GnRH analogues containing conformationally restricted amino acids in positions 3 and 6: differential impact on pituitary binding affinity and direct antiproliferative effect on breast cancer cells†

Abstract

Abstract: Analogues of GnRH have been widely used in oncology and gynaecology to induce reversible chemical castration. In addition to the classic hypophysiotropic action of GnRH, it has been shown that many malignant cells, such as breast cancer cells, secrete GnRH and express the GnRH receptor/s. In order to study the effect of modifications in position3and6of GnRH on both pituitary binding affinity and breast cancer cell proliferation, we synthesized eight new GnRH analogues. All GnRH analogues lacked the carboxy–terminal Gly10–amide of GnRH and an ethylamide residue was added to Pro9. Gly6was substituted by α,α‐dialkyl amino acids (Aib:α‐aminoisobutyric acid, Deg: diethylglycine) and Trp3by D–Trp, D‐ and L‐1,2,3,4,‐tetrahydro‐isoquinoline‐3‐carboxylic acid (Tic). During competition binding experiments in mouse anterior pituitaryαT3‐1 cells, [Aib6,desGly10]GnRH‐NHEt bound to the GnRH receptor with IC50values comparable to those of parent hormone in contrast to the analogues substituted at position3. However, [L‐Tic3,Deg6,desGly10]GnRH‐NHEt had high pituitary binding affinity. With the exception of GnRH and [Aib6,desGly10]GnRH‐NHEt, all GnRH analogues significantly inhibited the proliferation of human breast cancer cells (MCF‐7); higher inhibitory effect was observed for analogues modified at position3. Results show differential impact of the modifications on the binding affinity to the GnRH receptor in mouse pituitary cells and on the inhibition of human breast cancer cell proliferation and provide insight into structure‐activity relationship of GnRH in different biological systems.