Reversed-phase chromatography of urinary metabolites of paracetamol using ion suppression and ion pairing

Evaluation of urinary acetaminophen metabolites and its association with the genetic polymorphisms of the metabolising enzymes, and serum acetaminophen concentrations in preterm neonates with patent ductus arteriosus

Acetaminophen is gaining importance as a first-line drug for treating patent ductus arteriosus (PDA) in neonates. Predominant metabolites of acetaminophen in preterm neonates vary from that of adults; and the drug is predominantly metabolised by conjugation and partly by Cytochrome P450 (CYP) enzymes.We carried out the present study to identify the principal urine metabolites of acetaminophen (glucuronide/sulphate) in preterm neonates with hemodynamically significant PDA receiving intravenous acetaminophen, and to evaluate the prevalence of single nucleotide polymorphisms (SNPs) in the key CYP enzymes (CYP1A2*3, CYP1A2*4, CYP1A2*1C, CYP1A2*1K, CYP1A2*6, CYP2D6*10, CYP2E1*2, CYP2E1*5B, CYP3A4*1B, CYP3A4*2, CYP3A4*3, CYP3A5*3, CYP3A5*7, and CYP3A5*11) and their effect on urinary metabolites and serum acetaminophen concentrations.Nineteen (32.8%) neonates had heterozygous CYP1A2*1C, two (3.3%) with heterozygous CYP1A2*1K, 15 (27.8%) and two (3.7%) had heterozygous and homozygous CYP2D6*10, two (3.7%) had heterozygous CYP2E1*5B, seven (12.3%) and three (5.3%) had heterozygous and homozygous CYP3A4*1B, and three (5.5%) had CYP3A5*7 amongst the study population. Acetaminophen sulphate predominated over glucuronide metabolite at all time points. Postnatal days of life was significantly associated with an increase in the urine acetaminophen metabolites with decreased serum acetaminophen concentrations.A significant prevalence of SNPs in the key CYP enzymes related to acetaminophen metabolism was observed in our neonatal population. Population pharmacokinetic-pharmacodynamic modelling incorporating genetic and metabolite data is urgently needed for implementation of precision medicine in this vulnerable population.

Role of zinc and iron chelation in apoptosis mediated by tachpyridine, an anti-cancer iron chelator

Tachpyridine (N,N',N"-tris(2-pyridylmethyl)-cis,cis-1,3,5-triaminocyclohexane; tachpyr) is a potent hexadentate iron chelator under preclinical investigation as a potential anti-cancer agent. Tachpyridine induces apoptosis in cultured cancer cells by triggering a mitochondrial pathway of cell death that is p53-independent. To explore the relationship between the chelation chemistry of tachpyridine and its biological activity, a sensitive and specific reversed-phase high-performance liquid chromatography (RP-HPLC) method was devised and used to measure tachpyr and its metal complexes in cells and tissue culture media. Major species identified in cells treated with tachpyr were tachpyr itself, [Zn(tachpyr)](2+), and iron coordinated to two partially oxidized species of tachpyridine, [Fe(tachpyr-ox-2)](2+), and [Fe(tachpyr-ox-4)](2+). The kinetics of intracellular accumulation of [Zn(tachpyr)](2+) and [Fe(tachpyr-ox-2)](2+) were markedly different: [Zn(tachpyr)](2+) rapidly reached plateau levels, whereas intracellular levels of [Fe(tachpyr-ox-2)](2+) and free tachpyr rose steadily. At the last timepoint measured, 9% of total cellular iron and 13% of total cellular zinc were bound by tachpyridine. Taken together, [Zn(tachpyr)](2+), [Fe(tachpyr-ox-2)](2+), and free tachpyr accounted for virtually all of the tachpyr added, indicating that iron and zinc are the principal metals targeted by tachpyridine in cells. Consistent with these findings, activation of the apoptotic caspases 9 and 3 was blocked in cells pre-treated with either iron or zinc. Pretreatment with either of these metals also completely protected cells from the cytotoxic effects of tachpyridine. These results demonstrate a link between metal depletion and chelator cytotoxicity, and suggest that intracellular chelation of zinc as well as iron may play a role in the cytotoxicity of tachpyridine.

Complete separation of urinary metabolites of paracetamol and substituted paracetamols by reversed-phase ion-pair high-performance liquid chromatography

A reversed-phase high-performance liquid chromatographic procedure has been developed for the separation of thirteen urinary metabolites of the analgesic drug paracetamol. The method involved the use of radially compressed columns packed with octadecylsilica with a particle diameter of 5 micron. Metabolites were chromatographed by linear gradient elution using an ion-pair solvent system composed of tetrabutylammonium hydroxide and Tris buffered to pH 5.0 with phosphoric acid, and acetonitrile as the organic solvent. Analyses can be performed at the rate of three per hour. This method enables the direct identification of sulphate and glucuronide conjugates of 3-thiomethylparacetamol and 3-thiomethylparacetamol sulphoxide which have only previously been detected following enzyme hydrolysis of urine samples. The application of this fully optimised separation to the study of the metabolism of substituted paracetamols is also discussed.

Automated liquid chromatographic method for the determination of paracetamol and six metabolites in human urine

An automated liquid chromatographic method, with a coefficient of variation for total imprecision of less than 4%, has been developed for the quantitative determination of paracetamol, paracetamol-4-glucuronide, paracetamol-4-sulphate, paracetamol-3-cysteine, paracetamol-3-mercapturate, 3-hydroxyparacetamol-3-sulphate and 3-methoxyparacetamol-4-sulphate in urine samples. The gradient elution system was based on 0.067 M phosphate buffer (pH 2.0) and acetonitrile on an octadecylsilica column. The on-column detection limit using an ultraviolet detector at 254 nm for each of the compounds using 3-hydroxyacetanilide as internal standard was of the order of 10-50 ng from urine and 2-10 ng from water. Application of the method to 24-h urine samples from subjects who had received a therapeutic dose of the drug confirmed the findings of previous studies for the importance of the glucuronide, sulphate, mercapturate and cysteine conjugates. 3-Hydroxyparacetamol-3-sulphate was shown to be present in the urine of all volunteers and to account for up to 5% of the dose. 3-Methoxyparacetamol-4-sulphate was not detected in any urine samples and if present as a metabolite must account for less than 0.1% of the dose.

Impact of dilution on the pharmacokinetic behavior of acetaminophen in rabbits after oral administration

Seven rabbits received two different acetaminophen solutions by gavage in a randomized crossover fashion. In one administration the dose was given in a concentrated small volume of an ethanol-glycerol-water mixture (preparation I). In another administration an identical dose was given in a 10-fold water-diluted volume of the same mixture (preparation II). Three rabbits also received the same dose in a 10-fold original mixture volume (preparation III). The acetaminophen concentrations were measured by HPLC in plasma samples collected for 3.5 h after gavage. The lag times ranged from 2.5 to 23 min for preparation I and from 2.2 to 29 min for preparation II. The mean peak plasma concentrations (12.38 micrograms/mL for preparation I and 9.14 micrograms/mL for preparation II) and the mean time-to-peak concentrations (26.57 min for preparation I and 36.57 min for preparation II) were significantly different. The total area under the plasma concentration curve and the absorption and elimination half-lives did not, however, differ significantly. For the three rabbits receiving the acetaminophen doses in the 10-fold ethanol-glycerol-water mixture volumes (preparation III), the total area under the plasma concentration curve obviously was increased.

Improved high-performance liquid chromatographic separation of urinary paracetamol metabolites using radially compressed columns

Methods have been adapted for the high-performance liquid chromatographic (HPLC) analysis of urinary paracetamol metabolites on radial compression columns. Enhanced resolution and decreased analysis time were two major advances. Various modifications to existing methods were made to counter the effect of the different C18 surface. Thus in ion suppression HPLC the addition of triethylamine at pH 3.0 (phosphate buffer) was necessary to block residual hydroxyl sites, while in ion-pair HPLC a higher tetrabutyl-ammonium hydroxide concentration of 0.01 M at pH 5.0 was used to enhance selectivity. The methods were successfully applied to the study of the metabolism of paracetamol, its glutathione conjugate and 3-thiomethylparacetamol in Sprague-Dawley rats. 3-Thiomethyl-paracetamol sulphoxide and its glucuronide and sulphate conjugates were shown to be metabolites of both 3-thiomethylparacetamol and paracetamol. 3-Thiomethylparacetamol sulphate was unresolved from the sulphates of paracetamol and 3-methoxyparacetamol in ion-pair HPLC. This raises a previously unrecognised problem in which the peak normally attributed to paracetamol sulphate contains metabolites arising from an oxidative metabolic pathway. Elevated levels of 3-methoxyparacetamol conjugates were found in human overdose urine and to some extent in analgesic nephropathy.

A simple HPLC assay for urinary paracetamol metabolites and its use to characterize the C3H mouse as a model for paracetamol metabolism studies

A high performance liquid chromatographic (HPLC) assay for unchanged paracetamol and its glucuronide, sulphate, cysteine and mercapturic acid conjugates in the urine of man, mouse and rat is described. The method is simple, rapid and reproducible. The metabolite assay has been used to characterize the male C3H mouse, which shows sensitivity to paracetamol toxicity similar to man, as a model for paracetamol metabolism studies. In male C3H mice there was no evidence to suggest saturability of the glucuronidation pathway on increasing the paracetamol dose from 50 to 300 mg/kg. By contrast, the metabolic ratio and fractional excretion of both the sulphate and glutathione-derived conjugates decreased with increasing paracetamol dose. For animals administered a 200 mg/kg dose of paracetamol, pretreatment with phenobarbitone or 3-methylcholanthrene increased the fractional excretion and metabolic ratio of the glutathione-derived and glucuronic acid conjugates. Piperonyl butoxide pretreatment of animals administered the same dose of paracetamol inhibited glutathione and glucuronic acid conjugation.

3-thiomethylparacetamol sulphate and glucuronide: metabolites of paracetamol and N-hydroxyparacetamol

1. Two new metabolites of paracetamol, 3-thiomethylparacetamol sulphate and glucuronide, have been isolated and identified. 2. The metabolites occurred in both rat and mouse urine after administration of either paracetamol of N-hydroxyparacetamol. The amount excreted increased proportionally with the dose.

Metabolism of paracetamol by the isolated perfused kidney of the homozygous Gunn rat

1. Isolated kidneys from homozygous Gunn rats were perfused with paracetamol in concentrations lower and higher than Km for paracetamol oxidation in the albino rat kidney. 2. Glucuronylation of paracetamol was not detected at either concentration. 3. An increase in oxidative metabolism at the higher concentration, similar to that seen with the Sprague-Dawley rat kidney, did not occur with kidneys from homozygous Gunn rats. 4. This finding does not support the hypothesis that the enhanced nephrotoxicity of paracetamol observed in the homozygous Gunn rat in vivo is due to increased intrarenal formation of reactive metabolites.

Induction of paracetamol metabolism in the isolated perfused kidney

1. At a perfusate concn. of 3.5-4.0 mM, 59 plus or minus 9 nmol of paracetamol h per g wet wt. were oxidized by isolated rat kidney. 2. Approx. half the paracetamol undergoing oxidation was converted to a mercapturic acid metabolite and the remainder was covalently bound to kidney protein. 3. Addition of GSH to the perfusate decreased the level of covalent binding. Depletion of cellular GSH, by prior administration of diethyl maleate, significantly decreased formation of the mercapturic acid metabolite. 4. The metabolic pathways of glucoronylation, sulphation and mercapturic acid formation were induced either by 3-methylcholanthrene pretreatment or by prolonged feeding of aspirin or paracetamol; covalent binding of paracetamol to kidney protein was not increased.