Identification of stabilized dynorphin derivatives for suppressing tolerance in morphine-dependent rats

Purpose. Modulatory actions on morphine-induced effects, such as tolerance and withdrawal, have been noted for dynorphin A( 1-13) [Dyn A(1-13)] and similar peptides. These are currently of limited therapeutic potential due to extensive metabolism by human metabolic enzymes resulting in a half-life of less than 1 min in human plasma. The purpose of this study was to identify stabilized dynorphin A ( Dyn A) derivatives, to determine their metabolic routes in human plasma, and to assess whether the pharmacodynamic activity is retained. Methods. The stability of peptides in human plasma was tested using in vitro metabolism studies with and without enzyme inhibitors. Identification of the generated metabolites was performed by mass spectrometry after high performance liquid chromatography ( HPLC) separation. The in vivo activity of a stabilized dynorphin was tested by tail-flick assay in morphine-tolerant rats. Results. Though amidation of the Dyn A(1-13) was able to stop the majority of C-terminal degradation, metabolism of Dyn A(1-10) amide continued by captopril sensitive enzymes, suggesting that Dyn A(1-13) amide is a better candidate for additional stabilization. Two Dyn A(1-13) amide derivatives further stabilized at the N-terminal end, [D-Tyr(1)]-Dyn A(1- 13) amide and [N-Met-Tyr(1)]-Dyn A(1- 13) amide, showed half- lives in plasma of 70 and 130 min, respectively. The most stable derivative [N-Met-Tyr(1)]-Dyn A(1- 13) amide was tested successfully for retention of the pharmacological activity in modulating antinociceptive activity. Conclusions. [N-Met-Tyr(1)]-Dyn A(1- 13) amide showed significant stability and antinociceptive activity in the tail-flick test, thus pointing to the clinical potential of this derivative in the management of pain as well as its potential activity in suppressing opiate tolerance and withdrawal.