Pre-clinical metabolism & pharmacokinetic aspects are crucial in lead generation and optimization. The most important determinants of the pharmacokinetic profile of a drug are metabolism by the host organism. High metabolic charge regularly hints to poor bioavailability and high clearance. The development of active or toxic metabolites will have an impact on the pharmacological and toxicological outcomes and there will be also possible for drug-drug interactions with administered drugs due to inhibition and /or induction of drug metabolism pathways. Thereby, optimization of the metabolic liability and drug-drug interaction potential of the new chemical entities are certain of the greatest crucial steps during the drug discovery process. The rate and site of metabolism of new chemical entities by drug metabolizing enzymes are approachable to modulation by appropriate structural changes. In the same way, the potential for drug-drug interactions can also be lessened by appropriate structural modifications to the drug candidate. Yet, the optimization of the metabolic stability and drug-drug interaction potential during drug discovery stage has been mostly by empirical methods and by trial and error.