Introduction

Correct interpretation of testing results for amphetamines and related compounds is dependent on many factors including an understanding of the nomenclature, structure, and metabolism of these compounds. The class of phenethylamine compounds having varying degrees of sympathomimetic activity includes amphetamine, methamphetamine, and many other compounds known by several names including amphetamines (as a group) and sympathomimetic amines. The structure of many of

From: Handbook of Drug Monitoring Methods Edited by: A. Dasgupta © Humana Press Inc., Totowa, NJ

Fig. 1. Structures of sympathomimetic amines. Reprinted with permission from ref. (1).

these compounds is shown in Fig. 1. Amphetamine is a primary amine and metham-phetamine is a secondary amine. These compounds have a stereogenic center and have enantiomers or optical isomers designated d (or +) for dextrorotatory and l (or -) for levorotatory. In general, the d isomers are the more physiologically active compounds, but pharmaceutical preparations may consist of either isomer or a mixture of both isomers. When the mixture contains equal concentrations of the two enantiomers, it is known as a racemic mixture. The existence of enantiomers for amphetamines creates analytical and interpretative problems that will be discussed in sections 2.2 and 5.2 of this chapter. Two of the compounds, shown in Fig. 1, 3,4-methylenedioxymethamphetamine (MDMA) and 3,4-methylenedioxyamphetamine (MDA) are synthetic stimulants often called amphetamine designer drugs (see Chapter 19). Several sympathomimetic amines (ephedrine, pseudoephedrine, and phenylpropanolamine) contain a hydroxyl moiety on the carbon adjacent to the amine group (a-hydroxy) and are often ingredients in over-the-counter (OTC) drugs.

Metabolism and excretion of these compounds must be considered when discussing screening and confirmatory tests for their detection, identification, and quantification. Metabolism resulting in a phenolic hydroxyl group increases acidity of the compound and changes extraction properties. The principal urinary metabolites of methamphetamine are hydroxymethamphetamine (~15%) and amphetamine (4-7%), but approximately 40-50% is excreted unchanged (1). Renal excretion is dependent on urinary pH with acidification (to a pH <5.6) decreasing the plasma half-life and alkalization increasing the plasma half-life (1).

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