Idiosyncratic adverse drug reactions (IDR) induce severe medical complications or even death in patients. Alert structure in drugs can be metabolized as reactive metabolite (RM) in the bodies, which is one of the major factors to induce IDR. Structure modification and avoidance of alert structure in the drug candidates is an efficient method for reducing toxicity risks in drug design. This review briefly summarized the recent development of the methodologies for structure optimization strategy to reduce the toxicity risks of drug candidates. These methods include blocking metabolic site, altering metabolic pathway, reducing activity, bioisosterism, and prodrug.

The methyl group plays an important role in the rational drug design. Introducing methyl into small molecules has become an important strategy of lead compound optimization. The application of methyl in drug design is reviewed in this paper. Methyl can modulate the physicochemical, pharmacodynamic, and pharmacokinetic properties by ortho effect, inductive effect, and conformational effect. It also improves the metabolic stability as a soft metabolic point. In addition, introducing methyl into drug molecules can also be applied as a strategy in new uses of old drugs and generate me-too drugs.