Search

Objectives With the growing popularity of herbal and natural medicinal products, attention has turned to possible interactions between these products and pharmaceutical drugs. In this study, we examined whether astragaloside IV (AGS-IV) could inhibit the activity of CYP1A2 in rat liver microsomes in vitro and in vivo. Methods The effect of AGS-IV on CYP1A2 activity was investigated using probe substrates: phenacetin in vitro and theophylline in vivo. Phenacetin was incubated in rat liver microsomes with or without AGS-IV, and the mechanism, kinetics and type of inhibition were determined. The inhibitory effect of AGS-IV on CYP1A2 activity in rats was also determined using theophylline in vivo. The pharmacokinetics of theophylline were observed after a single or week-long treatment with AGS-IV. Key findings AGS-IV was found to be a competitive inhibitor with a Ki value of 6.29 μm in vitro. In the multiple-pretreatment rat group, it was found to have a significantly higher area under the concentration–time curve (AUC) for theophylline, as well as a lower apparent oral total body clearance value (CL/F). In contrast, no significant difference in metabolism of theophylline was found for the single pretreatment group. Conclusions These findings suggest that AGS-IV is a potent inhibitor of CYP1A2. This work offers a useful reference for the reasonable and safe use of clinically prescribed herbal or natural products to avoid unnecessary herb–drug interactions.

Objectives Glycyrrhetinic acid is the main metabolite of glycyrrhizin and the main active component of Licorice root. This study was designed to investigate the in-vitro metabolism of glycyrrhetinic acid by liver microsomes and to examine possible metabolic interactions that glycyrrhetinic acid may have with other cytochrome P450 (CYP) substrates. Methods Glycyrrhetinic acid was incubated with rat liver microsomes (RLM) and human liver microsomes (HLM). Liquid chromatography tandem mass spectrometry was used for glycyrrhetinic acid or substrates identification and quantification. Key findings The Km and Vmax values for HLM are 33.41 µm and 2.23 nmol/mg protein/min, respectively; for RLM the Km and Vmax were 24.24 µm and 6.86 nmol/mg protein/min, respectively. CYP3A4 is likely to be the major enzyme responsible for glycyrrhetinic acid metabolism in HLM while CYP2C9 and CYP2C19 are considerably less active. Other human CYP isoforms have minimal or no activity toward glycyrrhetinic acid. The interactions of glycyrrhetinic acid and six CYP substrates, such as phenacetin, diclofenac, (S)-mephenytoin, dextromethorphan, chlorzoxazone and midazolam were also investigated. The inhibitory action of glycyrrhetinic acid was observed in CYP2C9 for 4-hydroxylation of diclofenac, CYP2C19 for 4′-hydroxylation of (S)-mephenytoin and CYP3A4 for 1′-hydroxylation of midazolam with half maximal inhibitory concentration (IC50) values of 4.3-fold, 3.8-fold and 9.6-fold higher than specific inhibitors in HLM, respectively. However, glycyrrhetinic acid showed relatively little inhibitory effect (IC50 > 400 µm) on phenacetin O-deethylation, dextromethorphan O-demethylation and chlorzoxazone 6-hydroxylation. Conclusions The study indicated that CYP3A4 is likely to be the major enzyme responsible for glycyrrhetinic acid metabolism in HLM while CYP2C9 and CYP2C19 are considerably less active. The results suggest that glycyrrhetinic acid has the potential to interact with a wide range of xenobiotics or endogenous chemicals that are CYP2C9, CYP2C19 and CYP3A4 substrates.

Objectives The effects of nuciferine, a major active aporphine alkaloid from the leaves of Nelumbo nucifera Gaertn, on a cytochrome P450 1A2 (CYP1A2) probe substrate were investigated in vitro and in vivo. Methods Nuciferine and recombinant human CYP1A2 were incubated together to study the impact of nuciferine on CYP1A2 in vitro. Nuciferine was administered orally to Wistar rats at a dose of 20 mg/kg to further estimate the impact of nuciferine on CYP1A2 in vivo. A probe substrate, phenacetin, was used to index the activity of CYP1A2. Key findings The IC50 value for nuciferine was determined to be 2.12 mmol/l. When phenacetin was intravenously coadministered with nuciferine compared with phenacetin alone, the elimination rate constant and total body clearance of phenacetin were decreased by 24.0% (P < 0.01) and 43.0% (P < 0.05), respectively. The mean residence time, apparent elimination half-time and area under the plasma concentration–time curve were increased by 22% (P < 0.005), 26.9% (P < 0.02) and 74.6% (P < 0.05), respectively. Similarly, when phenacetin was coadministered orally with nuciferine, the apparent elimination half-time in the nuciferine pretreated group was increased by 16.7% (P < 0.05) and the elimination rate constant was decreased by 15.4% (P < 0.05). Conclusions The results suggest that nuciferine inhibited CYP1A2 activity in vitro and caused changes in the pharmacokinetic parameters of phenacetin in vivo.

Gastric erosions after oral administration of analgesics separately and in admixture have been examined in adult rats. After administration of acetylsalicylic acid (aspirin), phenacetin, paracetamol and caffeine as single drugs, gastric erosions were only observed with aspirin. The combination of aspirin with phenacetin did not change, that of aspirin with caffeine significantly increased, and aspirin with paracetamol significantly decreased the incidence of gastric lesions compared with aspirin alone. The results for aspirin with paracetamol did not differ from those for the vehicle. Addition of caffeine to the combination of aspirin and phenacetin caused a significant increase in erosions, but when given with aspirin and paracetamol no erosions occurred. The mechanisms underlying the effects of these drugs on aspirin-induced erosions are discussed.

Stearic acid or hard paraffin added to crystals of paracetamol and phenacetin reduced capping of tablets prepared by direct compression but did not produce acceptable tablets because the inter-particular bonds were very weak. The pressure cycle that can be constructed from the measurement of the axial pressure and the corresponding die wall pressures offers information that is useful in the formulation of tablets. The behaviour of paracetamol or phenacetin and their mixtures with gelatin hydrolysate or water or both shows a similarity to a Mohr body and it appears that the maximum die wall pressure is affected by the particle size of the material compressed and also by the additives present. Good transmission of radial force implies that the material can be initially consolidated, but alone it does not indicate that the tablet formed is physically stable. When the tablet formed remains coherent after the axial pressure is removed the residual die wall pressure remains high. Measurement of the residual die wall pressure might therefore be a useful indicator for identifying satisfactory formulations of substances that cap readily. Hydrolysed gelatin or water or both together produced paracetamol and phenacetin mixtures with satisfactory compression characteristics.

The metabolism of debrisoquine (5mg kg−1 orally) was investigated in females of 7 strains of rat. Two major metabolic pathways, those of 4- and 6- hydroxylation were found to be polymorphic. The DA strain eliminated in urine only 7–10% of the dose as 4-hydroxy-debrisoquine together with 31–55% debrisoquine while the corresponding values for the Lewis strain were 44–55% and 11–17% respectively. Accordingly, DA and Lewis rats were proposed as models for the human PM (poor metabolizer) and EM (extensive metabolizer) drug oxidation phenotypes. To further test this model, DA and Lewis rats were given phenacetin (200 mg kg−1 orally). This underwent O-de-ethylation to paracetamol (52–55%) and aromatic 2-hydroxylation (7–8%) in Lewis rats. The corresponding findings in DA rats were 35–40% O-de-ethylation and 12–13% 2-hydroxylation. It is suggested that, with respect to both debrisoquine and phenacetin, Lewis and DA inbred rat strains afford a model of oxidative drug metabolism for the human EM and PM phenotypes respectively.

A thin-layer chromatography procedure is described suitable as a limit test for p-chloroacetanilide in paracetamol and phenacetin. The sample is chromatographed on silica gel together with a standard using the solvent mixture cyclohexane: acetone: diisobutylketone: methanol: water (100:80:30:5:1) and detection is by irradiation with ultraviolet light 253.7 mμ followed by examination in light of wavelength 365 mμ. The procedure may be used to limit the p-chloroacetanilide content of tablets containing paracetamol or phenacetin. Other possible impurities in paracetamol and phenacetin are also detected and may be limited by similar procedures.

The binding of fifteen p-substituted acetanilides to bovine serum albumin is examined at pH 7·2. An excellent correlation is obtained between the binding enthalpy and Hammett’s substituent constant, s̀. This is interpreted to mean that the binding is non-specific in nature. A very good correlation is also obtained between the entropy of binding and s̀, which suggests that the extent of hydration of unbound drug is a function of the charge separation within the drug molecule. Of the compounds examined, those that have been used clinically as analgesics possess the best thermodynamic properties, being neither so fully bound as to give low free drug concentrations in the bloodstream, nor so little bound that there is no sustained action.

Methods are described for the gas-liquid chromatographic estimation of phenacetin and paracetamol in plasma and free and conjugated paracetamol in urine. p-Chloracetanilide and p-bromacetanilide were used as internal standards. The drugs were extracted with ethyl acetate and before chromatography were converted to trimethylsilyl derivatives with N,O-bis(trimethylsilyl)acetamide (simultaneous assay of phenacetin and free paracetamol in plasma), or N-trimethylsilylimidazole (assay of paracetamol alone). Phenacetin was extracted with chloroform and chromatographed directly. Paracetamol glucuronide and sulphate were hydrolysed enzymatically to the parent compound before extraction. Recovery of added phenacetin and paracetamol in plasma at concentrations of 1–100 μg/ml was complete, and the limit of detection of the drugs in plasma was 0.05 μg/ml.

Studies have been made of the potential effects of aspirin, salicylic acid, phenacetin, paracetamol, codeine, caffeine, theophylline and quinine on the spectrophotofluorometric assay of griseofulvin in blood. Aspirin, salicylic acid or quinine are likely to give falsely high values. Detection of this bias by inspecting the fluorescence spectra of the extracted griseofulvin is not always possible.

Pretreatment of rats and guinea-pigs with phenobarbitone or phenylbutazone leads to a decrease in the rate of hydrolysis of acetylsalicylic acid in vitro by liver microsomes, treatment with phenacetin does not.