Species difference in stereoselective involvement of CYP3A in the mono-N-dealkylation of disopyramide

The Gut-Brain Axis in Neurodegenerative Diseases and Relevance of the Canine Model: A Review

Identifying appropriate animal models is critical in developing translatable in vitro and in vivo systems for therapeutic drug development and investigating disease pathophysiology. These animal models should have direct biological and translational relevance to the underlying disease they are supposed to mimic. Aging dogs not only naturally develop a cognitive decline in many aspects including learning and memory deficits, but they also exhibit human-like individual variability in the aging process. Neurodegenerative processes that can be observed in both human and canine brains include the progressive accumulation of β-amyloid (Aβ) found as diffuse plaques in the prefrontal cortex (PFC), including the gyrus proreus (i.e., medial orbital PFC), as well as the hippocampus and the cerebral vasculature. Tau pathology, a marker of neurodegeneration and dementia progression, was also found in canine hippocampal synapses. Various epidemiological data show that human patients with neurodegenerative diseases have concurrent intestinal lesions, and histopathological changes in the gastrointestinal (GI) tract occurs decades before neurodegenerative changes. Gut microbiome alterations have also been reported in many neurodegenerative diseases including Alzheimer's (AD) and Parkinson's diseases, as well as inflammatory central nervous system (CNS) diseases. Interestingly, the dog gut microbiome more closely resembles human gut microbiome in composition and functional overlap compared to rodent models. This article reviews the physiology of the gut-brain axis (GBA) and its involvement with neurodegenerative diseases in humans. Additionally, we outline the advantages and weaknesses of current in vitro and in vivo models and discuss future research directions investigating major human neurodegenerative diseases such as AD and Parkinson's diseases using dogs.

Disopyramide and mianserin intoxication: a unique fatal case--review of the literature

Lethal occurrence is exceptional after disopyramide or mianserin poisoning. A case of intentional lethal intoxication with these drugs was reported, as well as a review of the literature. Pre- and postmortem blood concentrations of disopyramide or mianserin were assessed in a woman who died from acute cardiac failure after ingestion. The premortem blood concentration of disopyramide alone was considered lethal, and a toxic premortem concentration of mianserin was observed that may have increased cardiovascular failure induced by disopyramide because the metabolism of both drugs is mediated via cytochrome P450. Moreover, it was shown that the postmortem redistribution of disopyramide was limited, as pre- and postmortem concentrations were 48 and 65 mg/L, respectively. As regards mianserin, redistribution was observed after death with pre- and portmortem concentrations at 0.23 and 0.79 mg/L, respectively. This case illustrates that if postmortem blood concentration of disopyramide is known, the premortem concentration can be deduced.

Examination of CYP3A and P-glycoprotein-mediated drug-drug interactions using animal models

With the advent of polytherapy for cancer treatment it has become prudent to minimize, as much as possible, the potential for drug-drug interactions (DDI). Toward this end, the metabolic and transporter pathways involved in the disposition of a drug candidate (phenotyping) and potential for inhibition and induction of drug-metabolizing enzymes and transporters are evaluated in vitro. Such in vitro human data can be made available prior to human dosing and enable in vitro to in vivo-based predictions of clinical outcomes. Despite some success, however, in vitro systems are not dynamic and sometimes fail to predict drug-drug interactions for a variety of reasons. In comparison, relatively less effort has been made to evaluate predictions based on data derived from in vivo animal models. This chapter will attempt to summarize different examples from the literature where animal models have been used to predict cytochrome P450 3A (CYP3A)- and P-glycoprotein-based DDI. When employing data from animal models one needs to be aware of species differences in enzyme- and transporter-activity leading to differences in pharmacokinetics, clearance pathways as well as species differences in selectivity and affinity of probe substrates and inhibitors. Because of these differences, in vivo animal studies alone, cannot be predictive of human DDI. Despite these caveats, the information obtained from validated in vivo animal models may prove useful when used in conjunction with in vitro-in vivo extrapolation methods. Such an integrated data set can be used to select drug candidates with a reduced DDI potential.

Inhibition and induction of human cytochrome P450 enzymes: current status

Variability of drug metabolism, especially that of the most important phase I enzymes or cytochrome P450 (CYP) enzymes, is an important complicating factor in many areas of pharmacology and toxicology, in drug development, preclinical toxicity studies, clinical trials, drug therapy, environmental exposures and risk assessment. These frequently enormous consequences in mind, predictive and pre-emptying measures have been a top priority in both pharmacology and toxicology. This means the development of predictive in vitro approaches. The sound prediction is always based on the firm background of basic research on the phenomena of inhibition and induction and their underlying mechanisms; consequently the description of these aspects is the purpose of this review. We cover both inhibition and induction of CYP enzymes, always keeping in mind the basic mechanisms on which to build predictive and preventive in vitro approaches. Just because validation is an essential part of any in vitro-in vivo extrapolation scenario, we cover also necessary in vivo research and findings in order to provide a proper view to justify in vitro approaches and observations.

Drug disposition in three dimensions: an update on stereoselectivity in pharmacokinetics

Many marketed drugs are chiral and are administered as the racemate, a 50:50 combination of two enantiomers. Pharmacodynamic and pharmacokinetic differences between enantiomers are well documented. Because of enantioselectivity in pharmacokinetics, results of in vitro pharmacodynamic studies involving enantiomers may differ from those in vivo where pharmacokinetic processes will proceed. With respect to pharmacokinetics, disparate plasma concentration vs time curves of enantiomers may result from the pharmacokinetic processes proceeding at different rates for the two enantiomers. At their foundation, pharmacokinetic processes may be enantioselective at the levels of drug absorption, distribution, metabolism and excretion. In some circumstances, one enantiomer can be chemically or biochemically inverted to its antipode in a unidirectional or bidirectional manner. Genetic consideration such as polymorphic drug metabolism and gender, and patient factors such as age, disease state and concomitant drug intake can all play a role in determining the relative plasma concentrations of the enantiomers of a racemic drug. The use of a nonstereoselective assay method for a racemic compound can lead to difficulties in interpretation of data from, for example, bioequivalence or dose/concentration vs effect assessments. In this review data from a number of representative studies involving pharmacokinetics of chiral drugs are presented and discussed.

Drug-induced changes in P450 enzyme expression at the gene expression level: a new dimension to the analysis of drug-drug interactions

Drug-drug interactions (DDIs) caused by direct chemical inhibition of key drug-metabolizing cytochrome P450 enzymes by a co-administered drug have been well documented and well understood. However, many other well-documented DDIs cannot be so readily explained. Recent investigations into drug and other xenobiotic-mediated expression changes of P450 genes have broadened our understanding of drug metabolism and DDI. In order to gain additional information on DDI, we have integrated existing information on drugs that are substrates, inhibitors, or inducers of important drug-metabolizing P450s with new data on drug-mediated expression changes of the same set of cytochrome P450s from a large-scale microarray gene expression database of drug-treated rat tissues. Existing information on substrates and inhibitors has been updated and reorganized into drug-cytochrome P450 matrices in order to facilitate comparative analysis of new information on inducers and suppressors. When examined at the gene expression level, a total of 119 currently marketed drugs from 265 examined were found to be cytochrome P450 inducers, and 83 were found to be suppressors. The value of this new information is illustrated with a more detailed examination of the DDI between PPARalpha agonists and HMG-CoA reductase inhibitors. This paper proposes that the well-documented, but poorly understood, increase in incidence of rhabdomyolysis when a PPARalpha agonist is co-administered with a HMG-CoA reductase inhibitor is at least in part the result of PPARalpha-induced general suppression of drug metabolism enzymes in liver. The authors believe this type of information will provide insights to other poorly understood DDI questions and stimulate further laboratory and clinical investigations on xenobiotic-mediated induction and suppression of drug metabolism.

THEINTEGRATION OFPHARMACOKINETICS ANDPHARMACODYNAMICS: Understanding Dose-Response

Pharmacokinetic (PK) and pharmacodynamic (PD) studies have proven to be powerful and instructive tools, particularly in elucidating important aspects of human pharmacology. Nevertheless, they remain imperfect tools in that they only allow researchers to indirectly extrapolate, through computational modeling, the dynamic processes of drug action. Furthermore, neither tool alone provides a complete nor necessarily relevant picture of drug action. This review explores the utility and applications of PK and PD in the study of drugs, provides examples of lessons learned from their application to studies of human pharmacology, points out some of their limitations, and advances the thesis that these tools ideally should be employed together in an integrated approach. As we continue to apply these tools across the continuum of age and disease, they provide a powerful means to enhance our understanding of drug action, drug interactions, and intrinsic host factors that influence pharmacologic response.

Issues related to the use of canines in toxicologic pathology--issues with pharmacokinetics and metabolism

The dog is a commonly used animal model by virtue of its size, well-characterized physiology, and ease of handling. For these reasons and others, dogs are also useful in pharmacokinetic and metabolism studies during the development of both human and veterinary pharmaceutical products. In comparison with humans, or with other animals, dogs have some unique physiologic attributes that can affect the disposition of drugs. Species differences in gastrointestinal physiology, metabolism, renal function, and protein binding can affect the correlation of the pharmacokinetics and toxicology of dogs with those of other species. With the use of relevant examples, this article will provide an introduction to characteristics of dog physiology and their impact on pharmacokinetics, metabolism, drug disposition, toxicity, and dose selection.

Involvement of CYP3A in the metabolism of eplerenone in humans and dogs: differential metabolism by CYP3A4 and CYP3A5

In vitro studies were conducted to identify the major metabolites of eplerenone (EP) and the cytochrome p450 (p450) isozymes involved in its primary oxidative metabolism in humans and dogs. The major in vitro metabolites were identified as 6 beta-hydroxy EP and 21-hydroxy EP in both humans and dogs. EP was metabolized by cDNA-expressed human CYP3A4 and dog CYP3A12 but only minimally by human CYP3A5. In human microsomes, inhibition of total metabolism by the CYP3A-selective inhibitors ketoconazole, troleandomycin, and 6',7'-dihydroxybergamottin, each at 10 micro M concentration, was 83 to 95%, whereas inhibition with inhibitors selective for other p450 isozymes was minimal. In dog liver microsomes, the percentages of inhibition were 53 to 76% with the CYP3A-selective inhibitors. A monoclonal anti-CYP3A4 antibody inhibited EP metabolism by 84%, whereas other monoclonal antibodies had minimal effects. The formation of 6 beta-hydroxy and 21-hydroxy metabolites in human liver microsomes was best correlated with CYP3A-selective dextromethorphan N-demethylation and testosterone 6 beta-hydroxylation activities. EP moderately inhibited only CYP3A (testosterone 6 beta-hydroxylase) activity in human liver microsomes by 23, 34 and 45% at concentrations of 30, 100, and 300 micro M, respectively. With human microsomes, the V(max) and K(m) for 6 beta-hydroxylation and 21-hydroxylation were 0.973 nmol/min/mg and 217 micro M, and 0.143 nmol/min/mg and 211 micro M, respectively. The human hepatic clearance calculated from total in vitro EP metabolism was 2.30 ml/min/kg, which agrees with in vivo data. In conclusion, 6 beta- and 21-hydroxylation of EP is primarily catalyzed by CYP3A4 in humans and CYP3A12 in dogs. Also, it is unlikely that EP would substantially inhibit the metabolism of other drugs that are metabolized by CYP3A4 or other p450 isoforms.