Metabolism of [des-Gly 10,D-Trp 6 ]LHRH Ethylamide in the Rabbit Conjunctiva

ABSTRACT

It was the objective of this study to determine whether [des-Gly¹⁰, D-Trp⁶]LHRH ethylamide, an LHRH agonist known as deslorelin, is degraded by the rabbit conjunctiva. Intact conjunctiva was incubated with deslorelin either alone or in the presence of 0.1 mM ouabain, 0.1 % 2,4-dinitrophenol, 0.1 mM phosphoramidon, 0.1 mM N-tosyl-L-phenylalanine chloromethylketone (TPCK), 2% EDTA, 1% ZnCl₂, 0.1% dithiothreitol PTT), or 0.1% N-ethylmaleimide (NEM) at 37°C. Furthermore, deslorelin alone was incubated with conjunctiva at 4°C All incubation solutions were made isotonic, and the pH was adjusted to 5.0. A reversed-phase HPLC was used to analyze supernatants collected at the end of 90 min. Deslorelin metabolism was inhibited at low temperature, as suggested by the disappearance of metabolite peaks at low temperature. Both ouabain and dinitrophenol failed to alter the intact drug remaining in the supernatant, indicating that energy-dependent cellular uptake of deslorelin is unlikely in the conjunctiva. Phosphoramidon- and TPCK- sensitive endopeptidases did not contribute to the observed metabolism, as suggested by the lack of effect of phosphoramidon and TPCK on deslorelin levels. DTT and NEM also failed to affect deslorelin levels. On the other hand, EDTA and ZnCl2 significantly elevated the intact deslorelin levels by 61 and 53%, respectively, and almost completely abolished the metabolite peaks, indicating a possible role for either metal-dependent peptidases or metallo-peptidases in the conjunctival metabolism of deslorelin.