Phase-I-metabolites

The latter approach is used in the enantioselective determination of a Phase I metabolite of the antihistaminic drug, terfenadine. Terfenadine is metabolized to several Phase I compounds (Fig. 7-13), among which the carboxylic acid MDL 16.455 is an active metabolite for which plasma concentrations must often be determined. Although terfenadine can be separated directly on Chiralpak AD - an amy-lose-based CSP - the adsorption of the metabolite MDL 16.455 is too high to permit adequate resolution. By derivatizing the plasma sample with diazomethane, the carboxylic acid is converted selectively to the methyl ester, which can be separated in the presence of all other plasma compounds on the above-mentioned CSP Chiralpak AD [24] (Fig. 7-14). Recently, MDL 16.455 has been introduced as a new antihistaminic drug, fexofenadine. 

The complementary nature of APCI and ESI, APCI for less polar compounds (Phase I metabolites) and ESI for more polar compounds (Phase II metabolites). 

Phase II metabolism The reaction of a phase I metabolite with an endogenous compound, e.g. glucuronic acid, to form a polar compound that is eliminated from the body. 

Conjugate In biochemical toxicology, a structure (often an anion) formed by the combination of a xenobiotic (usually a phase I metabolite) with an endogenous component (e.g., glucuronate sulfate or glutathione). 

Rudaz, S., Veuthey, J. L., Desiderio, C., and Fanali, S. (1999). Simultaneous stereoselective analysis by capillary electrophoresis of tramadol enantiomers and their main phase I metabolites in urine.. Chromatogr. A 846, 227—237. 

Cherkaoui, S., Rudaz, S., and Veuthey, J. L. (2001). Nonaqueous capillary electrophoresis-mass spectrometry for separation of venlafaxine and its phase I metabolites. Electrophoresis 22, 491 —496. 

Madsen, K.G. et al. (2007) Development and evaluation of an electrochemical method for studying reactive phase-I metabolites correlation to in vitro drug metabolism. Chemical Research in Toxicology, 20 (5), 821-831. 

Fig. 3. Phase I adds a small reactive portion to the drug molecule, and Phase II conjugates the Phase I metabolite to an...

Phase I metabolic reactions involve oxidation, reduction, or hydrolysis of the parent molecule, resulting in the formation of a more polar compound. Phase 1 reactions are mediated by the cytochrome P450 (GYP) family of enzymes. While metabolism used to be thought of as the body s detoxification process, phase I metabolites may be equally or even more pharmacologically active than the parent compound. Drug metabolism in general, and CYP-based mechanisms in particular, are discussed in detail in Chapter 5. 

The water-soluble metabolites of sedative-hypnotics, mostly formed via the conjugation of phase I metabolites, are excreted mainly via the kidney. In most cases, changes in renal function do not have a marked effect on the elimination of parent drugs. Phenobarbital is excreted unchanged in the urine to a certain extent (20-30% in humans), and its elimination rate can be increased significantly by alkalinization of the urine. This is partly due to increased ionization at alkaline pH, since phenobarbital is a weak acid with a pKa of 7.4. 

Sundstrom, I., Hedeland, M., Bondesson, U., and Andren, P. E. (2002). Identification of glucuronide conjugates of ketobemidone and its phase I metabolites in human urine utilizing accurate mass and tandem time-of-flight mass spectrometry. J. Mass Spectrom. 37 414-420. 

The conjugates produced by phase II metabolism are considerably more water soluble than either the parent compound or the phase I metabolite(s) and hence are more excretable. 

Hepatic metabolism accounts for the clearance of all benzodiazepines. The patterns and rates of metabolism depend on the individual drugs. Most benzodiazepines undergo microsomal oxidation (phase I reactions), including TV-dealkylation and aliphatic hydroxylation. The metabolites are subsequently conjugated (phase II reactions) to form glucuronides that are excreted in the urine. However, many phase I metabolites of benzodiazepines are pharmacologically active, with long half-lives. 

Herron, W. J. Eadie, J. Penman, A. D. 1995. Estimation of ranolazine and eleven phase I metabolites in human plasma by liquid chromatography-atmospheric pressure chemical ionization mass spectrometry with selected-ion monitoring. J. Chromatogr. A, 712,55-60. 

The most significant instance of addition of a sulfur-containing group is the phase II conjugation to sulfate of a xenobiotic compound or its phase I metabolite (see Section 7.4.3) by the action of adenosine 3 -phosphate-5 -phosphosulfate, a sulfotransferase enzyme that acts as a sulfating agent ... 

One of the compounds commonly determined as its phase I metabolite is benzene,7 which undergoes the following reactions in the body (see Section 13.5) ... 

In addition to the examples discussed above, a number of other xenobiotics are measured by their phase I reaction products. These compounds and their metabolites are listed in Table 20.1. These methods are for metabolites in urine. Normally, the urine sample is acidified to release the phase I metabolites from phase II conjugates that they might have formed, and except where direct sample injection is employed, the analyte is collected as vapor or extracted into an organic solvent. In some cases, the analyte is reacted with a reagent that produces a volatile derivative that is readily separated and detected by gas chromatography. 

Compare the analysis of phase I and phase II metabolic products for biological monitoring. How are phase II products converted back to phase I metabolites for analysis ... 

Fig. 3 Chromatographic profiles of main tamoxifen phase I metabolites and some of its identified glucuronidated metabolites in a plasma sample from a breast cancer patient receiving tamoxifen 20 mg BID (modified elution gradient from reference [145]). See legends in Fig. 2 for abbreviations...

---