Glutathione S-transferase (GST) is a superfamily of detoxification enzymes, represented by GSTalpha, GSTmu, GSTpi, etc. GSTalpha is the predominant isoform of GST in human liver, playing important roles for our well being. GSTpi is overexpressed in many forms of cancer, thus presenting an opportunity for selective targeting of cancer cells. Our structure-based design of prodrugs intended to release cytotoxic levels of nitric oxide in GSTpi-overexpressing cancer cells yielded PABA/NO, which exhibited anticancer activity both in vitro and in vivo with a potency similar to that of cisplatin (Findlay et al. Mol. Pharmacol. 2004, 65, 1070-1079). Here, we present the details on structural modification, molecular modeling, and enzymatic characterization for the design of PABA/NO. The design was efficient because it was on the basis of the reaction mechanism and the structures of related GST isozymes at both the ground state and the transition state. The ground-state structures outlined the shape and property of the substrate-binding site in different isozymes, and the structural information at the transition-state indicated distinct conformations of the Meisenheimer complex of prodrugs in the active site of different isozymes, providing guidance for the modifications of the molecular structure of the prodrug molecules. Two key alterations of a GSTalpha -selective compound led to the GSTpi-selective PABA/NO.