Racemic therapeutics : ethical and regulatory aspects

Racemic therapeutics are fixed ratio mixtures of stereoisomers to be regarded biologically as different compounds. Usually only one of the isomers fully contributes to the therapeutics action, and the other is often classifiable as "isomeric ballast". Due to differences in turnover and pharmacokinetics, the proportion of enantiomers (1:1 in the racemate) continuously changes in plasma. The implications of the neglect of stereoselectivity for various levels in the investigation of racemic drugs are discussed and summarized in Table 2. The fact is that clinical investigators, Ethical Committees and regulatory authorities have for decades accepted invalid pharmacokinetic data on some 25% of therapeutics. That those racemates remain in use make the benefit of and necessity for kinetics generally questionable. Exposure of patients to the "isomeric ballast" present in about 50% of the most commonly used drugs will probably contain for many decades. As a result of a change in attitude of the regulatory authorities, however, for new drugs the choice in future between the racemic therapeutic or the single isomeric ballast-free drug will largely be based on a critical evaluation of the chiral characteristics with regard to their therapeutic, toxicological and pharmacokinetic aspects.