Bernhard Sperker

The effects of the human MDR1 genotype on the expression of duodenal P-glycoprotein and disposition of the probe drug talinolol

A single‐nucleotide polymorphism (SNP) of the human multidrug‐resistance gene in wobble position of exon 26 reportedly predicts expression and function of P‐glycoprotein in human enterocytes and lymphocytes. Several other allelic variants of MDR1 have been identified, some of which lead to amino acid exchange with as yet unknown functional relevance.

The role of β-glucuronidase in drug disposition and drug targeting in humans

SummaryGlucuronides of drugs often accumulate during long term therapy. The hydrolysis of glucuronides can be catalysed by β-glucuronidase, an enzyme expressed in many tissues and body fluids in humans. The possible contribution of β-glucuronidase to drug disposition in humans has not been assessed in a systematic manner, but this enzyme may be able to release, locally or systemically, the active or inactive parent compound from drug glucuronides, thereby modifying the disposition and action of these drugs.Based on the information available on the localisation, expression and variability of β-glucuronidase, the concept of β-glucuronidase-mediated drug metabolism is outlined in this article using examples from the literature. Since some issues surrounding the β-glucuronidase-mediated deconjugation of drug glucuronides still need to be clarified in humans, additional data from animal models supporting this concept have been included. Moreover, as β-glucuronidase has already been proven to be useful in tumour specific bioactivation of glucuronide prodrugs of anticancer agents, we also focus on anticancer prodrug approaches utilising β-glucuronidase. This review summarises the role of β-glucuronidase in drug disposition and drug targeting in humans.

Expression and Localization of P-glycoprotein in Human Heart: Effects of Cardiomyopathy

ABC-type transport proteins, such as P-glycoprotein (P-gp), modify intracellular concentrations of many substrate compounds. They serve as functional barriers against entry of xenobiotics (e.g., in the gut or the blood-brain barrier) or contribute to drug excretion. Expression of transport proteins in the heart could be an important factor modifying cardiac concentrations of drugs known to be transported by P-gp (e.g., β-blockers, cardiac glycosides, doxorubicin). We therefore investigated the expression and localization of P-gp in human heart. Samples from 15 human hearts (left ventricle; five non-failing, five dilated cardiomyopathy, and five ischemic cardiomyopathy) were analyzed for expression of P-gp using real-time RT-PCR, immunohistochemistry, and in situ hybridization. Immunohistochemistry revealed expression of P-gp in endothelium of both arterioles and capillaries of all heart samples. Although P-gp mRNA was detected in all samples, its expression level was significantly reduced in patients with dilated cardiomyopathy. We describe variable expression of P-gp in human heart and its localization in the endothelial wall. Thus, intracardiac concentrations of various compounds may be modified, depending on the individual P-gp level.

Overexpression of glutathione S‐transferase A1‐1 in ECV 304 cells protects against busulfan mediated G2‐arrest and induces tissue factor expression

The antineoplastic drug busulfan is frequently used in preconditioning regimens for bone marrow transplantation. Pharmacokinetics vary tremendously between patients due to extensive metabolism in the liver via conjugation to glutathione catalysed by glutathione S‐transferase (GST) A1‐1. Since elevated busulfan plasma levels have been reported to be a risk factor for developing veno‐occlusive disease (VOD), metabolism of busulfan may play a pivotal role in the induction of VOD. Therefore, we developed a cell model to investigate the influence of busulfan metabolism on its biological effects. GSTA1‐1 cDNA was transfected into the cell line ECV 304 and protein expression was demonstrated by Western blotting. Enzymatic activity could be detected by formation of tetrahydrothiophene. Additionally, effects of busulfan treatment on cell cycle and expression of tissue factor have been investigated. A busulfan‐induced G2‐arrest was reduced in GSTA1‐1‐transfected cells, which consequently displayed a significantly higher activity of cdc2 kinase (24.1±1.5 AU mg−1 protein) after busulfan treatment compared to controls (14.7±2.3 AU mg−1 protein; P<0.01). Elevated basal expression of tissue factor in GSTA1‐1‐transfected ECV 304 cells could be 4 fold increased by busulfan treatment. These data demonstrate that ECV 304 cells transfected with GSTA1‐1 provide a valuable tool to assess busulfan metabolism in vitro. Furthermore, overexpression of GSTA1‐1 leads to a partial protection against cell cycle effects of busulfan and affects tissue factor expression.

Determination of tetrahydrothiophene formation as a probe of in vitro busulfan metabolism by human glutathione S-transferase A1-1 : use of a highly sensitive gas chromatographic-mass spectrometric method

A method for the sensitive determination of tetrahydrothiophene (THT) in cytosolic incubation mixtures was developed. Busulfan conjugation with glutathione was predominantly catalysed by glutathione S-transferase A1-1 (GST A1-1) and THT was released from the primary metabolite by alkalization. After liquid-liquid extraction using n-pentane separation and quantification of the product was performed by gas chromatography with a mass-selective detector. The method showed good sensitivity, accuracy and reproducibility with a detection limit of 2 ng ml(-1) and a limit of quantification of 5 ng ml(-1). The suitability of the method is shown for enzyme kinetic studies in human liver cytosol as well as for determination of GST A1-1 activity.

In vitro cleavage of paracetamol glucuronide by human liver and kidney β-glucuronidase: determination of paracetamol by capillary electrophoresis

A capillary electrophoresis (CE) method was developed using paracetamol glucuronide as a novel probe for human beta-glucuronidase activity. Using UV detection without prior sample clean-up procedures, fast and reliable quantitation of the released paracetamol was possible. The method showed good precision, accuracy and sensitivity with a limit of detection of 0.25 microM (38 ng/ml) and a limit of quantitation of 1 microM (151 ng/ml). The suitability of the method has been shown for enzyme kinetic studies using different liver and kidney homogenates, respectively. Our data clearly demonstrate that paracetamol glucuronide is cleaved by human beta-glucuronidase thereby releasing paracetamol. The CE method presented is not only a valuable tool for measuring human beta-glucuronidase activity, but also allows investigation of the contribution of deglucuronidation of paracetamol glucuronide to the disposition of paracetamol.

Simultaneous high-performance liquid chromatographic determination of a glucuronyl prodrug of doxorubicin, doxorubicin and its metabolites in human lung tissue.

A rapid and sensitive method was developed for the simultaneous determination of the new doxorubicin glucuronide prodrug HMR 1826, the parent drug doxorubicin and its metabolites in human lung tissue samples. Homogenization of frozen tissue samples with the micro-dismembrator was followed by a silver nitrate precipitation step. By removing the exceeding silver ions with sodium chloride further purification steps could be omitted. Compounds were separated by isocratic high-performance liquid chromatography on a LiChrospher 100 RP18 column and a mobile phase consisting of citric acid buffer-acetonitrile-methanol-tetrahydrofuran within 30 min and quantified with fluorescence detection. The method showed good recoveries for all compounds (86-99%) and a linear calibration range of 20 ng/g-80 microg/g for doxorubicin and 1-600 microg/g for HMR 1826.

Expression of multidrug resistance proteins in rat and human chronic pancreatitis.

Background and aims The expression of the ABC-transporters MDR-1, MRP1, and MRP-2 was investigated in healthy pancreas and in chronic pancreatitis tissue samples in rats and humans to evaluate their possible involvement in a multidrug resistance of the pancreas with consequences for the pharmacologic treatment of pancreatic diseases. Methods Human pancreatic tissue samples of healthy tissue and chronic pancreatitis were collected during pancreas surgery. In rats, the time-course of the expression of transporter proteins was studied 14, 28, and 56 days after experimental induction of chronic pancreatitis. The expression of MDR-1, MRP-1, MRP-2, and furthermore, LRP and PAP was investigated by RT-PCR, Real Time TaqManPCR, and immunohistochemistry. Results In rat pancreas, MDR-1 (P-gp) and MRP-1 but in human pancreas MDR-1 (P-gp), MRP-1 and MRP-2 were found to be expressed. Chronic pancreatitis lead to an increased transcription of mRNA of MDR-1 (rat and human) and much lower, MRP-2 (human). Conclusions The expression of P-gp and related transporters could have impact on the metabolism, distribution, and availability of various compounds, including drugs, in the pancreas. The results indicate that this could be more pronounced in chronic pancreatitis.

High-performance liquid chromatographic quantification of 4-methylumbelliferyl-β-d-glucuronide as a probe for human β-glucuronidase activity in tissue homogenates

An internally standardized HPLC method to determine the concentration of 4-methylumbelliferone liberated from 4-methylumbelliferyl-beta-D-glucuronide by human beta-glucuronidase was developed. The assay allows the precise and rapid measurement of specific enzyme activity in human tissue homogenates. Without prior extraction the incubation mixture can be separated using a C8 column followed by fluorescence detection. The assay showed good accuracy and precision with a detection limit of 20 nM and a limit of quantification of 167 nM. The suitability of the method was shown in enzyme kinetic experiments with human liver homogenates.

Involvement of AP-2 binding sites in regulation of human beta-glucuronidase

The lysosomal hydrolase β-glucuronidase (β-gluc) can be used for the bioactivation of non-toxic glucuronide prodrugs of anticancer agents. The enzyme is present at high levels in many tumours and hence may lead to an enhanced drug targeting by tumour-selective release of the active anticancer drug. Individual expression and regulation of this enzyme is one factor modulating the bioactivation of glucuronide prodrugs. Nevertheless, in contrast to murine β-gluc, which is inducible by androgens, the human enzyme has been regarded as an unregulated housekeeping gene due to a lacking TATA box and high G+C contents within the putative promotor sequence. Despite these facts, we were able to demonstrate downregulation of human β-gluc expression by the calcium ionophore A23187 and the calcium ATPase inhibitor thapsigargin in the human hepatoma cell line HepG2. However, cis-acting elements responsible for this regulation have not yet been identified. We therefore characterised the 5′-untranslated region of the human β-gluc gene using transient transfection assays with promotor-luciferase constructs in HepG2 cells and cloned fragments between 3,770 bp and 107 bp. A23187 reduced the β-gluc promotor activity. This effect disappeared using fragments smaller than 356 bp. Using site-directed in vitro mutagenesis and gel-electrophoretic-mobility shift assays, we found evidence of an involvement of transcription factor activating protein-2 (AP-2) binding sites on the regulation of human β-glucuronidase by A23187. Our studies provide a basis for the understanding of the transcriptional regulation of the human β-glucuronidase gene and could be useful for the optimisation of glucuronide prodrug therapy.