Overcoming chemical challenges in the solid-phase synthesis of high-purity GnRH antagonist Degarelix. Part 2

The hydrolysis and rearrangement of the dihydroorotic (Hor) residue in the presence of bases, leading to the formation of the hydantoin (Hyd) impurity, represent one of the major problems in manufacturing of the gonadotropin-releasing hormone antagonist Degarelix. In an attempt to find efficient strategies to overcome this problem, we carried out a screening of organic bases in order to select those which afforded both the rapid Fmoc deprotection during the solid-phase synthesis and the absence of this peculiar rearrangement. Among the bases tested, only tert-butylamine did not affect the peptide molecule and was able to perform fast Fmoc removal. The use of tert-butylamine for the synthesis of Degarelix led to a product with excellent purity and yield without a detectable amount of the hydantoin impurity. Thus, we showed that tert-butylamine can be a suitable alternative to piperidine for industrial-scale production of Degarelix or other Hor-containing peptide pharmaceuticals.