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In a simplified manner, prodrugs are masked forms of active drugs that are designed to be activated once they have administered into the body. Prodrugs are considered to be inactive or at least significantly less active than the released real drugs and therefore they have to undergo an enzymatic or chemical biotransformation before eliciting their pharmacological effects. The rationale behind the use of prodrugs is generally to optimize physicochemical, pharmaceutical and pharmacokinetic properties (such as aqueous solubility, permeability, bioavailability, chemical stability and duration of action), which can lead, if unfavourable, to considerable problems in later drug development. Additionally, the prodrug strategy has been used to increase the selectivity of drugs for their intended target. This improves not only the efficacy of the drug, but also decreases systemic and/or unwanted tissue/organ-specific toxicity. Development of a prodrug with improved properties may also represent a life-cycle management opportunity. In recent years, numerous prodrugs, designed to overcome formulation, delivery, and toxicity barriers to drug utilization, have reached the market. Approximately 10% of all marketed small molecular drugs are prodrugs.