Metabolism, drugs

IRrstnage metabolism AUorallyariminlstenddru passUvou the liver to the stemlc circulation. 5ome are so completely metabolized they am inactive orally - (e.g. Ildocafne, glyceryl trinitrate  

1 The drug is made more hydruphilic—diis hastens iisexcrciion by llie kidneys (right, i because the less lipid-soluble meitdjoliic is not readily reabsorbed in the renal tubules. 

The liver is the main organ ofd g melabolism and is involved in two general lypes of reacUon. 

These involve ihc biotransfoniialion of a drug to a more polar mclabo-lilc (Icfl of tigurc) by introdudng or unmasking a functional group (e.g. -OH. 

Oxidations are the most common reactions and ihe.se arc catalysed by an importam class of enzymes called the mixed function oxidases (cytuchronic lM5l)s). The substrate specilicity of this enzyme complex is very low ind m.Tny dilTcrem drugs can be oxidized (examples, top let l). Other phase I reactions are reductions (middle left) and hydrolysis t bottom left). 

On entering the body, drugs are treated as if they are toxic substances, which need to be detoxified, if a mechanism exists, and eliminated as soon as possible. 

This means that most drugs are subjected to some kind of metabolism and then excreted. 

Metabolism involves changes to the molecular structure of a substance and these changes are produced by the action of enzymes. This has two important effects on drug molecules  

The drug is made more water soluble and therefore more easily excreted by the 

The metabolites are usually less pharmacologically active than the parent dmg. 

The processes involved in drug metabolism involve simple chemical reactions such as oxidation (the most common), reduction and dealkylation and are influenced by a number of factors including  

Environmental factors. Examples of these are co-administration of other drugs, which can affect the rate and extent of drug metabolism. This can become literally a matter of life and death as a number of potentially fatal drug interactions involve liver enzyme induction and competition for drug-metabolism enzymes. 

Cytochrome P450s accomplish all these metabolic transformations due to the presence of an ion of iron at the active site that can accept or donate electrons to allow oxidation reactions to take place. The iron in 

CYP450 is bound within a haem co-factor and can exist in a number of oxidation states, of which Fe2+ (ferrous) and Fe3+ (ferric) are the most important. 

A detailed description of the (fascinating) molecular mode(s) of action of this important enzyme system is beyond the scope of this book and the interested reader should consult textbooks of biochemistry, pharmacology, or medicinal chemistry for more information. What is important is that a student should understand the functional group interconversions brought about by CYP450 and appreciate the metabolic effects of these changes on the physicochemical properties of drugs. 

7 Mass Spectrometry in Bioanalysis - Methods, Principles and Instrumentation 

When working with non-radiolabeled drugs the major challenge is to find metabolites in the biological matrices. Because the enzymes responsible for metabolism are quite well characterized metabolic changes can partially be predicted. For example hydroxylation of the parent drug is in many cases the principal metabolic pathway. From a mass spectrometric point of view it results in an increase of 16 units in the mass spectrum. In the full-scan mode an extracted ion current profile can be used to screen for potential metabolites. In a second step a product ion spectrum is recorded for structural interpretation. Ideally, one would like to obtain relative molecular mass information and the corresponding product ion spectrum in the same LC-MS run. This information can be obtained by data dependant acquisition (DDA), as illustrated in Fig. 1.39. 

In this case the survey scan was set as a full scan and the dependent scan as a product ion scan. The problem with data dependent acquisition is to determine the selection criteria. In most cases the system picks up the most abundant ion in the full scan spectrum. An inclusion list with masses of potential metabolites or exclusion list of known interferences significantly improves the procedure. In the example shown in Fig. 1.39, a procedure called dynamic background subtraction (DBS) was applied. This procedure considers chromatographic peak shapes and monitors not the most abundant signal in the spectrum but the largest increase of an ion in a spectrum. The advantage is that once a signal of a peak has 

Most methods of metabolite identification are done with online LC-MS. As mentioned earlier there is no ideal mass spectrometer for this type of work and the sample has to be reanalyzed several times on different types of mass spectrometer. The consequence is that metabolic investigation is often time-consuming. A concept has been described by Staack et al. [82] (Fig. 1.40) where, during the LC-MS run, fractions are collected onto a 96-well plate. 

Either the information obtained during the data-dependent acquisition is sufficient or a fraction of interest can be re-analyzed by chip-based infusion at a flow rate ca. 200 nl min. Due to the miniaturization sample consumption is very low (typically 1-3 pi) and acquisition time is no longer critical. Therefore various MS experiments can be performed on various instruments, including MS and accurate mass measurements. An additional advantage is that the eluent can be removed and the infusion solvent can be optimized for positive or negative ion detection or for deuterium exchange measurements. 

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Accurate, precise isotope ratio measurements are important in a wide variety of applications, including dating, examination of environmental samples, and studies on drug metabolism. The degree of accuracy and precision required necessitates the use of special isotope mass spectrometers, which mostly use thermal ionization or inductively coupled plasma ionization, often together with multiple ion collectors. 

Accurate, precise isotope ratio measurements are used in a variety of applications including dating of artifacts or rocks, studies on drug metabolism, and investigations of environmental issues. Special mass spectrometers are needed for such accuracy and precision. 

LaDu, B. N., Mandel, A. G., and Way, E. L. Fundamentals of Drug Metabolism and Drug Disposition. Baltimore Williams Wilkins, 1972. 

The renal clearance can be under estimated in the case of renal drug metabolism. The total drug clearance depends on bioavailability. Therefore, the most reliable estimate for the fraction eliminated by the renal route (fren) is given by the normal clearance (Clnorm) and drug clearance in case of acute and/ or chronic renal failure (Clfail), or from half-lives (Tl/2norm) and (Tl/2fail). 

Pregnane X Receptor, Nuclear Receptor Regulation of Drug-metabolizing P450-Enzymes. 

It is important to monitor closely serum blood levels of chloramphenicol, particularly in patients with impaired liver or kidney function or when administering chloramphenicol with other drugs metabolized by the liver. Blood concentration levels exceeding 25 mcg/mL increase the risk of the patient developing bone marrow depression. 

One approach to drug metabolism studies is therefore to predict the molecular weights of possible metabolites of the drug under consideration, to use reconstructed ion chromatograms to locate any components that have the appropriate molecular weights and then use MS-MS to effect fragmentation of the (M - - H)+ ions from these metabolites, and then to finally link the m jz values of the ions observed with ions of known structure from the parent drug or from other metabolites whose structures have been elucidated. 

A slightly different use of accurate mass measurement in drug metabolism studies has been reported [31]. In this investigation, the accurate masses of the ions derived from Glyburide (Figure 5.48) and some of its metabolites were used to calculate the difference in mass, and thus the elemental composition, between certain of the ions observed in the spectra of the various compounds. 

Drug metabolism The process by which drugs are transformed in the body to a form that is more readily eliminated. 

Bulusu S, Chakravarty I. 1988. Profile on drug metabolizing enz5mies in rats treated with parathion, malathion, and phosalone under various conditions of protein energy malnutrition. Bull Environ Contam Toxicol 40 110-118. 

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