Dry heat forced degradation of buserelin peptide: kinetics and degradant profiling

Buserelin is a GnRH agonist peptide drug, comprising a nine amino acid sequence (pGlu-His-Trp-Ser-Tyr-D-Ser(tBu)-Leu-Arg-Pro-NH-Et) and most commonly known for its application in hormone dependent cancer therapy, e.g. prostate cancer. In order to evaluate its hot-melt extrusion (HME) capabilities, buserelin powder in its solid state was exposed to elevated temperatures for prolonged time periods. A stability indicating UPLC-PDA method was used for quantification of buserelin and the formed degradants. Different solid state kinetic models were statistically evaluated of which the Ginstling-Brounshtein model fitted the data best. Extrapolation to and experimental verification of typical HME-related conditions, i.e. 5 min at 100°C and 125°C, showed no significant degradation, thus demonstrating the HME capabilities of buserelin. Mass spectrometric identification of the buserelin-related degradants formed under solid state heat stress was performed. Based upon the identity of these degradants, different degradation hypotheses were raised. First, direct β-elimination of the hydroxyl moiety at the serine residue, followed by fragmentation into an amide (pGlu-His-Trp-NH2) and pyruvoyl (pyruvoyl-Tyr-D-Ser(tBu)-Leu-Arg-Pro-NH-Et) peptide fragments, was postulated. Alternatively, internal esterification due to nucleophilic attack of the unprotected serine residue, followed by β-elimination or hydrolysis would yield pGlu-His-Trp, pGlu-His-Trp-NH2 and the pyruvoyl peptide fragment. Degradant pGlu-His-Trp-Ser-Tyr-NH2 is believed to be formed in a similar way. Secondly, direct backbone hydrolysis would yield pGlu-His-Trp and Tyr-D-Ser(tBu)-Leu-Arg-Pro-NH-Et peptide fragments. Moreover, the presence of Ala-Tyr-D-Ser(tBu)-Leu-Arg-Pro-NH-Et can be explained by hydrolysis of the Trp-Ser peptide bond and conversion of the serine moiety to an alanine moiety. Third and finally, isomerisation of aforementioned peptide fragments and buserelin itself was also observed.