Many therapeutic agents are manufactured and administered in prodrug forms. In this paper, a new classification system for prodrugs is proposed to provide useful information about where in the body a prodrug is converted to the active drug. In this system, prodrugs are classified into Type I or Type II and the respective Subtypes IA, IB, IIA, IIB or IIC based on their sites of conversion into the final active drug form. For Type I prodrugs, conversion occurs intracellularly (e.g., antiviral nucleoside analogs, lipid-lowering statins), whereas conversion of Type II prodrugs occurs extracellularly, for examples in digestive fluids, systemic circulation or other extracellular body fluids (e.g., etoposide phosphate, valganciclovir, fosamprenavir). Type IA prodrugs refer to those that are converted at the cellular targets of therapeutic actions, whereas Type IB prodrugs' conversion occurs in the primary metabolic tissues such as liver, gut, or lung. For Type II prodrugs, the conversion process could either take place extracellularly in the milieu of gastrointestinal fluids (Type IIA), in the systemic circulation and/or other systemic extracellular fluid compartments (Type IIB), or near therapeutical target cells (Type IIC). A prodrug may belong to multiple categories and be recognized as a Mixed-Type prodrug. For example a prodrug may be converted both in target cells and metabolic tissues such as liver (i.e., named as a Type IA/IB prodrug), or one converted in both GI fluids and systemic circulations (i.e., named as a Type IIA/IIB prodrug). The Mixed-Type compound can be further distinguished as a Parallel Mixed-Type or Sequential Mixed-Type prodrug depending on the conversion processes that proceed with, either in concurrent or in sequential steps. Because traditional analysis of drug actions has always been focused on the site of action and mode of action, the proposed classification of prodrugs based on cellular locations of conversion is in line with current thought processes of regulatory review and risk assessment of both prodrug and active drug. By gaining insights regarding the site of action through prodrug nomenclature, risk benefit evaluation can be made more efficiently because both information on kinetics and impact of tissues involved are adequately revealed through prodrug subtype designated. In conclusion, the new system of classification will add to existing knowledge of prodrug classifications, and will provide improved insight into the contributory roles of both prodrug and active drug in the product's efficacy and safety, and their risk-benefit assessment.
Keywords: drug development; prodrug; prodrug classification.