Purpose, To evaluate the cellular permeation characteristics and the chemical and enzymatic stability of phenylpropionic acid-based cyclic prodrugs (1) under bar and (2) under bar of opioid peptides [Leu(5)]-enkephalin (H-Tyr-Gly-Gly-Phe-Leu-OH) and DADLE (H-Tyr-D-Ala-Gly-Phe-D-Leu-OH), respectively. Methods, The rates of conversion of cyclic prodrugs (1) under bar and (2) under bar to [Leu(5)]-enkephalin and DADLE, respectively, in HBSS, pH 7.4 (Caco-2 cell transport buffer) and in various biological media having measurable esterase activity were determined by HPLC. The cell permeation characteristics of [Leu(5)]-enkephalin, DADLE, and cyclic prodrugs (1) under bar and (2) under bar were measured using Caco-2 cell monolayers grown onto microporus membranes and monitored by HPLC. Results. In HBSS. pH 7.4, cyclic prodrugs (1) under bar and (2) under bar degraded to [Leu(5)]-enkephalin and DADLE, respectively, in stoichiometric amounts. In 90% human plasma, the rates of disappearance of cyclic prodrugs (1) under bar and 2 Were slightly faster than in HBSS, pH 7.4. These accelerated rates of disappearance in 90% human plasma could be reduced to the rates observed in HBSS. pH 7.4. by pretreatment of the plasma with paraoxon, a known inhibitor of serine-dependent esterases. In homogenates nates of Caco-2 cells and rat liver, accelerated rates of disappearance of cyclic prodrugs (1) under bar and (2) under bar were not observed. When applied to the AP side of a Caco-2 cell monolayer, cyclic prodrug (1) under bar exhibited significantly greater stability against peptidase metabolism than did [Leu(5)]-enkephalin. Cyclic prodrug (2) under bar and DADLE exhibited stability similar to prodrug (1) under bar when applied to the AP side of the Caco-2 cell monolayers. Prodrug (1) under bar was 1680 fold more able to permeate the Caco-2 cell monolayers than was [Leu5]-enkephalin, in part because of its increased enzymatic stability. Prodrug (2) under bar was shown to be approximately 77 fold more able to permeate a Caco-2 cell monolayer than was DADLE. Conclusions. Cyclic prodrugs (1) under bar and (2) under bar, prepared with the phenylpropionic acid promoiety, were substantially more able to permeate Caco-2 cell monolayers than were the corresponding opioid peptides. Prodrug (1) under bar exhibited increased stability to peptidase metabolism compared to [Leu(5)]-enkephalin. In 90% human plasma but not in Caco-2 cell and rat liver homogenates, the opioid peptides were released from the cyclic prodrugs by an esterase-catalyzed reaction that is sensitive to paraoxon inhibition. However, the rate of this bioconversion appears to be extremely slow.