Overcoming Chemical Challenges in the Solid-Phase Synthesis of High-Purity GnRH Antagonist Degarelix. Part 1

The highly potent, long-acting, gonadotropin-releasing hormone antagonist Degarelix is known to be very efficient for prostate cancer treatment. The synthesis of decapeptide Degarelix is complicated because of the presence in its sequence of several unnatural α-amino acids, which are prone to rearrangements and side reactions. In particular, the rearrangement of the dihydroorotic (Hor) moiety with following hydantoin formation in the presence of bases represents one of the major problems. In this study, we describe a novel chemical strategy to overcome this obstacle by the use of the corresponding p-nitrophenylalanine derivative, which is reduced on the solid support to p-aminophenylalanine and acylated with dihydroorotic acid at the end of the solid-phase synthesis. Thus, the contact of Hor with the bases required for Fmoc deprotection is completely avoided. This approach provides a superior purity of Degarelix when the synthesis is carried out in the industrial scale as well.