5-Aminolevulinic acid, an unusual amino acid, possesses high water solubility and corresponding low lipid solubility and thereby has limited ability to cross biological membranes. Various 5-aminolevulinic acid esters have been synthesized to obtain more lipophilic prodrugs. [4] 5-Aminolevulinic acid hydrochloride (ALA·HCl) is used as the starting material for the synthesis of the corresponding 5-aminolevulinic acid prodrugs in most published reports. 5-Aminolevulinic acid ...