Glutathione conjugation of chlorobenzylidene malononitriles in vitro and the biotransformation to mercapturic acids in rats

The glutathione conjugation of 2-chloro-, 3-chloro-, 4-chloro- and 2,6-dichlorobenzylidene malononitrile (chloroBMNs) was investigated in vitro. In incubation mixtures containing rat liver cytosol (9000 g), the decrease in the initial amount of glutathione due to the various chloroBMNs ranged from 40 to 60% and occurred both enzymatically and spontaneously at physiological conditions (37°C, pH7.4). 2,6-DichloroBMN, however, depleted glutathione largely spontaneously (38±3%). The steric hindrance of the two chlorosubstituents probably plays an important role during the glutathione-S-transferase catalyzed reaction.The hydrolysis of the chloroBMNs to the corresponding chlorobenzaldehydes and malononitrile was studied in a mixture of buffer pH 7.4 and ethanol. The rate of hydrolysis of 2,6-dichloroBMN was slower than those of the related chloroBMNs. This means that 2,6-dichloroBMN will be the most stable compound in the presence of water.Only IP administration of 2-chloroBMN (CS) to adult male Wistar rats gave enhancement of urinary thioether excretion. A thioether could be isolated and was identified as the N-acetyl-S-[2-chlorobenzyl]-L-cysteine. The quantity of this benzylmercapturic acid in the urine of rats amounted to 4.4% dose (0.07 mmol/kg, n=12).After IP administration of 2-chloro- and 3-chlorobenzaldehyde to rats benzylmercapturic acid excretion in the urine was found to be 7.6 and 1.1% of the dose, respectively. Administration of the related 4-chloro- and 2,6-dichlorobenzaldehyde, however, resulted in _no urinary mercapturic acid excretion.It is very likely that in rats the initial biotransformation of chloroBMNs is mainly hydrolysis to corresponding chlorobenzaldehydes, leading in the case of 3-chloro-, 4-chloro- and 2,6-dichloroBMN to no mercapturic acid excretion in the urine.Nevertheless, 2,6-dichloroBMN will be the most reactive compound with proteins and therefore the best haptene in comparison with the related chloroBMNs.This work was financially supported by a grant from the Dutch Foundation for Medical Research FUNGO, grant no. 13-28-57     

Test chamber exposure of humans to 1,6-hexamethylene diisocyanate and isophorone diisocyanate

An isocyanate generation apparatus was developed and stable isocyanate atmospheres were obtained. At a concentration of 5 g 1,6-hexamethylene diisocyanate (HDI) per m³ the precision was found to be 7% (n = 5). Three volunteers were each exposed to three different concentrations of HDI (11.9, 20.5, and 22.1 g/m³) and three concentrations of isophorone diisocyanate (IPDI) (12.1, 17.7, and 50.7 g/m³), in an exposure chamber. The duration of the exposure was 2 h. Urine and blood samples were collected, and hydrolysed under alkaline conditions to the HDI and IPDI corresponding amines, 1,6-hexamethylene diamine (HDA) and isophorone diamine (IPDA), determined as their pentafluoropropionic anhydride amides (HDA-PFPA and IPDA-PFPA). The HDA-and IPDA-PFPA derivatives were analysed using liquid chromatography mass spectrometry with thermospray monitoring negative ions. When working up samples from the exposed persons without hydrolysis, no HDA or IPDA was seen. The average urinary excretion of the corresponding amine was 39% for HDI and 27% for IPDI. An association between the estimated inhaled dose and the total excreted amount was seen. The average urinary elimination half-time for HDA was 2.5 h and for IPDA, 2.8 h. The hydrolysis condition giving the highest yield of HDA and IPDA in urine was found to be hydrolysis with 3 M sodium hydroxide during 4 h. No HDA or IPDA could be found in hydrolysed plasma (< ca 0.1 g/l).     

Dermal absorption and hydrolysis of methylparaben in different vehicles through intact and damaged skin: Using a pig-ear model in vitro

Currently, there is a trend to reduce of parabens use due to concern about the safety of their unmetabolised forms. This paper focused on dermal absorption rate and effectiveness of first-pass biotransformation of methylparaben (MP) under in-use conditions of skincare products. 24-h exposure of previously frozen intact and tapestripped (20 strips) pig-ear skin to nine vehicles containing 0.1% MP (AD, applied dose of 10 μg/cm²), resulted in 2.0–5.8%AD and 2.9–7.6%AD of unmetabolised MP, and 37.0–73.0%AD and 56.0–95.0%AD of p-hydroxybenzoic acid, respectively, in the receptor fluid. The absorption rate of MP was higher from emulsions than from hydrogels, from enhancer-containing vehicles than from enhancer-free vehicles, and when skin was damaged. Experiments confirmed that the freezing of pig-ear skin slightly reduces hydrolysis of MP.After 4-h exposure of intact freshly excised and intact frozen stored skin, amount of <LOQ-2.3%AD and 2.3–3.3%AD unmetabolised MP, respectively, were found in the receptor fluid. Taking into account the number of useful properties of MP, but also the potential of systemic availability of unmetabolised MP, we consider that MP is more suitable for preserving rinse-off topical products than for leave-on products. Risk of systemic absorption of parabens should also be explored via the skin with damaged barrier.     

A stability-indicating HPLC assay with diode array detection for the determination of a benzylpenicillin prodrug in aqueous solutions

The aim of this study was to develop a stability-indicating HPLC assay for the determination of penethamate (PNT), an ester prodrug of benzylpenicillin (BP), in aqueous solutions. The method was validated by subjecting PNT to forced decomposition under stress conditions of acid, alkali, water hydrolysis and oxidation. A quenching solution was developed to limit degradation to negligible levels before and during the analysis. Both PNT and BP were simultaneously determined and separated in presence of degradation products on a C₁₈ column using a mobile phase consisting of methanol–acetonitrile–acetate buffer. Different degradation products were formed in the stress conditions. The peak purity indexes of PNT and BP obtained by diode array detection were >0.999, confirming the absence of other co-eluting substances. The assay was linear for both analytes in the concentration range 1–100 μg mL⁻¹. The LOD and LOQ of PNT were 0.03 and 0.09 μg mL⁻¹ respectively. Degradation of PNT followed pseudo-first-order kinetics with t_1/2 of 43.6 min at pH 2.01 and 4.2 min at pH 9.31. In addition, the absence of BP in the acidic solutions of PNT emphasises the futility of monitoring BP to assess the stability of PNT. In conclusion, the assay is rapid and stability-indicating with adequate precision and accuracy, and in conjunction with the quenching solution, can be used for stability studies of PNT with simultaneous quantitation of BP. The degradation studies provide useful information for formulation development of PNT.     

Stability of ricobendazole in aqueous solutions

The chemical stability of ricobendazole (RBZ) was investigated using a stability-indicating high performance liquid chromatographic (HPLC) assay with ultraviolet detection. The degradation kinetics of RBZ in aqueous solution was evaluated as a function of pH, buffer strength and temperature. The oxidation reaction in hydrogen peroxide solution was also studied. Degradation products were analyzed by mass spectroscopy and degradation pathways are proposed. Degradation of RBZ followed pseudo first-order kinetics and Arrhenius behavior over the temperature range 24–55 °C. A V-shaped pH-rate profile over the pH range 2–12 was observed with maximum stability at pH 4.8. The shape of the pH-rate profile was rationalized by catalytic effects of various components in the solution on each RBZ species. At pH 11 the activation energy for hydrolysis was 79.5 kJ/mol, and phosphate catalysis was not observed. Oxidation occurred in hydrogen peroxide solutions and was catalyzed by the presence of copper (Cu²⁺) ions. Ricobendazole amine and albendazole sulfone were identified by MS assay to be the degradation products of hydrolysis and oxidation respectively.     

The kinetics and mechanism of the hydrolysis of cyclopentolate hydrochloride in alkaline solutions

Cyclopentolate hydrochloride (Cy · HC1) is an ester of a substituted benzeneacetic acid, having N,N-dimethyl-aminoethanol as the alcohol moiety. A reversed-phase HPLC assay was employed to investigate the kinetics of degradation of Cy · HCI. The influence of pH, buffers, and temperature was studied in alkaline solutions. The degradation follows (pseudo) first-order kinetics at 50°C. Results indicate that it degrades very rapidly at higher pH values. Phenylacetic acid and a-(1-hydroxycyclopentyl)benzeneacetic acid were isolated and identified as the degradation products. The reaction mechanism appears to follow a parallel scheme where phenylacetic acid and a-(1-hydroxycyclopentyl)benzeneacetic acid are formed simultaneously. It is proposed that -(1-hydroxycyclopentyl)benzeneacetic acid is formed by normal ester hydrolysis. Phenylacetic acid is formed via a six-membered transition state and its formation requires the assistance of the hydroxyl group from the adjacent cyclopentanol moiety.     

反胶束体系中猪胰脂肪酶水解橄榄油的动力学研究

目的：系统研究猪胰脂肪酶在二（2-乙基已基）丁二酸酯磺酸钠（AOT）/异辛烷反胶束体系中水解橄榄油的动力学行为。方法：考察温度，wo([H2O]/[AOT])，pH和表面活性剂的浓度对酶反应的影响以及酶的稳定性。结果：酶反应符合常规的一级反应动力不模型，活化有Ea=43.68kJ/mol；最佳pH值为7.7。结论：高浓度表面活性剂对酶表现非竞争性抑制作用；wo是决定酶稳定性的关键。