Interaction of [d-Trp6, Des-Gly10] LHRH Ethylamide and Hydroxy Propyl β-Cyclodextrin (HPβCD): Thermodynamics of Interaction and Protection from Degradation by α-Chymotrypsin

Abstract

Purpose: The purpose of this study is to investigate the mechanisms and thermodynamics of the interaction between hydroxypropyl β-cyclodextrin (HPβCD) and [d-Trp6, des-Gly10] LHRH ethylamide (deslorelin), a peptide drug. Methods: We used UV and Fluorescence spectroscopy to study the interaction of HPβCD and deslorelin. Circular dichroism was used to study the conformational changes induced in deslorelin upon interaction with HPβCD. The thermodynamics of the interaction of deslorelin and HPβCD was studied using isothermal titration calorimetry (ITC). We also determined the effect of HPβCD on the degradation of deslorelin by α-chymotrypsin. Results: UV and fluorescence spectroscopy indicated that HPβCD induced a change in polarity of the environment surrounding the chromophores of deslorelin. Wavelength selective fluorescence indicated an increase in the fluorescence polarization of deslorelin with an increase in excitation wavelength in the presence of HPβCD suggesting that tryptophan is present in a media of reduced mobility. Circular dichroism studies indicated that HPβCD stabilizes the conformation of deslorelin. In addition, ITC indicated an exothermic reaction between deslorelin and HPβCD with a low enthalpy of binding of ∼−600 cal/mol and a binding affinity of ∼1.25 × 102 M−1. Finally, the rate of degradation of deslorelin by α-chymotrypsin was decreased by 33% in the presence of HPβCD. Conclusions: These results indicate that there is an interaction between HPβCD and deslorelin, which involves the inclusion of aromatic amino acids of deslorelin into the hydrophobic cavity of the cyclodextrin. This inclusion, providing steric hindrance, may be one of the mechanisms by which HPβCD reduces enzymatic hydrolysis of deslorelin.