Stereoselectivity in pharmacodynamics and pharmacokinetics

Stereoisomers of bioactive products, particularly drugs and pesticides, usually differ greatly in their biological properties. Usually only one of the enantiomers, the eutomer, is largely responsible for the therapeutic action. The distomer, the "isomeric ballast", must be considered an impurity which often contributes to, or is even the main cause of, the undesired actions of the racemate. A fundamental requirement is to "reduce the xenobiotic load" to the feasible minimum. This implies "avoiding isomeric ballast". The implications of the use of racemic products are largely ignored in scientific publications and in the marketing of drugs and pesticides. The problem is illustrated by the results of pharmacological investigations into the racemic drugs labetalol and medroxalol.