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.