Disposition and metabolism of a novel antineoplastic agent, 4-tert-butyl-[3-(2-chloroethyl)ureido]benzene, in mice

1-Aryl-3-(2-chloroethyl)ureas are new agents that have shown promising cytotoxic and antineoplastic activities. In this work, we studied the disposition and metabolism of one of these molecules, 4-tert-butyl-[3-(2-chloroethyl)ureido]benzene (tBCEU). tBCEU was labeled with 14C and 13C in the urea function and in the chloroethyl moiety. After ip administration of the molecule labeled in the urea function, radioactivity was widely distributed in the whole organism, including the brain. HPLC analysis of plasma showed that tBCEU was extensively metabolized, with <20% being found in the plasma as unchanged tBCEU 1 hr after administration. One main metabolite was identified by NMR and MS analysis as N-[4-(2-hydroxy-1, 1-dimethylethyl)phenyl]urea, widely conjugated to glucuronic acid. The same metabolite was found in the urine. After administration of tBCEU labeled in the chloroethyl moiety, the same tissue affinities were observed, but the decrease of total radioactivity in blood and tissues was slower than that observed for the molecule labeled in the urea function. HPLC analysis of urine showed the presence of two main metabolites, identified by MS as thiodiacetic acid and its sulfoxide. From these results, we can deduce that the metabolic pathway of tBCEU involves N-dealkylation of the urea portion of the molecule and hydroxylation of the tert-butyl group. The strong cytochrome P450 reactivity of the carbon adjacent to the urea portion of tBCEU is probably related to particular sensitivity to oxidation at this position, based on the chemical structure of tBCEU. These results can explain the fact that the cytotoxic effect of tBCEU is not due to DNA alkylation, in contrast to that of its parent molecule, chloroethylnitrosourea.

Stereoselective disposition of propranolol in rabbits. Role of presystemic organs and dose

The kinetics of propranolol enantiomers are stereoselective when high doses of the racemic drug are given po. To document whether the dose and/or the route of administration determines the stereoselective kinetics of propranolol enantiomers, conscious rabbits received 40, 80, or 120 mg/kg po or 0.5 or 10 mg/kg iv doses of racemic propranolol, and serial blood samples were obtained to assay propranolol enantiomers. At low po and iv doses, the kinetics of the propranolol enantiomers were identical. After the 120 mg/kg po dose, the kinetics of the enantiomers were stereoselective, i.e. the AUC0-->infinity for (S)-(-)-propranolol was greater than the AUC0-->infinity for (R)-(+)-propranolol (p < 0.05). The iv injection of 10 mg/kg generated zero-order kinetics, and (S)-(-)-propranolol was eliminated faster than the antipode. Propranolol enantiomer plasma protein binding was not stereoselective. In vitro, after the incubation of 5.8 or 58 microM (RS)-propranolol with cells of the intestinal mucosa or the liver, (R)-(+)-propranolol was more rapidly metabolized than (S)-(-)-propranolol at both concentrations in the intestine and at the higher concentration in the liver. Incubation of the individual enantiomers (2.9 and 29 microM) showed that in the intestine the intrinsic clearance of (R)-(+)-propranolol was greater than that of (S)-(-)-propranolol but in the liver there was preferential saturation of (S)-(-)-propranolol clearance. In conclusion, at low po or iv doses the kinetics of (RS)-propranolol are not stereoselective because the liver overshadows the effect of the intestine, and at high po doses the kinetics of propranolol enantiomers are stereoselective because of hepatic saturation of (S)-(-)-propranolol clearance.

Enantioselectivity of the enterohepatic recycling of carprofen in the dog

The disposition of the two enantiomers of carprofen (CPF), the (R)-CPF and the (S)-CPF, was investigated after iv administration of the racemate (4 mg/kg) in dogs equipped with a chronic bile duct catheter. Studies in dogs with diverted bile flow showed that both enantiomers were extensively excreted in bile with 74% of the (R)-enantiomer and 92% of the (S)-enantiomer from the iv administered dose being recovered in the bile as the respective glucuronide conjugates. The direct administration of acidic bile containing acyl-glucuronides of CPF in the duodenum showed that both conjugated enantiomers led to high CPF enantiomer systemic availability. However, comparison of CPF pharmacokinetics between dogs with nondiverted bile flow and dogs with diverted bile flow suggested that CPF was subjected to enantioselective enterohepatic recycling (EHC) and that only the (S)-CPF was recycled. The absence of EHC for the (R)-CPF is hypothesized to be the result of formation of glucuronidase-resistant isoglucuronides (epimers) to a greater extent for the (R)-CPF than for the (S)-CPF.

Phase II-selective induction of hepatic drug-metabolizing enzymes by oltipraz -5-(2-pyrazinyl)-4-methyl-1,2-dithiol-3-thione-, 1,7-phenanthroline, and 2,2'-dipyridyl in rats is not accompanied by induction of intestinal enzymes

The induction of hepatic and intestinal cytochrome P450, NAD(P)H:quinone oxidoreductase (QOR), glutathione S-transferase (GST), and UDP-glucuronosyltransferase (UGT) activities by intragastric administration of 1,7-phenanthroline, 2,2'-dipyridyl, and oltipraz has been investigated in rats. In the liver, all three compounds induced phase II drug-metabolizing enzymes without inducing overall cytochrome P450 concentrations and, in a direct comparison, all agents induced the enzymes to a greater extent than did the same dose of tert-butyl-4-hydroxyanisole. With a 75 mg/kg daily, 3-day regimen, UGT, GST, and QOR activities were induced by all compounds. The changes in hepatic GST, QOR, and UGT activities induced by N-heterocyclic compounds were accompanied by increases in the amounts of mRNA for GST Ya (2-2.4-fold), QOR (1.6-2.8-fold), and the UGTs UGT2B1 (4-6-fold) and UGT1A6 (4-10-fold). Changes in the amounts of UGT2B1 mRNA and UGT1A6 mRNA were highly correlated (r = 0. 9), but there was no correlation between changes in either UGT2B1 or UGT1A6 mRNA and GST Ya mRNA. No significant mRNA changes were elicited by tert-butyl-4-hydroxyanisole. Neither GST nor UGT activities were induced in the small intestinal mucosa by any agent. QOR activity was slightly induced by oltipraz. The data suggest that requirements for induction of phase II enzymes in the intestine are markedly different from requirements in the liver.

Disposition and covalent binding of diflunisal and diflunisal acyl glucuronide in the isolated perfused rat liver

Acyl glucuronides are intrinsically reactive metabolites of carboxylate drugs, capable of undergoing hydrolysis, intramolecular rearrangement (isomerization via acyl migration), and intermolecular transacylation reactions. Transacylation with nucleophilic groups located on protein molecules leads to covalent drug-protein adducts. Protein adducts can also form from the rearrangement isomers via a glycation mechanism. In this study, the isolated perfused rat liver preparation was used to separately trace the dispositions of the nonsteroidal anti-inflammatory drug diflunisal (DF), its reactive acyl glucuronide metabolite (DAG), and a mixture of DAG rearrangement isomers (iso-DAG), each administered at 30-microg DF equivalents/ml perfusate (four recirculating perfusions each group). After administration of DF, the drug was eliminated in a log linear manner over 3 hr, with apparent elimination half-life (t1/2) of 2.6 +/- 0.4 hr. The sulfate conjugate (DS), excreted almost exclusively into perfusate, accounted for 14.2% of the dose, with the phenolic glucuronide (DPG) and DAG (11.1 and 7.9% of dose, respectively) excreted primarily in bile. Only a small portion (2.3%) of the dose was recovered as novel "diglucuronides" (D-2G, arising from phenolic glucuronidation of iso-DAG), excreted exclusively in bile. Covalent DF-protein adducts were found in both perfusate (0.98%) and liver (0. 14%). After administration of DAG, rapid hydrolysis occurred (initial DAG t1/2 17.3 +/- 4.2 min). At 3 hr, recoveries (in comparison to DF-dosed perfusions) were similar for DF (51.7%) and DAG (8.3%), significantly decreased for DS (10.6%) and DPG (6.4%), and significantly increased for iso-DAG (0.8%), D-2G (9.1%), and covalent adducts in perfusate (1.49%) and liver (0.30%). After administration of iso-DAG, elimination from perfusate was slower (t1/2 55 +/- 15 min), and hydrolysis to DF was modest by comparison with DAG-dosed perfusions. Recoveries as iso-DAG and D-2G in bile were greatly enhanced (8.2 and 36.4%, respectively). Adduct formation was higher in liver (0.76% of dose) but not in perfusate (1.03%). Immunoblots of liver homogenates revealed drug-modified proteins at ca. 110 and 120 kDa. The results show that (a) DAG undergoes avid systemic deconjugation-conjugation cycling and isomerization to iso-DAG; (b) iso-DAG is more resistant to hydrolysis, is readily taken up by hepatocytes and undergoes novel metabolism (phenolic glucuronidation); and (c) the glycation pathway (i.e. using iso-DAG as substrate) plays a major role in formation of covalent DF-protein adducts in liver.

Relationship between hepatic cytochrome P450 3A content and activity and the disposition of midazolam administered orally

It was recently shown by others that the clearance of midazolam/kg body weight after iv administration correlates with hepatic cytochrome P450 (CYP or P450) 3A content in liver transplant patients. However, after po administration midazolam undergoes significant first-pass metabolism, with significant intestinal extraction. The relationship between hepatic CYP3A and midazolam disposition after po administration had not previously been investigated. The aim of this study was to compare intraindividually hepatic CYP3A content and activity with the in vivo pharmacokinetics of midazolam (7.5 mg) administered po. For 15 patients scheduled for partial liver resection, the AUC values for the observed time period (AUC0-5hr) and to infinity (AUCinf) and the clearance were determined. In a macroscopically normal area of resected liver tissue, the microsomal CYP3A4 content (nanomoles per nanomole of total P450) was measured by immunoblot analysis and parameters (apparent Vmax, apparent KM, and intrinsic clearance) for the microsomal alpha-hydroxylation of midazolam were determined. Clearance/kg in vivo correlated with the apparent Vmax (r2 = 0.45, p < 0.01) and the CYP3A4 content (r2 = 0.29, p < 0.05). We conclude that interindividual variability in the pharmacokinetics of po administered midazolam is in part determined by interindividual variability in the hepatic microsomal Vmax for the alpha-hydroxylation of midazolam. However, the relationship between the disposition of midazolam administered po and hepatic CYP3A content is weaker than that reported after iv administration, indicating the importance of the contribution of intestinal CYP3A to the in vivo disposition of midazolam administered po.

Species differences in pharmacokinetics of a hepatoprotective agent, YH439, and its metabolites, M4, M5, and M7, after intravenous and oral administration to rats, rabbits, and dogs

Pharmacokinetic parameters of YH439 and its metabolites, M4, M5, and M7, were compared after iv administration of YH439 to rats (1-10 mg/kg), rabbits (1-10 mg/kg), and dogs (1-20 mg/kg) and oral administration of YH439 to rats (50-500 mg/kg) and dogs (0.5-2 g per whole body weight). After oral administration of YH439 to rats, the F values were 3.67, 1.33, and 0.859% for YH439 oral doses of 100, 300, and 500 mg/kg, respectively. However, the F value increased significantly, 21.2%, after oral administration of YH439-contained mixed micelles (10 mg as free YH439) to rats due to increased water solubility of YH439. Species differences in the pharmacokinetics of YH439 and its metabolites were found. First, M7 was detected in both plasma and urine after both iv and oral administration of YH439 to dogs, whereas it was detected neither in rats nor in rabbits, indicating that considerable amount of M7 was formed from YH439 only in dogs. Second, the AUC (or AUC0-->t) ratios of M4 to YH439 after iv administration of YH439 were 24.6-31.3, 42.2-49.2, and 2200-7640% for rats, rabbits, and dogs, respectively, indicating that formation of M4 after iv administration of YH439 was maximal in dogs. Third, the AUC (or AUC0-->t) ratios of M5 to YH439 after iv administration of YH439 were 103-127, 2.93-3.31, and 92.4-158% for rats, rabbits, and dogs, respectively, indicating that formation of M5 after iv administration of YH439 was minimal in rabbits.

Metabolic screening using on-line ultrafiltration mass spectrometry

An on-line mass spectrometric method has been developed to generate and identify drug metabolites formed by hepatic cytochromes P450. This method, pulsed ultrafiltration-mass spectrometry, may be used for rapid screening of drugs to determine their extent of metabolism by microsomal cytochromes P450 and to characterize the primary metabolites. Rat liver microsomes were trapped in a stirred ultrafiltration chamber fitted with a 100,000 molecular weight cut-off ultrafiltration membrane. A continuous-flow of ammonium acetate buffer was pumped through the chamber and into an electrospray mass spectrometer. Substrates for cytochromes P450 including imipramine, chlorpromazine, and pentoxyresorufin were flow injected through the chamber along with the cofactor, NADPH, and metabolites were detected on-line by using electrospray mass spectrometry. Identical control experiments carried out using boiled microsomes or without NADPH showed no metabolite formation. Naringenin and quinidine, which are inhibitors of some isozymes of cytochrome P450 and are not known to be extensively metabolized, showed no major metabolites. For comparison, imipramine metabolites were also generated by standard batch incubation with microsomes and NADPH, followed by extraction and LC-MS analysis. Similar metabolites were obtained using the flow-through ultrafiltration method and the standard batch microsomal incubation. Tandem mass spectrometry was used to confirm structures of imipramine metabolites including 10-hydroxyimipramine, 2-hydroxyimipramine, imipramine N-oxide, and N-desmethylimipramine. Finally, the feasibility of using ultrafiltration mass spectrometry for high throughput metabolic screening was demonstrated by using on-line mass spectrometry for only 3 min per incubation instead of monitoring the entire elution profile. By carrying out multiple ultrafiltration experiments in parallel, efficient use of the mass spectrometric detector may be obtained with a throughput of at least 20 incubations per hour. Throughputs of up to 60 profiles per hour should be possible. On-line ultrafiltration electrospray mass spectrometry offers a streamlined, higher-throughput method for in vitro formation and mass spectrometric characterization of microsomal drug metabolites.

Identification and characterization of in vitro metabolites of 2-[2'-(dimethylamino)ethyl]-1,2-dihydro-3H-dibenz [de,h]isoquinoline-1,3-dione (Azonafide)

Azonafide (2-[2'-(dimethylamino)ethyl]-1,2-dihydro-3H-dibenz[de, h]isoquinoline-1,3-dione) is the parent of a new series of anthracene-containing antitumor agents. Its structure is based on amonafide but lacks a primary amine and has an anthracene chromophore rather than a naphthalene chromophore. Using a rat liver cytosol incubation and HPLC/MS detection, we have identified four metabolites resulting from in vitro metabolism of azonafide. These alkyl-modified derivatives include a mono- and a di-N'-desmethyl metabolite, an N'-oxide metabolite, and a carboxylic acid metabolite. Purified samples of these metabolites were analyzed for cytotoxic activity using a 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyl-2H-tetrazolium vital dye (mitochondrial reductase) assay and for inhibition of topoisomerase II (TOPO II) using a cell-free enzymatic system. Each metabolite had decreased cytotoxicity relative to azonafide with the following relative potencies in descending order: the mono-N'-desmethyl metabolite, di-N'-desmethyl metabolite, the N-oxide metabolite, and the carboxylic acid metabolite. Similarly, the N'-desmethyl metabolites retained TOPO II inhibitory activity but with lower potency than azonafide. The N-oxide and carboxylic acid metabolites did not inhibit TOPO II at 0. 05 and 0.5 microg/ml, respectively. Thus, metabolism of azonafide by rat liver cytosol represents a detoxification pathway rather than a bioactivation scheme for this DNA intercalator.

Lipid changes in hepatic microsomes and its relationship to P-nitrophenol glucuronidation in an experimental model of portal hypertension

The liver is responsible for the most important metabolic pathway of non polar compounds. The aim of the present work was to study the p-nitrophenol glucuronidation and its relationship with lipidic composition of microsomal membrane in a model of hepatic portal hypertension and hepatocellular damage induced by monocrotaline. A global increment in liver microsomal phospholipids as well as changes in the phospholipid pattern (phosphatidylethanolamine and sphingomyelin increased up to 156 +/- 13 and 195 +/- 14% respectively) were detected in monocrotaline intoxicated rats when it were compared to control rats. The microsomal cholesterol content showed a decrease in monocrotaline intoxicated rats. (4.1 +/- 0.7 against 6.6 +/- 1.5 micrograms/mg of microsomal protein, in control rats). When p-nitrophenol activity was measured, Km from monocrotaline intoxicated rats was 0.137 mM, and Vmax was 2.9 nmol of p-nitrophenol/mg microsomal protein since in control group Km was 0.322 mM, and Vmax was 4.5 nmol of p-nitrophenol/mg microsomal protein. It is concluded that monocrotaline intoxicated rats showed a different behavior in the kinetics of p-nitrophenol UDP-glucuronyltransferase, as well as a different microsomal lipidic profile, when compared to control group.

Mutants of Saccharomyces cerevisiae defective in vacuolar function confirm a role for the vacuole in toxic metal ion detoxification

To directly define vacuolar role(s) in metal detoxification, we have examined the responses of vacuole-deficient mutants of Saccharomyces cerevisiae to several potentially toxic metals known to be mainly detoxified in the cytosol (Cu, Cd) or the vacuole (Co, Mn, Ni, Zn). Three mutants, deficient in targeting of vacuolar proteins, were used with JSR18 delta 1 being devoid of any vacuole-like structure while ScVatB and ScVatC were deficient in specific protein subunits of the V-ATPase. The results obtained show that the absence of a vacuole or a functional vacuolar H(+)-ATPase was associated with increased sensitivity and a largely decreased capacity of the vacuole-deficient strains to accumulate Zn, Mn, Co and Ni, confirming an essential role for the vacuole in detoxification of these metals. In addition, the lack of vacuolar involvement in detoxification of Cu and Cd was confirmed since these metals did not exhibit increased toxicity towards the vacuolar mutants nor were there significant differences in Cu or Cd accumulation between parental and mutant strains.

Detoxification mechanism in the rat extraorbital lacrimal gland: conjugation of brefeldin A to glutathione

In this work the existence of a glutathione based detoxification system in rat lacrimal glands is reported. We showed that brefeldin A, a drug used as a tool for the study of intracellular trafficking mechanisms, was inactivated by metabolization and converted into two derivatives. We purified them by high performance liquid chromatography and determined, by mass spectroscopy, that they correspond to glutathione and cysteine derivatives of BFA. The determination of the respective amounts of these derivatives in the medium and the tissue in different experimental conditions, revealed that glutathione-BFA is formed in the tissue, excreted from the cells, cleaved by gamma-glutamyl transpeptidase and finally converted to cysteine-BFA.

Hypercholesterolemia impairs a detoxification mechanism against peroxynitrite and renders the vascular tissue more susceptible to oxidative injury

Previous studies have shown that glutathione (GSH) plays a central role in the protection against peroxynitrite (ONOO-) toxicity. The present study evaluated the changes of the GSH cytoprotective system against ONOO- in hypercholesterolemia and determined the effects of carvedilol, a beta-blocker with free radical-scavenging activity, on these hypercholesterol-induced changes. New Zealand White rabbits were fed either a normal diet, a high-cholesterol diet, or a high-cholesterol diet supplemented with either carvedilol or propranolol. Eight weeks later, the rabbits were killed, and the thoracic aortas were isolated. Total GSH content of aortic tissue, vasorelaxation response of aortic rings to exogenous ONOO-, No regeneration from ONOO- by aortic homogenate, and ONOO(-)-induced aortic tissue injury were examined. Hypercholesterolemia decreased tissue GSH content (0.52 +/- 0.08 versus 0.86 +/- 0.04 mumol/g in control, P < .01), attenuated the vasorelaxation response to ONOO- (40 +/- 4.1% versus 76 +/- 3.2%, P < .01), reduced NO regeneration from ONOO- (387 +/- 40 versus 662 +/- 51 pmol, P < .01), and potentiated ONOO(-)-induced vascular tissue injury (37 +/- 4.4% versus 14 +/- 2.6% of increase in lactate dehydrogenase release after 3-morpholinosydnonimine exposure, P < .01). Treatment of the hypercholesterolemic rabbits with carvedilol, but not propranolol, significantly preserved tissue GSH content (0.79 +/- 0.05 mumol/g, P < .01 versus nontreated hypercholesterolemic rabbits), restored the vasorelaxation to ONOO- (61 +/- 2%, P < .01), increased NO regeneration from ONOO- (583 +/- 39 pmol, P < .01), and attenuated ONOO(-)-induced tissue injury (19 +/- 1.8%, P < .01). These results suggest that hypercholesterolemia impairs the GSH-mediated detoxification mechanism against ONOO- and renders the vascular tissue more susceptible to oxidative injury. Carvedilol, a novel vasodilating beta-blocker with antioxidant activity, significantly preserved this self-defense system and protected tissue from oxidant injury.

Plant glutathione S-transferases and herbicide detoxification

Glutathione S-transferases (GSTs) are a family of multifunctional enzymes involved in the metabolism of xenobiotics and reactive endogenous compounds. The interest in plant GSTs may be attributed to their agronomic value, since it has been demonstrated that glutathione conjugation for a variety of herbicides is the major resistance and selectivity factor in plants. The structure of the Arabidopsis thaliana isoenzyme, the first plant GST whose structure has been solved, may serve as a model system for the understanding of herbicide selectivity in crops.

Utilization of acute inpatient services for alcohol detoxification

This study indicates that the majority of patients admitted to VA hospitals for medical detoxification could have those services provided on an outpatient or less intensive basis. However, inpatient medical detoxification services appear to be appropriate for those alcoholics at risk for potential life-threatening complications of withdrawal such as delirium tremens, or those with concurrent associated medical conditions such as pancreatitis, gastrointestinal bleeding, or complications of cirrhosis. Data were obtained from a national random sample of hospitalizations in Department of Veterans Affairs (VA) inpatient medical and surgical units. Medical records for 144 alcoholism-related medical admissions to 35 VA medical centers were reviewed using the Appropriateness Evaluation Protocol (AEP), a clinically based utilization review instrument widely used in the private sector. The medical records for the admission and each day of medical/surgical inpatient stay were reviewed using clinical criteria for the appropriateness of acute inpatient care as opposed to lower levels of care.

Age-related changes in O-deethylase and aldrin epoxidase activity in mouse skin and liver microsomes

The metabolism of three model substrates for the cytochrome P450 dependent mono-oxygenase enzyme system (P450-MMO) was studied in microsomes isolated from skin and liver of young adult and senescent C57B1/6J mice. The substrates chosen were aldrin (AE), 7-ethoxycoumarin (EOC), and 7-ethoxyresorufin (EOR). Both EOC and EOR activities were lower in senescent skin. By contrast, no-age related changes were seen in senescent liver. AE was similar in young and old, in both tissues. We suggest that some important age-related differences in cutaneous xenobiotic metabolism do occur, but that these are not mirrored by hepatic differences, and are substrate specific. Previous work from these laboratories would also suggest significant species differences.

Production of metallothionein in copper- and cadmium-resistant strains of Saccharomyces cerevisiae

Certain mutants of the yeast Saccharomyces cerevisiae show copper or cadmium resistance. Both copper- and cadmium-resistant strains produce the same metallothionein with 53 amino acid residues which causes metal detoxification by chelating copper or cadmium. The metal detoxification role is the only known function of the metallothionein in yeast. The MT is encoded by the CUP1 gene on chromosome VIII which is expressed by induction with metals. The CUP1 is amplified to 3-14 copies with 2 kb-tandem-repeat units in the metal-resistant strains, whereas the wild-type strain contains only a single copy of the CUP1. Although transcription of CUP1 is inducible by metals, the ACE1 protein serves a dual function as a sensor for copper and an inducer for CUP1 transcription in the copper-resistant strain. In the cadmium-resistant strain, the heat-shock factor having a point mutation may be the regulator for CUP1 transcription. Therefore, it has been clarified that production of MT in yeast is controlled by two systems, the amplification of CUP1 and its transcriptional regulation.

Nickel resistance mechanisms in yeasts and other fungi

This review describes nickel toxicity and nickel resistance mechanisms in fungi. Nickel toxicity in fungi is influenced by environmental factors such as pH, temperature and the existence of organic matter and other ions. We describe resistance mechanisms in nickel-resistant mutants of yeasts and filamentous fungi which were obtained by exposure to a mutagen or by successive culture in media containing increasing concentrations of nickel ion. Nickel resistance may involve: (1) inactivation of nickel toxicity by the production of extracellular nickel-chelating substances such as glutathione; (2) reduced nickel accumulation, probably by modification of a magnesium transport system; (3) sequestration of nickel into a vacuole associated with free histidine and involving Ni-insensitivity of vacuolar membrane H(+)-ATPase.

Platinum resistance. Experimental and clinical status

It has become clear that multiple mechanisms of cellular resistance to platinum compounds exist; however, the knowledge that platinum resistance can not only be explained by processes such as reduced drug accumulation, and increased detoxification, has shifted the attention of researchers to more molecular mechanisms. The introduction of new techniques, such as the PCR technique for example, has opened the possibility to monitor genes involved in antitumor responses and may provide molecular information on the emergency of resistance.

Effects of chronic alcohol feeding and murine AIDS virus infection on liver antioxidant defense systems in mice

Whether ethanol (ETOH) abuse could contribute to the development of acquired immunodeficiency syndrome (AIDS) among human immunodeficiency virus (HIV)-positive drug abusers is a critical question for which little experimental information is available. This study was designed to determine if chronic ETOH feeding and murine AIDS virus infection cooperatively affected liver antioxidant defense systems in C57B1/6 female mice. Mice were divided into two groups and fed the Lieber-DeCarli liquid ETOH diet containing ETOH at a concentration to provide 31% of total caloric intake or an isocaloric liquid control (control) diet in which dextrin-maltose replaced ETOH. One week after the initiation of ETOH feeding, half of the mice in each diet group (8 mice) were injected intraperitoneally with murine retrovirus (MAIDS) stock. After 3 and 5 weeks of ETOH feeding, half of the mice in each of the four treatment groups (4 mice) were killed, and livers were excised for biochemical analysis. Liver reduced glutathione (GSH) levels and activities of glutathione peroxidase (GP), glutathione reductase (GR), glutathione transferase (GT), catalase and superoxide dismutase (SOD), and serum ETOH concentrations were determined. The results demonstrated that serum ETOH concentrations were significantly elevated in ETOH-MAIDS group when compared with the ETOH group. Moreover, chronic ETOH feeding and MAIDS infection independently depressed liver antioxidant defense capability, and together led to an additive inhibition of GSH and SOD activities. In addition, MAIDS infection inhibited an ETOH-induced increase in catalase and GT activities. These results suggest that alcohol abuse could contribute to the development of AIDS by inhibiting the protective capability of an infected individual against oxidative stress.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutathione-dependent bioactivation of xenobiotics

Glutathione conjugation has been identified as an important detoxication reaction. However, in recent years several glutathione-dependent bioactivation reactions have been identified. Current knowledge on the mechanisms and the possible biological importance of these reactions are discussed. 1. Dichloromethane is metabolized by glutathione conjugation to formaldehyde via S-(chloromethyl)glutathione. Both compounds are reactive intermediates and may be responsible for the dichloromethane-induced tumorigenesis in sensitive species. 2. Vicinal dihaloalkanes are transformed by glutathione S-transferase-catalyzed reactions to mutagenic and nephrotoxic S-(2-haloethyl)glutathione S-conjugates. Electrophilic episulphonium ions are the ultimate reactive intermediates formed. 3. Several polychlorinated alkenes are bioactivated in a complex, glutathione-dependent pathway. The first step is hepatic glutathione S-conjugate formation followed by cleavage to the corresponding cysteine S-conjugates, and, after translocation to the kidney, metabolism by renal cysteine conjugate beta-lyase. Beta-Lyase-dependent metabolism of halovinyl cysteine S-conjugates yields electrophilic thioketenes, whose covalent binding to cellular macromolecules is responsible for the observed toxicity of the parent compounds. 4. Finally, hepatic glutathione conjugate formation with hydroquinones and aminophenols yields conjugates that are directed to gamma-glutamyltransferase-rich tissues, such as the kidney, where they undergo alkylation or redox cycling reactions, or both, that cause organ-selective damage.

[Pregnancy, contraception and epilepsy]

Epilepsy is the most common neurological disease of females in reproductive age. Problems concerning contraception, reproduction, teratogenicity and antiepileptic therapy preceding and during pregnancy are discussed and recommendations made. We underline the advantages of a planned pregnancy with optimal adjustment of antiepileptic drug therapy and recommend prophylactic treatment with folic acid before and during, and with vitamin K towards the end of pregnancy.

Cellular and metabolic engineering. An overview

Metabolic engineering is defined as the purposeful modification of intermediary metabolism using recombinant DNA techniques. Cellular engineering, a more inclusive term, is defined as the purposeful modification of cell properties using the same techniques. Examples of cellular and metabolic engineering are divided into five categories: 1. Improved production of chemicals already produced by the host organism; 2. Extended substrate range for growth and product formation; 3. Addition of new catabolic activities for degradation of toxic chemicals; 4. Production of chemicals new to the host organism; and 5. Modification of cell properties. Over 100 examples of cellular and metabolic engineering are summarized. Several molecular biological, analytical chemistry, and mathematical and computational tools of relevance to cellular and metabolic engineering are reviewed. The importance of host selection and gene selection is emphasized. Finally, some future directions and emerging areas are presented.

Insect detoxifying enzymes: their importance in pesticide synergism and resistance

Pyrethroid esterases of Trichoplusia ni, Spodoptera littoralis and Bemisia tabaci hydrolyze the trans-isomers of various pyrethroids more extensively than the cis-isomers. Profenofos fed to T. ni larvae at a level inhibiting the gut pyrethroid esterases by 65% with trans-permethrin and of 95% with cis-cypermethrin increased the toxicity of topically applied trans-permethrin by fourfold and cis-cypermethrin by 20-fold. Similar assays with S. littoralis resulted in an increase of about threefold in the toxicity of both compounds. Monocrotophos, profenofos, acephate, and methidathion inhibited pyrethroid esterase activity in B. tabaci and synergized considerably the toxicity of cypermethrin. The remarkable tolerance of the predator Chrysopa carnea to pyrethroids is attributed to the presence of a high level of pyrethroid esterase activity with a unique specificity for hydrolyzing the cis-isomer. Phenyl saligenin cyclic phosphonate, a potent inhibitor for larval pyrethroid esterases synergized the toxicity of trans-permethrin by 68-fold from an LD50 of 17,000 micrograms/g to 250 micrograms/g. In contrast, oxidase inhibitors such as piperonyl butoxide, SV-1, and MPP synergized considerably the toxicity of pyrethroids in Tribolium castaneum and Musca domestica. Hence the predominant pathway for pyrethroid detoxification in insects, whether hydrolytic or oxidative, depends largely on the insect species. The high toxicity of the recent developed acylureas results from their high retention in the insects. Assays using radiolabeled diflubenzuron and chlorfluazuron applied to fourth instar T. castaneum larvae revealed a rapid elimination of diflubenzuron (T1/2 approximately equal to 7 h) as compared with chlorfluazuron (T1/2 > 100 h). Addition of 100 ppm DEF to the diet increased both the retention time and the toxicity of diflubenzuron in both T. castaneum and S. littoralis, which was due probably to the inhibition of diflubenzuron hydrolase activity. Esterases, hydrolyzing pyrethroids, and acylureas may serve as tools for evaluating potential synergists and for monitoring resistance in various agricultural pests due to increased metabolism.

Insecticide toxicity, glutathione transferases and carboxylesterase activities in the larva of the Aedes mosquito

1. Toxicity evaluations of DDT, lindane, abate and carbaryl were carried out in the larvae of two wild Aedes aegypti strains from Kuala Lumpur and Klang. The Kuala Lumpur strain was more susceptible to the insecticides than the Klang strain. 2. The lethal toxicity time was also determined. The insecticides were found to take a longer time to exert their effect in the Klang strain as compared to the Kuala Lumpur strain. 3. Carboxylesterase activity was determined to be higher in the Kuala Lumpur strain, but glutathione transferase activities were higher in the Klang strain.

Rat liver protein linking chemical and immunological detoxification systems

Mammals have separate enzymatic and cellularly mediated detoxification systems. Glutathione S-transferases (GSTs) protect against xenobiotic chemicals which continuously enter the body, largely through mucous membranes. These enzymes catalyse the conjugation of glutathione with a wide variety of electrophilic compounds rendering them non-toxic. Mammals also mount a cellular immunological response on entry of foreign cells, viruses or macromolecules into the body. T lymphocytes mobilize at the site of foreign body entry and secrete protein messengers called lymphokines. Secondary to T lymphocytes, macrophages concentrate at the infection site and function in antigen processing and phagocytosis. In vitro, macrophage movement is arrested by one class of lymphokines known as macrophage migration inhibitory factors (MIFs). We report here the purification of milligram quantities of a unique multifunctional protein from rat liver which links enzymatic and immunological detoxification systems. This protein actuates both GST and MIF activity and matches the primary structure of a human MIF in 25 out of 26 amino-terminal amino acids. Primary structure comparisons revealed significant similarity between GSTs and MIF. The glutathione affinity chromatography purification described here yields a 100-fold increase in obtaining MIF and will aid understanding of its precise biological function.

Drug-metabolizing enzymes in respiratory nasal mucosa and liver of cynomolgus monkey

1. Microsomal and cytosolic drug-metabolizing enzyme activities of respiratory mucosa of male and female monkeys have been determined and compared to those of monkey liver. The results demonstrated that cytochrome P-450, NADPH-cytochrome P-450 reductase and some monooxygenase activities, especially ethoxycoumarin O-deethylase activity, were present in respiratory epithelium, although at lower levels than in liver. 2. Activities of non-oxidative enzymes--namely, epoxide hydrolase, UDP-glucuronyltransferase, glutathione S-transferase, DT-diaphorase, carbonyl reductase, benzaldehyde and propionaldehyde dehydrogenases--were also detected in respiratory tissue, some at higher levels than in liver. 3. The enzymic activities found in monkey nasal mucosa are not very similar to those in corresponding human tissue where, for example, UDP-glucuronyltransferase activity is not detectable. This indicates that monkey is not necessarily the best animal model for studies of the human upper respiratory tract.

[Exogenous NADPH restores the detoxication function of isolated hepatocytes lost in the process of freezing-thawing]

Freezing and thawing have been studied for their effect on the rat hepatocyte detoxication system. Freezing-thawing of hepatocytes in the medium without cryoprotectors is to impair functioning of the biotransformation system of biphenyl, a hydrophobic xenobiotic. It is found that inhibition of the first stage of xenobiotics biotransformation is a result of the loss of pyridine nucleotides and substrates necessary for NADP+ reduction.

[Evaluation of hepatic microsomal enzyme activity using C-l4-labeled aminopyrine breath test in patients with diabetes mellitus type 2 treated with tolbutamide]

Among 116 diabetic patients of the District Diabetic Polyclinic in Zabrze (51 type II treated with tolbutamide++, 36 type I treated with insulin, 29 type II treated with diet only) and 30 health persons functional microsomal hepatic fraction with ABT was assessed. Classical hepatic function test were done too. Among 49% patients under examination with type II diabetes treated with tolbutamide++ statistically significant decrease of value of ABT was observed as compared with other groups. Values of classical hepatic function tests in investigated patients were within normal values. Decrease in demethylation of aminopyrine shown as the abnormality in values of ABT can be considered as early symptom of hepatocyte damage caused by tolbutamide++. ABT is valuable method of appraising the liver's detoxication function in type II diabetes. Patients treated with tolbutamide++ should not receive any other drug known as inhibitor of microsomal enzymes of the liver.

Reactions of the nitroso and hydroxylamine metabolites of sulfamethoxazole with reduced glutathione. Implications for idiosyncratic toxicity

N4-oxidation of sulfonamides has been implicated in the pathogenesis of idiosyncratic reactions to these antimicrobials. In vitro toxicity assays employing mononuclear leukocytes as target cells have shown that the toxicity of sulfamethoxazole hydroxylamine (SMX-HA) is inhibited by exogenous glutathione, suggesting that conjugation with glutathione is an important detoxification pathway. However, in these experiments, significant depletion of cellular glutathione only occurred at concentrations of SMX-HA greater than or equal to 300 microM. At concentrations of SMX-HA which produce 50% toxicity in mononuclear leukocytes (approximately 100 microM), there was not a significant loss of glutathione. SMX-HA also caused a small but significant increase in oxidized glutathione concentrations. In cell-free experiments, reduced glutathione (GSH) prevented the autooxidation of SMX-HA to nitrososulfamethoxazole (nitroso-SMX). During this process, oxidized glutathione was formed. GSH rapidly reacted with nitroso-SMX to form a labile semimercaptal conjugate. Physiologically relevant concentrations of GSH (i.e. 1 mM) favored thiolytic cleavage of the semimercaptal to form SMX-HA. Isomerization of the semimercaptal to the more stable sulfinamide occurred at low GSH concentrations. Purified glutathione transferases had no effect on the reaction of SMX-HA with GSH. Therefore, glutathione is important in protecting cells from the toxicity of SMX-HA largely by preventing its further oxidation to nitroso-SMX. Stable glutathione conjugates are likely to be formed only in small quantities under physiological conditions. Conjugation with glutathione would not be expected to be a major pathway for clearance of the hydroxylamine and nitroso metabolites of sulfonamides.

Proposed role of drug-metabolizing enzymes: regulation of steady state levels of the ligands that effect growth, homeostasis, differentiation, and neuroendocrine functions

Every ligand known to bind to a receptor in the nuclear hormone receptor superfamily is involved in a variety of signal transduction pathways effecting growth, morphogenesis, homeostasis, proliferation, and neuroendocrine functions. Often these ligands are associated with increases in particular subsets of cytochromes P450 and other drug-metabolizing enzymes. Interestingly, certain of these enzymes participate in the metabolism (synthesis as well as degradation) of these ligands. It appears that genes coding for certain drug-metabolizing enzymes might have existed on this planet at least 1 billion years before the presence of plants, animals, and drugs. An early role for oxidative enzymes in prokaryotes most likely involved energy substrate utilization: insertion of oxygen into various inaccessible carbon and other food sources, thereby rendering them accessible to further metabolism. It is proposed that a later development of these "drug-metabolizing enzymes" in prokaryotes and early eukaryotes might be related to their metabolic ability to control the steady state levels of the ligands that modulate cell division, growth, morphogenesis, and mating, and that this role has diversified in numerous additional signal transduction pathways and exists today in all eukaryotes.

Microencapsulated hepatocytes. Prospects for extracorporeal liver support

To assess the potential for encapsulated hepatocytes as a bioartificial liver support system, rabbit hepatocytes were encapsulated within multicomponent capsules using a complex coacervation technique, and cultured both on plates and in a perfusion reactor. The urea synthesis rate and antipyrine and diazepam degradation rates were evaluated in each system over a 10 day period, and compared with standard plate-cultured hepatocyte efficacy. Urea synthesis rates were significantly higher in the perfusion cultures than in either of the plate culture environments, whereas drug degradation rates were not significantly different in any of the systems.

The 1990 Pharmaceutical Manufacturers Association of Canada keynote lecture. The role of the cytochromes P450 in the detoxication and activation of drugs and other chemicals

The roles of the cytochromes P450 are reviewed, with emphasis on their involvement in the detoxication of drugs and chemicals, the activation of carcinogens, and the toxicity of drugs. Cytochromes P450 have different characteristics. P450I mostly activates carcinogens and other chemicals by forming oxygenated reactive intermediates, which are also associated with the formation of neoantigens and immunotoxicity. P450IIE has a propensity to form oxygen radicals, which are cytotoxic and carcinogenic; other cytochromes generate oxygen radicals by futile cycling when activated by difficulty metabolized substrates. Novel procedures for the safety evaluation of chemicals are described; COMPACT is based on the computer graphic determination of the spatial conformation and electronic structure of chemicals to enable their activating cytochromes P450, and hence their toxicity, to be established; ENACT is based on quantifying the induction of individual cytochromes P450, since the extent of induction of P450I, and possibly other activating cytochromes, is directly related to the carcinogenic potential of the chemical.

Intestinal glutathione transport system: a possible detoxication role

The epithelium of the small intestine act by the formation of GSH-S-conjugation, as a first line of defence against various ingested toxic chemicals. GSH and GSH-dependent enzymes are present in the gastrointestinal wall. We and others have characterized the GSH-specific transport systems in intestinal brush-border and in basolateral membrane vesicles, in which gamma-glutamyltranspeptidase (gamma-GT) activity was inactivated by AT-125. In the present study we use inhibition experiments, kinetic studies, trans-stimulation of GSH uptake and HPLC determination to demonstrate (for the first time) that GSH and two GSH-S-conjugates (chosen as model compounds) share a common transport system. Plasma GSH-S-conjugates that may enter the intestinal cells via basolateral membrane, and GSH-S-conjugates that form in intestinal cells, may be eliminated directly by this GSH transporter across brush-border membranes or transported into lumen to the active site of gamma-GT; they are then further metabolized and excreted by various routes. This transport system may thus contribute to the intestinal detoxication role.

Detoxification pathways in the liver

The liver plays an important rôle in protecting the organism from potentially toxic chemical insults through its capacity to convert lipophiles into more water-soluble metabolites which can be efficiently eliminated from the body via the urine. This protective ability of the liver stems from the expression of a wide variety of xenobiotic biotransforming enzymes whose common underlying feature is their ability to catalyse the oxidation, reduction and hydrolysis (Phase I) and/or conjugation (Phase II) of functional groups on drug and chemical molecules. The broad substrate specificity, isoenzyme multiplicity and inducibility of many of these enzyme systems make them particularly well adapted to handling the vast array of different chemical structures in the environment to which we are exposed daily. However, some chemicals may also be converted to more toxic metabolites by certain of these enzymes, implying that variations in the latter may be important predisposing factors for toxicity. Pharmacogenetic defects of xenobiotic biotransformation enzymes, a subclass of inborn errors of metabolism which are manifested only upon drug challenge, introduce marked variation into human populations for the pharmacokinetics and pharmacodynamics of therapeutic and toxic agents, and thus may have important clinical consequences for drug efficacy and toxicity.

Binding and detoxification of heavy metals in lower vertebrates with reference to metallothionein

1. The metabolism of Cu, Zn, Cd and Hg in lower vertebrates is described, using fish as a model. 2. The main part of this review deals with metallothionein and the role of this protein for the storage and detoxification of these metals. 3. Factors influencing the bioavailability and probable uptake routes are identified. 4. The distribution of the metals within the organism is outlined. The distribution between tissues is described and the subcellular distribution discussed with reference to metallothionein.

Metabolism of benzo[a]pyrene and persistence of DNA adducts in the brown bullhead (Ictalurus nebulosus)

1. The in vitro metabolism of [3H]benzo[a]pyrene (BP) and [14C]benzo[a]pyrene-7,8-dihydrodiol (BP-7,8-diol) by liver of brown bullhead (Ictalurus nebulosus) was characterized, as was the formation and persistence of BP-DNA adducts in vivo. 2. Compared to rat liver microsomes, bullhead liver microsomes produced relatively larger amounts of BP-7,8-diol (predominantly the [-] enantiomer) and smaller amounts of of BP-7,8-diol (predominantly the [-] enantiomer) and smaller amounts of BP-4,5-diol. 3. BP phase I metabolites were efficiently converted by freshly isolated bullhead hepatocytes to conjugates, predominantly glucuronides. 4. BP-7,8-diol was metabolized by hepatocytes 4-fold more rapidly than was BP and was converted to approximately equal amounts of glucuronides, glutathione conjugates and sulfates. 5. BP-DNA adducts formed in bullhead liver with a lag time of several days and maximum adduct formation at 25-30 days. The major adduct was anti-BPDE-deoxyguanosine.

The association of age with aspirin esterase activity in human liver

The activity of the phase 1 enzyme aspirin esterase was studied in liver tissue from 16 patients, aged 45-88 years. No correlation between age and enzyme activity was found in post-mitochondrial, cytosolic and microsomal fractions. These results provide further evidence that age is not a major determinant of the activity of hepatic drug-metabolizing enzymes in man.

Hereditary variation of liver enzymes involved with detoxification and neurodegenerative disease

Enzymes involved with the metabolic transformation of xenobiotics have recently been studied in patients with the neurodegenerative diseases, Alzheimer's disease, Parkinson's disease and motor neurone disease. Defects were detected in sulphur pathways and also, in the case of Parkinson's disease, in monoamine oxidase B. The possibility exists that the ability to cope safely with endogenous and exogenous substances which have neurotoxic properties is important in the pathogenesis of these diseases. Potentially such individuals could be identified preclinically and these diseases postponed by reduction in the load of toxin or modification of the relevant enzymic activity.

[Age related changes in the liver. Consequences for drug therapy]

The most significant morphological peculiarity of the liver in old age is "brown atrophy", with a reduction in weight and volume of the organ. The physiological metabolic functions of the liver are extremely well preserved right up into very old age. There is, however, some impairment of function with respect to the elimination of exogenous substances and drugs. The major cause of this is a decrease in hepatic perfusion, the volume of the liver, and the number of functioning hepatocytes. Thus, since the elimination capacity of both the kidneys and the liver can be reduced in old age, drugs should initially be administered at lower doses in geriatric patients.

[Indices of structure and function of erythrocyte membranes in rats with vitamin E deficiency and their correction with antioxidant drugs]

It is found that the amount of saturated fatty acids grows, while that of unsaturated--falls in the erythrocyte membranes of rats maintained on a diet without vitamin E. In this case transmembrane calcium transport catalyzed by Ca2(+)-ATPase (EC 3.6.1.3) is not broken and activity of antioxidant enzyme superoxide dismutase (EC 1.15.1.11) in lysate of erythrocytes falls. The found shifts are corrected with administration of antioxidant preparations.

Possible role of C-reactive protein in detoxication of mercury

In mercury-treated rats, C-reactive protein (CRP) has been found to be synthesized in the liver which, in turn, sequesters mercury resulting in the denaturation of the protein into subunits. The subunits retain the mercury and are released into the serum from where it gets excreted. Native CRP and CRP-subunits have been found to have different antigenicity. Thus one of the physiological roles of CRP seems to be detoxication of heavy metals.

Altered benzo[a]pyrene metabolism in C3H/10T1/2 cells transformed by aflatoxin B1 or 3-methylcholanthrene

C3H/10T1/2 clone 8 (10T1/2) cells possess Phase I and Phase II xenobiotic metabolizing enzymes associated with the metabolism of polycyclic aromatic hydrocarbons to activated or detoxified species. We compared the metabolism of benzo[a]pyrene (BaP) by these cells to an aflatoxin B1 (AFB1)-transformed line (7SA) and a 3-methylcholanthrene (3-MC)-transformed line (MCA) isolated from carcinogen-treated 10T1/2 cells. Relative to 10T1/2 cells, basal levels of cytochrome P450-mediated aryl hydrocarbon hydroxylase (AHH) were significantly depressed in 7SA cells by about 30%. The inducibility of AHH by BaP treatment was depressed by 30-70% in MCA and 7SA cells over a 36-hr time course. 10T1/2 and MCA cells accumulated similar intracellular amounts of 3-OH-BaP by 12 and 24 hr, respectively; in contrast the accumulation of 3-OH-BaP in 7SA cells was 70% lower. During 36 hr of BaP treatment, total BaP-DNA adduct levels formed in 7SA and MCA cells, determined by 32P-postlabeling analysis, were 90 and 83% lower, respectively, than those found in 10T1/2 cells. These differences in response to BaP treatment were not related to cellular differences in the uptake or efflux of BaP. Relative to 10T1/2 or MCA cells, 7SA cells were found to have at least a twofold increase in UDP-glucuronyltransferase activity, which correlated with the lower intracellular accumulation of 3-OH-BaP and enhanced formation of extracellular polar metabolites. MCA cells had an almost twofold increase in glutathione S-transferase activity relative to parental 10T1/2 cells but produced lower levels of extracellular polar metabolites. These results demonstrate an association between chemical transformation of 10T1/2 cells and altered xenobiotic metabolism. This system may provide an in vitro model for examining the molecular events responsible for the biochemically altered phenotype of the malignantly transformed cell.