formation.     

In vitro metabolism of clenbuterol and bromobuterol by pig liver microsomes

1. Clenbuterol (CBL) and bromobuterol (BBL) were both extensively metabolized by hepatic microsomes of swine to only one polar metabolite which was separated by hplc and purified to perform mass analysis. 2. LC-MS analysis by direct infusion into an ion trap system and after reverse-phase chromatograpy into a triple quadrupole system showed that the metabolites were the hydroxylamine-derivatives of CBL and BBL. GC-MS analysis by the CI and EI modes confirmed that the hydroxyl group was bound to the aniline nitrogen. The chemical instability of those metabolites probably as a consequence of spontaneous oxidation and reduction gave rise during the analysis to the corresponding nitroso and nitro derivatives, together with the original compound. 3. Thermal inactivation and CO complex formation were used selectively to inactivate flavin monooxygenase and cytochrome P450, respectively. Both inactivation procedures significantly reduced the formation of the hydroxyl metabolite.     

Primaquine metabolism by human liver microsomes: effect of other antimalarial drugs.

A number of drugs have been studied for their effect on the metabolism of the antimalarial drug primaquine by human liver microsomes (N = 4) in vitro. The only metabolite generated was identified as carboxyprimaquine by co-chromatography with the authentic standard. Ketoconazole, a known inhibitor of cytochrome P450 isozymes, caused marked inhibition of carboxyprimaquine formation with IC50 and K(i) values of 15 and 6.7 microM, respectively. This finding and the dependency of metabolite formation on NADPH indicates that cytochrome P450 isozyme(s) catalysed metabolite production. Of compounds actually or likely to be coadministered with primaquine to malaria patients, only mefloquine produced any inhibition (K(i) = 52.5 microM). Quinine, artemether, artesunate, halofantrine and chloroquine did not significantly inhibit metabolite formation. It seems unlikely that the concurrent administration of mefloquine, or other antimalarials, with primaquine will lead to appreciably altered disposition.     

Cyclosporin metabolism in human liver microsomes and its inhibition by other drugs

The metabolism of cyclosporin was studied in human liver microsomes. There was no metabolism in the presence of cytochrome C or carbon monoxide or in the absence of cofactors, suggesting metabolism by cytochrome P-450 enzymes. The metabolism was inhibited by ketoconazole and erythromycin, by the steroids methylprednisolone and oestradiol, and by the calcium antagonists diltiazem, nifedipine, prenylamine and verapamil. These in vitro findings correlate well with previously published clinical reports suggesting that these drugs may inhibit the metabolism of cyclosporin in vivo. Our observations suggest that metabolic interactions between cyclosporin and other drugs in vivo may be predicted in vitro under proper experimental conditions.     

Mechanism of 2-naphthylamine oxidation catalysed by pig liver microsomes.

1. In pig liver microsomes 2-naphthylamine-dependent NADPH oxidation, oxygen reduction, and hydroxylamine formation are linear with time for several minutes. A sharp increase in NADPH oxidation and oxygen uptake then coincides with an abrupt loss of hydroxylamine from the medium. 2. The initial rate of 2-naphthylamine N-oxidation correlates with the microsomal concentration of mixed-function amine oxidase and the extent of linear accumulation of hydroxylamine is dependent on microsomal NADPH-cytochrome c reductase activity and concentration of lipid (microsomes). 3. Antisera to NADPH-cytochrome c reductase markedly decreased hydroxylamine accumulation during incubation but had no effect on the rate of 2-naphthylamine N-oxidation. 4. A system duplicating all of the kinetic properties of the microsomal 2-naphthylamine oxidase was constructed with two purified flavoproteins, (mixed-function amine oxidase and NADPH-cytochrome c reductase) and a lipid phase (erythrocyte ghosts or synthetic lecithin liposomes). 5. By independently varying the concentrations of each component in the reconstituted system, the contribution of each to the observed kinetics was defined. 6. In addition to the initial N-oxidation of 2-naphthylamine, at least six other reactions contribute to the kinetic patterns of 2-naphthylamine oxidation catalysed by the reconstituted system.     

Metabolism of prostaglandin E analogs in guinea pig and rat liver microsomes

Tritium-labelled 16,16-dimethyl-PGE2, 9-methylene-PGE2 (9-deoxo-16,16-dimethyl-9-methylene-prostaglandin E2) and tetranor-9-methylene-PGE2 were incubated with guinea pig liver microsomes. All three compounds were converted to omega-oxidized products in yields of a few per cent. In addition, from incubations with 9-methylene-PGE2 and tetranor-9-methylene-PGE2 were also obtained metabolites with the methylene group transformed into a dihydrodiol. In a comparative study with rat liver microsomes, it was found that these converted tetranor-9-methylene-PGE2 in a 50 per cent yield to omega-oxidized products. Finally, 20.000 X G supernatants from guinea pig and rat liver were compared with respect to omega-oxidation. The rat liver 20.000 X G supernatant was found to convert the substrate to the same extent as washed microsomes. By contrast, the guinea pig liver 20.000 X G supernatant was considerably more efficient than washed microsomes.     

Interconversions of haloperidol and reduced haloperidol in guinea pig and rat liver microsomes

An alcohol metabolite of haloperidol, reduced haloperidol, is present in the tissues of haloperidol-treated patients. We have studied whether rat and guinea pig liver microsomes have the capability to reduce haloperidol and thus serve as models for human haloperidol metabolism. Interestingly, the rat microsomes did not reduce haloperidol, but possessed an NADPH-dependent, carbon monoxide-inhibited mechanism to oxidize the reduced haloperidol back to haloperidol. Guinea pig microsomes efficiently reduced haloperidol molecules in a fashion not dependent on nicotinamide cofactors and not inhibited by carbon monoxide. Both of these activities were confined to the microsomal fraction. In guinea pigs, reduction of haloperidol was observed also in kidney slices, whereas brain slices proved inactive. Reduced haloperidol was also oxidized to haloperidol to a small extent in guinea pig microsomes. These in vitro experiments confirm our findings in vivo, which showed that in rats haloperidol is not reduced, while guinea pigs have a very active mechanism for reducing haloperidol. Thus, guinea pigs constitute a model for human haloperidol metabolism, and they should be used for further characterization of the reductive drug-metabolizing system.     

In vitro inhibition of prostaglandin synthesis by antithyroid drugs.

Methimazole inhibited the release of PGE₂ and PGF_2α by dog thyroid slices: at 1 mM, the inhibition was 70% for PGE₂ and 63% for PGF_2α. The concentrations required to inhibit prostaglandin synthesis (0.1–1 mM) were much higher than those which depressed protein iodination (1–10 μM). A similar inhibition of prostaglandin synthesis was obtained with carbimazole, propylthiouracil and aminotriazole, but not with imidazole. Basal release and release stimulated by ionophore A₂₃₁₈₇, carbamylcholine and epinephrine were decreased to the same extent. Methimazole also inhibited the release of PGE₂ and PGF_2α by slices of dog renal cortex and inner medulla. The conversion of arachidonic acid to PGF_2α and PGE₂ by a dog kidney medulla homogenate was depressed by methimazole. The antithyroid drugs constitute a new class of prostaglandin synthesis inhibitors.     

Glucuronide formation of various drugs in liver microsomes and in isolated hepatocytes from phenobarbital- and 3-methylcholanthrene-treated rats

Various substrates of rat liver microsomal UDP-glucuronosyltransferase were classified in vitro as preferred substrates of either 3-methylcholanthrene- or phenobarbital-inducible enzyme forms. Microsomal UDP-glucuronosyltransferase activities towards a third group of substrates (including oestrone, phenolphthalein, paracetamol and oxazepam) are not markedly altered by treatment with either 3-methylcholanthrene or phenobarbital. Some substrates of the 3-methylcholanthrene- and phenobarbital-inducible enzyme activities were selected to evaluate the importance of multiple enzyme forms for glucuronide formation in the intact cell. The metabolism of these compounds was compared in isolated hepatocytes from untreated controls and from rats treated with 3-methylcholanthrene (MC-hepatocytes) or phenobarbital (PB-hepatocytes). Glucuronidation of 1-naphthol and 3-hydroxybenzo[a]pyrene was chiefly enhanced in MC-hepatocytes (greater than 2-fold), whereas glucuronidation of chloramphenicol and bilirubin was chiefly enhanced in PB-hepatocytes. These observations are in agreement with differential induction of UDP-glucuronosyltransferase activities in vitro suggesting that, besides other factors such as cofactor supply, physiological activators, etc., the levels of the multiple enzyme forms are critically determining glucuronide formation in the intact cell.     

Hydroxylation of 4,4'-methylenebis(2-chloroaniline) by canine, guinea pig, and rat liver microsomes

An in vitro microsomal mixed function oxidase enzyme system was used to study the phase I metabolism of 4,4'-methylenebis(2-chloroaniline) (MBOCA) by dog, guinea pig, and rat liver. TLC with color development and autoradiography, and HPLC with detection by UV absorbance and radioactivity flow monitoring were utilized to isolate metabolites. Reference standards of the N-oxidized metabolites were prepared by oxidation of MBOCA with 3-chloroperoxybenzoic acid and structures confirmed by mass spectrometry and proton NMR. These were utilized to identify the N-hydroxy and nitroso metabolites of MBOCA isolated from the microsomal incubations by comparison of their HPLC retention times and mass spectra. The structure of the o-hydroxy metabolite (ring, ortho to the amine) isolated from the microsomal incubations was elucidated by mass spectrometry and proton NMR. N- and o-hydroxylations of MBOCA were shown to increase with incubation time, microsomal protein, substrate, and NADPH concentration, and were inhibited by 2,3-dichloro-6-phenylphenoxyethylamine, an inhibitor of the microsomal mixed function oxidase enzyme system. Guinea pig liver microsomes oxidized MBOCA to the N-hydroxy metabolite predominantly, whereas the dog liver formed predominantly the o-hydroxylated metabolite, with significant amounts of the hydroxylamine as well. The rat liver formed lesser amounts of the N- and o-hydroxylated metabolites, but larger numbers of other polar compounds.     

Bufuralol metabolism by guinea pig adrenal and hepatic microsomes

Previous investigations demonstrated that CYP2D16 was expressed at high levels in guinea pig adrenal microsomes. The studies presented here were done to determine whether adrenal metabolism of bufuralol (BUF), a model CYP2D substrate, was similar to that in the liver. Guinea pig adrenal microsomes converted BUF to 1'-hydroxybufuralol (1'-OH-BUF) as the major metabolite and smaller amounts of a compound identified as 6-hydroxybufuralol (6-OH-BUF). In contrast, 6-OH-BUF was the major product formed by hepatic microsomal preparations. The apparent Km values were similar for 1'-OH-BUF and 6-OH-BUF production in each tissue. Quinidine, a selective CYP2D inhibitor, decreased the production of both BUF metabolites equally in liver and adrenal microsomes. Cortisol also caused equivalent decreases in the rates of 1'-OH-BUF and 6-OH-BUF formation by adrenal microsomes, but had no effect on hepatic BUF metabolism. Although both BUF metabolites may be produced by CYP2D16, unknown factors appear to effect some differences in the catalytic characteristics of BUF metabolism in adrenal and liver. The large amount of 6-OH-BUF produced distinguishes BUF metabolism in guinea pigs from that in other species previously studied.