Current Drug Metabolism - Volume 5, Issue 6, 2004

Most drugs exert their effects not within the plasma compartment, but in defined target tissues into which drugs have to distribute from the central compartment. Unfortunately, a complete and lasting equilibration between blood and tissue cannot always be taken for granted. Drug distribution processes may be characterized by a high intertissue- and intersubject variability and target site drug levels may substantially differ from corresponding plasma levels. Suboptimal target site concentrations may have important clinical implications, as it is a potential explanation for therapeutic failures. Therefore, determination of drug tissue penetration plays an important role in clinical drug development. In recent years, the assessment of tissue concentrations after administration of very low, sub-pharmacological drug amounts (microdosing) has attracted major interest in early clinical drug development, which calls for the availability of highly sensitive analytical methods. The present article will review the most important techniques that are currently available for studying drug disposition in humans. These can be classified as semi-invasive (microdialysis, MD) and non-invasive (positron emission tomography, PET, and magnetic resonance spectroscopy, MRS). We will discuss individual strengths and shortcomings of each method and provide some recent examples with particular focus on antiinfective and anticancer drugs. Whereas MD and MRS also lend themselves to the assessment of tissue-specific drug metabolism, PET usually does not provide metabolic information. For some drugs, such as the anticancer agents 5-fluorouracil and capecitabine, measurement of drug metabolites is particularly important as these represent the therapeutically active species.

Cytochrome P450 3A4 (CYP3A4), an enzyme that is highly expressed in the human liver and small intestine, plays a major role in the metabolism of a large variety of xenobiotics, including an estimated 50% of therapeutic drugs, as well as many endogenous compounds. The expression of CYP3A4 can be induced by xenobiotics. Such induction leads to accelerated metabolism of the xenobiotics themselves (autoinduction) or of concomitantly administered CYP3A4 substrates / drugs, thereby significantly altering their pharmacokinetic and pharmacodynamic profiles. During the past decade, much progress has been made in our understanding of the biological mechanisms responsible for regulation of CYP3A4 expression. It is now known that many xenobiotics induce CYP3A4 expression via the pregnane X receptor (PXR) pathway, while others are thought to act through the constitutive androstane receptor (CAR) and the vitamin D receptor (VDR). As a result, most pharmaceutical companies have recognized that it is important to evaluate CYP3A4 induction potential preclinically and are using primary cultures of human hepatocytes and / or PXR reporter gene assays. In general, the results from these two assay methods correlate well. The reporter gene assays in particular can be used to rapidly screen hundreds of drug candidates, whereas methods using primary human hepatocyte cultures may more accurately assess the potential for CYP3A4 induction in vivo. Although it is important to consider CYP3A4 induction in the early stages of the drug development process, it should be recognized that the assessment of induction potential preclinically is a difficult and imprecise endeavor and can be complicated by many factors.

Metabolism is one of the primary routes of drug elimination from the body. This process comprises of mechanisms, such as oxidation and conjugation, which lead to inactivation and / or elimination from hepatic, biliary, pulmonary, renal and ocular tissues. Enzymes involved in metabolism are expressed in various tissues of the body, liver being the primary site. Studies involving ocular tissues have demonstrated the expression of several metabolic enzymes such as esterases, peptidases, ketone reductases, and CYP-450's in these tissues. These enzymes play an important role in ocular homeostasis by preventing entry and / or eliminating xenobiotics from the ocular tissues. Scientists have targeted these enzymes in drug design and delivery through prodrug derivatization. The prodrugs undergo biotransformation to the parent drug by ocular enzymatic degradation. This review examines the distribution pattern of various metabolic enzymes in the ocular tissues, their physiological role and utility in targeted prodrug delivery.

2-Arylpropionic acid derivatives are probably the most frequently cited drugs exhibiting the phenomenon that is best known as chiral inversion. One enantiomer of drug is converted into its antipode either in the presence of a solvent or more often in inner environment of an organism. Mechanistic studies of the metabolic chiral inversion were carried out for several drugs from NSAIDs, and a model of this inversion was suggested and subsequently confirmed. The chiral inversion of NSAIDs has been intensively studied in the context of the pharmacological and toxicological consequences. However, the group of NSAIDs is not the sole group of drugs in which the inversion phenomenon can be observed. There exist several other drugs that also display chiral inversion of one or even both of their enantiomers. These drugs belong to different pharmacotherapeutic groups as monoamine oxidase inhibitors, antiepileptic drugs, drugs used in the treatment of hyperlipoproteinemia or drugs that are effective in the treatment of leprosy. Moreover, some chiral or prochiral drugs are metabolized to give chiral metabolites that undergo chiral inversion too, which can have direct impact on pharmacological properties or toxicity of the drug. As the process of chiral inversion is affected by several factors, so the intensity of chiral inversion of individual substances and at different conditions can differ considerably. Interspecies differences and types of tissue are reported to be the main factors that were recognized to play the key role in the process of chiral inversion. Some of more recent studies have revealed that several other factors, such as the route of administration or interaction with other xenobiotics, can influence the enantiomeric conversion, too. Chiral inversion does not seem to be a phenomenon connected with only several drugs from some unique group of 2- arylpropionic acid derivatives: it is also observed in drugs with rather different chemical structures and is much more frequent than it can be realized.

A family of three nitric oxide synthase 1(NOS) isoforms produces nitric oxide (NO ) and L-citrulline via a stepwise oxidation of the guanidinium nitrogen of L-arginine. Akin to cytochrome P450 (P450)-catalyzed monoxygenations, the NOS reactions are dependent on both molecular oxygen and NADPH. Although all three NOS isoforms (two constitutively-expressed isoforms; neuronal (nNOS) and endothelial (eNOS), and one inducible isoform; (iNOS)) produce identical products, the function of the NO varies widely in terms of physiological functions due to the varied localization of the isoforms within different cell populations of the body. The NOS isoforms are homodimeric, bi-domain enzymes. Each monomer consisting of a flavin-containing reductase domain linked to a hemecontaining oxygenase domain by a calmodulin (CaM)-binding sequence located between the two domains. The reductase domains of the NOS isoforms not only possess close sequence and structural homology to NADPH-cytochrome P450 oxidoreductase (CPOR), but also, owing to the flavins, resemble CPOR in function in that the reductase permits the passage of electrons from a NADPH-derived hydride, one at a time, to the NOS oxygenase domain. Although possessing very little structural resemblance to P450, the oxygenase domain of NOS is referred to as being “P450-like” due to the presence of iron protoporphyrin IX (heme), linked axially by a cysteine to the NOS protein, which carries out “P450-like” mono-oxygenation reactions. Considering the importance of NO in regulating many physiological functions, the NOS isoforms are important drug targets. Modulators (both inhibitors as well as possible activators) of enzymatic function are needed to either decrease or increase NO production, respectively, in various disease states. In addition to the widely known beneficial processes where NO plays a role (i.e. neurotransmission, blood pressure regulation, immunomodulation), under certain conditions production of NO (as well as other NOS-derived oxidants) can be detrimental by altering enzyme activity; for example P450-mediated drug metabolism. Furthermore, because of the close structural and functional similarities between NOS reductase and CPOR, many of the same redox-cycling and reductive reactions that occur with CPOR can also occur with NOS. The final focus of this review article is the less-well-recognized reactions mediated by NOS, which have recently begun to receive the attention they deserve. We hope to highlight reasons for concern regarding both altered drug metabolism as well as extra-hepatic, and target-organ toxicities in response to both altered NO production as well as the detrimental interaction of NOS with certain xenobiotics.

Antiarrhythmics are a group of drugs that manage the irregular electrical activity of the heart. Their use in the clinic is made difficult by their narrow therapeutic index. The disposition of antiarrhythmics is dependent on many factors, such as administration route, stereoselectivity in the first-pass effect, inhibition of enzymes, polymorphisms, etc. Consequently, the pharmacological activity of drugs may be interindividually variable. Experiments using organ homogenates or hepatic microsome fractions were used for simulating the biotransformation of the drug in vivo. The classical approaches, such as correlation analysis, specifically the inhibitory effect, or induction of chemicals, and immunoinhibition, may be combined with the use of recombinant enzymes for identifying the enzymes involved in the drug metabolism. The fate of the antiarrhythmics may also be investigated in live animals. A speciesdependent metabolism was often observed. The pre-treatment with chemicals, which influences the change (inhibition or induction) in the drug disposition, may provide insights into the enzymes involved in vivo. However, published data indicated that the data obtained from animals should not be extrapolated directly to humans. Nevertheless, animal models are useful for investigating the mechanism of clinical observations. The clinical use of the antiarrhythmics becomes complex, when the drug metabolism is genetically / phenotypically dependent and active metabolites are formed. Furthermore, the stereoselectivity may also modify the disposition and the pharmacodynamic profile of a therapeutic agent. Only the knowledge of the drug metabolism and the status of each individual may allow the use of antiarrhythmics safely.