Biotransformation of the antiemetic 5-HT3 antagonist tropisetron in liver and kidney slices of human, rat and dog with a comparison to in vivo

The metabolism of the 5HT3 antagonists, ondansetron, alosetron and GR87442 II: investigation into the in vitro methods used to predict the in vivo hepatic clearance of ondansetron, alosetron and GR87442 in the rat, dog and human

The in vitro clearances of the 5HT3 antagonists, ondansetron, alosetron and GR87442 were investigated. Intrinsic clearances using either metabolite formation or substrate depletion methods were equivalent (R2 = 0.95). Hepatocytes from preclinical species were superior to microsomes for the prediction of hepatic clearance (CL(H)), whereas the predictions from human microsomes and hepatocytes were similar. Using a non-restrictive model, seven of the nine CL(H) predictions using hepatocytes were within 2-fold of the in vivo CL(H) values. If the unbound fraction was included, the clearance of the compounds was generally under-predicted by both in vitro models. However, for the most metabolically stable compound, GR87442, the non-restrictive model over-predicted CLp. This and the possibility of extrahepatic metabolism indicate that the restrictive model is more appropriate for prediction of CL(H). The rank order of metabolic stability correlated with that in vivo. All three compounds were more metabolically stable in human than in the preclinical animal species examined.

Pharmacology, pharmacogenetics, and clinical efficacy of 5-hydroxytryptamine type 3 receptor antagonists for postoperative nausea and vomiting

PURPOSE OF REVIEW

The use of selective 5-hydroxytryptamine type 3 receptor antagonists has improved the management of postoperative nausea and vomiting, but has not completely eliminated it. In this article, we discuss the pharmacology of 5-hydroxytryptamine type 3 receptor antagonists and the impact of pharmacogenetics on postoperative nausea and vomiting.

RECENT FINDINGS

Dolasetron, granisetron, ondansetron, palonosetron, and tropisetron have similar mechanisms of action but different pharmacokinetic and pharmacodynamic properties. Genetic polymorphism in the cytochrome P450 mono-oxygenase system, drug efflux transporter adenosine triphosphate-binding cassette subfamily B member 1 and 5-hydroxytryptamine type 3 receptor subunits also contribute to the interindividual variation in response to different 5-hydroxytryptamine type 3 receptor antagonists. These differences account for differences in the duration of action and clinical efficacy of these agents.

SUMMARY

Pharmacogenetics testing in patients may help differentiate responders to 5-hydroxytryptamine type 3 receptor antagonists from non-responders and allow the anesthesiologist to individualize antiemetic therapy. The cost-effectiveness of such screening in postoperative nausea and vomiting management has, however, not been evaluated. Given the multifactorial nature of postoperative nausea and vomiting, a multimodal approach to reduce or eliminate risk factors will be most successful in its management.

Precision-cut organ slices to investigate target organ injury

Drug-induced organ injury is a multifaceted process, involving numerous cell types and mediators, and remains a significant safety issue in pharmaceutical development and clinical therapy. Organ slices, an in vitro model representing the multicellular, structural and functional features of in vivo tissue, is a promising model for elucidating mechanisms of drug-induced organ injury and for characterising species susceptibilities. Time- and concentration-dependent drug-induced effects on organ slice gene expression, function and morphology are providing insight into the molecular and biochemical pathways leading to organ dysfunction, an altered morphology and the induction of repair pathways. Human organ slice studies are valuable for bridging the extrapolation of animal-derived data and for identifying mechanisms relevant for humans. The liver is the major organ used in organ slice studies; however, the utility of extrahepatic-derived slices, as well as cocultures for investigating multiple organ involvement in tissue injury is increasing. Organ slice investigations can further our understanding of the cell types and cell interactions involved in drug-induced injury and the consequences of drug-induced off-target effects for identifying compound liabilities that will impact safety.

Selective serotonin 5-HT3 receptor antagonists for postoperative nausea and vomiting: are they all the same?

Selective serotonin 5-HT(3) receptor antagonists have proven safe and effective for the management of postoperative nausea and vomiting. Dolasetron, granisetron, ondansetron and tropisetron selectively and competitively bind to 5-HT(3) receptors, blocking serotonin binding at vagal afferents in the gut and in the regions of the CNS involved in emesis, including the chemoreceptor trigger zone and the nucleus tractus solitarii. Despite their shared mechanism of action, 5-HT(3) receptor antagonists have different chemical structures and exhibit differences in receptor binding affinity, dose response and duration of effect. Furthermore, although dolasetron, granisetron, ondansetron and tropisetron are all extensively metabolised by the cytochrome P450 (CYP) system, different components of this system predominate in the metabolism of each of these agents. Hence, although these agents are considered equally effective in the overall population, their pharmacokinetic and pharmacodynamic differences may explain the variability in individual responses to these drugs. This review discusses the pharmacological profiles of dolasetron, granisetron, ondansetron and tropisetron, and the clinical implications of differences in their profiles.

Effects of renal diseases on the regulation and expression of renal and hepatic drug-metabolizing enzymes: a review

1. The activity of drug-metabolizing enzymes (DMEs) in extrahepatic organs is highest in the kidneys. Generally, the kidneys contain most, if not all, of the DMEs found in the liver. Surprisingly, some of these DMEs show higher activity in the kidneys than in the liver. 2. Most of the renal DMEs are localized in the cortex of the kidneys, especially in the proximal tubules. DMEs are also found in the distal tubules and collecting ducts. 3. Renal diseases such as acute and chronic renal failure and renal cell carcinoma alter the regulation of both hepatic and extrahepatic phase I and II DMEs. Changes in the expression of these DMEs seem to be tissue and species specific. 4. Generally, there is significant down-regulation of most of the phase I and a few of phase II DMEs at the protein, mRNA and activity levels. Unfortunately, the mechanisms leading to the alteration in DMEs in renal diseases remain unclear, although many theories have been made. 5. The presence of some circulating factors such as cytokines, nitric oxide, parathyroid hormones and increased intracellular calcium play a role in the regulation of DMEs in renal diseases.

Strategies and molecular probes to investigate the role of cytochrome P450 in drug metabolism: focus on in vitro studies

Drug metabolism is the major determinant of drug clearance and, because of polymorphic or inducible expression of drug-metabolising cytochrome P450s (CYPs), is the factor most frequently responsible for interindividual differences in pharmacokinetics. A number of well characterised CYP substrates and inhibitors have been identified that allow precise measurements of individual CYP isoforms. Their use, alone or in combination, facilitates the phenotype characterisation of hepatocytes in vitro and in vivo. Two procedures are used for in vitro investigation of the metabolic profile of a drug: incubation with microsomes and incubation with metabolically competent cells. The major limitation of microsomes is that they express phase I activities, but only part of phase II activities, and can only be used for short incubation times. When intact cells are used, gene expression, metabolic pathways, cofactors/enzymes and plasma membrane are largely preserved, but fully differentiated cells such as primary cultured hepatocytes need to be used, since hepatoma cell lines have only very low and partial CYP expression. CYP-engineered cells or their microsomes ('supersomes') have made the identification of the CYPs involved in the metabolism of a drug candidate straightforward and easier. Inhibition of CYP is an undesirable feature for a drug candidate, and needs to be addressed by examining whether the drug candidate inhibits the metabolism of other compounds or whether other compounds inhibit the metabolism of the drug candidate. Such experiments can be conducted both with microsomes and in cells. The major limitation of microsomes is that inhibition parameters may not accurately reflect the situation in vivo, since the contribution of drug transport is not considered. The best picture of a potential drug-drug interaction can be obtained in metabolically competent hepatocytes. Screening of CYP inducers cannot be done in microsomes. It requires the use of a cellular system fully capable of transcribing and translating CYP genes, and can be monitored in vitro as an increase in enzyme mRNA or activity. Human hepatocytes in primary culture respond well to enzyme inducers during the first few days; this ability is lost thereafter. Rat hepatocytes are much less stable and soon become unresponsive to inducers. Hepatoma cell lines respond poorly to inducers, although the induction of a few isoenzymes has been reported. Primary cultured hepatocytes are still the unique in vitro model that allows global examination of the inductive potential of a drug.

Viability of liver slices exhibiting absorption, metabolism, and elimination of substrates in culture medium

In an attempt to improve liver slice culture for in vitro tests of newly developed drugs, we found that a relatively thick 2-mm slice was much more viable, long-lived, and metabolically functional than the conventionally used 300-microm-thick slices. Results revealed that some ionic and nonionic compounds (Na(125)I, [(14)C]Ala, and [(14)C]Xyl) dissolved in culture medium were absorbed into the slices, reaching from the cut edges deep into the inner part, in only a short period. Moreover, the slices could be observed for 24 h after incubation so that a normal histological image could be obtained. Each semi-microautoradiographic image was also clearly positive not only at the margin but also in the inner part of each slice. Furthermore, we determined the specific functioning of each donated liver for each substrate added, in uptake, metabolism, and excretion.

Use and value of metabolism databases

A key objective during drug discovery is to ensure selection of lead compounds for development that have the optimum pharmacodynamic profile without undesirable toxicity. Metabolism plays a key role in determining the biological activity of compounds in vivo, and is therefore an important factor to consider during the discovery phase. The ability to predict the biotransformation pathways for specific chemical structures provides the opportunity to implement structural modifications that might advantageously modify these processes. Potentially valuable tools include knowledge databases that contain searchable information on the known metabolites of existing compounds and databases that are designed to predict the metabolites.

Cytochrome P450-dependent metabolic pathways and glucuronidation in trout liver slices

We investigated the capacity of trout precision-cut liver slices to metabolize xenobiotics and steroids. As a first approach, liver slices were compared with freshly isolated trout hepatocytes, using 7-ethoxycoumarin (7-EC) and testosterone as substrates. Trout liver slices and freshly isolated hepatocytes had a similar capacity for conducting cytochrome P450-dependent metabolism, as indicated by the rate of oxidative metabolism of 7-EC and testosterone, and by the metabolic profile of these substrates. A lower rate of glucuronidation in slices compared with hepatocytes was observed with testosterone (50 microM), whereas the opposite situation occurred with 7-EC used at higher concentration (100 microM). In a second step, we investigated the effect of beta-naphthoflavone on 7-EC and testosterone biotransformation, using slices maintained in culture for 24 h, with or without the inducer added. The results were compared with the metabolic rates of these substrates incubated with liver slices originating from trout pretreated in vivo with beta-naphthoflavone. Cytochrome P450-mediated rates of 7-EC dealkylation and testosterone hydroxylation decreased to 38 and 55% of the control value, respectively, when incubations were performed in 24-h cultured slices instead of freshly cut slices. Exposure of the slices to 50 microM beta-naphthoflavone resulted in about 3 times higher deethylation rate of 7-EC. A similar value was obtained when treatment occurred in vivo. As demonstrated in rat by several authors, liver slices seem a useful and simple tool for studying the metabolic pathways of xenobiotics and steroids and for the assessment of inducers of the CYP1A1 family.