Many currently marketed drugs produce metabolites that are pharmacologically active, enhancing efficacy or toxicity, or exhibiting a completely different type of pharmacological profile. Aberra Fura has suggested that approximately 22% of the top 50 drugs prescribed in the US in 2003 produced metabolites that significantly affected their pharmacodynamics (PD), pharmacokinetics (PK), or both [1]. In our own assessment of the literature for the top 100 prescribed drugs reported for 2013–2014, we found that ~39% produce at least one active metabolite. Given the importance of active metabolites in drug safety and efficacy, extending the patent life of a drug or becoming drug leads in their own right, it is important to identify active metabolites early in the drug development process.