Metabolites INDEX

The comparatively straightforward link between 5-HT and its primary metabolite, 5-HIAA, encouraged many researchers to use changes in the ratio of tissue concentrations of 5-HIAA and 5-HT as an index of the rate of release of 5-HT ex vivo. However, it has been clear for some time that the majority of 5-HT is metabolised in the cytoplasm by MAO before it is released from 5-HT nerve terminals. Consequently, the reliability of the 5-HIAA 5-HT ratio as an index of transmitter release is rather dubious, although it could be used as an acceptable measure of MAO activity. In any case, the development of in vivo microdialysis means that changes in the concentration of extracellular 5-HT can now be monitored directly which, under drug-free conditions, provides a far more reliable indication of any changes in the rate of release of 5-HT. 

Griffiths, H.R, Lunec, J. and Blake, D.R. (1992). Oxygen radical-induced fluorescence in proteins identification of the fluorescent tryptophan metabolite N formyl kynurenine as a biological index of radical damage. Amino Acids 3, 183-194. 

US Food Drug Adminstration (2008) Guidance for Industry, Safety Testing of Drug Metabolites, http //www.fda. gov/cder/guidance/index.htm (last access October 2008). 

Zinc protoporphyrin IX is a normal metabolite that is formed in trace amounts during haem biosynthesis. However, in iron deficiency or in impaired iron utilization, zinc becomes an alternative substrate for ferrochelatase and elevated levels of zinc protoporphyrin IX, which has a known low affinity for oxygen, are formed. This zinc-for-iron substitution is one of the first biochemical responses to iron depletion, and erythrocyte zinc protoporphyrin is therefore a very sensitive index of bone-marrow iron status (Labbe et ah, 1999). In addition, zinc protoporphyrin may regulate haem catabolism by acting as a competitive inhibitor of haem oxygenase, the key enzyme of the haem degradation pathway. However, it has been reported... 

Serotonergic function has been investigated by using multiple methods. Assaying the major metabolite of serotonin, 5-hydroxyindoleacetic acid (5-HIAA) in cerebrospinal fluid (CSF) has been widely used (Ch. 13). This method assumes that CSF 5-HIAA is related to brain serotonin activity. This premise is supported by the rostral-caudal concentration gradient of CSF 5-HIAA and the observation in postmortem studies that CSF 5-HIAA correlates with levels of 5-HIAA in prefrontal cortex [16], both of which suggest that CSF 5-HIAA is a reasonable index of prefrontal serotonin turnover. ... 

C. Wagner, M. Sefkow, and J. Kopka, Construction and application of a mass spectral and retention time index database generated from plant GC/EI TOF MS metabolite profiles. 

Alcohol consumption is very difficult to assess. There is widespread belief that individuals underreport their intake and there are no reliable laboratory tests available for definitive diagnosis of alcohol abuse. A combination of abnormalities in the plasma activity of gamma-glutamyl transferase (GGT or yGT), AST and reduction in erythrocyte mean cell volume (MCV) maybe useful and all are routine lab. tests. A potential marker of interest is carbohydrate-deficient transferrin (CDT) which is an abnormal isoform of serum transferrin arising due to defects in the attachment of carbohydrate chains to the protein core. Unfortunately, CDT is a somewhat specialized test, not performed by most laboratories. Other markers which have attracted some research interest are ethyl sulphate and ethyl glucuronide. Excretion in the urine of these metabolites occurs for up to 50 hours after binge drinking so they offer a useful index of recent heavy alcohol intake. 

Direct isolation of sufficient quantities of each metabolite for structural characterization, assay validation and pharmacological or toxicological testing from in vivo studies using biological specimens is, therefore, often impossible, particularly from dmgs with a low therapeutic index. Furthermore, many metabolites have structural modifications which are difficult to replicate by traditional chemical methods. A number of synthetic steps may be required to prepare such metabolites from the API, or, in the worst case, a completely new synthetic route may need to be developed. 

Dempsey D, Jacob P, 3rd, Benowitz NL (2002) Accelerated metabolism of nicotine and cotinine in pregnant smokers. J Pharmacol Exp Ther 301(2) 594-598 Dempsey D, Tutka P, Jacob P, 3rd, Allen F, Schoedel K, Tyndale RF, Benowitz NL (2004) Nicotine metabolite ratio as an index of cytochrome P450 2A6 metabolic activity. Clin Pharmacol Ther 76 64-72... 

In a study of workers engaged in synthesizing or otherwise handling p-dichlorobenzene, it was concluded that urinary excretion of 2,5-dichlorophenol (a metabolite of p-dichlorobenzene) can serve as an index of exposure."... 

Breath analysis for TCE has provided a more accurate index of exposure than the measurement of metabolites (trichloroethanol and trichloroacetic acid) in the urine. ... 

The clinical value of monitoring drug therapy by measuring plasma levels is probably best exemplified by reference to lithium (F6). It is a useful drug, which has a narrow therapeutic index, and treatment without reference to plasma levels is probably not ethically justified. Toxic side effects are predictable and severe. It has an acceptably long plasma half-life, and its measurement both in blood and urine is comparatively simple. Moreover, there is no problem of interference from either active or inactive metabolites. 

From a purely pragmatic perspective, it is clear that reactive metabolites are linked with toxicity and that a circumstantial link can be made to idiosyncratic toxicides. Consequently, even though the mechanism of this toxicity is not fully understood, since assays are available to measure the potential for bioactivation in an ideal world one would not carry this liability forward. Conversely, it is not an ideal world, all drug molecules have challenges and the definition of therapeutic index (i.e., the ratio between the toxic exposure and the therapeutic exposure) is critical. Covalent binding of reactive metabolites to macromolecules is a crude measure and not a full predictor of toxicity and it is well known that toxicity can be ameliorated by a lower dose. Furthermore, the so-called definitive assays require radiolabeled drug material which is expensive and generally slow to produce. 

We (K1) attempted to develop a noncompetitive assay based on the anti-idiotype antibodies for a conjugated bile acid metabolite, ursodeoxycholic acid 7-A-acetyl-glucosaminide (UDCA 7-NAG), which is expected to serve as a diagnostic index for an autoimmune disease, primary biliary cirrhosis. In our assay, the hapten UDCA 7-NAG, a /3-type antibody, and a biotin-labeled a-type antibody were simultaneously added to a microtiter plate coated with an F(ab )2 fragment of a specific anti-UDCA 7-NAG antibody, then incubated at room temperature for 8 h. Bound biotin was then detected with HRP-labeled streptavidin, whose enzyme activity was measured using o-phenylenediamine/H202 as a substrate. This noncompetitive assay system provided a subfemtomole-order sensitivity (detection limit 118 amol) that was 7 times lower than the competitive immunoassay using the same anti-hapten antibody (K2), even with a common colorimetric detection (Fig. 13). Somewhat improved specificity was also obtained namely, better... 

Theophylline, a dimethylxanthine, causes broncho-dilation, possibly by inhibiting the enzyme phosphodiesterase in smooth muscle of the bronchioli. An other proposed mechanism of action is that of adenosine receptor antagonism. It has positive chronotropic and inotropic, CNS stimulant and weak diuretic properties. In obstructive lung disease sustained release tablets are to be preferred. Theophy-line has a narrow therapeutic index. Therapeutic plasma concentrations are between 7-15 mg/1. Theophylline undergoes N-demethylation via CYPl A2 in the liver and is eliminated in the urine as metabolites... 

Cyclophosphamide (Cytoxan) is the most versatile and useful of the nitrogen mustards. Preclinical testing showed it to have a favorable therapeutic index and to possess the broadest spectrum of antitumor activity of all alkylating agents. As with the other nitrogen mustards, cyclophosphamide administration results in the formation of cross-links within DNA due to a reaction of the two chloroethyl moieties of cyclophosphamide with adjacent nucleotide bases. Cyclophosphamide must be activated metabofically by microsomal enzymes of the cytochrome P450 system before ionization of the chloride atoms and formation of the cyclic ethylenimmonium ion can occur. The metabolites phosphoramide mustard and acrolein are thought to be the ultimate active cytotoxic moiety derived from cyclophosphamide. 

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