Effects of valproic acid on organic acid metabolism in children: a metabolic profiling study.

Young children are at increased risk for valproic acid (VPA) hepatotoxicity. Urinary organic acid profiles, as a surrogate of mitochondrial function, were obtained in children 1.9 to 17.3 years of age (n = 52) who were undergoing treatment with VPA for seizure disorders. Age-matched patients receiving treatment with carbamazepine (CBZ; n = 50) and healthy children not undergoing treatment (n = 22) served as controls. Age-related changes in organic acid profiles were observed in all three groups. Although the untreated and CBZ control groups were indistinguishable from each other with respect to the principal-component analysis (PCA) score plots of the subjects, a distinct boundary was apparent between the VPA and each of the control groups. Interindividual variability was observed in the VPA-induced alterations in endogenous pathways corresponding to branched-chain amino acid metabolism and oxidative stress. The data suggest that more detailed metabolomic analysis may provide novel insights into biological mechanisms and predictive biomarkers for children at highest risk for serious toxicity.

Influence of CYP2C9 genotypes on the formation of a hepatotoxic metabolite of valproic acid in human liver microsomes.

The present study investigated the effect of cytochrome P450 2C9 (CYP2C9) genetic polymorphism on the biotransformation of valproic acid (VPA) to its hepatotoxic metabolite, 4-ene-VPA, and compared that to the formation of the inactive 4-OH-VPA and 5-OH-VPA. cDNA-expressed CYP2C9(*)2 and CYP2C9(*)3 variants were less efficient than the CYP2C9(*)1 wild type in catalyzing the formation of these metabolites, as assessed by the ratio of Vmax and apparent Km (in vitro intrinsic clearance). The reduced efficiency by CYP2C9(*)2 was due to a reduced Vmax, whereas, in the case of CYP2C9(*)3, it was the result of increased apparent Km. The formation rates of 4-ene-VPA, 4-OH-VPA, and 5-OH-VPA in human liver microsomes were reduced by 29, 28, and 31%, respectively, in samples with one mutated CYP2C9 allele, and by 61, 73, and 58%, respectively, in samples with two mutated CYP2C9 alleles. Overall, the homozygote and heterozygote CYP2C9(*)2 and CYP2C9(*)3 genotypes may compromise hepatic VPA biotransformation.

Amino acid conjugates: metabolites of 2-propylpentanoic acid (valproic acid) in epileptic patients.

In this study, spectroscopic and chromatographic evidence is presented for the identification and characterization of the metabolites, valproyl glutamate (2-propylpentanoyl glutamate, VPA-GLU) and valproyl glutamine (2-propylpentanoyl glutamine, VPA-GLN) in the urine, serum, and cerebrospinal fluid (CSF) of patients on valproic acid (VPA) therapy. Moreover, the identification of valproyl glycine (2-propylpentanoyl glycine, VPA-GLY) in the serum and urine of patients on VPA, albeit in trace concentrations, is also reported here. The three amino acid conjugates excreted in urine accounted for about 1% of the VPA dose in four patients who were on VPA therapy chronically and had reached steady state. VPA-GLU was quantitatively the most prominent metabolite (0.66-13.1 microg/mg creatinine) compared with VPA-GLN (0.78-9.93 microg/mg creatinine) and VPA-GLY (trace-1.0 microg/mg creatinine) in overnight urine samples of all patients studied (n = 29). The relatively low serum concentrations of the three amino acid conjugates of VPA in six patients suggest that the metabolites are readily excreted once formed. In contrast, whereas VPA GLY was absent in the CSF of one patient on VPA, the concentrations of VPA-GLU and VPA-GLN in this CSF sample were 9 and 5 times, respectively, their corresponding serum concentrations.

Determination of an arylether antiarrhythmic and its N-dealkyl metabolite in rat plasma and hepatic microsomal incubates using liquid chromatography-tandem mass spectrometry.

A method was developed and validated for the quantification of (+/-)-trans-[2-morpholino-1-(1-naphthalene-ethyloxy]cyclohexane monohydrochloride (RSD1070) and its N-dealkyl metabolite in rat plasma and hepatic microsomal incubates. Chromatographic separations were achieved using reversed-phase high-performance liquid chromatography coupled with positive ion electrospray ionization and detection by tandem mass spectrometry. The assay was linear from 2.5 to 100 ng/ml and this range was used for validation. Inter- and intra-assay variability (n=6), extraction recovery, and stability in plasma were assessed. The estimated limit of quantitation was in the range 2.5-3 ng/ml for both analytes in rat plasma. The analytical method was used in a pharmacokinetic study of RSD1070 in rats after a single i.v. bolus of 12 mg/kg.

In vitro investigation of the hepatic extraction of RSD1070, a novel antiarrhythmic compound.

PURPOSE: The hepatic extraction of a novel antiarrhythmic, RSD1070, was investigated to test the hypothesis that the poor bioavailability observed in rats is due to high hepatic metabolism. METHODS: The pharmacokinetics of RSD1070 was examined in rats (n=8) and its metabolism was investigated using pooled rat hepatic microsomes. The free fraction in plasma and microsomal matrices was determined by equilibrium dialysis. Hepatic extraction was predicted by scaling-up of the microsomal kinetic data using the well-stirred liver model. RESULTS: RSD1070 demonstrated tri-exponential decay following single iv bolus administration of a dose of 12 mg/kg. RSD1070 exhibited a rapid elimination, t1/2 of 25 +/- 8 min and a CL(tot) of 71 +/- 9 mL/min/kg. Renal clearance based on 24 h urinary recovery was determined to be insignificant (<< 1% of CL(tot)). A Michaelis-Menten model described the elimination of RSD1070 with a K(m) of 0.45 microg/mL and Vmax of 2.81 microg/min/mg microsomal protein. Taking the V(max)/K(m) ratio (CL(int)) as the basis for scaling, the data from the microsomal kinetic studies (75 mL/min/kg) closely approximated the apparent CL(tot). In the scale-up of the in vitro CL(int), plasma free fraction (1.5%) and microsomal free fraction (15%) were determined and incorporated into the well-stirred liver model. CONCLUSION: RSD1070 is a high hepatic extraction compound (E = 0.94) with a predicted CL(h) value that accounted for the CL(tot) observed in rats.

Dose-dependent pharmacokinetics and metabolism of valproic acid in newborn lambs and adult sheep.

Dose-dependent pharmacokinetics and metabolism of valproic acid (VPA) were studied in newborn and adult sheep to assess age-related differences in plasma protein binding and metabolic elimination. Newborn lambs received either a 10- (n = 8), 50- (n = 5), 100- (n = 4), or 250-mg/kg (n = 4) VPA i.v. bolus. Individual adult sheep (n = 5) received all four doses in a random order with an appropriate washout period between experiments. Unbound or metabolic clearance of VPA was significantly higher in adult sheep at the two lower doses when compared with lambs, and similar to the lambs at the two higher doses. Plasma protein binding was nonlinear at all doses. Estimates of binding capacity (B(max1)) at the saturable site were higher in adults (91.8 microg/ml) when compared with lambs (44.9 microg/ml), whereas the opposite trend was observed for binding affinity [K(d1) = 9.6 microg/ml (adult) versus 3.2 microg/ml (lambs)]. Characterization of developmental differences in overall VPA metabolic elimination involved fitting of unbound VPA plasma concentration data to a two-compartment model with Michaelis-Menten elimination. This resulted in similar in vivo estimates of apparent V(max) [445.0 microg/min/kg (adult) versus 429.9 microg/min/kg (lambs)]. However, apparent K(m) estimates appeared to be higher in lambs [30.0 microg/ml (adult) versus 69.6 microg/ml (lambs)]. Similar findings were obtained from in vivo estimates of V(max) and K(m) for VPA glucuronidation obtained from VPA-glucuronide metabolite urinary excretion data. Thus, it appears that age-related differences in metabolic clearance may be related to differences in the apparent in vivo K(m) as opposed to V(max) of VPA glucuronidation.

Metabolic profiling of valproic acid by cDNA-expressed human cytochrome P450 enzymes using negative-ion chemical ionization gas chromatography-mass spectrometry.

A sensitive negative ion chemical ionization (NCI) gas chromatographic-mass spectrometric (GC-MS) method was modified for the quantitation of valproic acid (VPA) metabolites generated from in vitro cDNA-expressed human microsomal cytochrome P450 incubations. The use of the inherent soft ionization nature of electron-capture NCI to achieve high sensitivity enabled us to conduct kinetic studies using small amounts of recombinant human P450 enzymes. The assay is based on the selective ion monitoring of the intense [M-181] fragments of pentafluorobenzyl (PFB) esters in the NCI mode, and has the following features: (1) a micro-extraction procedure to isolate VPA metabolites from small incubation volumes (100 microl); (2) a second step derivatization with tert.-butyldimethylsilylating reagents to enhance sensitivity for hydroxylated metabolites; (3) a short run-time (<30 min) while maintaining full separation of 15 VPA metabolites by using a narrow-bore non-polar DB-1 column plus a new temperature gradient; and (4) good reproducibility and accuracy (intra- and inter-assay RSDs <15%, bias <15%) by using seven deuterated derivatives of analytes as internal standards. The derivatives of mono-and diunsaturated metabolites, like the parent drug, produced abundant [M-181](-) ions while the hydroxylated metabolites gave an ion at m/z of 273, corresponding to the [M-181](-) ion of the tert.-butyldimethylsilyl ethers. In conclusion, the GC-NCI-MS analysis of valproate metabolites provided us with a high resolution and sensitivity necessary to conduct metabolic and kinetic studies of valproic acid in small volume samples typical of the in vitro cDNA-expressed micro-incubation enzymatic systems.

Ontogeny of valproic acid disposition and metabolism: a developmental study in postnatal lambs and adult sheep.

The ontogeny of valproic acid (VPA) disposition and metabolism was investigated in developing lambs and adult sheep (Dorset or Suffolk breed). Specifically, we wished to investigate the role of glucuronidation and beta-oxidation on VPA elimination during development. Catheters were implanted in a carotid artery, a jugular vein, and the urinary bladder in 10-day-old (10 d; n = 8), 1-month-old (1 M; n = 4), and 2-month-old lambs (2 M; n = 5). In adult sheep (n = 5), catheters were implanted in a femoral artery and vein. After the administration of a 10 mg/kg VPA i.v. bolus, serial blood samples and cumulative urine samples were collected for 36 h in the adult ewes and for 72 h in the lambs. Due to saturable protein binding, age-related differences in VPA clearance were more obvious when examining the total body clearance of unbound drug (Cl(u)tb). Mean Cl(u)tb increased significantly with age up to 2 months (10 d = 2.65 +/- 1.16 ml/min/kg; 1 M = 5.11 +/- 2.49 ml/min/kg; 2 M = 12.84 +/- 3.88 ml/min/kg) before decreasing to adult levels (7.73 +/- 2.64 ml/min/kg). Similarly, the urinary recovery of the major metabolite, VPA-glucuronide, was significantly less in 10 d lambs (29.2 +/- 16.0% of the dose) when compared with the adult and 2 M groups (both approximately 74% of the dose). No differences with age were observed in the portion of the dose excreted as the beta-oxidation metabolite, 2-n-propyl-3-oxopentanoic acid. The results suggest that alterations in Cl(u)tb with age may be attributable to postnatal development of enzymes involved in VPA glucuronidation.

Identification and characterization of N-acetylcysteine conjugates of valproic acid in humans and animals.

Reactive and hepatotoxic metabolites formed from the biotransformation of valproic acid (VPA) are normally detoxified by conjugating with GSH and followed by mercapturic acid metabolism to produce their respective N-acetylcysteine (NAC) conjugates. Hence, the levels of NAC conjugates of VPA in human urine are an indirect measure of exposure of the liver toward reactive metabolites of the anticonvulsant drug. We report here the synthesis, identification, and characterization of a second NAC conjugate of (E)-2-propyl-2, 4-pentadienoic acid in the urine samples (n = 39) of humans on VPA therapy, namely, (E)-5-(N-acetylcystein-S-yl)-2-ene VPA by gas chromatography/mass spectrometry and liquid chromatography with tandem mass spectrometry. In this study, we were able to separate the diastereomers of (E)-5-(N-acetylcystein-S-yl)-3-ene VPA by HPLC. The NAC conjugate of 4,5-epoxy VPA, namely, 5-NAC-4-OH-VPA gamma-lactone, previously identified in rats treated with 2-propyl-4-pentenoic acid (4-ene VPA), was not detected in any of the human urine samples studied. This suggests that in humans, the P-450 metabolism of 4-ene VPA to the reactive epoxide is not a significant pathway. The excretion of the NAC conjugate of (E)-2, 4-diene VPA glucuronide in the urine of seven patients on VPA was also examined and was not detected. The limit of detection of 5-NAC-3-keto VPA and its decarboxylated product, 1-NAC-3-heptanone, was estimated at 25 ng (signal to noise ratio > 3). Neither 5-NAC-3-keto VPA nor 1-NAC-3-heptanone was detected in the urine of patients on VPA therapy or 4-ene VPA-treated guinea pigs, but 1-NAC-3-heptanone was detected in the urine of 4-ene VPA-treated rats.

Disposition of valproic acid in maternal, fetal, and newborn sheep. I: placental transfer, plasma protein binding, and clearance.

Separate 24-h maternal and fetal infusions of valproic acid (VPA) were administered to five pregnant sheep at 125 to 138 days gestation (term approximately 145 days) to determine maternal-fetal disposition. The pharmacokinetics of VPA were also investigated in five newborn 1-day-old lambs after a 6-h drug infusion. Plasma, urine, and amniotic and fetal tracheal fluid samples were analyzed for VPA using gas chromatography-mass spectrometry. During maternal drug infusion, the average steady-state fetal/maternal unbound VPA plasma concentration ratio was 0.81 +/- 0.09. Unbound maternal-to-fetal VPA placental clearance (69.0 +/- 20.2 ml/min/kg) was similar to that in the other direction (61.9 +/- 24.2 ml/min/kg); this indicates passive placental diffusion and intermediate placental permeability of VPA in sheep. Newborn unbound VPA clearance (0.66 +/- 0.28 ml/min/kg) was much lower than in the mother (5.4 +/- 2.7 ml/min/kg) or the fetus (62.1 +/- 22.4 ml/min/kg), and exhibited pronounced Michaelis-Menten characteristics. The elimination half-life of the drug was much longer in the newborn (18.6 +/- 2.6 h) relative to the mother (5.6 +/- 1.4 h) and the fetus (4.6 +/- 1.9 h). Thus, VPA elimination in newborn lambs is much slower as compared with adult sheep, a situation similar to that in humans. Plasma protein binding of VPA was saturable, with similar VPA binding capacities and affinities in maternal and fetal plasma. VPA was extensively displaced from binding sites in the newborn lamb during the first 1 to 2 days of life, possibly because of increased plasma free fatty acid concentrations at birth. Thereafter, newborn plasma appeared to have a similar VPA binding capacity but lower affinity compared with the mother and the fetus.

Disposition of valproic acid in maternal, fetal, and newborn sheep. II: metabolism and renal elimination.

Metabolism and renal excretion of valproic acid (VPA) were examined in maternal, fetal, and newborn sheep to identify the underlying reasons for the previously observed reduced VPA clearance in newborn lambs. Plasma and urine from VPA infusion studies in maternal, fetal, and newborn sheep were analyzed for VPA and its metabolites [VPA-glucuronide; beta-oxidation products: (E)-2-ene, (E)-3-ene, and 3-keto VPA; hydroxylated metabolites: 3-hydroxy, 4-hydroxy, and 5-hydroxy VPA (5-OH VPA); and 4-ene VPA, 4-keto VPA, 2-propylglutaric acid, and 2-propylsuccinic acid] using gas chromatography-mass spectrometry. All measured metabolites were detectable in maternal and fetal plasma, with 3-keto and 5-OH VPA being at higher concentrations in the fetus. Plasma concentrations of (E)-2-ene, (E)-3-ene, 3-keto, and 5-OH VPA were higher in the newborn compared with the mother, whereas those of the other metabolites were similar. A smaller percentage of the dose was excreted as VPA-glucuronide in newborn lamb urine (28.3 +/- 12.0%) compared with the mother (77.0 +/- 7.8%). Similarly, a lower fraction of the dose was excreted unchanged in newborn urine (11.0 +/- 5.8%) relative to the urine of the mother (19.3 +/- 5.8%); however, significantly larger percentages were excreted as (E)-2-ene (0.11 +/- 0.04 versus 0.02 +/- 0.01%), 3-keto (11.6 +/- 3.5 versus 1. 6 +/- 0.8%), 4-hydroxy (6.1 +/- 3.2 versus 2.3 +/- 1.3%), and 5-OH VPA (2.2 +/- 0.6 versus. 0.8 +/- 0.6%). The major reason for the reduced VPA elimination in newborn lambs appears to be impaired renal excretion and glucuronidation capacity. As a result, a larger fraction of the dose is channeled to beta-oxidation and hydroxylation pathways. The beta-oxidation activities are high at birth; this may explain the high plasma concentrations of (E)-2-ene and 3-keto VPA observed in newborn lambs and human newborns exposed to VPA.

Gas chromatography/negative ion chemical ionization mass spectrometry and liquid chromatography/electrospray ionization tandem mass spectrometry quantitative profiling of N-acetylcysteine conjugates of valproic acid in urine: application in drug metabolism studies in humans.

We report a GC/NICI-MS assay and a LC/ESI-MS/MS assay for the analysis of N-acetylcysteine (NAC) conjugates of (E)-2,4-diene VPA (NAC I and NAC II) identified in humans. The assay also includes the analysis of the NAC conjugate of 4,5-epoxy VPA (NAC III), an identified metabolite in rats treated with 4-ene VPA for its use in metabolic studies in animals. The highly sensitive GC/MS assay was designed to monitor selectively the diagnostic and most abundant [M - 181](-) fragment anion of the di-PFB derivatives of NAC I, NAC II, and NAC IV, the internal standard (IS) and the PFB derivative of NAC III. The higher selectivity of LC/MS/MS methodology was the basis for an assay which could identify and quantitate the underivatized conjugates simultaneously using MRM of the diagnostic ions m/z 130 and 123 arising from the CID of their protonated molecular ions [MH](+). The GC/MS assay employed liquid-liquid extraction whereas the LC/MS/MS assay used a solid-phase extraction procedure. Linearity ranges of the calibration curves were 0.10-5.0microg ml(-1) by GC/MS and 0.10-1.0microg ml(-1) by LC/MS/MS for NAC I, NAC II and NAC III (r(2) = 0.999 or better). Both assays were validated for NAC I and NAC II and provided good inter- and intra-assay precision and accuracy for NAC I and NAC II. The LOQ by LC/MS/MS was 0.1microg ml(-1), representing 1 ng of NAC I and NAC II. The same LOQ (0.1microg ml(-1)) was observed by GC/MS and was equivalent to 100 pg of each metabolite. NAC III was detected at concentrations as low as 0.01 microg ml(-1) by both methods. The total urinary excretion of the NAC conjugates in four patients on VPA therapy was determined to be 0.004-0.088% of a VPA dose by GC/MS and 0.004-0. 109% of a VPA dose by LC/MS/MS.

LC/MS analysis of hydroxylation products of salicylate as an indicator of in vivo oxidative stress.

Hydroxyl radical attack upon salicylate leads to the generation of 2,3-dihydroxybenzoic acid (2,3-DHBA) and therefore can be used to assess hydroxyl radical formation both in vitro and in vivo. Evidence is presented for a highly sensitive LC/MS assay for the quantification of 2,3-DHBA. Calibration curves showed linearity within the concentration range tested (0.5-6.5 pmol/microl rat plasma) with a coefficient of determination (r2) greater than 0.99. A detection limit of less than 0.25 pmol for 2,3-DHBA has been achieved. The intra-assay and inter-assay variability were determined to be 4.1% and 12.5%, respectively. This method was evaluated for the determination of drug-induced in vivo generation of oxidative stress by means of 1,1,1-trichloroethane (TCE) a compound that is a pseudosubstrate for cytochrome P450 and is known to induce oxygen reductase activity of this enzyme(s). TCE treated rats had a 6.4-fold increase in the mean maximal plasma 2,3-DHBA concentration as compared to the saline treated rats (p = .009). The developed LC/MS assay requires minimal sample preparation and provides a rapid and sensitive method for quantification of 2,3-DHBA as a specific indicator of hydroxyl radical generation.

Assessing the mechanism of metabolism-dependent valproic acid-induced in vitro cytotoxicity.

This study was designed to distinguish and evaluate the contribution of reactive metabolite and reactive oxygen species as the mechanism of metabolism-dependent valproic acid-induced in vitro cytotoxicity. The involvement of reactive oxygen species in the mechanism of in vitro cytotoxicity was examined by the addition of a series of antioxidant enzymes and iron chelators to the reaction mixture. Addition of catalase to the reaction mixture resulted in a complete prevention of valproic acid-induced cytotoxicity. Co-incubation of a cell impermeable iron chelator deferoxamine did not effect cytotoxicity, whereas 1,10-phenanthroline, a chelator with the ability to traverse cell membranes at low concentrations, afforded significant protection against valproic acid-induced cytotoxicity. A possible inhibitory effect of catalase and 1,10-phenanthroline on the microsomal metabolism of valproic acid was disproved by the quantification of valproic acid metabolites in the presence and absence of these compounds. To assess the specificity of the mechanism of in vitro valproic acid-induced cytotoxicity, prevention of in vitro acetaminophen-induced cytotoxicity by antioxidant enzymes and iron chelators was also evaluated. Addition of catalase to the reaction mixture in the presence of acetaminophen resulted in a moderate reduction in the level of but a lack of complete protection of cytotoxicity. Addition of 1,10-phenanthroline to the reaction mixture in the presence of acetaminophen did not result in a detectable change in acetaminophen-induced cytotoxicity. These data suggest the involvement of reactive oxygen species in the mechanism of toxicity of valproic acid and perhaps reactive metabolites as the major cause of cytotoxicity in the case of acetaminophen in the in vitro model investigated. Inhibition of poly(ADP-ribose) polymerase activity by various antagonists resulted in complete prevention of valproic acid-induced in vitro cytotoxicity. The cytoprotective effects of known poly(ADP-ribose) polymerase antagonists implicate poly(ADP-ribose) polymerase in the mechanism of in vitro metabolism-dependent valproic acid-induced cytotoxicity under these conditions. These results further point to nuclear DNA as the intracellular site of insult by the generated oxygen radicals. Overall, the data obtained support the hypothesis that the metabolism-dependent valproic acid-induced in vitro cytotoxicity is the result of generation of hydrogen peroxide in the medium that can readily cross cell membranes and subsequently interact intracellularly with iron to produce the highly reactive hydroxyl free radicals.

Time course of alpha-fluorinated valproic acid in mouse brain and serum and its effect on synaptosomal gamma-aminobutyric acid levels in comparison to valproic acid.

To prevent the hepatotoxicity of valproic acid (VPA), a fluorine substituent was introduced at the alpha-position to eliminate the formation of putative toxic metabolites through mitochondrial beta-oxidation. Although the alpha-fluorinated VPA analogue (alpha-fluoro VPA) is more acidic (pK(a) = 3.55) than VPA (pK(a) = 4.80), the lipophilicity of these two compounds, as determined by their log P values, were similar when compared at pH 2.5. Brain, serum and urine samples were prepared from mature male CD-1 mice treated with either alpha-fluoro VPA or VPA for quantitation of drug concentrations. Brain synaptosomes were isolated to determine gamma-aminobutyric acid levels. After equivalent doses of 0.83 mmol/kg, alpha-fluoro VPA was characterized by its slower access into mouse brain, compared to VPA. The peak concentration of alpha-fluoro VPA in mouse brain was achieved 45 min later than in the serum, whereas the peak brain level of VPA coincided with the peak serum level occurring within 15 min. Simultaneous curve fitting of both brain and serum drug concentrations using a two-compartment model indicated that alpha-fluoro VPA, like VPA, may be asymmetrically transported across the blood-brain-barrier. This property of alpha-fluoro VPA was also reflected in its low brain-to-serum concentration ratio of 0.09 at the peak brain drug concentration (0.16 for VPA). The primary beta-oxidation metabolite of VPA was not found in the serum and urine of mice treated with alpha-fluoro VPA. Although the glucuronide was a major metabolite of VPA (28.5% of the dose), alpha-fluoro VPA was observed to conjugate extensively with L-glutamine (33.3% of the dose). Alpha-fluoro VPA appeared to persist in the general circulation, which, in turn, may contribute to the apparent slow elimination of the drug from the brain. The fluorinated compound was demonstrated to have anticonvulsant activity in the 1,5-pentamethylenetetrazole seizure test and to be capable of increasing brain synaptic gamma-aminobutyric acid, the ED50 being 1.70 mmol/kg. These results suggest that alpha-fluoro VPA has potential as a new anticonvulsant drug.

A rapid in vitro assay for evaluation of metabolism-dependent cytotoxicity of antiepileptic drugs on isolated human lymphocytes.

In vitro assessment of human lymphocyte viability by trypan blue dye exclusion in the presence of an external metabolizing system (microsomes plus NADPH) has been shown to be a useful method in assessing predisposition to idiopathic toxicity in response to various anticonvulsant drugs. The trypan blue method, however, is labor intensive, is time consuming, is prone to human error, is not suitable for high-volume toxicity screening, and excludes autolysed cells. The objective of this study was to develop a rapid, high-capacity, objective, and easy in vitro cytotoxicity method for the detection of metabolism-dependent cytotoxicity of a test chemical. The in vitro system uses an external metabolizing system (rabbit microsomes) in conjunction with isolated human lymphocytes as the target cells. Cellular toxicity was determined by assessing plasma membrane integrity using a membrane-impermeant fluorescent nucleic acid dye (YO-PRO-1) and a multiwell plate scanner for fluorescence. Using this system, cells incubated with either acetaminophen (1500 micrograms/ml), carbamazepine (62.5 microM), phenytoin (62.5 microM), or phenobarbital (62.5 microM) showed net increases in percentage cell death of 31 +/- 5, 11 +/- 4, 0 +/- 3, and 2 +/- 3, respectively. A metabolism-dependent concentration-response was observed for valproic acid-induced cytotoxicity, which approached a plateau at a concentration of 4000 micrograms/ml with a net percentage cell death of 31 +/- 4. This technique resolves various technical difficulties inherent in viability determinations by the trypan blue exclusion method. The YO-PRO-1 method also may be useful in a clinical setting for the assessment of patients with a genetically determined susceptibility to certain drugs and for identifying the responsible drug in patients with idiopathic toxicity undergoing multiple-drug therapy.

A comparative investigation of 2-propyl-4-pentenoic acid (4-ene VPA) and its alpha-fluorinated analogue: phase II metabolism and pharmacokinetics.

An earlier study in rats revealed that alpha-fluorination of 2-propyl-4-pentenoic acid (4-ene VPA), a toxic metabolite of the anticonvulsant drug valproic acid, would avert its metabolism via beta-oxidation and eliminate the drug-related hepatotoxicity. This investigation was carried out to compare 4-ene VPA and the alpha-fluorinated analogue (alpha-fluoro-4-ene VPA) for their pharmacokinetic and protein binding properties. Male Sprague-Dawley rats were dosed with either 4-ene VPA or alpha-fluoro-4-ene VPA i.p. at 1.4 mmol/kg. Blood was collected from the tail vein at various time points and serum samples were prepared. Urine was collected for 24 hr. A second set of rats was treated the same but sacrificed 1 hr post dose, and the livers were homogenized in a Tris-buffer. Protein binding was assessed via ultrafiltration of the naive serum samples spiked with either of the drugs. The serum drug concentration-time profiles of 4-ene VPA and alpha-fluoro-4-ene VPA seemed to resemble one another during the initial 200 min within which differences were reported for their effects on mitochondrial GSH. A second serum peak concentration was observed for 4-ene VPA at approximately 300 min, which was attributed to the extensive glucuronidation of the drug and enterohepatic circulation. The alpha-fluoro-4-ene VPA, on the other hand, did not show these properties with its major phase II metabolite being the corresponding L-glutamine conjugate. The toxic metabolite (E)-2-propyl-2,4-pentadienoic acid and its N-acetylcysteine conjugate were detected only in 4-ene VPA treated rats. Liver concentrations of 4-ene VPA and alpha-fluoro-4-ene VPA were 0.96 +/- 0.11 and 0.89 +/- 0.19 micromol/g of wet liver, respectively, at 1 hr after the dose. Comparable and parallel serum free drug levels were apparent for the two drugs over a concentration range of 0.25 to 2.9 micromol/ml. Taken together, the data seem to suggest that the reported distinction in the ability of 4-ene VPA and alpha-fluoro-4-ene VPA to produce liver toxicity in the rat resides in differences in their metabolism rather than in their pharmacokinetic properties.

Cytotoxicity of unsaturated metabolites of valproic acid and protection by vitamins C and E in glutathione-depleted rat hepatocytes.

Valproic acid (VPA) and the unsaturated metabolites, 2-ene VPA and (E)-2,(Z)-3'-diene VPA, demonstrated dose-dependent cytotoxicity in primary cultures of rat hepatocytes, as evaluated by lactate dehydrogenase (LDH) leakage. Cellular glutathione (GSH) was depleted by adding buthionine sulfoximine (BSO) to the culture medium. Induction of cytochrome P450 by pretreatment of rats with phenobarbital or pregnenolone-16 alpha-carbonitrile enhanced the cytotoxicity of parent VPA in BSO-treated hepatocytes. The cytotoxicity of 4-ene VPA was apparent in BSO-treated hepatocytes with detectable loss of cell viability at 1 microM of added 4-ene VPA. Depletion of cellular GSH also increased the cytotoxicities of 2-ene VPA and (E)-2,(Z)-3'-diene VPA. The cytotoxicity of 2-ene VPA was comparable to or higher than that of VPA, producing loss of viability at concentrations > or = 5 mM. Time-course evaluation of hepatocyte response to 4-ene VPA in the GSH-depleted state revealed a delayed cytotoxicity with no effect during the first 12 h of exposure followed by a pronounced toxicity between 12 and 14 h. Two major GSH conjugates of 4-ene VPA metabolites, namely 5-GS-4-hydroxy VPA lactone and 5-GS-3-ene VPA, were detected in 4-ene VPA treated hepatocytes. Consistent with this finding, a 50% decrease in cellular GSH levels was observed following 4-ene VPA treatment. Under similar conditions, neither toxicity nor the GSH conjugated metabolite were detected in cells treated with the alpha-fluorinated 4-ene VPA analogue (alpha-F-4-ene VPA). The antioxidants, vitamin C and vitamin E, demonstrated a cytoprotective effect against 4-ene VPA-induced injury in GSH-depleted hepatocytes. These results are in support of hepatocellular bioactivation of VPA via 4-ene VPA to highly reactive species, which are detoxified by GSH. The susceptibility of hepatocytes to VPA metabolite-mediated cytotoxicity depends on cellular GSH homeostasis.

Characterization of thiol-conjugated metabolites of 2-propylpent-4-enoic acid (4-ene VPA), a toxic metabolite of valproic acid, by electrospray tandem mass spectrometry.

The hepatotoxicity of the anticonvulsant drug valproic acid (VPA) is most likely associated with the bioactivation of its metabolite 2-propylpent-4-enoic acid (4-ene VPA), which is known to induce hepatic microvesicular steatosis in rats. This paper presents an on-line liquid chromatographic/tandem mass spectrometric (LC/MS/MS) identification of new glutathione (GSH)-related conjugates of the reactive metabolites of 4-ene VPA. Bile samples collected from male Sprague-Dawley rats dosed intraperitoneally with 4-ene VPA or its [2H7]-analogue (100 mg kg-1) were injected on to an ODS column interfaced to a LC/MS/MS instrument using electrospray ionization. LC was developed such that no overlapping of peaks occurred among those metabolites which may potentially produce common fragment ions of interest. Subsequent comparison of LC retention times and MS/MS full fragment ion spectra generated for putative metabolites with that of authentic reference compounds made available by chemical synthesis confirmed the presence of the GSH, cysteinylglycine, cysteine and N-acetylcysteine (NAC) conjugates of 2-(2'-carboxypentanyl)oxirane (4,5-epoxy VPA) and (E)-2-propylpenta-2,4-dienoic acid ((E)-2,4-diene VPA), respectively. Quantitatively, the biliary thiol conjugates accounted for 5% of the dose. This observation is novel for 4-ene VPA metabolism in terms of the degradation of GSH conjugates to the corresponding mercapturic acids possibly occurring within the liver as opposed to an inter-organ process which involves the kidney. In addition, the GSH- and NAC-glucuronide di-conjugates of (E)-2,4-diene VPA were also identified as the biliary metabolites with the GSH-glucuronide di-conjugate being 10% of the corresponding mono-GSH conjugate. Taken together, these data clearly indicate that reactive metabolites of VPA can react with hepatic GSH via several different metabolic pathways and may subsequently produce depletion of GSH that leads to toxic consequences.

Comparative effects of carbamazepine and carbamazepine-10,11-epoxide on hepatic cytochromes P450 in the rat.

The effects of treatment with carbamazepine (CBZ), carbamazepine-10, 11-epoxide (CBZE), and phenobarbital (PB) on total hepatic cytochrome P450 and on cytochrome P450-mediated enzyme activity and protein levels were determined and compared in the present study. Adult male Long Evans rats were treated intraperitoneally with either CBZ (100 mg/kg) or CBZE (50 mg/kg) every 12 hr for 3, 7, 10, or 14 days, or PB (75 mg/kg/day) for 4 days. The dose of CBZE selected was half that of CBZ because serum levels of CBZE in patients on CBZ therapy are generally less than half those of the parent compound. The mean hepatic cytochrome P450 content for the CBZ treatment groups over the 14-day treatment period was 1.9-fold to 2.3-fold greater compared with the untreated group, whereas treatment with CBZE resulted in a more modest increase in total hepatic cytochrome P450. Pentoxyresorufin O-dealkylase and testosterone 2 beta-hydroxylase and 16 beta-hydroxylase activities, as well as androstenedione formation, were increased to a similar extent in CBZ-treated and PB-treated rats relative to the control groups. Immunoblot analysis indicated that hepatic levels of cytochromes P4502B1 and P4502B2 were highly induced, whereas cytochrome P4503A levels were increased slightly, by treatment with CBZ. In comparison, treatment with CBZE was approximately half as effective as CBZ for induction of pentoxyresorufin O-dealkylase activity, and for induction of cytochromes P4502B1/2B2 and cytochrome P4503A levels. In contrast, cytochrome P4501A1 and P4502E1 levels were not altered by treatment with CBZ, CBZE, or PB. The possibility that autoinduction of CBZ metabolism occurred was investigated, but urinary recoveries of CBZ, CBZE, and carbamazepine diol were too low to confirm this finding. In summary, the results demonstrated that cytochromes P4502B1 and P4502B2 are highly inducible by CBZ and CBZE. Maximal induction by CBZ occurred after 3 days of treatment at a dose of 100 mg/kg intraperitoneally every 12 hr and CBZ, when used at this dosage, was as effective as PB for inducing hepatic cytochrome P450. Maximal induction by CBZE also occurred after 3 days of treatment, but the extent of induction was less than that produced by CBZ, possibly because of the smaller dose used.