Prodrugs are pharmacologically inactive chemical derivatives of a drug molecule that require a transformation within the body in order to release the active drug. They are designed to overcome pharmaceutical and/or pharmacokinetically based problems associated with the parent drug molecule that would otherwise limit the clinical usefulness of the drug. The scientific rationale, based on clinical, pharmaceutical and chemical experience, for the design of various currently used prodrugs is presented in this review. The examples presented are by no means comprehensive, but are representative of the different ways in which the prodrug approach has been used to enhance the clinical efficacy of various drug molecules.

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A proposed mechanism of spherical particle formation of the 3:1 zinc peptide encompasses the freezing of a TBA/water solution (20-70% TBA) causing the formation of a TBA hydrate phase ("dispersed TBA hydrate"). Decreasing the temperature further causes the formation of a eutectic mixture between TBA hydrate ("eutectic TBA hydrate") and water. Due to its low aqueous solubility, the zinc peptide adduct accumulates in both of the dispersed and eutectic TBA hydrate phases to form a hydrophobic "oil" phase. Since the eutectic TBA hydrate phase is surrounded by ice, a "solid emulsion" forms to lower the interfacial energy, and gives rise to spherical zinc peptide particles upon solvent sublimation. Possibility of liquid-liquid phase separation during freeze-drying was also investigated, and no evidence was found to support this alternative mechanism.

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